21 Phosphorus Removal

Babita Khosla

22.1 Phosphorous in wastewater

The major sources of phosphorus in raw wastewater are derived from human, domestic and industrial wastes, atmospheric deposition and run-off from phosphorus-rich fertilised land. Often a high proportion of phosphorus originates from detergents and cleaning compounds. Phosphorus occurs in natural waters and wastewaters almost solely as phosphates. These phosphates include organic phosphate, particulate polyphosphate and inorganic orthophosphate. Orthophosphates are readily utilized by aquatic organisms. Some organisms may store excess phosphorus in the form of polyphosphates for future use. At the same time, some phosphate is continually lost in the sediments where it is locked up in insoluble precipitates. The usual forms of phosphorous found in aqueous solutions include:

Organic phosphates (with -P-O-C- bonds) are usually the constituents of dissolved and suspended compounds of wastewater. In aqueous environment these compounds are degraded chemically and/or biologically to orthophosphates. The different forms of compounds having organic phosphate are the parts of cell walls, phospholipids, phosphoamides, toxic phosphate esters, phospho organic insecticides.

Particulate poly phosphates (with -P-O-P- bonds), include both chain-bonded polyphosphates and cyclic-bonded metaphosphates. Polyphosphates being basic constituents of detergents and water softeners are the most common inorganic condensed polyphosphates in municipal wastewater. Usually polyphosphates undergo hydrolysis to the orthophosphate forms.

Inorganic orthophosphates PO43- ions: This form of phosphorus is available for biological metabolism without further breakdown.

Municipal wastewaters may contain phosphorous, of which 20-25% is organic and the rest in inorganic. When the input of phosphorus to waters is higher than it can be assimilated by a population of living organisms the problem of excess phosphorus content occurs. The excess content of phosphorus in receiving waters leads to extensive algae growth called Eutrophication. The phenomenon of eutrophication usually decreases the water quality and as a result it may increase significantly the cost of water treatment for surface water. Conventional biological wastewater treatment does not remove phosphorus effectively enough so a large excess of phosphorous is discharged in the final effluent, causing eutrophication in surface waters. There is a continuing effort to control the amount of P compounds that enter surface waters. Therefore, application of advanced wastewater treatment techniques, either chemical or biological is required to reduce phosphorus discharges to eutrophic and potentially eutrophic water bodies.

The general purpose of phosphorus removal is to eliminate the excess phosphorus content from wastewater discharged to receiving waters and then to utilize this excluded phosphorus load in the way which is the most proper for the natural phosphorus cycle in nature. Phosphorus can be chemically or biologically removed. Chemical treatment involves direct precipitation (with the use of coagulant) for the physical removal of P compounds while the biological removal involves bacteria removing phosphorus via their metabolic pathways (USEPA 2009).

22.2 Chemical phosphorus removal: The chemical treatment for phosphorus removal involves the addition of lime or metal salts to react with soluble phosphate to form solid precipitates that are removed by solids separation processes including clarification and filtration. Chemical treatment is the most common method used for phosphorus removal to meet effluent concentrations below 1.0 mg/L. Chemical phosphorus removal, called also “removal by a salt addition” (EPA 1987), can be applied as:

1. Direct precipitation
2. Pre-precipitation
3. Co-precipitation
4. Post – precipitation

1. Direct Precipitation: The chemicals most often employed are compounds of calcium, aluminium, and iron.

Calcium: Lime is the most common calcium salt used for phosphorus precipitation. Calcium oxide (CaO) commonly named as quicklime or burnt lime is added to waste water. The reaction between lime and phosphorus precipitates out hydroxyapatite according to the following reactions:

CaO + H2O → Ca(OH)2

Ca(OH)2 →Ca 2+ + 2OH-

10Ca 2+ + 6PO4 3- + 2OH-  ↔ Cal0(PO4)6(OH)2

This precipitation usually occurs within the pH range of 8 to 11. The retention times in treatment plants were not long enough for the formation of pure crystalline hydroxyapatite or tricalcium phosphate, but the presence of nucleating material  allowed adequate reaction times for precipitation. Seeding in any reaction system could increase phosphorus removal due to the increase in surface area and rate of calcium phosphate crystal growth. Calcium is first precipitated as bicarbonate from carbon dioxide and any excess calcium then precipitates out the phosphate. Hence lime requirements are independent of phosphate concentration, but are directly related to wastewater alkalinity. Some high alkalinity wastewaters needing three times as much lime for effective precipitation.

Aluminium: The major chemical used for precipitation is aluminium sulphate (alum), which undergoes the following reaction:

A12(SO4)3.14H20 + 2PO4 3- → 2A1PO4 + 3SO42- + 14H20

The dosage of aluminium depends upon the concentration of soluble phosphate and colloidal particles. Aluminium phosphate is formed because the precipitation is thermodynamically and kinetically favoured over aluminium hydroxide formation. At low phosphate concentrations (< 10 mg P litre-1) there is competition for the formation of hydroxides preventing the metal phosphate precipitation. Once formed, the precipitate probably has an amorphous composition intermediate between the crystalline aluminium phosphate and hydroxide solids. For effective removal of P from the waste water the alkalinity must be high enough to buffer the aluminium sulphate; with acidic wastewaters, the alkalinity has to be adjusted.

