Enhancement of granulation and start-up in the anaerobic sequencing batch reactor
The phenomenon of granulation in the anaerobic sequencing batch reactor (ASBR) was studied in four laboratory-scale reactors at 35°C. The reactors were fed a synthetic substrate consisting of either sucrose or a mixture of beef extract and glucose as the sole carbon source(s), supplemented with ammonia and other nutrients and trace metals. The reactors were operated at an initial COD load of 1 g/L/day and an HRT of 2 days. The cycle time for the ASBRs was 6 hours, resulting in four sequences per day. The objective was to increase the COD load and decrease the HRT as quickly as possible without inhibiting the system. Automated image analysis was used to characterize the average size of the particles over time, which served as a basis for determining the progression of granulation. Specific methanogenic activity (volume of methane produced per unit biomass per time) was also measured over the course of the experiments to determine the relative activities of the flocculent and granular consortia;To artificially enhance granulation and to aid in start-up, several attachment matrices and coagulants were tested (individually) in the ASBR. Attachment matrices were added only one time at the beginning of each test. These included powdered activated carbon (PAC), granular activated carbon (GAC), silica sand, and garnet. In other experiments, coagulants were added to the ASBR during the final mixing cycle just prior to the settle phase;It was hypothesized that (1) the attachment matrices would serve as a growth surface for the anaerobic bacteria, thereby artificially producing granules, and (2) the coagulants would enhance flocculation of the biomass during the critical start-up period, resulting in granulation more quickly than could otherwise be obtained;PAC, GAC, and cationic polymer addition were found to have a stimulatory effect on granulation and start-up. Cationic polymer addition was the most promising approach. The ASBR receiving the cationic polymer was able to attain high COD loads (6 g/L/day) and granulation within 2 months of operation.