Waste Activated Sludge Acidification Research Articles

Using heat-activated persulfate to accelerate short-chain fatty acids production from waste activated sludge fermentation triggered by sulfate-reducing microbial consortium

Persulfate has been applied extensively for waste activated sludge (WAS) decomposition due to the strong oxidizing sulfate radical generated as a product. However, the efficiency is not improved without activation to produce free radicals. In this study, a novel coupling strategy of heat-activated persulfate (Heat_PS) pretreatment and sulfate-reducing bacteria (SRB) triggering was explored to enhance short-chain fatty acids (SCFAs) produced by WAS fermentation. The remaining sulfate acts as an essential acceptor of electrons for the metabolism of synergistic SRB, thereby boosting WAS acidification by energetic cooperation with anaerobic fermenters. The results showed that SCFAs yield in the Heat_PS + SRB group peaked at 431.89 mg COD/gVSS, with the proportion of acetate reaching 57.8 %. This was 6.33 and 1.75 times higher than that in raw and single Heat_PS treated WAS, respectively. Carbon balance revealed a conversion rate of 26.1 % of carbon content in WAS to SCFAs, with 4.5 % lower CO2 equivalents emitted than that in raw WAS fermentation by the assessments of environmental impacts. This was partially attributed to the strong decomposition of WAS by SO4•− and •OH oxidation from heat-activated PS and the SRB trigger. In addition, the synergistic relationship among acidogenic/fermentative bacteria and SRB consortia was further verified by the positive correlation among Desulfovibrio, the hydrolytic Escherichia-Shigella, Morganella and the fermetative Macellibacteroides and Bacteroides, as revealed by molecular ecological networks (MENs) analysis. The results of this study may highlight the cooperation of the synergistic micribial consortia as an additional perspective for the recovery of value-added biological metabolites from complex biotransformation.

Long-term effects of copper nanoparticles on volatile fatty acids production from sludge fermentation: Roles of copper species and bacterial community structure

The long-term effects of copper nanoparticles (Cu NPs) on volatile fatty acids (VFAs) production during the waste activated sludge (WAS) fermentation, and the underlying mechanisms regarding copper species distribution and bacterial community evolution were explored. The yield of VFAs in the control was 1086 mg COD/L, whereas those were inhibited by 11.1%, 56.0% and 83.1%, with 25, 50, and 100 mg/g-TSS Cu NPs, respectively. Further investigation indicated that Cu NPs severely affected hydrolysis and acidification of WAS in a dose-dependent manner, while had little impact on solubilization. Besides, Cu NPs enriched the acid-consuming anaerobe while reducing the acid-forming bacteria. The metabolic pathways, microbial function, and enzymatic activities involved were inhibited at all tested dosages. Moreover, soluble and acid-extractable fractions dominated the copper speciation, which were also the main factors inhibiting the VFA production. This study provides a new perspective to interpret the long-term impacts of Cu NPs on WAS fermentation.

Potential promotion of activated carbon supported nano zero-valent iron on anaerobic digestion of waste activated sludge

ABSTRACT A large amount of waste activated sludge (WAS) harms the ecological environment, and anaerobic digestion (AD) is an effective method for WAS treatment. In this study, activated carbon (AC)/ nano zero-valent iron (NZVI) was synthesized by a liquid-phase reduction method, and was used to boost methane production. The associated mechanisms and effects of additives on AD during the addition and removal stage were investigated systematically. Compared to the blank group, the cumulative methane production was increased by 14.3%, 26.3% and 34.1% in the groups of AC, NZVI and AC/NZVI, respectively. The addition of AC/NZVI significantly increased the concentration of VFAs and promoted the hydrolysis and acidification of WAS. After the AD of the additives addition stage was finished, the additives were removed and the sludge was replenished in all groups, the methanogenesis performance of the experimental groups was significantly inhibited. The cumulative methane production in the AC and AC/NZVI groups was 21.7% and 13.5% lower than the blank group, respectively. The experimental results have a good correlation with curve fitting by the modified Gompertz model. The modified Gompertz model found that AC, NZVI and AC/NZVI increased the methanogenic potential and maximum methane production rate of WAS, but also prolonged the lag-phase time. AC/NZVI might play a role in coupling effects. It could not only maintain the original characteristics of NZVI and increase its stability, but also develop the advantages of AC promoting direct interspecies electron transfer. Microbial community analysis indicated that the abundance of hydrogenotrophic methanogens was enriched by AC/NZVI.

