In the small town of Elmira in southwestern Ontario, building foundation drains were connected to the sewer system by sump pumps, and the aging sewers were being swamped with inflow during storms.The ...
In the small town of Elmira in southwestern Ontario, building foundation drains were connected to the sewer system by sump pumps, and the aging sewers were being swamped with inflow during storms.
The Regional Municipality of Waterloo asked Stantec to quantify the problem in the sewers and also to investigate what should be done to improve the existing wastewater treatment plant. During 1997, for example, the peak wet weather inflow was 40,000 m3/d, more than eight times the plant’s average daily capacity. Overflows were running into the nearby Canagigue Creek. In addition, the rapid changes in flow were diluting the wastewater and reducing the level of treatment.
Stantec designed a “total solution” that involves both an expansion of the plant and the separation of the inflow-infiltration groundwater portions of the sewage into a new dedicated sewer of six kilometres that discharges directly into the creek. Normally these would be approached as standalone projects, but here they were treated as interrelated issues with a common solution.
For the plant, Stantec devised an adaptable treatment system that can quickly be ramped up to cope with the peak flows. Key components include:
dual on-site equalization basins to store peak flows temporarily and dampen the hydraulic surges;
a multi-compartment bio-reactor with internal recycles permitting biological nutrient removal (BNR), activated sludge, or step feed modes of operation;
oversized secondary clarifiers sized to capture and store excess biomass to have this available for recycling during peak flow events.
The normal, low-flow operating condition is the BNR mode. Although BNR — which has energy and chemical efficiencies — has been regularly used in western Canada, this is the first BNR project in Ontario. The plant also uses ultraviolet disinfection.
The treatment process is robust enough to allow for a sustained peak of 2.5 times the average influent for a period of 48 hours. Return activated sludge is continually returned to provide the necessary biomass to ensure nitrification takes place.
The site was small, measuring only 160 by 80 metres — a factor that influenced the decision to adapt the existing plant. About half the existing process tanks needed to be modified, requiring partial demolition.
Construction on the plant was completed in 2002 for $14 million, approximately $10 million less than the cost of a new greenfield plant. The savings could then be directed to separating the inflow and infiltration from the sanitary sewers, the original source of the problem. Once the sewer replacement program is complete in 2004, the expanded plant should be able to handle Elmira’s wastewater treatment for 40 years.
Client: Regional Municipality of Waterloo
Prime consultant (design and project management):
Stantec Consulting, Waterloo, Ont. (Guy Le Patourel, P.Eng., Richard Waite, P.Eng.)
Geotechnical: Naylor Engineering Associates
Contractor: Wellington Construction