AD digestate separation and recirculation

GreenerGrass · May 16, 2019

We get a fair bit of crusting on our digestate storage tank and so are thinking of investing in a separator so the liquor goes into storage and the solids can be stacked.

What I am wondering is about recirculation of the separated solids into the digester again. We have just a single tank, and if we fed them through feeder it would work out as a longer retention time wouldn't it? Also it should reduce the solid fraction further?

Does anyone else do this, what I am specifically thinking is separating solids and refeeding for a certain period, and then separating and storing/spreading solids for a period to remove material that is non digestable and when the gas yield of refeeding it drops- say when average retention is X2 where it is presently.

Alternatively could you just keep recycling it into the digester?

We are feeding grass silage, slurry and some maize/wholecrop.

Be interested of anyone elses experience?

The Son · May 16, 2019

Yes I do it, separate out of the main digester tank, into the digestate tank, then recirculate separate back into the digester to help with DM. I have a dedicated pump from the digestate tank into the digester tank and dose in as required. At the moment the pump runs for two minutes every hour.

Pros:

You get solid digestate which can be stacked which is a very good soil conditioner.
It makes your storage bigger as you are not putting so much into the digestate tank, which helps me especially in the winter.
You can recirculate liquid into the digester to reduce DM.
You can pump your digestate much further, and it is much easier to spread.
You get reduced crusting on your digestate tank, and when you do mix it up it is much easier to get rid of the crust.

Cons.

It is expensive to run, on my 400kW plant it costs £10-£12k in wearing parts (a FAN screen seperator) every year
It can be very fickle to run, and every time it stops you get covered putting it right.
As I use poultry litter I cannot recirculate as much as I would like as the N rises too much in the tank.

GreenerGrass · May 16, 2019

Thanks, I remember seeing your setup, as you very kindly gave me a tour pre our construction! I am coming at a different angle and thinking of putting the solids back into the digester, rather than liquids Our tank is nice and liquid as we put a lot of slurry through so no issue with DM, the liquid would just go into storage tank as does currently.

Just feel it might keep things simpler for when we are unable to spread if we can only store the liquid and increase the tank retention time of the solids (which is presumably where extra gas yield would come from)

sjt01 · May 16, 2019

GreenerGrass said:
Thanks, I remember seeing your setup, as you very kindly gave me a tour pre our construction! I am coming at a different angle and thinking of putting the solids back into the digester, rather than liquids Our tank is nice and liquid as we put a lot of slurry through so no issue with DM, the liquid would just go into storage tank as does currently.

Just feel it might keep things simpler for when we are unable to spread if we can only store the liquid and increase the tank retention time of the solids (which is presumably where extra gas yield would come from)

When our digester goes below about 8.5% total solids we feed back separated solid digestate. When slurry is wet, and when we were feeding fodder beet rather than energy beet (18% DM vs 24% DM) we ran for 9 months without taking any solid digestate away.
There is quite a bit of straw going in with the slurry, and my theory is that while it is out of the digester waiting to be re-fed (we do it by the trailer load), the fungal mycelium that developed on the solids had started to break the lignin down so the digester bugs could utilise it.

The Son · May 16, 2019

I have never put solids back through as DM is always an issue, others do and I have been told to make sure mushrooms are growing on it before it goes back in.

What gas do you think you get per tonne @sjt01 of the solid digestate?

sjt01 · May 16, 2019

The Son said:
I have never put solids back through as DM is always an issue, others do and I have been told to make sure mushrooms are growing on it before it goes back in.

What gas do you think you get per tonne @sjt01 of the solid digestate?

Difficult to attribute, as we feed a mix of whatever slurry there is, whatever whey there is, digestate at about the rate of production and maize silage / energy beet to top up. The control algorithm modulates feed to achieve the desired gas production ( approximately, with a bit of lag)

thesilentone · May 19, 2019

You must consider what the OLR relative to the size of digester you have, it is not a simple as just ' increasing retention time'

It really depends who you supplier was, and what spare capacity was designed in from the start.

If you continually re-circulate, there is a risk of N build-up, which may exceed the overall TAN.

The other option is to add a gas tight roof (and maybe another agitator) to your storage tank and increase the operating temp in the main digester so you get the last amount of Biogas (which will more than cover the cost of gas tight roof and membrane) from your feed-stock.

