Separating the fact from fiction

Gareth Samuel of Hepworth Drainage presents a complete profile of oil separators, from full retention to bypass, and covering issues such as the EA's guidelines for performance

Numerous constructions require the use of oil separators to prevent contamination of ground waters by petrol, oil and other hydrocarbons. There is currently no European standard governing the specification of these products, although work is under way on such a standard - BS EN 858.

Clear guidance is provided by the Environment Agency (EA) in its Pollution Prevention Guidelines (PPG3). However, since by no means all projects require its installation, it is understandable familiarity with the available options is far from universal. As a broad indication, separators should be used wherever there is a significant risk of oils being washed into the drainage system either as a result of normal precipitation or extreme weather conditions. Petrol station forecourts are prime examples but industrial units, workshops and car parks will also frequently require some form of protection. Individual car park spaces associated with private houses do not normally fall within this bracket.

However, apartment blocks and, for example, housing association developments where spaces for a number of vehicles may be provided within relatively close proximity would. Separators all work on the same principle. Essentially they store water for a time before it is discharged into the main drainage system. During this storage period oil floats to the top of the separator and road grit, etc, settles. Clean water is then drawn off from the centre of the tank and fed into the drainage network.

The length of time over which water can be stored is one of the principal factors that determines the correct choice of separator. These devices are commonly manufactured either in glass reinforced plastic (GRP) or medium-density polyethylene.

Hepworth Drainage uses the more robust medium-density polyethylene and a one-piece, seamless moulding technique to create a leak-proof construction offering long-term performance, even under the most arduous of conditions.

types of separators

The EA guideline identifies three different types and two principal classifications of separator. All are readily available from suppliers like Hepworth, which offers separator sizes from 1,000-9,000 litres capacity to suit a broad range of operating requirements. Separators are described as full retention or by-pass. There are also higher performance products developed specifically to deal with the conditions encountered on garage forecourts.

Full retention separators are capable of treating the full flow delivered by the drainage system, based on a 50mm/h rainstorm. They can incorporate additional filters and automatically shut-off when the tank is full to prevent overflowing of contaminated waste. By-pass separators are design to treat about 10% of the flow from extreme rainfall intensities to which full retention designs are tested. Flows above this level are allowed to by-pass the separator. They are generally used where there is a lower risk of pollution for the drained area. Forecourt separators are full retention designs capable of accommodating the contents of one compartment of a road tanker - up to 7,600 litres. Products are described as being either Class 1 or Class 2. Class 1 separators are designed to achieve a concentration of less than 5mg/l of oil under standard test conditions. Class 1 separators would normally be used in high risk areas, which discharge directly to rivers or water courses. Class 2 separators must achieve a concentration of less than 100mg/l under the same test conditions.

This lower requirement applies where less pure discharges can be tolerated, for example, for trapping spillages and where effluent will ultimately pass to a foul sewer.

the right decisions

Specification decisions, therefore, are based on the nature of likely run-off from the site, the degree of risk and whether the drainage is linked to a foul or surface water sewer. (The EA PPG includes a helpful flow chart to assist in determining the correct solution for different conditions.) While all separators work in the same way, considerations when determining the best product will include such factors as versatility in terms of inlet and outlet pipe sizes and the ability to accommodate different vent pipe positions. Simple features like wide, flat bases and moulded lifting eyes can also contribute to ease of installation. Manufacturers normally supply a range of turret units to accommodate installation depths greater than 1m and up to 1.75m. The size of separator specified will be based primarily on flow calculations. However, some local authorities also set additional requirements. Separator installation is straightforward but will, to an extent, be dependent upon ground conditions. In dry ground conditions the trench bottom is covered with a wet concrete bed to a minimum of 150mm and haunched around the base of the separator.

The remainder of the trench is then backfilled with pea gravel. In wet ground conditions it is normal to use 200mm hardcore and blinding as necessary and then to lay 500 gauge polythene sheeting. A wet concrete bed is haunched up around the base of the tank and concrete, rather than pea gravel, backfill is used. It is apparent that, over time, separators will become full of oil so periodic maintenance and emptying is required. There is no set time-frame for maintenance as this will vary considerably according to the installation circumstances. However, the EA guidelines require oil level alarms to be fitted and these provide a simple means of establishing when the separator needs to be cleaned. These devices can be mains or battery-powered and fitted directly to the separator or at some remote location. They produce both a visual and an audible warning when the oil level reaches a critical level.

This is usually when 90% of the available storage volume has been used, for example, when a 75mm thick layer of oil has been retained or when there is an emergency spillage.

taking care of units

Irrespective of the activation of oil warning level alarms, it is good practice to inspect separators every six months and maintain a log of such inspections. Effective maintenance can also be facilitated by the addition of sampling chambers. These units are designed to be installed downstream of oil separators and allow easy access for effluent sampling. Hepworth's standard sampling chamber is designed for drain invert depths to 1m but it can be cut down for shallower depths.

The responsibility for inspection and maintenance lies clearly with the property owner. However, water companies and the EA do carry out occasional spot-checks. These relatively simple approaches to oil treatment have a key role to play in protecting our watercourses and offer long-term performance at minimal cost. They are the unsung heroes of water protection - but ones which we ignore at our peril



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