Enhanced dewaterability of sludge during anaerobic digestion with thermal hydrolysis pretreatment: New insights through structure evolution

Rheological properties of thermally treated and digested sludge: Implications for energy requirements of pumps and agitators

Understanding sludge rheology and optimizing equipment performance is crucial for energy efficiency in wastewater treatment plants (WWTPs). This study examined sludge rheology after thermal hydrolysis pre-treatment (THP) at 60, 80, and 120 °C for 2 h, followed by anaerobic digestion (AD) at 37 °C for 20 days, and assessed impacts on pump and agitator performance. Post-treatment, sludge showed reduced viscosity and improved flowability, indicated by changes in Herschel-Bulkley parameters, enhancing pump and agitator efficiency, particularly at 120 °C. These rheological improvements were correlated to the solubilization of sludge components after THP and solids reduction after AD, highlighting the interconnectedness of rheology and treatment outcomes. Despite high heat demands, an energy balance showed that THP scenarios, especially at 120 °C, had lower energy requirements for pumps and agitators, leading to energy savings without increased heat consumption. These findings underscore the influence of rheological changes in improving energy efficiency in WWTPs.

Characteristics and aging of microplastics in waste activated sludge under persulfate and hydrothermal co-treatment: Impact of solid content and temperature

Activated persulfate and hydrothermal treatment (HTT) are often employed to treat waste activated sludge, which can improve the efficiency of subsequent sludge treatment and change the distribution of pollutants in the sludge. However, the impact of sludge solid content and temperature on the occurrence and aging of microplastics (MPs) during HTT remains poorly understood. This study investigated the effects of persulfate-HTT (SPS-HTT) co-treatment on the migration, occurrence, and aging of MPs in sludge with different solid contents (2% and 5% solid content). The results indicated that SPS-HTT co-treatment triggers both the disruption of sludge flocs and the melting deformation of MPs at high temperatures, leading to variations in the increasing trend of MP concentration in the solid-liquid phase at different solid contents. 5% solid content sludge showed a weak release of MPs from the solid phase. The proportion of fiber MPs first increased and then decreased with increasing temperature, while no significant changes were observed in the color and type of MPs. Higher temperature and solid content induced the melting deformation of MPs, exacerbated the aging of polypropylene MPs, and resulted in rough surfaces, higher carbonyl index, and variations in crystallinity. Moreover, the correlation between the carbonyl index and aging indicators increased with increasing solid content. The MP-derived dissolved organic matter under HTT primarily comprised soluble microbial by-products and humic acid-like substances. These findings underscore the significance of sludge solid content in affecting the migration and aging of MPs during HTT, and offer novel insights into the application of HTT to MP management in sludge treatment.

Enhanced methane production from waste activated sludge by potassium ferrate combined with thermal hydrolysis pretreatment

Anaerobic digestion of waste activated sludge (WAS) was one of the directions of sludge treatment, but how to effectively improve the production of methane as a resource product of anaerobic digestion of sludge still needs further research. The study examined how the combination of potassium ferrate (PF) and thermal hydrolysis (TH) pretreatment affected methane production from sludge. The results demonstrated a positive synergistic effect on methane production with PF-TH pretreatment. Specifically, by employing a 0.05 g/g TSS (total suspended solids) PF in conjunction with TH at 80 °C for 30 min, the methane yield increased from 170.66 ± 0.92 to 232.73 ± 2.21 mL/g VSS (volatile suspended solids). The co-pretreatment of PF and TH has been substantiated by mechanism studies to effectively enhance the disintegration and biodegradability of sludge. Additionally, the variation of microbial community revealed an enrichment of active microorganisms associated with anaerobic digestion after treated with PF + TH, resulting in a total abundance increase from 11.87 to 20.45% in the PF + TH group.

Enhancing Phosphorus Release from Sewage Sludge in Anaerobic Digestion via Thermal Hydrolysis Pretreatment: Insights from Phosphorus Speciation and Molecular Biological Pathways

This study explores the mechanisms enhancing phosphorus (P) release from sludge in anaerobic digestion (AD) with thermal hydrolysis pretreatment (THP) using sequential chemical extraction, X-ray absorption near-edge structure spectroscopy (XANES), 31P NMR, and multiomics. THP-treated sludge notably increased liquid-phase P by 53.8% over 3 days compared to sewage sludge (SS), identifying solid-phase Fe-P as the primary P source. The THP+AD also provided a higher abundance of bacteria that contributed to P release through multiple pathways (MPRPB), whereas SS+AD enriched some microbial species with single P release pathway. Moreover, species co-occurrence network analysis underlined the pivotal role of P-releasing bacteria in THP+AD, with 8 out of 16 keystones being P-releasers. Among the 63 screened genes that were related to P transformations and release, the poly beta-hydroxybutyrate (PHB) synthesis genes associated with polyphosphate bacteria-mediated P release were more abundant in THP+AD than in SS+AD. Furthermore, the upregulation of genes involved in methyl phosphonate metabolism in the THP-treated sludge enhanced the methane production potential of the AD process. These findings suggested that MPRPB were indeed the main contributors to P release, and enrichment in the THP+AD process enhanced their capability for P liberation.

Yield stress Measurement of municipal sludge: A comprehensive evaluation of testing methods and concentration effects using a rotational rheometer

Accurate prediction and measurement of yield stress are crucial for optimizing sludge treatment and disposal. However, the differences and applicability of various methods for measuring yield stress are subjects of ongoing debate. Meanwhile, literature on measuring sludge yield stress is limited to low solid concentrations (TS <10%), understanding and studying the yield stress of medium to high solid concentration sludge is crucial due to increasingly stringent standards for sludge treatment and disposal. So, this study employed a rotational rheometer to measure sludge yield stress across a wide range of TS (4-50%) using steady shear, dynamic oscillatory shear, and transient shear. The study derived significant conclusions by comparing and summarizing the applicability and limitations of each testing method: Dynamic oscillatory shear methods, including G'-σ curve method, γ-σ curve method, and G**γc method can measure sludge yield stress ranging from 4% to 40% TS, while other methods are restricted to low or limited solid concentrations; The G' = G″ method, utilizing the intersection of G' and G″ curves, consistently yields the highest value for yield stress when 4%≤ TS ≤ 12%; The rotational rheometer cannot measure sludge yield stress when the solid concentration exceeds 40% TS; The relationship between sludge yield stress and solid concentration is stronger as a power-law for TS ≤ 25%, transitioning to linear for higher concentrations (28%≤ TS <40%). This study systematically explores the applicability and limitations of various measurement methods for characterizing sludge yield stress across a wide range of solid concentrations, providing valuable guidance for scientific measurement and highlighting challenging research issues.

Research progress in improving sludge dewaterability: sludge characteristics, chemical conditioning and influencing factors

Sludge from wastewater treatment processes with high water content and large volume has become an inevitable issue in environmental management. Due to the challenging dewatering properties of sludge, current mechanical dewatering methods are no longer sufficient to meet the escalating water content standards of sludge. This paper summarizes the characteristics of various sludge and raises reasons for the their dewaterability differences. Affected by extracellular polymeric substances, biological sludge is hydrophilic and negatively charged, which limits the dewatering degree. The rheological properties, flocs, ionic composition, and solid phase concentration of the sludge also influence the dewatering to some extent. For these factors, the chemical conditioning measures with simple operation and excellent effect improve its dewaterability, which mainly include flocculation/coagulation, acid/alkali treatment, advanced oxidation, surfactant treatment and combined treatment. There is a growing necessity to explore the development of new chemical conditioning agents, even though traditional agents continue to remain widely used. However, the development of these new agents should prioritize finding a balance between various factors such as efficiency, effectiveness, ease of operation, environmental safety, and cost-effectiveness. Electrochemical dewatering enhances solid-liquid separation, and its coupling with chemical conditioning is also an excellent means to further reduce water content. In addition, the improvement of press filter is an effective way, which is influenced by pressure, processing time, sludge cake thickness and pore structure, filter media etc. In general, it is essential to develop new conditioning agents and enhance mechanical filtration press technology based on a thorough understanding of various sludge properties. Concurrently, an in-depth study of the principles of mechanical pressure filtration will contribute to establishing a theoretical foundation for effective deep sludge dewatering and propel further advancements in this field.

