Farm Digesters

Anaerobic digesters produce clean renewable biogas, and reduce greenhouse emissions, water pollution and dependence on artificial fertilizers

Jonathan Letcher

• Publisher: Green Books
• Year: 2015
• ISBN:
  9780857842374
• Topic: Technology & Engineering
• Subtopic: Sustainable Agriculture
• Index: Technology & Engineering

As well as producing commercially valuable biogas, anaerobic farm digesters have many other benefits. They enable the livestock farmer to recycle fertilizer from organic waste, to produce saleable compost as a byproduct, and reduce the environmental impact on water and atmosphere at the same time. Digesters let us run farms more economically, and make our energy supplies and our food production more sustainable.. This straight-forward book provides a wealth of useful information, including the benefits of using a digester, how to go about installing one and how policy makers can influence a farm digester programme.

Chapter One

WHAT DIGESTERS DO, AND WHY THEY ARE SO IMPORTANT

Anaerobes are the naturally occurring bacteria that break down organic wastes in the absence of air – in a marsh, for instance. An anaerobic digester is essentially a machine in which this natural process is encouraged and controlled. In nature, anaerobic bacteria help to reduce organic materials such as faeces or dead plants and animals into their basic elements – including water, nutrients and gases – allowing them to be recycled effectively and safely back into the ecosystem, rather than causing it to be clogged and poisoned. The bacteria in a digester have the same beneficial effect on the organic wastes produced by our modern society – such as livestock slurry and manure, sewage sludge, and the food waste in domestic rubbish. In their raw untreated state, these wastes, which we produce in very large and increasing volumes, can cause serious problems to our health or our environment. They are, in particular, a major source of greenhouse gas emissions. Digesters allow us to control those problems, while turning the waste materials themselves into three valuable by-products – renewable biogas, recycled organic fertilizer, and a fibre which can be used as the basis for peat-free horticultural composts.

The ability of digesters to help reduce greenhouse gas emissions – not only by producing renewable energy, but in a number of other ways – has become increasingly significant in recent years. As a result, many countries are now planning to greatly expand their farm digester programmes. But other benefits of the process have been important to mankind for hundreds, if not thousands of years. Simple digesters have been used in India and China for many centuries, allowing farmers to produce their own source of both energy and fertilizer from human and livestock waste. Many people in rural Britain, including my own grandfather in Suffolk, also traditionally used what was in effect an even simpler digester – known locally as a ‘bumby hole’ – in their cottage gardens. The daily contents of the lavatory would be thrown into this small pit, together with some straw or plant waste from the garden, covered over with a few shovelfuls of earth, left to mature for a year, and then spread back onto the soil as fertilizer. My grandfather had no idea that the faint smell coming from the pit on warm days was caused by biogas, which could potentially have been harnessed as a source of energy. But he did know, as people had known from time immemorial, that the nutrients and organic fibre in human or animal waste were essential to good husbandry. He also knew that the natural processes at work in the bumby hole allowed him to use those materials more effectively, and without causing disease.

It was a simple, practical system, perfectly effective on that scale. But if the people in the big housing estates which were spreading towards his cottage from the nearby town had all adopted this system, there might soon have been an outbreak of cholera. To deal with the same materials on such a large, modern scale, a more sophisticated approach was needed – the sewage works, and the sewage sludge digester. The same thing is true of farms. It is common knowledge that in traditional mixed farming systems, the manure from livestock was a vital source of fertilizer. The slurry or manure from modern intensive livestock farms is an equally rich source of nutrients, and of biogas. But the large number of animals that we need to help feed our rapidly growing population has led to such an increase in the volume and concentration of livestock waste that its very richness now causes serious environmental problems. Digesters can reduce these problems and turn the otherwise noxious wastes into a valuable, sustainable resource.

The benefits of the digester process in detail

Some of the benefits of the process, which are summarized below, are clearly of national or even global importance. Others may seem relatively minor matters, or seem to have little direct relevance to farms. But they are all part of the overall picture. They either give society as a whole a reason to support or encourage the use of farm digesters, or help to make them affordable by reducing a farm’s costs or improving its returns.

