Partial ozonation of returned sludge via high-concentration ozone to reduce excess sludge production: A pilot study

Towards zero excess sludge discharge with built-in ozonation for wastewater biological treatment

In this study, a biological treatment process, which used a built-in ozonation bypass to achieve sludge reduction, was built to treat the industrial antifreeze production wastewater (mainly composed of ethylene glycol). The results indicated there is a positive correlation between ozone dosage and sludge reduction. At the laboratory level, the MLSS in the system can be stably controlled at around 3400 mg MLSS L-1 under the dosage of 0.18 g O3 g-1 MLSS. Ozonation can increase the compactness of sludge flocs (fractal dimension increased from 1.89 to 1.92). Ozone destroys microbial cell membranes and alters the structure of sludge flocs through direct oxidation through electrophilic reactions. It leads to the release of intracellular polysaccharides, proteins, and other biological macromolecules in microorganisms, thereby promoting the implicit growth of microbial populations. Some bacteria such as g_Pseudomonas, g_Gemmobacter, etc. have strong ethylene glycol degradation ability and tolerance to ozonation. The removal of ethylene glycol includes the glyoxylate cycle, glycine serine carbon cycle, and the glutamate-cysteine ligase pathway of assimilation. Gene KatG and gpx may be key factors in improving microbial tolerance to ozonation. The comprehensive evaluation from the perspectives of cost and carbon emission shows that choosing ozone cracking-implicit growth in wastewater treatment systems has significant cost advantages and application value.

Structure and composition of dissolved organic matters in sludge by ultrasonic treatment

The leaching of dissolved organic matter (DOM) from the sludge into the liquid phase is induced by ultrasound. However, there is limited investigation into the structure and molecular composition of sludge DOM in this process. The molecular structure and composition of sludge DOM in ultrasonic treatment were comprehensively elucidated in this study. The sludge dissolved organic carbon (DOC) and three-dimensional fluorescence spectroscopy (3D-EEM) image had most significant change at 15-min ultrasonic time and 1.2 W/mL ultrasonic density, respectively. Gas Chromatography-Mass Spectrometry (GC-MS) analysis indicated that ultrasonic treatment of sludge reduced the macromolecules to small molecules in DOM. Then, electrospray ionization Fourier-transform ion cyclotron resonance mass spectrometry (ESI FT-ICR-MS) analysis revealed that lignin, tannins, and carbohydrates were the main components of sludge DOMs after ultrasound treatment. analysis revealed that lignin, tannins, and carbohydrates were the main components of sludge DOMs after ultrasound treatment. Furthermore, through the Van Krevelen analysis, the major components were CHO (48.50%) and CHOS (23.20%) in the DOM of ultrasonicated sludge. This research provides the basis for the practical application of ultrasonic treatment of sludge and provides basic information for DOM components.

Abatement of antibiotics and resistance genes during catalytic ozonation enhanced sludge dewatering process: Synchronized in volume and hazardousness reduction

Based on the efficiency of the catalytic ozonation techniques (HDWS+O3 and MnFe2O4 @SBC+O3) in enhancing the sludge dewaterability, the effectiveness in synchronized abatement antibiotics and antibiotic resistance genes (ARGs) was conducted to determine. The results revealed that catalytic ozonation conditioning altered the distribution of target antibiotics (tetracycline (TC), oxytetracycline (OTC), norfloxacin (NOR), ofloxacin (OFL)) in the dewatered filtrate, the dewatered sludge cake and the extra-microcolony/cellular polymers (EMPS/ECPS) layers, achieving the redistribution from solid-phase adsorption to liquid-phase dissolution. The total degradation rate was over 90% for TC and OTC, 72-78% for NOR and OFL; the abatement efficiency of eleven ARGs reached 1.47-3.01 log and 1.64-3.59 log, respectively, and more than four eARGs were eliminated. The effective abatement of the absolute abundance of Mobile genetic elements (MGEs) (0.91-1.89 log) demonstrated that catalytic ozonation conditioning could also significantly inhibit horizontal gene transfer (HGT). The abundance of resistant bacteria was greatly reduced and the signal transduction of the typical ARGs host bacteria was inhibited. The highly reactive oxidation species (ROS) generated were responsible for the abatement of antibiotics and ARGs. These findings provided new insights into the sludge conditioning for ideal and synchronized reduction in volume and hazardousness by catalytic ozonation processes in sludge treatment.

