Enhancement of sludge dewaterability via the thermal hydrolysis anaerobic digestion mechanism based on moisture and organic matter interactions

Seasonal variations in nutrient changes during sludge anaerobic digestion combined with thermal hydrolysis and deep dewatering

The quality of sludge is significantly influenced by seasonal variations and specific treatment stages, particularly through changes in nutrient levels and organic matter composition. However, the detailed mechanisms of these effects remain poorly understood. In this study, sludge samples were collected from a large-scale wastewater treatment plant in Beijing during spring, summer, autumn and winter, across each process node of the "pre-dewatering + thermal hydrolysis + anaerobic digestion + deep dewatering". A systematic analysis of key soil nutrients (C, N, P, K, Na, Ca and Mg) was conducted on the collected sludge samples. The results demonstrated the concentration of C, N and P in sludge was higher in autumn, with TOC reaching 29.23 %, TN at 44,470.70 mg/kg and TP at 28,572.89 mg/kg. The concentrations of C, N and Na decreased by 34.9 %, 20.4 % and 12.0 % following treatment. Conversely, the concentrations of P, Ca and Mg increased by 20.4 %, 52.7 % and 39.9 %, respectively. Additionally, the reducible fraction contents of K, Na, Ca and Mg exhibited a general increase of 41.68 %, 14.01 %, 31.30 % and 27.13 %, while their oxidizable fractions increased by 101.03 %, 57.27 %, 101.68 % and 59.38 %. The sludge fertilizer quality index (SFQI) showed that the fertility of the treated sludge increased by 1.6 times This study is the first time to use SFQI to quantify sludge quality and systematically evaluate the impact of sludge management on nutrient dynamics. It provides a scientific basis for sludge land utilization and resource utilization.

Effects of anaerobic digestion on the dewaterability of sonicated biosludge

Studies on the impact of anaerobic digestion on biosludge dewatering are conflicting and mechanisms are poorly understood, with significant implications to both wastewater costs and environmental impact. To identify the mechanisms that improve dewaterability and to amplify the impacts of anaerobic digestion, biosludge was first sonicated to deteriorate dewaterability. Sonicated biosludge was next anaerobically digested under mesophilic conditions in three experiments, and the roles of extracellular polymeric substances (EPS) and supracolloidal particles on dewaterability were explored. Dewaterability, as measured by capillary suction time, improved with anaerobic digestion from 84.7 ± 11.3 s-L/g to 31.6 ± 3.2 s-L/g. The loosely-bound fraction of EPS saw only a minor decrease from 6.4 % ± 1.1 %-1.2 % ± 0.3 % and total EPS protein did not change significantly. On the other hand, in a separate experiment, significant removal of supracolloidal particles under 10 μm was observed within 6 days of anaerobic digestion, coinciding with an improvement in a capillary suction time from 116.0 ± 6.7 s-L/g to 69.8 ± 1.1 s-L/g and Crown Press filtrate total solids from 13.7 ± 0.2 g/L to 8.4 ± 0.5 g/L. In addition, in a third experiment, raw biosludge as well as sonicated at five different intensities, saw significantly faster capillary suction time and decreased Crown Press filtrate total solids which coincided with a decrease in the volume % of particles under 16 μm. The results suggest the net removal of small supracolloidal particles is a mechanism that occurs during anaerobic digestion to improve biosludge dewaterability.

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.

Comprehensive hydrothermal pretreatment of municipal sewage sludge: A systematic approach

This study provides a comprehensive analysis of the potential impact of hydrothermal pretreatment (HTP) on municipal thickened waste-activated sludge (TWAS) and its integration with anaerobic digestion (AD). The research demonstrates that HTP conditions (170 °C, 3 bars for 30 min) can increase the solubilization of macromolecular organic compounds by 41%, which enhances biodegradability in semicontinuous bioreactors. This treatment also results in a 50% reduction in chemical oxygen demand (COD) and a 63% increase in the destruction of volatile solids (VS). The combination of HTP with AD significantly boosts methane yields by 51%, reaching 176 ml/g COD, and improves the digestate dewaterability, doubling the solid content in the dewatered cake. However, a higher polymer dose is required compared to conventional AD. Microbial community analysis correlates the observed performance and alterations; it indicates that HTP enhances resilience to stress conditions such as ammonia toxicity. This comprehensive study provides valuable insights into the transition from wastewater treatment plants (WWTPs) to resource recovery facilities (RRF) in line with circular economy principles.