Effects of sludge age on anaerobic acidification of waste activated sludge: Volatile fatty acids production and phosphorus release

Effects of sludge age on volatile fatty acids (VFAs) production and Phosphorus (P) release during anaerobic acidification of waste activated sludge (WAS) were investigated. Sequencing batch reactors (SBR) fed with simulating domestic sewage were applied to produce WAS of different sludge ages, and batch tests were used for anaerobic acidification. The maximum dissolved total organic carbon, release of PO43+−P, and accumulation of acetate (C2), propionate (C3), butyrate (C4), and valerate (C5) decreased by 56.2%, 55.8%, 52.6%, 43.7%, 82.4% and 84.8%, respectively, as the sludge age of WAS increased from 5 to 40 days. Limited degradation of protein played a dominating role in decreasing DTOC and VFAs production. Moreover, the increase in molecular weight of organics and organic nitrogen content in the supernatant after acidification suggested that the refractory protein in WAS increased as sludge age extended. Although the production of C2, C3, C4, and C5 from WAS decreased as the sludge age increased, the proportions of C2 and C3 in VFAs increased, which might be due to the declined production of C5 from protein and the faded genus Dechlorobacter. Keeping sludge age of WAS at a relatively low level (<10 days) is more appropriate for anaerobic acidification of WAS as internal carbon sources and P resource.

Enhancing acidogenic fermentation of waste activated sludge via isoelectric-point pretreatment: Insights from physical structure and interfacial thermodynamics

The poor biodegradability of waste activated sludge (WAS) is widely regarded as one of the main bottlenecks in the fermentation of sludge and is attributed mainly to the complex nature of sludge. In this study, the physical structure and interfacial thermodynamics of sludge, which reflect its complex nature, were explored to reveal the effects of isoelectric-point (pI) pretreatment on enhancing the production of volatile fatty acids (VFA). It was observed that the maximum VFA production and the initial VFA production rate increased by 151.2% and 46.6%, respectively, after pI pretreatment, which indicates that pI pretreatment significantly improved the generation efficiency of VFA. The experimental results of 12-day acidogenic fermentation assays following pI pretreatment show that the maximum concentrations of soluble total organic carbon, soluble protein and soluble polysaccharide increased by 209.8%, 148.9% and 84.5%, respectively, and the maximal proportion of low molecular weight (<1 kDa) soluble organic substances increased by 92.4%, thus confirming that pI pretreatment can promote organic solubilisation and hydrolysis in sludge. The analyses of changes in the fractal dimension (Df), the spatial configuration of extracellular polymeric substances, and the interfacial non-covalent interaction energy of sludge during the fermentation process reveal that pI pretreatment can loosen the physical structure, promote the spatial extension of biopolymer molecular chains, and increase the driving forces of solid-liquid interfacial enzymatic reactions. It is thus hypothesised that these changes could be responsible for the high degree of organic solubilisation, hydrolysis and acidification of WAS, which is further confirmed by correlation analyses of the Df and interfacial free energy versus VFA production. These findings are expected to provide a possible means to improve the biodegradability of sludge via its pI to trigger dismantling of the sludge structure and increase the driving forces of interfacial enzymatic reactions.

Effect of initial pH on the sludge fermentation performance enhanced by aged refuse at low temperature of 10 °C.