In-situ sludge reduction based on Mn2+-catalytic ozonation conditioning: Feasibility study and microbial mechanisms

To improve the sludge conditioning efficiency without increasing the ozone dose, an in-situ sludge reduction process based on Mn2+-catalytic ozonation conditioning was proposed. Using ozone conditioning alone as a control, a lab-scale sequencing batch reactor coupled with ozonated sludge recycle was evaluated for its operating performance at an ozone dose of 75 mg O3/g VSS and 1.5 mmol/L Mn2+ addition. The results showed a 39.4% reduction in MLSS and an observed sludge yield of 0.236 kg MLSS/kg COD for the O3+Mn2+ group compared to the O3 group (15.3% and 0.292 kg MLSS/kg COD), accompanied by better COD, NH4+-N, TN and TP removal, improved effluent SS and limited impact on excess sludge properties. Subsequently, activity tests, BIOLOG ECO microplates and 16S rRNA sequencing were applied to elucidate the changing mechanisms of Mn2+-catalytic ozonation related to microbial action: (1) Dehydrogenase activity reached a higher peak. (2) Microbial utilization of total carbon sources had an elevated effect, up to approximately 18%, and metabolic levels of six carbon sources were also increased, especially for sugars and amino acids most pronounced. (3) The abundance of Defluviicoccus under the phylum Proteobacteria was enhanced to 12.0% and dominated in the sludge, they had strong hydrolytic activity and metabolic capacity. Denitrifying bacteria of the genus Ferruginibacter also showed an abundance of 7.6%, they contributed to the solubilization and reduction of sludge biomass. These results could guide researchers to further reduce ozonation conditioning costs, improve sludge management and provide theoretical support.

Effects of ozone on activated sludge: performance of anaerobic digestion and structure of the microbial community

The treatment and disposal of activated sludge are currently challenging tasks in the world. As a common biological engineering technology, biological fermentation exists with disadvantages such as low efficiency and complex process. Ozone pretreatments are commonly applied to improve this problem due to their high efficiency and low cost. In this study, the significant function of ozone in anaerobic fermentation gas production was verified with excess sludge. Compared with other untreated sludge, ozone pretreatment can effectively degrade activated sludge. After ozone treatment and mixing with primary sludge, the methane production of excess sludge increased by 49.30 and 50.78%, and the methanogenic activity increased by 69.99 and 73.83%, respectively. The results indicated that the mixing of primary sludge with excess sludge possessed synergistic effects, which contributed to the anaerobic fermentation of excess sludge. The results of microbial community structure exhibited that methanogenic processes mainly involve hydrogenogens, acidogens and methanogens. The relative abundance of both bacteria and microorganisms changed significantly in the early stage of hydraulic retention time, which coincided exactly with the gas production stage. This study provided a feasible pretreatment strategy to improve sludge biodegradability and revealed the role of microorganisms during anaerobic digestion.

Footprints of total coliforms, faecal coliforms and E. coli in a wastewater treatment plant and the probabilistic assessment and reduction of E. coli infection risks

Wastewater contains various pathogenic microorganisms, and the disease of workers caused by exposure to wastewater at the wastewater treatment plants (WWTPs) is a growing concern. The footprints of total coliforms (TC), faecal coliforms (FC) and Escherichia coli (E. coli) in a conventional activated sludge WWTP during 12 consecutive months were clarified. It was found that TC, FC and E.coli in influent were significantly removed (log 4.71, log 4.43 and log 4.62, respectively) by WWTP with sand filtration playing a key role, and excess sludge was a major potential pathway for them flowing to the environment. Through quantitative microbial risk assessment (QMRA), hand-to-mouth ingestion of untreated wastewater and wastewater in secondary/efficient sedimentation tanks, as well as accidental ingestion of sludge in dewatering workshop presented the highest infection risks of pathogenic E.coli in the WWTP, considerably exceeded the U.S. EPA benchmark (≤1 × 10-4 pppy). PPE application and E.coli concentration reduction in wastewater or sludge were recommended to reduce the infection risks at these stages. Further, partial ozonation and dissolved ozone flotation thickening were investigated able to reduce the infection risks at the stages of secondary and tertiary treatment of wastewater or sludge treatment by 90- 98 %. The findings of this study would assist in selecting appropriate processes for the further sanitation of WWTPs.

