Category (e.g., SW, CSO, STUDY, CSO/SW): CSO/SW
Name of study: Pollution Prevention and Control Planning Study
Area of concern: St. Clair River
Keywords: combined sewer overflow, detention tank, sewage treatment
Pollution control objectives for the Pollution Control Plan (PCP) were established by the Technical Steering Committee as being:
|To reduce the overall pollutant loading from Sarnia to the St. Clair River;|
|To improve the water quality in Sarnia Bay and at the waterfront, thereby increasing the opportunities for recreational activities.|
The City of Sarnia is located at the southern end of Lake Huron, on the shore of St. Clair River, and on the border between Ontario and the State of Michigan. The study area covers approximately 14,000 ha, and the city core serviced by older combined sewers, shown in Figure 1, covers an area of 650 ha. The predominant land use in portion of Sarnia serviced with sewers is urban residential. Industrial land use comprises approximately 30% of the sewered area, with majority of the heavy industries being involved in petrochemical processing and storage. Approximately 5% of the sewered area is used as parks or open space, while the agricultural land covers approximately 50% of the overall study area.
Figure 1. City of Sarnia Study Area
All three municipal sources of pollution, the Sarnia Water Pollution Control Center (WPCC), the combined sewer overflows and storm sewer outflows need to be addressed to achieve the pollution control objectives. Information related to a broad range of toxic and non-toxic pollutants were examined. Suspended solids, BOD5, total phosphorus and faecal coliforms were selected as pollution indicators of importance. The conclusions were:
|The most important source of pollution to the St. Clair River in the reach between Lake Huron and Sarnia industrial sector, is the Sarnia WPCC. On average, the effluent from the primary sewage treatment plant contributes most of the suspended solids, BOD, nutrients and coliforms discharged to the River from Sarnia. It also contains trace metals and organics of concern, which may originate from domestic, commercial and industrial sources.|
|Based on wet weather days only, storm sewers contribute 70% of suspended solids load and associated pollutants originating from municipal outfalls.|
|Combined sewage overflows contributes a small percentage of the pollution load to the St. Clair River. However, they are responsible in large part for the bacterial contamination of the Sarnia waterfront.|
|Pollution from the municipal sources in Sarnia are responsible for restrictions on recreational activities, and may be partly responsible for the restrictions on fish consumption and the degradation of the downstream benthic communities.|
|A reduction in the pollution from the combined sewer overflows and storm sewer outfalls will significantly improve the aesthetic and recreational value of the downtown Sarnia waterfront. A significant reduction of the pollution to the St. Clair River will only be achieved by improving the quality of the Sarnia WPCC effluent.|
Pollution Control Planning Alternatives
The following Pollution Control Planning Alternatives were identified and assessed to address pollution from Combined Sewer Overflows (CSOs):
The distributed storage option was selected over the other sewer overflow alternatives, mainly on the basis of cost-benefit analysis. It was found that a distributed storage approach would achieve, at a relatively low cost, acceptable pollutant reductions at the waterfront. This would be achieved by intercepting a significant percentage of the sewage and treating it at the central treatment facility. From an economic comparison of various combinations of storage volume and interception/treatment rates, it was found that the existing sewage interception rate to the WPCC should be maintained. One of the storage tanks, the Devine Street storage tank, with a 10,740 m3 volume designed to intercept and defer overflow from 40% of the citys combined area, was constructed in 1996. Figure 2 shows the construction and the finished look of the Devine Street storage tank.
Figure 2. Devine Street Storage Tank
Five sewage treatment planning alternatives were considered:
An upgrade of the existing treatment plant to provide secondary treatment was the only option which would allow the pollution control objectives to be met. Two options were analyzed for the provision of secondary treatment: using the conventional activated sludge process and using a modified (step feed) version of the activated sludge process which gives more operational flexibility. The latter alternative was selected as it is better adapted to the variations in flows from a combined sewer system such as the one in Sarnia. Figure 3 presents a schematic diagram of the Sarnia WPCC with the required additional facilities.
Figure 3. Sarnia WPCC Expansion Schematic
Alternatives for addressing the pollution from storm sewer outfalls were quite varied in their approach and effect. Solutions considered included:
The stormwater option pond was selected as it provides the highest level of treatment and will assist in reducing the pollutant loads to both the waterfront and the overall St. Clair River. It represents an opportunity for the establishment of a natural setting within the urban core. It also captures combined sewage flow intercepted by the storm sewers at overflow structures upstream in the sewer system.
The PCP for Sarnia integrates pollution prevention measures with the preferred structural abatement solutions described above. The pollution prevention measures that are recommended for the Sarnia PCP encompass runoff quantity and quality control, water conservation and residential and industrial liquid waste management. The Sarnia pollution prevention strategy includes:
|combined sewer flushing programs;|
|street cleaning programs;|
|monitoring and enforcement of the sewer-use-by-law;|
|public education programs and policies to promote: water conservation, disconnection of foundation drains from sanitary and combined sewers, disconnection of roof drains from storm sewers, reduced use and proper disposal of domestic products containing hazardous materials;|
|education and information program to assist small industries and commercial activities in the use of pollution prevention technologies and practices.|
The overall performance of the project, in terms of pollutant reduction to the St. Clair River, is predicted to be 56% for suspended solids, 55% for BOD, 26% for phosphorus and 99% for faecal bacteria. The preliminary capital cost of the total project was estimated at $48.5 million, excluding land costs and municipal programs for pollution prevention, The proposed implementation plan is shown in Table 2, which may be affected by the economic conditions.
Table 2. Sarnia PCP Implementation Plan
Contact: Peter Seto, Burlington Environmental Technology Office