The most important factor that affects the quality of effluents and overall organic matter removal in biological treatment processes is the presence of soluble microbial products (SMP) that are produced during biological treatment and remain in effluent. High strength wastewater from fermentation industry (FIE) was used as a slowly degradable substrate to investigate SMP formation in aerobic and anaerobic treatment. For SMP determination which forms a major part of residual soluble COD of the wastewater, the initial inert soluble COD (SI) was determined. It was found that SI/So is 0.122 and 0.164 for aerobic and anaerobic processes, respectively. During aerobic treatment of FIE for initialCOD (So) values varying between 1540–6645 mg COD/L, SMP are expected to vary within the range of 12.5–8.6% of So, respectively. In anaerobic treatment of the FIE, for values of 2670–6585 mg COD/L, SMP are expected to vary within the range of 9.8–6.2% of So, respectively. Molecular weight distribution analyses of raw wastewater, aerobic and anaerobic treatment effluent indicate that in both systems the effluent of FIE presents similar molecular weight fractions and are very high compared to raw wastewater.

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