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AD digestate storage/spreading costs

Discussion in 'Renewable Energy' started by LoamRanger, Jan 13, 2015.

  1. Chalky

    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!
  2. The Son

    The Son Member

    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
  3. e3120

    e3120 Member

    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.
  4. warksfarmer

    warksfarmer Member

  5. thesilentone

    thesilentone Member


    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).
  6. thesilentone

    thesilentone Member

    Yes, but it is quite expensive and has high maintenance costs.

    see: http://www.hrs-heatexchangers.com/en/applications/evaporation.aspx
  7. Whynot

    Whynot Member

  8. MickMoor

    MickMoor Member

    Bonsall, UK

    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.
  9. warksfarmer

    warksfarmer Member

    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.
  10. thesilentone

    thesilentone Member

    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.
  11. Thomo7930

    Thomo7930 Member

  12. Whynot

    Whynot Member

    Very well. PM me your mobile and I'll give you a call, or you're more than welcome to visit.
  13. thesilentone

    thesilentone Member

  14. Whynot

    Whynot Member

    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.
  15. Rob_AD

    Rob_AD Member

    Why are your round tanks so expensive? PM on its way..

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