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

Evaluating river health through respirogram metrics: Insights from the Weihe River basin, China

Human activities pose a significant threat to rivers, requiring robust assessment methods for effective river management. This study focuses on the Weihe River Basin in Shaanxi province and introduces the respirogram as an innovative assessment technique. The respirogram allows the simultaneous assessment of river health from two important aspects: pollution levels and microbial status. Specifically, the in-situ respiration ratio (Rs/t) serves as an indicator of pollution, with higher Rs/t values correlating with increased pollution levels. Conversely, the recovery index (RI) measures microbial vitality, with values below 0.15 indicating greater microbial activity and recovery potential. Using predefined thresholds of Rs/t = 0.3 and RI = 0.15, water bodies were categorized into four types. For example, rivers with Rs/t > 0.3 and RI > 0.15 were identified as receiving sewage, characterized by high pollution and low microbial vitality. Similarly, different assessment criteria delineated urban rivers, natural rivers, and wastewater treatment plants. Based on these classifications, targeted engineering measures were proposed to enhance the self-purification capabilities of rivers of different statuses.

Strengthen high-loading operation of wastewater treatment plants by composite micron powder carrier: Microscale control of carbon, nitrogen, and sulfur metabolic pathways

The concentration of activated sludge is a crucial factor influencing the capacity and efficiency of sewage wastewater treatment plants (WWTPs). However, high sludge concentrations can lead to sludge loss in the secondary sedimentation tank, resulting in reduced processing capacity, particularly during low-temperature stages and sludge bulking. This study investigated the impact of adding composite micron powder carriers (CMPC) in high-concentration powder carrier biofluidized bed (HPB) technology to the biochemical units of WWTPs on sludge concentration and settling performance. For the traditional activated sludge method (ASM), its hydraulic retention time (HRT) was 8 h, with an average effluent total nitrogen (TN) of 15.14 mg/L. Sludge bulking was prone to occur in low-temperature environments, resulting in a high average sludge volume index (SVI) of 560 mL/g. Conversely, with a CMPC dosage of 4 g/L, the HRT of HPB technology was 4.8 h, and the average effluent TN was 11.40 mg/L, with a removal efficiency of 67.43 %. During operation of HPB technology under high sludge concentration conditions (8 g/L), the average SVI remained at 85 mL/g, indicating excellent settling characteristics. Moreover, in the sequencing batch reactor (SBR), the SVI value of bulking sludge decreased from the original 695 to 111 mL/g by the 9th day of operation with the CMPC dosage of 2 g/L. At the same time, the filamentous bacteria almost disappeared, suggesting that CMPC inhibit the growth of filamentous bacteria. Metagenomic analysis demonstrated that CPMC enhance the utilization of small molecular fatty acids in activated sludge and promote electron transfer between nitrate and nitrite, thereby improving wastewater treatment capacity. Additionally, CMPC enhanced the relative abundance of Saprospiraceae in sludge, which accelerate the degradation of polysaccharides in extracellular polymeric substances, weaken sludge's hydrophilic properties, and improve sludge's settling performance. Overall, these findings suggested that CMPC effectively strengthen the high-loading operation of WWTPs by improving sludge concentration and sedimentation performance.

Insight into sludge dewatering by periodate driven directly with Fe(Ⅱ): Extracellular polymeric substances solubilization and mineralization