The efficiency of anaerobic fermentation of waste activated sludge (WAS) under low-temperature condition is usually low. This work reported a new strategy to enhance the low-temperature fermentation of WAS by using aged refuse (AR), and explored the effect of initial pH 4 till pH 12 on the production of short-chain fatty acid (SCFA) from AR-enhanced sludge fermentation. The results showed that AR improved WAS fermentation under low-temperature condition (10 °C), and the maximum accumulation of SCFA was 75.6 ± 3.5 mg/g, which was significantly higher than that of the blank (without AR). In addition, when the initial pH was 7, the maximum yield of methane was 102.2 ± 4.8 mL/g, whereas when the initial pH was 11, the maximum yield of SCFA was 85.6 ± 2.8 mg/g. Weak acid or alkali pH benefited hydrolysis and acidification of WAS, but inhibited methanogenesis. The release of NH4+-N and phosphate in a strong alkaline environment (pH 11 and 12) was lower than that in other alkaline environments (pH 10).

Influence of sulfadiazine on anaerobic fermentation of waste activated sludge for volatile fatty acids production: Focusing on microbial responses

Extensive studies on anaerobic fermentation of waste activated sludge (WAS) for volatile fatty acids (VFAs) production focused on the effects of operating parameters and pretreatment methods, and little information is available for those of organic pollutants which were absorbed on sludge. The influence of sulfadiazine (SDZ), a typical antibiotic pollutant in WAS, on VFAs production during anaerobic fermentation was investigated in this study. The accumulation of VFAs was remarkably affected in the presence of SDZ. When the content of SDZ was 50 mg per kilogram dry sludge the concentration of VFAs from sludge was 2032.8 mg COD/L, much higher than that of control (1540.2 mg COD/L). Mechanism investigation revealed that the content of extracellular polymeric substances (EPS) from sludge was increased due to the presence of SDZ, which provided more substrates, i.e., protein and carbohydrate, and created a favorable environment for anaerobes. The hydrolysis and acidification of WAS were stimulated by SDZ, and the functional microorganisms were advantageous to VFAs production. The activities of protease, α-glucosidase and acetate kinase were promoted when SDZ occurred, which were beneficial for hydrolysis and acidification. The effect of SDZ on pure strains further confirmed that the formation of VFAs during anaerobic fermentation was stimulated by SDZ.

Comparison of enhancement of anaerobic digestion of waste activated sludge through adding nano-zero valent iron and zero valent iron.

The feasibility of adding nano-zero valent iron (NZVI: 0.6, 1.0, 4.0, 10.0 g L−1) to enhance anaerobic digestion of waste activated sludge (WAS) was examined by comparison with ZVI, and the mechanisms of NZVI enhancement of the hydrolysis and methanogenesis processes were elucidated. NZVI could enhance hydrolysis–acidification of WAS by destroying the integrity of microbial cells. Both volatile fatty acids production and the acetic acid portion were greatly improved by NZVI additions, peaking at 4.0 g L−1 NZVI. In anaerobic digestion, CH4 production was promoted at a NZVI dosage ≤1.0 g L−1. The optimum dosage of NZVI for methanogenesis was 1.0 g L−1, and further addition of NZVI could cause inhibition of methanogenesis because of long-term accumulation of H2. ZVI could also improve hydrolysis–acidification and the CH4 yield, but its efficiency was relatively low compared with NZVI, and it could not induce cell wall rupture. 16S rDNA high-throughput sequencing results showed that NZVI addition at appropriate dosage facilitated increasing the proportion of microorganisms involved in WAS hydrolysis–acidification and methanogenesis.

