The following article will guide you about how to remove phosphorous from sewage effluents.
Introduction:
Phosphorus is present in domestic sewage and in agricultural return water. It is often cited as culprit responsible for the stimulation of aquatic plants and for contributing to eutrophication in general.
ADVERTISEMENTS:
Phosphorus in sewage may be present in three form:
(i) Orthophosphate;
(ii) Polyphosphate; and
(iii) Organic phosphorus.
Ortho and polyphosphates are the inorganic forms of phosphorus. About 70 per cent of the total phosphorus present in the domestic sewage is in the form of ortho and polyphosphates, and about 30 per cent is in the form of organic phosphorus. Typically, phosphorus enters the domestic sewage from human body wastes, from food wastes discharged to the sewers, from kitchen grinders, and from the condensed inorganic phosphate compounds used in various household detergents. Commercial washing and cleaning compounds are also a source of phosphates.
In general about 10 per cent of the total phosphorus present in the domestic sewage is insoluble and the same is normally removed by primary settling. All the phosphorus present in the sewage after primary sedimentation is soluble and hence except for the amount taken up for incorporation into cell tissue, the additional removal of phosphorus achieved in conventional biological treatment is minimal.
ADVERTISEMENTS:
None of the forms of phosphorus present in sewage are gaseous at normal temperatures and pressures, so removal of phosphorus may be accomplished by the addition of certain chemicals and formation of an insoluble precipitate that can be removed by gravity settling. The principal chemicals used for this purpose are lime, alum and ferric chloride or ferric sulphate. Polymers have also been used effectively in conjunction with lime and alum.
The chemical reactions between phosphate ions (PO3-4) and the various multivalent metal ions viz., calcium (Ca++), aluminium (Al3+) and iron (Fe3+) ions are presented in the following equations:
Chemicals can be added at a variety of different points in the treatment process as shown in Fig. 18.10. Since polyphosphates and organic phosphorus are less easily removed than orthophosphorus, adding aluminium or iron salts after secondary treatment (where organic phosphorus and polyphosphorus are transformed into orthophosphorus) usually result in the best removal.
Chemical Addition to Primary Sedimentation Facilities:
When aluminium or iron salts are added to untreated sewage, they react with the soluble orthophosphate to produce a precipitate. When lime is used, both the calcium and the hydroxide react with the orthophosphorus to form an insoluble hydroxylapatite.
Organic phosphorus and polyphosphate are removed by more complex reactions and by absorption onto floc particles. The insolubilized phosphorus, as well as considerable quantities of BOD and suspended solids are removed from the system as primary sludge.
Adequate mixing and flocculation are necessary upstream of primary facilities. Additions of polymer may be required to aid in settling. A base is sometimes necessary in low-alkalinity sewage to keep pH in the range of 5 to 7 with mineral addition. The rate of application of mineral salts varies with the characteristics of the sewage and the desired phosphorus, BOD, or suspended solids removals. Hence the exact application rate may be determined by on-site testing.
Both low-lime and high-lime treatment can be used to precipitate a portion of the phosphorus (usually about 65 to 80 percent) at pH values equal to or less than 10. In the trickling filter process, recarbonation is generally required before biological treatment. In the activated sludge process, the carbon dioxide generated during treatment is sufficient to lower the pH without recarbonation.
ADVERTISEMENTS:
The residual phosphorus level of 1.0 mg/l can be readily achieved with the addition of effluent filtration facilities to which chemicals can be added. In the high-lime system, sufficient lime is added to raise the pH to about 11. After precipitation the effluent must be recarbonated before biological treatment.
When lime is used, the principal variables controlling the dosage are the degree of removal required and the alkalinity of the sewage. The operating dosage may be determined by on-site testing.
The additional BOD and suspended solids removals achieved by the addition of these chemicals to primary treatment may solve overloading problems on downstream biological treatment systems, or may allow nitrification.