The order in which the benefits have been listed has been chosen to follow a natural sequence, and to reduce repetition, rather than to reflect their relative importance. Most of the key aspects have been dealt with only very briefly here, as they will be considered more fully later on. The three by-products, for example, have each been given a chapter of their own.

Anaerobic digestion systems:

1) Produce an on-farm source of recycled nutrients, reducing our reliance on artificial fertilizers.

2) Produce a rich, potentially valuable organic fibre.

3) Produce renewable energy in the form of biogas.

4) Kill most of the pathogens (organisms responsible for causing disease) found in organic waste.

5) Greatly reduce its smell.

6) Allow more efficient management of grassland.

7) Make livestock waste easier to store, handle and spread.

8) Kill most weed seeds.

9) Encourage beneficial earthworms and clovers.

10) Reduce water pollution.

11) Reduce greenhouse gas emissions in a number of ways.

1) Nutrient recycling through farm digesters

Most organic wastes, especially livestock manure and slurry, contain substantial amounts of the nutrients which are vital to the growth of grass and other crops. In the raw, untreated waste, some of those nutrients are not in a form in which they are readily available to plants, so they are often lost into the groundwater or streams before they can be of use. This can cause a variety of environmental problems, especially as the farm then has to add extra artificial fertilizers to make up for the loss. Digester systems help to make the nutrients in organic waste more accessible to plants. This allows those nutrients to be used more effectively as a source of fertilizer, and many dairy farms with digesters no longer have to apply any artificial fertilizers at all. This helps to reduce farm costs, as well as helping with environmental problems, including water and atmospheric pollution.

The manufacture and transport of artificial fertilizers, especially nitrates, depends heavily on our diminishing supplies of fossil fuel. So the recycling of nutrients through farm digesters can also help to make our food production more sustainable.

2) Producing compostable organic fibre

A typical digester system generally includes a means of mechanically separating the processed material into a thin liquid, used as fertilizer, and a nutrient-rich fibre. This fibre can be spread on fields, like the liquid, as a source of nutrients and to replace vital organic matter. It is also ideal for turning into compost products. Some farms with digester systems have produced and marketed their own soil-conditioning products based on this digested fibre. It has also been used successfully, on a commercial scale, as the basis of multipurpose peat-free seed and potting composts. Using the fibre to make compost can give farms a substantial extra income. It can also help to reduce the environmental problems caused by the use of peat in horticulture.

Digester plant built in 2010 by Fre-Energy Ltd on a mixed livestock and arable farm in Devon. To the left of the digester are the separated fibre store, generator shed and digested liquid store, which is covered by a flexible triple-skinned air-inflated roof. The liquid provides almost all the fertilizer used on the farm’s 600 acres of grassland and arable crops. The slurry passes straight from the cattle shed to the digester reception pit, so no waste accumulates in the yard. This simplifies waste management and – like other aspects of the process – reduces the risk of pollution. Most of the gas is used to generate electricity for the farm itself and for sale to the grid; but some is also used to provide heating for the farmhouse, and hot water for the house and dairy. (© Author)

3) Producing biogas

Modern farm digesters were first developed and used in Britain, as in many other countries, in response to the global energy crisis of the 1970s. Digesters produce biogas both from organic waste materials, such as slurry, manure and food wastes, and from specially-grown energy crops like sugar beet and maize. Biogas is a very versatile form of energy. Most digester plants being installed in Britain today use the biogas they produce to generate electricity. But the gas is also widely used around the world for many other purposes – for heating and cooking, on a farm or in a domestic environment, for injecting into national gas grids as a replacement for fossil natural gas, and as a vehicle fuel for cars, buses and even trains.

