The environmental footprint of mono-incineration of sewage plant sludge

In the Netherlands sewage sludge is processed by a variety of routes. One of these is mono-incineration: high-temperature incineration of dewatered sludge, with flue-gas treatment and maximum recycling of waste products, including fly ash. With its high phosphate content, the fly ash can be used as a substitute for rock phosphate.  

At the request of N.V. Slibverwerking Noord-Brabant (SNB) CE Delft carried out a ‘screening LCA’ to analyse the environmental footprint of the current mono-incineration processing route and the improvements potentially available via a range of measures. One of the results to emerge is that if all the measures are implemented the net environmental footprint switches from negative to positive. 

The environmental impact of closing the phosphorus cycle cannot be characterised using conventional LCA methods like ReCiPe, since phosphate scarcity (depletion of rock phosphate stocks) is not included, although that is the case for depletion of fossil energy and metal resources. In this study we therefore make a first move towards remedying this situation, incorporating P-scarcity in LCA by calculating a shadow price for use of remaining rock phosphate stocks and then using these to weight environmental impacts.

The shadow prices for rock phosphate use are valid under the assumption that continued use of finite stocks will ultimately lead to a shortage of P-fertilisers, and consequently to higher food prices and famine. The associated loss of human life – assigned a value according to Western methodology – yields a relatively high worst-case shadow price for phosphate scarcity. 

These shadow prices are many times higher than current rock phosphate market prices. Given the high dependence on the estimated time of ‘peak phosphate’ and the impacts assumed to follow, we present multiple values: the further into the future the impacts occur, the less  the financial damage discounted to 2012 euros. As a conservative estimate, peak phosphate can be assumed to occur in the year 2200, giving an associated shadow price of € 69 per kg P.

Erratum, July 5, 2017
In the course of a new life cycle assessment on the SNB process it came to light that the original report contains a calculation error on the environmental benefits of phosphate ore substitution with respect to savings relating to extraction and transport. Our latest understanding indicates that the measure ‘100% use of sludge incineration ash as a phosphate ore substitute’ has a favourable impact on the ReCiPe environmental footprint of -1 Pt/tonne sludge cake. This report was not updated for this error. In the new report for SNB ‘The environmental footprint of SNB sludge processing: 2015 and 2017 update’ (CE Delft, July 2017) this error has been rectified. This new report on the life cycle assessment of the SNB process is to be taken as definitive.