Sludge dewaterability improvement with microbial fuel cell powered electro-Fenton system (MFCⓅEFs): Performance and mechanisms

Throughout the entire process of sludge treatment and disposal, it is crucial to explore stable and efficient techniques to improve sludge dewaterability, which can facilitate subsequent resource utilization and space and cost savings. Traditional Fenton oxidation has been widely researched to enhance the performance of sludge dewaterability, which was limited by the additional energy input and the instabilities of Fe2+ and H2O2. To reduce the consumption of energy and chemicals and further break the rate-limiting step of the iron cycle, a novel and feasible method that constructed microbial fuel cell powered electro-Fenton systems (MFCⓅEFs) with ferrite and biochar electrode (MgFe2O4@BC/CF) was successfully demonstrated. The MFCⓅEFs with MgFe2O4@BC/CF electrode achieved specific resistance filtration and sludge cake water content of 2.52 × 1012 m/kg and 66.54 %. Cellular structure and extracellular polymeric substances (EPS) were disrupted, releasing partially bound water and destroying hydrophilic structures to facilitate sludge flocs aggregation, which was attributed to the oxidation of hydroxyl radicals. The consistent electron supply supplied by MFCⓅEFs and catalytically active sites on the surface of the multifunctional functional group electrode was responsible for producing more hydroxyl radicals and possessing a better oxidizing ability. The study provided an innovative process for sludge dewaterability improvement with high efficiency and low energy consumption, which presented new insights into the green treatment of sludge.

Process modelling and techno-economic analysis of anaerobic digestion of sewage sludge integrated with wet oxidation using a gravity pressure vessel and thermal hydrolysis

Anaerobic digestion (AD) of sewage sludge is used to biodegrade sewage sludge into biomethane and digestate. With the addition of thermal processes such as thermal hydrolysis (TH) and wet oxidation (WO), AD biodegradability generally improves. Implementation of additional treatment is challenging due to the limitation in the mass and energy balances. Hence, tools such as process simulation can be utilized to predict the input and output around the process. In addition, an economic analysis needs to be conducted to check the economic feasibility. The techno-economic analysis (TEA), an integrated method to evaluate a process scheme through simulation and subsequent economic analysis, is effective in providing a systematic understanding of economic implications and the feasibility of a process by identifying the bottlenecks and uncertainties that have a significant impact on the technology. TEA of AD, especially incorporating the TH or WO using gravity pressure vessel (GPV) technology, is limited in the literature. A comprehensive TEA of the AD and the pre- and post-treatment schemes can be utilized to determine the most feasible pathway for sludge treatment for implementation in the wastewater industry. In this study, TEA for four different scenarios of AD was conducted using Aspen Plus and economic analysis tools: (1) without any pre- or post-treatment, (2) with TH pre-treatment, (3) with 100 % WO post-treatment, and (4) with 20 % partial wet oxidation (PWO) and acid hydrolysis pre- or post-treatment. A simulation model (GPVM) was developed using Aspen Plus to mimic the GPV reactor. The study outcomes showed that Scenario 3 with 100 % WO post-treatment was the most suitable for processing parameters and sludge treatment cost. The sensitivity analysis concluded that operating cost and plant capacity are the dominant factors that impact the plant feasibility significantly.

