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A low volatile fatty acid wastewater from a Barcelona residential area was prefermented in a laboratory-scale primary clarifier operated as a prefermenter -an activated primary tank. Total suspended solids (TSS), oxidation-reduction potential (ORP) and temperature were measured in the prefermenter. Influent and effluent were characterised through chemical oxygen demand (COD), soluble COD, volatile fatty acids (VFA), VFA potential, soluble PO 4-P, NH4-N, pH and alkalinity. Solids retention times (SRT) of 5 d and 10 d were tested. Best results were obtained for the 5 d SRT with the prefermenter covered for better temperature an d ORP control. For these conditions, COD solubilisation was measured as 22 mg COD/ l, 66 mg COD/g influent particulate COD, or 91 mg COD/g influent VSS. VFA-formation was measured as 34 mg VFA-COD/l, 142 mg VFA-COD/g influent VSS, or 77 mg VFA-COD/g influent COD. These values indicate remarkable solubilisation and fermentation in the prefermenter. The VFA/PO4-P ratio was improved from 0.9 to 5.5 mg VFA-COD/mg PO4-P, but did not approach the recommended value for biological P removal (20 mg/mg). VFA production could not reach the influent VFA-potential either (110 mg VFA-COD/ l) and VFA-potential was lower in effluent than in influent. With a 5 d SRT and the prefermenter uncovered, a small VFA formation and no solubilisation were observed. This was interpreted as the VFA being formed from the influent soluble COD. With a 10 d SRT, a very low ORP was measured. Neither solubilisation nor VFA production were detected in the prefermenter. Concurrent acidogenic fermentation and methanogenesis are compatible with these results. P and N solubilisation was low or moderate in the preferment er over all periods, and increased with increasing SRT and TSS, and decreasing ORP, and pH and alkalinity were quite stable, due to the high influent alkalinity and the moderate VFA formation and N solubilisation.

Fermentation of a low VFA wastewater in an activated primary tank