Diffused air from turbocompressor blowers is used to strip the dissolved sulphide (and other odourous compounds) from the solution. This also adds dissolved oxygen into the wastewater, prior to discharge into the east entrance chamber, where it is mixed with City of Red Deer wastewater. It is estimated that up to 90% of the dissolved sulphide could be stripped and released in the chamber.
The stripping chamber was designed for the 25-year (2035) peak pump cycle volume and to maximize contact time, which increases stripping process efficiency. While this provides an oversized chamber for current projected flows, it will accommodate future growth without the need for expansion.
The odour and corrosion potential within the stripping chamber is extremely high because of the nature of its intended function, that is, to strip dissolved sulphide through turbulent aeration. Also, other organic odour compounds are present because of the severe anaerobic environment in the force main. To protect against hydrogen sulphide related corrosion, an integral high-density polyethylene (HDPE) liner was installed on all exposed concrete surfaces.
Sizing for the stripping chamber included a cast-in-place concrete chamber 21 m long x 6 m wide. Two turbocompressor blowers (one duty, one standby) and the accompanying aeration equipment were sized for approximately 3,250 m3/h, to provide the air necessary to strip sulphide from solution.
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Air is withdrawn from the stripping chamber via two fibreglass reinforced plastic (FRP) exhaust fans (one duty, one standby) at a rate greater than the input aeration rate. This keeps a negative pressure inside the chamber while pulling air from the stripping chamber, attached east entrance chamber and 1050 mm CRD gravity trunk. This air is then sent to a vapour phase odour control system for treatment. The total airflow pulled from the stripping chamber is 4,600 m3/h.
Given the high hydrogen sulphide concentrations from the sulphide stripping chamber, a single biofilter was not adequate to accommodate expected odour concentrations. Therefore, a two-stage system was required. The first stage is an inorganic media biofilter sized for approximately 30 seconds of media contact time (empty bed resident time), requiring an overall footprint of 100 m2.
This stage is used as a hydrogen sulphide roughing filter to drastically reduce the inlet hydrogen sulphide concentration. Inorganic media was required here to prolong the media life and maximize hydrogen sulphide removal. An organic media biofilter was not a good application for these high hydrogen sulphide concentrations as the media would quickly degrade and would require frequent changes.
While a bioscrubber (bio-tower) could have been used in place of the inorganic media biofilter to accomplish this, on-site chemical (nutrient) storage and more complex operation precluded this from consideration.
This inorganic biofilter consists of two concrete chambers with a HDPE flooring system to accommodate air distribution. The inorganic expanded clay media depth is 1.2 m and each cell has both in-bed drip irrigation and ceiling mounted top spray irrigation. The units were fitted with low-profile insulated and removable FRP covers. Air leaving the first stage biofilters is captured in a pipe and sent directly to the second stage for further treatment.
