Anaerobic Digest

UPDATE: 28/11/13 - Industry welcomes small scale FIT review by Greg Barker. This is a huge step forward and a very positive move for both farmers and UK Climate change targets. Background to the review is detailed below:

After months of talks between industry and government, DECC have decided that they are able to do nothing about a year-end announcement of an Anaerobic Digestion (AD) Feed-In Tariff degression (reduction) of 20% for the under 500kWe sector. This will have a disproportionate effect on British companies and British jobs, as many of the British AD companies concentrate at this end of the scale. It will also mean that the majority of UK farmers will not be able to access the most effective GHG reduction and pollution mitigation agri-technology available to them.

This tariff reduction will favour the larger, primarily crop-based digesters built by the well-established European companies who have had years of consistent government support, but whose governments are now beginning to realise (and incentivise) the myriad benefits of slurry/manure-based AD at scales less than 100kWe, far more appropriate to the size of the average UK farm; for example, a 500 cow dairy farm can produce 50kWe using slurry. Unlike Defra, who recently launched a loan fund to encourage farm AD of manures and slurries, DECC is effectively discouraging the inclusion of these feedstocks into AD and jeopardising investor confidence in the sector.

A 20% degression would be understandable if 9MW of effective plant had actually been built; however, the degression is triggered on those plants which have got pre-accreditation, ie the plant has planning permission and grid connection approval, but has not been built. Records show that, in the sector’s very short history, only 15% of these pre-accredited plants are built.

The relatively tiny capacity of 9MW for the sub-500kWe (see table below) has been further eroded by the Extensions Rule, where large plants apply for the first 500kWe at the higher tariff (which has lower capacity), subsequently installing further capacity at the >500kWe tariff.

Tariff rates are currently 15.16p/kWh for AD plants less than or equal to 250kWe, 14.07p/kWh for plants > 250 & less than or equal to 500kWe and 9.24p/kWe for plants over 500kWe.

Table 1 - Degression triggers (click table to enlarge)

Currently, only around 20% of UK farm digesters are over 500kWe and more than half of the digesters in the UK are farm digesters, so it is apparent that the capacity bands do not reflect reality.

The UK AD industry has agreed that a crisis can be averted if DECC carries out the following relatively minor changes to policy:

Consolidated Degression: Merging the 2 FIT bands (for under and above 500kW) to create a single overall degression trigger.  Consolidating total AD capacity in a single budget will create a simpler and less wasteful FIT mechanism. 

Change Pre-accreditation:  Combining consolidation with allocation of capacity to the year of actual deployment will not increase net FIT expenditure. Pre- accredited capacity must be counted in the year the installation is deployed (i.e. when payments will be made) not the year of accreditation.  The current situation means that plants (including larger ones) that are not yet producing biogas or receiving FIT payments count towards the degression for under 500kW.  If capacity is allocated when a plant is ready to begin producing biogas, DECC’s FIT expenditure will be more in line with deployment.  But pre-accreditation must be retained to give certainty to investors.

Revert to the old ‘extensions rule’: The current FIT rules allow the opportunity for “gaming” (increasing the cost to the taxpayer) as they encourage developers building larger plants to split their CHP capacity.  Hence, a 1MWe AD site can accredit a 499 kWe CHP unit at 14.02p per kW, then immediately install a further CHP unit at 9.24p, at added cost to the FIT budget.  Reverting to the old extensions rule that required a delay of a year before plants were extended – would provide better value for money.

The greenhouse gas (GHG) emissions from the storage, handling and spreading of the 90 million tonnes of slurry in the UK are considerable and include methane, oxides of nitrogen, ammonia and carbon dioxide, not to mention issues surrounding potential pollution from nitrates and phosphates in ground water and water courses, from both these and from the use of fossil fuel fertilisers. Anaerobic digesters can not only produce renewable energy (in the form of heat, vehicle fuel or electricity), but they produce an excellent fertiliser, reducing fossil fuel reliance and considerably reducing GHG emissions.

