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Finding our way through the FOG

News & Opinion
24 February 2016

Fats, oils and greases (FOG) in the sewer network are like excess cholesterol in the human body. We understand by now the need to reduce the amount of cholesterol in our diets, but why don’t we appreciate the need to reduce FOG? Keith Hutchings attended a recent conference on the subject and provides his thoughts.

Currently FOG is flushed down the drains into the sewers, eventually ending up in the wastewater treatment works. However, it causes untold problems by the time it gets there. So what can we do to solve this issue?

The vast majority of FOG comes from kitchens, whether at restaurants and fast food outlets, or at factories where food is prepared. The United Nations estimates that each person disposes of 22 kgs of vegetable oil per year – that’s 1.4 million tonnes per year in the UK alone. This should provide a sense of the scale of the problem. The issues caused by FOG have even made the news recently – you may have heard of the ‘fatbergs’ that are clogging up London’s sewers.

It is estimated that between 70% and 80% of all combined sewer overflows (CSO) – discharges from our sewers into our rivers and seas – are directly attributable to FOG build up. This is a significant problem for municipal water companies, as discharges such as these are environmentally damaging, generate negative publicity and may result in fines from the national environment authority.

FOG isn't just fat, oil and grease

One problem is that FOG comprises more than just fat, oil and grease; once in the sewer it mixes with detergents, soaps and other surfactants, which change its chemistry, and it also collects the ever-present ‘flushable’ wipes, as well as grit and other solids. It clings to the walls of the pipes and restricts the flow, and over time the pipes get smaller and smaller – similar to the way that cholesterol clogs and restricts blood vessels in the human body – until there is nowhere for the flow to go but out to the local watercourse or out through a manhole.

Any FOG that does not accumulate in the pipes passes through to the wastewater treatment plant, carrying the collected wipes and solids with it, and adding significantly to the cost of treatment – to continue the cholesterol analogy, you might think of the treatment plant as the liver – Thames Water have predicted that if they could prevent 60% of the FOG getting to the works then it could save them £12 million per year.

Prevention rather than cure

The overwhelming desire amongst delegates at the recent FOG: Means & Opportunities conference seemed to be that we should prevent the FOG from entering the sewer in the first place. The trouble is, how do we achieve this? Every restaurant and commercial kitchen may be fitted with a grease trap, but considering the amount of FOG in our sewers it is highly likely that they are poorly maintained and are not regularly emptied. There is no regulation at the moment to enforce this maintenance because it is difficult to point the finger of blame at one restaurant in a long line of them when the sewer blocks.

If we are committed to prevention then perhaps monitoring is the key – perhaps we need a ‘cholesterol test’ for FOG. If there were a way of logging the amount of grease that passed through a grease trap then the kitchen could see how it was contributing to this problem and be held to account by the relevant local authority for a pollution event.

Until such time as these measures are put in place, however, it is difficult to see how we can be anything more than reactive.

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