Rethinking the relationships between gel like structure and sludge dewaterability based on a binary gel like structure model: Implications for the online sensing of dewaterability

The digital transformation of sludge treatment processes requires online sensing of dewaterability. This topic has been attempted for many years based on macroscopic shear rheology. However, the relationship between rheological behavior and dewaterability remains noncommittal, and the reason is unclear. Herein, a binary gel-like structure model was proposed including the interactions network at the supra-flocs level and the gel-like structure at the flocs level. Multiple advanced techniques including optical tweezers were employed to precisely understand the binary gel-like structure and to classify the correlation mechanism between this gel-like structure, rheological behavior, and dewaterability. The analysis of sludge from eight wastewater treatment plants showed the binary gel-like structures at both supra-flocs and flocs levels have significant relationships with sludge dewaterability (p < 0.05). Further deconstruction of the sludge viscoelastic behavior illustrated that the gel-like structure at the supra-flocs level dominates the rheological behavior of sludge. Moreover, the direct description of the binary gel-like structure in four typical sludge treatment processes highlighted the importance of the flocs level's structure in determining the dewaterability. Overall, this study revealed that shear rheology may prefer to stress the interactions network at the supra-flocs level but mask the flocs level's structure, although the latter is important. This observation may provide a general guideline for the design of robust sensors for dewaterability.

Anaerobic co-digestion of sewage sludge pretreated by thermal hydrolysis and food waste: gas production, dewatering performance, and community structure

Anaerobic co-digestion can effectively break the limitations of mono-digestion. However, there are still some problems such as long residence time, unsatisfactory methane yield, and unstable performance for co-digestion of sewage sludge (SS) and food waste (FW). Therefore, the SS in the reactor treating co-digestion of SS and FW is considered to be pretreated by thermal hydrolysis. In this work, the anaerobic co-digestion of SS of thermal hydrolysis pretreatment (THP) and FW significantly improved the stability, methane production of the digestive reactor, and dewaterability of the digested sludge. The R6 obtained the most cumulative methane production (315.76 mL/g VS). In addition, compared to R3, the cumulative methane production and maximum methane production rate of R5 increased by 9.93% and 14.56%, respectively. The dewaterability of R4, R5, and R6 was improved, while the dewatering performance of the R3 decreased to a greater extent. The results of the kinetic model fitting were consistent with the experimental results. Among them, the hydrolysis constants (Kh) of anaerobic co-digestion of THP-SS and FW were 0.121, 0.130, and 0.114 d-1, respectively, which were higher than those of other groups. And the estimated lag time (λ) of co-digestion was also lower than that of mono-digestion groups. Microbial community analysis indicated that the bacterial diversity and richness of anaerobic co-digested groups of THP-SS and FW were enhanced, while the methanogens with acetoclastic pathway became the main methanogenic microorganisms. This work provides essential information on anaerobic co-digestion containing different THP-SS contents.

Characterization of melanoidins in thermal hydrolysis sludge and effects on dewatering performance

Thermal hydrolysis pretreatment (THP) of sludge can form the refractory brown melanoidins due to the occurrence of the Maillard reaction, which adversely involves the subsequent sludge anaerobic digestion (AD) process. However, details of the generation pattern of melanoidins and how they affect the sludge dewaterability remain largely unknown. This work aims to determine an approach to characterize and quantify the melanoidins created by THP of sludge. On this basis, the effect of melanoidins on sludge dewatering performance was revealed by adding synthetic melanoidins to the mixed sludge. Experimental results showed that three-dimensional fluorescence-region integration (3DEEM-FRI) could effectively distinguish melanoidins from other organic substances and achieve semi-quantitative characterization in sludge. The melanoidins significantly deteriorated the sludge dewaterability, and the lowest solids content of the filter cake (TS) was only 17.78% at the addition of 480 mg (g TS)-1, which was a drop of about 20% compared to the control group. The mechanism investigations indicated that the internal structure of sludge becoming particularly complicated and the opportunities for molecules to collide with each other enlarged because of the contribution of melanoidins, resulting in the increment of the sludge apparent viscosity and consistency coefficient (k), a decline of the flow behavior index (n) and a weakening of flowability. Melanoidins could capture massive water molecules and carry negative charges with the decrease of sludge particle size and zeta potential value, which enhanced the electrostatic repulsion between sludge particles and abated the flocculation ability, thus further aggravating the sludge dewatering performance.

A quantitative theory integrating solid surface hydrophilicity and pore structure features for non-phase-change drying of sewage sludge through gradient increase of ultrahigh filtration pressure

The sustainable application of thermal sludge drying process is limited by the high energy consumption due to the phase-change latent heat of moisture. This study proposed that the ultrahigh pressure filtration could realize the non-phase-change sludge drying. The lowest water content of 28.12 wt.% was realized by the filtration pressure of 21 MPa for the excess sludge with polyaluminium chloride as the conditioning agent. With the stepwise increase of filtration pressure employed (5-21 MPa), the diameter of solid pores was gradually narrowed to the same order of magnitude with the thickness of vicinal water film (i.e., 1-10 nm). As a result, the capillary water was transformed into the vicinal water, and the solid-water interface interaction played more crucial roles in water occurrence states. However, Hagen-Poiseuille equation was introduced to estimate the pore water outflow based on the pore wall hydrophilicity and the external filtration pressure, which implied that there can be always a sufficiently large driving force to maintain the water outflow rate no matter how the pore diameter is small and the sidewall is hydrophilic. Typically, the fitting results of excess sludge (R2=0.985, p-value<0.01) indicated that the pressure gradient of 2.11 × 109 Pa/m was required to maintain the pore water flow rate of 1.38 × 10-15 m3/s with the median pore diameter of 5.33 × 10-7 m. All these findings broke through the conventional cognition that only thermal drying process can decrease the sludge water content below 60 wt.%, and facilitated energy saving of sludge dewatering process through non-phase-change separation, i.e., ultrahigh pressure filtration.

Volatile fatty acids bio-production using extracellular polymeric substances disengaged from sludge for carbon source recycling

Excessive waste-activated sludge (WAS) and insufficient carbon source (CS) for biological nitrogen removal (BNR) often coexist in municipal sewage treatment. Although the production of volatile fatty acids (VFAs) from WAS has been recognized as a promising solution, the development is limited by low VFAs production efficiency and dewatering deterioration of sludge. This study extracted the extracellular polymeric substances (EPS) from sludge by low-temperature thermal-hydrolysis (LTH) and high-speed hydro-cyclone (HSHC) pretreatment and recovered it for high-quality VFAs bio-production in thermophilic fermentation. Microbial mechanism analysis disclosed that interspecific interaction networks composed of functional flora, which accumulate VFAs by bio-converting EPS primarily and supplemented by EPS synthesis, guaranteed the efficient bio-production of VFAs. This process scheme shows promise in providing alternative denitrification CSs and avoiding deterioration of sludge dewaterability.

Effect of exogenous CaO addition on H2S production from waste activated sludge and its influence mechanism

Hydrogen sulfide (H₂S) production from waste activated sludge (WAS) is the main reason for odor emission during anaerobic fermentation system. CaO has been reported to effectively improve the resources recovery of WAS, but its potential effect on H₂S production in anaerobic fermentation process remains unrecognized. In present study, it was found that the addition of 60 mg/g VSS CaO greatly inhibited H₂S production and the maximum yield of H₂S was 60.1 ± 1.8% lower than the control. Mechanism investigation demonstrated that CaO destroyed sludge structure and increased the release of intracellular organic matter with hydrogen bonding networks destroying, but had a mild effect on the transformation of sulfur containing organic matters and inorganic sulfate reduction. Additionally, the enhancement in H⁺ and S^2- consumption by alkaline condition and metal ions release was another reason for the inhibition of H₂S production in CaO addition reactors. Furthermore, microbial analysis showed that CaO addition importantly reduced the hydrolysis microorganism, particularly denitrification hydrolytic bacterias (e.g., unclassified_f_Chitinophagaceae and Dechloromonas), sulfate reducing bacterias (SRBs) (e.g., unclassified_c_Deltaproteobacteria and Desulfosarcina) and genes (e.g., PepD, cysN/D, CysH/C and Sir) involved in organic sulfur hydrolysis and sulfate reduction. Results from this study provides theoretical insights into the practical applications of CaO.

