Introduction

Phosphorus is a non-renewable source and is the vital element in the Earth’s crust forming the basis for all life. It occurs in pentavalent forms such as orthophosphate, polyphosphates, pyrophosphate, organic phosphonates, organic phosphate esters and phosphate diesters in aquatic ecosystems. Functions of phosphorus include: i) major constituent of the skeletal bones and teeth (as calcium phosphates), ii) crucial role in the energy transport system in cells, iii) essential for photosynthesis, iv) forms a part of structural component of cell membranes (phospholipids), v) forms a part of DNA and RNA structure, helps in cell division and development of new tissue, vi) helps in the synthesis of proteins, vitamins and is a component in enzymes etc.^1,2

Demand for phosphorus for agricultural activities, medicinal and industrial uses varies depending on population, food habit, soil quality, etc. However, the flow of phosphorus to the environment had increased substantially over decades with the increased use of phosphate fertilizers, manures and detergents with burgeoning population.

Global phosphate demand

Globally, phosphate rocks have been used for the manufacture of fertilizers, animal feed, detergents, pharmaceutical products, insecticides, beverages, toothpaste, matches, fireworks, military operations, bombs, etc.

Fig. 1: Phosphate reserves (%) across many regions in the globe

Phosphate rocks/reserves occurs in few countries like United States, Algeria, Australia, Brazil, Canada, China, Egypt, Finland, Jordan, Morocco and Western Sahara, Russia, Peru, South Africa, Syria, Saudi Arabia and others. According to U.S. Geological Survey 2019³, Morocco and Western Sahara has 50,000 million tonnes ~72% of world phosphate rock reserves (figure 1). Phosphate reserves across countries given in Figure 2, which indicates that most part of globe (India, Israel, Kazakhstan, Mexico, Senegal, Togo, Tunisia, Uzbekistan and Vietnam) has less than 1% of phosphate reserves and depend on imports.

Phosphorus is a very limited and non-renewable source. But, an increase in population, agricultural production, demand for more protein diet (like meat and dairy products), crop production, etc. have escalated the demand for phosphorus⁴. Factors such as income, price, prices of substitutes, technological change, consumer preferences, government activities, etc. need to be considered for assessing the demand for phosphate⁵. World consumption of phosphorus pentoxide (P₂O₅) projected to increase to 50.5 million tons in 2022 from 47.0 million tons in 2018. The countries such as Africa, India and South America would account for about 75% of the projected growth, whereas P₂O₅ consumption in U.S. remains as ~5 million tons per year³. According to IFASTAT 2019 data⁶, the production, consumption and export of Grand total P₂O₅ is highest in China than other countries. Countries such as Brazil, followed by India and United States, are the largest importer of Grand total P₂O₅ (figure 3).

P dependency in India

India has very limited reserves/resources of rock phosphates (RP) and is of low to medium grade quality and hence imports P in the form of rock phosphate/phosphoric acid/direct fertilizers. India is the largest importer of RP (about 30% of global trade) in the world⁷. The total reserves/resources of rock phosphate accounts to 312.67 million tonnes (2015) and about 34% of it is in Jharkhand, 31% in Rajasthan, 19% in Madhya Pradesh, 8% in Uttar Pradesh and 8% in Uttarakhand, while states like Gujarat and Meghalaya has meager quantities of resources⁸. In India, demand for phosphatic fertilizer has increased gradually and as per the Fertiliser Association of India, the consumption of phosphorus pentoxide (P₂O₅) during 2008 to 2018 exceeded the production necessitating imports (figure 4).

Fig. 2: Global phosphate reserves in million tonnes

Fig. 3: Country wise production, consumption, export and import of total P₂O₅ (in 000 metric tonnes of nutrients)

Fig. 4: Overall consumption and production of P₂O₅ (’000 tonnes) in India

Chemical fertilizers in India include Urea, Diammonium Phosphate (DAP), Single Super Phosphate (SSP), Muriate of Potash (MOP) and other Complex fertilizers like Calcium Ammonium Nitrate (CAN) and various grades of NPK fertilizers. Indian Fertilizer Scenario, 2017 reported⁹ about 37 nitrogenous fertilizers (Urea, ASP and CAN) plants, 17 DAP and Complex Fertilizers plants and 105 Single Super Phosphate (SSP) fertilizer plants in India. As per the Department of Fertilizers, GoI¹⁰, about 25% of Urea requirement, 90% of phosphate either as raw material or as finished fertilizers ((Diammonium Phosphate (DAP) or Monoammonium Phosphate (MAP) or Triple Superphosphate (TSP)) and potash requirement during the year 2017-2018 in India is completely met through imports. During 2016-17, the total production of urea, DAP and NP/NPK complex fertilizers was 24.33, 4.25 and 8.57 million tonnes respectively⁸.

Phosphate fertilizer consumption in India includes both phosphoric acid based fertilizers and non-phosphoric acid based fertilizers. The non-phosphoric acid based fertilizers include phosphate in nitric acid based fertilizers and super phosphate. India is the third largest producer and consumer of fertilizers as per CARE Rating (2018)¹¹. Recently, India has increased the domestic capacity and with easy availability of acid, the production of DAP has increased by 7.3% (38 to 46 LMT in FY16-18) whereas import of DAP had decreased by 3.8% during FY18. DAP is mainly imported to India from China (45%), Saudi Arabia (31%), USA (13%) and Jordan (5%).

The phosphate rocks stocks are fast dwindling and there is no substitute for phosphorus, which is essential for agriculture and medicine, and hence it is necessary to minimize wasteful use of P for detergents manufacture apart from recovering from wastes. The main objective of the study is to assess (i) the water quality to understand the implication of sustained inflow of untreated sewage (rich in N and P), (ii) biomonitoring through assessment of plankton diversity, (iii) to understand the causal factors of pollution in Varthur lake leading to foam formation and (iv) to suggest remedial measures for lake protection.