From the June-July issue, page 15
The Vauxhall Pollution Control Plant in London, Ontario was one of three in the city that were being overwhelmed by the extra stormwater flows during wet weather. Major flows from storms would discharge directly into the Thames River when it was too much for the plant to handle. The average and peak flow treatment capacity of the treatment plant is 20,900 cu.m/d and 34,640 cu.m/d, respectively.
Sending untreated sewage flows into the river was a major environmental concern, especially since the river was designated a Canadian Heritage River in 2000. Periodically wet weather overflows into the Thames had exceeded 100,000 cubic metres per day.
Dillon Consulting was retained by the city to design the upgrades to the plant to resolve the situation. The key objective was to reduce the number of bypass events, and to improve the level of treatment of the effluent that was discharged into the river.
The engineers were able to upgrade the facility, some sections of which date back more than 80 years, into a state of the art operation. It is now one of the first Ontario municipal wastewater treatment plants to have a wet weather strategy that involves flow diversion, chemically enhanced primary treatment, and chemically enhanced secondary clarification processes. Furthermore, the $5.6-million upgrades pulled together technologies already in use in the industry, but reimagined them in such a way that minimal new equipment was required, giving the city cost savings. The new peak plant flows are nearly 10 times the plant’s average daily capacity.
The wet weather operating strategy is considered so innovative that it has been adopted in other Ontario plants, including Toronto’s Ashbridge’s Bay Wastewater Treatment Plant and the City of London’s Greenway Pollution Control Centre.
At the Vauxhall plant, the new inlet works incorporating screens and grit removal are sized to handle maximum wet weather flows up to 200,000 m3/d. Under dry weather flow conditions the effluent from screening and grit removal is sent to the primary clarifiers before biological treatment. During wet weather flows, effluent from screening and grit removal is split between the biological treatment step and the chemically enhanced primary clarification stage.
During high wet weather flow conditions flows up to the maximum capacity of the biological treatment step are directed to this process step. The maximum peak wet weather treatment capacity of the biological process is 50,000 cu.m/d. The increased peak wet weather treatment capacity is achieved by the flow proportional addition of a cationic polymer and iron salt to the mixed liquor ahead of the secondary clarifiers. Chemical addition enhances the peak wet weather treatment capacity of the process by retaining the mixed liquor solids in the biological treatment step. The wet weather operating strategy involves the gradual increase of flows to the biological treatment to avoid “washing out” and losing the biomass inventory in the aeration tank. This strategy requires a sophisticated level of flow control as the wet weather flows increase. Effluent from the biological treatment process is discharged to the River Thames.
Flows in excess of the maximum wet weather treatment capacity of the biological process are diverted to the existing primary clarifiers. These were upgraded to facilitate chemically enhanced primary clarification, which increases the removal of solids and associated contaminants. An anionic polymer and iron salt are added proportionally to the inflow to enhance the efficiency of the primary clarifiers, and the treated effluent is discharged to the river. The chemically enhanced primary clarification process has a maximum treatment capacity of 150,000 cu.m/d.
In summary, by using the chemically enhanced primary clarification for wet weather flows and chemically enhanced secondary clarification to increase the treatment capacity of the biological treatment process, the overall maximum wet weather treatment capacity of the plant is increased to approximately 200,000 cu.m/d. Of this flow 50,000 m3/d receives full biological treatment while the rest of the flow is treated to achieve high effluent quality for excess wet weather flows.
At the new inlet works an innovative but easy-to-operate ozonation system was installed for odour control. It includes heat recovery, thus reducing the consumption of chemicals and energy.
The new inlet works building’s main heating and ventilation unit includes an air-to-air heat recovery feature. The heat recovery system generally recaptures 40-45% of the energy for make-up air heating. To further enhance the energy efficiency of the heating system, dual ventilation rates are used along with occupancy sensors and gas detectors. cce