In general, sewer odours are caused by the action of naturally occurring bacteria which feed on the contents of sanitary sewers. If enough oxygen is not provided for bacteria in the sewers, the result may be the production of odourous gases. This can occur in forcemains where the sewage is pumped for long distances in the absence of air. It can also occur in long flat sewers where long retention times and lack of aeration can cause oxygen depletion.
Generation of Sewer Odour in Sewage Collection System
Edmonton’s sewage collection system has two characteristics that generally affect the nature of sewer odour generation, and hence the approach in mitigating the problems:
- The collection system consists of deep, large diameter trunk sewers that flow by gravity to sewage treatment facilities. A shallower collection system of smaller diameter sewers drops sewage from neighbourhoods into this system of trunk sewers. As a result of this system design, Edmonton has several hundred drop shafts ranging in drop heights of 6 to 40 meters.
- The older central area of the city was designed with combined sewers, which means these trunk systems take both sanitary and storm flows. There are many openings to the streets to take surface storm water. These openings provide many passages for air movement from sewers to ground level.
Drop shafts provide a constant aeration of the sewage to reduce odour generation potential. As a result, the gases in Edmonton’s sewer system have relatively low concentrations of offensive compounds.
Many drop shafts, however, are also the source of odour problems. By the action of falling liquid, the drop shafts push large volumes of air into the deep tunnel system. This excess air volume builds up air pressure and is generally dragged downstream of drop shafts and can be released at street level catch basins in the combined sewer areas or from manhole lids.
Solutions to Sewer Odours
In order to limit the build-up of odourous gases in sewage, the City has design standards that limit the retention times for sewage in forcemains and pump stations. The City has also implemented design requirements for new drop shafts, with the intent of limiting the effect that these shafts have on air pressurization in downstream trunks.
In Edmonton, the ventilation of our sewer system is the issue that most effects the release of odour. Several retrofits have been constructed to existing drops shafts with the intent of optimizing the design of recirculation systems, where pressurized air at the bottom of drop shafts is recirculated to the top of drop shafts.
The City has avoided using chemical treatment to mitigate sewer odours. A biological treatment has been applied at some pump stations to deal with odour and grease. This has been reasonably successful.
It is the City’s approach to consider constructing odour control facilities only when other options of treating the causes are not possible. These facilities target the air phase of the problem by extracting air and treating it.
Two larger air extraction facilities in the Kenilworth and King Edward Park neighbourhoods have been constructed, as well as smaller units in other areas.
Recently a small facility has been constructed in north Edmonton to control odours from Lake Valencia. This is an unusual case where odours from a storm water pond are treated with an air extraction facility.
The City’s odour control program is based on complaints from citizens. Initially, complaints are investigated by Drainage Operations.If odours can be corrected by maintenance such as flushing, they can often reduce the problem. If the problem is systematic in nature and requires engineered solutions, the Design and Construction group will analyze the causes, propose solutions and put the area on a priority list. Most cases will require consulting with the community or residents to arrive at acceptable solutions.