Rheological properties and dewaterability of anaerobic co-digestion with sewage sludge and food waste: effect of thermal hydrolysis pretreatment and mixing ratios

Anaerobic co-digestion (co-AD) of sewage sludge (SS) and food waste (FW) converts municipal organic waste into renewable energy, which plays an important role in achieving carbon emissions reduction. The existing anaerobic digestion (AD) treatment projects often have problems such as low organic conversion and unstable performance. SS and FW were used as raw materials to explore the effects of thermal hydrolysis pretreatment (THP) and mixing ratios on the dewaterability and rheological properties of the digestate. The results showed that co-digestion of FW and SS in a ratio of 1:1 obtained the highest biogas production (255.14 mL/g VS), which was 1.53 times and 14.5 times higher than that of mono-digestion of FW and thermal hydrolysis pretreatment sewage sludge (THSS), respectively. However, the dewaterability of this ratio deteriorated sharply after co-digestion, with a decrease of 54.92%. The groups containing a higher proportion of THSS had improved dewaterability after AD. The apparent viscosity and shear stress were reduced by co-digestion compared with mono-digestion of THSS and FW, indicating a higher flow property of the co-digestion matrix. After the Herschel-Bulkley model fitting, there were linear correlations between rheological indices and soluble chemical oxygen demand (SCOD), and digestate dewaterability.

The effect of repeated energy inputs on the release profiles of extracellular organic substances in sewage sludge

Organic matter (OM) recovery from sewage sludge is critical for sustainable development. Extracellular organic substances (EOS) are the main organic components of sludge, and the release of EOS from sludge is usually the rate-limiting step for OM recovery. However, a poor understanding of the intrinsic characteristics of binding strength (BS) of EOS usually restricts the release of OM from sludge. To reveal the underlying mechanism that how the intrinsic characteristics of EOS limit its release, in this study, the BS of EOS in sludge was quantitatively characterised by 10 rounds of energy input (E_in) with the same magnitude per round; the corresponding changes in the main components, floc structures and rheological properties of sludge after different numbers of E_in were also explored. Results showed that relationships between the release of EOS and the main multivalent metals, median diameters, fractal dimensions, elastic modulus and viscous modulus in the linear viscoelastic region of sludge versus the number of E_in, highlighted that the power-law distribution of BS in EOS was responsible for the occurrence state of organic molecules, stability of floc structures and maintenance of rheological properties. The result of hierarchical cluster analysis (HCA) further revealed three BS levels of the EOS in sludge, indicating that the release or recovery of OM from sludge occurred in three stages. To the best of our knowledge, this is the first study that explores the release profiles of EOS in sludge by repeated E_in for assessing the BS. Our findings may provide an important theoretical basis for the development target methods about the release and recovery of OM from sludge.

Towards the carbon neutrality of sludge treatment and disposal in China: A nationwide analysis based on life cycle assessment and scenario discovery

Motivated by the carbon neutrality target, strategic planning for a low-carbon transition of sludge treatment and disposal in China is challenging due to the unpredictability of technical, regional, socioeconomic, and political factors affecting greenhouse gas (GHG) emissions. This study combines the use of a Life Cycle Assessment and the Patient Rule Induction Method, accounting for possibilities that could achieve net-zero carbon emissions by exploring multiple plausible future profiles of sludge treatment and disposal. Results show that reducing sludge landfill and increasing anaerobic digestion are effective methods to facilitate GHG reduction. Achieving carbon neutrality is closely linked to developing a cleaner electricity mix. Based on a cascaded scenario analysis considering regional differences for 31 Chinese provinces, results demonstrated a maximum cumulative reduction potential of 371 Mt CO₂ equivalents from 2020 to 2050, equal to 59.84% of the business-as-usual scenario. Together with GHG reductions, terrestrial acidification and ecotoxicity as well as freshwater ecotoxicity are synergistically reduced. However, the shifting environmental burden results in freshwater eutrophication, human toxicity, marine ecotoxicity, marine eutrophication, and photochemical oxidant formation. This study presents a novel method for systematically identifying possible future development paths toward carbon neutrality. The findings may support policy designs for achieving target carbon reduction effects for sludge disposal.

Strategies for energy conversion from sludge to methane through pretreatment coupled anaerobic digestion: Potential energy loss or gain

Anaerobic digestion (AD) of wasted activated sludge from wastewater plants is recognized as an effective method to reclaim energy in the form of methane. AD performance has been enhanced by coupling various pretreatments that impact energy conversion from sludge. This paper mainly reviewed the development of pretreatments based on different technologies reported in recent years and evaluated their energy benefit. Significant increases in methane yield are generally obtained in AD with pretreatments demanding energy input, including thermal- and ultrasound-based methods. However, these energy-intense pretreatments usually gained negative energy benefit that the increase in methane yield consumed extra energy input. The unbalanced relationship counts against the goal of energy reclamation from sludge. Combined pretreatment consisting of multiple technologies normally outcompetes the single pretreatment, and the combination of energy-intense methods and chemicals potentially reduces energy input and simultaneously ensure high methane yield. For determining whether the energy reclamation from sludge via AD contribute to mitigating global warming, integrating greenhouse gas emission into the evaluation system of pretreated AD is further warranted.

The function of "Cambi® thermal hydrolysis + anaerobic digestion" on heavy metal behavior and risks in a full-scale sludge treatment plant based on four seasons investigation

The environmental risk of heavy metals in sewage sludge from a full-scale "Cambi® thermal hydrolysis + anaerobic digestion" sludge treatment plant was discussed based on four seasons' data. Results showed that the order of heavy metal concentration in sludge was Zn > Cu > Cr > Ni > As > Pb > Hg > Cd, which all increased significantly due to the "enrichment effect" caused by the degradation of organics. Nevertheless, the mass of heavy metals except for Cd decreased. Chemical fractions of different heavy metals in raw sludge varied greatly. The proportion of their residual fraction all increased slightly after treatment. Thermal hydrolysis and anaerobic digestion led to the transformation of some heavy metal fractions. Deep dehydration process reduced the mass of heavy metals from sludge (less than 10%). Potential ecological risk of heavy metals was low (RI <150) when sludge is applied 0.75 kg/m² to soil according to GB 4284-2018, in which the risk of Hg and Cd was highest. Furthermore, the accumulation amounts of heavy metals in test soil and rural soil with the annual sludge application amount of 0.75 kg/m² for 15 years were calculated, which did not exceed GB 36600-2018 and GB 15618-2018 respectively.

Evaluation of impact of sludge types and solids content on sludge treatment using microwave enhanced advanced oxidation process

The microwave enhanced advanced oxidation process (MW-AOP) has been applied to pre-treat different sludge types and high solids content. Secondary sludge not only had the highest solids and nutrient content but also yielded higher treatment efficiency than primary or mixed sludge. In the case of secondary sludge with 4% total solids (TS), the total suspended solids (TSS) concentration was reduced by 32% while soluble chemical oxygen demand concentration increased from 1% to 40% after treatment at 110°C. A high level of nutrient release was also achieved; about 65% total phosphate (TP) solubilized at 110°C. The degree of secondary sludge disintegration was dictated by temperature and hydrogen peroxide dosage. The optimal operating temperature for the system was 110°C, and sludge containing TS up to 8% was treated effectively. Secondary sludge with 8% TS had a TSS reduction of 41% after treatment at 110°C while COD solubilization was about 45%; about 55% TP was solubilized at 10 min holding time. Treatment of sludge with higher solids content would allow for handling larger amounts of sludge at a given period and reduce heating cost per unit of treated sludge. The inter-relationship between the degree of sludge disintegration and changes in chemical and physical properties was also clearly demonstrated here. The treated sludge would be an ideal substrate for anaerobic digestion or phosphorous recovery processes. High levels of nutrients (phosphorus and nitrogen) and metal release, and solids disintegration from sludge containing high solids content would make subsequent resource recovery processes more effective and economical.

Impact of hydrophilic functional groups of macromolecular organic fractions on food waste digestate dewaterability

：Deterioration of dewaterability is one of challenges faced by anaerobic digestion (AD) of food waste (FW). The underlying mechanism of the effect of AD on digestate dewaterability remains unclear. Thus, the effect of hydrophilic functional groups of macromolecular organic on FW digestate dewaterability in different stages during AD was studied. Results showed that the dewaterability first improved at the acidification stage, and then worsened at the gasification and stabilization stages. The correlations between normalized capillary suction time (NCST), bound moisture (BM) and extracellular protein (extra-PN) were significant (R = 0.736, p < 0.05, R = 0.637, p < 0.05). Macromolecular extra-PN that enhance the bonding between organic fractions and moisture via peptide bonds. In addition, carbonyl, phenolic and amide groups increased after AD, resulting in the enhancement of the digestate hydrophilicity. Furthermore, the evolution of microbial community during AD resulting in the wrapping of BM by increased organic fractions. Therefore, higher organic fractions with hydrophilic functional groups in digestate strongly hinder moisture removal. The findings obtained deepen our understanding of hydrophilic functional groups of macromolecular organic affecting FW digestate dewaterability and provide strong supports to treatment and disposal of FW digestate.