Iron(lII): Ferric ions are mainly responsible for phosphorus removal and the salt most commonly used in wastewater treatment is ferric chloride. lron(II) ions can be used only if they are first oxidised to the iron(III) form that form strong complexes with pyrophosphate and tripolyphosphates, which are then probably removed by adsorption onto iron(III) hydroxo-phosphate surfaces.

2. Pre-precipitation: This process includes the ttreatment of raw/primary wastewater and falls in the general category of chemical precipitation processes. Phosphorous is removed with 90% efficiency and the final P concentration is lower than 0.5 mg/l. The chemical dosage for P removal is the same as the dosage needed for organic matter (BOD) and suspended solids removal, which uses the main part of these chemicals. As mentioned above lime consumption is dependent on the alkalinity of the wastewater: only 10% of the lime fed is used in the phosphorous removal reaction and the remaining amount reacts with water alkalinity, with softening.

3. Co-precipitation: The co-precipitation process is particularly suitable for active sludge plants, where the chemicals are fed directly in the aeration tank or before it. The continuous sludge recirculation, together with the coagulation-flocculation and adsorption process due to active sludge, allows a reduction in chemical consumption of the coagulant used for removal of P. Moreover the costs for the plant are lower, since there is no need for big post-precipitation ponds. In this process the chemical added are only iron and aluminium, lime is added only for pH correction. Lower costs and more simplicity are contrasted by phosphorous removal efficiency lower than with post-precipitation (below 85%). The phosphorous concentration in the final effluent is about 1 mg/l. Another disadvantage is that biological and chemical sludge are mixed, so they cannot be used separately in next stages and bigger sedimentation tanks are needed than activated sludge.

4. Post-precipitation: The post-precipitation is a standard treatment of a secondary effluent, usually involving the metallic reagents for removal of P. It is the process that gives the highest efficiency in phosphorous removal. Efficiency can reach 95%, and P concentration in the effluent can be lower than 0.5 mg/l. Post-precipitation gives also a good removal of the suspended solids that escape the final sedimentation of the secondary process. This process also guarantees the better purification efficiency even if the biological process is not efficient for some reason. The chemical action is stronger, since the previous biologic treatment had converted part of the organic phosphates in orthophosphates. The disadvantages of the process are high costs for the treatment plant (big ponds and mixing devices are needed) and sometimes a too dilute effluent. In case ferric salts are used in the post precipitation process, there is also the risk of having some iron in the effluent, with residual coloration. The metallic ions dosage is about 1.5-2.5 ions for every phosphorus ion (on average about 10-30 g/m3 of water).

removal.htm Initially the techniques used for phosphorus removal were all based on physical-chemical treatment methods, especially the addition of metal salts or lime. This results in the precipitation of metal-phosphorus complexes such as ferric phosphate (FePO4), calcium phosphate (Ca3(PO4)2), apatite (Ca5(OH)(PO4)3) and struvite (NH4MgPO4). There are three important disadvantages associated to this chemical method of phosphorus removal strategy:

The production of additional sludge, which can be dramatic, especially if the method selected is lime application during primary treatment. Use of alum after secondary  treatment can be predicted to produce much less sludge, but the increase could still be problematic.

A certain overdosing of metal salts is necessary to obtain the required low effluent phosphorus values, resulting in high costs of chemicals and a significant increase of excess sludge production.

The accumulation of ions (increased salt content) may seriously restrict the reuse possibilities of the effluent.

22.3 Biological process removal

Over the past 20 years, several biological suspended growth process configurations have been used to accomplish biological phosphorous removal. In the biological removal of phosphorous, the phosphorous in the influent wastewater is incorporated into cell biomass. The principal advantages of biological phosphorous removal are reduced chemical costs and less sludge production as compared to chemical precipitation.

The commonly employed biological processes for P removal include:

1. Assimilation: Phosphorus removal from wastewater has long been achieved through biological assimilation – incorporation of the P as an essential element in biomass, particularly through growth of photosynthetic organisms (plants, algae, and some bacteria, such as cyanobacteria). Traditionally, this was achieved through treatment ponds containing planktonic or attached algae, rooted plants, or even floating plants (e.g., water hyacinths, duckweed). Land application of effluent during the growing season has also been used, and constructed wetlands are now an established practice as well. In all of these cases, however, it is necessary to remove the net biomass growth in order to prevent eventual decay of the biomass and re-release of the phosphorus.

