Active Volume Estimation of Anaerobic Sludge Digesters under Different Cleaning Intervals

Anaerobic digestion has been widely used to treat sludge since it reduces the sludge mass and produces a renewable source of energy. Unfortunately, solids tend to accumulate inside the digesters, reducing the active volume available for the digestion process. In this study, the active volumes of three minimally mixed digesters at a particular wastewater treatment plant were determined. The active volume was calculated by conducting a tracer study using lithium chloride (LiCl). The tracer study was conducted in about a 20-day period for the three digesters. The three digesters represent different cleaning intervals, each assumed to have different amounts of accumulation of debris. The tracer study results have shown that the active volume for the three digesters range from 25 to 61 percent. As expected, the recently cleaned digester yielded the highest active volume. Mass of the tracer recovered was computed for all three digesters. Mass recoveries ranged from 34 to 62 percent. The recently cleaned digester yielded the highest mass recovery. The low mixing energy may have caused uneven discharge of tracer concentrations in the overflow based on the mechanical configuration of the digesters, resulting in reduced tracer recovery. The best fit of the mass tracer versus time for the digester that has not been cleaned the longest, digester C, revealed a 99.7 percent mass recovery within 30 days with the bulk of recovery in the first 5 days. The estimated hydraulic retention time (HRT) based on the best-fit curve is 4.4 days. The estimated HRT based on best-fit curves were also computed for digesters A and B. Digester A had not been cleaned for nearly a year and digester B have not been cleaned for approximately 3 years. Active volumes were also recomputed based on the recomputed HRTs yielding 24, 45 and 82 percent for digesters C, B and A, respectively. Thus, the results of the tracer study have shown that the calculated HRT is consistent with cleaning history; the most recently cleaned digester had the greatest calculated HRT. In order to verify the conclusions derived from the study, a follow-up study will be conducted. The next study will include a longer sampling duration, higher tracer dosage, and better tracer distribution in the digesters.