The environment team visits the anaerobic digestor that turns our food waste into biogas
“Like a stomach”… is how our guide, Alan, describes the anaerobic digestion process that is used to break down our food waste into biogas, liquid fertilizer and nutrient-rich compost.
Standing next to the tall, twin vessel AD (anaerobic digestion) plant operated by TEG in Glenfarg, on the border of Fife and Perth and Kinross councils, I listen as Alan elaborates. He explains how organic matter – our potato peelings and plate scrapings – are broken down by anaerobic bacteria under controlled temperatures in much the same way food is processed in our stomachs. If the AD plant is not ‘fed’ properly (eg, with food waste with too high a carbon content) the process will be disrupted. Yes, I get it, like a stomach!
It takes only 60 days for food waste to be processed in this way starting from the time of collection at recycling points across the University of St Andrews, to the electricity generated and sent straight to the grid by using the biogas to power a generator. With the anaerobic process monitored and controlled to produce a constant supply of 40-65% rich CH4 (methane) gas, which would otherwise have escaped from a landfill site if not recovered, our food waste is being utilised as a clean energy resource. Now that’s impressive!
Here at the University we’ve been composting our raw (uncooked) food waste in a small in-vessel composter, affectionately known as “Hamish”, since 2007. Now, as of January 1st 2014, TEG’s AD plant processes the remaining food waste from our student catered halls, our retail outlets and delivered catering in special green collection bins. This is in line the with the Waste Scotland regulations that stipulate all businesses must recycle food waste produced, prepared or distributed in excess of 50 kg per week by catering units.
TEG’s massive twin-chambered AD plant is rather unique. Designed to collect a wide range of food waste – including meat products – TEG was the 1st plant licensed for operation after the foot & mouth scandal which saw much stricter legislation come into effect in 2003. Since then, TEG has been well underway converting the food we chuck out into useful natural bi-products. These include nutrient rich compost and liquid fertiliser for use by local farms, as well as biogas for generating electricity.
We visited the site in early January to see exactly how an anaerobic digestion system works.
The process begins when food waste is collected here in St Andrews by special food waste collection vehicles. The food waste then travels a mere 26 miles to TEG’s plant located in Glenfarg at the edge of Fife where it undergoes de-packaging and pre-treatment. This is when any unwanted plastic, metal, or other non-food materials are screened and sent through their own recycling systems. At this point, it is absolutely crucial that any non-food items are kept out of the AD process. Glass, cutlery and fabric clothe are notable offenders that can severely damage the pre-screening machinery – ever more reason to make sure we only dispose of food waste in the correct collection bins!
After pre-screening is complete the food waste enters two large containers where the anaerobic digestion process occurs. In this oxygen-less environment, food waste is churned around in a digestive “soup”, where it is broken down by natural bacteria and releases gases that rise to the surface. These gases are then collected and stored in an expandable gas holder for later use as fuel for powering an electric generator.
The “soup” which is still inside the anaerobic containers is then pumped through a pasteurisation system and separated into dry and wet components. Dry components are mixed with garden waste and aged as nutrient-rich compost. Wet components are stored as liquid biofertiliser and sold to local farms as an alternative to petrochemical fertilisers.
In the end, the nutrient and calorific values of ordinary food waste is converted into three highly useful products without producing any further environmentally damaging outputs. No further pollution or un-used waste sent to landfill. Now that is worth being excited about!
It’s sobering to remember that this process all begins back home in our kitchens and halls of residence. We can improve this cycle of reusing food waste in a local, environmentally friendly way by following these 3 simple reminders.
- Think before you toss out uneaten food. Can this be used as leftovers for tomorrow’s lunch? Do I need to plan my portion sizes smaller?
- Plan before you shop. Don’t buy too much before you can reasonably eat it before the sell by date.
- Read directions. Please, please, please only put food waste into the food waste bins! Take a closer look at the instructions for what can and cannot be place in the bins, and ask if your uncertain.
For more information relating to the recycling facilities provided at the University, please visit our waste and recycling pages.