FOG Management and Control: Martin Fairley, Research Director for ACO, looks at the problems surrounding the disposal of fats, oils and greases and explores the practical solutions that can be adopted to help tackle the issue.

The disposal of fats, oils and greases, or FOG as they are otherwise known, requires careful control and management. Indeed, when designing a drainage system, particularly in commercial hot food premises, factoring in an effective solution for the correct disposal of FOG is of paramount importance. Essentially, to use a medical metaphor, FOG blocks the arteries, with the arteries in this case being the building’s drainage and ongoing sewerage system. With much of the UK depending on combined surface and foul water drainage systems, the effects of such blockages can result in significant damage to property and business. Furthermore, the problem has been exacerbated by environmental, social and economic change, with increasing rainfall intensities, a predilection for fast food and eating out in general, and the geographic concentration of commercial food premises, all having a dramatic adverse impact on our sewerage infrastructures ability to cope.

Accordingly, UK industry has responded with a push toward best practice in the commercial kitchen, with advice from leading bodies, such as Water UK, focusing on the message that prevention is better than cure. Ultimately, this means minimising the amount of FOG entering the waste stream at source, through appropriate education and management at a kitchen level. In reality, this means taking a commonsense approach to the problem by dry wiping plates and avoiding gross oil disposal to the drain, for example. Despite even the best of intentions, there will, however, always be residual FOG entering the drainage system through the washing process. That is where further inline defence comes to hand in the form of grease separators, dosing units and mechanical devices.

Conversely, guidance for the designer, client and contractor is still minimal. Even the British European standard that relates to FOG management, BS EN 1825, reflects only a small proportion of the product technologies now available to deal with the problem. It is clear that more can and should be done, and a new FOG initiative has been introduced to show the way forward, which should be coupled with a greater understanding of the methodology needed when designing a system that complies with BS EN 1825.

BS EN 1825 Grease separators: Principles of design, performance and testing, marking and quality control

Introduced in 2002, the standard covers selection, installation, operation and maintenance, with a further document, Part 1, covering design, performance, testing and quality control. Although 84 pages in total, the standard relates to one class of product – the grease separator. As such there are currently no recognised British or European Standards for mechanical skimming traps, often referred to as grease retention units, or the various biological products which can be directly injected into the waste pipe, or into a separator.

In many respects separators are the simplest of the FOG management options, relying on relative density differences to separate the FOG, which floats, and the denser solids, which tend to sink. The separation process requires time, and this directly relates to the physical size of the product for a given inflow rate. Part 1 gives all the information necessary for a manufacture to design a viable separator. Beyond the basic construction described, commercial separators now combine other technologies to enhance performance, typically automating or enabling critical maintenance functions to be performed quickly and simply. Although it is not required by the standard, systems that enable content evacuation without opening the separator, or which have high pressure water jets to break the often solid grease ‘cake’ make eminent practical sense in a busy working environment. Such add-ons can be selected as required at a later stage from the various manufacturers offering BS EN 1825 certified products.

Do I need a grease separator?

In new build or some refurbishment situations for commercial hot food premises a grease separator complying with BS EN 1825 or other effective means of grease removal is suggested in the Building Regulations approved document H – drainage and waste disposal. If the kitchen is operating already, without such provision, then installation of a separator will help keep private drainage clear, as well as the ongoing sewer.

The standard’s coverage, however, is not concerned with mineral oils, and accepts that stable emulsions are beyond its scope. In general, it is easy to appreciate that kitchen waste water quality will be improved by the use of a separator, but it is important to understand that the device should not be connected to receive effluent from the foul drain, nor should macerator waste be directed to the separator.

Selecting a grease separator

The process of selection aims to determine the size, expressed using the term nominal size (NS), which relates to the physical capacity of the unit in units of litres per second (l/s). Beyond this simple designation the designer can prescribe a greater volume available for solid settlement, as would be the case if the units were serving an abattoir with far greater solids potential than a general kitchen.

Anticipated flow is the basis for further calculation which introduces modifications for variables such as oil density, as greater density impedes separation equating to a larger unit; influent temperature, as higher temperatures equate to a larger unit; and the use of cleaning agents, as any cleaning agent use at all also introduces a size multiplying factor. To simplify the task, some manufacturers may provide a template to work through these variables.

Evaluating flow ought to be straightforward in new build, except information on equipment discharge rate is surprisingly difficult to amass, even more so in refurbishment situations. The standard helps to calculate this by providing three methodologies, including by known discharge rate as specified by the supplier or measured in use, by equipment type, where a number of typical equipment items are given together with flow rate, or by establishment type.

In the case of specification using the known discharge rate or the equipment type, the flow should be modified by a usage factor, acknowledging that devices will seldom be operated in tandem for appreciable periods of time. Adding the output together provides the core nominal size (NS), which is then multiplied by the factors pertaining to density (1.5 max); temperature (1.3 max) and cleaning agent (normally 1.3 but factors equal or greater than 1.5 are acceptable).

When sizing a separator by establishment type, the designer has to stipulate how many meals are produced per day and the operational time. Calculated NS is then modified according to the factors given above. Although apparently subjective, this method remains practically useful when discussing requirements with owners and managers who often have such information to hand.

In all cases the NS arrived at is rounded up, so a calculated size of NS 3.23 would be rounded to NS 4. Once the grease separator has been sized, decisions then relate to the available area and installation options, such as selecting products for internal above ground, free standing or below ground – often external to the building – installations. The more sophisticated automated products tend to be internal, where minimal work disruption or containment of odours and bacteria is desired.

Future developments

As stated, there is only one product standard pertaining to FOG management – BS EN 1825 – and this is limited to gravity based separation, with its design methodology being based on peak flow. Consequently, products work extremely well but can occupy significant space. Other technologies exist and may provide tenable solutions, but in the absence of objective performance, assessments rely on proprietary advice from manufacturers. Selection of a grease separator remains just one aspect of overall performance, with installation and maintenance being of similar important.

The issues of FOG management have been taken up in a recent forum following a successful conference at Cranfield University. The forum intends to form an association enabling input from all stakeholders, and connecting the issues raised with solution providers and academia in order to develop understanding, influence regulations and ultimately reduce FOG in the sewer.

For further information contact ACO Building Drainage, ACO Business Centre, Caxton Road, Elms Farm Industrial Estate, Bedford, MK41 0LF.  Tel: 01462 816666.  Email:

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