Two-phase anaerobic digestion of three different dairy effluents using a hybrid bioreactor
The South African dairy industry is a major water user and as a result has to reconsider current effluent treatment and disposal methods. The effluents from three dairy factories (cheese, fresh milk and milk powder/butter factories) were analysed and the chemical oxygen demand (COD), pH and effluent volumes were found to be highly variable over short time intervals during the daily production cycles. The pH was found to vary between 2.2 and 11.8 units and the COD values ranged from 800 to 15 000 mg · l-1 over a period of 2 h. The average COD of the effluents emerging from the three factories varied between 1 908 and 5 340 mg · l-1. Significant differences were also found in the composition of the effluents from the three factories. In this study, a mesophilic laboratory-scale hybrid bioreactor was used in conjunction with a pre-acidification step to treat the three dairy factory effluents. It was clear from the data obtained on the cheese, fresh milk and milk powder/butter waste waters that dairy effluents are suitable for treatment by means of the anaerobic digestion process and the use of a hybrid anaerobic bioreactor can be seen as a viable treatment option. The COD values of the three pro-digested dairy waste waters were reduced by between 91 and 97% at organic loading rates of between 0.97 and 2.82 kgCOD · m-3 · d-1 and subsequent methane yields varied from 0.287 to 0.359 m-3 CH4 · kg-1 COD(removed) (73 and 91% of the theoretical maximum yield) during anaerobic digestion. The pH values of all the digester effluents were >7.5 units. The data clearly indicated that anaerobic treatment of the different dairy effluents was successful and that this particular type of bioreactor would be suitable for the anaerobic treatment of dairy effluents. An important consequence of the data from this study is that a two-phase set-up will be required to protect the methanogens in the bioreactor from prohibitively low pH values and high VFA concentrations produced during the acidogenic phase. The two-phase system will allow pH control in the acidogenic phase should it be needed in a full-scale or pilot-scale treatment plant.