Phosphorus recycling in urban areas. A comparative study of nutrient recycling from urban wastewater in two Danish cities.

摘要

Using a comparative study, this research investigates the foundation for phosphorus (P) recycling (and reuse in agriculture) in two Danish urban areas (Copenhagen, and Roskilde), via selected nutrient recycling stakeholders (Wastewater Treatment Plants - WWTP). The Sewage treatment works (STW) chosen for the study are Avedøre WWTP (Copenhagen) operated by BIOFOS A/S, and Bjergmarken WWTP (Roskilde) operated by Forsyning Roskilde A/S. The STW selected for the study were chosen due to the timespan available for the master thesis research, and the consideration of other stakeholders involved in the phosphorus recycling agenda. The results show that phosphorus recycling is proactively being pursued in Roskilde, compared to Copenhagen. Avedøre WWTP incinerates sewage sludge (containing the p), and the resulting ash is deposited on site within their facility while the Bjergmarken WWTP recycles phosphorus via drying and pelletizing sewage sludge. Pelletized sludge is distributed to farmers, inherently contributing to the nutrients available for their agricultural activities. Momentarily, the STWs are enhanced for sanitation but not optimized for nutrient recycling, and there is a growing call for a shift in wastewater treatment paradigm (from the conventional sanitation model to a sanitation and recovery model). A potentially promising option for phosphorus recycling optimization within the STW could be achieved through controlled struvite precipitation. Struvite (magnesium- ammonium-phosphate hexahydrate - Mg·NH4·PO4· 6H2O) can be precipitated with the supernatant liquor from the sludge dewatering centrifuge, using a supplementary addition of magnesium. Also, sludge gasification produces a residue that can be used for agricultural purposes. Furthermore, decentralized phosphorus recycling alternatives are feasible, and can be achieved through urine separation at the source for direct use or struvite precipitation. This dissertation also reflects swiftly, and theoretically, on the possibility of technology knowledge transfer (centralized and decentralized nutrientrecovery) to a sub-Saharan African country case study (Wupa WWTP) in Abuja.