The agricultural sector is one of the most significant producers of GHG emissions. Whilst much public focus has been on food waste, the environmental impact of the hugely larger volumes of manures and slurries is much greater and more diffuse. Without consistent policy which allows for sustained growth in the farm AD sector, the UK will continue to struggle to meet its climate change commitments and farmers will be unable to de-carbonise using this agri-technology, recognised worldwide as being the best way to turn the ‘slurry problem’ into a solution.

The digestate trials are continuing apace, with spring, late spring and early summer all crowding in together on the growing season. All at once, I seem to have strawberries, tomatoes, cucumbers, salad crops, herbs, cauliflower, cabbages, carrots, potatoes and peas. Large quantities of digestate have improved the soil so much more than my beloved compost or wormcasts ever did. Additionally, I have poured the liquid fraction of the digestate onto many of these crops with utter impunity, much to the horror of my colleague. Miraculous and bizarre things have happened, such as my tomatoes developing double trusses of 45 tomatoes and more, the rather sickly Pieris bursting into beautiful fresh growth, the tarragon deciding to become a Triffid and cucumbers deciding to place themselves at 4" intervals along the cucumber stem. A few pictures are included below, with more to come.

Normally April is gardening month in UK, but this year there was still snow in some places, so much of the work has dragged over to May. However, the four tomato varieties have been warm in the greenhouse and apparently love the digestate in any mix, although there are size variations between the mixes- for example, it can be seen in the photo below that the Ailsa Craig in 2:1 mix is slightly bigger than the 1:1, but the plant on the right in the proprietary compost is the smallest. Across all varieties, the digestate performed better than the proprietary compost: the leaves were greener, the plants taller and bushier and the flowers earlier. The beans don't like the strong digestate mixes much: but more on that later.

In the early 90’s, I was part of a company called Waste Refineries International and we were trying to develop peat-free compost using digestate as a raw material. The main business was to install anaerobic digesters to improve nutrient recovery and to produce energy for the farm. The by-product was separated fibre. The compost was marketed under the ‘Heritage’ name and remained in production for about 15 years, achieving prestigious environmental awards along the way.     

The development of this compost was to help farmers to find an extra source of income to help pay for these anaerobic digester advanced slurry management systems.

In 1987, Margaret Thatcher’s government brought in subsidies to help farmers pay the high cost of protecting the environment, but these were removed in 1995. Nearly 20 years later, the UK is still slow to develop capacity to deal with all aspects of organic waste. Some countries have romped ahead, installing digesters to process a multitude of feedstocks. Germany now has about 8000 and the UK has a little over 100. Using wastes and crops, Germany has installed enough digesters in a single year to produce output similar to a nuclear reactor.

For the past two years, a Wiltshire micro-digester has been turning smelly food waste from four families into gas to cook on and a black sludge. Going back to my earlier experience, I decided to blend this sludge with dry coir. The result is amazing: a beautiful, friable growing medium. So, growing trials have started. 4 varieties of tomatoes, 4 strengths of medium with sieved soil and one bought-in potting compost.

Murcott's Musings: Dispelling the ‘Expert’ Myths of Digesting Chicken Muck

If we could get £120 worth of electricity per tonne of chicken muck (with the UK feed in tariff) via anaerobic digestion, should we be wasting this energy source by throwing it straight onto land as a very inefficient fertiliser?

Some experts say that chicken muck cannot be digested without the addition of large amounts of water, making it unviable to spread as a fertiliser. Other experts say we should only use it as a fertiliser, whilst others say that it should be burnt as a fuel source.

My own experience suggests that we can have all three. We can gain the majority of the energy potential by anaerobically digesting it. The destruction of the dry matter helps to yield a thin, free-flowing liquid as a bio-fertiliser which can be easily and accurately applied (and stored), and using fibre separation can give us a wonderful fibre soil enricher.

By gaining the value of these separate fractions of chicken litter, we can gain far greater benefit from it than any other single-path disposal route. In my experience, chicken litter can be digested with the minimal addition of water and equally it digests extremely well when added to cattle or pig slurry.

