Eutrophication due to excess nutrient availability
Nutrient pollution is the primary cause of eutrophication of surface waters where excess nutrients, generally nitrogen and/or phosphorus, stimulate algal growth. This algal growth clouds water and sunlight, causing underwater grass to die. Moreover when algae die and decompose, more dissolved oxygen is consumed, thus causing further harm to aquatic life.
Although eutrophication is a natural process, industrial activity over the decades has accelerated the release of pollutants into water bodies leading to an unfit habitat for aquatic organisms. According to the Environment Protection Agency (EPA), USA, the permissible limit for phosphates in effluents from industriesis 0.05mg/L if the streams discharge into lakes or reservoirs.
Chemical treatment for phosphorus removal involves the addition of chemicals to react with soluble phosphate to form solid precipitates that are removed by solids separation processes including clarification and filtration. Although this method of treatment results in low phosphorus effluent, it has many disadvantages such as high cost of chemicals, increased inorganic waste sludge content(upto 40 percent), and corrosion of the reactor.
A better alternative to the chemical treatment is a process called Enhanced Biological Phosphorus Removal (EBPR) which involves less chemical costs, and better sludge digestibility, with a preferably higher pollutant removal capacity. EBPR is a process that promotes the accumulation of bacteria called Phosphate Accumulating Organisms (PAOs) that use Phosphorus as an energy storage mechanism, concentrating it in the form of Waste Activated Sludge(WAS). Anaerobic digestion is used to carry out the degradation of the WAS, thus releasing some phosphates back into the soluble form. Recycling this liquor(obtained after degradation of WAS) can result in unintentional formation of struvite (MgNH4 PO4 •6H2O)crystals in the sludge treatment infrastructure, reducing process efficiency and increasing treatment costs.
The Pearl Process – an innovative technology
The formation of the struvite crystals can be controlled by the Pearl process, a technology that combines the need for the removal of phosphorus and nitrogen from waste water with the requirement of these nutrients in plant fertilizers (in the form of struvite).The name “pearl” is assigned to this process owing to the production of the crystalline struvite pellets which are used in the plant fertilizer.
In this process the production of struvite is kept in check by controlling the hydraulic conditions, fluidization energy and pH inside the fluidized bed reactor in which the pearl process occurs. With the controlled chemical precipitation inside the reactor, microscopic struvite crystals are created, which grow on the particle surface. The pellets are allowed to grow sufficiently till they can be collected and used as a plant fertilizer. The pellet removal process requires operation in batch mode so that the nutrient removal process is not disturbed.
In a typical wastewater treatment plant, upto 90% of phosphorus load and 40%of the ammonia load is reduced by the Pearl process. >Most phosphorus is obtained from mining phosphate rock, which occurs as marine sedimentary phosphate deposits, and in igneous rocks that are rich in phosphate minerals. Estimates of global reserves of phosphate suggest that supplies may last from 50 – 130 years.
India, with its agricultural cover above 60 percent, will benefit from the use of phosphate fertilizers obtained as a by-product of nutrient removal process, thus reducing the load on the world supply of phosphates.