Waste Activated Sludge Hydrolysis Research Articles

Effect of Initial pH Adjustment on Hydrolysis and Acidification of Sludge by Ultrasonic Pretreatment

Batch tests were carried out to analyze the effect of pH in the range of 7–12 on the hydrolysis and acidification of sludge at room temperature. Ultrasonic pretreatment was conducted to disintegrate waste activated sludge (WAS) and accelerate WAS hydrolysis. The experimental results showed that the sludge with ultrasonic pretreatment was readily degraded in the hydrolysis and acidification process, and more organic substances were released with increase of initial pH. The optimal initial pH for sludge acidification was 11, and the short chain fatty acids (SCFAs) concentration reached about 1751 mg/L after 15 days of hydrolysis and acidification. Phosphorus release was investigated during hydrolysis and acidification at any pH adjustment. The SCFAs mainly consisted of acetic and propionic, accounting for 74.6–84% of total SCFAs. Three microbial species, including Bacteroidetes, γ-proteobacteria, and β-proteobacteria, were involved in different pH environments of the hydrolysis and acidification process, an...

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.

New Sludge Pretreatment Method to Improve Methane Production in Waste Activated Sludge Digestion

During two-phase sludge anaerobic digestion, sludge is usually hydrolyzed and acidified in the first phase, then methane is produced in the second stage. To get more methane from sludge, most studies in literature focused on the increase of sludge hydrolysis. In this paper a different sludge pretreatment method, i.e., pretreating sludge at pH 10 for 8 d is reported, by which both waste activated sludge hydrolysis and acidification were increased, and the methane production was significantly improved. First, the effect of different sludge pretreatment methods on methane yield was compared. The pH 10 pretreated sludge showed the highest accumulative methane yield (398 mL per g of volatile suspended solids), which was 4.4-, 3.5-, 3.1-, and 2.3-fold of the blank (unpretreated), ultrasonic, thermal, and thermal-alkaline pretreated sludge, respectively. Nevertheless, its total time involved in the first (hydrolysis and acidification) and second (methanogenesis) stages was 17 (8 + 9) d, which was almost the same as other pretreatments. Then, the mechanisms for pH 10 pretreatment significantly improving methane yield were investigated. It was found that pretreating sludge at pH 10 caused the greatest sludge hydrolysis, acidification, soluble C:N and C:P ratios, and Fe(3+) concentration with a suitable short-chain fatty acids composition in the first stage, which resulted in the highest microorganism activity (ATP) and methane production in the second phase. Further investigation on the second phase microorganisms with fluorescence in situ hybridization (FISH) and scanning electron microscopy (SEM) indicated that there were much greater active methanogenesis Archaea when methane was produced with the pH 10 pretreated sludge, and the predominant morphology of the microcolonies suggest a shift to Methanosarcina sp. like.

Ultrasonic enhancement of waste activated sludge hydrolysis and volatile fatty acids accumulation at pH 10.0

Volatile fatty acids (VFA), the preferred carbon source for biological nutrients removal, can be produced by waste activated sludge (WAS) anaerobic fermentation. However, because the rate of VFA accumulation is limited by that of WAS hydrolysis and VFA is always consumed by methanogens at acidic or neutral pHs, the ultrasonic pretreatment which can accelerate the rate of WAS hydrolysis, and alkaline adjustment which can inhibit the activities of methanogens, were, therefore, used to improve WAS hydrolysis and VFA accumulation in this study. Experiment results showed that the combination of ultrasonic pretreatment and alkaline adjustment caused significant enhancements of WAS hydrolysis and VFA accumulation. The study of ultrasonic energy density effect revealed that energy density influenced not only the total VFA accumulation but also the percentage of individual VFA. The maximal VFA accumulation (3109.8 mg COD/L) occurred at ultrasonic energy density of 1.0 kW/L and fermentation time of 72 h, which was more than two times that without ultrasonic treatment (1275.0 mg COD/L). The analysis of VFA composition showed that the percentage of acetic acid ranked the first (more than 40%) and those of iso-valeric and propionic acids located at the second and third places, respectively. Thus, the suitable ultrasonic conditions combined with alkaline adjustment for VFA accumulation from WAS were ultrasonic energy density of 1.0 kW/L and fermentation time of 72 h. Also, the key enzymes related to VFA formation exhibited the highest activities at ultrasonic energy density of 1.0 kW/L, which resulted in the greatest VFA production during WAS fermentation at pH 10.0.