Characterization of biocarbon-source recovery and microbial community shifts from waste activated sludge by conditioning with cornstover: Assessment of cellulosic compositions

Most studies on the production of volatile fatty acids (VFAs) from waste activated sludge (WAS) digestion have focused on operating conditions, pretreatments and characteristic adjustments. Conditioning by extra carbon sources (ECS), normally added in a solid form, has been reported to be an efficient approach. However, this has caused considerable waste of monomeric sugars in the hydrolysate. In this study, the effects of two added forms (pretreated straw (S) and hydrolyzed liquid (L)) of cornstover (CS) on WAS acidification were investigated. To obtain different cellulosic compositions of CS, low-thermal or autoclaved assisted alkaline (TA or AA) pretreatments were conducted. The results showed that AA-L test achieved the highest VFAs value (653 mg COD/g VSS), followed by AA-S (613 mg COD/g VSS). These values were 12% and 28% higher, respectively, than that obtained in the TA-L and TA-S tests. Meanwhile, higher percentages of acetic acid were observed after AA pretreatment (~62% versus ~53% in TA). The added forms of CS played an important role in structuring the innate microbial community in the WAS, as shown by high-throughput sequencing and canonical correspondence analysis. The findings obtained in this work may provide a scientific basis for the potential implementation of co-digesting WAS with ECS simultaneously obtaining energy and high value-added products.

What could the entire cornstover contribute to the enhancement of waste activated sludge acidification? Performance assessment and microbial community analysis.

BackgroundVolatile fatty acids (VFAs) production from waste activated sludge (WAS) digestion is constrained by unbalanced nutrient composition (low carbon-to-nitrogen ratio). Characteristics conditioning by extra carbon sources, normally in the mixture of raw solid, has been reported to be an efficient approach to enhance WAS acidification. However, little attention has been paid to the contributions of other adjustment forms. Moreover, the corresponding ecological estimation has not been investigated yet.ResultsIn this study, the feasibility of corn stover (CS) conditioning with three adjustment forms [pretreated straw (S), hydrolysate (H) and hydrolysate + straw (HS)] in improving VFAs production from WAS was demonstrated. It was observed that the highest VFAs yield was achieved in H co-digesting test (574 mg COD/g VSS), while it was only 392 mg COD/g VSS for WAS digesting alone. VFAs composition was strongly adjustment form-dependent, as more acetic (HAc) and propionic (HPr) acids were generated in CS_HS and S, respectively. High-throughput sequencing analysis illustrated that acid (especially HAc)-producing characteristic genera (Bacteroides, Proteiniclasticum and Fluviicola) and HPr-producing characteristic genera (Mangroviflexus and Paludibacter) were detected by CS_HS and S conditioning, respectively.ConclusionsCorn stover conditioning greatly upgraded the WAS acidification performance, especially for the CS_H adjustment form, and the VFAs yield gained was considerably larger than that previously reported. CS adjustment forms played an important role in structuring the innate microbial community in WAS. Canonical correlation analysis illustrated that characteristic genera, with better hydrolysis and acidification abilities, could be enriched by the feedstocks with certain content of cellulose, hemicellulose or their saccharification hydrolysates. Moreover, ecological estimation revealed that, as far as the entire CS (including S and H) per acre was concerned, the capacity of WAS treatment would reach that produced in a one million mts capacity wastewater treatment plants (WWTPs) per day. These findings may have crucial implications for the operation of WWTPs.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-016-0659-y) contains supplementary material, which is available to authorized users.

Effect of nonylphenol on volatile fatty acids accumulation during anaerobic fermentation of waste activated sludge

Most of the reported studies on anaerobic fermentation of sludge focused on the influences of operating conditions, pretreatment methods, and its characteristics, and little attention was paid to those of persistent organic pollutants (POPs) which widespreadly appeared in sludge. In this study, the effect of nonylphenol, a typical POPs in waste activated sludge (WAS), on anaerobic fermentation for volatile fatty acids (VFAs) accumulation was investigated. The concentration of VFAs during WAS anaerobic fermentation was found to be affected positively from 2856 mg COD/L in the control (without NP) to 5620 mg COD/L with NP of 200 mg/kg dry sludge. Mechanism exploration exhibited that the main reason for the enhanced VFAs accumulation in the presence of NP was that more acetic acid was generated during the acidification of WAS, which was increased by almost three times (3790 versus 1310 mg COD/L). In WAS fermentation systems, the abundance of anaerobic functional microorganisms was advantageous to the accumulation of acetic acid. Further investigation by the pure acetogen revealed that both the viability and activity of Proteiniphilum acetatigenes were improved by NP during anaerobic fermentation, resulting in more production of acetic acid and showing good agreement with that in the real WAS fermentation systems.

