Nitrate and Phosphate - Page 3

After it was shown that phosphates from detergents were a key factor, legislation banning the use of phosphates in-home laundry detergents were passed in many areas. Phosphate legislation usually includes exemptions for products such as hard-surface cleaner and automatic dishwashing detergents used in the public health sector. Polyphosphates may also be added to water supplies during culinary water treatment and during the treatment of boiler water. Polyphosphates slowly undergo hydrolysis in aqueous solutions and are converted to orthophosphate forms. Organic phosphates are formed primarily by biological processes. They enter sewage water through body wastes and food residues. They may also be formed from orthophosphates in biological treatment processes and by receiving water organisms. Like polyphosphates, they are biologically transformed back to orthophosphates. One means of surface water protection from phosphorus addition in both domestic and industrial wastewaters is the use of phosphorus removal processes in wastewater treatment.

Phosphates are typically present in raw wastewaters at concentrations near 10 mg/l as P. During wastewater treatment, about 10-30% of the phosphates in raw wastewater is utilized during secondary biological treatment for microbial cell synthesis and energy transport. Additional removal is required to achieve low effluent concentration levels from the wastewater treatment process. Effluent limits usually range from 0.1-2 mg/l as P, with many established at 1.0 mg/l. Removal processes for phosphates from wastewaters utilize incorporation into suspended solids and the subsequent removal of those solids. Phosphates can be incorporated into chemical precipitates that are insoluble or of low solubility or into biological solids, (e.g, microorganisms). Chemical precipitation is accomplished by the addition of metal salts or lime, with polymers often used as flocculant aids.

The precipitation of phosphates from wastewater can occur during different phases of the wastewater treatment process. Pre-precipitation, where the chemicals are added to raw wastewater in primary sedimentation facilities, removes the precipitated phosphates with the primary sludge. In co-precipitation, the chemicals are added during secondary treatment to the effluent from the primary sedimentation facilities; to the mixed liquor in the activated-sludge process; or to the effluent from a biological treatment process before secondary sedimentation. They are removed with the waste biological sludge. In post-precipitation, the chemicals are added to the effluent from secondary sedimentation facilities and are removed in separate sedimentation facilities or in effluent filters.