Insights into thermal hydrolysis pretreatment temperature for enhancing volatile fatty acids production from sludge fermentation: Performance and mechanism

To reveal the impact of thermal hydrolysis pretreatment (THP) temperature on the unclear mechanisms of volatile fatty acids (VFAs) production, four groups were established with different temperatures (100, 120, 140 and 160 °C), and high throughput sequencing technology was utilized. The results indicated that the optimal VFAs production occurred at 140 °C. Moreover, as the THP temperature increased, the proportion of acetic acid also increased, accounting for 10.8% to 26.7% of the VFAs, compared to only 4.9% in the control group. Mechanism investigations revealed that THP facilitated the hydrolysis and release of biodegradable organic matter. Moreover, the abundance of VFAs production and hydrolytic microorganisms and related metabolic functional genes expression were evidently improved by THP. Overall, this study deepens the understanding of the mechanisms through which different THP temperatures stimulate the production of VFAs through acidogenic fermentation, providing technical support for future THP application in sludge treatment.

Effect of KMnO4/pH adjustment of extracellular polymeric substances under waste activated sludge on sludge dewatering

In this study, we examine the dewaterability of sludge after treatment by KMnO4 at various pH levels, with the goal of understanding the dewaterability of strong oxidizers to waste activated sludge. Good dewatering performance is observed, with capillary suction times (CST) reduced from 263.4 to 30.1 s, and specific resistance to filtration (SRF) falling by 19.6%. Proteins and polysaccharides in tightly bound extracellular polymeric substances (EPS) were also significantly reduced. Based on spectroscopic and electrochemical analysis, we propose mechanisms for the improved dewatering in terms of changes to the sludge's physicochemical properties and EPS. Under strong oxidation, the structure surrounding the bound water is oxidized and bound water is released, so the dewaterability of the sludge is improved.Weiliang Pan and Jiaoni Li contributed equally to this work.

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.

Implications for assessing sludge hygienization: Differential responses of the bacterial community, human pathogenic bacteria, and fecal indicator bacteria to sludge pretreatment-anaerobic digestion

Sewage sludge is the byproduct of wastewater treatment plants, which host enormous diversity of microbes including potential pathogens. However, there are still challenges in assessing hygienization during sludge stabilization due to the complex relationships between dominant microbes and human pathogenic bacteria (HPB), and the accuracy of fecal indicator bacteria (FIB) is also disputed. Here, the responses of the bacterial community, HPB, and FIB to sludge pretreatment-anaerobic digestion (AD) were comprehensively compared using culture-based and 16S rRNA gene molecular analysis methodologies. Bacterial and HPB communities differed in response to sludge pretreatment-AD. AD drove the variation of bacterial community, but led to the convergence of HPB communities in pretreated sludge, indicating the existence of ecological niches that favors HPB dissemination in digesters. The correlation analysis indicated that FIB was suitable for characterizing general pathogen removal instead of showing the real pattern of HPB (i.e., each HPB), implying the need for comprehensive assessment approaches. Moreover, AD-related parameters including pH, total solids destruction, and methane yield were found to play important role in assessing pathogen inactivation given their correlation. This work provides theoretical basis for the selection of appropriate sludge stabilization approaches and future supervision of biosolids biosafety, which finally benefits human health.

Enhanced production of short-chain fatty acids from sludge by thermal hydrolysis and acidogenic fermentation for organic resource recovery

