Biodegradability Of Waste Activated Sludge Research Articles

Enhancing anaerobic digestion of waste activated sludge by dielectric barrier discharge pretreatment: Nutrient release, sludge hydrolysis and microbial community succession

A dielectric barrier discharge (DBD) pretreatment of waste activated sludge (WAS) was developed for boosting the releasing of organic matter from floc structures and microbial cells. The changes of protein and polysaccharide in different extracellular polymeric substances layers of SCOD releasing from sludge were determined by using three-dimensional fluorescence spectra. Significantly improved cumulative methane production was achieved via enhancing anaerobic digestion, which was 83.5% higher than that of the control. The formed ∙OH radicals play a key role in sludge hydrolysis, thus improving the biodegradability of WAS. Furthermore, high-throughput sequencing analysis showed that DBD pretreatment improved the relative abundances of the main functional microbes with respect to sludge hydrolysis and anaerobic digestion. Therefore, this study verifies the feasibility of DBD pretreatment in sludge hydrolysis and is expected to provide a practical new technology for anaerobic digestion of WAS.

Biochemical assays of intensified methane content in biogas from low-temperature processing of waste activated sludge

Anaerobic digestion (AD) is implemented as an important ‘waste to energy’ approach for converting organic-rich byproducts such as sewage sludge into biogas and nutrient-rich digestate. Sewage sludge consists of primary sludge and waste activated sludge (WAS), and low biodegradability of WAS limits methane production. This study presents the influence of the low-temperature pretreatment (LT-PT) of WAS on the efficiency of AD. Different conditions of LT-PT process were tested in terms of temperature (45 °C/50 °C/55 °C/60 °C) and exposure time (24h/48h). LT-PT caused the increase of biomethane potential, which was even 43% higher if compared with the untreated WAS; 90% of all methane production was reached between 6 and 12 day of the LT-PT process. The highest improvements were observed in samples pretreated at 55 °C and 60 °C for 48 h (198.8 ± 5.5 NmL CH4/g VS and 194.3 ± 4.2 NmL CH4/g VS, respectively). Changes in the microbial community during LT-PT of WAS showed an oxygen-driven decrease among the predominant phyla except for Firmicutes. Taxonomic diversity and numerous rare taxa with the potential for functional compensation were observed. It was also proved that AD of pretreated WAS, combined with the heat recovery, achieved a positive net energy balance (improved by over 40%) and better energy performance, than AD without LT-PT.

Sulfite pretreatment enhances the medium-chain fatty acids production from waste activated sludge anaerobic fermentation

Production of high-value medium chain fatty acids (MCFAs) from anaerobic fermentation of waste activated sludge (WAS) has been considered as a promising alternative for renewable energy resources. However, the low biodegradability of WAS greatly limits the anaerobic fermentation performance. This study proposed and demonstrated a novel approach, sulfite pretreatment, to efficiently produce MCFAs through anaerobic fermentation of WAS. Pretreatment of WAS at a sulfite concentration of 100–500 mg S/L for 24 h effectively improved the MCFAs production and MCFAs selectivity and the promotion effect was positively correlated with the sulfite concentration used in pretreatment (Pearson's R > 0.9). The maximum MCFAs production of 6.84 g COD/L and MCFAs selectivity of 39.1 % were both achieved under 500 mg S/L sulfite pretreatment, which accounts for 2.6 times and 2.4 times of the control, respectively (MCFAs production of 2.62 g COD/L and MCFAs selectivity of 16.4 % in the control). Sulfite pretreatment also enhanced the WAS degradation from 25 ± 2 % in the control to a maximum of 39 ± 2 % under 500 mg S/L sulfite pretreatment. The electron transfer efficiency and COD flows from the substrate to products were enhanced by up to 25 % due to the sulfite pretreatment, which supports the enhanced WAS degradation. Sulfite pretreatment also promoted the solubilization, hydrolysis, and acidification processes during the anaerobic fermentation by up to 200 %, 60 %, and 45 %, respectively, which subsequently makes more substrates available for MCFAs production. The findings from this study provide a potential solution of using industrial sulfite-laden wastes for WAS pretreatment, to enhance the MCFAs production at a minimized cost.

