Current Research and Trends in Alum Treatment of Stormwater Runoff

Alum treatment of runoff has been used as a stormwater retrofit option for the past 20 years. This technology has evolved from the initial demonstration research projects to a viable retrofit option for urban areas. A considerable amount of data has now been collected on the water quality and ecological impacts of alum treatment systems. Alum treatment of stormwater consistently provides removal efficiencies of 85-95% for total phosphorus, >95% for total suspended solids (TSS), 35-70% for total nitrogen, 60-90% for metals, and 90->99% for total and fecal coliform bacteria. Although only positive chemical and ecological impacts have been reported in waterbodies receiving alum floc, current state policies require collection and disposal of the generated floc, and this issue has received considerable attention in recent years. A variety of floc collection and disposal techniques have been evaluated, including settling ponds, in-lake floc traps, underground vaults, and CDS units. Current floc disposal techniques include disposal to sanitary sewer systems and drying ponds. Chemical characterization of floc suggests that the material can be used as fill or applied to soil surfaces to reduce release of phosphorus, metals, and organics under flooded conditions. System reliability has been substantially enhanced in recent years, but commitment to long-term maintenance is a concern with many systems. However, in spite of the additional costs associated with floc disposal and maintenance, alum treatment continues to provide pollutant removal at a substantially lower unit cost ($/kg removed) than traditional treatment systems such as ponds.