GreenerGrass · May 21, 2019

thesilentone said:
You must consider what the OLR relative to the size of digester you have, it is not a simple as just ' increasing retention time'

If you continually re-circulate, there is a risk of N build-up, which may exceed the overall TAN.

The other option is to add a gas tight roof (and maybe another agitator) to your storage tank and increase the operating temp in the main digester so you get the last amount of Biogas (which will more than cover the cost of gas tight roof and membrane) from your feed-stock.

Sorry I'm not sure you have understood what we were planning. The main issue we are solving here is the crusting that forms on the digestate storage tank (despite having a gas bubbler in it). We have recently had to build ramps to get a mechanical over the top mixer in so that we can break it up in order to empty and landspread. So fitting a gas tight roof will not address the issue.

We have a single stage tank, and because the digester was designed to take a lot of slurry, it is very large relative to the solids we are feeding it and power output.

I didn't think that the recirculation of the solid portion would lead to N build-up, because the majority of the N is in the liquid portion which is going straight into the storage tanks?

My question is whether anyone is feeding the separated solids back into the digester - for example during months when it is not possible to spread it directly to fields. We could then give it a clearout in the Spring by spreading solids for a month, and close the loop once by refeeding solids to the digester once more until late summer when another portion is directly land spread. Whether this would deliver ancillary benefit of further digestation of that portion - which presumably is the portion with the most undigested material and thus a higher potential gas yield than the liquid.

I appreciate the comments regarding fungi from @The Son and @sjt01 for I presume lignin breakdown, but that would mean the digestate would have to be transported, stacked and then transported and fed back again. As it stands we don't have a suitable storage area for it. We could potentially build one. But I am just seeing if we can feed the solids back it would quite neatly solve a couple of issues. I guess in some ways an increase in gas yield is not the main driver for this - but it must have some positive impact I would have thought?

sjt01 · May 21, 2019

GreenerGrass said:
Sorry I'm not sure you have understood what we were planning. The main issue we are solving here is the crusting that forms on the digestate storage tank (despite having a gas bubbler in it). We have recently had to build ramps to get a mechanical over the top mixer in so that we can break it up in order to empty and landspread. So fitting a gas tight roof will not address the issue.

We have a single stage tank, and because the digester was designed to take a lot of slurry, it is very large relative to the solids we are feeding it and power output.

I didn't think that the recirculation of the solid portion would lead to N build-up, because the majority of the N is in the liquid portion which is going straight into the storage tanks?

My question is whether anyone is feeding the separated solids back into the digester - for example during months when it is not possible to spread it directly to fields. We could then give it a clearout in the Spring by spreading solids for a month, and close the loop once by refeeding solids to the digester once more until late summer when another portion is directly land spread. Whether this would deliver ancillary benefit of further digestation of that portion - which presumably is the portion with the most undigested material and thus a higher potential gas yield than the liquid.

I appreciate the comments regarding fungi from @The Son and @sjt01 for I presume lignin breakdown, but that would mean the digestate would have to be transported, stacked and then transported and fed back again. As it stands we don't have a suitable storage area for it. We could potentially build one. But I am just seeing if we can feed the solids back it would quite neatly solve a couple of issues. I guess in some ways an increase in gas yield is not the main driver for this - but it must have some positive impact I would have thought?

If you are not allowing the fungi to start the lignin breakdown, you will find that the solids will build up in the digester, to the extent that mixing will be inhibited and pumping made next to impossible.
What total solids level are you working at, and what margin do you have for increasing it?
I am not sure I understand your system - are you storing whole digestate in a lagoon then separating it before spreading? At what point do you get the separated solids, or are you just thinking about fitting a separator?

GreenerGrass · May 21, 2019

Thinking about fitting a separator in-line between AD plant and the large permastore tank that stores digestate, so that it intercepts the solids, and only liquid portion pumped into the permastore. At present whole digestate is pumped through.

7.4% DM, but that's from an old report will get a new sample!

maen · May 21, 2019

To answer your question we use a Carrier separator and treat all digestate prior to storage in lagoon. We never have a crust on lagoon and suck from the bottom of the lagoon without any blockage. We also mix back separated digestate into AD when growing mushrooms.

Will Wilson · May 21, 2019

I might be daft but would Slurry bugs (or similar) make any difference to the crusting? Would a product like this work - https://www.aegltd.co.uk/aerotop-covers/

AEG are very helpful on these topics.

thesilentone · May 21, 2019

The question you asked was "We have just a single tank, and if we fed them through feeder it would work out as a longer retention time wouldn't it?"

