smells
High-solid waste handled – & no
smells
By Andy Collings
ANAEROBIC digestion – the use of bacteria to degrade organic materials in airless conditions – is not a new process. Countless numbers have tried it, and countless numbers have given up.
The carrot which fuels such exercises is, of course, the production of combustible methane which is then used to produce heat and/or electricity. Muck converted to useable energy is clearly an attractive option.
But there has been a problem. Digesters, in the main, have only been developed to handle liquid wastes – wastes having less than 5% total solids – slurry or sewage.
Attempts to increase solids percentage can result in an accumulation of insoluble particles in the digester vessel which, as a result, then requires more power to agitate. There can also be expensive problems with pumping systems because pumps designed for liquids do not like stones and there could be additional bad news for temperature control systems as levels of grit, stones and other unwanted material builds up.
Many who have persevered with such units discover the maintenance required outweighs the potential rewards of the system.
So, if all classes of waste – solid manure, household waste and green industry waste – are to be exploited successfully, there clearly has to be a way of digesting material having a high percentage of solids. The advantage of such a process would extend further than methane production and would mean vast volumes of waste, which currently require equally large volumes of landfill sites could, in theory at least, be reduced to safe, odourless material, some of which could be used as organic fertiliser.
The answer may have been discovered by Christopher Reynell who, based at Windover Farm at Longstock, Hants, has developed a two-stage digester system.
Designated the Portagester, the system keeps the solids separate from the liquids and, as such, is claimed to overcome problems experienced by other designs.
To describe it first in its simplest format, a trailer load of green waste – manure fresh from the yard in this case – is parked near a liquid anaerobic digester tank. Liqueur from this tank is then pumped into the manure – about 25% by volume – and left there during the 4-12 day digestive period. The enhanced liqueur is then released back into the anaerobic digester where further action produces methane gas, while the now odourless manure can now be used for compost.
Had industrial green waste been used, the remaining material – the digestate – could be dried, graded and sold as organic fertiliser.
But now for the finer details. The trailer used by Mr Reynell has been modified to cater for the extra heat required for the digestive process to be achieved.
Water pipes plumbed into the bottom of the trailer are circulated with hot water to bring the temperature of the manure up to 35 degrees C to create what is known as a mesophilic action. A higher temperature of 55C (thermophillic) could be possible and would enable a more valuable compost free of weed seeds, diseases and parasites to be created.
The trailer is also fitted with an airtight lid, while other plumbing allows the liqueur to drain back into the liquid digester.
The liquid digester tank also requires some heat. In this case the actual liqueur is circulated through a heat exchanger. In Mr Reynells pilot plant, heat is supplied by a mains-powered heater, but he concedes that in a commercial plant all heating and electricity demands would be produced from the methane.
Even so, to demonstrate the potential output of the system a gas/diesel engine is on site which, initially fired up on diesel, switches to run on 66% methane and 33% diesel. Methane collected from the top of the liquid digester is held in a mini gasometer and fed into the single-cylinder engine as required. Output from this particular unit is rated at 6kW hours.
Mr Reynell, who has a patent on the Portagester system, has started a new company, Bioplex, to market it.
"The fact that we can process individual loads of waste without cross-contamination in mobile modules – trailers, skips or whatever – means that different types of waste can be digested at the same time," he says. "A farmer installing a system could, for example, also process skip loads of green council waste if the capacity was there.
"Such a system may also be important for those who farm in areas sensitive to smells – the digestate is odourless and, if dried, can be stored more easily than raw manure or slurry."
Mr Reynell also points out that, for slurry at least, the system is immune from excessive rainwater and all the hassle that situation can cause.
Mr Reynell calculates that an installation which could handle 30t of waste/week would cost about £50,000. This, he says would produce 250-300cu m of methane each day which, when converted via a generator to electricity makes 500kW hours worth about £50 – a pay-back, on paper at least, of under four years.