Short-chain Fatty Acids Production From Waste Activated Sludge Ermentation Stimulated By Pretreatments

Waste activated sludge (WAS), as the main by-product of wastewater treatment process, its production yield reached to18million tons a year (80%moisture content) in2008. Meanwhile, its annual production rate was even up to10%, the treatment and disposal cost accounted for20~50%of the whole operating costs of sewage treatment. Hence, it is a pressing problem to reasonably handle and dispose the generated WAS in the biological wastewater treatment process. WAS is carbonaceous in nature and rich in organic carbon, which has the potential to be utilized as important, renewable and utilizable resource rather than be discarded as a waste. Cost-effective microbial conversion from WAS to specific valuable products is an innovative and promising way to gain social and economic benefits. In this study, a method for recovering available bio-carbon in WAS to short chain fatty acids (SCFAs) by anaerobic fermentation and acidification was proposed. Strategies for the improvement of SCFAs conversion and protein degradation by quick and low-cost pretreatments and external carbon sourse conditioning were also conducted, which provided a novel pathway for WAS reutilization.In order to improve the lysis rate of microbe cells in WAS and further enhance the anaerobic digestibility of WAS, the effects of three pretreatments (bi-frequency ultrasonic, ramnolipid and saponin biosurfactants) on granular organics hydrolysis and acidification were studied. The lysis mechanisms of pretreatments were also investigated. Experimental results showed that SCOD concentration increased from363(raw sludge) to10810mgCOD/L after28+40kHz bifrequency ultrasonic pretreatment. It was1.53and1.44folds than28kHz and40kHz single-frequency ultrasonic pretreatments. As a microorganism-derived surfactant, rhamnolipid (RL) can significantly promote the efficiency of the hydrolysis and acidification of the sludge, SCFAs production reaches3840mgCOD/L, which was4.24times of that without any pretreatment at dosage of0.04g RL/g TSS. Pyrosequencing analysis showed that dominant population in different surfactant pretreatment tests was significantly changed. Megasphaera was only observed in RL samples, which was related to the production of acetic (HAc), propionic (HPr), butyric (HBu) and valeric (HVa) by carbohydrate and lactic acid fermentation. Oscillibacter was proved had a direct relationship with D-glucose and partly pentaglucose metabolism and VFAs production. Meanwhile, it has been confirmed that rhamnolipid could in situ synthesis during WAS fermentation (96h, the concentration increased to1312±7mg/L from initial880±92mg/L). Pseudomonas, which was related to the RL production, its distribution proportion was~1%higher than that in the chemical surfactants pretreated WAS. As a plant-derived surfactant, saponins pretreatment effectively stripped LB-EPS floc layer, thereby breaking the floc which located inside the TB-EPS layers. Compared to the un-pretreated test, SCFAs yield significantly improved1.56-fold at the dosage of0.05g saponin/g TSS.According to the issues of carbon-nitrogen out-of-balance and lower protein degradation during WAS fermentation, we focused on the research of the conditioning performance of agricultural residues, such as straws (corn and rice) and spent mushroom substrates (SMS)(Agaricus bisporus and Lentinus edodes), as additional carbon sourse to WAS fermentation. Apparently, the addition of carbohydrate substrate (CS) improved the conversion of carbon source from WAS fermentation, especially protein, and led to a significant increase in organic release. The average concentrations of soluble chemical oxygen demand (SCOD) with the addition of corn straw, rice straw, agaricus bisporus spent mushroom substrate and shitake mushroom substrate were increased3.6-fold,3.8-fold,2.6-fold and2.7-fold, compared with control test (SRT10d), respectively. The increased soluble organics was substance for further acidification and fermentation of WAS, which was the one of the main reasons for higher conversion rate of carbon source. The significant increasing concentration of NH4+-N indicated that co-digestion enhanced the protein conversion. Compared with individual WAS digestion, the concentrations of NH4+-N were increased3.7-fold and1.8-fold with addition of straw and mushroom substrate (SRT8d). FTIR spectra analysis showed a significant increasing in amide peaks at1720,1658,1550and1230cm-1, respectively. Moreover, the degradation of total suspended solids (TSS) and volatile suspended solids (VSS) was enhanced with CS feedstock, and straw feedstock was remarkably better than mushroom substrate feedstock.WAS fermentation liquid enriched VFAs with different pretreatments was used as substrates for hydrogen production in microbial electrolysis cell (MEC) to achieve efficient utilization. The effects of substrates concentration by bi-frequency ultrasonic pretreatment on hydrogen production in MEC were investigated. The experiment tests with twice diluted SFL2obtained the maximal H2yield (1.2mL H2/mg COD), the total H2recovery rate reached10mmol H2/mg VSS from WAS. Meanwhile, the effects of different surfactant pretreatments on hydrogen production in MEC were also investigated. Considering the utilization of partly substrates (higher than C4VFAs and protein), the degradation rate with SDS addition was higher. Lower than C3VFAs and carbohydrate was utilized efficiently with RL addition. Compared with the results of H2yield and energy efficiency, biological surfactant was better than chemical surfactant on hydrogen production in MEC. The maximal H2yield (0.92mL H2/mg COD) was obtained in biological surfactant addition test, which was0.01and0.75mL H2/mg COD in SDBS and SDS addition tests. The corresponding energy efficiency were2.0%for SDBS,51%for SDS and 64%for RL, respectively.