The production of waste activated sludge is expanding in tandem with the significant growth in the global population. It is important to explore sludge pretreatment technology to achieve sludge reduction. In this study, deep sludge dewatering was achieved by using Fe2+-catalyzed periodate (Fe2+/PI) conditioning. The result showed that capillary suction time was reduced by 48.27% under the optimum Fe2+ and PI dosages. ·OH, FeⅣ, O2·-, 1O2, and IO3· generated from the reaction between Fe2+ and PI, while ·OH (49.79%) and FeⅣ (47.76%) contributed significantly to sludge dewatering. Investigations of the mechanism revealed that the synergistic action of radical species oxidation and iron species flocculation in Fe2+/PI conditioning led to the mineralization and aggregation of hydrophilic substances in extracellular polymeric substances. The hydrophobic groups on the protein surface were more exposed to soluble extracellular polymeric substances and reduced protein-water interaction. The variations in zeta potential and particle size also verified the presence of a synergistic effect of oxidation and flocculation. The morphology observations revealed that the increased frictional forces generated when water flowed over the raw sludge (RS) surface prevented the rapid passage of internal water. In addition, the hydrophobic and electrostatic interactions in the sludge samples were essential influences that promoted flocculation and sedimentation of the sludge. This research aids engineers by providing a new option to better optimize sludge management while also deepening understanding of the Fe2+/PI conditioning involved in sludge dewatering.

Roles of high/low nucleic acid bacteria in flocs and probing their dynamic migrations with respirogram

Bacterial migration is crucial for the stability of activated sludge but rarely reported. The static distribution was explored by changes in bacteria concentration with extracellular polymeric substances (EPS) extractions. Next, denitrification and aeration were conducted as normal running conditions for examining the bacterial migration between floc-attached and dispersed growth. Above observations were further explored by conducting copper ion (Cu2+) shock as an extreme running condition. After extracting EPS, low nucleic acid (LNA) bacteria migrated from the sludge to the supernatant primarily, and high nucleic acid (HNA) bacteria remained in the residual sludge, suggesting that HNA bacteria mainly distributed inside the sludge while LNA bacteria outside the sludge. During the denitrification process, LNA bacteria migrated out of flocs, which increased by 6.94 × 106 events/mL in the supernatant. During the feast phase of aeration, LNA bacteria grew attached to flocs, causing the increased flocs diameter from 45.60 to 47.40 μm. During the following aerobic famine phase, LNA bacteria grew dispersedly, but HNA bacteria remained unchanged. However, a further severe famine phase drove HNA bacteria to be dispersed, breaking flocs with the decreased diameter from 48.10 to 46.50 μm. When the Cu2+ shock was employed, LNA and HNA bacteria increased but the LNA/HNA ratio decreased in the supernatant, indicating more HNA bacteria migrating to the dispersed phase. From a structural perspective, HNA bacteria distributed inside the sludge and functioned as the backbone of flocs, undertaking the maintenance of flocs stability primarily; while LNA bacteria distributed outside the sludge and functioned as filling materials, having a secondary influence on flocs stability. These processes were also probed by respirogram exactly, correlating the system-scale measurement and microscale migrations and providing an early warning signal under abnormal circumstances. The processed HNA-backbone theory is promising for regulating the stability of activated sludge based on bacterial migrations.

Under-loaded operation of an anaerobic-anoxic-aerobic system in dry and wet weather dynamics to prevent overflow pollution: Impacts on process performance and microbial community

Effects of low hydraulic loading rate (HLR) in dry weather and high HLR in wet weather on pollutant removal, microbial community, and sludge properties of a full-scale wastewater treatment plant (WWTP) were extensively studied to explore the risk of under-loaded operation for overflow pollution control. Long-term low HLR operation had an insignificant effect on the pollutant removal performance of the full-scale WWTP, and the system could withstand high-load shocks in wet weather. Low HLR resulted in higher oxygen and nitrate uptake rate due to the storage mechanism under the alternating feast/famine condition, and lower nitrifying rate. Low HLR operation enlarged particle size, deteriorated floc aggregation and sludge settleability, and reduced sludge viscosity due to the overgrowth of filamentous bacteria and inhibition of floc-forming bacteria. The remarkable increase in Thuricola and the contract morphology of Vorticella in microfauna observation confirmed the risk of flocs disintegration in low HLR operation.

