During recent years it has become clear that, particularly to protect the quality of sea waters, nitrogen and phosphorus discharges have to be substantially reduced. Nitrogen reduction can take place by conventional biological treatment. However, the problem can be partly, or perhaps completely, resolved by treating the returned liquors resulting from sludge dewatering. These normally create a substantial load on sewagetreatment works; in fact the nitrogen in the returned liquors can contribute 15–25% of the total nitrogen load entering the works. It therefore seems appropriate, particularly with a view to future nitrogenreduction requirements, to treat the returned liquors before they are returned to the works’inlet. Since 1987, Watergroup has been working on these problems, and the company now has full‐scale plants at Frederikshavn, Denmark (population equivalent 130 000) and Eslöv, Sweden (population equivalent 250 000). Normally this treatment can be carried out at a considerably lower cost, per kg nitrogen removed, than when applying traditional methods. An additional advantage is that the method makes it possible to reuse the nitrogen content of the returned liquor, e.g. in the form of ammonium sulphate which is an excellent fertilizer.
The secondary clarifier is a very important part of the biological treatment. The better the secondary settlement works, the higher sludge concentration can be achieved in the process tanks, and the higher the sludge concentration, the better purification, or perhaps the less process volume is needed. But in many cases keeping the emission standards with regard to suspended substance creates problems to the sewage works. The traditional solution is installation of a new sedimentation tank or perhaps a sand filter, and these solutions are generally expensive. Another usual problem often comes up during rain at a sewage works which apart from wastewater also receives the load of rainwater. Such a hydraulic overload means a substantial risk of washing out of the active biological sludge through the secondary clarifier and into the recipient where the washed out sludge may create a substantial pollution problem. Furthermore, washing out of the biological sludge will delay the biological treatment as it takes some time before the biological treatment becomes effective again. These problems can, however, be solved by installing an upward flow clarifier which secures a considerable reduction of the emission of suspended substance and will, to a certain extent, stop the washing out of biological sludge during rain.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.