Editorial Emphasis: Wastewater Treatment
Exfiltration system uses distribution chambers to protect sensitive Lake Erie
The Port Burwell Sewage Treatment Plant sits on the shore of Big Otter Creek near Lake Erie in Bayham, Ontario. The population in the surrounding area is growing steadily, creating a need for additional wastewater treatment to accommodate the growth. After a review of the possibilities, the experts decided that the most efficient option was to connect the surrounding communities to the area’s Port Burwell Sewage Treatment Plant.
This however, presented a challenge to community government officials charged with protecting the lake environment, while providing for future growth and development.
In December 1998, engineers were hired to evaluate the expansion options for the facility. The resulting conceptual design report for the plant was based on adding the projected future populations for Eden, Straffordville, and Vienna. The report determined that the treatment capacity of the existing Port Burwell plant would have to be expanded from 528 m3 per day to 1060 m3 per day in order to handle the future demand.
Prepared by Acres & Associated Environmental Limited of Niagara Falls, Ontario, the report presented three options for the expansion of the facility. Any of these three options would enable the treatment of the additional sewage flows generated due to the connection of Eden, Straffordville, and Vienna to the existing treatment plant in Port Burwell.
The first option was to construct a duplicate of the existing extended aeration plant. Another possibility presented was to construct a second facility capable of treating the additional sewage flows using new treatment technologies such as sequencing batch reactors (SBR). The third was to retrofit the existing extended aeration facility with new treatment technology (SBRs) and Infiltrator® High Capacity H-20 distribution chambers to treat total flows generated by both the existing and newly connected populations.
With any of these three options, the expanded plant needed upgrading to provide sludge digestion and storage facilities to replace the existing sludge-holding tank. This was necessary to allow for the continued application of processed biosolids to the land in compliance with Ontario Ministry of the Environment (MOE) policy. MOE policy also included a provision for 180 days of sludge storage so that biosolids are not land-applied on frozen or snow-covered ground.
Based on the evaluation carried out in the report, engineers from Acres & Associated Environmental Limited recommended that the existing plant be retrofitted with SBRs to expand the treatment capacity of the plant to 1060 m3 per day. The MOE approved the modified installation with the stipulation that an upgraded outfall structure that would not discharge directly into the mouth of Big Otter Creek be included as part of the expansion. This stipulation was in response to the limitations of the existing outfall structure, comprised of a gravity sewer discharging to a pump station approximately 30 meters west of the harbor wall. The pump station then pumps the effluent directly into the mouth of Big Otter Creek.
“The original late 1970s concept for the Port Burwell plant was to include an outfall pipe extending out into Lake Erie with a multi-port diffuser. This facility was not part of the construction when the plant was built in 1984. In its place, a two-port diffuser was installed at the mouth of Big Otter Creek and it was subsequently upgraded to a pumped discharge,” commented Geoff Burn, Manager of the Municipal Infrastructure Division at Acres & Associated and Port Burwell project manager.
Burn added that construction of a new outfall into the lake would be expensive because of the uncertain nature of the lake bottom around the mouth of the river. A submerged pipe would have to be placed deep into the lake bottom to provide protection against the migration of sand. Because the water is shallow for some considerable distance into the lake, the practicality of effluent dispersion from a multi-port diffuser was also unclear unless the diffuser could be located in an area where the depth exceeded 8 to 10 meters. It was determined that both of these factors would add considerably to the project cost.
Following detailed investigation and discussions with the London office of the Ontario Ministry of the Environment, Acres and Associated recommended that the outfall from the treatment plant be converted to an exfiltration gallery. This gallery would provide sufficient capacity to discharge the effluent from the SBRs on a sequential basis including a 25 percent surcharge.
“The benefit of providing effluent filtration at a treatment plant is the typical reduction in effluent phosphorous levels. Where filters have been used, effluent levels can be achieved to less than 0.3mg/l. At Port Burwell, additional phosphorous will be removed from the effluent through the filtering action provided by the natural sand found on the beach beside the plant,” Burn said. The exfiltration gallery will allow for absorption of the suspended solids in the effluent. This will contain the phosphorous in the sand, thereby removing its impact from the lake water. The proposed effluent discharge method will provide some environmental benefits over the traditional multi-port lake diffuser.
The exfiltration gallery at Port Burwell includes approximately 220 meters of Infiltrator High Capacity H-20 distribution chambers and uses bottomless culverts over a 2000 m2 area. The gallery will be located in the existing plant outfall easement adjacent to the treatment facility. As proposed, the exfiltration gallery will have capacity to contain one full discharge from one SBR including a 25 percent surcharge. It is proposed to divide the bed into two sections, one for each SBR, with interconnections so that one bed can be used by both SBRs if desired. The design will include inspection ports and manholes so the bed can be physically inspected.
The construction of the bed includes 300 mm of clear stone as a base. The chambers are installed on top of the stone and are covered with filter fabric followed by native sand, topsoil, and vegetation that are slightly mounded over the bed. The entire bed area is excavated to a depth of approximately 1.2 meters.
As the groundwater level fluctuates, it is possible that the bed will become submerged. In this scenario, the hydraulics of the plant will ensure that the plant effluent will enter the bed and filter into the groundwater. Depending on the level of the groundwater, a surface breakout of the bed may be possible. If this ever occurs, it would be very infrequent. Given the quality of the plant effluent and the fact that ultraviolet disinfection will take place, this is not considered a major concern. In the case of such a surface breakout, the effluent would be sampled and tested for contaminants. The existing outfall will be maintained for emergency purposes only and requires manual operation of interconnecting valves. It could be used whenever there may be an occurrence of breakout in the exfiltration system.
Contractors for the plant expansion were Stone Town Construction of St. Marys, Ontario. The Infiltrator chambers used in the project were provided by C. J. Pink Ltd., of London, Ontario.
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