Improved Short-chain Fatty Acids (SCFAs) Production From Waste Activated Sludge By Surfactant

Short-chain fatty acids (SCFAs), the preferred carbon sources for biological nutrient removal, are the important intermediate products of sludge anaerobic fermentation. Surfactant is widely used in industry and daily lives, and can be easily found in waste-water and waste sludges. In this investigation the effect of surfactant on SCFAs production from waste activated sludge (WAS) was investigated to find out a technique for improving SCFAs biological production from WAS.Experimental results of the effects of different kinds of surfactants on WAS acidification showed that all surfactants investigated could improve SCFAs production from WAS. However, the SCFAs production with anion surfactant was the largest under the same environmental and operational conditions. Thus, the anion surfactant was selected as a model to investigate the optimum conditions for WAS fermentation, the mechanism of SCFAs production enhanced by surfactant and the hydrolysis and acidification dynamics of WAS.At room temperature the total SCFAs production increased significantly in the presence of anion surfactant sodium dodecyl sulfate (SDS) or sodium dodecylbenzene sulfonate (SDBS). The optimum conditions for SCFAs production were SDS dosage of 0.1 g/g or SDBS dosage of 0.02 g/g and fermentation time of 6 days. The concentrations of both NH₄⁺-N and PO₄^3--P in fermentation liquor increased in the presence of SDS or SDBS. During the entire fermentation time pH values kept increasing no matter in the presence or absence of SDS or SDBS, and the same pH variations were observed. In addition, reduction of WAS was observed in the presence of surfactant.At SDBS dosage of 0.02 g/g, the alkaline pH (8.0-11.0) was beneficial to SCFAs production, while the acidic pH (4.0-6.0) showed adverse effects on WAS acidification. The increase of temperature up to 35℃and sludge concentration up to 18000mg/L were both favorable to improve SCFAs production.The mechanism of SCFAs production enhanced by surfactant was studied with SDBS as the model surfactant. It was found that during sludge anaerobic fermentation, the solubilization of sludge particulate organic-carbon and hydrolysis of solubilized substrate (protein and carbohydrate) as well as acidification of hydrolyzed products (amino acid and monosaccharide) were all increased in the presence of SDBS, while the methane formation was decreased, the SCFAs production was therefore remarkably improved.Comparisons of SCFAs production and hydrolytic enzymes activities between autoclaved and unautoclaved sludge showed that the production of SCFAs enhanced by SDBS was caused mainly by biological effects, rather than by chemical ones. The negligible removal of SDBS during the whole fermentation time indicated that most of SCFAs produced during WAS fermentation was not from the biodegradation of SDBS although SDBS enhanced SCFAs production significantly.Investigations on the effects of SDBS on hydrolytic enzymes activities showed that some of the hydrolytic enzymes activities could be promoted in the presence of SDBS. But their activities decreased with the increase of fermentation time no matter with or without SDBS.Studies on the hydrolysis and acidification dynamics of WAS at SDBS dosage of 0.02 g/g were also conducted. Results showed that no matter in the presence or absence of SDBS, the hydrolysis of sludge protein and carbohydrate could be descried by first-order kinetics when temperature was in the range of 15～35℃. In the presence of SDBS, the increased activities of protease andα-glucosidase resulted in the reduction of activation energy of sludge protein and carbohydrate hydrolysis, which made their hydrolysis much easier. Results also showed that acidification process of WAS in the absence of SDBS could be described by first-order kinetics, while acidification in the presence of SDBS followed zero-order kinetics.