Acidogenic Fermentation of Organic Residual Solids: Effect of Different Alkaline Sources on pH, Alkalinity, and Fermentation Performance

Acidogenic fermentation (AF) of wastes provides a biological route to volatile fatty acids (VFA) production as an alternative to fossil hydrocarbons. As VFA are acidic in nature, the pH in AF typically drops, affecting the fermentation performance. The effect of different alkaline substances such as urea, NaOH, or CaCO3 on AF of organic residual solids (ORS), a food waste-rich fraction of municipal solid waste (MSW), was studied. Enzymes were used as an additive in simultaneous hydrolysis and fermentation (SHF) experiments. Use of urea (66 mg Urea g−1 TS) in SHF resulted in a VFA yield of 0.14 g VFA g−1 VS after 6 days, 177% higher than the control (no urea addition). pH and ammonia concentrations significantly influenced the bacterial population distribution. In SHF using NaOH/CaCO3, added alkalinity of 11 g CaCO3 L−1 did not influence VFA yields or pH control with NaOH addition at pH 7 or higher. This study demonstrates that, while increasing pH up to 8.5 using different alkaline substances improves VFA production, higher pH is detrimental to SHF due to enzyme inhibition. Finally, the use of urea as an alkaline source must be considered carefully as the acidic pH naturally reached by AF can inhibit urea hydrolysis.