Waste activated sludge hydrolysis and short-chain fatty acids accumulation under mesophilic and thermophilic conditions: Effect of pH

The effect of pH (4.0–11.0) on waste activated sludge (WAS) hydrolysis and short-chain fatty acids (SCFAs) accumulation under mesophilic and thermophilic conditions were investigated. The WAS hydrolysis increased markedly in thermophilic fermentation compared to mesophilic fermentation at any pH investigated. The hydrolysis at alkaline pHs (8.0–11.0) was greater than that at acidic pHs, but both of the acidic and alkaline hydrolysis was higher than that pH uncontrolled under either mesophilic or thermophilic conditions. No matter in mesophilic or thermophilic fermentation, the accumulation of SCFAs at alkaline pHs was greater than at acidic or uncontrolled pHs. The optimum SCFAs accumulation was 0.298 g COD/g volatile suspended solids (VSS) with mesophilic fermentation, and 0.368 with thermophilic fermentation, which was observed respectively at pH 9.0 and fermentation time 5 d and pH 8.0 and time 9 d. The maximum SCFAs productions reported in this study were much greater than that in the literature. The analysis of the SCFAs composition showed that acetic acid was the prevalent acid in the accumulated SCFAs at any pH investigated under both temperatures, followed by propionic acid and n-valeric acid. Nevertheless, during the entire mesophilic and thermophilic fermentation the activity of methanogens was inhibited severely at acid or alkaline pHs, and the highest methane concentration was obtained at pH 7.0 in most cases. The studies of carbon mass balance showed that during WAS fermentation the reduction of VSS decreased with the increase of pH, and the thermophilic VSS reduction was greater than the mesophilic one. Further investigation indicated that most of the reduced VSS was converted to soluble protein and carbohydrate and SCFAs in two fermentations systems, while little formed methane and carbon dioxide.

Kinetic analysis of waste activated sludge hydrolysis and short-chain fatty acids production at pH 10

The accumulation of short-chain fatty acids (SCFAs), a preferred carbon source for enhanced biological phosphorus removal microbes, was significantly improved when waste activated sludge (WAS) was fermented at pH 10. The kinetics of WAS hydrolysis and SCFAs production at pH 10 was investigated. It was observed that during WAS anaerobic fermentation the accumulation of SCFAs was limited by the hydrolysis process, and both the hydrolysis of WAS particulate COD and the accumulation of SCFAs followed first-order kinetics. The hydrolysis and SCFAs accumulation rate constants increased with increasing temperature from 10 to 35°C, which could be described by the Arrhenius equation. The kinetic data further indicated that SCFAs production at pH 10 was a biological process. Compared with the experiment of pH uncontrolled (blank test), both the rate constants of WAS hydrolysis and SCFAs accumulation at 20°C were improved significantly when WAS was fermented at pH 10.

Kinetic analysis of waste activated sludge hydrolysis and short-chain fatty acids accumulation under alkaline conditions

Kinetic analysis of waste activated sludge hydrolysis and short-chain fatty acids accumulation under alkaline conditions

Waste activated sludge hydrolysis and short-chain fatty acids accumulation in the presence of SDBS in semi-continuous flow reactors: Effect of solids retention time and temperature

Abstract In the presence of surfactant sodium dodecylbenzene sulfonate (SDBS), the effects of sludge retention time (SRT) and temperature on the hydrolysis and short-chain fatty acids (SCFA) accumulation during waste activated sludge (WAS) anaerobic fermentation were investigated in semi-continuous flow reactors. Results showed that the presence of SDBS remarkably increased WAS hydrolysis and SCFA accumulation. A longer SRT or higher temperature accelerated WAS hydrolysis. The SCFA accumulation was increased as SRT increased from 3 to 12 d, but decreased as SRT further increased to 18 d. The presence of SDBS increased the total SCFA concentration remarkably, but did not change the SCFA composition significantly. The SCFA production was improved with the enhancement of temperature from 10 to 35 °C. During WAS fermentation in the presence of SDBS, the concentration of released NH 4 + -N increased linearly with SRT or temperature, but that of PO 4 3 -P remained relatively stable. Further investigation revealed that the presence of SDBS caused not only more total SCFA accumulation than the blank (1149.8 against 246.9 mgCOD/L), but also more volatile suspended solids reduction (33.8% versus 17.9%).

