Phosphate Recovery from Waste Source: A Sustainable Approach

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Phosphorus (P) is the 11th most abundant element on earth crust and highly reactive in nature. It is mostly found in its oxidized state as inorganic phosphate rocks specifically fluroapatite [3Ca3 (PO4).CaF2], hydroxyapatite [3Ca3(PO4)2.Ca(OH2)], and chloroapatite [3Ca3(PO4)2.CaCl2]. Phosphorus circulates through the environment in three natural cycles viz., inorganic cycles, biospheric phosphorus flows, and organic cycles. In the inorganic cycle, it moves slowly starting from rocks weathering to form soil and gradually leached from lands to river and aquatic systems. The second cycle involves little phosphorus that gets into the atmosphere as dust or sea spray. The organic cycle involves P movements in living organisms as a part of food chain.

P is extensively used in agriculture and industrial processes and is obtained from phosphate rich sedimentary rocks, which are unevenly distributed across the globe. P is an immobile nutrient and plants can uptake it in mobile form only, deficiency in P can result in stunted growth, early leaves senescence, etc. Food is the primary source of phosphorus in animals. Extraction of P from phosphates rocks results in deterioration of landfills and also have many other environmental consequences like soil erosion, deforestation, loss of habitat, etc.  P is a non-renewable and a vital resource for both plants and animals.

Only few countries control the worlds remaining reserves (Morroco, China, Algeria, Syria, and South Africa). The extracted P is used in production of H3PO4, P4S10, P4S3, etc. Annual mining rate of P is about 19.5 MMT per year, and at this rate the world’s known phosphate reserves could be exhausted in about 120 years.   P has no alternative substitute and hence to lessen the stress on mined phosphate rocks.

P recovery from waste sources provides an alternative route for recycling and reuse in land applications and will also reduce the burden on mined rocks. P in wastewater arises due to human excrement, detergents, food wastes, domestic and commercial discharges. Untreated disposal of wastewater results in accelerated eutrophication causing harm to aquatic ecosystem. Comprehensive research studies are ongoing around the world to recover and reuse Phosphorus from wastewater. Conventional methods used to recover P from waste resources includes processes like crystallization and precipitation where transformation of soluble phosphates from a liquid phase into solid phase.

P precipitation can occur spontaneously although it is mostly initiated by addition of metal ions such as Mg2+, Ca2+ etc. When P is precipitated using Mg2+, a white crystalline substance called struvite [MgNH4PO4.6H2O] is formed, which constitute P in its mineral form and can be used as slow release of P rich fertilizer. Wet-Chemical treatment releases P from chemical or biological sludge and sludge ash using acids or bases and thermo-Chemical treatment helps in removal of P and heavy metals from sludge and sludge ashes by destructing and reduction of solids using heat. Apart from these, biological routes like Enhanced Biological Phosphate Removal (EBPR) can also be used for P recovery using Phosphate Accumulating Organisms (PAOs) by assimilating the P for metabolism and growth as nucleic acids, phospholipids, nucleotide followed by P storage as poly-P and finally precipitating and adsorption thus recovering it in a soluble form for easy plant uptakes.

Recovery of P from these waste sources can help alleviate reliance on imported phosphate, reduce vulnerability to fluctuating prices, and also increase efficiency of waste sources, providing an alternative renewable route for recovery of not only P but many other nutrients and valuable products that are otherwise wasted.   

Author: Miyon Moyong, JRF, Bioengineering and Environmental Sciences Lab, CEEFF, CSIR-IICT.

Image Source: Y. Hodges. 2013. Infographic of Phosphorus Cycle.


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