Effect of sulfate concentration and scraping on aerobic fixed biofilm reactors

The influence of the sulfate concentration and biofilm scraping on the treatment performance and biofilm structure of aerobic fixed film reactors was studied in rotating tubular reactors (RTRs). The RTRs operated at 22(±3)°C treating diluted milk powder (0.9 g COD/1; COD/SO2−4 ratio of 20) at an organic loading rate of 30 g COD per m2 carrier material per day. The reference RTR gave the following removal efficiencies: CODtot, 76%; CODsol, 91%; Ntot---N, 51%; Norg---N, 70%; PO3−4---P, 45%. COD removal was linear as a function of the reactor length (2 mg COD/cm reactor). The biomass yield coefficient amounted to 0.44(±0.13) g DM/g COD removed. High sulfate concentrations appreciably reduced the yield to 0.28(±0.06) g DM/g CODrem but did not significantly influence the COD and nutrient removal capacity, despite the longer hydraulic retention time (12′07″) compared to the reference RTR (6′25″). Daily removal of about 15% of the biofilm by biomass scraping did not significantly affect the reactor performance. Biofilm samples growing in the reference and the sulfate amended reactor showed comparable specific oxygen uptake rates (15 mg O2/g VS • h). In the reactor treating sulfate-rich wastewater, large populations (6.3–7.5 log CFU/g VS) of lactate, acetate, propionate and butyrate oxidizing sulfate reducing bacteria (SRB) were present. Their number as well as their sulfate reducing activity (13.5 ± 9.5 mg SO2−4---S/g VS • h) was higher compared to the reference RTR. The addition of molybdate (30 mM) to the influent showed that SRB could account for 50% and 10% of the COD removal in the sulfate amended and reference RTR, respectively. It is concluded that wastewaters with a low COD/SO2−4 ratio are particularly suited for aerobic fixed film processes because of the low waste biomass production.