Predicting the impact of hydraulic retention time and biodegradability on the performance of sludge acidogenesis using an artificial neural network

This study aimed to predict volatile fatty acids (VFAs) production from SDBS-pretreated waste-activated sludge (WAS). A lab-scale continuous experiment was conducted at varying hydraulic retention times (HRTs) of 7 d to 1 d. The highest VFA yield considering the WAS biodegradability was 86.8 % based on COD at an HRT of 2 d, where the hydrolysis and acidogenesis showed the highest microbial activities. According to 16S rRNA gene analysis, the most abundant bacterial class and genus at an HRT of 2 d were Synergistia and Aminobacterium, respectively. Training regression (R) for TVFA and VFA yield was 0.9321 and 0.9679, respectively, verifying the efficiency of the ANN model in learning the relationship between the input variables and reactor performance. The prediction outcome was verified with R2 values of 0.9416 and 0.8906 for TVFA and VFA yield, respectively. These results would be useful in designing, operating, and controlling WAS treatment processes.

New insight into mechanisms of ferroferric oxide enhancing medium-chain fatty acids production from waste activated sludge through anaerobic fermentation

Medium chain fatty acids (MCFAs) production from waste activated sludge (WAS) is restricted by poor biodegradability of WAS and low electron transfer efficiency. Herein, a novel ferroferric oxide (Fe3O4) technique was proposed. Results indicated that the MCFAs yield and selectivity were respectively enhanced by 155.4% and 66.7% in the Fe3O4-mediated WAS. Mechanistic studies disclosed that Fe3O4 promoted substrates degradation through conducting dissimilatory iron reduction (DIR) and stimulating hydrolase activity, providing precursors for chain elongation (CE). Generally, Fe3O4 improved the key processes for MCFA production at different degrees, i.e., hydrolysis, acidification and CE. Interestingly, MCFAs yield enhancement was primarily ascribed to facilitated electron transfer rather than DIR or produced ferrous iron, which could be supported by the analyses of electrochemical properties, electron transfer system activity and morphology. Further, Fe3O4 shifted the key microorganisms in favor of MCFAs production. Overall, this strategy could improve MCFAs production, sludge dewatering and phosphorus removal, concurrently.

Reduction of refractory Maillard reaction products by Fe3+ during thermal hydrolysis pretreatment and enhanced sludge biodegradability

Refractory Maillard reaction products (MRPs) produced during thermal hydrolysis pretreatment (THP) of waste activated sludge (WAS) may negatively impact the performance of downstream anaerobic digestion (AD) and nitrogen removal processes. Operating THP at lower temperature can mitigate the production of MRPs and improve biodegradability of WAS, while solubilization of WAS is reduced. This study intends to develop a method to reduce the refractory MRPs of WAS without compromising on the solubilization. Fe3+ was introduced into THP process (165 °C, 30 min) to mitigate Maillard reaction. Effects of Fe3+ on solubilization of WAS, reduction of refractory residuals, accumulative methane production, and microbial community shift were studied. Results confirm that solubilization of WAS was improved and refractory residuals were reduced with the amendment of 10 mg-Fe/L FeCl3. MRPs mitigation mechanisms were investigated and mainly attributed to Fe3+-triggered Fenton-like reactions. Methane production was enhanced by 10.4 ± 0.8% and attributed to the improved biodegradability of THP liquor, as well as to the enrichment of protein degradation and methane production related microbial community. This work provides a simple, economical, and safe strategy to reduce refractory residuals discharged from THP-AD system and to enhance methane production for more energy recovery.

