Green Solution to Waste Woes
City of Saskatoon tackles phosphorus problem through Canadian-first nutrient recovery process
Saskatchewan’s largest city has turned a problem of excess phosphorus accumulating in its wastewater treatment system into a slow-release, environmentally friendly fertilizer. Saskatoon’s wastewater treatment plant, built in 1971, has expanded several times to serve the city’s growing population of more than 300,000, and comply with environmental
regulations. In 1991, an enhanced biological phosphorus removal (EBPR) process was installed to reduce phosphorus to meet discharge permit limits for the South Saskatchewan River. In the EBPR process, microbes known as phosphorus accumulating organisms (PAOs) remove phosphorus from the incoming waste water stream of 85 million litres per day. The resulting biomass is piped 12 kilometres to settling lagoons where it is aerobically digested and dewatered. The resulting biosolids from the lagoons are applied to farmland, and the supernatant liquid is pumped back through a second 12-kilometre pipeline to the wastewater plant inflow. Although EBPR has many advantages over chemical phosphorus removal, the accumulating sludge was releasing phosphorus back into solution, resulting in greater loads of phosphorus and other nutrients (i.e., ammonia and magnesium) circling back to the main treatment process. The nutrient overload promoted formation of a precipitate called struvite (magnesium ammonium phosphate hexahydrate) that was coating pipes, valves and other equipment, reducing plant flow capacities and increasing maintenance requirements.
“Struvite formation was challenging operational reliability and reducing process efficiency in the sludge treatment stream, impacting digesters, dewatering, and associated biosolids infrastructure,” says Derek Lycke, director of Ostara Nutrient Recovery Technologies Inc., which was commissioned by the City of Saskatoon to install Canada’s first commercial facility that could recover phosphorus and nitrogen, transforming them into more than 725 tonnes per year of a slow-release, environmentally responsible, enhanced efficiency Crystal Green fertilizer.
Like an oyster forms a pearl The heart of the nutrient recovery process is a fluidized bed reactor into which the nitrogen (i.e. ammonia) and phosphorus-rich sludge water is fed. Magnesium chloride is separately added. Additional phosphorus and magnesium are fed to the reactor by a waste-activated sludge stripping process that strips both nutrients from the sludge. The stripping produces as much as 40 per cent more phosphorus for recovery, and further controls struvite scale formation throughout the sludge treatment stream. The reactor removes 90 per cent of phosphorus and 40 per cent of nitrogen from its feed water, and converts them into the high-value fertilizer. Microscopic crystals of struvite (each crystal containing both nitrogen and phosphorus) begin to form in the reactor, like a pearl, and grow until they reach desired particle size of one to 3.5 millimeters in diameter for the fertilizer product. >>
Ostara Nutrient Recovery CH-Process West January 2016