Estimation of the competent biomass concentration for the degradation of synthetic organic compounds in an activated sludge culture receiving a multicomponent feed

Abstract

Biomass samples from two completely mixed activated sludge (CMAS) systems fed an array of biogenic and synthetic organic chemicals (SOCs) were subjected to microbial enumeration using substrate-specific most probable number techniques to quantify the biomass fractions degrading isophorone, phenol, 4-chlorophenol, 4-nitrophenol, m-toluate, and m-xylene, which individually constituted 1.88–4.30% of the total chemical oxygen demand (COD) in the CMAS feed. The competent biomass fractions ranged from 3.1–6.4%, showing that only a small fraction of the total biomass was responsible for degrading the test SOCs. The efficacy of the influent COD fraction as a predictor of the competent biomass fraction was evaluated. Generally, the influent COD fraction underestimated the competent biomass fraction by a factor of about two. The convergence of degradation pathways for different SOCs could contribute to the discrepancy between the influent COD fraction and the competent biomass fraction for each SOC. An accurate estimate of the competent biomass fraction is essential for correctly predicting system response to transient SOC loads. Using the influent COD fraction to approximate the competent biomass fraction, despite the twofold difference, is still more realistic than assuming that the total biomass is responsible for SOC removal.