Waternet, Amsterdam, has signed a contract with ICL Fertilisers, who will take the recovered struvite produced in the sewage treatment works for use in fertiliser production.
The Waternet sewage works, treats sewage from one million person equivalents (biological nutrient removal) and sludge from two million, incoming from other sewage works (anaerobic digestion). Struvite precipitation (magnesium ammonium phosphate), using the AirPrex process / magnesium chloride dosing in the digestate (anaerobic digester outflow) upstream of the sludge filter press, was installed to address major nuisance incrustation problems in pipes and other installations: operational savings of 200 000 €/year.
The struvite precipitation also improves methane production in the anaerobic digester (possibly by reducing ammonium return streams) and allows biological nutrient removal to achieve phosphorus discharge consent without chemical dosing. The biggest cost benefit comes from improved digestate dewatering (from 20% to 23% dry matter): this results in 300 000 – 400 000 €/year cost savings (gate fee for incineration of sludge in adjacent municipal waste incinerator and lower polymer dosage requirement), effectively resolving dewatering problems which resulted from moving from chemical to biological P removal in the plant.
The Waternet struvite plant started operation in 2013, and has a capacity of 900 tonnes/year. The struvite will now be used in the nearby ICL Fertilisers factory to produce fertilisers, after combination with other minerals.