Acidogenic fermentation (FM treatment) converts organics in waste sludge to valuable short-chain fatty acids (SCFAs). To maintain a favorable condition for the production of SCFAs, an alkali is often added continuously to maintain an alkaline pH in the fermenter. However, this chemical adjustment is costly and biotic hydrolysis is slow. In this research, thermal hydrolysis (TH) was introduced as a pretreatment to enhance fermentation and SCFA production. The results were compared with those obtained from the untreated sludge that underwent fermentation with a daily pH 10 adjustment (NT-FM_pH10). The TH pretreatment resulted in rapid abiotic hydrolysis within a short period (1 h), releasing more than 30.5% of organics into the liquid phase of the sludge. These dissolved organics in sludge promoted rapid acidogenesis and SCFA production. TH together with a one-time alkali pretreatment further increased the production of SCFAs during sludge fermentation (TH&Alk-FM): it produced 22.8% more SCFAs than the non-treated NT-FM_pH10 sludge with alkaline pH control during fermentation. Semicontinuous fermentation further showed the advantage of the TH&Alk-FM process, as a rapid and high production of SCFAs was achieved when the fermentation time was shortened from 5 d to 2 d. The microbial community analysis revealed that TH&Alk-FM and NT-FM_pH10 sludge samples had simple but varied microbial communities. The dominant genera in the TH&Alk-FM sludge were unclassified Ruminococcaceae (18.9%) and unclassified Porphyromonadaceae (22.3%), belonging to the classes Clostridia and Bacteroidia, respectively. NT-FM_pH10 was dominated by Tissierella (23.7%) and Proteiniborus (13.5%), which belong to Clostridia. Compared with NT-FM_pH10, the microbial consortia in TH&Alk-FM were supplied with sufficient soluble organics and performed better in fermentation and SCFA production, without the need for the daily alkali addition to control pH.

Stepwise freezing-thawing treatment promotes short-chain fatty acids production from waste activated sludge

Waste activated sludge (WAS), as the byproducts of wastewater treatment plants, has been greatly produced. With high cost and environmental risk of WAS disposal, to explore a low-cost and environment-friendly technology has been a great challenge. Considering that WAS is a collection of organic matters, anaerobic fermentation has been selected as a sustainable way to simultaneously recover resources and reduce environmental pollution. To recover short-chain fatty acids (SCFAs) has gained great concern because of the high value-added application and high-efficiency production process. Considering the temperature in some areas of the world can reach to below 0 °C, this study proposed an efficient strategy, i.e., stepwise freezing and thawing treatment, to promote SCFAs production. The maximal production of SCFAs, i.e., 246 mg COD/g volatile suspended solid, was obtained with the shortened retention time of five days. Mechanistic studies showed that the solubilization of both extracellular polymeric substances (EPSs) and microbial cells could be accelerated, with the EPSs removal of 58.3% for proteins and 59.0% for polysaccharides. Also, the hydrolysis process was promoted to provide more substrates for subsequent acidogenisis, and the functional microorganisms, such as Romboutsia, Paraclostridium, Macellibacteroides and Conexibacter, were greatly enriched, with a total abundance of 26.2%. Moreover, compared to control, methanogenesis was inhibited at a shortened sludge retention time (e.g., five days), which benefited to the accumulation of SCFAs, but the methane production was increased by 25.2% at a longer sludge retention time (e.g., ten days). Thus, these findings of this work may provide some new solutions for the enhanced resource recovery from WAS, and further for carbon-neutral operation of wastewater treatment plants.

Freezing-low temperature treatment facilitates short-chain fatty acids production from waste activated sludge with short-term fermentation

This study proposed a novel and high-efficiency strategy, i.e., freezing followed by low-temperature thermal treatment, to significantly promote short-chain fatty acids (SCFAs) production from waste activated sludge compared to traditional freezing/thawing treatment. The maximal production of SCFAs was 212 mg COD/g VSS with a shortened retention time of five days, and the potentially recovered carbon source, including SCFAs, soluble polysaccharides and proteins, reached 321 mg COD/g VSS, increased by 92.1 and 28.3% compared to sole freezing and thermal treatment. Both the solubilization and hydrolysis steps of WAS were accelerated, and the acid-producing microorganisms, such as Macellibacteroides, Romboutsia and Paraclostridium, were greatly enriched, with a total abundance of 13.9%, which was only 0.54% in control. Interestingly, the methane production was inhibited at a shortened retention time, resulting in SCFAs accumulation, whereas it was increased by 32.0% at a longer sludge retention time, providing a potential solution for energy recovery from WAS.