4) Killing pathogens in organic waste

The bacteria in a digester destroy many of the disease organisms found in organic wastes such as livestock slurry and sewage sludge. (Sewage sludge is the noxious, concentrated material left behind after most of the liquid has been removed from the waste water and sewage coming into a treatment works.) The ability of digesters to control pathogens is one of the main reasons why they have been used, for over a century, as a standard method of treating sewage sludge. The operating temperature of the kind of digesters normally used on both farms and sewage works is not high enough to completely destroy the pathogens, but it reduces them by up to 90 per cent.¹ This allows the digested sludge to be safely spread onto farmland, as a valued source of recycled organic fertilizer. More than half the sludge produced by Britain’s sewage works is now processed in digesters.

The digester’s ability to destroy pathogens can also be important in other ways. In the future, the processing of non-farm organic wastes – especially domestic food waste – in on-farm digesters, together with the farm’s own livestock wastes, may become much more common in Britain, as it already is in some other countries. This integrated approach can provide a cost-effective and environmentally benign method of recycling and disposing of these wastes. It also helps to improve the economics of farm digesters, by creating an ongoing reliable income (known in the UK as a gate fee) for processing these materials on our behalf. In these cases, high-temperature (called ‘thermophilic’) digesters may be used in combination with the mid-temperature (‘mesosphilic’) type usually found on farms or sewage works. This will ensure that an even higher proportion of the pathogens which may be present in some wastes are destroyed.

5) Odour reduction

Another reason why digesters are used on sewage works is that the process greatly reduces the smell of organic waste. Smell can also be a problem for some livestock farms, especially those which are close to a village. In Britain, smell tends to be a problem mainly for pig and poultry farms, and most of the digesters which have been installed on our pig farms were intended mainly for this purpose. But cattle waste can also create a powerful smell, especially when raw untreated slurry has been stored for some months in a slurry store or lagoon before spreading, and has partially decomposed. In the USA, in particular, a number of cattle farms have installed digesters mainly for odour control.

There are other methods of controlling the smell of pig or cattle slurry, such as aeration, which involves pumping large volumes of air through the slurry. The capital cost of an aeration system is lower than for an anaerobic digester, so it was often recommended by government farm waste advisors in the past, when atmospheric pollution was not seen as a major problem. But aerators consume large amounts of electricity, and allow greenhouse gases to escape into the atmosphere; whereas anaerobic digesters reduce greenhouse gas emissions, and are net producers, rather than consumers, of energy.

The ability of digesters to control odour is also important when they are used to process domestic or commercial food waste. This – together with their other environmental benefits – helps to give them a significant advantage over alternative methods of disposing of such waste, such as composting or landfill. In Europe, many composting plants installed to process domestic food and other organic wastes are now also using anaerobic digesters.

6) More efficient management of grassland

All animals instinctively avoid their own waste materials, as a potential source of disease. But the material coming out of a digester, after being subjected to intense bacterial activity – often for twenty days or more – is very different to raw slurry or manure. The animals no longer regard it as their own waste matter. It has very little smell, and contains fewer disease organisms. And as the process breaks down or removes most of the organic matter, it is thin enough to sink easily into the surface, rather than capping and contaminating the grass or soil. So it is possible to use a field for grazing or silage making much sooner, after it has been spread with digested liquid, than would be possible with raw slurry or manure. This allows the grassland – which is one of the most important assets of most cattle farms – to be managed more efficiently.

7) Easier storage, handling and spreading of livestock waste

Even a modest-sized herd of 100 dairy cows can produce several tonnes of slurry in a single day. Raw, undigested cattle slurry is a very difficult material to deal with. It is too thick to be easily pumped, but too close to liquid form to be moved with a front-end loader, like manure. When raw slurry is put into a storage tank or lagoon, the solids tend to rise to the top and form a crust, which makes it difficult to remove with a vacuum tanker for spreading. But mixing it with a mechanical paddle driven by electricity or by a tractor can consume a lot of time and energy.

With a digester-based system the problems of waste management, handling and storage are greatly reduced. The bacteria break down much of the organic matter, and the fibre and thin liquid produced by a digestion and separation system are much easier to deal with than untreated slurry. The fibre can be stored in a heap, and later either spread by a muck spreader or used to make compost. The liquid is easily pumped, can be...