Wastewater stabilization pond sludge: Accumulation, characteristics, and pathogen inactivation

The accumulation and characteristics of sludge in three facultative and one anaerobic primary wastewater stabilization pond (WSP) in central Mexico were studied with particular emphasis on the concentration and inactivation of Ascaris eggs in the sludge layer. Commonly used reagents for enumerating Ascaris eggs were evaluated to ensure that they did not adversely affect the eggs' viability. None of the reagents studied inactivated the eggs except the acid alcohol solution (35% ethanol in 0.1 N H₂SO₄). In the method used for the remainder of the study, inactivation of the eggs during enumeration was avoided by limiting direct exposure to the acid alcohol to 30 min and diluting the concentration by a factor of 100 before incubation. In three of the ponds, stratified sludge cores were taken from several locations including near the inlet, middle, and outlet. The average concentration of viable helminth eggs in the sludge cores ranged from 25 to 48 eggs/g total solids (TS), with a maximum concentration of 257 eggs/g TS measured in recently deposited sludge near the inlet of one of the ponds. As determined from the sludge cores, the average, long-term, first-order rate constant describing the inactivation of Ascaris eggs was approximately 0.001 d−1, although the initial rate measured using dialysis chambers was faster. The first-order rate constants for the inactivation of fecal coliform bacteria and F+ coliphage were determined independently by two methods, sludge cores and a batch test, and found to be approximately 0.1 and 0.01 d−1, respectively. During the first year, at least 25 to 30% of the organic matter in the sludge layer was degraded, after which the rate decreased significantly. Based on the estimated degradation and inactivation rates, a significant improvement in sludge quality could be achieved by taking a pond out of operation for a period of time before removing the sludge. The average rate of sludge accumulation ranged from 0.021 to 0.036 m3/person/yr. The majority of sludge accumulated directly in front of the single inlet in the facultative ponds, whereas in the anaerobic pond with multiple inlets the sludge was distributed evenly throughout the pond.