Rural communities are frequently disadvantaged by the lack of commercial services typically available in larger municipalities. The Thornton Crossing Commercial Plaza, built in a rural community North of Toronto, is an example of an environmentally sustainable development project. The project is distinguished by the construction of an onsite wastewater treatment plant that incorporates advanced tertiary wastewater treatment processes.
On a small property in the rural community of Thornton, Ontario, Unifay Fedar Investments of Mississauga wanted to develop Thornton Crossing, a commercial plaza housing a variety of restaurants, including a Tim Hortons, a gas station and some shops.
Thornton is not serviced by a municipal wastewater treatment facility and a condition of property development required the construction of an onsite treatment plant to treat wastewater. The most significant contributor of wastewater strength to the system is undoubtedly the Tim Hortons restaurant. To accurately estimate the loading on the system, data from several existing locations was analyzed and
the wastewater characteristics were defined. This analysis is critical because wastewater strengths in commercial facilities are typically four to 10 times that of typical municipal wastewater.
Following extensive consultation with the Ministry of the Environment (MOE), it was determined that the effluent was to be disposed of on the property through subsurface discharge. The complete Certificate of Approval
(C of A) effluent requirements are shown in Table 1, with the major challenge being a nitrate level of 3.0 mg/l or below. Inherent in such a stringent requirement is a need for tertiary treatment processes for both nitrification and denitrification.
The company selected to provide the process design, engineering and equipment supply was Seprotech Systems Inc. of Ottawa. This environmental technology company owns a number of proprietary technologies and, based on the client requirement, the engineers selected a combination of aerobic and anoxic treatment technologies. At the heart of the aerobic process is an extensively modified Rotating Biological Contactor (RBC) design trademarked Rotordisk.™ This is biological process whereby bacteria are grown on a media that is mounted on a shaft and rotated into and out of the wastewater. The process is extensively modified over a conventional RBC by incorporating a series of sludge digestion, recirculation and aeration functions. This technology was selected based on the advantages of low energy consumption and low operator and maintenance needs.
The client wanted an operator to visit the plant no more than twice per week
for no more than two hours at a time. The anoxic portion of the plant that performs the denitrification uses a submerged fixed-film biological growth system. In this case, the bacteria are grown on rectangular sections of media that are submerged and a motor maintains a forward and backward movement that provides conditions of controlled growth of bacteria on the media. This technology is known as Submerged Dynamic Media.
The Rotordisk™ integrated with Submerged Dynamic Media was chosen as it was an ideal choice for the Thornton Crossing installation for two important reasons. First, it could be custom designed to treat the very high strength waste streams generated by the restaurants and commercial facilities. Second, the plant design required a small footprint and it needed to be architecturally designed into the complex and have the appearance of a small outbuilding emitting no noise or odours. The client’s engineering firm selected
subsurface discharge through ‘shallow buried trenching,’ an engineered dispersion technology that allows for tile beds to take on any shape desired by the engineer. This important design detail has allowed the footprint of the tile field and plant to be only a fraction of the area that is normally required for a similar sized operation.
The treatment process is shown in Figure 1 and summarized as follows:
- Wastewater is pumped from a wetwell to the Primary Settling Tank (PST).
- The PST removes readily-settleable solids and floating material and thus reduces the suspended solids content of the wastewater.
- The wastewater flows by gravity to the Rotorzone of the Rotordisk.™ The first two stages of the Rotorzone reduce organic strength (Biological Oxygen Demand BOD) to an advanced tertiary standard using naturally cultivated microorganisms, which form a film on the surface of each disc. As the discs rotate, the inactive film of microorganisms sloughs off and a new active film is regenerated in a continuing cycle. A portion of the microorganisms slough off and the remaining BOD consumed converts into carbon dioxide as it is used in the metabolism of the bacteria.
- The third and fourth stages of the Rotorzone are for nitrification – the bacteriological conversion of ammonia nitrogen to nitrates. In the nitrification reaction ammonia is oxidized first to nitrite and then to nitrate.
- The wastewater exits the Rotorzone, is dosed with acetic acid, and enters an anoxic Submerged Dynamic Media zone where nitrates (NO3-) are oxidized into nitrogen gas (N2).
- The wastewater flows into a Final Settling Tank (FST).
- The wastewater is pumped through multi-media filters (MMFs) to eliminate any remaining solids and flocculants. From there, the wastewater is pumped into a holding tank that discharges into the shallow buried trench system.
Despite the extremely high strength of the wastewater, the system has performed well. The tenant base in the plaza continues to evolve and the treatment plant is able to adjust and accommodate the changes. This project is a solid example of how the selection and application of advanced treatment technologies results in environmentally friendly and sustainable treatment of wastes, while at the same time giving the developer of the shopping mall the ability to develop the site in exactly the same way as in an area serviced by municipal infrastructure. The greatest beneficiary is the local community that has access to services they would otherwise lack.
Article originally published in Influents Magazine, Summer 2009
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