Effect of ultrasonication on waste activated sludge rheological properties and process economics

The present study provides an overall view of the effect of the ultrasound treatment on waste activated sludge (WAS) rheological and dewatering properties as well as its impact on the economic balance of a theoretical wastewater treatment plant. The results showed that ultrasonication at 27,000 kJ/kg TS increased the soluble protein concentration (> 100%), bound water content (∼25%), and capillary suction time (> 100%) of WAS. The molecular weight distribution of the extracellular polymeric substances (EPS) revealed that the ultrasound treatment solubilised a portion of the peptides and low-molecular-weight proteins. The thixotropic behaviour of the WAS was analysed by means of a rheological structural model that defines the time evolution of a structural parameter as a function of kinetic coefficients for the breakdown and build-up processes. The ultrasound treatment reduced the kinetic coefficients for the breakdown process and changed the fast speed of alignment of flocs because of the reduction of WAS structures. Similarly, the creep tests revealed that the ultrasound treatment at 27,000 kJ/kg TS reduced the initial elasticity (∼80%) and the zero-shear rate viscosity (∼60%), which means that the internal structure of the WAS loosened and disrupted. Finally, a techno-economic analysis showed that ultrasonication was not yet economically favourable since its implementation increased 14% the net cost for WAS treatment and disposal. However, a sensitivity analysis illustrated that increasing electricity revenue and reducing biosolids disposal costs through improvement in WAS biodegradability is important to make ultrasound implementation economically attractive.

The biodegradability of aquatic worm predated waste activated sludge: A sequential aerobic and anaerobic treatment approach

The objective of this study was to investigate the effect of waste activated sludge (WAS) predation by the aquatic worm Tubifex tubifex (T. tubifex) on the overall biodegradability of WAS. The initial WAS biodegradability potential was determined in 80 days sequential batch-fed anaerobic and aerobic treatment combinations. These treatment combinations were used as a reference for comparison with the effect of 5-day predation and 40-day anaerobic treatment combinations. Predation and the subsequent anaerobic digestion of the predated solids shows superior solids removal and superior overall conversion rates compared to solely conventional anaerobic digestion. Strikingly, the predation and anaerobic treatment combinations reached the same chemical oxygen demand (COD) and volatile solids (VS) reduction as the reference processes, i.e. 58% and 49% for COD and VS, respectively. Our results show that predation and anaerobic treatment combinations increase solids removal rates, but do not alter the overall biodegradability potential of WAS.

Octylphenol facilitates fermentative volatile fatty acids recovery from waste activated sludge

The presence of endocrine disruptor compounds (EDCs) in wastewater treatment plants has attracted widespread attention, but their potential impact on anaerobic fermentation of waste activated sludge (WAS) remains unclear. Therefore, this study aims to reveal the effect of typical EDC octylphenol (OP) on the recovery of volatile fatty acids (VFAs) in anaerobic fermentation. The results show that OP has a positive effect on the recovery of VFAs from WAS. The presence of 200 mg/kg dry sludge of OP increased the cumulative amount of VFAs from 3245 in the control (without OP) to 6828 mg COD/L. The increase in VFA production was mainly attributed to the accumulation of acetic acid, which rose from 1511 to 4425 mg COD/L, almost tripled. Further research found that OP promoted solubilization and hydrolysis by improving the biodegradability of WAS, and severely inhibited the methanogenesis process by inhibiting the activity of coenzyme F420, thereby significantly increasing the accumulation of acetic acid. These findings are of great significance to clarify the role of OP in anaerobic fermentation, and provide theoretical basis and guidance for the selection of target products in anaerobic fermentation of WAS containing OP.