Effects of ceramsite derived from sewage sludge combined with the O3-FeCl3/PAM process on the dewatering of waste-activated sludge and investigation of dewatering mechanisms

The high water content of waste-activated sludge restricts the subsequent disposal of sludge. The dehydration properties of O₃, ferric chloride (FeCl₃)/polyacrylamide, and sludge ceramsite sand (SCS) were studied. Simultaneously, the effect of combining the three was investigated to support the deep dehydration of waste-activated sludge. Experimental results showed that with13.42 mg/(g dry solids (DS)) of O₃, 109.89 mg/(g DS) of FeCl₃, and 100 mesh dosage of 70% DS of sludge ceramsite on weight, the highest sludge net yield was 7.13 kg/(m²·h) and the minimum specific resistance to filtration of sludge cake was 1.02 × 10¹² (m/kg). Compared with the compressibility of the raw sludge, the compressibility of the sludge cake decreased by 37.48%. Moreover, the Y_N (net yield) increased by 73.55%. The results demonstrate that the structure of cracking, flocculation, and hydrophobic framework is the mechanism of sludge dewatering in this combined process. This combined treatment process provides a new perspective for the realization of deep dewatering of sludge and is anticipated to be a successful sludge dehydration method.

Potential of non-thermal discharge plasmas for activated sludge settling: effects and underlying mechanisms

With increase in the construction of urban sewage treatment plants, the output of sludge also surges. Therefore, it is highly important to explore effective ways to reduce the production of sludge. In this study, non-thermal discharge plasmas were proposed to crack the excess sludge. High sludge settling performance was obtained, and the settling velocity (SV₃₀) dramatically decreased from the initial value of 96% to 36% after 60 min of treatment at 20 kV, accompanied by 28.6%, 47.5%, and 76.7% decreases in mixed liquor suspended solids (MLSS), sludge volume index (SVI), and sludge viscosity, respectively. Acidic conditions improved the sludge settling performance. The presence of Cl^- and NO₃^- slightly promoted the SV₃₀, but CO₃^2- has adverse effects. ·OH and O₂˙^- in the non-thermal discharge plasma system contributed to the sludge cracking, especially for ·OH. These reactive oxygen species destroyed the sludge floc structure; as a result, the total organic carbon and dissolved chemical oxygen demand obviously increased, the average particle size of the sludge decreased, and the number of coliform bacteria was also reduced. Furthermore, the microbial community abundance and diversity both decreased in the sludge after the plasma treatment.

From waste-activated sludge to algae: a self-reliant cultivation process in photoreactors using saline conditions

In this study, microalgae-bacteria (MB) systems using saline conditions (3 and 5% salinity) were built in order to use waste-activated sludge (AS) as raw material for cultivating lipid-rich microalgae. Algae were observed to be flourishing in 60 days of operation, which totally used the N and P released from the sludge biomass. A prominent improvement of lipid content in MB consortia was obtained under algae growth and salinity stimulation, which occupied 119-136 mg/g-SS rather than a low content of 12.1 mg/g-SS in AS. Lipid enrichment also brought a 3.1-3.3 times total heat release (THR) in the MB biomass. The marine spherical algae Porphyridium, as well as filamentous Geitlerinema, Nodularia, Leptolyngbya were found to be the main lipid producers and self-flocculated to 23.0% (R1) and 33.5% (R2) volume under the effect of residue EPS. This study had a big meaning in not only waste sludge reduction but also in manufacturing useful bioenergy products.

