Properties of Waste Resulting from Arsenic Removal Processes in Drinking Water Treatment
The arsenic maximum contaminant level (MCL) for drinking water is likely to be lowered sometime in 2001 or 2002. If the MCL is lowered, it is speculated that there will be stricter limits imposed on the disposal and handling of arsenic-containing residuals. The purpose of this study is to determine the properties of drinking water residuals, including the chemical characteristics, the amount of arsenic that leaches in common residual handling and dewatering processes, and the hazardous potential of the residuals.
Residual samples were collected from seven utilities with high arsenic concentrations in their raw water. Included in the study were four plants that coagulate with ferric chloride, two with aluminum hydroxide, and one softening plant. The residuals from each facility were acid digested and chemically characterized using ICP-AES to determine the total arsenic, aluminum and iron present. TCLP and California WET were performed to verify if the residuals were hazardous.
Simulated lagoons were set up and monitored over a six-month period to determine the amount of arsenic that was leached to the liquid portion over time. Toxicity testing was performed on the residuals at 20% and 100% after two months and six months of storage. Bench-scale sand drying beds were used to dewater residuals, and the leachates were analyzed to determine if arsenic leached from residuals.
The residuals were found to contain high levels of arsenic during the chemical characterization. However, all of the facilities passed the current TCLP limit of 5 mg/L. For all residuals, the California WET values were much higher than the TCLP values.
In the lagoon study, redox potentials decreased and the arsenic and iron concentrations in the liquid portions increased. In the sand drying beds, very little arsenic leached; arsenic concentrations in the leachate were less than 10 ppb. Lagoon storage may not be a safe alternative for residuals containing arsenic. However, sand drying does not appear to present any threats.
There were differences between the toxicity tests performed at 100% solids and 20% solids in the lagoon study. Some of the values increased and others decreased. Additionally, toxicity testing conducted after lagoon aging differed from earlier toxicity testing, due to the changing soluble arsenic. These discrepancies suggest that the test results can be affected by the percent solids and age of the residuals, and specific instructions should be given for consistent residuals testing.