Hydrolysis and decomposition of waste activated sludge with combined lysozyme and rhamnolipid treatment: Effect of pH

Effect of pH on waste activated sludge (WAS) hydrolysis and decomposition treating with lysozyme and rhamnolipid combined (Ly + RL) was investigated in this study. Results showed that Ly + RL system could significantly improve the release of soluble organic matters at the optimal RL dosage of 0.3 g/gSS and lysozyme dosage of 0.15 g/gSS. Alkali conditions showed better effect than that of acid on the release of soluble organics, improvement of WAS biodegradability and reduction of big floc size within Ly + RL treatment system and the optimal pH was 10. And 9591.6 mg/L soluble chemical oxygen demand (SCOD), 1612.0 mg/L protein and 1211.6 mg/L polysaccharide were released at pH10 after 12 h co-digestion. 83.7% bacteria and 92.2% archaea were decomposed at pH10. Class Gammaproteobacteria (82.4%) was the predominant bacteria after treated by Ly + RL system, and the treated WAS was beneficial for the subsequent organics bio-degradation and volatile fatty acids accumulation.

How does free ammonia-based sludge pretreatment improve methane production from anaerobic digestion of waste activated sludge

Previous studies reported that free ammonia (FA) pretreatment could improve methane production from anaerobic digestion of waste activated sludge (WAS) effectively. However, details of how FA pretreatment improves methane production are poorly understood. This study therefore aims to reveal the underlying mechanisms of FA pretreatment affecting anaerobic digestion of WAS through a series of batch tests using either real sludge or synthetic media as the digestion substrates at different pH values. At pH 8.5 level, with an increase of FA level from 18.5 to 92.5 mg/L (i.e., NH+ 4-N: 100–500 mg/L; pH 8.5) the maximum methane yield varied between 194.0 ± 3.9 and 196.9 ± 7.7 mL/g of VSS (25 °C, 1 atm). At pH 9.5 or 10 level, however, with an increase of initial FA level from 103.2 to 516.2 mg/L, the maximal methane yield increased linearly. The mechanism studies revealed that FA pretreatment at high levels not only accelerated the disintegration of WAS but also enhanced the biodegradability of WAS. Although pH in the digesters was adjusted to 7.0 ± 0.1, the high levels of NH+ 4-N added or released led to substantial levels of residual FA ranging from 4.4 to 11.6 mg/L. It was found that this level of FA inhibited homoacetogenesis and methanogenesis significantly, though hydrolysis, acidogenesis, and acetogenesis processes were unaffected largely. Further analyses showed that the inhibition constant of FA to substrate degradation was in the sequence of dextran > glucose > hydrogen > acetate, indicating the methanogenesis process was more sensitive to FA.

Free nitrous acid pre-treatment of waste activated sludge enhances volatile solids destruction and improves sludge dewaterability in continuous anaerobic digestion

Previous work has demonstrated that pre-treatment of waste activated sludge (WAS) with free nitrous acid (FNA i.e. HNO2) enhances the biodegradability of WAS, identified by a 20–50% increase in specific methane production in biochemical methane potential (BMP) tests. This suggests that FNA pre-treatment would enhance the destruction of volatile solids (VS) in an anaerobic sludge digester, and reduce overall sludge disposal costs, provided that the dewaterability of the digested sludge is not negatively affected. This study experimentally evaluates the impact of FNA pre-treatment on the VS destruction in anaerobic sludge digestion and on the dewaterability of digested sludge, using continuously operated bench-scale anaerobic digesters. Pre-treatment of full-scale WAS for 24 h at an FNA concentration of 1.8 mg NN/L enhanced VS destruction by 17 ± 1% (from 29.2 ± 0.9% to 34.2 ± 1.1%) and increased dewaterability (centrifuge test) from 12.4 ± 0.4% to 14.1 ± 0.4%. Supporting the VS destruction data, methane production increased by 16 ± 1%. Biochemical methane potential tests indicated that the final digestate stability was also improved with a lower potential from FNA treated digestate. Further, a 2.1 ± 0.2 log improvement in pathogen reduction was also achieved. With inorganic solids representing 15–22% of the full-scale WAS used, FNA pre-treatment resulted in a 16–17% reduction in the volume of dewatered sludge for final disposal. This results in significantly reduced costs as assessed by economic analysis.

