AD digestate storage/spreading costs

Chalky

Member
Yes. One of our estates is 'drying' the solid digestate fraction in its under construction AD plant. It also qualifies for RHI- all using 'waste' heat. The fluids are still fluids though. Normally they are approx. 3-4% DM- so to get a stackable heap of product would need a hell of a lot of energy!
 

The Son

Member
Location
Herefordshire
From my experience I would look into a separator, ours was out of commision for a month last winter, and caused us real problems in the digestate tank, which meant we spent a lot of man hours and diesel mixing it so we could spread it.
Our digester tank runs at about 9% dm, and we do have problems putting that product through the umbilical any distance
 

e3120

Member
Location
Northumberland
Can digestate liquid be evaporated to form field stackable cake, (ignoring fact nitrogen is wasted as ammonium to atmosphere) using something like a milk powder plate dryer with waste heat from AD?
I'd love to hear the answer, especially if the RHI would help along the way...

My best hope was to get a syrup, rather than a stackable output, but that would still reduce storage/spreading volumes a lot. The evaporated water could be re-introduced at the front end but this might not be worth the hassle and virgin water could be simpler.

My main worry would be that the nitrogen would all disappear during the evaporation process as ammonia.
 
I'm happy to be corrected. I did say, "I believe....." but please tell me why you say so, because it is important to me and my customers. I don't get to invloved in the why's and wherefore's, my business is supplying storage, but I can't help being inquisitive.
As regards separation, some operators have an issue with settlement in the buffer tank as digestate comes out of the separators, after pasteurisation. This is with a base of maize silage and digesting food waste.
I have been saying for a few years that the technology is not yet proven, but of course everybody cites Germany and creates the impression that everything is hunky dory there, so it will be here.
The fact remains that there is no 'blueprint' for an AD system. It has to be tailored exactly to the inputs. Some people have been lucky, and stupidly shoot their mouths off; others less so and are, sadly, ridiculed. Unfortunately there are too many shiny suits in this industry. Their companies may go bankrupt, after they and their employees have taken a nice fat salary for a few years, and the customer, having spent a six or seven figure sum, is left with a problem.
I may be cynical, but I've been in business for 30 years, and I built digesters way back in 1992.
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If PAS110 is required due to what is classified as waste feedstock, then pasteurisation is required. However, unless front end pasteurisation is carried out, at the back end you have Digestate entering the settlement tank just below 70 degrees C. PAS110 is a UK regulation only. The settlement tank is to help cool the digestate and is BEFORE the separator and requires agitators to avoid settlement . Many factors come into play here, not least the amount of VS still in the DM. New PAS110 regulations have a strict requirement for VS levels, if this is met, then little to no further gas production should take place.
On farm (holding) feedstocks, no pasteurisation is required, so now we have a question of cost, those being. 1) The cost of sealed covered storage tank which will maintain the N and protect against evaporation v a lagoon 2) The lower cost of a lagoon v potential N losses. 3) All systems should have two digesters due to wash through everytime fresh material is added. 3) The cost of a separator v the cost of the additional liquid storage required. In addition what is the digestate to be used for. (eg: N or soil conditioning or both).
 

MickMoor

Member
Location
Bonsall, UK
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If PAS110 is required due to what is classified as waste feedstock, then pasteurisation is required. However, unless front end pasteurisation is carried out, at the back end you have Digestate entering the settlement tank just below 70 degrees C. PAS110 is a UK regulation only. The settlement tank is to help cool the digestate and is BEFORE the separator and requires agitators to avoid settlement . Many factors come into play here, not least the amount of VS still in the DM. New PAS110 regulations have a strict requirement for VS levels, if this is met, then little to no further gas production should take place.
On farm (holding) feedstocks, no pasteurisation is required, so now we have a question of cost, those being. 1) The cost of sealed covered storage tank which will maintain the N and protect against evaporation v a lagoon 2) The lower cost of a lagoon v potential N losses. 3) All systems should have two digesters due to wash through everytime fresh material is added. 3) The cost of a separator v the cost of the additional liquid storage required. In addition what is the digestate to be used for. (eg: N or soil conditioning or both).

You still have not said why I am wrong to say that storage of digestate is limited to 3,000 cubic metres on one site.
Furthermore, you are talking about settlement before separation, I am talking about settlement after pasteurisation and separation! Could you be confusing treatment, which is dependant on feedstock, with storage?
The OP was about storage of digestate.
 
You still have not said why I am wrong to say that storage of digestate is limited to 3,000 cubic metres on one site.
Furthermore, you are talking about settlement before separation, I am talking about settlement after pasteurisation and separation! Could you be confusing treatment, which is dependant on feedstock, with storage?
The OP was about storage of digestate.
Digestate is quoted around here as muck spreader stuff.

The liquor is dribble bar / splash plate stuff.

I think there's some confusion going on so I'm with mick on this one.
 
There appears to be some confusion here.

Digestate is the terminology used for the bi-product of AD. However the digestate can be managed in two ways: 1) A liquid (around 7% DM) or after separation as both a liquid (around 2.5% DM) and a dry fraction (around 25% DM) these depend on the separation equipment also (either screw press or centrifuge). Is is still digestate whether liquid or dry.

The issue over storage very much depends on the type of EA permit required:

The options are:

1) EA Permit exemption, this is for Ag plants using ag feed-stock, the only restriction is if NVZ regulations apply whereby the period of closed spreading time storage is required.
2) EA Standard permit, this applies where waste-feedstock is being used, it is a simply but inflexible permit. This has restrictions on the amount of dry fraction storage, as under this permit the dry fraction should be stored inside and usually requires some sort of Odour management.
3) EA Bespoke permit, This I can best describe as a site/area specific permit that takes into consideration design and local area considerations. This is time consuming and expensive but could be more flexible than a Standard permit.

The limit on storage is for the dry fraction on a standard or bespoke permit.

Finally, after separation you may have some settlement in the liquid storage tank or the lagoon IF no agitation is provided. In a storage tank it is usually one single submersible agitator. In a lagoon a whisk is used.

Hope this helps.
 

Whynot

Member
Location
Rugby
I maybe missed a alteration to the new RHI conditions, however those announced in Dec removed support from Drying Digestate. Bottom of page 32

https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/577024/RHI_Reform_Government_response_FINAL.pdf
I agree. I was of the view that digestate drying was no longer an eligible use for digestate drying on NEW installations.
One plant I know is trying to get accredited using the argument that is was in the initial planning. I would be very doubtful.
 

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World Food Day: NFU Cymru celebrates Welsh food producers at the Senedd

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Written by Rachel Martin

NFU Cymru members and Assembly Members have been celebrating the role that Welsh farmers play in producing nutritious, high quality, safe affordable food during an event at the Senedd today on World Food Day (October 16).

The lunchtime event, which was sponsored by Llyr Gruffydd AM, included a special menu of fine Welsh produce.

Speaking at the event, NFU Cymru...
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