Treatment of settled piggery waste by a down-flow anaerobic fixed bed reactor

Abstract

A study of the effect of organic volumetric loading rate (B V ) on the performance of a down-flow anaerobic fixed bed reactor (DFAFBR) treating settled piggery waste was carried out at a range of between 1.1 and 6.8 g COD dm -3 d -1 . The reactor operated at good removal efficiencies and stability under the operational conditions studied. Logarithmic empirical equations described adequately the removal efficiency for different parameters studied (COD, SCOD, BOD, TS, VS, TSS, VSS and phosphorous). Although process stability was affected by the increase of B V , process failure was not observed. A logarithmic relationship was found to describe the influence of B V on the TVFA/alkalinity ratio (p). A linear correlation was found between the effluent substrate concentration and the values of p and between p and the CO 2 /CH 4 ratio in the biogas. The effect of the hydraulic volumetric loading rate (H v ) on the flow pattern of the reactor was evaluated. Dispersion number (D n ) was in the range of 0.17-0.37 for the maximum and minimum values of H V studied, respectively. The ratio between the real and theoretical HRT increased as the H V decreased. These results demonstrate that axial dispersion increased as the H V and the Reynolds number decreased. Due to the hydraulic behaviour of the reactor, the kinetic model developed by Lawrence and McCarty was used for describing the experimental results obtained. Maximum specific substrate removal rate (K), specific organic loading rate constant (K L ), microbial decay coefficient (K d ), microbial yield coefficient (Y), maximum microbial growth rate (U M ) and saturation constant (K S ) were found to be: 3.1 (g COD g VSS -1 d -1 ), 3.0 (g COD g VSS -1 d -1 ), 0.062 (d -1 ), 0.15 (g VSS g COD removed -1 ), 0.39 (d -1 ) and 2.6 (g SCOD dm -3 ), respectively.