Phosphorrückgewinnung bei der Abwasserreinigung : Entwicklung eines Verfahrens zur Integration in kommunale Kläranlagen

• Phosphorus recovery during wastewater treatment : development of a process for integration in municipal wastewater treatment plants

Montag, David Markus; Pinnekamp, Johannes (Thesis advisor)

Aachen : Publikationsserver der RWTH Aachen University (2008)
Dissertation / PhD Thesis

Aachen, Techn. Hochsch., Diss., 2008

Abstract

Economically exploitable phosphate ore resources are finite and non-renewable. Due to the fact that phosphorus is, besides nitrogen, the main nutrient for plant growth, most of the phosphorus world-wide is used for the production of mineral fertilisers. Because of the increasing world population the demand for phosphorus will increase as well. Phosphates cannot be substituted by other substances as far as plant nutrition is concerned; thus we are faced with an increasing scarcity of this raw material. Even nowadays, deposits with increasing pollutants and decreasing phosphate concentrations are exploited. In Germany, at least a part of agricultural phosphate demand is fulfilled by phosphates contained in the sewage sludge from waste water treatment. But within the last few years, agricultural utilisation of the sludge has become less important. Further reductions are expected by the amendment of the German Sewage Sludge Regulation, which is up for discussion. Hence, the gap between agricultural phosphate demand and the contribution of sewage sludge to fulfil this demand will rise. The PRISA process, which has been developed and tested within this dissertation, enables a recycling of phosphorus to agriculture that is independent of the sludge disposal. The process is based on well-known techniques of municipal wastewater treatment plants (wwtp) and can easily be integrated into existing wwtps. The first step of the PRISA process is the feeding of the pre-thickener with excess sludge of a wwtp with enhanced biological phosphorus removal. This thickener is operated with a longer hydraulic retention time and the sludge is mixed periodically by recirculation. Due to a longer retention time and the anaerobic conditions in the thickener, the release of phosphorus, which has been bound biologically, is higher than in conventional thickeners. Periodical mixing ensures the transfer of released phosphates into the supernatant liquor. This is mixed with the sludge liquor from post-thickening and mechanical dewatering. Solids become removed from the nutrient-rich process water and in a coagulation and crystallisation reactor, phosphate is recovered as magnesium ammonium phosphate (MAP, struvite). Finally, the valuable phosphate product is dewatered by bag filter housings. The following results have been gained from the scientific pilot scale examination of the PRISA process in detail:- By a three-day hydraulic retention time in the pre-thickener and a mixing of the sludge once or twice a day, 20% of the phosphorus that was in the inflow to the wwtp can be released. - The total load in sludge liquor amounts to approx. 0.7 g P/(I•d).- For MAP crystallisation, magnesium oxide (Mg:P = 1.5:1.0) as reaction agent and caustic soda in order to optimise the pH value (pH=9.5) are added to the sludge liquor. In total, in the coagulation and crystallisation reactor 60 minutes hydraulic retention time is necessary for a phosphate removal efficiency of >90% at optimised settings. Finally, using the PRISA process, 30% of the phosphorus inflow load to the wwtp can be recovered as secondary phosphate.- Concentrations of the examined heavy metals (lead, cadmium, chromium, copper, nickel, zinc, mercury and uranium) and organic pollutants (musk compounds, organic tin compounds, surfactants, polycyclic aromatic hydrocarbons) are lower than in sewage sludge. The concentrations fulfil the actual limit values and the stricter limit values that are supposed to become valid by the novel Sewage Sludge Regulation. The pollutants in the secondary phosphate present no obstacle for its use as fertiliser in agriculture. - The technical devices that are necessary for the PRISA process are standard equipment for municipal wwtps. The operation of the process and the exposure to the chemicals needed, are - compared to other processes for phosphate recovery - not complicated and easily handled by wwtp staff. - A municipal wwtp with a capacity of 100,000 I integrating and using the PRISA process will be faced with annual costs (capital and operation costs) between 1.34 €/(I•a) and 1.60 €/(I•a) after revenues. Costs of wastewater treatment would rise by approx. 0.04 €/m³ treated wastewater.