2. Enhanced biological phosphorus removal (EBPR): The basic theory for enhanced biological phosphorus removal relies on selecting P accumulating organisms (PAO) capable of storing more phosphorus than other bacteria naturally found in activated sludge through alternating anaerobic and aerobic zones. Some aqueous microorganisms (e.g. Acinetobacter sp. Arthrobacter globiformis and Klebsiella pneumonia) can store an excess amount of phosphorus compounds within an intracellular compartment.

The reactor configuration in EBPR is comprised of an anaerobic tank (nitrate and oxygen are absent) and an aerobic sludge activated tank. Under these conditions a group of heterotrophic bacteria, called polyphosphate-accumulating organisms (PAO) are selectively enriched in the bacterial community within the activated sludge. These bacteria accumulate polyphosphate in large quantities as an energy reserve in intracellular granules within their cells and the removal of phosphorus is said to be enhanced.

Generally speaking, all bacteria contain a fraction (1-2%) of phosphorus in their biomass due to its presence in cellular components, such as membrane phospholipids and DNA. Therefore as bacteria in a wastewater treatment plant consume nutrients in the wastewater, they grow and phosphorus is incorporated into the bacterial biomass. When PAOs grow they not only consume phosphorus for cellular components but also accumulate large quantities of polyphosphate within their cells. Thus, the phosphorus fraction of phosphorus accumulating biomass is 5-7%. This biomass is then separated from the treated water at end of the process and the phosphorus is thus removed. If PAOs are selectively enriched by the EBPR configuration, considerably more phosphorus is removed, compared to the relatively poor phosphorus removal in conventional activated sludge systems. So, the present interest has been in enhanced biological phosphorus removal to achieve very low (<0.1 mg/L) effluent P levels at modest cost and with minimal additional sludge production. The removal of organic contaminants (BOD), nitrogen, and phosphorus can all be achieved in a single system, although it can be challenging to achieve very low concentrations of both total N and P in such systems.

The potential for EBPR depends on successful cultivation of phosphate accumulating organisms (PAO) which require three conditions for growth: excess phosphate in the wastewater, alternating aerobic and anaerobic conditions, and the availability of a particular type of organic carbon called volatile fatty acids (VFA). Under anaerobic conditions, PAOs accumulate volatile fatty acids which are stored polyhydroxy alkanoate (PHA) and orthophosphate is released outside. Under aerobic conditions, the PAOs then grow on the stored organic material PHAs. The energy released from PHAs oxidation is used to form polyphosphate bonds in cell storage. The soluble orthophosphate is removed from solution and incorporated into polyphosphates within the bacterial cell. PHAs utilisation also enhances cell growth and this new biomass with high polyphosphate storage accounts for phosphorous removal. As a portion of the biomass is wasted, the stored phosphorous is removed from the bioreactor for ultimate disposal with the waste sludge.

Thus PAOs, although strictly aerobic, are selected for by having an upfront anaerobic zone in an activated sludge type of biological treatment process. The PAOs are able to compete with other aerobes under these conditions because of their ability to sequester a fraction of the available organic material under the initial anaerobic conditions, while out-competing the anaerobes because of the much higher energy yield from aerobic vs. fermentative metabolism. The phosphate in EBPR is removed in the waste activated sludge, which might have 5% or more P (dry weight) as opposed to only 2-3% in non-EBPR sludge. Simultaneous biological nutrient removal (SBNR) has also been observed in treatment systems.

22.4 Membrane technologies: Typically, conventional phosphorus removal processes using

biological and tertiary treatment steps are capable of providing effluent total phosphorous in

the range of 0.5 to 1 mg/L, levels which exceed many regulatory requirements. So, membrane technologies have been of growing interest for wastewater treatment in general, and most recently, for phosphorus removal in particular. In addition to removing the suspended phosphorus, membranes also can remove dissolved phosphorus. Membrane filtration is used as the liquid solids separation method in membrane bioreactors (MBR) system to capture solids and results in a very low level of phosphorous in the treated water. Membrane bioreactors (MBRs) are becoming the advanced solution for water reuse as fresh water demands continue to increase and environmental contamination of drinking water sources becomes of greater concern. MBRs, which incorporate membrane technology in a suspended growth secondary treatment process, tertiary membrane filtration (after secondary treatment) and reverse osmosis systems have all been used in full-scale plants with good results. The biggest disadvantage of the membrane filtration technique is the high cost and fouling of the membranes, which are the heart of the whole process.

References:

• Environmental Protection Agency 1987 Design manual. Phosphorus removal. US EPA Proceedings 625/1-87-001.
• USEPA 2009 Nutrient control design manual state of technology: review report. US Environmental Protection Agency, National Risk Management Research Laboratory, Water Supply and Water Resources Division.
• Wachtmeister A., Kuba T., Van Loosdrecht M.C.M., Heijnen, 1997. A sludge characterization assay for aerobic denitrifying phosphorus removing sludge, Water Research, vol. 31, no. 3, pp. 471-478.