Improving sludge dewaterability by free nitrous acid and lysozyme pretreatment: Performances and mechanisms

Reducing the water content of waste activated sludge (WAS) is critical for sludge treatment and disposal in wastewater treatment plants (WWTPs). In this study, a new combined conditioning processes by using lysozyme (LZM) and free nitrous acid (FNA) were proposed and demonstrated to enhance the dewaterability of WAS. The water content of sludge cake dropped from 82.82 % to 68.42 % (1 h FNA treatment + 1 h LZM treatment) and 69.52 % (6 h FNA treatment + 1 h LZM treatment) with the combined FNA and LZM treatment; and the corresponding capillary suction time (CST) reduction efficiency increased 49.29 % (1 h FNA treatment + 1 h LZM treatment) and 52.98 % (6 h FNA treatment + 1 h LZM treatment). A comprehensive investigation conducted in this study revealed the underlying mechanism of dewaterability improvement lies in the transformations of extracellular polymeric substances (EPS). The combined conditioning led to enhanced hydrophobicity in the sludge, as suggested by FTIR protein secondary structure and interfacial free energy. The reduced zeta potential and the potential barrier indicated the reduction of the repulsive force of sludge particles and the bound water content in the conditioned floc. The hydrophobicity, flow permeability and flocculability were enhanced after combined treatment, leading to the release of bound water.

Influential mechanism of water occurrence states of waste-activated sludge: Over-focused significance of cell lysis to bound water reduction

The rigid cell membrane structure is widely thought to retain the intracellular water and positively contributes to the presence of bound water in waste-activated sludge (WAS), which is the main obstacle of its highly-efficient dewatering. However, few studies realized the quantification of intracellular water fraction in the total bound water. Thus, there still may be some debates on whether and what extent of cell lysis is optimal for the dewaterability improvement. This study specifically focused on the effect of microbial cell lysis on the water occurrence states of WAS. The sonication, cyclic freezing-thawing and dimethyl sulfoxide (DMSO) amendment were used as the non-chemical means for cell lysis without altering the chemical compositions of WAS. The extent of cell lysis was quantified by the aqueous lactate dehydrogenase (LDH) released from intracellular cytoplasm and the water occurrence states of WAS were characterized by the transverse relaxation time (T₂) spectra of low-field nuclear magnetic resonance (NMR). The results indicated that 8 h sonication (60 W/g dry matter, solid content of WAS: 23.10±0.30 g/L) completely lysed the microbial cells, but only increased the moderately mobile water fraction from 0.555% to 2.370%; similarly, it could be estimated that nearly 15% of cells were destructed after 5 times of freezing-thawing, but the fraction of moderately mobile water only rose from 0.555% to 0.805%. The transmission electron microscope (TEM) with ultrathin sections visually tracked the WAS micro-morphology accompanied with the cell lysis; the sonication caused the notable lysis of microbial cells and dispersed the external encapsulating components, which originally surrounded microbial cells closely; most of the microbial cells could be deformed but wasn't lysed by cyclic freezing-thawing; DMSO amendment made the outer edge of microbial cells tend to be rough, which may reflect the DMSO-enhanced permeability of cell membrane. The correlative analysis further indicated that the capillary suction time (CST) had the close correlation with particle size/zeta potential (Pearson coefficient>0.85, p-value<0.05), but no strong correlation was identified between CST and slightly reduced bound water contents (Pearson coefficient<0.9, p-value≥0.05). Instead of the cell integrity, the compositional aggregation states dominated the water occurrence states of WAS. Highly-efficient conditioning approaches should rely on the reduction of bio-floc porosity through eliminating solid-liquid interfacial affinity instead of damaging the cell membrane structure.

A unified operating procedure is crucial to evaluate sludge dewaterability, taking the setup of refrigerated storage time as an example

Although extensive efforts have been carried out to study sludge dewatering mechanism, the lack of universal operating procedures makes it never be satisfactorily explained. This study evaluated the impact of a unified operating procedure on waste activated sludge (WAS) dewaterability by taking the setup of refrigerated storage time as an example. It was found that storage time played an important role in determining WAS dewaterability and sampled WAS should be refrigerated within 2 days. The results showed that after 2-d storage, sludge filterability was deteriorated significantly while the extent of dewatering efficiency had little change. Meanwhile, increasing storage time greatly increased the release of extracellular polymeric substances (EPS) and heavy metals, decreased sludge viscosity and weakened its network strength, but had little impact on the floc size and zeta potential of the sludge samples. It can hardly reveal the mechanism of storage time on sludge dewaterability due to the non-uniformity of operating procedures in literatures, which is normally ignored. This study emphasizes a unified operating procedure is crucial to evaluate WAS dewaterability. Therefore, more efforts shall be focused on establishing the uniform operating procedure while advancing applied research in the field of sludge dewatering.

Effects of sludge thermal hydrolysis pretreatment on anaerobic digestion and downstream processes: mechanism, challenges and solutions

Thermal hydrolysis pretreatment (THP), as a step prior to sludge anaerobic digestion (AD), is widely applied due to its effectiveness in enhancing organic solids hydrolysis and subsequent biogas productivity. However, THP also induces a series of problems including formation of refractory compounds in THP cylinder, high residual ammonia and organic in the AD centrate, inhibition on downstream nitrogen removal process and reduction in UV-disinfection effectiveness during post-treatment. More attention should be paid on how to mitigate these negative effects. Despite intensive studies were carried out to reduce refractory compounds formation and enhance biological performance, there is limited effort to discuss the solutions to tackle the THP associated problems in a holistic manner. This paper summarizes the solutions developed to date and analyzes their technology readiness to assess application potential in full-scale settings. The content highlights the limitations of THP and proposes potential solutions to address the technological challenges.

Medium chain fatty acids production from simple substrate and waste activated sludge with ethanol as the electron donor

Production of MCFAs (Medium-chain fatty acids) from simple substrate (i.e., ethanol and acetate) and WAS with chain elongation microbiome was investigated in this study. The results showed that rapid production of MCFAs was observed when simple substrate was utilized. 1889 mg/L of caproate and 3434 mg/L of butyrate were achieved after 10 d's reaction. H₂ proportion in the headspace could reach as high as 10.1% on day 8 and then declined quickly. However, when WAS was used, the bacterial consortia was not able to hydrolyze WAS efficiently, which resulted in poor MCFAs production performance. Presence of ethanol could improve the hydrolysis process to a limited degree, which resulted in solubilization of a small fraction of protein and carbohydrate. Around 33.8% and 36.9% of the total detected electrons on day 6 in the 50 mM and 100 mM tests were extracted from WAS respectively. Those results indicate that the chain elongation microbial consortia tended to receive electrons form ethanol directly other than the complex WAS.

Post-treatment options for anaerobically digested sludge: Current status and future prospect

Anaerobic digestion is the most commonly used sludge treatment technology in large-scale wastewater treatment plants (WWTPs), generating two main products, i.e., biogas and anaerobically digested (AD) sludge. Biogas can be used as a source of renewable energy, and AD sludge is often transported for agricultural land application. Land application of AD sludge is confronted with ever-increasing economic and regulatory pressures due to its high water content, high organic content and related odour and pathogen content (if poorly stabilized), as well as potential toxic metal and organic contaminants. To address these challenges, a number of technologies have been developed for the further treatment of AD sludge before final disposal. This review aims to critically evaluate these state-of-the-art technologies. These technologies were categorized based on their primary aims: 1) dewaterability enhancement; 2) solids reduction and stabilization; 3) toxic metals removal. At present, the goal of post-treatment mainly focuses on dewaterability enhancement, to reduce transport costs. In future, we propose that the post-treatment of AD sludge should orient towards multiple aims, i.e., an integrated approach enabling sludge volume reduction, stabilization (including pathogen removal), and metal solubilization simultaneously. Two promising technical routes are suggested as examples, i.e. physio-chemical iron-based advanced oxidation and biological acidic aerobic digestion, while more approaches need to be developed in future studies. We concluded that post-treatment of AD sludge will promote the AD sludge management towards a more economically favourable, socially acceptable, and environmentally sustainable way; however, further development and rigorous evaluation are required for a wider adoption.

