Phosphorus recovery from municipal wastewater : a case study: the wastewater treatment plant of Linz Asten

Abstract

Phosphorus (P) is an essential nutrient, which is vital for living organisms. Before the industrial revolution, it used to follow a natural life cycle, which is now broken: the Pcycle has become a linear process starting in the phosphate-rock mines, travelling through wastewater treatment plants (WWTP) and ending in landfill and in the aquatic environment. Phosphorus is mostly used in agriculture, entering in the composition of fertilisers. Due to the fact that it is a broken cycle as well as the fact that its resources are concentrated in three non-European countries, it is becoming a critical raw material in Europe. P is also a pollutant, which is at the origin of eutrophication problems in surface waters. In order to limit eutrophication, more and more WWTPs in Europe treat phosphorous, which ends in sludge and not in effluent. The P-flow estimation at a global scale, as well as the European and Austrian scale shows the great possibilities of recycling through wastewater treatment. A view on the European and Austrian legislation shows that the concern is traditionally on phosphorus as a fertiliser and pollutant. However, a new EU regulation on fertilisers - which is still in draft form - tends to begin to consider P as a critical raw material that must be saved and recycled. This thesis aims to test the possibilities of installing recycling process on the WWTP of Linz (Austria), which has a capacity of 950 000 population equivalent and a phosphorus treatment process. Three main techniques exist, depending on their location: the treatment of digested supernatant, the sludge or the ashes. All techniques have advantages and disadvantages in terms of costs, establishment and implementation easiness. Moreover, the final products, struvite, calcium phosphate or ashes, usually sold as fertilisers, have different plant availability. The techniques tested in this thesis take part in the European project P-REX: Struvia, Ostara/Pearl, AirPrex, Gifhorn, Stuttgart, Leachphos, Ecophos, AshDec. As a methodology, several MFAs with the software STAN were built on the flows of phosphorus at Linz WWTP. It includes multiple scenarios: the actual state, as well as the projected state with the setup of the different recovery techniques. A cost-benefit analysis and a multi-criteria analysis were used to rank the techniques that are the most adaptable to Linz WWTP. Almost all the techniques are economically feasible, and the most relevant ones including all the criteria are ECOPHOS (ash leaching) and STRUVIA (liquor precipitation). The implementation of Ecophos would require a change in the wastewater management practices in Austria, with a mono-incineration plant instead of the current co-incineration. The thesis concludes with the importance of the political willingness and an adapted legislation in order to close the phosphorus cycle.