Treatment of dilute soluble and colloidal wastewater using an anaerobic baffled reactor: influence of hydraulic retention time

The start-up and performance of four anaerobic baffled reactors (ABR, each of 10 l and eight compartments) treating a complex dilute soluble or colloidal wastewater (milk and colloidal rice and dog food — 500 mg COD/l) was studied. All four were started with a hydraulic retention time (HRT) of 80 h at 35°C and this was steadily reduced to 6 h; all the HRTs tested resulted in more than 80% COD removal. In one ABR an HRT as low as 1.3 h was tested over a period of 2 d and even at this high flow rate a COD removal of 40% was obtained. The production of soluble microbial products (SMP) was found to be greater in the reactors being fed colloidal material, possibly due to ‘differences’ in solubility and composition, and these were potentially refractory resulting in slightly lower COD removals than in the milk reactor. The biomass content of the reactors did not appear to influence SMP production, however, as HRTs were reduced, more SMPs were produced. Anaerobic bioassays were used to evaluate the activity of the biomass on the different wastewaters and their biodegradability. It was found that the initial activity on both the soluble and colloidal wastewater were of the same order of magnitude and hence solid hydrolysis was not limiting the rate or extent of degradation. Residence time distribution studies were carried out to monitor the effect of system characteristics such as HRT, solids content and feed composition on the hydrodynamic flow characteristics and dead space in each ABR. Little difference in mixing or dead space was observed under the different conditions tested. The average dead space measured was 20–37% and the flow pattern within the reactors showed an intermediate behaviour between plug–flow and ideally mixed.