Phosphates removal from sewage

T15.3 Note that traditionally phosphates have been removed from sewage waters by precipitation. Recently, however, new methods have been developed, including biological treatment of waters. Try to analyse the chemical precipitation reactions, look at the metals used and issues related to each (including cost and potential side-effects). Any textbook covering water treatment and analysis will have both chemical and engineering sides of the process covered. 

Effluents from sewage plants in Sweden and Switzerland are treated with soluble aluminium or iron salts to precipitate insoluble aluminium or iron phosphates. Alum, Al2(S04)3 I4H2O is much favoured as an additive to precipitate AIPO4. Over 90% of the phosphate can be removed by treatment with FeClj, but this may lead to sludge problems. Up to 99.9% of phosphate can be removed from sewage sludge by absorption on to activated alumina. 

The aluminum chloride catalyst used in the alkylation is finally rejected from the system as an acidic aqueous solution. This by-product AICI3 is usually best used for waste water treatment. Sale or use of this material is often as an alum or copperas substitute in water clarification. It is also used in U.S. municipal sewage treatment plants, where a valuable side effect is phosphate removal. In Japan it is converted to polyaluminum chloride (PAC) and used for water treatment. Other uses for this aqueous aluminum chloride stream exist. 

Recent research into the removal of phosphate from sewage [28.10] indicates that phosphorous concentrations can be reduced to below 2 mg P/1 at pH levels in the range of 8.0 to 9.2, and to below 1 mg P/1 at pH levels above 10.5. 

Phosphate in the form of H2P04 and HPO/ ions is the substance usually removed from secondary sewage effluent to prevent excessive algal growth and eutrophication in receiving waters. Of several possible algal nutrients, why is phosphate chosen Show with a chemical reaction the most common means of removal. 

The applications of fly ash zeolites discussed in this chapter are summarized here in brief. Fly ash zeolites A, 4A, P and 4X are suitable for detergent builder and common substitute to phosphate builders. Other types of zeolites A, Y, P, Na-Pl, 4A, X, KM, Chabazite, Herschelite and Faujasite are effective for removal of heavy metals and ammonia from waste water. Zeolite A and Faujasite are proven cation exchange materials for removal of from sewage. Zeolite Na-P,... 

Sodium alumiaate is an effective precipitant for soluble phosphate ia sewage and is especially useful ia wastewater having low alkaliaity (20,21). Sodium alumiaate hydrolyzes ia water to Al(OH)2 and Al" which precipitate soluble phosphate as aluminum phosphate [7784-30-7], AlPO. Sodium alumiaate has also been described as an effective aid for the removal of fluorides from some iadustrial waste waters (22). Combiaations of sodium alumiaate and other chemicals are being used to improve the detackification of paint particles ia water from spray-painting operations (23). 

A by-product aqueous stream is generated in the first washing step that contains dissolved HCl, alkylate and AICI3. The uses for this byproduct depend upon its purity. It has been used directly in industrial waste water treating and it has been shown to effectively remove phosphate from municipal sewage. In Japan it is converted to polyaluminum chloride, a highly desirable coagulant for waste water. 

---