Packed-bed biofilm reactor for semi-continuous anaerobic digestion of olive mill wastewater: performances and COD mass balance analysis.

In the present study, the treatability of olive mill wastewater (OMWW) using an anaerobic fixed bed biofilm reactor packed with granular activated carbon (GAC) and inoculated with non-acclimated biomass was performed in a semi-continuous mode under mesophilic conditions. Three organic loading rates (OLR) varied from 0.94 to 2.81 g COD/(L d) were applied. The results of batch adsorption tests on GAC and the experimental data from PBBR-GAC operation were used to set up a COD mass balance in order to investigate the effect of adsorption on the COD removal during the three anaerobic treatment steps. Despite the slight accumulation of volatile fatty acids (VFAs) during the second and the third steps, between 735 and 1135 mg COD/L (as acetic acid), a stable environment for methanogens was maintained for a period of 104 days. During the three steps, degradation levels were up to 80% of COD and 85% of phenolic compounds. An averaged specific biogas production of 1.77 L/d and a methane (CH) concentration of about 60%, corresponding to a CH yield of 0.31 L CH/g COD, were reached at an OLR of 2.81 g COD/(L d). The results show that the COD mass balance was not closed during the first two steps, while in the third step, it could be around 96%. This finding suggests that the adsorption of organic substances on activated carbon occur just during the two first steps, while at 2.81 g COD/(L d) OLR no adsorption is occurring and the introduced COD becomes completely available for CH production.