Enzymes catalyzing pre-hydrolysis facilitated the anaerobic fermentation of waste activated sludge with acidogenic and microbiological perspectives

This study investigated acidogenic and microbiological perspectives in the anaerobic fermentation (AF) of waste activated sludge (WAS) pre-hydrolyzed by enzymes catalysis. The enzymes catalysis boosted WAS biodegradability dramatically with nearly 8500 mg/L soluble chemical oxygen demand (SCOD) increase just within 4 h. The volatile fatty acids (VFAs) in the acidogenesis were accumulated effectively with over 3200 mg COD/L in 12 d, which reached 0.687 kWh/kg VSS electricity conversion efficiency (2.5 times higher than the control test). The fermentation process favored the compression of fermentative sludge with the distribution spread index (DSI) rising. The core populations of bacteria and archaea shifting enlarged the dissimilarity of communities at different fermentation stages. Increase of community diversity contributed to VFAs accumulation stability. Moreover, the intermediate bacterial community evenness favored VFAs accumulation potentially. The enzymes catalysis might be a promising solution for strengthening VFAs accumulation in the WAS fermentation with boosting the electricity conversion potential.

Upgrading the hydrolytic potential of immobilized bacterial pretreatment to boost biogas production.

In this study, surfactant dioctyl sodium sulphosuccinate (DOSS)-mediated immobilized bacterial pretreatment of waste activated sludge (WAS) was experimentally proved to be an efficient and economically feasible process for enhancing the biodegradability of WAS. The maximal floc disruption with negligible cell cleavage was achieved at surfactant dosage of 0.009g/g SS. Results of the outcome of bacterial pretreatment of sludge biomass revealed that chemical oxygen demand (COD) solubilization for deflocculated (EPS removed-bacterially pretreated) sludge was 20%, which was higher than that of flocculated (14%) or control (5%). The pretreatment was swift in deflocculated sludge with a rate constant of about 0.064h-1. Biochemical methane potential (BMP) assay resulted in significant methane yield at 0.24 gCOD/gCOD for deflocculated sludge. Economic assessment of the proposed method showed a net profit of about 57.39 USD/ton of sludge.

Prediction of Methane Generation of Ozone-Treated Sludge from a Wastewater Treatment Plant

ABSTRACTPrior to anaerobic digestion of waste activated sludge (WAS), the pre-treatment of sludge including ozonation is utilized to enhance the rate of methane generation and turn it to feasible bio-resource energy. Biochemical methanogenic potential (BMP) test is a common tool to estimate the biodegradability of raw and ozonated WAS, although, it is a time consuming test requiring expensive gas analysis. The objective of this research is to recognize key parameters essential to predict biodegradability of WAS and create a valid Artificial Neural Networks (ANN) for estimating sludge biodegradability. The results reflect that ANN is an effective estimation tool.

Impact of thermal treatment on the rheological properties and composition of waste activated sludge: COD solubilisation as a footprint of rheological changes

The rheological properties of sludge are key parameters in the design and optimisation of wastewater treatment processes. However, these properties are irreversibly altered when the sludge is exposed to heat (e.g. either passing through heat exchangers of anaerobic digesters or during thermal pre-treatment process before digesters). Literature showed that thermal pre-treatment of waste activated sludge (WAS) solubilises particulate organic matter in sludge and increases the biodegradability of WAS in anaerobic digesters.This paper reports the similarity between the kinetic of rheological enhancements due to thermal treatment and the rate of organic matter solubilisation in the WAS when it is subjected to cyclic thermal treatment between 20 and 80°C up to 1h. This work observed the evolution of rheological characteristics and soluble oxygen demand (sCOD) of 3.5–9.9wt.% WAS at different temperatures and for different thermal histories (1h thermal treatment at 50, 60, 70 and 80°C).Results showed that, the rate of yield stress and infinite viscosity reductions due to thermal treatment (for 1h treatment at 50, 60, 70 and 80°C) were linearly proportional to the rate of solubilisation of organic matter. Because similar results were observed in the literature with digested sludge, these results confirmed that rheological measurement (e.g. infinite viscosity and yield stress) can be used to predict the level of solubilisation of organic matter in any type of sludge when it is exposed to thermal treatment. Vice versa, the rate of COD changes can be used as an indicator of the rate of rheological changes.

