By Bob Dabkowski, Hach Company
Phosphorous promotes eutrophication in surface waters and helps create conditions for algal blooming and oxygen depletion. Some phosphorous removal can occur naturally in a conventional biological wastewater treatment facility, but the result is not reliably compliant with increasingly strict limits on permissible phosphate levels in effluent discharged to receiving streams, ponds and lakes.
Coping with eutrophication is costly: surface waters lose their recreational value, and where the algal bloom is extreme, the water can become toxic to animals and humans. Federal and state regulatory agencies have established limits on the phosphorous concentration in any effluent that reaches surface waters—and, in some cases, ground water reservoirs—typically ranging from 1.0 mg/L to 3.0 mg/L. Where the discharges are made to particularly sensitive waters, 2.0 mg/L or less is the standard.
Phosphorous removal through chemical and physical methods can only take place when the phosphorous is in the orthophosphate form. Most biological wastewater treatment processes naturally convert organic and condensed phosphates to orthophosphate. The orthophosphate can then be precipitated into salt particles through treatment with alum, lime, or iron salts. When one or more of these chemicals is added, the smaller particles flocculate into large masses. The flocculated particles settle in the sedimentation tank, adding to the sludge.