Meeting Stormwater Quality Objectives with Oil Grit Separators

Oil grit separators are devices which are typically installed on a private storm sewer system that assist in the removal of sediments from storm drainage and are designed to separate oil from stormwater and keep it contained onsite for proper disposal.
There are a variety of proprietary and non-proprietary OGS’s on the market with the majority using gravity based separation for sediment removal and oil separation. OGS’s are designed for areas under 2 hectares and are typically constructed as a chamber or manhole. OGS’s are well suited for constrained spaces due to their small footprints making them the preferred solution to achieve the quality control objectives for industrial, commercial and institutional type of developments. OGS’s also provide an effective solution for containing specific types of spills onsite and are typically required by either the local municipality or the Ministry of Environment for sites which are susceptible to contaminated spills such as fueling stations and high risk industrial developments.
The quality control objectives which are stipulated by municipal and provincial approval agencies are based on downstream receiving waters and are defined under three separate protection levels. The protection levels are listed as Enhanced, Normal and Basic and are based on the long-term suspended solids removal from stormwater. In most sites within the GTA, enhanced protection is stipulated which requires removal of 80% of the total suspended solids from the stormwater for 90% of the runoff volume on a long-term average basis. The vast majority of the smaller more frequent storm events provide the majority of the long-term runoff volume.

It is important to note that this requirement is often misinterpreted by review agencies with the assumption that 90% of peak flow for every single event is required to be treated. The reality is that OGS’s are designed to treat the more frequent smaller 5 to 10mm storm events known as first flush events and allow the high flows generated from the less frequent storm events to by-pass the system. The first flush events are typically ladened with sediments and oils which are trapped by the OGS and the additional flows from the less frequent events assisting in keeping the sediments trapped within the OGS during by-pass.
Depending on site specific stormwater management objectives, OGS’s can be used as a pre-treatment option. For example, most municipalities have water balance objectives for private sites where they require a volume of water to be retained on site through reuse or infiltration. By installing an OGS upstream of an infiltration pit, the OGS will act as a filter reducing the amount of sediment to the pit, increasing its performance life.
The technical-effectiveness of an OGS can be increased and the cost of an OGS reduced by using a multi-component approach. Additional upstream lot level measures such as bio-swales, enhanced swales, rain gardens, etc. can reduce the amount of sediment draining to an OGS. It is important that the designer consider the site specific constraints and opportunities at the preliminary design stage to effectively meet the stormwater quality objectives from both a technical and cost effective perspective.

Inspection and maintenance is fundamental to the operation of any OGS. Inspections should be completed on a 6 month basis for the first year with a sediment cleanout target of sediment depths reaching 15% of storage capacity and immediately upon and oil, fuel or chemical spill.
For additional information on this topic, contact MGM directly.