On-farm or what are also known as farm-scale plants are most often built to provide an income from the that is made from manure. The manure which is treated may be from stock litter waste created when stock is kept under cover, plus farm slurries and farmyard run off. There may be other sources of organic feedstock added to these plants, as the most cases they are primarily intended to take manure.

Due to economic constraints, and the need to avoid time spent running these units and maintaining them, these anaerobic digestion systems are invariably the simplest of designs and built to the lowest budget’s possible. In fact, in many cases these plants are, what is known as a “distress purchase”, because the main motivation towards the installation, may be a need to reduce farm run-off pollution from the working areas of farm buildings, and also in highly nitrate sensitive watercourse areas, pollution of nearby watercourses. Another reason for installing these plants can also be for odour reduction.

Many of these plants are appearing in the landscape, throughout much of Europe and the U.S. They are single stage, continuously stirred tank reactors (CSTRs). In other words they comprise of essentially just one large mixed digester vessel. The mixture of the feedstock, which is made into a slurry by cominution and the addition of recirculated digestate, or in the case of existing slurry just a slurry itself, is pumped into the large reactor tank. The mixed liquid, which is known as the substrate in these plants has a solids concentration of approximately 4% to 5%, and the temperature is normally 35 C. It is known as mesophilic digestion.

The financial viability of these plants is often finely balanced. That being the case, that being the case there have been a number of cases where budgets have been paired and the resulting newly installed biogas installations have been almost too cheap for viable operation. Cheap tank construction, and rushed installation, with the aim of keeping costs down, and minimizing construction periods so that plant owner can commission the facility rapidly and begin to get pay back very quickly, can play so much pressure on installation contractors, are too many corners are cut. Such plants, seldom meet expectations, and the design life of them can by experience, the remarkably short. It is unfortunate, that in current times, when there are many anaerobic digestion EPC contractors vying for work, that each is being forced to compete on price. That means that we can expect a surprisingly large rate of plant failures in the next few years, for the owners of budget on-farm biogas digesters.

Many biogas plants of this type currently being put forward in bids by the owners of on-farm biogas plants, to accept additional household food waste. While this may be a salvation for the farming business, as accepting say to up to 10 per cent of its throughput as food waste, will greatly increase the biogas yield, low cost budget biogas plants seldom possess sufficiently sophisticated pipework and pumping equipment, for this use.

The result, seen in many such food waste co-generation contracts, has been that adding the food waste has resulted in plant blockages, lost biogas production due to excessive downtime, and unhappy council waste disposal departments. The owners of on-farm biogas digesters need to take professional advice from expert biogas plant designers before they bid, and include in their prices for upgrading their biogas plant systems, before they start to add food waste.

Them we all should applaud farming businesses that are taking on board a real financial risk in installing farm biogas plants. Not only are they are improving the environment of their farms for the good of the general public, but they are providing much needed renewable energy, without which governments would be unable to reduce the rate of global warming caused by greenhouse gases emitted by fossil fuel consumption.

On-Farm Anaerobic Digestion Plants
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