My answer was: You must consider what the OLR relative to the size of digester you have, it is not a simple as just ' increasing retention time'

When your system was designed, the digester size was calculated based on the ammount of input (tons/day) and what the organic loading rate would be (kg/VS/M3 digester space). This calculation varies based on the types of feed-stocks used and how long they take to decompose.

Several other factors are also used such as the N content and operating temp of the system and (and this is the 64 dollar question) what additional capacity your supplier designed in for future proofing.

If your system is already near the OLR limit (tons/vs/m3 digester space) then adding further VS (your separated fraction) then you may exceed the OLR and make the digested unstable. If you have Biogas available in the dry fraction, (not fully digested) it could be harvested in the storage tank with a few additions.

If you do not have any Biogas in the dry fraction, then what is the point of putting it through the process again ?

sjt01 · May 21, 2019

GreenerGrass said:
Thinking about fitting a separator in-line between AD plant and the large permastore tank that stores digestate, so that it intercepts the solids, and only liquid portion pumped into the permastore. At present whole digestate is pumped through.

7.4% DM, but that's from an old report will get a new sample!

I suspect you will be able to go up to about 9% without problems with most types of pump, if your throughput is anything like ours it will take you a couple of months to get there.
If you are fitting a separator, then our way is to put a trailer underneath the output and take a load to the field heap every week or two.

Chippy · May 21, 2019

I put slurry bugs into digester to prevent crust and I presume they also come out into my digestate tank which also crusts like yours. I think it crusts less now that I use bugs but think more bugs are needed to completely eradicate crust

thesilentone · May 21, 2019

Chippy said:
I put slurry bugs into digester to prevent crust and I presume they also come out into my digestate tank which also crusts like yours. I think it crusts less now that I use bugs but think more bugs are needed to completely eradicate crust

Take care with bugs, bear in mind adding O2 to the gas store to clean the biogas is assisted by the H2S eating bacteria on the floating layer inside your storage tank (so a small crust is beneficial).

sjt01 · May 21, 2019

Chippy said:
I put slurry bugs into digester to prevent crust and I presume they also come out into my digestate tank which also crusts like yours. I think it crusts less now that I use bugs but think more bugs are needed to completely eradicate crust

Unless you are going to put in about 10% or more of the existing bug population, the existing bugs will compete out your slurry bugs and they will disappear. If you put them in the aerobic digestate store they would stand a better chance as the anaerobes would not be active

Slurry Solutions · Jun 4, 2019

The Son said:
Yes I do it, separate out of the main digester tank, into the digestate tank, then recirculate separate back into the digester to help with DM. I have a dedicated pump from the digestate tank into the digester tank and dose in as required. At the moment the pump runs for two minutes every hour.

Pros:

You get solid digestate which can be stacked which is a very good soil conditioner.
It makes your storage bigger as you are not putting so much into the digestate tank, which helps me especially in the winter.
You can recirculate liquid into the digester to reduce DM.
You can pump your digestate much further, and it is much easier to spread.
You get reduced crusting on your digestate tank, and when you do mix it up it is much easier to get rid of the crust.

Cons.

It is expensive to run, on my 400kW plant it costs £10-£12k in wearing parts (a FAN screen seperator) every year
It can be very fickle to run, and every time it stops you get covered putting it right.
As I use poultry litter I cannot recirculate as much as I would like as the N rises too much in the tank.

Interested to know the run hours you are putting on your separator in a year as your wearing parts costs seems high for a 400kw plant although poultry litter does increase wear due to the increase in grit / abrasion.

Regards

Rob

The Son · Jun 4, 2019

Currently running at one hour on, 15 minutes on and is just about keeping up.

I use both sugar beet and potatoes, so I don't think this helps the situiation, but last year I bought a beet washer and washed all the beet, so hopefully that might prolong the life of the auger and screen

thesilentone · Jun 5, 2019

When sizing the separator, it is usual not to exceed 60 % operational time in hours (around 5000 hr/yr). So, as an example, a FAN screw press would need to be specified at - (for 20k tpa input - 1 MW) you would require a model 520 which should allow for any additional liquid if your feeding maize.

The operational cost could be calculated based on the FAN motor capacity x efficiency (80% approx) calculated in kwhr/yr.

AD digestate separation and recirculation

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