The mobile AD tea strainer

Anaerobic digestion (AD) has been used for many years to treat organic waste.

Bioplex Technologies has introduced a mobile system that aims to let communities use AD to generate their own eco energy. Geraldine Faulkner (Editor Sustainable Solutions)reports.

It is not every day that a biochemist mentions the Clangers and the Wombles in a business context, but then Chris Reynell, a director of Bioplex Technologies , is not an everyday businessman.

And bearing in mind that Reynell is discussing waste collection and how to make the best use of it, it therefore makes perfect sense for him to refer to the clangers and the wombles and the way they collected unwanted materials.

However there are certain waste materials that no self-respecting clanger or womble would pick up. For instance, you wouldn't have seen Great Uncle Bulgaria or Orinoco collecting sewage sludge.

Yet this is only one of the waste materials that Bioplex Technologies, consultants in anaerobic digestion (AD), deal with along with horse stable, farm manure, as well as food and drink processing residues.

How do they go about processing these particular waste materials? Well, it is all down to the Bioplex AD system that Reynell says is very flexible.

"We take a client's problem and we build our kit around it rather than take a piece of kit and work around their problem," explains the biochemist.

So how does it work?

The Bioplex AD system operates as a batch process in two phases. The first, or solid phase, uses a digester vessel called a Portagester which acts as an anaerobic filter/leathate bed reactor that pasteurises and stablises the feedstock.

The vessel is covered and positioned at the digestion site; liquor from the liquid phase (stage 2 digester) is circulated through the contents during the digestion period to wash
out organic fraction as well as being heated.

Typically, this solids phase has a three-day (or less) retention time during which the stabilisation known as acid phase hydrolysis occurs.

At the end of the process period, the liquid is screened, drained out and stored in the second stage digester.

The methane generated from the second stage digester process is used for energy either to heat the first stage or it is put through a generator to produce electricity. The solids left from stage one can be spread on to farmland
or composted to help break it down more effectively. For anyone who hasn't heard of the AD process, it uses naturally occurring microorganisms to digest/degrade organic materials in the absence of air.

According to records, the AD process has been used for thousands of years to treat organic waste, including the ancient city of Ur in Mesopotamia where the digesters also
provided gas for cooking.

The AD process is defined according to the temperature range at which it operates. Basically, there are three options. First there is the psycrophilic range which operates at an ambient temperature, followed by mesophilic which functions between 30 and 38 degrees
Celcius and lastly, thermophilic which operates
between 52 and 60 degrees Celcius.

"Anaerobic digesters currently used to treat farm wastes and sewage sludge are generally mesophilic completely stirred tank reactor (CST R) digesters," says Will Kirkman, director with Bioplex, which forms part of the Burdens Environmental group, suppliers of civil engineering and building materials. Kirkman goes on to explain that CST R digesters are designed to treat liquid wastes and slurries - typically no more than 5% total solids content.

"Higher total solid content liquids can be introduced, but problems arise through grit and crystals accumulating within the vessel and the system requires ever-increasing power and
sophistication to mix the contents. This can lead to pumping difficulties and blanketing of
internal heating apparatus which can then lead to loss of temperature control," he continues.

Therefore mesophilic temperatures are widely used in CST R digesters as the biogas yield usually matches heating energy requirements.

Kirkman adds there are other types of digestion systems such as plug-flow, batch and anaerobic filter. These however are not as widely used as CST R digesters.

What is Bioplex's unique selling point?

"AD meets many requirements but one of the challenges has been how to deal with the sediment that builds up? It usually adds up to around 4% of liquid solids," explains Reynell.
"In what is usually referred to a 'penny drop moment', Bioplex added a mobile tea strainer,
i.e. the Portagester."

The Portagester is a mobile and modular anaerobic fermenter, which treats waste
material in less than four days.

The beauty of this piece of kit is that it collects the feedstock from one site, travels to
a central location where the contents (still in the Portagester) are treated and then transports the treated material to the end user.

Who is best suited for systems like this?

Reynell points to Longstock, the village in Hampshire where he lives with his wife, Jane,
an organic farmer.

"Longstock is a rural parish that lies on the western bank of the River Test to the north and west of Stockbridge and has a population of around 450 people," he says. "Assume there comes a day where there are no more fossil fuels. How do we survive? By installing our own anaerobic digester, it's possible we could generate enough methane to help the village be self-sufficient and if there is surplus power to requirement, we could sell it to the local electricity supplier. Another bonus with our AD system is that it's self-contained and despite being a few hundred yards away from one of the most famous trout fishing rivers in the world, there is no danger of leachate contaminating the river."

Another example of how Bioplex's system contributes to a location's self-containment
is HMP Guys Marsh in Dorset where AD has been judged a great success.

"They are even about to run an NVQ on AD at the prison," says Kirkman. "After all, they are the ones who are going to run this in the future." He pauses a moment before adding: "Another advantage of having the system in the prison is that the average 'waste distance' is only 100 feet."

What does the director think about AD's future in the waste hierarchy?

His response is unequivocal.

"The market is pushing us to the bottom of the pyramid and favouring the biggest facilities, such as one million tpa incinerators. What is needed is a good mix of waste technologies with a government laying down a funding strategy to encourage big and small facilities otherwise there is a danger of stifling innovation."

Despite Kirkman's warning about the government's approach to AD, the process appears to be rapidly becoming the darling of the waste management world.

The 2007 Waste Strategy recognised that: "AD has significant environmental benefits
over other options for food waste" and, more recently, Jane Kennedy, the minister with
responsibility for waste and recycling, stated that "by 2020 AD will be an established
technology in this country".

In February 2009, Defra produced 'Anaerobic Digestion - Shared Goals', which it called its 'vision' for AD in the UK. As part of this, a task group has been set up to develop an implementation plan, expected in summer 2009.

Despite the government's proclaimed enthusiasm for the process and its potential in helping the UK meet its ambitious recycling targets, Kirkman emphasises the need for an integrated approach; particularly in the rural economy where a mobile, relatively low cost system that
deals efficiently with the most polluting components of domestic and food processing wastes clearly has an important role to play.

Benefits of AD on farms

Reduce odour from land-applied slurry by 75%
Enable the sale of electricity and provide a heat source to the farm
Improve the handling and solids separating characteristics of manure
Captures or absorbs CO2
Conserve water and produce marketable digester fibre
Reduce transportation costs by reducing manure solids by 70% to 95%.