Assessing the Effects of Ozonation on the Concentrations of Personal Care Products and Acute Toxicity in Sludges of Wastewater Treatment Plants

The aim of this study was to understand the distribution of the personal care products nonylphenol (NP), triclosan (TCS), benzophenone-3 (BP-3), and caffeine in the sludges from three wastewater treatment plants (WWTP-A, -B, and -C) in southern Taiwan. The four compounds were analyzed from activated sludge and dewatered sludge samples, and then the samples were treated with pressure-assisted ozonation under different conditions and removal efficiencies. All four target compounds were detected, especially NP, which was detected in the highest concentrations in the activated sludges of WWTP-A and dewatered sludges of WWTP-C at 17.19 ± 4.10 and 2.41 ± 1.93 µg/g, respectively. TCS was dominant in dewatered sludges from WWTP-B, and the highest detected concentration was 13.29 ± 6.36 µg/g. Removals of 70% and 90% were attained under 150 psi at 40 cycles for NP and TCS, respectively, with 5 min of ozonation reaction time, a solid/water ratio of 1:20, and 2% ozone concentration. Ecological risk quotients (RQs) were calculated by the ratios of the 10-day Hyalella azteca (freshwater amphipod) LC50 to the environmental concentrations of the target compounds. High RQs were found to be >10 for NP, TCS, and BP-3 in untreated sludges, resulting in significant ecological risks to aquatic organisms when the sludges are arbitrarily disposed. However, the toxic effects on Hyalella azteca were not significantly different among ozone sludge treatments. The reason for this may be related to the formation of toxic oxidation by-products and incomplete mineralization of organic compounds. This could also be true for unknown intermediates. The relatively high detection frequencies of these emerging compounds in WWTP sludges requires further applications and treatments.

Insights on mechanisms of excess sludge minimization in an oxic-settling-anaerobic process under different operating conditions and plant configurations

In the present research, insights about the mechanisms of excess sludge minimization occurring in an oxic-settling-anaerobic (OSA) were provided. The investigation involved two systems operating in parallel. In particular, a conventional activated sludge (CAS) system as control and a system implementing the OSA process both having a pre-denitrification scheme were considered. Five periods (P1-P5) were studied, during which several operating conditions and configurations were tested. Specifically, the hydraulic retention time (HRT) in the anaerobic reactor of the OSA system (P1 8 h, P2-P3 12 h, P4 8 h, P5 12 h) and the return sludge from the anaerobic to the anoxic (scheme A) (P1-P2) or aerobic (scheme B) mainstream reactors (P3-P5) were investigated. The results highlighted that the excess sludge production in the OSA was lower in all the configurations (12-41%). In more detail, the observed yield (Y_obs) was reduced from 0.50-0.89 gTSS gCOD^-1 (control) to 0.22 -0.34 gTSS gCOD^-1 in the OSA process. The highest excess sludge reduction (40%) was achieved when the OSA was operated according to scheme B and HRT of 12 h in the anaerobic reactor (P3). Generally, scheme A enabled the establishment of cell lysis and extracellular polymeric substances (EPS) destructuration, leading to a worsening of process performances when high anaerobic HRT (>8 h) was imposed. In contrast, scheme B enabled the establishment of maintenance metabolism in addition to the uncoupling metabolism, while cell lysis and EPS destruction were minimized. This allowed obtaining higher sludge reduction yield without compromising the effluent quality.

PPCPs and heavy metals from hydrothermal sewage sludge-derived biochar: migration in wheat and physiological response

Once the sludge was directly used in the farmland, it will have a negative impact on human health through the food chain because sludge contains pollutants. Sewage sludge pyrolysis into biochar is an effective way to realize sludge harmless and resourceful utilization. This research used hydrothermal carbonization method to convert sludge into sludge biochar (SLBC) to reduce the types and contents of pharmaceuticals and personal care products (PPCPs) and available heavy metals. Furthermore, migration of the residual caffeine (Caf), acetaminophen (Ace), and heavy metals (Cr, Pb, Cu, Zn) released from the SLBC in the wheat was assessed. The results showed that the levels of Caf, Ace, Pb, Cu, and Zn accumulated in the shoots were lower than the limit regulated by Drug and Food Additive Use Standard in China (Caf: 150 mg/kg; Ace: 2.5 ~ 5 mg/kg; Pb: 0.3 mg/kg; Cu: 10 mg/kg; Zn: 20 mg/kg). The migration of Cr from roots to shoots was also significantly controlled by SBLC. SBLC delayed the germination time of wheat seeds with increasing in hydrothermal temperature, the germination rate and root length showed a decreasing trend. Evans blue and O₂^- fluorescence staining of root tips also confirmed this conclusion. When the wheat was exposed to the low temperature and dose of SLBC, the chlorophyll contents and growth of wheat can be significantly increased; the oxidative damage of cell plasma membrane and net photosynthetic rate were reduced. However, 0.8 g/L of SLBC made plants suffer abiotic stress and caused oxidative damage to plants, and decreased membrane system stability. The study provides some parameters for sludge to realize resource utilization in the agricultural system.

