Physicochemical Removal of Phosphate

There are several physicochemical phosphate removal processes that can be used with conventional secondary wastewater treatment. These processes generally involve using chemicals to facilitate precipitation of phosphate and a primary clarifier (sedimentation tank or basin) to separate phosphate-containing sludge from the treated wastewater; or in some cases, dissolved air flotation is used to remove the phosphorus-containing complexes. The principal chemicals employed in this type of removal are alum, sodium alimunate, ferrous chloride or sulfate, ferric chloride or sulfate, and lime. Ferrous sulfate and ferrous chloride are available as by-products of steel-making operations (calledpickle liquor) and thus have cost advantage in use. Polyelectrolytes have been used effectively along with alum or lime as flocculant aids. The assortment of chemicals used in precipitation of phosphate has a striking similarity to those flocculents employed in the formation of flocs from colloidal dispersion of particulates.

The chemistry of chemical precipitation reactions involved may be reviewed from the descriptive materials in Chapter 3. The choice of using any of the chemicals mentioned above for phosphorus removal is determined by the following factors:

• Incoming flow phosphorus level

• Suspended solids and colloids

• Alkalinity

• Chemical cost including transportation

• Reliability of chemical availability

• Sludge handling facilities and method as well as cost including disposal

• Compatibilities with other treatment processes (primary and secondary treatment processes)

Iron and alum salts can be added at various points in primary and secondary treatment processes. However, in order to achieve maximum removal of phosphorus, alum and iron salts are best added after the secondary treatment in organic phosphorus-containing wastewater streams (where organic phosphorus is transformed as orthophosphorus). Additional nitrogen removal might occur as a result of this sequence of adding alum or iron salts. The various chemical addition points and sequences are dependent on characteristics of wastewater treatment objectives.

The obvious drawback of physicochemical processes of phosphorus removal is the cost associated with the chemicals or coagulants. Additionally, the increase in sludge volume due to addition of chemicals and inability of the physicochemical processes to remove nitrogen compounds are also known disadvantages.

Lime precipitation of phosphate

A lime phosphate precipitation system is similar to a primary settling tank or basin. Wastewater inflow is mixed with lime and flocculated to cause the precipitation of phosphate in the wastewater in the form of insoluble calcium salt. The main reaction required to determine the quantity of sludge produced during the precipitation of phosphorus with lime is expressed as the following (Equation 5.1):

However, the same lime may also cause precipitations of Mg(OH)2 and CaCO3 in addition to orthophosphorus.

The lime sludge from the clarifier is often recycled for reducing the cost of lime in the treatment. The process generally achieves 80-95% removal rate of phosphate in wastewater. Lime can also soften the water; however, the large amount of lime used presents a new challenge: dealing with the increased amount of sludge in the treatment.

Ferrous precipitation of phosphate

The process uses ferrous chloride to convert soluble phosphate in waste-water streams into an insoluble form. The relevant reaction of phosphorus precipitation in the presence of ferrous salts can be written as the following (Equation 5.2):

It is possible Fe(OH)2 may also generate and precipitate when ferrous salts are added.

Polyelectrolyte is also used to aid flocculation of phosphate complex with ferrous chloride in the clarifier. Other salts can also be used in lieu of ferrous chloride; ferric chloride and sodium aluminate are among them. Recall that in Chapter 3, we discussed flocculation using iron salts and polyelectrolytes to reduce colloidal particulates. This process can be viewed as an extension to the conventional primary wastewater treatment processes.

Like all processes involving chemical precipitation reaction, the cost of ferrous precipitation of phosphate depends on the cost of metal salts. If the metal salts can be economically had, the cost of the phosphate removal operation will be reasonable. Ferrous chloride used in wastewater treatment can be obtained from steel plants in which ferrous chloride is a waste of steel production. Application of ferrous chloride to the primary sedimentation facilities for phosphate removal also has an unintended benefit: it reduces the BOD load on secondary treatment facilities because it causes flocculation of and subsequent settling of organic colloids from waste-water. The downside of the use of ferrous chloride as flocculent is that it requires the use of corrosion-resistant equipment that the ferrous or ferric sludge may inflict damage on; the sludge may not be compatible with some dewatering techniques described in Chapter 6.

Alum and ferric precipitation of phosphate

A representative reaction expression for phosphorus removal with addition of alum is as follows (Equation 5.3):

It is likely that Al(OH)3 may also precipitate from the wastewater.

A pertinent reaction for phosphorus removal with addition of ferric salts is expressed as the following (Equation 5.4):

Again, it is almost unavoidable that Fe(OH)3 will form and precipitate out of the wastewater stream if the pH value is alkaline.

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