Phosphorus Removal in Biological Treatment of Sewage:
Phosphorus is required for the growth of micro-organisms and hence it is removed in biological treatment of sewage by means of incorporation into cell tissue. The total amount of phosphorus removed depends on the net amount of solids produced.
It can be estimated by assuming that the phosphorus content of the cell tissue is about one-fifth of the nitrogen content. The actual phosphorus content may vary from about one-seventh to one-third of the nitrogen value, depending on specific environmental conditions.
It has been observed that the degree of phosphorus removal at some activated sludge treatment plants is considerably higher than would be predicted on the basis of the requirements for the growth of micro-organisms.
To account for this observation two different theories have been proposed as indicated below-
The first theory is that the removal of phosphate is brought about by chemical precipitation, as described by Menar and Jenkins.
The required conditions which may lead to the removal of phosphorus are as follows:
(1) Hydrolysis of complex phosphates to orthophosphates;
(2) Decreasing carbon dioxide production as the sewage passes through a plug flow reactor; (3) An increase in pH because less carbon dioxide is being produced and more is being removed by aeration; and
(4) The development of conditions favouring the precipitation of calcium phosphate.
As noted from these conditions, a long plug flow reactor would be required.
The second theory is that the removal of phosphorus is accomplished by biological means. It is believed that under certain ideal conditions, the micro-organisms in the activated-sludge mixed liquor are able to remove an excess amount of phosphorus over that required for their growth. This phenomenon has been termed as luxury uptake. It is, however, not clear whether the excess phosphorus is incorporated (stored) within the cell or adsorbed on the bacterial cells, or a combination of both.
Metal-Salt Addition to Secondary Treatment:
Metal salts can be added to the untreated sewage, in the activated sludge aeration tank, or the final clarifier influent channel. In trickling filter systems the salts are added to the untreated sewage or to the filter effluent. Multipoint additions have also been used. Phosphorus is removed from the liquid phase through a combination of precipitation, adsorption, exchange and agglomeration, and it is wasted with either the primary or secondary sludges or both.
Theoretically the minimum solubility of AlPO4 occurs at pH 6.3, and that of FePO4 occurs at pH 5.3; however, practical applications have yielded good phosphorus removal in the range of pH 5.5 to 7.0, which is compatible with mixed-liquor organisms.
The use of lime or ferrous salts is limited because they produce low phosphorus levels only at high pH values. In low alkalinity sewage either sodium aluminate and alum or ferric plus lime or both can be used to maintain the pH higher than 5.5. Improved settling and lower effluent BOD result from chemical addition, particularly if polymer is also added to the final clarifier. Dosages generally fall in the range of a 1 to 3 metal ion-phosphorus molar ratio.
Chemical Polymer Addition to Secondary Clarifiers:
In certain cases such as trickling filtration and extended aeration activated sludge processes, solids may not flocculate and settle well in the secondary clarifier. This problem may become acute in sewage treatment plants that are overloaded. The addition of aluminium or iron salts will cause the precipitation of metallic hydroxides or phosphates, or both.
Aluminium and iron salts along with certain organic polymers can also be used to destabilize colloidal particles. The resultant destabilized colloids and precipitates will settle readily in the secondary clarifier, thereby reducing the suspended solids in the effluent and effecting phosphorus removal. Dosages of aluminium and iron salts usually fall in the range of a 1 to 3 metal ion-phosphorus molar ratio.
Tertiary Lime Coagulation Filtration:
Lime can be added to the waste stream after biological treatment to reduce the level of phosphorus and suspended solids (see Fig 18.10). In the secondary clarifier sufficient lime is added to raise the pH above 11 to precipitate the soluble phosphorus as basic calcium phosphate (apatite). The excess soluble calcium is removed in the clarifier as a calcium carbonate precipitate by adding carbon dioxide gas to reduce the pH to about 10.
The calcium carbonate precipitate formed in the process acts as a coagulant for suspended-solids removal. To remove the residual levels of suspended solids and phosphorus the secondary clarifier effluent is passed through a multimedia filter.