Environmental evaluation of phosphorus recovery processes from wastewater sludge using life cycle assessment

One of the functions of wastewater treatment plants is to recover phosphorus from wastewater in order to limit the eutrophication phenomena that can severely damage ecosystems. This extracted phosphorus is mainly concentrated in the wastewater sludge. Nowadays with the prohibition on land spreading of this sludge, treatment options such as incineration are becoming more and more common. One of the problems with this method of sludge valorisation is that it breaks the cycle of phosphorus, which ends up in the residual ash, often sent to landfill.
In order to address this problem, the European Phos4You project aimed to develop technologies for phosphorus recovery from sludge. Various processes were developed and promoted during this project, ranging from thermo-chemical treatments to bio-acidification followed by precipitation directly integrated into the water treatment plant.
An environmental assessment was carried out on four of these phosphorus recovery processes using life cycle assessment. This method was used to quantify the environmental impacts of the processes and to identify the most polluting steps within each process.
The particularity of this environmental study is to include the wastewater treatment plant in the systems studied and therefore not opt for the zero-burden assumption of the sludge. Sewage sludge is therefore not considered as waste and has an environmental cost. In order to avoid allocation problems between the water treatment function and the sludge production function of the treatment plant, two options have been studied. The first methodological approach is the extension of the system boundaries to take into account the wastewater treatment plant in addition to the means of treating the sludge produced. The system studied with this option in the framework of the Phos4You project therefore has two functions, wastewater treatment and phosphorus fertiliser production. The second methodological approach used is the avoided burden. This approach studies a system with only the wastewater treatment function and takes into account the fertilisers produced by recovering phosphorus from the sludge as avoided mineral fertiliser production.
The environmental study showed that two of the four recovery technologies had an environmental advantage compare to the baseline scenario of sludge incineration.