Treating Fat, Oil and Grease in Wastewater

Treating Fat, Oil and Grease in Wastewater

Fat, oil and grease (FOG) is a by-product of cooking foods such as vegetable oils, meats and dairy products. Food Service Establishments such as restaurants, takeaways and pubs are a prime source of FOG, which can pass into the sewer system during cleaning processes. When hot, liquid FOG can easily enter sinks and drains, however when FOG cools, it solidifies, blocking and damaging sewer pipes and causing problems during the wastewater treatment process.

In this article, we examine the challenges associated with FOG, and the various treatment methods available to disperse or eliminate it.

Challenges caused by FOG in wastewater

FOG entering the wastewater system causes several problems, both in the drains and wastewater collection system, and in wastewater treatment plants themselves.

Challenge: FOG in drains and in the wastewater collection system

When fats, oils and grease enter the drainage system they mix with food and other sanitary waste, congeal and harden in the pipe. In addition to unpleasant odours, FOG-related blockages can result in sewer overflows due to reduced capacity or burst drain and sewer pipes. This can have significant environmental consequences including land contamination and risk to public health.

To avoid blockages, problematic sewer lines require frequent jetting and chemical usage to disperse FOG. Unfortunately, this can cause problems with lift stations and sewage pumps downstream.

Challenge: FOG in the wastewater treatment plant

Once FOG reaches the wastewater treatment plant, it can cause further issues. Grease is more difficult to biologically degrade than other common components of municipal sewage.

Moreover, FOG can congeal and form deposits on the surface of settling tanks, digesters, pipes, pumps and sensors. Grease may also partially block screens and trickle filter systems, clog sludge pumps and, in large volumes, inhibit the activity of sludge digesting micro-organisms. Other problems include excessive foaming, increased sludge volume and issues with floc forming Nocardia bacteria.

Treatment solutions for FOG in wastewater

Treatment for FOG is tri-fold. The most effective solution is to ensure that FOG is stopped at source and does not enter the wastewater system. Thereafter it should be removed from the sewer network before it reaches the wastewater treatment plant. Finally, there are methods to deal with FOG at the wastewater treatment plant. Traditional methods include introducing bacteria or enzymes to attack FOG, while newer methods involve stimulating existing bacteria, prompting FOG to break itself down.

Removal at source

The most effective way to control FOG is at source. Many Food Service Establishments employ a passive grease interceptor unit, installed in the kitchen greywater outlet to separate FOG from the restaurants’ sullage. The brown grease collected in the trap or GI must be disposed of appropriately. To maintain their efficacy, grease interceptors must be regularly emptied and cleaned. Find out more about maintaining grease interceptor efficiency in our Interceptor Maintenance Guide.

Removal from drains

Once FOG has coalesced in the sewer system, it may be necessary to manually remove it, particularly from deposition ‘hotspots.’ These locations will be more prone to FOG blockages due to: proximity to residential, commercial or industrial areas; the characteristics of the sewer pipe, such as its diameter or condition; and the characteristics of the sewer network, such as the number of inflows and effluent volume. Where pipe repairs, or changes to cleaning and maintenance remain ineffective, the only solution may be manual collection, which carries a high man-power requirement and associated health and safety risks.

Removal at the WWTP

At the wastewater treatment plant, FOG removal traditionally involves the addition of biological additives to break down and consume grease buildup. This process, known as bioaugmentation, uses bacteria-forming enzymes to break the bond between glycerol and fatty acids. These elements can then be disposed of by biodegradation, whereby bacteria micro-organisms are able to feed on fat, sugar and starch wastes. Long-term, bioaugmentation typically requires the regular injection of specialised organisms, since introduced organisms do not tend to keep pace with a system’s main population.

Newer treatment methods focus on the stimulation of existing bacteria to break down FOG, rather than introducing foreign enzymes and bacteria. This approach utilises optimised fermentation-based yeast proteins, micro-nutrients and specialised surfactant chemistry, dosed at only a few parts per million, to stimulate the indigenous bacteria population and accelerate natural biodegradation. This biocatalytic effect can provide various operational benefits at the wastewater treatment plant, including reduced sludge production, increased nutrient uptake and improved settlement.

Fat, oil and grease present a serious challenge for our wastewater treatment networks. Ideally FOG should not enter the network, but when it does the appropriate treatment must be selected to control the problem. Enva have partnered with Next Filtration UK to bring advanced NFS Next FOG Stop technology to the market, read more about this solution here.