Hydrolysis and volatile fatty acids accumulation of waste activated sludge enhanced by the combined use of nitrite and alkaline pH.

Volatile fatty acids (VFAs) production from anaerobic digestion of waste activated sludge (WAS) is often limited by the slow hydrolysis and/or poor substrate availability. Increased attention has been given to enhance the hydrolysis and acidification of WAS recently. This study presented an efficient and green strategy based on the combined use of nitrite pretreatment and alkaline pH to stimulate hydrolysis and VFA accumulation from WAS. Results showed that both proteins and polysaccharides increased in the presence of nitrite, indicating the enhancement of sludge solubilization and hydrolysis processes. Mechanism investigations showed that nitrite pretreatment could disintegrate the sludge particle and disperse extracellular polymeric substances (EPS). Then, anaerobic digestion tests demonstrated VFA production increased with nitrite treatment. The maximal VFA accumulation was achieved with 0.1 g N/L nitrite dosage and pH 10.0 at a sludge retention time (SRT) of 7 days, which was much higher VFA production in comparison with the blank, sole nitrite pretreatment, or sole pH 10. The potential analysis suggested that the combined nitrite pretreatment and alkaline pH is capable of enhancing WAS digestion with a great benefit for biological nutrient removal (BNR).

Enhancing the quantity and quality of short-chain fatty acids production from waste activated sludge using CaO2 as an additive

The effect of calcium peroxide (CaO2) addition on anaerobic fermentation of waste activated sludge (WAS) was investigated. The lab-scale experiments were conducted at 35 °C with CaO2 doses ranging from 0.05 to 0.3 g/g VSS. The performances of hydrolysis and acidification of WAS were significantly enhanced by CaO2 addition, whereas the production of methane was inhibited. Maximum total short-chain fatty acids (TSCFA) production (284 mg COD/g VSS) occurred at a CaO2 dose of 0.2 g/g VSS and fermentation time of 7 d, which was 3.9 times higher than the control tests. Further, CaO2 addition led to the conversion of other SCFAs to acetic acid. Acetic acid comprised 60.2% of TSCFA with the addition of 0.2 g CaO2/g VSS compared with 45.1% in the control tests. The mechanism of improved SCFAs generation was analyzed from the view of both chemical and biological effects. Chemical effect facilitated the disintegration of WAS, and improved the activities of both hydrolytic enzymes and acid-forming enzymes. Illumina MiSeq sequencing analysis revealed that bacteria within phylum Firmicutes increased significantly due to CaO2 addition, which played an important role in the hydrolysis and acidification of WAS. In addition, CaO2 oxidized most refractory organic contaminants, which were difficult to biodegrade under the ordinary anaerobic condition. Hydroxyl radicals were the most abundant reactive oxygen species released by CaO2, which played a key role in the removal of refractory organic compounds. We developed a promising technology to produce a valuable carbon source from WAS.

Evaluating properties of protein in waste activated sludge for volatile fatty acid production: effect of pH

Protein is the primary organic constituent of waste activated sludge (WAS) and its properties are related to pH. In order to investigate the effect of the sludge protein on the production of volatile fatty acid (VFA), the hydrolysate from the pretreated WAS with thermo-alkaline method was used as the substrate to conduct batch anaerobic acidification over a wide pH range 3–12. Results showed that the percentage of consumed soluble protein in consumed volatile organic substance was more than 75 % at various pHs. The soluble protein was responsible for VFA production. When the pH value decreased from 12.0 to 3.0, some soluble proteins were precipitated and a decrease of 42.2 % of soluble protein was observed. There was no significant decrease in the amount of precipitated protein after anaerobic acidification. Differential scanning calorimeter analysis showed that the enthalpy change (∆H over 1.470 J g−1) of precipitated protein was higher than that of soluble protein except at pH 12. A high level of aggregation between/among precipitated protein particles might be the main reason that precipitated protein displayed a poor bio-degradability and disfavored the accumulation of VFAs during 240 h of anaerobic acidification. WAS acidification for VFA production could be an attractive waste treatment practice in which both pollution control and VFA recovery can be achieved.