Identifying the key sludge properties characteristics in Fe2+-activated persulfate conditioning for dewaterability amelioration and engineering implementation

Fe²⁺-activated persulfate process has been introduced into sludge conditioning currently, however the key sludge properties characteristics are worthwhile comprehensively considering for the engineering implementation and management. The results indicated that both the optimal dosages of persulfate and Fe²⁺ were 0.6 mmol/gTS for sludge dewaterability amelioration, and the reduction efficiencies of capillary suction time (CST), specific resistance of filtration (SRF), and water content (Wc) of dewatered sludge cake reached to 90.5%, 97.2%, and 22.4%, respectively. Significantly, the persulfate and Fe²⁺ exerted distinctive roles in the conditioning process. The increased persulfate could promote the oxidatively disintegrated effect on sludge flocs, rendering the decrease of particle size. With the oxidative decomposition of the negatively charged biopolymers, sludge zeta potential rose gradually. However, Fe²⁺ contributed to more persulfate activation to generate free radicals, and the produced Fe³⁺ could further electrically neutralize the broken sludge fragments. The core mechanism of Fe²⁺-activated persulfate conditioning is "destroying and re-building" of sludge flocs. Noteworthily, EPS protein was oxidatively degraded more preferentially than EPS polysaccharide, and the decrease of the α-helix content of EPS protein was conducive to the enhancement of sludge dewaterability. Furthermore, the hydrophilic functional groups reduced clearly and element chemical states on sludge flocs altered pronouncedly, also the destroyed structure and microchannel facilitated the flowability of water. These findings provide theoretical and technical support for the practical engineering implementation of the Fe²⁺-activated persulfate conditioning process.

In-depth research on percarbonate expediting zero-valent iron corrosion for conditioning anaerobically digested sludge

Anaerobically digested sludge (ADS) is commonly hard to dewater for the presence of extracellular polymeric substances (EPS) and the liberation of glutinous soluble microbic products during anaerobic digestion. Sodium percarbonate (SPC) expediting zero-valent iron (ZVI) corrosion (SPC/ZVI) process firstly conditioned ADS to amend its dewaterability. Results showed that SPC/ZVI conditioning decreased moisture content of dewatered cake from 90.5% (control) to 69.9% with addition of 0.10 g/g TS SPC and 0.20 g/g TS ZVI. Mechanistic research indicated that the enhanced ADS dewaterability mainly resulted from •OH and Fe(III)/iron polymers yielded in SPC/ZVI. •OH disrupted EPS, damaged cytoderm & cytomembrane, and lysed intracellular substances, unbinding the bound water. Meanwhile, the breakage and inactivation of microbe by •OH prompted the production of macro-pores in ADS. •OH adjusted the conformation of extracellular/intracellular proteins by intervening in the H-bonds and S-S bonds, availing the hydrophobicity and slight flocculation of ADS. •OH further facilitated the despiralization of α-helical to β-sheet structure in ADS pellets, benefiting cell-to-cell aggregation. Additionally, Fe(III)/iron polymers from ZVI corrosion accelerated to gather ADS and maintained its floc structure. Consequently, SPC/ZVI conditioning not only adjusted the natures of ADS and its EPS but also the features of residual pellets, which further induced the advancement of ADS dewaterability. In addition, SPC/ZVI conditioning possibly surmounts some limitations existing in ZVI/Peroxide or ZVI/Persulfate technique.

Comprehensive role of thermal combined ultrasonic pre-treatment in sewage sludge disposal

Thermal/ultrasonic pre-treatment of sludge has been proven to break the hydrolysis barriers of sewage sludge (SS) and improve the performance of anaerobic digestion (AD). In this study, the objective was to investigate whether the combination of two pre-treatment methods can achieve better results on the AD of SS. The results indicated that, compared with the control group and separate pre-treatment groups, the thermal combined ultrasonic pre-treatment presented more obvious solubilization of soluble proteins, polysaccharides, and other organic matters in SS. The combined method promoted the dissolution of protein-like substances more effectively, with biogas production increased by 19% and the volatile solid (VS) removal rate improved to above 50% compared with the control group. The capillary suction time is reduced by about 85%, which greatly improved the dewatering performance of SS. In addition, the combined method has advantages in degrading sulfonamide antibiotics, roxithromycin and tetracycline. Particularly, by analyzing the interaction between the degradation of different antibiotics and the composition of dissolved organic matters (DOMs), it was found that the composition of DOMs could affect the degradability of different antibiotics. Among them, the high content of tyrosine-like and tryptophan-like was conducive to the degradation of sulfamethoxazole, and the high content of fulvic acid-like and humic acid-like was conducive to the degradation of roxithromycin and tetracycline. This work evaluated the comprehensive effect of thermal combined ultrasonic pre-treatment on SS disposal and provided useful information for its engineering.

New methods for quantification of Fenton's reagent addition based on aged sludge indicators to improve filterability

Fenton oxidation can effectively improve the dewaterability of aged sludge. Quantification of the addition of optimal reagents is central to the conditioning and dewatering of aged sludge. Improving the accuracy of quantification is significant to promote cost effectiveness. The effects of reagent addition and the mechanism governing the improved filterability of the aged sludge need to be understood uniformly. In this study, the optimal reagent additions have been determined using the response surface method (RSM) for five out of the eight aged sludges that were investigated. The physicochemical characteristics of eight aged sludges, including the extracellular polymer substance, undissolved organic matter, and suspension structure network, were investigated. Meanwhile, a comprehensive correlation analysis of critical indicators was conducted to investigate the interactions among the properties of the aged sludge. The effects of these interactions on the conditioning and filtration processes were examined, and a unified understanding of the combination of factors affecting the optimal reagent addition was obtained. The key factors were aggregate size, dewatering extent, yield stress, and organic substance content. Based on these results, a new reagent addition quantification method was developed along with an empirical model of the relationship between physicochemical properties and the economically optimal reagent addition.

Effect of high-temperature thermal hydrolysis on rheological properties and dewaterability of sludge

The disposal processes like dewatering and anaerobic digestion (AD) are commonly utilized to reduce the volume of sludge and recover energy. Thermal hydrolysis process is widely used as a pretreatment for sludge AD, which can change rheological properties and dehydration of sludge irreversibly. The experiment was designed to evaluate the effect of high-temperature thermal hydrolysis (120, 130, 145, 160 and 170°C) for 60 min on sludge rheological properties, as well as its dewaterability. Rheological tests were performed at (20 ± 0.1)°C with a HAAKE Viscotester 550 Rotary Viscometer. Both raw and thermal hydrolysis sludge has a considerable reduction on apparent viscosity and yield stress, but a somewhat increase in thixotropy. With the increase of temperature, the sludge flow behaviour index n increases linearly, while the consistency coefficient k follows the law of linear decline, showing that thermal hydrolysis can weaken the non-Newtonian fluid properties and then improve sludge fluidity. The dewaterability increases linearly with the temperature. Besides, the dewaterability of high-temperature thermal hydrolysis processes sludge was always significantly better than raw sludge as its mud cake has a much larger solid content. Notably, the increase in flow performance index n and the decrease of thixotropy kinetic coefficient K caused by thermal hydrolysis are all linear with the enhancement of dewaterability, which demonstrates that rheological indicators can be a new tool to evaluate the dewaterability of sludge.

Influential mechanism of water occurrence states of waste-activated sludge: specifically focusing on the roles of EPS micro-spatial distribution and cation-dominated interfacial properties

The highly hydrated colloidal structure of waste-activated sludge (WAS) is the main obstacle of enhanced dewatering for sludge volume minimization. Extracellular polymeric substances (EPS) maintain the colloidal stability of bio-flocs in a three-dimensional matrix due to bindings with bivalent cations (i.e., Ca²⁺ and Mg²⁺) and hydrophobic interactions. However, few studies specifically focused on the quantitative relationships among spatial distribution of EPS, microstructure of bio-flocs and fractions of bound water (e.g. vicinal water and interstitial water). Thus, there may be still some debates on whether and what extent of the lysis or flocculation of sludge flocs is optimal for the dewaterability improvement. This study applied the gradient addition of cation exchange resin (CER) to remove EPS-complexed cations and loosen the spatial distribution of EPS. Consequently, how the spatial extension of EPS layers with relief of complex cations influenced the particle size distribution, fractal dimension, interfacial free energy and water occurrence states of WAS was systematically investigated. The quartz crystal microbalance with dissipation (QCM-D) was also applied to analyze the water-EPS interactions with and without the presence of Ca²⁺ and Mg²⁺. All the results confirmed that the dispersed EPS adhering layers led to the higher fractal dimension (D_f) but the lower space filling degree of bio-flocs. Also, the 4-fold reduction in the polar/acid-based interfacial free energy could be induced by the removal of cations from EPS matrix, which indicated the significant increase in hydrophobicity. Predictably, the fractions of vicinal water and interstitial water were dominated by the polar/acid-based interfacial free energy and pore structure of microbial aggregates, respectively, which were confirmed by the strong Pearson correlation (R_p>0.80, p-value<0.04). These findings are expected to provide the improved mechanistic insights into the relationship between water occurrence states and colloidal structure of WAS, and can serve as the basis for the optimal combination of various sludge conditioning approaches towards regulating aggregation states of bio-flocs.

