Anaerobic hydrolysis of a municipal wastewater in a pilot-scale digester J.A. Álvarez*, C.A. Zapico** , M. Gómez**, J. Presas** and M. Soto* * Dept. of Química Física e Enxeñeria Química I. University of A Coruña. A Zapateira s/n, 15071 A Coruña. Galiza (Spain) (E-mail: sotoc@udc.es) ** Aquagest S.A. EDAR Silvouta – 15896 Santiago de Compostela. Galiza (Spain) Abstract Raw domestic wastewater from the city of Santiago de Compostela (Northwest Spain) was fed into a pilot-scale hydrolytic up flow sludge bed (HUSB) digester with an active volume of 25.5 m 3 . The total influent chemical oxygen demand (COD) ranged from 360 to 470 mg/l, the influent SS varied from 190 to 370 mg/l, and the temperature was between 17° and 20°C. The organic load rate (OLR) applied increased step by step from 1.2 to 3.9 kgCOD/m 3 .d, while the hydraulic retention time (HRT) decreased from 7.1 h to 2.9 h. A high suspended solids (SS) removal of about 82–85% from the influent was reached, most of which (81 to 88%) was eliminated by hydrolysis, while the rest remained in the purge stream. The total COD removal ranged from 46 to 59%. On the other hand, a high acidification of the COD remaining in the effluent was obtained, so the percent COD in the form of volatile fatty acids (VFA COD ) with respect to total effluent COD was about 43% for the highest HRT applied, and about 27% for the lowest HRT. The soluble to total COD ratio (CODs/CODt) increased from 25–32% for the influent to 71–86% for the effluent. The results obtained confirm the viability and interest of direct anaerobic hydrolytic pre-treatment of domestic wastewater. Keywords Anaerobic digestion; hydrolysis and acidification; urban wastewater Introduction Compliance with the criteria as stipulated in Directive 91/271/CE of the Council of Europe involves the construction of new treatment facilities in some countries, with the need for new economic and efficient treatment methods. During the last decade different anaerobic technologies have been applied to the treatment of low concentrated effluents, such as domestic wastewater and some industrial effluents, providing good treatment efficiencies at low or very low hydraulic retention times. The principal application of anaerobic digestion in urban wastewater treatment consists of the utilisation of single-step methanogenic digesters for organic load removal (Lettinga et al., 1993; Ruiz et al., 1998; Foresti, 2001). Another option is the separation of phases, in which wastewater undergoes a pre- hydrolysis–acidification step before anaerobic digestion. Two-step systems permit the optimisation of each of the individual process steps that are involved in anaerobic digestion. However, the first step should be considered as a wastewater pre-treatment, as the hydrolysed effluent could be followed by an aerobic treatment, such as activated sludge, lagoon or wetland systems, instead of the final methanogenic step. Some advantages of the direct hydrolytic pre-treatment of domestic wastewaters are the following (Wang, 1994; Gonçalves et al., 1994; Ligero, 2001a,b): it serves to remove an elevated percentage of SS, substituting the primary settler at a similar HRT; it stabilises the sludge, totally or partially; and increases the biodegradability of the remaining COD, which favours the subsequent biological elimination of nutrients (N, P). Different variables, such as wastewater characteristics, type of digester, up flow veloci- ty (v), mixing mechanism, HRT and solids retention time (SRT) influence this process. The Water Science and Technology Vol 47 No 12 pp 223–230 © IWA Publishing 2003 223