Short sludge age denitrification as alternative process for energy and nutrient recovery

High rate activated sludge (HRAS) systems redirect organics into highly biodegradable sludge and nutrients into microbial proteins. This study evaluates anoxic HRAS for nitrogen and carbon recovery. The reactor treated synthetic wastewater at solids retention times (SRTs) of 5, 3 and 1 days. Denitrification rates varied between 0.15 and 0.19 g-NO₃-N g-TSS^-1 d^-1 (total suspended solids per day) and all conditions showed favourable settling. The highest sludge yield, obtained at SRT 1 d, was 0.75 g-TSS g-COD_removed^-1, double that observed for aerobic HRAS. The highest methane yield (322 mL-CH₄ g-VS_sludge^-1) was obtained from sludge wasted at 3 d SRT. Both 1 d and 3 d SRTs showed favourable energy recovery, with 14 % of the organics recovered as methane. All conditions yielded sludge with protein content ranging between 24 and 27 % of dry weight and similar amino acid profile, comparable to traditional proteins. Thus, denitrifying HRAS recovers resources as its aerobic counterpart, allowing for nitrogen removal via denitrification, more stable compared to mainstream partial nitritation anammox typically combined with aerobic HRAS.

A novel approach to estimate and control denitrification performance in activated sludge systems with respirogram technology

Respirogram technology has been widely applied for aerobic process, however, the response of respirogram to anoxic denitrification is still unclear. To reveal such response may help to design a new method for the evaluation of the performance of denitrification. The size distribution of flocs measured at different denitrification moments demonstrated a clear expansion of flocs triggered by denitrification, during which higher specific endogenous and quasi-endogenous respiration rates (SOUR_e and SOUR_q) were also observed. Furthermore, SOUR_q increases exponentially with the specific denitrification rate (SDNR), suggesting that there should be a maximum SDNR in conventional activated sludge systems. Based on these findings, an index R_q/t, defined as the ratio of quasi-endogenous (OUR_q) to maximum respiration rate (OUR_t), is proposed to estimate the denitrification capacity that higher R_q/t indicates higher denitrification potential, which can be readily obtained without complex measurement or analysis, and it offers a novel and promising respirogram-based approach for denitrification estimation and control by taking measures to extend anoxic time to maintain its value at a high level within a certain range.

Long-term effects of hydrocyclone operation on activated sludge morphology and full-scale secondary settling tank wet-weather operation in long sludge age WWTP

This paper presents the study concerning long-term effects of a full scale hydrocyclone unit implemented in a continuous flow long sludge age system, on sedimentation, treatment efficiency and sludge morphology. The research concentrates on identifying the mechanisms of sludge behaviour within the system. The gravimetric selection of activated sludge via a hydrocyclone is a recent development for enhancing sludge separation, where heavier flocs are retained in the system, and lighter ones are discarded as waste sludge. The effects of implementing hydroclyclones were analysed with the use of SEM imagining and fractal dimensioning through the frequent assessment of sludge settling capabilities, effluent quality, and floc properties. Over the course of 60 weeks of hydrocyclone operation, sedimentation efficiency varied significantly. Sludge volume index values of 40 mL/g, achieved during the warm season, were not sustained when the temperature decreased and an overgrowth of filamentous bacteria occurred. Good settling efficiency was also observed in batch tests, where settling velocity of experimental sludge was app. 1 m/h higher than for the reference train at the same concentrations. This was confirmed during wet weather, as the experimental train sustained safe sludge blanket height in secondary clarifiers. SEM imaging and fractal dimension analysis revealed that the underflow that returned to the system had a more compact and spherical shape, which led to an increased content of granule-like particles in the reactor. The presence of flocs with a diameter exceeding 900 μm in the underflow, which is not observed in the feed, indicated agglomeration within the hydrocyclone. This is contradictory to most of the literature data coming from laboratory experiments. This phenomenon was attributed to differences in the size and geometry of the used hydrocyclones, and the potential process mechanism was presented.

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.