Effect of solids retention time and temperature on waste activated sludge hydrolysis and short-chain fatty acids accumulation under alkaline conditions in continuous-flow reactors

The effects of solids retention time (SRT) and temperature on waste activated sludge (WAS) hydrolysis and short-chain fatty acids (SCFAs) accumulation were investigated in a series of continuous-flow reactors at pH 10. The experimental results showed that the increase of either SRT or temperature benefited the hydrolysis of WAS and the production of SCFAs. The changes in SRT gave also impact on the percentage of acetic and propionic acids in the fermentative SCFAs, but little influence on that of the slightly long-chain SCFAs, such as n-butyric, iso-butyric, n-valeric and iso-valeric acids. Compared with the control (pH unadjusted) experiment, at SRT of 12 d and temperature of 20 °C the concentration of SCFAs produced at pH 10 increased from 261.2 to 933.5 mg COD/L, and the propionic acid percentage improved from 11.7 to 16.0%. It can be concluded from this investigation that the efficient continuous production of SCFAs at pH 10 is feasible.

Effect of sodium dodecyl sulfate on waste activated sludge hydrolysis and acidification

Abstract The effect of sodium dodecyl sulfate (SDS) on hydrolysis and acidification of waste activated sludge (WAS) was investigated, which has not been documented in the literature. Results showed that SDS improved the solubility of protein and carbohydrate in aqueous phase, and the concentrations of these two substrates increased with the amount of SDS. On the second day of fermentation the enzyme activities of protease and alkaline phosphatase were improved by SDS with its dosage in the range of 0.02–0.3 g/g, but the activities of α-glucosidase and acidic phosphatase decreased at SDS dosage above 0.05 and 0.2 g/g, respectively. The volatile fatty acids (VFAs) production was also enhanced by SDS under room temperature. On the sixth day of fermentation, concentration of total VFAs was 2243.04 mg COD/L at SDS dosage 0.1 g/g, while it was only 191.10 mg COD/L in the blank test. The maximum VFAs production increased with the amount of SDS. However, longer time was needed for WAS treated with higher SDS dosage to reach its own maximum VFAs production. The composition of VFAs was analyzed, and acetic acid was observed the most prevalent product. Further studies showed that methanogenesis was inhibited in the presence of SDS. Almost the same pH variations in the presence and absence of SDS indicated that the improvement of VFAs production and inhibition of methane production in WAS fermentation was more due to the effects of SDS than due to the variations of pH.

Application of oxidation–reduction potential as a controlling parameter in waste activated sludge hydrolysis

In order to destroy the refractory structure of waste activated sludge (WAS) and increase its biodegradability, chemical pretreatment by adding NaOH was used in this study to enhance the hydrolysis of the WAS collected from the Taipei Min-Sheng Sewage Municipal Wastewater Treatment Plant located in northern Taiwan. Experiments were conducted with four total suspended solids (TSS) concentrations (0.5–2.0%) and four NaOH dosages (20–80 meq/l) at 25 °C. It was found that the optimal condition for WAS hydrolysis was TSS of 1% and NaOH dosage of 40 meq/l; 45% of soluble chemical oxygen demand (SCOD) was solubilized in the pretreatment time of 10 h. When the WAS was hydrolyzed by adding NaOH, profiles of oxidation–reduction potential (ORP) values were monitored on-line; also, a model covering the relationship between the change of ORP value and increase in SCOD was developed. This verified that the ORP monitoring technique is not only useful to investigate the solubilization rate but also suitable for the determination of optimal solubilization in WAS treatment. Considering the release of nitrogen by alkaline pretreatment, profiles of total Kjeldahl nitrogen, ammonia-nitrogen and amino acids were investigated.

Studies on Anaerobic Digestion Mechanism: Influence of Pretreatment Temperature on Biodegradation of Waste Activated Sludge

The effect of thermal pretreatment on hydrolysis, acidogenesis and methanogenesis during anaerobic digestion of waste activated sludge (WAS) was investigated. Pretreatment temperature of WAS ranged from 60°C to 120°C and the holding times were 5, 10, 20, 30 and 60 minutes. WAS was fed into the laboratory-scale anaerobic digesters, after thermal pretreatment. The preliminary conversion step (such as death/lysis of cell and hydrolysis of WAS) was promoted through thermal pretreatment, and consequently efficiencies of acidogenic and methanogenic steps were also markedly enhanced. Thermal pretreatment at lower temperature resulted in a significant increase in methane generation and destruction of organic matter. At 60°C, when hydraulic retention time (HRT) was 8 days, an increase in approximately 52.1 % in methane generation (ml. l−1. d−1) and 26.6% in organic matter destruction was achieved (Series-I). In addition, HRT necessary for anaerobic digestion could be reduced by thermal pretreatment.