Having written many an article that touches upon anaerobic digestion (AD), and a couple that tackle explaining it, I knew it was a tried and tested technology capable of treating many different kinds of waste: food waste, of course, but also slurry, crop residues and sewage. I probably wouldn’t have put black bag waste in a list of potential feedstock, though – until I visited Biffa’s AD plant at Wanlip near Leicester, that is.

Martyn Evans, Plant Manager for Biffa’s Bursom Recycling and Wanlip AD facilities (which service the Leicester City Council contract), explains it to me when I visit the sites on a sultry day in August. We stop off first at the Bursom site, where all of the city’s black bag waste arrives following collection. Thirty-five ‘conventional’ bin lorries deliver what residents consider to be residual waste to the site, where it is shredded and sorted for further use.

Leicester does not currently have a dedicated food waste service, and so before I see the use that the organic fraction is put to, I learn how it is separated from the other items that fill up the black bags (sadly, including a number of recylables). Once the bags have been ‘flailed and shredded’, the waste continues on belts, and ferrous and non-ferrous metals are removed by overband magnets and eddy currents, respectively, before the remaining waste enters into a ‘ball mill’, a large, slatted drum filled with 30 tonnes of nine-kilogramme shot puts. Evans sums up the latter process succinctly: “Basically, what it does is grind everything to an absolute pulp… It gets incredibly hot when it’s in operation – you can imagine that the food in the black bag waste generates an incredible amount of heat as a result of natural, biological breakdown. So, we try and process waste as quickly as we can because age then generates offensive odours, and some of the waste is already a week old by the time it gets here.” And indeed, the heat coming off the giant rotating drum is oppressive – around 42 degrees – and standing next to it is a bit like being in a malodorous sauna.

But that’s inside the plant. To combat the odours outside, both the recycling site and the AD facility are equipped with odour suppression systems involving extraction fans, scrubbing units and bag filters. The final stage of the system, Evans explains, consists of cubicles filled with wood chip: “That wood chip is sprayed periodically with water and nutrients, and volatile organic carbons are passed through it. The retention time is literally around about a minute, but it’s sufficient for bacteria to basically feed off it and break down the smells.” The air is finally passed through a venting stack where fragrance is introduced with aerosols, and a great deal of the site is lined with sprayers giving off a fine mist with a perfumed additive.

After the ball mill, the waste passes on to a trommel, which Evans describes as ‘basically like a spin dryer’. “The organic waste that’s been ground up into what can only be described as a compost then falls through 40-millimetre apertures and goes onto a separate belt where any plastics that have been entrained are removed. It does have some plastics in the mix, but it’s a very low percentage.”

Wanlip's AD silos with the odour systems and gas storage tank in the foreground

The plastics present mean that the solid residue, or ‘cake’, from the AD process cannot be used on agricultural land as it could if the organic feedstock were source-segregated in origin. Evans says that a dedicated food waste service would “be hugely beneficial”, but tells me that although the limited applications for the cake are ‘restrictive’, they still find uses for it, including as landfill capping or land reclamation to create golf courses.

But I’m getting ahead of myself – the organic waste hasn’t even arrived at the AD plant yet. Once it’s separated from the recyclables and the genuine residual waste (which currently goes to Rotterdam as refuse-derived fuel), it’s transported to the Wanlip plant in roll-on, roll-off bins (or ‘ro-ros’) and piled in the reception hall. The day I visit, there’s a bit of a backlog as Bursom’s shredder’s hydraulic pump has had to be replaced, meaning both sites were down for a day and a half. There are roughly 500 tonnes of organic waste, which looks like soil flecked with plastic, piled in the Wanlip reception hall, but Evans assures me they’ll soon work through it. Indeed, the facility at Bursom treats 1,600 tonnes a week.

From the reception hall floor, the organic waste is fed onto an incline belt into ‘Tank 1’, where it’s blended with processed waters and mixed with a large paddle. “The organic waste is complemented with what we call third-party liquid influx”, Evans explains, “which consists of alcohol and glycerols and sometimes food waste suspension. The big business at the moment is converting vegetable oils to biodiesel, so we take the byproduct from that and feed that in conjunction with the organic suspension into the digester to maximise on the power.” While I’m there, Evans gets a call from a transport firm he sometimes uses at short notice, asking him to take thickened milk waste, which he accepts as a gesture of goodwill, explaining that the calorific value of milk washings isn’t as high as they’d like and so won’t produce as much energy.

The aim, ahead of the ‘wet’ AD process, is to get 10 per cent dry solids in suspension using process water. Before the sludge is fed into the digestion tanks, though, it goes into a conical hopper (or ‘all flux’ unit) and cyclone units where contaminants are removed. The cyclone units separate items by density, with lighter items (the organic waste) going forward and heavier contaminants, like sand, grit and glass being removed (“We find we have black bags with a lot of glass in them”, Evans laments – the glass makes the wear and tear on equipment even more ‘aggressive’ than it would be otherwise.)

The AD feedstock looks like soil flecked with plastic

The organic material then goes into a hydrolysis tank, and is held at 70 degrees Celsius for at least an hour to meet animal by-products regulations and kill off pathogens. From there, it’s fed into one of the three digesters (with a total capacity of 7.5 thousand cubic metres), where it’s treated at 39-40 degrees and has a retention period of 19 days. As in any other AD plant, the process involves microorganisms feeding off the organic material in the absence of air and producing biogas as they digest it. I’m told that the Biffa plant received a starter batch of liquids containing microorganisms from Severn Trent Water (sewage treatment works of which are located on the same site), but that the process is now self-sustaining.

Severn Trent also treats any effluent the plant must discharge after the digested sludge is centrifuged and de-watered (although liquids go back into the processed water tank at the front of the plant and can be used multiple times). The gas, meanwhile, consists of 65 per cent methane and 35 per cent CO2 and is stored in a gas holder prior to being burned in one of two combined heat and power (CHP) engines. “We produce up to 32 megawatts of electricity a day when everything’s working at peak, at optimum”, Evans tells me. The outside temperature, unfortunately, means the CHP engines cannot run optimally during the barbecue summer, resulting in power reductions of up to 15 per cent.

Other things can go wrong, but Evans insists the 18-strong workforce does everything it can to prevent problems, with a night shift consisting of two engineers doing planned maintenance four nights a week (“If you fail to plan, you plan to fail”, Evans says). And yet, problems can still arise: “You can have a spillage or a digester circulation problem – it can block. It’s fully bundled off and environmentally protected, but some of the spillages need cleaning up as quickly as possible. When things go horribly, horribly wrong, we’ll all get stuck in and start cleaning up everything to keep the place tidy”, he explains. “Our basic ethos is we work with food waste and waste, but there’s no need to roll in it.”