Effect of extracellular polymeric substances disintegration by ultrasonic pretreatment on waste activated sludge acidification

In this study, batch tests were conducted to investigate the effect of extracellular polymeric substances (EPS) disintegration on anaerobic fermentation of waste activated sludge (WAS). Ultrasonic pretreatment was used to disintegrate EPS and accelerate WAS hydrolysis. The experimental results showed that more organic substances were released from sludge with increase of ultrasonic density, and the EPS were completely disintegrated at ultrasonic parameters beyond 2.0 W/mL and 15 min. The optimal ultrasonic density for short chain fatty acids (SCFAs) accumulation was 2.0 W/mL, and the SCFAs concentration reached 3166 mg/L after 5 days of anaerobic fermentation. The SCFAs mainly consisted of acetic and propionic acids, accounting for 88% of total SCFAs. Denaturing gradient gel electrophoresis (DGGE) analysis indicated that ultrasonic density significantly altered the bacterial communities by affecting the EPS disintegration degree. Firmicutes, Proteobacteria and Bacteroidetes were main species, contributing to proteins (PN) and polysaccharides (PS) degradation and SCFAs production.

Integrating waste activated sludge (WAS) acidification with denitrification by adding nitrite (NO2−)

Adding nitrite (NO2−) to waste activated sludge (WAS) fermentation systems is an efficient approach to integrate fermentation with denitrification, and utilize volatile fatty acids (VFAs) to achieve a high nitrogen removal rate even at low C/N ratios. In this study, the effect of nitrite on the integrated WAS fermentation and denitrification (termed as WASFD) was investigated under acidic and alkaline conditions. The results indicated that adding nitrite achieved a most reduction of nitrite to N2 and improved the acidification of WAS with high VFAs production. Under acidic condition (pH = 5), the maximum VFAs produced with nitrite addition was 3.3 times that without nitrite addition. Higher concentration of free nitrous acid (FNA) at the pH of 5 increased WAS particulates, improved the hydrolysis of organic substrates, and finally promoted VFAs yields. Under alkaline condition (pH = 9), adding nitrite only increased the VFAs production by 1.5 times, indicating that acidic condition was preferable for acidification than alkaline condition.

Hydrolysis and acidification of waste-activated sludge in the presence of biosurfactant rhamnolipid: effect of pH

In this investigation, the effect of pH (4.0-11.0) on waste-activated sludge (WAS) hydrolysis and acidification in the presence of a biosurfactant rhamnolipid (RL) were studied. The results showed that the hydrolysis and acidification of WAS in the presence of RL at alkaline pH values were more efficient than that at acidic and near-neutral pH values. After 6h of hydrolysis, the soluble protein and carbohydrate were 1,654.7 and 675.9mg/L (pH11.0), and 825.6 and 376.0mg/L (pH7.0), whereas the values were only 315.0 and 84.0mg/L at pH4.0 and 164.1 and 32.0mg/L for the blank, respectively. After 2 or 3days of fermentation, the accumulated short-chain fatty acids (SCFAs) reached the highest and then decreased with a further increase in time at all investigated pH values. The analysis of SCFA compositions showed that acetic, propionic, and iso-valeric acids were the three main products at any pH value. A higher pH contributed to a greater proportion of acetic acid and a lesser proportion of iso-valeric acid; a lower pH resulted in a greater proportion of iso-valeric and lesser proportion of acetic acid in the initial fermentation. The proportions of acetic acid for the system with biosurfactant RL addition were 16.65, 36.33, and 62.94%, respectively, at pH4.0, 7.0, and 11.0 after 1day. Correspondingly, the proportions were 40.34, 12.60, and 11.01% for iso-valeric acid.

