Simulating the Accidental Alum Release at a Wastewater Treatment Plant: A Model Defense

An accidental release of 18.43 m3 of liquid alum from a storage tank containment to the third of three parallel activated trains occurred over a period of eight hours. Investigating environmental officers believed the elevated effluent aluminum (Al) level discharged to the river had potential to cause water quality impairment. Legal action by the provincial regulatory authority was considered probable, despite the municipality’s insistence most of the Al was retained in-plant. To bolster its case, the municipality retained Hydromantis to develop a dynamic process model of the WWTP, incorporating Al chemistry in wastewater treatment, for quantifying the mass of Al discharged to the river during the accidental release. Modeling indicated that the maximum concentrations of soluble and total Al in the effluent were 0.68 mg /L and 0.9 mg/L, respectively. The masses of soluble and total Al discharged during the release were estimated to be 36 kg and 52 kg, respectively, or 3.3% and 4.9% respectively of the total mass of 1060 kg Al released to the Section 3 aeration tank. A simplified river transport model indicated that the effluent plume would move past a point in the river within a 14 hour span. Based on historical surface water data, the maximum Al concentration of 89 μg/L in the river due to the effluent plume might be expected to occur naturally three times per month.