Anaerobic Digester Silage and Clamp Design

What is Good silage?
What is good silage for an anaerobic plant? In truth, it is little different from good silage for a dairy cow. The principals for good silage production and ensiling remain the same. We should be aiming to grow and harvest the crop at the target dry matter and energy levels to optimize the silage quality as with any traditional silage plan. So here we will consider the clamp design and operations to ensure we make the most of the incoming crop.

Maize silage for AD plants tends to be chopped to shorter lengths than for cattle feed and contractors will often use a special Biogas knife cylinder within the forage harvester to achieve this. From here the rules of good ensiling are no different from those followed by livestock farmers for years. Put simply, speed, good compaction and the minimization of possible contaminants are the key.

Ensiling & Compaction
Speed of ensiling is vital to the preservation of the crop. The bacteria types Lactobacillus and Streptococcus are already present on the crop and these under the correct conditions will convert carbohydrates into lactic acid. This is a relatively strong acid and as levels within the clamp rise, the action of the bacteria will cease and the material will become stable and effectively pickled. These bacteria are anaerobic and will not tolerate high oxygen levels. It is therefore imperative that the levels of oxygen in the clamp are reduced as quickly as possible, and the clamp then sealed. If we produce conditions with higher oxygen levels then butyric acid producing bacteria can thrive and this does not produce stable crop conditions. The aim should be to fill the clamp as fast as possible and seal the silage with an airtight cover.

Compaction of the crop has little to do with minimizing the storage space and a lot to do with oxygen reduction. Whilst good tight compaction is the goal, there is more to this than might first appear. Contrary to the "bigger is always better" rule, good compaction is not achieved by simply using a massive vehicle. Material should be ensiled in thin layers and compacted. The use of larger vehicles can work against the aims here. As the wheels of the vehicle pass over an area, the crop is compressed and the oxygen rich air is squeezed out. As the wheel passes onwards the crop recovers and rebounds slightly sucking air back in again. The anaerobic bacteria within the clamp produce carbon dioxide; this can be squeezed out and replaced by oxygen rich air in cases of "over rolling". This is the complete opposite of what we are trying to achieve.

Keep It Clean
Minimization of contaminants within the crop needs to be considered right back to the field. With maize harvesting this is less of an issue than with wilted grass silage but the principals are the same. Soil contamination is the common enemy here. The header on the forage harvester need to be set up to minimize the amount of soil going in the front end, but the tyres of the haulage traffic also need to be considered. In Europe it is common for the haulage trailer to drive up onto the clamp to discharge. Whilst this poses all sorts of stability issues, its also introduces soil contamination if this is the same tractor and trailer that travelled in the field. Good practice would see the trailers discharging onto clean areas and a "clean" vehicle transferring the crop to the heap. Consideration to wheel washes should be made in wet muddy harvest conditions.

The Importance of Clamp Design
So how does the design of the clamp influence the operator's chances of meeting the goals for high quality silage? Layout is important to give the best conditions for ensiling, but the key element is in minimizing losses once the clamps have been filled, and more importantly when they are being emptied. The most crucial element that needs to be determined is the area of the silage face as the crop is being used. As soon as the face is opened, the silage is exposed to high levels of oxygen and the bacteria that
have been dormant find conditions where they can start to break down the energy stored in the silage to form a secondary fermentation. To minimize this, we need to minimize the exposed face. Good compaction will limit the depth of this fermentation but reducing the exposed face is the aim to minimize in clamp losses. The age-old rule observed by dairy farmers was a target use of 2' per day. For a typical 6 month winter, this means a dairy needs around 360' of clamp or about 110m. This is not too hard to achieve, as the summer cropping will result in 2 or 3 grass silage cuts together with a maize harvest later in the season. To store and get access to this, most livestock farms would use a 3 bays clamp system with each bay about 36m long.

An AD plant runs 365 days a year so the requirements are twice those of a traditional dairy farm. That suggests we need 220m of clamp space but the crop is usually all harvested in a single operation. It is conceivable that someone may build a single space 220m long clamp but it is hardly practicable or economic. A three or four clamp system is more common. So we will assume a 4 clamp system with each around 55m long. It should be noted that no allowance has been made for the loss of storage associated with the ramp up onto the clamp. This will generally add 4-5m to the overall clamp length. We now need to calculate the height and width of the clamps to achieve the target storage tonnage.

Height v Footprint?
Obviously the greater the height, the smaller the footprint of the clamps. This obviously saves costs in construction but also collects significantly less rainwater. This rainwater needs to be treated as dirty water and is usually a greater volume than can be accommodated by the AD plant so needs to be handled in a separate system. The aim should be to reduce the footprint where possible so higher walls should be considered. But just how high should we go? Traditionally clamps were built 2-3m high but today's AD clamps go much higher. Experience suggests that around 5.0m is the sensible maximum depth. Above this and the vertical flow of effluent though the crop during the initial ensiling process leads to increased losses at the base of the clamp.

We can now calculate the size options for a silo. Assuming a target capacity of 10,000 tonnes of maize silage, at a density of 725kg/m³ we need 13,800m³ of storage space. Using the 4 bunker system with each clamp 55m long and lets assume 15m wide, the walls need to be 4.2m high to get the required tonnage. If we increase the height to 5.0m we can reduce the widths to 12.6m. This reduces the amount of collected rainfall by around 375,000 litres over the 4.2m high clamps (allowing for an additional 10m wide apron area). In a 4 bunker system it is common to leave both ends of one bay open to allow silage to be fed to the digester whilst ensiling takes place. Leaving all the clamps open at both ends increases the overall footprint without any significant operational benefits.

Wall Design
We should now consider the design of the walls. The design criteria for silo walling in the UK has been used for the least 20 years and is set out in BS 5502 part 22. This is now becoming out-dated since the upper limit given for compacting vehicle weight is 10 tonnes. Today's silage contractors are regularly using industrial loading shovels for filling and compacting clamps and these regularly exceed 20 tonnes. It is clear that the BSI code is not suitable to give us the design information we need. As a result ACP (Concrete) Ltd has been utilizing a North American code to overcome this issue. The loads generated in clamps with high walls and high compaction loads are massive. Traditional methods of wall construction cannot be adapted to suit these pressures so ACP have designed an entirely new product range using a 280mm prestressed panel. The enormous leap in unit load capacity offered by these panels has been achieved whilst still retaining all the economic and practical benefits of using prestressed wall panels. The rapid construction and relatively slender section of prestressed concrete wall panels have been appreciated by livestock farmers for the last 30 years, now the requirements of the AD plant operator can also be met by these new 280mm panels. A good silage clamp for an AD plant is perhaps not that different from a good silage clamp for a livestock farmer.

For further information please email ACP (Concrete) Ltd

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