What do we mean by On-site and Decentralized wastewater treatment?

Although the terms on-site and decentralized are often used interchangeably when discussing small off-grid wastewater treatment solutions, there are significant differences in what we mean by the two terms. Decentralized wastewater systems treat wastewater at or near the source of generation either for safe discharge into the environment or, increasingly, for local, non-potable reuse. Importantly, across the spectrum of decentralized solutions there are significant differences in various technologies are designed, operated, maintained and regulated.

On-site systems are a sub-set of decentralized solutions, designed to treat the sanitary wastewater from residential property or properties on a single lot and usually for sub-surface disposal on that property. These on-site technologies range from very basic septic systems to more advanced aerobic processes designed to reduce the organic, solids and nutrient loading of the final effluent significantly. In many areas, on-site solutions are regulated by local building codes and may have certification by various agencies. Typically, there is also a maximum daily capacity of on-site systems that may be regulated by these local codes.

The term Decentralized is a more appropriate description where:

1. The wastewater is a sanitary discharge from more than the residence(s) on a single property, for example from a cluster of homes, a motel or similar, or

2. The daily volume of sanitary wastewater is greater than the upper limit prescribed by the local authorities, for example if over 10,000L per day in the Province of Ontario, or

3. The wastewater is other than a sanitary discharge, for example from a shopping mall, a craft brewery or similar, or

4. The final effluent is discharged into a surface water body.

For this broader range of applications, solutions range from simple septic system to advanced processes that may include advanced aerobic or anaerobic technologies such as Membrane BioReactors (MBR’s), anaerobic digesters, ultra-violet disinfection and resource recovery options. In most cases, permits to construct and operate these systems will require project-specific assessment and approval by a state/provincial or federal environmental agency.