Anaerobic digestion of total raw olive-oil wastewater in a two-stage pilot-plant (up-flow and fixed-bed bioreactors)

Abstract

A study was undertaken to evaluate the anaerobic digestion of total raw olive-oil wastewater in a two-stage pilot-plant with reactors connected in series. Two different types of anaerobic digesters were used, an up-flow type and a fixed-bed type. The main pupose of the study was to evaluate the performance of an up-flow-type reactor working in series with a fixed-bed-type reactor, which was used as a complementary treatment. The pilot-plant system operated in the mesophilic range (35 ± 1°C) during approximately 390 days, and with organic loading levels that ranged between 2.8 and 12.7 g COD/l.day. Concentrated aqueous ammonia was added to the total raw wastewater to adjust the C/N ratio to the optimum value of 20/1, and this also achieved stabilisation of the pH values in the digesters within a range about neutrality. In a series of seven consecutive experiments, for the first stage (up-flow digester) optimum values of specific biogas production rate stabilised at a value of 2.1 litres/litre digester.day with a very satisfactory COD reduction of 83% (with a volumetric load of 11 g COD/l.day). For the second stage (fixed-bed digester), the biogas production rate stabilised at a value of 0.22 litres/litre digester.day with a COD reduction of 8% (with a volumetric load of 0.19 g COD/l.d). According to the results reported in the literature for total raw wastewater, the performance of the applied system is amongst the best for biogas production, COD reduction and loading rate reported so far, especially for the up-flow digester. Phenols were greatly reduced during the anaerobic digestion process in both digesters, with a concentration reduction which reached 75% in the up-flow digester; with the use of the second stage (fixed-bed reactor) a further reduction of 45% was obtained. With the above encouraging results we may suggest the employment of the up-flow type digester as an economical and effective treatment for significantly reducing the organic load of total raw wastewater. More satisfactory results might be expected from the use of a fixed-bed-type digester connected in series with a previous one, as a second treatment stage.