There appears to be very little experience with using chicken muck as a feedstock for anaerobic digesters, but I heated my house for 10 years using only chicken muck in the digester. Experts say that this cannot be done because of the high ammonia levels but, after a period of acclimatisation, the digester worked fine.

As I had a day job, the digester was fed at the weekends and took me about half an hour to load – much easier than hauling the amount of wood necessary for the same amount of heating.

A recent report also said that loading more than 6% chicken muck into a digester on a daily basis is not advised. The innovative Fre-Energy digester at Lodge Farm in Wrexham has been running successfully for a long time with at least three times that much chicken muck going into the digester, producing good gas and excellent digestate. The picture below shows a closeup of the digestate. The majority of the rest of the feedstock into the digester is slurry from cows bedded on ash and woodchip – two further feedstocks which apparently cannot work in a digester.

Plenty of Gas and Yummy Stew

I don't know what I had put into the food waste bin this week, but overnight gas production seems to have doubled. I suspect it was likely the vinegar left over after all the gherkins were eaten - acetic acid is like rocket fuel for anaerobic digesters. So, plenty of gas and a cold, wet evening means only one thing: a yummy stew! As I have put the burner in the greenhouse (adds heat and carbon dioxide, is my theory), the best thing to do is to throw everything in a pan, cover it up and leave it burning on low. 

If it is not raining too hard or I feel I am losing the 'critter battle', this gives me a great opportunity to go slug and snail hunting. I find that the neighbours don't appreciate having such creatures slung into their gardens and I begin to feel quite ill if I have to crush them or hear them crash as they hit the road out front. And, in any case, the slugs this year are so huge that they are big enough to argue, if I try to throw one. In true 'greenie' style, I have tried having harsh words with them, but I think they are territorial and the biggest ones get the opportunity to decimate the juciest garden vegetables. However, it is no problem to gather them up on a trowel and plop them into the nice warm bath that is the digester hopper. More on how they come out in another installment.

This probably has the unfortunate side effect of turning my digester into some kind of Animal By-Products processing equipment under ABPR regulations, but I feel I might actually be winning the Slug Wars.

Kevin McCloud's Man Made Home

Treflach Farm Small Scale Energy Event - 29 Jul 12

We had a great day with Ian Steele and some very interesting speakers who were talking about small-scale energy generation at Treflach Farm. Steve Jones of Sector 39 had spoke about the possible futures of our energy supply in an interesting and accessible way. Dave the Pig talked about small-scale hydro, thermal solar and wind with a great deal of practical hands-on experience and Matt Swarbrick focussed on small-scale PV.

After our disaster with our newly refurbished digester trailer breaking, it was nice to be able to go to Treflach with the Baby Biogastronome on a new trailer. More information on the day can be found here, and if you are interested in attending such a course, do contact Ian.

Digester at Queen's Jubilee Cosford

We Become Mobile

We have now given the 'nome some wheels and are able to transport him to any events where you think Anaerobic Digesters need to be seen. These were taken at the really good Wrexham Rural Environmental Project event this weekend at Glyndwr University.

The really exciting moment for anyone with a digester is when you produce the first burnable gas. You may have spent several millions on a plant or a few thousand or you may have cobbled one up from a 45 gallon drum. Shown here is the 'Kealan' digester - a 600 litre digester currently running in Newcastle. This was taken when it was being tested more than 18 months ago--teabags in one end and tea being cooked out the other! We at Methanogen named it after Kealan Gell, it's owner, who is doing inspirational work on renewables, specifically anaerobic digestion, in Canada: Off-grid Earth and Water.

After showing itself off for a CCN Open Day later this month, it is due to head to Canada to re-join Kealan.

Well, it has taken a bit of time for us to get the baby biogastronome anaerobic digester running at my house. From the original design (deep in the brain of digester engineer and innovator, James Murcott, shown above with the patterns for the range) to inception has been a challenge since, every time we build one for ourselves, it seems to disappear to another project!

This blog will try to give advice, information and opinion on a number of subject areas to do with anaerobic digestion. We hope you enjoy it and feel able to contribute your thoughts and experiences with small digester technology, as well.