Modified Anaerobic Digestion Test Produces Reliable Results with Decreased Experimental Complexity

This study focused on evaluating additional volatile solids reduction (VSR) using a modified digestion test based on the recommended procedure by the USEPA (USEPA, 1992). The modified procedure uses fewer volumetric flasks (reactors), provides for continuous mixing, and increases the number of determinations. These modifications allowed the laboratory to: (1) reduce the number of flasks from 15 per sample to 3 per sample, (2) change from every day swirling of the reactors to continuous mixing, and (3) increase the number of solids determinations from only 3 (at 0, 20, and 40 days) to early and more frequent determinations (at 0, 5, 11, 20, and 40 days). The modified procedure required bigger reactors (1 L versus 100 mL) and used 30-mL aliquots of the reactors for solids determinations instead of the entire 50-mL volume.The study results indicate that the average 40-day additional VSR of 17.6 percent using the USEPA digestion test was greater than the average 40-day additional VSR of 14.0 percent using the modified procedure, suggesting that the modified procedure may have underestimated the 40-day additional VSR. The differences observed, however, were within the experimental error (around 20 percent). In addition, early and more frequent suspended solids determinations provided data to estimate the first order anaerobic sludge digestion rate coefficient as 0.125 grams of volatile suspended solids degraded per gram of degradable volatile suspended solids per day at 35.7°C.This study shows that the modified procedure for VSR testing: (1) estimates VSR conservatively, but differences from the recommended procedure are within the experimental error of the test, (2) significantly reduces the number of reactors, (3) provides continuous mixing of the reactor contents, and (4) provides operational flexibility allowing more frequent sampling, at any time.