Phosphates - they have been blamed for the decline of the Broods and recently NACA, Wesum Valley Project and Environment Agency have made efforts to persuade Anglian Water to install phosphate strippers along the Upper Wensum. But the question on many lips is "What are phosphates and why strip them?" Simon Johnson now provides the answers.
What is phosphorus ?
Phosphorus is , on average, the most scarce element in the earth's crust of those essential for algal and higher plant growth. As this element is rare it is termed a 'limiting' nutrient because without it plants cannot grow. We can therefore say that the potential for plant growth is directly proportional to phosphorus levels and that phosphorus levels set the upper limit for plant growth. This case has ben proven in many experiments where lakes have been divided in two, the half which has had phosphate added to it consequently showed a dramatic increase in phytoplankton (alagae) growth.
Why the problem?
Expansion of villages, towns and cities has led to increasing amounts of human sewage. Sewage is a mixture of wastes from laundry, bathing, cooking and the flushing of faeces and urine in lavatories. Sewage contains much organic matter and can render the watercourse anoxic. In isolated houses or hamlets a very simple system of allowing the sewage to decompose in an underground tank and the consequent effluent to seep away into the soil and ground
Sewage treatment removes organic matter from the sewage but leaves an effluent, still containing some soluble and particulate organic matter, and greatly enriched in ammonium, nitrate and phosphate from the decomposition of the sewage. In western countries about half of the phosphate content of this effluent comes from domestic detergents in which sodium tripolyphosphate is used to remove calcium from solution and increase the efficiency of the surface
Sewage effluent is released, into a stream near the treatment works, usually at rates of only a few percent of the total system discharge. At such a dilution the stream community should normally decompose any remaining organic matter without severe problems but the added phosphate in particular will greatly increase the stream phosphate concentration over that expected in water draining from natural vegetation or farmland.
Increases in phosphate concentration from sewage effluent are now very prominent in drainage waters and in many lowland areas the bulk of the phosphorus comes from this source, just as the bulk of the nitrogen comes from the use of fertilizers on the catchment area of the river.
Phosphate stripping. What is it and what does it do?
Sewage treatment works can have a phosphorus stripping plant installed relatively cheaply. The effluent, after passing through conventional sewage treatments, is dosed with a precipitant to remove the phosphate. The costs of the process are largely in chemicals rather than installations and up to 95% of the phosphate can be removed.
Phosphate stripping is unfortunately only part of the answer to the problem. In the Norfolk Broads and its associated river systems phosphate has been locked up in the sediment over many decades and represents a major sink of this 'limiting' nutrient.
Currently there are various studies being conducted where broads are being manipulated by biological means (biomanipulation) to restore them to a 'natural' state. This is proving to be a complex and very costly operation. (Barton Broad £+ million just to pump nutrient rich sediment away).