Electrical impedance spectroscopy as a potential tool to investigate the structure and size of aggregates during water and wastewater treatment

Microscopic structure and size are important metrics for estimating aggregates environmental behaviors during water and wastewater treatment. However, in-situ determination of these characteristics is still a challenge. Here, we drew inspiration from a block disassembly process to propose an electrical impedance spectroscopy (EIS) method and constructed a generalized framework to associate macroscale electrical properties with microscopic structure and size-related characteristics of aggregates of different hierarchies. Extracted via EIS, the proposed models were verified to be capable of describing the self-similarity of aggregates and capturing the fractal and size information. Further, the proposed models exhibited a wide range of applications, which agrees well with the data gathered from various activated sludges, other colloids, and microgels in water and wastewater treatment. Finally, the EIS method was achieved online monitoring of fractal dimension and floc size during a sludge pre-oxidation conditioning process, which was elected as an example to illustrate the potential online applications of this EIS method in real water and wastewater environment. The obtained on-line data were used to indicate the potential suitable oxidation time during sludge pre-oxidation conditioning. These observations may inspire new methods of quantifying the aggregate structure and promote intelligent and dynamic decision-making during water and wastewater treatment.

Compact wastewater treatment process based on abiotic nitrogen management achieved high-rate and facile pollutants removal

Chemically enhanced primary treatment (CEPT), ammonium ion exchange and regeneration (AIR) and membrane bioreactor (MBR) were coupled as CAIRM to treat domestic wastewater compactly and efficiently. CAIRM achieved efficient removal of chemical oxygen demand, ammonia nitrogen, total nitrogen (TN) and total phosphorus with total hydraulic retention time of 4.6 h, and obtained 2.3 ± 0.9 mg/L TN in the effluent. CEPT removed phosphate and impurities and prevented AIR from pollution. AIR maintained excellent nitrogen removal with a slight decrease in the exchange capacity of ion exchangers. MBR polished the effluent from AIR, and the larger particle size and better dewaterability of sludge mitigated the membrane fouling. Many heterotrophic genera, such as Rhodobacter and Defluviimonas, were enriched in the oligotrophic MBR. This study demonstrates the viability and stability of CAIRM in efficient wastewater treatment, which will address critical challenges in insufficient nitrogen removal and high land occupancy of current processes.

Thermal pre-treatment: Getting some insights on the formation of recalcitrant compounds and their effects on anaerobic digestion

Thermal hydrolysis is a common pre-treatment, used before anaerobic digestion processes, to enhance the hydrolysis rate. However, formation of inhibitory compounds and the increase of liquid fraction colour have been identified as potential drawbacks. This study was oriented to study the methane production from simple substrates, subjected to thermal hydrolysis. A mixture of glycine and glucose at different concentrations was prepared, at a ratio similar to proteins and carbohydrates found in activated sludge. Two temperatures were tested. At 120 °C a decrease on biogas production rate was observed. On the other hand, at 165 °C generation of recalcitrant material was observed, causing a decrease in methane potential and COD degradation, when a mixture of glycine and glucose was used as substrate. This was atributed to the formation of recalcitrant compounds via Maillard reaction, hyphothesis supported by FTIR-ATR, which indicated the formation of amide II Bonds.

Coupling hydrothermal carbonization and anaerobic digestion for sewage digestate management: Influence of hydrothermal treatment time on dewaterability and bio-methane production

Hydrothermal carbonization (HTC) technology is addressed in the framework of sewage digestate management. HTC converts digestate into a stabilized and sterilized solid (the hydrochar) and a liquor (HTCL) rich in organic carbon. This study aims to optimize the HTC operating parameters, namely the treatment time, in terms of hydrochar production, HTC slurry dewaterability, HTCL bio-methane yields in anaerobic digestion (AD), and process energy consumption. Digestate slurry was processed through HTC at different treatment times (0.5, 1, 2 and 3 h) at 190 °C, and the dewaterability of the treated slurries was addressed through capillary suction time and centrifuge lab-testing. In addition, biochemical methane potential (BMP) tests were conducted for HTCL under mesophilic conditions. Results show that by increasing the HTC treatment time the dewaterability was further improved, ammonium concentration in HTCL increased, and methane potential of HTCL decreased. 0.5 h HTCL had the highest bio-methane potential of 142 ± 3 mL CH₄/g COD yet the treatment time was not sufficient for improving the slurry's dewaterability. HTC treatment time of 1 h at 190 °C was identified as the optimum trade-off for improved dewaterability and utilisation of HTCL for biogas production. 1 h HTCL bio-methane potential can cover around 25% of the HTC and AD thermal and electrical energy needs without considering the eventual use of the hydrochar as a biofuel.

Insights into thermal hydrolyzed sludge liquor - Identification of plant-growth-promoting compounds

This study proposes a new path to utilize thermal hydrolyzed sludge (TH sludge) as fertilizer given high value chemical compounds that can promote plant growth were identified in the liquid fraction of TH sludge (TH liquor). Together with micro- and macro-nutrients released/synthesized during thermal hydrolysis, the feasibility of using TH liquor as organic fertilizer was evaluated. Besides high contents of N, P and K, total free amino acids (FAAs) and plant-growth-promoting FAAs (including glutamic acid, leucine and cystine) also presented in high concentration (4.98-6.48 and 1.12-2.73 g/100 g) in the TH liquor. For the first time, phytohormone compound, indole-3-acetic acid, was observed and the content was the highest in TH liquor with 165 °C treatment (165 °C TH liquor). Meantime, 165 °C TH liquor did not have negative impact on the growth of soil microbes, and this product, instead, demonstrated stimulating effect on the plant growth. These results suggest that 165 °C TH liquor has a great potential to be an organic fertilizer. The remaining solids of TH sludge could be converted to valuable biochar. The holistic approach of using TH liquor as organic fertilizer and producing biochar could realize nearly zero-waste discharge in sludge management.

A comprehensive review of the recent development and challenges of a solar-assisted biodigester system

The extensive use of fossil fuels and the environmental effect of their combustion products have attracted researchers to look into renewable energy sources. In addition, global mass production of waste has motivated communities to recycle and reuse the waste in a sustainable way to lower landfill waste and associated problems. The development of waste to energy (WtE) technology including the production of bioenergy, e.g. biogas produced from various waste through Anaerobic Digestion (AD), is considered one of the potential measures to achieve the sustainable development goals of the United Nations (UN). Therefore, this study reviews the most recent studies from relevant academic literature on WtE technology (particularly AD technology) for biogas production and the application of a solar-assisted biodigester (SAB) system aimed at improving performance. In addition, socio-economic factors, challenges, and perspectives have been reported. From the analysis of different technologies, further work on effective low-cost technologies is recommended, especially using SAB system upgrading and leveraging the opportunities of this system. The study found that the performance of the AD system is affected by a variety of factors and that different approaches can be applied to improve performance. It has also been found that solar energy systems efficiently raise the biogas digester temperature and through this, they maximize the biogas yield under optimum conditions. The study revealed that the solar-assisted AD system produces less pollution and improves performance compared to the conventional AD system.

