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Water: Best Management Practices

Alum Injection

Minimum Measure: Post-Construction Stormwater Management in New Development and Redevelopment

Subcategory: Other 

Description

The process of adding aluminum sulfate salt, otherwise known as alum, to stormwater is called alum injection.  Alum causes fine particles to coalesce (or flocculate) into larger particles.  The process can also be applied to other pollutants.  Alum injection can help meet downstream pollutant concentration loads by reducing concentrations of fine particles and soluble phosphorus.  Alum treatment systems generally consist of three parts, a flow-weighted dosing system that fits inside a storm sewer manhole, remotely located storage tanks that provide alum to the doser, and a downstream pond that allows the alum, pollutants and sediments to settle out (Kurz, 1998).  When injected into stormwater, alum forms the harmless precipitates aluminum phosphate and aluminum hydroxide.  These precipitates combine with heavy metals and phosphorus and sink into the sediment in a stable, inactive state (WEF, 1992).  The collected mass of alum precipitates, pollutants and sediments is commonly referred to as floc.         

Applicability

Liquid alum injected into storm sewers can reduce the harmful water quality effects of phosphorus, a chemical frequently found in stormwater runoff.  Because of high installation and operation costs, alum injection is most effective when applied to large volumes of water stored in one area, such as combined sewer overflow (CSO) storage areas at wastewater treatment plants.  Alum treatment can also be used as a pretreatment step to reduce turbidity and total suspended solids (TSS) (Kurz, 1998). 

Siting and Design Considerations

To properly apply alum and dispose of floc, alum injection systems need to incorporate several design features.  The design needs to incorporate a doser system, as well as sufficient chemical storage in tanks to minimize the frequency with which they need to be refilled.  Dosage rates, which range from 5 to 10 mg of A1 per liter, are determined on a flow-weighted basis during storm events (Harper, 1996).  Other chemicals, such as lime, may also be added during the process to enhance pollutant settling.  However, this often increases the pH to between 8 and 11. 

It's important to dispose of the floc that settles in downstream basins because it contains high concentrations of dissolved chemicals, as well as viable bacteria and viruses (Kurz, 1998).  In addition to the settling pond, a separate floc collection pump-out facility should be installed to reduce the chance of re-suspension and transport of floc to receiving waterbodies.  The facility's pumps dispose of the floc into a sanitary sewer system, a nearby upland area, or a sludge drying bed.  Pumping into a sanitary sewer system requires a permit, however.  The quantity of sludge produced at a site can be as much as 0.5 percent of the volume of water treated (Gibb et al., 1991). 

Limitations

While alum shows some potential as a stormwater treatment method, it has several limitations, including:

  • Alum injection is an experimental practice, and little is known about its long-term performance.
  • In addition to maintenance, alum injection requires ongoing operation, unlike most other post-construction stormwater treatment practices.
  • While alum injection can reduce pollutant loads, it cannot control flows or protect downstream channels from erosion.
  • Chemicals added during the alum injection process may have negative effects on downstream waters.
  • The precipitates from the alum increase the solids that must be disposed of.
Maintenance Considerations

Operation and maintenance for alum treatment is critical. Some typical items include:

  • Routine inspection and repair of equipment, including the doser and pump-out facility.
  • A trained operator should be on-site to adjust the dosage of alum and other chemicals, and possibly to regulate flows through the basin.
  • Floc stored on-site in drying beds will need to be disposed of regularly.
  • The settling basin must be dredged periodically to dispose of accumulated floc.
Effectiveness

Limited performance data of alum injection is available in Table 1. One study (Harper and Herr, 1996) found high removal rates for TSS and fecal coliform bacteria. This and another study (Carr, 1998) showed mixed results on total phosphorus and ortho-phosphorus.

Table 1. Alum injection removal rates

Study

TSS

TP

Ortho-phosphorus

TN

Fecal Coliform Bacteria

Heavy Metals

Zinc

Ammonia

Harper and Herr, 1996

95-99

85-95

90-95

60-70

99

50-90

-

-

Carr, 1998

-

37

42

52.2

-

-

41

24.5

Cost Considerations

Alum injection is a relatively expensive practice. Construction costs for alum treatment systems range from $135,000 to $400,000, depending on the watershed size and the number of outfall locations treated. Generally, alum treatment is applied to large drainage areas. In one study (Kurz, 1998), an alum treatment system was a successful stormwater retrofit for a 460-acre urbanized watershed in downtown Tampa. Operation and maintenance costs, including routine and chemical inspections, range from $6,500 to $25,000 per year (Harper and Herr, 1996).

References

Carr, D. 1998. An Assessment of an In-Line Injection Facility Used to Treat Stormwater Runoff in Pinellas County, Florida. Southwest Florida Water Management District, Brooksville, FL.

Gibb, A., B. Bennet, and A. Birkbeck. 1991. Urban Runoff Quality and Treatment: A Comprehensive Review. Prepared for the Greater Vancouver Regional District, the Municipality of Surrey, British Columbia, Ministry of Transportation and Highways, and British Columbia Ministry of Advanced Education and Training. Document No. 2-51-246 (242).

Harper, H.H. and J.L. Herr. 1996. Alum Treatment of Stormwater Runoff: The First Ten Years. Environmental Research and Design, Orlando, FL.

Kurz, R. 1998. Removal of Microbial Indicators from Stormwater Using Sand Filtration, Wet Detention, and Alum Treatment Best Management Practices. Southwest Florida Water Management District, Brooksville, FL.

Water Environmental Federation and the American Society of Civil Engineers. 1992. Design and Construction of Urban Stormwater Management Systems. Water Environmental Federation, Alexandria, VA, and American Society of Civil Engineers, Washington, DC.


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