Chemical Phosphorous Removal
Wastewater can contain substantial inorganic phosphorous resulting from synthetic detergents, industrial processes, and high phosphate raw water. Industrial wastewater may contain a variety of phosphate compounds. (Some industrial waste may not contain sufficient phosphorous to promote the growth of the organisms used in wastewater treatment. See the application summary entitled “Nutrient Balancing” for a discussion of this issue). Polyphosphates gradually hydrolyze in wastewater and revert to orthophosphate form. This reversion is a function of pH, temperature and the presence of bacterial enzymes.
Many wastewater treatment plants are required to remove phosphorous during the treatment process. This is done in order to inhibit the formation of algae and cyanobacteria in surface waters by limiting the amount of this essential nutrient in the treated wastewater.
There are two basic approaches for removal of phosphorous from wastewater. One approach relies on naturally occurring microorganisms that release stored phosphorous under anaerobic conditions and subsequently remove soluble phosphorous under aerobic conditions. (For additional discussion see the application summary entitled “Biological Phosphorous Removal”.) The other approach uses chemical addition of metal salts to the of soluble phosphorous to form an insoluble precipitate which can be removed by sedimentation or filtration (chemical phosphorous removal). Some biological phosphorous removal processes also have supplemental chemical phosphorous removal capability for tertiary treatment.
Chemical Phosphorous Removal
Coagulants such as aluminum sulfate (alum), lime, ferrous sulfate, ferric chloride or organic polymers are typically added to the wastewater in a rapid mix tank, followed by flocculation to form a precipitate with soluble phosphorous.
One strategy is to add coagulants to the raw wastewater as it enters the plant, with sedimentation in the primary clarifier. If this is done, care must be taken to assure that sufficient phosphorous remains after sedimentation so that biological treatment of the wastewater is not inhibited during subsequent stages of treatment.
Another strategy is to add the coagulant in the activated sludge aeration tank, where the agitation from aeration can provide flocculation of the phosphate precipitate. The resulting floc is removed in the secondary clarifier.
Coagulants can also be added in a separate tertiary treatment process, which can include chemical addition and mixing, followed by tertiary sedimentation or filtration.
The most important component of a control strategy for chemical phosphorous removal is the calculation of coagulant dosage. Dosage rates for aluminum salts or for iron salts are based on the molar ratio of available metal ion to phosphorous.
Insufficient coagulant dosages can produce an effluent with excessive turbidity, but excessive coagulant dosage can also produce the same result. Surplus coagulants can also have an adverse effect on disinfection processes, by exerting an oxidation demand. Also, surplus metal salts can coat ultraviolet disinfection tube surfaces. Thus, it is important that the process be well controlled.
Proper control is difficult to achieve using manual techniques such as grab samples and periodic jar tests. There are several reasons for this difficulty:
The monitoring strategy will depend on the chemical addition point and on the control strategy to be employed.
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