Peroxide/Zero-valent iron (Fe0) pretreatment for promoting dewaterability of anaerobically digested sludge: A mechanistic study

Peroxide/Zero-valent iron (Fe⁰) was reported to promote dewaterability of anaerobically digested sludge (ADS), but the mechanism of how Peroxide/Fe⁰ facilitates ADS dewatering is unknown. This study therefore aims to uncover the details of how Peroxide/Fe° elevates ADS dewaterability. Experimental results showed that with 0.6 g Fe⁰/g TSS and 0.08 g peroxide/g TSS, capillary suction time, specific resistance to filtration, and time to filtration of ADS was 50.7 %, 41.4 %, and 54.4 % of that in the control, respectively. In this condition, water content of sludge cake decreased from 91.2 % ± 0.5 % (the control) to 68.6 % ± 1.3 %. The mechanism explorations revealed that the elevated dewaterability was mainly caused by role of OH and Fe(II)/Fe(III) species during Peroxide/Fe° pretreatment. OH decreased the polysaccharides and proteins in extracellular polymeric substance (EPS), then injured the cytoderm & cytomembrane through the releases of lactate dehydrogenase and N-acetylglucosamine, and further facilitated the decrease of intracellular substances, which disengaged the water trapped in ADS. In addition, the cell lysis caused by OH facilitated forming macro-pores. Moreover, OH converted the conformational structure of extracellular proteins, which may strengthen the ADS hydrophobicity, promoting the discharge of unbound water and ADS flocculation. Meanwhile, Fe(II)/Fe(III) benefited aggregating the denatured ADS particulates.

Revealing the variations in physicochemical, morphological, fractal, and rheological properties of digestate during the mesophilic anaerobic digestion of iron-rich waste activated sludge

Dosing of iron (Fe)-salts in sewers to control odour and corrosion problems have proven to be effective on phosphate and sulfide removal in downstream treatment units. However, the interaction of Fe with sludge may impact the sludge properties during wastewater treatment and sludge digestion. Herein, we investigated the downstream impacts of sewer-dosed Fe-salt on key digestate properties including digestate dewaterability. For this, Fe-salt was dosed to a sewer reactor and resultant iron-rich waste activated sludge (Fe-WAS) was digested in an anaerobic digester (AD) in the experimental line of integrated laboratory system running in parallel to a control system. Iron containing and non-iron containing digestates were sourced from the respective AD reactors of experimental and control lines. Results showed improved dewaterability in iron containing digestate than non-iron containing digestate, which was attributed to the variations in key digestate properties. Compared to non-iron containing digestate, iron containing digestate exhibited the decreased contents of bound water, soluble extracellular polymeric substances (S-EPS), protein, polysaccharide, and monovalent-to-divalent (M⁺/D⁺⁺) cations ratio. Likewise, we observed the increased mean particle size (D_v50) for iron containing digestate than the non-iron containing digestate, but fractal dimension (D_f) values were comparable. Besides, iron containing digestate exhibited a reduced degree of thixotropy, relative sludge network strength, viscosity, yield stress, flow stress, and storage/loss/complex (G'/G''/G∗) moduli but increased creep compliance and shear strain (%) than non-iron containing digestate. The combined synergistic effects of such favorable changes amongst the key properties of iron containing digestate, might have been responsible for improving it's dewaterability.

Critical review on dewatering of sewage sludge: Influential mechanism, conditioning technologies and implications to sludge re-utilizations

Sewage sludge (mainly composed of excessive bio-sludge) is an inevitable by-product of biological wastewater treatment process and contains various toxic substances, such as pathogens, heavy metals, and organic contaminants. The production of sewage sludge may cause serious pollution risks without appropriate disposals. As the essential step of sludge treatment, dewatering plays significant roles in minimizing the sludge volume, facilitating the transportation, increasing the calorific value and even reducing the leachate production in landfill sites. This paper presents a comprehensive review on the issues related to dewatering of sewage sludge. Section 1 starts with the environmental implications of sludge dewatering. Section 2 deals with the concepts and challenges about differentiation of bound water fractions, and also reviews the recent progress of in-situ visualization of water occurrence states in bio-flocs. Section 3 discusses about how various physiochemical properties influence the sludge dewaterability, and the insufficiency in in-situ micro-characterization of sludge constituents is pointed out. Section 4 reviews the existing conditioning technologies for sludge dewaterability improvement, and the advantages/disadvantages of each technology in terms of applicable occasions, material consumption, energy consumption and environmental impacts are evaluated. The last section (section 5) specifically analyzes the feasibility of integrating sludge dewatering and re-utilization, and raises attention to the potential environmental risks of dewatering conditioning. Based on the above discussion, we propose that a unified theory for sludge dewaterability improvement remains to be established. Especially, how the molecular structures of sludge compositions affect the solid-water interface behavior requires to be deepened, which will further unravel the mechanism behind strong water-holding capacities of bio-flocs. Additionally, we believe that the key challenges for sludge dewatering is how to select the appropriate conditioning technique according to the physiochemical properties of target sludge. The reliable indicators for real-time control of conditioning operations are still deficient, e.g., dynamic dosage control of conditioning chemicals. Accordingly, the potential environmental risks of excessive conditioning chemicals should be taken into more consideration.

Simultaneous evaluation of bioactivity and settleability of activated sludge using fractal dimension as an intermediate variable

Bioactivity and settleability of activated sludge are essential for the operation of activated sludge systems in wastewater treatment. In this work, the fractal dimension of sludge image is proposed as a tool to evaluate these two factors. The specific endogenous respiration rate (SOUR_e) and the specific quasi-endogenous respiration rate (SOUR_q) are found to be more dependent on the 3D structure of sludge than the specific total respiration rate (SOUR_t). The relationship between the fractal structure and bioactivity suggests that the bioactivity governs the acceptable upper bound of the fractal dimension (D_f), as at its theoretical maximum of 2.0, the non-porous compact flocs are predominant. The settleability or the biomass concentration determines the acceptable lower bound of D_f, as at its theoretical minimum of 1.0, the free-swimming microbes are predominant. Our data reveal that the activated sludge has an acceptable fractal dimension D_f in a range of 1.07-1.68. In practice, the fractal dimension should be controlled at a reasonable value as there is a trade-off between the bioactivity and physical structure to achieve better performance. A decrease or increase in the fractal dimension can serve as a signal for the change of the operational status, and this is further elucidated from the perspective of settling tanks using state point analysis. Compared with respirogram measurement, measuring fractal dimension is a complex process and its online implementation is challenging. Also, the measured value varies with the methods used. In addition, the difference in their theoretical values depends on the homogeneity of the sludge structure. Since the fractal dimension D_f reflects both bioactivity and settleability of the sludge but is difficult to measure, in this work a relationship between D_f and the easily measurable respirogram is established, and a method using the respirogram as a proxy of D_f is proposed to control the bioactivity and settleability simultaneously. This respiration-based method is able to simultaneously control aeration and settling tanks, and could serve as an efficient tool for the management of wastewater treatment plants.

Free nitrous acid pre-treatment enhances anaerobic digestion of waste activated sludge and rheological properties of digested sludge: A pilot-scale study

In this study, the effects of free nitrous acid (FNA) pre-treatment on the rheological properties of digested sludge were investigated at a pilot-scale, along with the improvement in volatile solids (VS) destruction and biogas production. Two pilot-scale anaerobic sludge digesters were operated for one year, one receiving thickened waste activated sludge (TWAS) without pre-treatment (control) and one receiving TWAS pre-treated for 24 h at an FNA concentration of 4.9-6.1 mgN/L (nitrite = 250 mgN/L, pH = 5.0, T = 22-30 °C). The results confirmed the enhancing effect of FNA pre-treatment on methane production (37 ± 1%), consistent with previous laboratory studies. Equally importantly, FNA pre-treatment substantially reduced the shear viscosity of TWAS by 51 ± 8% at 100 s^-1 and 49 ± 7% at 250 s^-1, likely due to the solubilization of the TWAS (11.1 ± 0.8%). Similarly, FNA pre-treatment also reduced these viscosity parameters of the digested sludge by 80 ± 4% and 78 ± 4%, respectively, caused by both enhanced VS destruction and disintegration of the digested sludge. The dewaterability of digested sludge, assessed by dewatered solids content, capillary suction time and specific resistance to filtration, was not improved by FNA pre-treatment. The polymer requirement for dewatering was reduced by 24 ± 0.6% due to the lower solids concentration in the digested sludge achieved with FNA pre-treatment. The changes to sludge rheological properties revealed in this study further enhances the business case for the FNA pre-treatment technology.