Optimization of dissolution and fermentation acid production of rhamnolipid-alkali-heat synergistic pretreatment of sludge

With the development of the urban sewage treatment industry, the sludge output has increased year by year, and it has the characteristics of large output and high organic content, and has great potential for resource recovery. However, the biodegradability of sludge is poor, resulting in low added value of resource products. Therefore, it is necessary to treat sludge efficiently and improve the utilization. Based on this, the effects on sludge characteristics and acid-producing fermentation were investigated, and optimal conditions were determined by response surface method. The results showed that: The optimal conditions for experimental optimization are rhamnolipid (RL: 40 mg/gVS) alkali (Alk: 35 mg/gVS), heat: 80 °C. Response surface design optimization results are RL (28.44 mg/gVS), NaOH (35 mg/gVS), heat: 80 °C. In the process of RL-Alk-Heat pretreatment, the organic matter dissolution is Heat > Alk. Also, RL, Alk and Heat all promoted the content of fluorescent substances. From the results of the optimal combination verification test, it showed that SC (Soluble carbohydrate) and SP (Soluble protein) increase. Among them, three-factor treatment is higher than two-factor treatment than single-factor treatment.

Simultaneous methanogenesis and denitrification coupled with nitrifying biofilm for high-strength wastewater treatment: Performance and microbial mechanisms

A combined system consisting of an upflow blanket filter (UBF) and a moving-bed biofilm reactor (MBBR) was developed for the simultaneous removal of organic matters and ammonia from high-strength wastewater. With a constant COD of approximately 2000 mg/L and ammonium nitrogen in a series of concentrations (e.g., 50, 200 and 400 mg/L in stages I to III) of the influent wastewater, the removal efficiencies of COD, ammonium nitrogen and total nitrogen reached 96.10%-98.19%, 100%, and 79.12%-82.15%, respectively. With the increase of influent ammonia nitrogen concentration, the specific methanogenic activity of the UBF granules decreased significantly, while the specific denitrification rates of the UBF granules and specific nitrification rates of the MBBR biofilms increased significantly. Microbial community analysis showed that Methanobacterium and Methanosaeta were the dominant methanogens in the UBF granules, while Candidatus Competibacter, Thauera and Acinetobacter were identified as dominant denitrifiers. In addition, nitrifiers were enriched in MBBR biofilms at 11.33% and 13.87% of the average abundance of Nitrosomonas and Nitrospira, respectively, at stage III (influent ammonium at 400 mg/L, COD/NH₄⁺-N =  5). The ecological network analysis, including full-networks and sub-networks, indicated that the interactions between methanogens and denitrifiers in the UBF granules were strong when the influent ammonium concentration reached 400 mg/L. No intensive interactions were observed among the functional bacteria in the MBBR biofilms over the entire operation. Overall, this study provides a new strategy for the application and construction of efficient biological processes to achieve simultaneous removal of organic matter and nitrogen for high-strength wastewater treatment.

A novel application of dissolved ozone flotation on sewage sludge thickening: Performance and mechanism investigation

Sludge thickening in sewage treatment plants is an essential step to reduce the sewage sludge volume and provide space for collaborative anaerobic digestion of sludge and other urban organic wastes. Dissolved ozone flotation (DOF) is a novel choice worthy of application in the field of sludge thickening. In order to investigate the effect of DOF thickening, the total solid content (TS %) was used to characterize the thickening performance under different O₃ dosage. The optimal condition was determined to be polyacrylamide (PAM) dosage = 3 ‰ TS, air floatation time = 2 h and O₃ dosage = 12 mg/g TS, under which the TS % of raw-sewage sludge (RS) increased from 0.33 ± 0.01 % to 8.03 ± 0.06 %. In this study, the relationship between the sludge thickening performance, physicochemical properties, and the changes of organic matter (content, structure and molecular weight) in extracellular polymers (EPS) was systematically clarified. The results indicated that the DOF couple with PAM could change the sludge surface properties, destroy the sludge floc structure, release intracellular organic matter, and increase moisture fluidity. The surface hydrophilicity/hydrophobicity, protein (PN) secondary structure and moisture distribution were mainly responsible for sludge thickening performance. Moreover, the change of TS % during the DOF thickening process was mainly caused by the variations of the organic matter content in EMPS layer. The identification of key influencing factors was conducive to the further regulation and upgrading of the novel application for enhanced sludge thickening in sewage treatment plants.

