Continuous Volatile Fatty Acid Production from Waste Activated Sludge Hydrolyzed at pH 12

Excess amount of waste activated sludge (WAS) produced in the biological wastewater treatment practice contains high levels of proteins, polysaccharides, and lipids. Efficient hydrolysis of sludge biomass can generate soluble organic carbons (DOC) that assist recovery of valuable resources from waste sludge. Proteins were readily hydrolyzed by alkaline pretreatments. Alkaline pretreatment at different pH’s affects the compositions of hydrolysate, hence leading to distinct volatile fatty acid (VFA) compositions in the subsequent fermentation process.

Previous studies on sludge fermentation for VFA production were mostly conducted in batch mode. In practice continuous process is desired for reduced fixed and operational costs. Additionally, the practical use of produced VFA should be demonstrated together with the hydrolysis and fermentation stages that us it so the VFA is confirmed as a resource than another waste. The purposes of this study are three folds. First, effects of solution pH for maximizing hydrolysis efficiency of a waste activated sludge were revealed. Then the hydrolysates were used in continuous fermentation process for VFA production. The microbial communities involved in the fermentation process were probed using high throughput sequencing technique. Finally, the so-yielded fermentation liquor was applied to demonstration the feasibility to be the substrate for struvite production and DNPR operation.

This study adopted rapid alkaline treatment at pH 12 to hydrolyze 66% of total chemical oxygen demands. Then the hydrolyzed liquor was fermented in a continuous-flow stirred reactor to produce volatile fatty acids (VFAs) at 8-h hydraulic retention time and at 35°C. The maximum VFA productivity reached 365 mg VFAs g^–1 volatile suspended solids in a 45-d operation, with most produced VFAs being acetate and propionate, principally produced by protein degradation. The Bacteroidia, e-proteobacteria and the Clostridia were identified to be the classes correlating with the fermentation processes. The fermented liquor was applied to denitrifying phosphorus removal process as alternative carbon source after excess phosphorus and nitrogen being recycled via struvite precipitation. Fermented liquors from alkaline hydrolysis–acid fermentation on waste activated sludge are a potential renewable resource for applications that need organic carbons.

This study was published in Bioresource Technology in September 2014.