By Peter Davey
Sewer clogging “fatbergs” that are made up of congealed fats, oils, grease and garbage are a menace to sewage systems, costing utilities time and money to break down and remove. Yet fats, oils and grease (FOG) are a tremendous energy source that can be broken down into biogas through anaerobic digestion. New research from the University of British Columbia (UBC) shows that FOG can be “busted” with a pre-treatment process that allows more FOG to be loaded to digesters, while keeping it a liquid at lower temperatures.
The process uses microwave radiation to heat FOG to temperatures of 90°C to 110°C. While heating, hydrogen peroxide is simultaneously added in small doses. At high temperatures. hydrogen peroxide disassociates into hydroxyl radicals, which are very strong oxidants. The combination of high temperatures and hydroxyls rapidly break down organic matter in a process that the researchers call microwave-enhanced advanced oxidation (MW-AOP).
UBC research associate Asha Srinivasan and her team looked at applications of MW-AOP technology in municipal sewage sludge and dairy manure treatment. FOG is often added as a co-digestant with dairy manure since manure itself is not a good substrate, owing to the efficiency of the cow’s digestion system.
However, Srinivasan said that when manure is co-digested with FOG, typically no more than 30% of the feed can be FOG. Any more might inhibit microbial growth in the digester, clog pipelines and create other nuisances. Yet, when FOG is treated with MW-AOP, it can make up to 75% of digester feed.
According to Srinivasan, the process breaks down and reduces the volume of solids in FOG by as much as 80% and breaks down complex organic molecules into volatile fatty acids. Micro-organisms in anaerobic digesters can more easily break these down, boosting biogas production. Another benefit is that treated FOG remains in a liquid state at temperatures even slightly lower than room temperature.
Municipal FOG and sludge treatment
While the UBC research team initially looked at dairy manure and their recent paper focused on treating FOG on its own, Srinivasan said she believes the benefits will hold true for digestion at municipal wastewater treatment plants (WWTPs).
One of the benefits for WWTPs is the ability to process collected FOG and used cooking oil and use it to produce more biogas, which in some cases can be sold to energy utilities. “This process can be directly retrofitted into an existing sewage treatment plant and go before the anaerobic digester unit,” said Srinivasan. “Without any huge modifications, we can directly retrofit the plant.”
MW-AOP can also be used to treat municipal sewage sludge, breaking down suspended solids content and reducing sludge volume by up to 80%, according to Srinivasan. It also increases the volatile fatty acid content in sludge, allowing anaerobic digestion to speed up.
With faster processing, WWTPs can lower their hydraulic retention time (HRT), which is how long they hold their sludge for. According to Srinivasan, HRT is typically between 20-40 days depending on operating conditions and the type of treatment plant.”With MW-AOP, you don’t have to hold the sludge for that long,” said Srinivasan. “You can lower the HRT by 5-15 days.”
This means that new sludge storage doesn’t have to be constructed to hold 40 days of liquid, and existing digesters and storage don’t have to be expanded for future growth. Since anaerobic digesters are very expensive structures, reducing their size or keeping existing dimensions is a huge savings.
Importance of FOG diversion
While MW-AOP technology improves the solubility of FOG so it may clog less, keeping fats, oils and grease out of sewer systems in the first place remains a priority according to Barry Orr, sewer outreach and control inspector with the City of London, Ontario.
London’s Your Turn pilot program encourages residents to collect kitchen FOG in compostable paper cups that can be dropped off at collection centres when filled. The cups are then delivered to a contractor that processes the waste in an anaerobic digester.
Orr said that in 2017, the program collected over 6,000 kg of residential FOG at four drop-off depots in the city. “That’s a very large number of people that are bringing things to us,” said Orr, adding that this program is community-based and spread mostly by word of mouth or social media.
In addition to keeping clog-forming FOG out of the sewer system, Orr said London is looking at diverted FOG as a valuable green energy source. “We know that people use a lot of oil, bacon and other meats and, if we could just capture that, there is a great opportunity for us getting it to the digesters,” said Orr.
If the Your Turn FOG diversion program catches on in other cities, UBC’s research on microwave-enhanced advanced oxidation may be a huge benefit to anaerobic digestion plants that are handling more fats, oils and grease.
“We know there is good value in the anaerobic digestion of FOG, so it’s very promising to see this research,” said Orr.
For more information on microwave-enhanced advanced oxidation, read the UBC research paper “Microwave-Enhanced Advanced Oxidation Treatment of Lipids and Food Wastes,” published in the journal Water, Air, & Soil Pollution.
For more information on the City of London’s Your Turn program, read the article “London evaluates a novel approach to FOG source diversion,” published in ES&E Magazine’s July/August 2015 issue, or visit the Your Turn webpage.
Peter Davey is with Environmental Science & Engineering Magazine.