About eutrophication
EUTROPHICATION A WORLD PROBLEM
Sweden has a long tradition in phosphorus removal to protect against eutrophication. Phosphorus is at present removed in an efficient and reliable way in Sweden, and the concern is now directed towards recycling of phosphorus as a limiting substance.
Stringent phosphorus requirements, and inexpensive precipitation chemicals have led to chemical precipitation as the dominating process although about 20 small or medium sized plants in Sweden are operated with biological phosphorus removal.
As of today, virtually all households in urban areas are connected to municipal sewer networks, and 95% of urban wastewater undergoes both biological and chemical treatment.
Discharge of eutrophying substances (mainly nitrogen, and phosphorus) has significantly decreased in recent decades. Agriculture is the largest single source (45% for nitrogen; 48% for phosphorus), and WWTPs are the source of 23% of nitrogen and 15% of phosphorus. Small-scale treatment plants contribute 15% of phosphorus (as high as phosphorus released in WWTPs), and 4% of nitrogen. Total phosphorus discharge was 1930 tons, and 83,500 tons for nitrogen (2009).
Polonite closes the nutrient loop
Using the Phosphorus (P) from wastewater as a fertilizer would turn P into a valuable resource, thereby eliminating pollution problems.
Phosphorus is an essential macro-nutrient for crop growth and should therefore be recycled, especially since raw materials for manufacturing P-fertilizers are limited and non-renewable. Precipitation of P by chemicals and subsequent sludge storage in landfills does not solve the pollution problem, rather it postpones it. Several recently conducted investigations on different materials show that P can be trapped efficiently.
The mechanisms of P retention by these materials are not known in detail, but adsorption to particle surfaces is one of them. The term ‘‘sorption’’ is generally used as a synonym for retention, which covers physical, chemical and biological processes in which P is immobilised (see e.g. Zhu et al., 1997). The material that is enriched with P can be used as a fertilizer, provided that P is available to the plants, and contents of toxic compounds and pathogens do not restrict such use. Most of the materials are characterised by high pH values (9–12), which create an unfavourable environment or bacteria (Renman et al., 2004). Phosphorus-sorbing materials with additional beneficial characteristics may be used. For example, lime can be used to counteract acidification to improve soil structure. Potential availability of P to plants provides also a clear benefit to the user.
Of all the recently studied materials, Polonite had the largest sorption capacity for P. Polonite used in wastewater filters can be recycled as a combined fertilizer for plant production and liming agent on acidic soils.