Energy feasibility and life cycle assessment of sludge pretreatment methods for advanced anaerobic digestion

Energy sustainability is one of the critical parameters to be studied for the successful application of pretreatment processes. This study critically analyzes the energy efficiency of different energy-demanding sludge pretreatment techniques. Conventional thermal pretreatment of sludge (∼5% total solids, TS) produced 244 mL CH₄/gTS, which could result in a positive energy balance of 2.6 kJ/kg TS. However, microwave pretreatment could generate only 178 mL CH₄/gTS with a negative energy balance of -15.62 kJ/kg TS. In CAMBI process, the heat requirements can be compensated using exhaust gases and hot water from combined heat and power, and electricity requirements are managed by the use of cogeneration. The study concluded that <100 ℃ pretreatment effectively enhances the efficiency of anaerobic digestion and shows positive energy balance over microwave and ultrasonication. Moreover, microwave pretreatment has the highest global warming potential than thermal and ultrasonic pretreatments.

Pilot-Scale Anaerobic Treatment of Printing and Dyeing Wastewater and Performance Prediction Based on Support Vector Regression

Printing and dyeing wastewater is characterized with complex water quality and poor biodegradability. In this study, a pilot-scale anaerobic baffled reactor (ABR) with packing was verified to effectively degrade the complex organic pollutants in the wastewater through the hydrolysis and acidification of anaerobic microorganisms. At a hydraulic retention time (HRT) of 12 h and an organic loading rate (OLR) of 2.0–2.5 kg COD/(m3·d), the ABR stabilized the fluctuation range of pH and achieved an average colority removal rate of 10.5%, which provided favorable conditions for subsequent aerobic treatment. During the early operation period, the reactor increased the alkalinity of the wastewater; after 97 days of operation, the volatile fatty acid (VFA) content in the wastewater decreased. To demonstrate the suitability of the support vector regression (SVR) technology in predicting the performance of the reactor, two SVR algorithms with three kernel functions were employed to relate the chemical oxygen demand (COD) removal rate to its influencing factors, and the predictions of both the training and validation groups agreed with the measurements. The results obtained from this study can contribute to the design and optimal operation of the anaerobic treatment project of the industrial wastewater treatment plant.

Techno-Economic Evaluation of Ozone Application to Reduce Sludge Production in Small Urban WWTPs

In Chile, small wastewater treatment plants (WWTPs) (treatment capacity of less than 4,800 m3/d) are normally not designed with consideration for the potential valorization of generated sludge. For this reason, they are generally operated at high solids residence times (SRT) (15 d) to promote the decay of biomass, promoting less sludge production and reducing the costs associated with biomass management. Operation at high SRT implies the need for a larger activated sludge system, increasing capital costs. The implementation of a sludge-disintegration unit by ozonation in future WWTPs could enable operation at an SRT of 3 d, with low sludge generation. In this work, we evaluate how the implementation of a sludge-ozonation system in small WWTPs (200–4000 m3/d) would affect treatment costs. Four scenarios were studied: (1) a current WWTP operated at an SRT of 15 d, without a sludge ozonation system; (2) a WWTP operated at an SRT of 15 d, with a sludge-ozonation system that would achieve zero sludge production; (3) a WWTP operated at an SRT of 3 d, with a sludge-ozonation system that would provide the same sludge production as scenario 1; (4) a WWTP operated at an SRT of 15 d, with a sludge-ozonation system that would achieve zero sludge production. Economic analysis shows that the treatment costs for scenarios 1 and 2 are similar, while a reduction in cost of up to 47% is obtained for scenarios 3 and 4.