Pretreatment of urban wastewaters in a hydrolytic upflow digester

Domestic wastewater was fed continuously to a laboratory-scale upflow digester operated at short hydraulic retention times (HRT < 4 h). The digester acts as a system for sedimentation and hydrolysis of suspended solids (SS) and for acidification of solubilised substances. Optimum results were obtained at an HRT of 2.3 h. Over 60% SS are retained in the digester and hydrolysed. Average influent SS content is 230 mg/l, whereas effluent SS is 90 mg/l. Effluent SS concentrations shows very stable behaviour, varying little with influent concentration, as with HRT. Retention and hydrolysis of SS causes an increase in volatile fatty acid (VFA) concentration, from about 20 mg/l in the influent to above 100 mg/l in the effluent, also contributing toward soluble fraction acidification. Thus, VFA reached 25% of effluent chemical oxygen demand (COD), while influent VFA COD was below 4%. During the process, a reduction in domestic wastewater COD of between 30 and 40% was observed. WaterSA Vol.27(3) 2001: 399-404