Phosphates
Phosphorus is essential to the growth of biological organisms, including both their metabolic and photosynthetic processes. Phosphorus occurs naturally in bodies of water mainly in the form of phosphate (i.e., a compound of phosphorus and oxygen). Addition of phosphates through the activities of humans can accelerate the eutrophication process of nutrient enrichment that results in accelerated ecological aging of lakes and streams. Phosphorus, especially in inland waters, is often the nutrient that limits growth of aquatic plants. Thus when it is added to a body of water, it may result in increased plant growth that gradually fills in the lake. Critical levels of phosphorus in water, above which eutrophication is likely to be triggered, are approximately 0.03 mg/l of dissolved phosphorus and 0.1 mg/l of total phosphorus. The discharge of raw or treated wastewater, agricultural drainage, or certain industrial wastes that contain phosphates to a surface water body may result in a highly eutrophic state, in which the growth of photosynthetic aquatic micro- and macroorganisms are stimulated to nuisance levels. Aquatic plants and mats of algal scum may cover the surface of the water. As these algal mats and aquatic plants die, they sink to the bottom, where their decomposition by microorganisms uses most of the oxygen dissolved in the water. The decrease in oxygen severely inhibits the growth of many aquatic organisms, especially more desirable fish (e.g., recreational catch fish such as trout) and in extreme cases may lead to massive fish kills. Excessive input of phosphorus can change clear, oxygen-rich, good-tasting water into cloudy, oxygen-poor, foul smelling, and possibly toxic water. Therefore, control of the amount of phosphates entering surface waters from domestic and industrial waste discharges, natural runoff, and erosion may be required to prevent eutrophication.
Rocks may contain phosphates; those with calcium phosphate upon heat treatment with sulfuric acid serve as a source of fertilizers. Phosphates, in addition to those found in fertilizers, are also present in such consumer products as detergent, baking powders, toothpastes, cured meats, evaporated milk, soft drinks, processed cheeses, pharmaceuticals, and water softeners. Phosphates are classified as orthophosphates (PO -3 4 , HPO -2, H 4 2PO4 , and H3PO4); condensed phosphates, or polyphosphates, which are molecules with two or more phosphorus atoms, oxygen atoms and in some cases, hydrogen atoms, combined in a complex molecule; and organically-bound phosphates.
Orthophosphates in an aqueous solution can be used for biological metabolism without further breakdown. Orthophosphates applied to agricultural or residential cultivated land as fertilizers may be carried into surface waters with storm runoff and melting snow.
Polyphosphates can be added to water when it is used for laundering or cleaning, for polyphosphates are present as builders of some commercial cleaning preparations for the public health sector. Massive algal blooms in lakes and floating foam on rivers aroused alarm in the United States in the 1970s. After it was shown that phosphates from detergents were a key factor, legislation banning the use of phosphates in home laundry detergents was 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 treatment of boiler water. Polyphosphates slowly undergo hydrolysis in aqueous solutions and are converted to the 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 within 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.
The most commonly used metal salts are ferric chloride and aluminum sulfate (alum), which combine with phosphate to form the precipitates aluminum phosphate and iron phosphate, with one mole of iron or aluminum precipitating one mole of phosphate. However, because of competing reactions and the effects of alkalinity, pH, trace elements, and ligands found in wastewater, appropriate dosages are established on the basis of bench-scale or full-scale testing. Less commonly used metal salts are ferrous sulfate and ferrous chloride, which are available as by-products of steel production operations. Because polyphosphates and organic phosphorus are less easily removed than orthophosphates, adding metal salts after secondary treatment, where organic phosphorus and polyphosphates are transformed into orthophosphates, usually results in the best control.
Lime (CaOH)2 is used less frequently because of the larger amounts of sludge produced as compared with the use of metal salts. Also, operating and maintenance problems are associated with the handling, storage, and feeding of lime. As lime is added to water, it reacts with natural bicarbonate alkalinity to precipitate CaCO3. As the pH of the wastewater increases beyond about 10, excess calcium ions react with phosphates to precipitate hydroxylapatite, Ca10(PO4)6(OH)2. The amount of lime required will be independent of the amount of phosphate present and will depend primarily on the alkalinity of the wastewater and the degree of phosphate removal required. The pH of the wastewater must be adjusted by recarbonation with carbon dioxide (CO2) before subsequent treatment or disposal. Lime can be added to primary sedimentation tanks or following secondary treatment.
Biological phosphate removal is accomplished by sequencing and producing appropriate environmental conditions in a reactor system in which microorganisms absorb and store phosphorus. The Acinetobacter bacteria are one of the primary microorganisms responsible for phosphorus uptake. The sludge containing the microorganisms with the excess phosphorus can either be wasted or removed and treated in a side stream to release the excess phosphate, which can then be treated with chemical precipitation processes. Wasted sludge, if it has a relatively high phosphorus content (3-5%), may have fertilizer value.
In natural treatment systems for wastewater, which include managed land-farms, landscape irrigation sites, groundwater recharge sites, and constructed wetlands, phosphates are removed by sorption with clay minerals in the soil matrix and by chemical precipitation. Sorbed phosphates are held tightly and are generally resistant to leaching until the soil sorptive capacity for phosphates becomes saturated. Chemical precipitation with calcium (in soils with neutral or alkaline pH) or with iron or aluminum (at acid pH) occurs at a slower rate than sorption, but is also an important removal mechanism. The degree of phosphorus removal in a natural treatment system depends on the degree of wastewater contact with the soil matrix.
Transport of phosphates into surface waters from nonpoint sources is controlled by practices that minimize soil erosion and runoff water. Phosphate losses from a watershed can be increased by a number of human activities, including timber harvest, intensive livestock grazing, soil tillage, and soil application of animal manures and phosphate-containing fertilizers. Since most soils, except for very sandy soils or soils with high levels of organic matter, retain phosphates through sorption and precipitation, techniques that prevent soil transport will also prevent transport of phosphates. Control of runoff water is required to prevent dissolved phosphates from entering rivers and lakes.
Resources
Books
Metcalf & Eddy. Wastewater Engineering: Treatment, Disposal, and Reuse. 3rd ed. New York: McGraw-Hill, 1991.
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