When Bigger Isn’t Better: Decentralized Wastewater Treatment Systems
Many small or rural communities across Ontario are faced with the need to overhaul wastewater treatment to accommodate an increasing population, climate change or stricter environmental regulations. Historically, the solution was one of three options: build a new centralized wastewater treatment system; connect to or upgrade an existing system; or continue to rely on septic or on-site systems. However, there is another option for these communities – decentralized wastewater systems.
Centralized systems collect wastewater from entire communities where it is usually treated at a large facility and discharged into a lake or river, typically from one location. For example, the York Durham Sewer System or ‘Big Pipe’ collects wastewater from communities from as far north as Newmarket and transports it over 120 km in sewer pipes to a large sewage treatment plant in Pickering where treated effluent is discharged into Lake Ontario. In contrast, standard on-site or septic systems collect, treat and discharge wastewater where it is generated, usually from a single home. Many rural and small communities in Ontario rely upon septic systems to treat wastewater until a sewage treatment plant is built or connected to.
Decentralized wastewater treatment systems use a combination of onsite or cluster systems to treat and dispose of wastewater from houses and businesses that are located relatively close together. Cluster systems collect and treat wastewater from two or more homes, but typically less than 100. In a decentralized system, individual septic tanks or aerobic units may pre-treat wastewater from several homes onsite before it is transported though small sewers to a local treatment unit. The effluent can be discharged into the groundwater or surface water after treatment. For example, the communities of the Fields of St. Croix and Jackson Meadow in Minnesota use constructed wetlands to provide onsite wastewater treatment in unsewered areas, instead of standard septic systems. This approach allowed development of a small cluster of residential lots and the community to maintain larger areas for permanent open space. In Ontario, the Thornton Crossing Commercial Plaza, in a rural community north of Toronto, uses an onsite plant to treat wastewater from restaurants, a gas station and some shops in an unserviced area. Decentralized systems keep wastewater within the watershed, can recharge groundwater supplies and maintain stream base flows, can recover nutrients (nitrogen and phosphorus) from wastewater for use as fertilizers in landscaping and agriculture and can reuse treated wastewater (e.g., for toilets, irrigation and firefighting).
While centralized wastewater treatment plants are appropriate for many larger communities and those with industrial sewer users, decentralized systems provide an option for some small communities where large sewage treatment plants are not desired or feasible. In 1997, the US Environmental Protection Agency reported to Congress that decentralized systems can protect public health and the environment (compared to septic systems), typically have lower capital and maintenance costs for rural communities (compared to centralized systems), are appropriate for a variety of site conditions and are suitable for ecologically sensitive areas when adequately managed. The US Environmental Protection Agency created voluntary guidelines for the management of decentralized wastewater treatment systems, but no such guidance exists in Ontario.
Decentralized approaches are by no means a panacea. Unless harnessed within strong land use planning rules, misuse of this option could lead to rampant, unsustainable development on rural lands. On the other hand, traditional centralized systems have certainly not been able to prevent urban sprawl, and long-distance “big pipe” systems have encouraged urban development well beyond the carrying capacity of local watersheds. If the two approaches are compared on the basis of overall system resilience, the centralized approach has a distinct disadvantage: whenever very large populations become dependent on a single central treatment system, they also become vulnerable to catastrophic failure of that system.
The time is ripe for a public policy discussion in Ontario on the relative merits, downsides and appropriate uses of both decentralized and centralized wastewater treatment approaches. Ontario’s existing rules and policies would certainly discourage communities from considering decentralized wastewater systems. For example, most government funding programs in Ontario favour the establishment of a centralized system approach. Typically, centralized systems are funded by municipalities who often seek and receive government grants or subsidized loans to help with the costs. Conversely, developers usually fund the installation of decentralized systems and then transfer the responsibility to a management entity (normally a municipality) once it is completed. Decentralized systems tend to have more components to manage than centralized systems and, therefore, proper management (e.g., planning, siting, design, installation, operation, maintenance and monitoring) is critical to their performance.
There are many legitimate questions regarding where, how and why decentralized wastewater systems could be employed sustainably in Ontario. This is one example where the Ministry of the Environment could show leadership, and could engage the public in a dialogue on the implications, barriers and options of this approach.
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| This is an article from the 2009/10 Annual Report to the Legislature from the Environmental Commissioner of Ontario. |
Citing This Article:
Environmental Commissioner of Ontario. 2010. "When Bigger Isn't Better: Decentralized Wastewater Treatment Systems." Redefining Conservation, ECO Annual Report, 2009/10. Toronto, ON : Environmental Commissioner of Ontario. 89-90.
