In the Bags

An Ontario treatment plant finds a cost-effective biosolids dewatering solution in flexible and permeable geotextile tubes
In the Bags
The Eganville (Ont.) Sewage Treatment Plant and nutrient management facility. The domed structure is the greenhouse.

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The Eganville Sewage Treatment Plant in Bonnechere Valley, Ont., paid $27,500 per year to handle 350,000 gallons of digested biosolids at 2.5 percent solids. In summer it was land-applied; in winter it was thickened, dewatered and hauled to the Robert O. Pickard Environmental Center (wastewater treatment plant) in Ottawa for storage and ultimately land application in spring.

To comply with pending provincial legislative restrictions on land-applying untreated sludge and septage, and to increase storage capacity to eliminate winter hauling, the County of Renfrew and Bonnechere Valley Township built a nutrient management facility across the road from the treatment plant.

After a pilot project to evaluate Geotube® dewatering units from TenCate Geosynthetics, officials chose the technology for the full-scale permanent facility. The flexible tubes are made of high-strength, permeable, engineered textile.

Darryl Verch, environmental officer for the township, runs the facility with two plant operators as part of regular duties. From June 2008 to January 2011, they dewatered 4,300 cubic yards of biosolids from the Eganville plant, 890 cubic yards of septage, and 670 cubic yards of biosolids from neighboring municipalities.

In April 2010, Bonnechere Valley Township received the Exemplary Biosolids Management Award from the Water Environment Association of Ontario.

 

Simple process

Bishop Water Technologies in Eganville provided information on infrastructure needs, process management, biosolids quality, and beneficial use opportunities. The facility’s major components include a receiving area, subsurface holding tank, process control building, three 30- by 50-foot concrete dewatering pads with a greenhouse on one pad, and a filtrate storage tank.

Haulers deliver biosolids from municipal treatment plants and limited amounts of septage to avoid conflict with the nutrient management operation. They discharge septage into a chamber with a half-inch bar screen that connects to the 10,000-gallon concrete holding tank. Biosolids are pumped directly into the tank.

During offloading, a Purafil odor-control system from F.J. Nugent & Associates creates a negative pressure in the tank. Odorous air is drawn through a carbon-based filter before discharging through a vent to the outside. A direct line from the Eganville plant transfers digested biosolids to the tank (about 265,000 gallons per year).

A 5 hp Flygt submersible pump in the holding tank circulates the contents, producing a homogenized mixture. The staff collects samples to observe the condition of the liquid and to perform jar tests that determine how much Drewfloc 2445 polymer (Ashland Inc.) to add.

The make-down system mixes water with polymer concentrated at 0.4 to 0.8 percent in a 5,800-gallon tank. When the holding tank pump activates, a progressive cavity pump injects the polymer solution into the stream before it enters a manifold for additional mixing and reaction time. Then the chemically conditioned solution is pumped to one of six 30- by 46-foot GT500 Geotube units through a 4-inch hose with quick-disconnect fittings. It takes three to four hours to process an 8,000-gallon batch.

 

Drip dry

“As soon as a container begins to fill, clear, decanted water drains through pores in the textile, which retain more than 99 percent of solids,” says Andrew Polley, environmental project manager.

The bottoms of the containment pads slope 1 percent to a trough that channels filtrate through underground piping to a 10,000-gallon concrete storage tank. When the tank is full, float controls trigger a 3 hp Flygt submersible pump that sends the liquid to the headworks.

Each containment pad holds two dewatering containers laid on top of mesh to promote drainage. The containers, which rise to 7.5 feet tall when filled with 132,000 gallons of conditioned liquid biosolids, take months to dewater. During the process, the staff submits samples to the provincial Ministry of Environment (MOE) Toronto laboratory for analysis. “Metals, pathogens, and nutrients have always been within MOE biosolids guidelines,” says Polley.

A greenhouse enables dewatering to continue through winter. The structure has two heaters supplemented by solar heating, and a ventilation system to remove excess heat and moisture. “A significant portion of our cost is heating the greenhouse, and it fluctuates,” says Polley. “The winter of 2008-09 was so cold that we added more heaters, but we had extended periods of mild weather and sunshine the next year.”

A year after topping off a container, the staff opened it for a technology demonstration. “The outer layer was a dry friable brownish material, and the core more grayish,” says Polley. “The biosolids, in the 10 to 30 percent solids range, loaded easily into a dump truck and had no significant odor.” The material was applied to farm fields.

Dewatering, including equipment and labor, costs on average $17 or less per cubic yard, saving the township $8 per cubic yard or some $50,000 per year. The containers also provide more than a year of storage capacity, enabling the staff to supply the biosolids when the farmer needs them most.

Note: This article was complied from a report by Andrew Polley, environmental project manager, Township of Bonnechere Valley, Pembroke, Ont.



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