Effects of alkali types on waste activated sludge (WAS) fermentation and microbial communities.

The effects of two alkali agents, NaOH and Ca(OH)2, on enhancing waste activated sludge (WAS) fermentation and short chain fatty acids (SCFAs) accumulation were studied in semi-continuous stirred tank reactors (semi-CSTR) at different sludge retention time (SRT) (2-10 d). The optimum SRT for SCFAs accumulation of NaOH and Ca(OH)2 adding system was 8 d and 10 d, respectively. Results showed that the average organics yields including soluble chemical oxygen demand (SCOD), protein, and carbohydrate in the NaOH system were as almost twice as that in the Ca(OH)2 system. For Ca(OH)2 system, sludge hydrolysis and protein acidification efficiencies were negatively affected by Ca2+ precipitation, which was revealed by the decrease of Ca2+ concentration, the rise of zeta potential and better sludge dewaterability in Ca(OH)2 system. In addition, Firmicutes, Proteobacteria and Actinobacteria were the main microbial functional groups in both types of alkali systems. NaOH system obtained higher microbial quantities which led to better acidification. For application, however, Ca(OH)2 was more economically feasible owning to its lower price and better dewaterability of residual sludge.