Encapsulation of bacteria in different stratified extracellular polymeric substances and its implications for performance enhancement and resource recovery

Simultaneous recovery of biopolymers and enhanced bio-reactor performance are promising options for sustainable wastewater treatment, and the bioactivity of sludge after biopolymer extraction is thus critical for the performance of the system. To this end, stratified extracellular polymeric substances (EPS), including slime, loosely bound EPS (LB-EPS), and tightly bound EPS (TB-EPS), were extracted, and the bioactivities of the consequent extraction residues were assessed using aerobic respirogram, kinetic, and flow cytometry (FCM). After the initial weak extraction of slime, the particle size distribution of the sludge significantly decreased, and subsequent extractions of LB-EPS and TB-EPS produced an equivalent size distribution. In contrast, the fractal dimension decreased after each extraction, suggesting that LB-EPS and TB-EPS affected the compactness of flocs rather than the size. The aerobic bacteria distribution estimated using respirogram shows that slime mainly encapsulated heterotrophs while LB-EPS mainly encapsulated nitrifiers. In addition, the ammonia-nitrogen affinity coefficient decreased from 1.79 to 0.28 mg/L when slime was removed, thereby encouraging the activities of autotrophic nitrifiers. Further removal of LB-EPS induced high energy dispersion as the maintenance coefficient m and the metabolic dispersion index μ/m increased from 0.11 to 0.22 and 0.44 to 0.63, respectively. Meanwhile, the yield rate decreased from 0.77 to 0.66. Although pellets that resulted from TB-EPS extraction were not aerobically active as described by respirogram and growth curves, they were still metabolically active as measured by live/dead cell counting and redox sensor green signal. These pellets used more energy for maintenance as indicated by the high maintenance coefficient than those residual after either slime or LB-EPS extraction. In addition, the variation in bacteria community distribution across flocs was related to the variation in temperatures, suggesting that the inner part of a floc might be hotter than the outer side. Therefore, compared to bacteria in the raw sludge, the viable bacteria bounded in LB-EPS and TB-EPS convert more energy to heat rather than growth. These results indicate that energy was dispersed as metabolic heat for the LB-EPS extracted sludge, and removal of LB-EPS favored thermogenesis and sludge reduction. Based on the above findings, a simultaneously EPS-recovery and performance enhancement configuration is thus proposed, which holds great promise for the integration of next-generation wastewater treatment plants.

Highly efficient solid-liquid separation of anaerobically digested liquor of food waste: Conditioning approach screening and mechanistic analysis

Anaerobic digestion is known for its effectiveness and environmental friendliness in treating food waste. However, it produces anaerobically digested liquor (ADL). ADL usually has a high solid content and high concentrations of nitrogen (N), phosphorus (P), and chemical oxygen demand (COD) carried by suspended solids (SS). Thus, when ADL with amounts of SS reaches the subsequent biochemical treatment units, they negatively impact the microbial stability of corresponding processes, causing unstable effluent qualities. For this reason, the solid-liquid separation of ADL acts as a crucial step for the wide application of anaerobic digestion. In this work, the typical sludge conditioning approaches, including flocculation/coagulation, coagulation, oxidation and advanced oxidation processes (AOPs), were systematically screened for their feasibility in enhancing the solid-liquid separation of ADL. The modified Fenton treatment combined with centrifugation was found to be the most effective approach, which realized the removal of 91.36% SS with FeSO₄•7 H₂O (5.96 g/L) and H₂O₂ (2.79 g/L) but without pH adjustment of ADL. The mechanism analysis showed that the modified Fenton promoted ADL colloidal aggregates to form looser medium-sized flocs with pores, increased the zeta potential to -17.6 mV, and highly reduced the total interfacial free energy. Also, extracellular polymeric substances (EPS) were released into liquid phase, which further eliminated the water-retaining properties of solid compositions. The analysis of surface chemical composition suggested that the modified Fenton decreased the hydrophilic component from 53.37% to 43.81% and the relative content of protein-N from 45.43% to 23.57%, while increased carbon chain and hydrocarbyl species. Furthermore, principal component analysis (PCA) suggested that SS, zeta potential, Lewis acid-base interfacial free energy, two-dimensional fractal index (2-D_f) and C-(N, O) relative content were more sensitive to variations in conditioning approaches than protein-N relative content, and hydroxyl free radical (•OH) played the key role for the modified Fenton to improve SS removal from ADL.

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.