Improving Methane Production During the Anaerobic Digestion of Waste Activated Sludge: Cao-ultrasonic Pretreatment and Using Different Seed Sludges

Three individual seed sludges, which domesticated by filter paper (SS1), food waste (SS2) and grease (SS3), respectively, were used for enhancing the methane production of waste activated sludge (WAS). Also CaO-ultrasonic pretreatment was performed on WAS to evaluate the effectiveness on improving efficient anaerobic digestion (AD). The results showed that WAS being acidated for 24h after CaO-ultrasonic pretreatment was an effective method for increasing initial methane production rate. The daily concentration of volatile fatty acids (VFAs) during the AD course showed that the propionic was easier to be reduced after adding seed sludge. The optimum seed sludge for improving methane production and biodegradability of WAS was SS3, which led to an increase in the methane production of 68.92% and VS reduction of 69.20% higher than the control. This pretreatment combined with adding optimum seed sludge can greatly improve clean energy generation from WAS.

Toxicity and biogas production potential of refinery waste sludge for anaerobic digestion

Two waste streams from an oil refinery wastewater treatment system, float from a dissolved air flotation unit (DAF sludge) and waste activated sludge (WAS), were investigated to determine toxicity and biogas production potential for anaerobic digestion through batch testing methods. Ozonation as a pretreatment was investigated to observe the impacts of waste solubilization on both toxicity and biodegradability. Anaerobic toxicity assays resulted in no detectible inhibition from WAS, neither with nor without ozonation. Untreated DAF sludge exhibited inhibition that amplified with the increases in DAF sludge inclusion. Ozone treatment effectively reduces this inhibition. The biodegradability of WAS, measured by biochemical methane potential tests, doubled with low dose ozonation. DAF sludge biodegradability was negligible prior to treatment and was successfully enhanced through ozonation.

Effect of NaCl induced floc disruption on biological disintegration of sludge for enhanced biogas production

In the present study, the influence of NaCl mediated bacterial disintegration of waste activated sludge (WAS) was evaluated in terms of disintegration and biodegradability of WAS. Floc disruption was efficient at 0.03g/g SS of NaCl, promoting the shifts of extracellular proteins and carbohydrates from inner layers to extractable – soluble layers (90mg/L), respectively. Outcomes of sludge disintegration reveal that the maximum solubilization achieved was found to be 23%, respectively. The model elucidating the parameter evaluation, explicates that floc disrupted – bacterially disintegrated sludge (S3) showed superior biodegradability of about 0.23 (gCOD/gCOD) than the bacterially disintegrated (S2) and control (S3) sludges of about 0.13 (gCOD/gCOD) and 0.05 (gCOD/gCOD), respectively. Cost evaluation of the present study affords net profits of approximately 2.5 USD and −21.5 USD in S3 and S2 sludge.

Performance of the lysozyme for promoting the waste activated sludge biodegradability

The fresh waste activated sludge (WAS) from a lab-scale sequencing batch reactor was used to determine the performance of the lysozyme for promoting its biodegradability. The results showed that a strict linear relationship presented between the degree of disintegration (DDM) of WAS and the lysozyme incubation time from 0 to 240min (R2 was 0.992, 0.995 and 0.999 in accordance with the corresponding lysozyme/TS, respectively). Ratio of net SCOD increase augmented significantly by lysozyme digestion for evaluating the sludge biodegradability changes. Moreover, the protein dominated both in the EPS and SMP. In addition, the logarithm of SMP contents in supernatant presented an increasing trend similar with the ascending logarithmic relation with the lysozyme incubation time from 0 to 240min (R2 was 0.960, 0.959 and 0.947, respectively). The SMP, especially the soluble protein, had an important contribution to the improvement of WAS biodegradability.