For sustainable phosphorus recycling, the phosphorus in sewage sludge needs to be recovered, as it is a major phosphorus-source. However, all sludge is not suitable for direct recycling on agricultural land, e.g. when the content of heavy metals is too high. To achieve an increased recycling of phosphorus from sewage sludge there is a need of research and development of alternative methods to return the phosphorus from sewage sludge to agricultural land.
The aim of the project is to show that sewage sludge can be co-combusted with demolition wood in existing boilers, and that the ash can be processed for extraction of phosphorus, together with valuable by-products from the process. Further effects of the project will be the development of new knowledge about combustion with a high sludge to demolition wood ratio in a grate boiler (up to 45 % sludge with respect to wet mixture), which has not been studied in full-scale combustion earlier. The project's goal is to demonstrate mixing and combustion of fuel mixtures of sludge and demolition wood, and subsequent extraction of phosphorus from ash. The target groups for this work are energy companies, suppliers of grate boilers, municipalities, wastewater treatment plants and authorities.
The project involves the mixing of sewage sludge and demolition wood, combustion-tests of the fuel mixtures and laboratory experiments on bottom ash and fly ash for phosphorus extraction using EasyMining Sweden's method for phosphorus extraction from ash. Extensive analyses have been carried out on fuel, bottom ash and fly ash, as well as thermodynamic equilibrium calculations on the formed compounds containing phosphorus.
The experience of preparing sludge fuel mixtures shows that the mixtures should be prepared in the near future to when they are to be combusted, to avoid a composting process in the fuel. The project shows that it is possible to combust sludge fuel mixtures with an admixture of up to 45 % sludge relative wet fuel, in an existing grate boiler during one day. However, to combust sludge fuel mixtures over a longer period of time and at a higher load, modifications of the de-ashing system and the flue gas cleaning system are needed. The results indicate a reduced tendency of slagging on the fuel bed at the combustion of sludge mixtures. The analyses of the fly ash showed a trend towards less corrosive ash at combustion of sludge mixtures. The phosphorus content in the ash in this work was 3-4 %. For process economics and from the phosphorus extraction point, as high phosphorus content as possible in the ash is preferable. Higher phosphorus content can be accomplished by mixing sludge with a fuel with lower ash content than demolition wood, co-combustion with a fuel with higher phosphorus content than demolition wood, or by increasing the percentage of sewage sludge in the fuel mixture. Still, a higher proportion of sludge in the mixture would require a higher dry matter content of the sludge in order to get proper combustion conditions. It is primarily bottom ash that is suitable for phosphorus extraction using Easy Mining Sweden's method. If fly ash are to be used for phosphorus extraction, an optimization of the dissolution conditions is required, which was not included in this project. In this project, phosphorus is extracted as ammonium phosphate. The purity of ammonium phosphate is very high, which means that unwanted substances and heavy metals in sewage sludge will not be returned to contaminate fields.
Borås: SP Sveriges Tekniska Forskningsinstitut , 2013. , 49 p.
sewage sludge, combustion, phosphorus extraction, phosphorus recycling