New insight into the effect of thermal hydrolysis on high solid sludge anaerobic digestion: Conversion pathway of volatile sulphur compounds

Untreated sludge (total solids = 10%) and thermally hydrolysed sludge (total solids = 10%) were subjected to high-solid anaerobic digestion (HSAD) to study the effect of thermal hydrolysis pre-treatment (THP) on the conversion pathways of volatile sulphur compounds (VSCs). Typical VSCs were detected in the gas produced by THP at 160 °C for 30 min, including H₂S, methyl mercaptan (MM), dimethyl sulphide (DMS) and dimethyl disulphide (DMDS). After THP, the organic sulphide ratio in the treated sludge had decreased from 96% to 90%, and inorganic sulphide had increased from 4% to 10%. In the THS (THP + HSAD) group, the productivity and total volume of VSCs were significantly increased. These results suggest that THP directly promotes converting organic sulphur (OS) into VSCs. Further tests revealed that THP increased the activity of reductases (adenine phosphate sulphate reductase and sulphite reductase), OS hydrolysis was the main source of VSCs in biogas, and MM could be converted into H₂S (78%), DMS (18%) and DMDS (4%). These findings are used to elucidate the conversion pathway of sulphides in HSAD.

Liquid and solids separation for target resource recovery from thermal hydrolyzed sludge

This study proposed an integrated process for biogas generation and biochar production from thermal hydrolysis pretreated sludge (THP sludge). In this study, the liquid and solids fractions of THP sludge were separately processed for the first time. The liquid fraction of THP sludge (THP-L) reached the biodegradability (262.6 ± 5.1 mL CH₄/g tCOD_feed) on the 15th day during anaerobic treatment, while the solids fraction of THP sludge (THP-S) only contributed 31.0% to the total methane production and required more than 30 days digestion time. We investigated the feasibility to convert THP-S into biochar to realize the higher value of the solids fraction. The results prove the produced biochar can be used as slow-release fertilizer. Preliminary energy analysis was performed to evaluate the energy efficiency of the integrated approach, namely, methane generation from THP-L coupled with biochar production from THP-S. The process realized energy surplus of 0.81 MWh/tonne dry sludge. In addition, THP-L digested sludge showed better dewaterability, lower yield stress and reduced viscosity during digestion. The proposed new sludge treatment process therefore has lower operating cost and higher value returns.

Digestate management for high-solid anaerobic digestion of organic wastes: A review

Digestate management for anaerobic digestion (AD) is becoming a bottleneck of the sustainability of AD plants when the use of digestate for agricultural application is restricted due to nutrient surplus and low market acceptance. Digestate quality and treatment in high solid anaerobic digestion (HSAD) can be better than conventional low-solid system. The rheological behavior of digestate in high solid anaerobic digestion (HSAD) can have a great impact on the energy consumption of digestate management. After post-conditioning guided by rheological parameters, the solid digestate can be further treated based on the integrated solutions to enhance the energy efficiency or nutrients recovery. The environmental impacts for some core parts of those integrated systems were also evaluated in this study. This article presented a critical review of recent investigations of digestate management for HSAD, especially focusing on the rheology of HSAD digestate, integrated solutions and their environmental performances.

Comparison between Thermo-Alkaline and Electro-Fenton Disintegration Effect on Waste Activated Sludge Anaerobic Digestion

Disintegration of municipal waste activated sludge (WAS) using thermo-alkaline (TA) and electro-Fenton (EF) methods was investigated and compared in terms of the efficiency of sludge solubilisation and enhancement of anaerobic biodegradability. Performance of organic matter solubilisation (soluble COD, proteins, polysaccharides) of sludge pretreated with EF was proved to be better than that with TA pretreatment, which resulted in the enhancement of anaerobic biodegradability. Comparison of results indicated that percentages of PN and PS release obtained after EF pretreatment (68.95 and 65.22%) were higher than those obtained by TA method (45.25 and 35.22%) respectively. An improvement of biogas potential about 2 and 1.6 times was achieved respectively by EF and TA pretreatment in comparison to raw sludge. During semi-continuous fermentation study in continuous stirred tank reactor, EF pretreated sludge gave the best biogas yield (0.6 L biogas/g COD) at an OLR of 2.5 g COD/L. d in comparison to TA pretreated sludge (0.3 L biogas/g COD), where low biogas yield about 0.1 L biogas/g COD was registered by raw sludge in the same CSTR. Therefore, the integration of EF process to anaerobic digestion might be a promising process for sludge reduction and biogas recovery.

Electrochemical pretreatment for stabilization of waste activated sludge: Simultaneously enhancing dewaterability, inactivating pathogens and mitigating hydrogen sulfide

Stabilization of waste activated sludge (WAS) is an essential step for the disposal or reuse. In this study, WAS stabilization via electrochemical pretreatment (EPT) at 0-15V was evaluated for simultaneous dewaterability enhancement, pathogen removal and H₂S mitigation. The mechanism underlying EPT was investigated and discussed based on the changes in the physicochemical (e.g., particle size, zeta potential, hydrophobicity and extracellular polymeric substances) and biological characteristics (i.e. cell morphology, and distribution and percentages of live/dead cells) of WAS with different EPT voltages. The results revealed that EPT disintegrated WAS flocs and disrupted the cell walls leading to a reduction in particle size (by up to 50%), increased release of extracellular and intracellular substances (by up to 4 times) to facilitate WAS stabilization. With EPT at 15V, the capillary suction time of WAS decreased by 42%, and the concentrations of E. coli and indicator pathogens (Salmonella spp. and Streptococcus faecalis) fell by nearly 5 log₁₀ reaching U.S. EPA hygienization levels. Furthermore, EPT at 12V or higher suppressed the amounts of dissolved sulfide and H₂S_(g) produced from the WAS under anaerobic conditions by over 99%. This study demonstrates the feasibility of EPT for simultaneous WAS dewaterability enhancement, pathogen inactivation and H₂S mitigation, providing a one-step alternative for sludge stabilization.

Comprehensively evaluating the digestive performance of sludge with different lignocellulosic components in mesophilic anaerobic digester

In excess sludge digestion, organic matters cannot be digested adequately due to its high lignocellulose content. This study attempted to comprehensively evaluate the digestive performances of sludge with different lignocellulosic components (hemicellulose, cellulose and lignin). Results show that hemicellulose/dealkaline lignin addition (S6) presents the highest methane yield of 203.6 mL/gVS. Compared to hemicellulose, dealkaline lignin is hardly degraded (lower than 10%), while its participation can promote the degradation of other organics in the system. Additionally, solo cellulose feedstock is difficult to be hydrolyzed (only 40.1%) without hemicellulose and dealkaline lignin addition. VFAs composition analysis indicates that VFA inhibition occurs in the digester with hemicellulose, cellulose and dealkaline lignin addition (S8). Microbial diversities of different digestive systems show that the relative abundance of Euryarchaeota in the digester S6 (7.2%) is much higher than others, and some specific microbes (Bacteroidetas and Firmicutes) are enriched in the S5 (74.1%) and S8 (54.7%) digesters.

Achievements of biochar application for enhanced anaerobic digestion: A review

Anaerobic digestion (AD) and pyrolysis are two promising technologies used worldwide for waste biomass treatment. Interests on intensification techniques of AD has been increasing to obtain sufficient and sustainable methane production with stable digester performance. For instance, considerable attention has been devoted to the coupling of AD with biochar, which is produced by biomass thermochemical conversion. This manuscript presents a comprehensive review about recent achievements in enhancing AD efficiency with the utilization of biochar. The key roles of biochar include enhancing and equilibrating hydrolysis, acidogenesis-acetogenesis, and methanogenesis, as well as alleviating inhibitor stress were summarized. Biochar can promote biomethane process mainly by serving as a provision for bioelectrical connections between fermentative bacteria and methanogens, a support for microbial colonies, and a reinforcer for buffer capacity. Through an overview of the early applications, this paper aims to pinpoint the potential mechanism and future explorative directions of biochar enhancing AD performance.

Effects of stepwise thermal hydrolysis and solid-liquid separation on three different sludge organic matter solubilization and biodegradability

Separate digestion after pretreatment could be a promising process for sludge treatment. In this study, a novel process coupling two-step thermal hydrolysis pretreatment (THP) and separate digestion of liquid and solid fractions was applied on three different sludge to investigate the organic components solubilization and methane production potentials based on mass balance. Results showed that 61.94-69.08% of protein and 84.19-86.75% of polysaccharides were dissolved, while only 35.20-38.55% and 61.61-69.92% were detected in the liquid fraction, respectively. This indicated that not only disintegration and dissolution, but also hydrolysis occurred during THP. Organic matter was mainly dissolved in the first-step THP, and protein and VFAs were the main components. Although separate digestion did not improve the total methane production, the liquid fractions contributed 51.08-73.92% to the sum of liquid and solid fractions. The organic solubilization and biogas production are essentially related to the organic components, instead of the total organic content.