The South Red Deer Regional Wastewater System (SRDRWS) is a transmission system that extends from the Town of Olds to the City of Red Deer Wastewater Treatment Plant (CRD WWTP).
It includes the transmission of wastewater from the communities of Olds, Bowden, Innisfail, Penhold, Mountain View County, and Red Deer County to the CRD WWTP through 90 km of force main. Wastewater is treated at the plant to meet Alberta Environment’s standards. Major upgrades to the WWTP were undertaken to accommodate the increased flows.
Due to the length of the transmission system, extended sewage retention times were anticipated, with the potential for odour release, particularly where the SRDRWS force main joins the City of Red Deer gravity system at the east entrance chamber, and when wastewater reaches the CRD WWTP. The South Red Deer Regional Wastewater Commission (SRDRWC) had made requests to provide odour treatment in order to reduce its impact.
It was determined at the conceptual design stage that the east entrance chamber was the best location to implement an odour management system. Objectives included:
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- Preventing fugitive odour emissions from the proposed east entrance chamber.
- Effectively accommodating sulphide generation in the SRDRWS flow component.
- The SRDRWS should have little or no negative impact on the operation of the WWTP.
The impact of hydrogen sulphide
One of the most odourous substances produced in domestic wastewater is hydrogen sulphide. It can exist as a gas and as a dissolved ion. Hydrogen sulphide is found in almost all wastewater systems to some extent, and even in low concentrations can be detrimental to human health.
Sulphide generation is a biological process occurring in the submerged portion of sanitary sewers. Fresh domestic sewage entering a wastewater collection system is usually free of sulphide. However, a dissolved form of sulphide soon appears as a result of low dissolved oxygen content, long detention time, elevated temperatures, high BOD and high sulphate.
The first step in this biological process is the growth of a slime layer below the water level in a sewer pipe or force main. This slime layer is composed of bacteria and inert solids held together by a biologically secreted polysaccharide “glue”.
When this biofilm becomes thick enough to prevent the penetration of dissolved oxygen, an anoxic zone develops within it. Approximately two weeks is required to establish a fully productive slime layer in new pipes and, once established, it never goes away.
Within this slime layer, sulphate reducing bacteria use the sulphate ion (SO4=), a common component of wastewater, as an oxygen source for the assimilation of organic matter, in the same way dissolved oxygen is used by aerobic bacteria. When sulphate is utilized by these bacteria, sulphide (S=) is the byproduct. The rate at which sulphide is produced by the slime layer depends on a variety of conditions.
These include the concentration of organic food source (BOD), dissolved oxygen concentration, temperature, wastewater velocity, and the area of the normally wetted surface of the pipe. Because sulphide generation in sewers is a biological process, elevation of temperature also causes an increase in metabolic activity and an increase in sulphide production.
Regardless of the rate of sulphide generation, when sulphide is released back into the wastewater stream it immediately establishes a dynamic chemical equilibrium between four forms of sulphide. These are the sulphide ion (S=), the bisulphide or hydrosulphide ion (HS-), aqueous hydrogen sulphide (H2S (aq)), and hydrogen sulphide gas (H2S(g)).
The range of dissolved sulphide produced in the SRDRWS force main was high. It resulted in high atmospheric hydrogen sulphide (average H2S concentration of 200 – 400 ppm at the 25-year condition) at the odour management facility stripping chamber.
The solution: A two stage odour management facility
The solution to eliminating odour from the SRDRWS force main was to design and construct a sulphide stripping chamber with vapour phase odour control treatment. It accepts foul air from both the east entrance chamber and the sulphide stripping chamber. A stripping chamber is essentially a concrete structure with a serpentine flow arrangement (similar to a typical chlorine contact chamber) that is fitted with coarse bubble diffusers.
