Concentrating Digestate

[thesilentone]

Given the current and future situation regarding the supply of N Fertilizer, surely now is the time to start looking at existing technologies to improve the quality of digestate.

Exploring the use of dried and prilled, or concentrated liquid fraction to increase N values could be a possible stepping-stone towards satisfying the short-term demand.

 

[PSQ]

I’m guessing it would take a lot of energy to evaporate the water out of a million litres. Probably a lot more efficient to use that energy for haulage and application.

 

[thesilentone]

I’m guessing it would take a lot of energy to evaporate the water out of a million litres. Probably a lot more efficient to use that energy for haulage and application.

Better still, keep the liquid and improve it.

 

[PSQ]

Better still, keep the liquid and improve it.

Veolia were pelletising sewage waste at a local depot 10 years ago, but the drying cost was crippling.
Fast forward to current energy prices and it’s a complete non starter. The only way would be to use excess heat from the fermenter, possibly concentrate it through a heat pump, but it still comes at a significant cost with the risk of losing the ammonia N fraction (if any) to the atmosphere.

 

[thesilentone]

Veolia were pelletising sewage waste at a local depot 10 years ago, but the drying cost was crippling.
Fast forward to current energy prices and it’s a complete non starter. The only way would be to use excess heat from the fermenter, possibly concentrate it through a heat pump, but it still comes at a significant cost with the risk of losing the ammonia N fraction (if any) to the atmosphere.

Not if the steam off the process is put through a condenser first.

 

[PSQ]

Not if the steam off the process is put through a condenser first.

Where is the steam coming from in the process?

The only efficient way to evaporate the water content would be to use something like a modified ‘trickling bed’ usually found on a sewage farm, with spinning dribble bars over a coarse gravel layer (large surface area of stone). But it would have to be contained to prevent odour issues, which would preclude evaporation from the sun and wind.

The only free heat would be from using the coolant of any ‘gas engines’ attached to generators, but if the methane is pumped direct to the gas grid that’s not an option.

 

[sjt01]

Any drying off-gasses need scrubbing with sulphuric acid to capture the ammonia, otherwise you are throwing away a good part of the fertiliser value.
See https://agroenergien.de/en/mississippi-digestate-dryer/

 

[Exfarmer]

Surely would be easy with an reverse osmosis

 

[thesilentone]

Where is the steam coming from in the process?

The only efficient way to evaporate the water content would be to use something like a modified ‘trickling bed’ usually found on a sewage farm, with spinning dribble bars over a coarse gravel layer (large surface area of stone). But it would have to be contained to prevent odour issues, which would preclude evaporation from the sun and wind.

The only free heat would be from using the coolant of any ‘gas engines’ attached to generators, but if the methane is pumped direct to the gas grid that’s not an option.

Given it is the N we are primarily trying to capture, and re-concentrate, what about pressurised heat explosion and condensing the Nh3 in the steam ?

 

[renewablejohn]

Should be easy enough to do with woodchip filter.

Have been looking at digestate as a way of increasing calorific value of woodchip in briquettes and pellets. This article put me off but might be useful for what the op is looking at.

https://eprints.whiterose.ac.uk/183721/1/58%20Parmar%20et%20al.%2C%202022.pdf

 

[thesilentone]

One solution would be to strip off the majority of Nh2 as Nh3 prior to the digestion process via heat explosion ?

Capture then condensate the steam, then concentrate further with acid.

This liquid product could then be added to the digestate at the back end and provide a much higher quality product.

The AD process would be more stable and have much more flexibility for mono digestion, as well as allowing higher N feed-stocks to be used in larger quantities without dilution.

There maybe a need to use an evaporation unit to get rid of some of the liquid, hence a larger plant would be required, however there are millions of tons of sh1t available, so scaling should not be the issue.

 

[Wisconsonian]

None of this is remotely feasible with labor and energy prices. Reverse osmosis requires huge amounts of power, pressure drives the concentration (reverses the osmotic pressure) and power is not cheap. The basic problem is that water is a very good solvent, has a huge heat of vaporization, and the nutrients are very dilute.

The common practice is to spread waste water on a water and nutrient hungry crop, such as maize or reed canary grass. Then harvest and remove the crop, and spread the resulting manure elsewhere, simple. The sun does the water removal.

 

[thesilentone]

None of this is remotely feasible with labor and energy prices. Reverse osmosis requires huge amounts of power, pressure drives the concentration (reverses the osmotic pressure) and power is not cheap. The basic problem is that water is a very good solvent, has a huge heat of vaporization, and the nutrients are very dilute.

The common practice is to spread waste water on a water and nutrient hungry crop, such as maize or reed canary grass. Then harvest and remove the crop, and spread the resulting manure elsewhere, simple. The sun does the water removal.

The objective is to improve the quality of the digestate, to ensure 1) It is more saleable and offers better value. 2) To find a solution to the nitrogen shortage currently driven by energy (oil and gas) prices and future shortages.
Technolgies already exist off the shelf, however scale is the driver, as any feasibility would require very high tonnage throughputs.

 

[Wisconsonian]

I disagree, scale is only the driver if the process is fundamentally feasible and profitable. If the process is can not work economically, then scale just makes the losses bigger.

 

[thesilentone]

I disagree, scale is only the driver if the process is fundamentally feasible and profitable. If the process is can not work economically, then scale just makes the losses bigger.

Of course.

 

[Bridget]

The environmental permit application for Willand Biogas in Devon mentioned this,
“The resulting liquid digestate will be stored in three number tanks and be tested to ensure it meets PAS110 prior to being spread or de-watered using the “Morticia” plant which combines pressing cavitation and vacuum evaporation to produce a solid digestate for application to land and clean water which is to be discharged to Spratford Stream under a discharge consent.”

I am unsure whether they ever installed the ‘Morticia’ plant.

 

[thesilentone]

Having an economical and feasible plan is key, however large-scale would be important to ensure there is sufficient supply of product to make it interesting and worthwhile.

If we are to search and find solutions for a good regular supply of organic N as an alternative to the current traditional supply chain, at a price that is both affordable and available, now is the time to act.

The environmental light has been shining brightly on chemical N for some time, the recent events surrounding sky-rocketing energy prices, the destruction of major manufacturers in the Ukraine and environmental pressures combined, mean the days of low-cost chemical based N are long gone, never to return, the politicians will see to that.

We have no shortage of waste product, most in liquid form, now is the time to think big.

 

[deleted user 436632456]

CCS & BCS 21 annual report was out last week which is fairly interesting reading if you are that way inclined.

https://www.qualitycompost.org.uk/images/upload/news_150_CCS_Annual_Report_2021_final.pdf

"The mean nitrogen content of digestate sampled in 2021 was 1.01% m/m. The mean phosphorus and potassium content of digestate sampled in 2021 was 1731 mg/kg and 3142 mg/kg respectively."