Digestate storage: tanks, lagoons, covers and required capacity

Digestate is stored so that it can be applied to fields at the right time, and not when the tank is full. The best solution in terms of emissions and energy is a closed tank with a gas-tight roof, connected to the gas system, because it captures the methane released from the residual potential of the digestate. The capacity should cover the whole production in the period when application is prohibited, impossible (frozen soil, no vegetation) or uneconomic.

Why such a reserve

The digestate stream arises throughout the year, while the demand of fields is seasonal. In addition, the regulations on protecting waters against nitrates set periods when livestock manures may not be applied. The result is the need for storage over many months; in practice capacity is planned in months of production, usually at least six, and often more. The exact requirement for a given plant is set on the basis of the applicable national regulations and the conditions of the permit — do not adopt values "off the top of your head".

Types of stores

Emissions from the store

Ammonia escapes from digestate (depending on pH, temperature and the surface of contact with air), as does methane, which is formed from incompletely degraded organic matter. The residual methane potential of digestate depends on the HRT, temperature and substrates; the shorter the retention time, the more gas can form in the store. This is the reason why the post-digestion tank (the second or third digester) is often designed as a digester with biogas recovery. The residual potential can be measured in a test (usually incubation of a digestate sample for several dozen days at the storage temperature).

For open tanks, methane and ammonia also depend on temperature: in summer emissions rise many times over compared with winter.

How to calculate capacity — an example

Assumptions (illustrative): the plant receives 65 t/d of substrates, and the loss of mass due to degassing is about 5% (part of the VS converted to gas), so digestate arises in an amount of about 62 t/d, i.e. about 62 m³/d (density about 1 t/m³). We assume a storage period of 6 months (180 days) and a 10% safety reserve:

• 62 m³/d × 180 d = 11,160 m³,
• after adding the 10% reserve ≈ 12,300 m³.

If part of the digestate is divided into a solid and a liquid fraction (separation), the volume of the tank for the liquid part is smaller, but space is needed for the solid fraction (slab, shelter). For an open tank, net rainfall onto the surface is added (mm × area), as well as freeboard (a reserve for foam and waves). For recirculation to the mix, the volume returned to the digester is subtracted.

Good practice

1. Design two or more tanks to be able to carry out repairs and to have flexibility when emptying.
2. Provide mixers or sediment discharge systems, because without mixing sediments (sand, struvite, carbonates) form on the bottom.
3. Provide a safe station for loading tankers and for spreading, with drainage of leakage to a sealed tank.
4. Carry out a leak test and plan periodic inspections (visual, leak sensors, inspection drainage).
5. Gas-tight tanks require overpressure and underpressure protection and always a classification of explosion hazard zones.
6. Equip with level measurement and an overfill alarm.

Most common mistakes

• understated capacity leading to transport at an inappropriate time,
• no cover and high emissions, conflicts in dealings with neighbours,
• a tank without the ability to mix and remove sediments,
• omitting rainfall in the balance of open stores,
• no level measurement and overfill protection.

The digestate store is part of the plant, not an add-on: it affects the energy balance, emissions, relations with the surroundings and the fertiliser value of the product. It is worth planning it together with the digester.

Residual methane potential — how to assess it

Good practice is to take a digestate sample from the outlet of the last digester and incubate it at a temperature close to the storage temperature for several dozen days, measuring the volume and composition of the gas. The result expressed in m³ CH₄ per tonne of digestate makes it possible to calculate the energy loss and the emission from an open store. An illustrative example: if a test showed 5 m³ CH₄/t, then with 22,000 t of digestate per year an open store could emit up to 110,000 m³ of methane per year. Such a figure justifies a detailed cost–benefit analysis of a gas-tight cover; the size of the emission alone does not, however, settle the profitability of the investment. In climate calculations methane has a many times greater warming potential than CO₂, so even a small percentage of losses matters.

Safety

A gas-tight tank is an element of the gas system: it requires protection against overpressure and underpressure, classification of explosion zones, leak checks and entry procedures. An open tank requires fencing and protection against people and animals falling in, and consideration of the H₂S risk during mixing.