Urban phosphorus mining in the canton of Zurich: Phosphoric acid from sewage sludge ash

Abstract

The constitutional duty of sustainability in Switzerland requires economical use of valuable and scarce raw materials such as phosphorus. The government of Canton of Zurich recognised this as an opportunity around 10 years ago with respect to the bottlenecks in waste disposal threatening to materialise in 2015 in the existing sewage sludge disposal plan. In 2007, it already gave the Department of Public Works the assignment to design the future sewage sludge disposal so that phosphorus recovery is possible. In 2009, an evaluation of all then-known phosphorus recovery procedures as well as their integration into different sewage sludge disposal pathways showed that the procedures with P recovery from sewage sludge ash are clearly superior to P recovery from sludge and sludge water. The first milestone in the implementation was that it was possible within 6 years and with the involvement of all parties impacted to realign the existing sewage sludge disposal concept completely with respect to the new framing conditions. Since mid-2015, a new central sewage sludge treatment plant at the most optimal location in the Canton has been producing high-phosphorus ash from incinerated sewage sludge. It contains more than 90% of the phosphorus potentials of the entire potential in untreated community waste water from the Canton. By switching over the sewage sludge disposal system from an inefficient, decentralised one to an efficient, centralised system, it has been possible to cut the average sewage sludge treatment costs by more than half, including ash disposal. No modifications to the waste water treatment plants were needed. The Canton has worked with the Foundation ZAR and selected development partners on this implementation of the large-scale engineering of phosphorus recovery from sewage sludge ash since 2011 (Phosphorus-Mining- Project). The initial focus lay on the production of high-grade raw material for fertiliser. This led, among other risks in product sales, to settling on the production of technically pure, conventional phosphoric acid as an already established product. Currently there is development work going on using the Phos4life® procedure by Técnicas Reunidas. The attempts to this point demonstrate a P-recovery rate of > 95% from the ash, a material recycling of the minerals and the separated metals as well as the use of iron as a precipitant.