Acidification of Waste Activated Sludge During Thermophilic Anaerobic Digestion

Abstract The effects of pH value, temperature, adding ratio of waste activated sludge (WAS) and hydraulic retention time (HRT) on WAS acidification in thermophilic anaerobic digestion system were investigated by batch experiments. The results indicated that the WAS acidification at pH 9.0 was the greatest (VFAs content was about 148 mg/L), followed by the pH uncontrolled (105 mg/L) and the neutral pH (87 mg/L). Compared to the conditions of pH uncontrolled, the VFAs accumulation was retarded at acidic (pH 3.0 and pH 5.0), neutral (pH 7.0) or strong alkaline pH values (pH 11.0). Optimal temperature of thermophilic anaerobic digestion on VFAs accumulation was shown to be 60°C (VFAs content was 148 mg/L), better than lower temperature 50°C (79 mg/L) or higher temperature 70°C (75 mg/L). Each adding ratio has its own optimal HRT to producing maximum VFAs. The VFAs accumulation was attained peak value at dosing 12.5% and HRT 8d (about 184 mg/L). In conclusion, in order to maximize the accumulation of VFAs for providing more carbon, the optimal operating conditions for WAS thermophilic anaerobic digestion observed in this study was pH 9.0, 60°C, adding ratio 12.5% and HRT 8d, revealing that the performance of WAS acidification was sensitive to operating conditions.

Combined effect of sodium dodecyl sulfate and enzyme on waste activated sludge hydrolysis and acidification

The combined effect of sodium dodecyl sulfate (SDS) and enzyme system on hydrolysis and acidification of waste activated sludge (WAS) was investigated. The results showed that the combined system was more effective in the promotion of sludge hydrolysis than sole SDS and sole enzyme, and the SDS+mixed-enzymes (ME) system had better hydrolysis performance than SDS+single enzyme system. Compared with SDS+protease and SDS+amylase systems, the soluble protein concentration in SDS+ME system increased respectively by 20.0% and 44.4%, and the soluble carbohydrate concentration increased by 78.3% and 37.0%, respectively. During the WAS acidification stage, the SDS, ME and SDS+ME system could make the maximum short-chain fatty acids (SCFAs) concentration increased by 1.82 (6th day), 2.04 (5th day), 2.32 (7th day) times, respectively. The composition analysis of SCFAs produced in SDS+ME system indicated that acetic acid was the most prevalent product and propionic acid was the second one.

Enhancement of solubilization and acidification of waste activated sludge by pretreatment

The effects of various pretreatment methods, including thermo-acid, thermo-alkaline, ultrasonic-alkaline and ultrasonic-acid, on solubilization and subsequent acidification efficiency of waste activated sludge (WAS) were investigated. The results showed that both thermo-alkaline and ultrasonic-alkaline significantly improved the solubilization of WAS at a high concentration (7.4% of total solids). Solubilization of volatile solids (VS) and crude protein was 60.2-61.6% and 66.8-67.5%, respectively. Thermo-alkaline or ultrasonic-alkaline pretreatment also enhanced the efficiency of subsequent WAS acidification. Experimental results showed that they had similar effects on the acidification of WAS. The conversion ratio from VS to total volatile fatty acids (TVFAs) was 0.230 in the case of the ultrasonic-alkaline pretreatment. Through this pretreatment, the level of TVFAs was 68% more than the untreated WAS after 252 h of fermentation. The mechanism of TVFAs production under different pretreatment methods was further investigated and the results indicated that soluble protein was the main substrate for TVFAs production from the WAS after the thermo-alkaline or ultrasonic-alkaline pretreatment.