Demand characteristics of alkalinity for anaerobic sequencing batch reactor

A considerable amounts of alkalinity added into the anaerobic reactor is a major cost I and related directly to the economic feasibility in the anaerobic treatment processes for high-strength carbohydrate wastewater. In order to minimize this cost, the anaerobic reactor is expected to operate stably under lower pH and lower alkalinity. The objectives of this paper were to investigate the demand characteristics of alkalinity for anaerobic sequencing batch reactor (ASBR) to treat high-strength. carbohydrate wastewater, as well as the determination of the minimum alkalinity and pH compatible with stable long-term operation. The experiment was carried out in a lab-scale ASBR reactor of 6L effective capacity, with granulated anaerobic sludge of 1-2mm diameter. Instantaneous inflow was used in whole experiment. Using glucose as substrate, the substrate concentration was adjusted at a COD value of 6700 mg/L by addition of tap water. The organic loading rate(OLR) Was 10.0gCOD/(g.d), with COD removal efficiencies being higher than 90%. Sodium bicarbonate was added into the influent to control and adjust pH and alkalinity of digesting liquid, and alkalinity levels were characterized by the ratio of COD to alkalinity of the influent, (i.e. COD/Alk). The Dosages of the alkalinity added into the influent were decreased step by step in the experiment. Different ratios of COD/Alk, 1.0:1, 2.0:1, 2.5:1, 3.4:1, 4.2:1 and 4.8: 1, were respectively studied. After more than 2 years of operation, a successful anaerobic treatment operation under lower pH and lower alkalinity was achieved. The results showed significant effects of alkalinity levels on volatile fatty acids(VFA) accumulated concentrations, especially acid-production rate within the first 0.5h of reaction periods. The maximum concentration of VFA accumulated in the digesting liquid increased to 1626mg/L, at the same time the minimum pH value decreased to 6.10 at the COD/Alk ratios of 4.2:1 and 4.8: 1, which indicated that the digestion liquid in the reactor was acidic. Alkalinity levels had influences on CO2%, bicarbonate alkalinity (BAlk) and pH too. The variation of CO2 partial pressure was between 44% and 55% at the 2.0:1 alkalinity level, but the variation was between 41% and 55% at the ratio of 4.8:1. The minimum BAlk in the reactor was merely 423mg/L when the ratio of COD/Alk was 4.8:1, and the ratio of BAlk to (total alkalinity)TAlk was 0.4 correspondingly, simultaneously, the pH values in whole reaction period were almost always below 6.5, which was less than the lowest limit value recommended for the continuous-flow anaerobic processes. During operation of all alkalinity levels, the behavior of process was steady with COD removal efficiency higher than 97% and COD concentration of effluents lower than 198mg/L, and no signs of instability. The results proved feasibility at the lowest alkalinity for ASBR because of its intrinsic characteristics. The minimum alkalinity demand dose expressed by the ratio of COD/Alk was 4.8:1(0.2 1 gCaCO(3)/gCOD).