Wastewater converted to eco-friendly fertilizer

handful of phosphorus fertilizer pellets
The harvested struvite forms pure, crystalline pellets.

By Henry Alamzad

The City of Saskatoon’s wastewater treatment plant, built in 1971, has expanded several times to serve the city’s growing population of over 300,000, and to comply with environmental regulations. In 1991, an enhanced biological phosphorus removal (EBPR) process was installed to reduce phosphorus and 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 wastewater stream of 84 million litres per day. The resulting biomass is piped 12 km 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 km pipeline to the wastewater plant inflow.

Although EBPR has many advantages over chemical phosphorus removal, the accumulating sludge was releasing phosphorus back into solution. This resulted 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, MgNH4PO4·6H2O) that was coating pipes, valves and other equipment, reducing plant flow capacities, and increasing maintenance requirements.

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“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, Ostara’s director of engineering.

To eliminate these problems, the City of Saskatoon commissioned Ostara Nutrient Recovery Technologies Inc. to install Canada’s first commercial facility to recover phosphorus and nitrogen, and transform them into more than 72.5 tonnes per year of a slow-release, environmentally responsible, enhanced efficiency fertilizer called Crystal Green®.

Like an oyster forms a pearl

The heart of the nutrient recovery process is Ostara’s Pearl® 2000 fluidized bed reactor into which the nitrogen (i.e., ammonia) and phosphorus-rich sludge water is fed. Magnesium chloride is added separately.

Additional phosphorus and magnesium are fed into the reactor by Ostara’s waste activated sludge stripping (WASSTRIP®) process, which strips both nutrients from the sludge. This makes as much as 40% more phosphorus available for recovery, and further controls struvite scale formation throughout the sludge treatment stream.

The Pearl reactor removes 90% of phosphorus and 40% of nitrogen from its feed water, and converts them into high-value fertilizer. Microscopic crystals of struvite (each crystal containing both nitrogen and phosphorus) begin to form in the reactor, like a pearl. They grow until they reach the desired particle size of 1.0 mm to 3.5 mm diameter for the fertilizer product.

The result is struvite in the form of extremely pure, crystalline pellets (prills). They are batch harvested from the reactor in a slurry consisting of approximately 10% solids by weight at a rate of 1.0 to 1.5 kg/min.

Circular screener dewaters pellets

Following harvest, fertilizer finishing occurs automatically in batch mode. First, the Kason Vibroscreen® 762 mm diameter single deck screener dewaters the prills to a moisture content of 18% to 20%. The screener also helps spread out and equalize flow to the horizontal fluid bed dryer.

In operation, an imbalanced-weight gyratory motor imparts multi-plane inertial vibration to the screen deck, causing solid particles to migrate across the 35 mesh (445 mm) screen and exit through the upper discharge spout. The liquid flows through the screen apertures, exiting through the lower discharge spout, and returning to the head end of the treatment process. Particles smaller than 35 mesh are captured.

seperating the fertilizer pellets
The motion of the rings sliding across and atop of the perforated steel plate shears and scrapes clumping particles, so they can pass through the screen.

Because the solids tend to agglomerate, a Kleen Screen ring anti-blinding device is fitted on top of the screen. It consists of plastic rings that move continuously, by the screener’s vibration, across a perforated stainless steel plate having apertures slightly larger than those of the screen beneath. The motion of the rings across the steel plate shears and scrapes the clumping particles so they can pass through the screen. Because the rings are hollow, they promote product flow over the entire screen surface, maximizing screening efficiency.

Screened and dewatered particles pass to the horizontal fluid bed dryer, which further reduces their moisture content to 0.5%. Using heat, airflow and vibration, the unit separates and fluidizes individual particles to maximize drying.

Fertilizer pellets separated into commercial sizes

Following the fluid bed dryer, the four-deck, 813 mm diameter classifier separates the prills into five fractions ranging from 1.0 mm to 3.5 mm. The top deck has a 6 mesh (3.5 mm aperture) screen. Each deck below has a screen with smaller apertures than the one above it: 8 mesh (2.4 mm), 14 mesh (1.5 mm) and 18 mesh (1.0 mm). Particles smaller than 18 mesh exit the bottom deck’s discharge spout.

Each screening deck is fitted with the same Kleen Screen anti-blinding rings as the dewatering screener. The classifier separates the pellets based on the same multi-plane inertial vibration principle as the single-deck screener.

Enhanced efficiency fertilizer

According to Ostara, Crystal Green fertilizer is the first to provide slow-release of plant-available phosphorus, nitrogen and magnesium in one citrate-soluble product. Releasing these nutrients slowly over 160 to 200 days promotes plant health, while reducing the risk of leaching and runoff.

The process helps the City of Saskatoon dispose of an otherwise troublesome byproduct. Furthermore, the City receives a share of the revenue generated from fertilizer sales, which helps offset the costs of the system.

Henry Alamzad is with Kason Corporation. This article appears in ES&E Magazine’s October 2016 issue.

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