Anaerobic Digestion of Domestic Wastewater: The Role of Alkalinity in the Rate and Extent of Digestion

For an anaerobic baffled reactor (ABR) treating domestic wastewater, the major variables affecting the rate and extent of digestion are the organic loading/residence time, the upflow velocity and the pH/alkalinity characteristics of the wastewater. In this study, an ABR was operated at two different flow rates; in Phase A, a higher loading rate of 0.74 kgCOD/m³.d was employed, while in Phase B, the loading rate was reduced to 0.40 kgCOD/m³.d. At the higher organic loading rate, significantly higher solids accumulation rates per kgCOD treated were obtained, and it was estimated that only 30% of the influent COD was converted to CH₄. At the lower loading rate the estimated conversion to methane was 60%.

The most probable cause for the poor COD removal was failure of stable anaerobic microbial consortia to establish under the relatively high selection pressure experienced at the high loading rate, due to washout of anaerobic species. It was concluded that low wastewater alkalinity (240 mgCaCO₃/l) characteristic of the KwaZulu-Natal East Coast region resulted in low compartment pH values and associated inhibition of microbial activity, causing slower digestion of organics, and greater washout, particularly of methanogenic organisms, at higher up-flow velocities. The implication was that critical value for up-flow velocity, above which washout of anaerobic species occurs, depends on the organic loading and the prevailing pH and alkalinity.

This has implications for the stability of any anaerobic system, and the extent of treatment of the effluent, specifically, that low wastewater alkalinity has the potential to significantly affect reactor design in any anaerobic system.