The demonstration.

In her quest for sustainability, the owner of Trill Farm has an aspiration to turn any waste product into a valuable feedstock.  To illustrate this principle she invited BREG to demonstrate anaerobic digestion of food waste during their Summer Festival over the week end of 26-28th July.  On our part, we were keen to put the process in the hands of people unfamiliar with the technology, and to serve the needs of a real live situation.  The catering kitchen on the farm provides food throughout the festival, and at its peak on the Saturday evening, served a meal to approximately 500 people.  That, and the campsites set up for the festival, provided the cooked food waste.

I’m stressing the cooked food waste because we hold AD as a sister process to composting.  Uncooked food waste eg peelings, skins etc are consigned to the compost heap, and the cooked food waste, which would normally be collected by the Council on a Monday morning, was sorted and made available to the digester.

The process implemented at Trill was illustrated as follows:

The pictogram of the process prepared for Trill Festival. At the Festival it was not possible either to burn the gas, which was stored, or to apply digestate to the crops - it went on the compost heap.

The main components were the digester,  gas bladder, and electricity generating station for heating the digester:

The AD system installed at Trill. The digester on the right is of 220 litres capacity, the bladder (behind the sun screen) a nominal 3 cu M, and the pv station 380 Wp.

 

The food waste was collected from the kitchen and from the recycling centres dotted around the campsite, transferred to a drum and kept under water, so that the acidification stage of digestion could commence:

The food waste is kept under water to aid acidisation, reduce smell and keep flies at bay. It is chopped with a spade before being fed to the digester. What you're looking at is mainly bread.

 

About 60 litres of kitchen waste were digested, and a further 60 litres of campsite waste were sent to the Council.  There were two reasons for this: concern that the digestion process might be over-fed and collapse if all the waste was added over such a short period, and the fact that the waste from the campsites was not well sorted:

One of the recycling centres - how could recycling be made sufficiently attractive that people take care when sorting their waste?

 

Before arriving on site the digester had been heated with solar hot water (see earlier blog), and was converted to electric heating for the Trill demonstration.  It was installed on site on 9th July, heated from cold to 37 deg C and inoculated on 15th .  Gas production started immediately, with the quantity indicated on the gas meter:

The gas meter reads in litres. In top right of the picture is the readout for the digester internal temperature.

During the festival the digester was fed daily which required a set of operating instructions to be prepared:  

Laura adds the food waste...

By Sunday 26th the level of liquid inside the digester had risen to near the gas outlet, and it was necessary to siphon out some digestate:

...and siphons out some digestate.

The digestate from food waste - yellow in colour and rather unpleasant to smell.

The smell of this was rather acrid and consequently the digestate on offer was taken from the previous year’s operations, which had been fed with grass and smelled quite sweet (by comparison).  Dave has suggested that the smell of the food waste digestate might well sweeten as the process settles down and the bacteria become more acclimatised.

Digestate from grass - nectar by comparison! This was used in the Growing Trials (see previous blog).

 

The biogas was routed to the bladder for storage.  During the period of the festival there was insufficient gas to support a cooking demonstration, and it has been suggested that we might set up a refreshment station (tea, coffee, hot water) at a later date.

The biogas bladder. We've mixed feelings about this - it seems oversized, and even when about a third inflated as shown here the system pressure is still only 0.5 mB, too low to feed the gas burners.

 

What did we learn?

  • Recycling is not sexy, and most people just can’t be bothered to make the effort.  This suggests that we need to reinvent the recycling station, so that it becomes fun to do, and that waste does not need to be re-sorted behind the scenes. 
  • The feeding and digestate siphoning operations need to be sufficiently straightforward for lay people to carry out, without getting splashed, and instructions need to be posted in an appropriate place.
  • The surge in supply of feedstock over such a short period could be dealt with by having a number of drums to store it meantime.  In this state the acidification can safely progress for a week or so until such time as it can be fed to the digester.  For the amount of food waste produced I suggest 5 small drums would do, with one at each recycling station for the duration of the festival.
  • The digestate from food waste has an unattractive smell, and whilst it might well sweeten over time, we need to have an acceptable way of dealing with it.  At the moment it goes straight on to the compost heap, but we have no information on the makeup eg N P K and trace elements, and its potential benefit to crops.
  • If the digestate proves to be as nutrient rich as that from grass, then the benefits would be maximised by applying it to soil that has only recently been brought into cultivation (see the Growing Trials blog) – at Trill this might be the recently established fruit area.
  • The gas bladder is large and cumbersome, and for a number of reasons we now prefer a tractor tyre inner tube.  We need a better understanding of inner tube life on biogas duty, and might install one at Trill in place of the bladder.
  • The electric heating has proved very reliable compared to the solar water heating results.  The plot of temperatures over the period is shown below.
  • There were sausages served at one of the meals, and the possibility exists that some meat might have ended up in the digester.  To deal with this strictly in accordance with the Animal By-product Regulations would introduce a number of complications into the process, and we need to carry out a risk assessment to determine an adequate response.
  • If the refreshment station idea is to be implemented then any requirement for a fridge (eg for milk) should take into account the capacity of the electricity generating station.

 

OK you techies, I know this is what you've been waiting for! This is a plot of temperature against time for the 17 days from 19th July until 5th August. Red is the ambient Temp on the outside of the digester and shows the daily variation. Green is the Temp of the air cavity inside the thermal jacket, and the drop-outs are when the digester was opened for feeding. Blue is the digester internal Temp. Up to Minutes = 7000 the heating was continuous at about 10W, at which point a further 2.5 W was added and the thermostat kicked in. The two overcast days of the festival (roughly 11000 to 14000) meant we couldn't maintain temperature so that at 14500 the digester was taken off line and an additional 50W of heating capacity installed. From this point on the temp has risen steadily to the set point which has since been reduced by 1.5 degrees. The dead patch from 11000 to 12000 is when the heating ceased because we showed a movie and flattened the batteries (I think), and again the next day following a PowerPoint presentation.

In conclusion.

What a great learning opportunity!  There’s nothing like putting the process into use to advance the development.  Thanks to everyone at Trill for a lovely weekend, especially to Romy for inviting us, Zoe and Laura for their commitment to the needs of the process, and Godfrey for the beer.