Turning Poultry Manure into Biogas

Why poultry manure is harder than cow dung

Cow dung is the textbook feedstock for a biogas digester. Poultry manure is the one farmers try next, and it is the one that surprises them most in the first year. The bacteria inside a digester do not care how much manure goes in. They care about the balance of carbon and nitrogen, the moisture and the ammonia that builds up as the manure breaks down.

Poultry manure has a lot of nitrogen and not much carbon compared with dung. That makes it rich fertiliser, but it also means the digester can fill with ammonia and stop working. It is also often dry. Layer manure from a cage house can be almost solid, and litter from a broiler house carries shavings that float, clump and block pipes. The same qualities that make it good on a maize field make it awkward in a tank.

None of this rules poultry manure out. It means you have to design the system around it, and you have to be honest about how much manure you really produce each day.

What a digester does, in plain words

A biogas digester is a sealed tank where bacteria break down organic matter without oxygen. The process gives off a gas that is mostly methane, with carbon dioxide and smaller amounts of other gases, including hydrogen sulphide. The gas collects at the top of the tank or in a separate holder, and a pipe takes it to a burner. The material left behind, called digestate, comes out at the other end.

The tank has to stay warm, at a steady temperature, for the bacteria to work well. In Ghana the ambient air is warm for most of the year, which helps, but night-time temperatures drop, and a tank above ground loses heat quickly. Buried or well-insulated tanks keep the temperature steadier. The material also has to stay in the tank for long enough. Small digesters are often designed for a retention time of a few weeks, and your builder should tell you the figure for your design.

Think of the digester as a living process that needs regular feeding. Miss a few days and the bacteria slow down. Overfeed it and the ammonia climbs. The gas you see is only the visible part of a process that needs daily attention.

Feed it daily, or do not bother

This is where most farms that try biogas go wrong. A digester wants manure every day. Weekly clearing, which suits a drying pad and a fly-control routine, does not suit a digester at all. If you clear the manure once a week and load it all at once, you get a shock of nitrogen and a sluggish tank.

Here is a worked example for a 3,000-hen layer house, using rough figures that you should check on your own farm. Fresh layer manure is often in the region of 100 grams per bird per day, so the house produces about 300 kilograms of fresh manure a day. Fresh manure is roughly a quarter solid matter, so that is about 75 kilograms of dry matter a day. To bring that to a pumpable slurry of around 9 per cent solids, you need about 830 kilograms of total mixture, which means adding roughly 530 litres of water every day.

That is a lot of water and a lot of handling. It also tells you the honest question to ask first: can you collect 300 kilograms of manure every single day, carry it to the tank and mix it with water without it becoming a second job? If the answer is no, a digester on a small flock will not earn its keep. Our guide to using and selling poultry manure covers the drying route, which may suit a flock of that size better.

If you do go ahead, collect the manure into a sealed store each day and feed the tank from it at a steady rate. Keep the store covered so flies do not breed in it. The same fly rules apply as on any manure pile, and our fly control guide sets them out.

Dilution and the ammonia problem

Dilution is the first thing to get right. Most poultry digesters are fed with a slurry around 8 to 10 per cent total solids, though the exact target depends on the design and the builder. Thicker mixes block the inlet and settle in the bottom of the tank. Thinner mixes waste space and slow the process. Test a sample by drying it, weighing it and comparing the figures, rather than guessing by eye.

Ammonia is the second thing. Too much nitrogen in the tank pushes the pH and the free ammonia up, and the bacteria slow or stop. You see the signs in the gas first: the flame burns weaker, the gas output falls and the slurry smells sharply of ammonia. If that happens, stop feeding, check the mix, and let the tank recover before you feed it again. Adding more manure to a sick tank makes things worse.

Some farms add carbon-rich material such as chopped plant waste to balance the nitrogen. This works in theory, but it needs careful testing and adds a second feedstock to manage. Ask your digester supplier what they recommend for your mix, and get it in writing.

Three digester types a small farm can consider

There is no single best design. The right type depends on your feeding routine, the space you have, the ground conditions and the skills of the local builder you trust. These are the three designs we hear farmers discuss most.

• Fixed dome. A brick or concrete tank with a fixed gas dome on top. Long-lived and well insulated when buried, but it needs a skilled builder and sound construction. Repairs can be difficult.
• Floating drum. A steel drum that rises and falls with the gas inside a concrete tank. Easy to read the gas level, but steel corrodes, and the drum needs regular painting and checking.
• Plastic or bag digester. A flexible tube or bag, usually laid in a trench. Cheaper to install and easy to move, but it is more vulnerable to damage from sharp objects, rodents and the sun, and it needs a careful inlet and outlet.

Whichever you choose, ask the builder to show you a working installation with the same manure type and a similar flock size. A design that works for cattle dung may not work for poultry, and a design that works in one district may need changes in another because of the soil.

What the gas can do on the farm

The gas is most useful when a farm already uses a lot of energy in one place. Cooking for farm workers is the simplest use. A biogas burner in the kitchen or the staff room replaces charcoal or gas cylinders, and it burns cleanly when the tank is working well.

Lighting is the second use, and a gas lamp can replace some of the diesel or paraffin you burn at night. Heating a brooder is possible, but only with a burner designed for biogas. A standard gas ring from a shop will not give a safe or steady flame. Converting a generator to run on biogas is a specialist job. Do not attempt it without an engineer who knows the make and model of your set.

Before you decide, measure what you really use. Write down the cooking hours, the lamp hours and the generator hours for two weeks. Then compare that with the gas the digester can make from your daily manure, using the figure your supplier gives you, not a number from a brochure. A digester sized to a dream of gas will disappoint on the first cloudy month.

The digestate: the part that often pays for the trouble

Everything that comes out of the digester is digestate, and it is worth more than many farmers expect. The nitrogen in it is largely in a form that plants can take up quickly. It has less smell than fresh manure, and when it is applied to the field soon after it leaves the tank, it causes fewer complaints from neighbours than a heap left to rot.

Handle it with the same care as any fertiliser. Apply it to the land, never into a waterway, and keep it well away from any borehole. Spread it when the crop can use it, and do not pile it on bare ground to sit in the rain. Our guide to poultry manure as fertiliser covers the crops that respond well and how much to use.

Digestate can also feed other enterprises. Some farms use it in fish ponds, and our guide to poultry manure in fish ponds sets out the balance you need. If you are also raising insects, the rules change, and the black soldier fly guide explains how larvae handle manure and what to watch for.

Feedstock by feedstock: what to do with each kind of manure

Not all poultry manure behaves the same. The table below gives the usual problem with each feedstock and the first thing to do about it. It is a starting point for a talk with your digester supplier, not a substitute for a test on your own mix.

If you are unsure what your mix is, sample it, take it to a laboratory that tests agricultural material, and ask for total solids, volatile solids and total nitrogen. A few tests at the start save many months of guesswork.

Safety that people skip

A digester is not a safe place to be careless. Methane is flammable and can explode when mixed with air in the right proportions. Hydrogen sulphide is toxic, and at high levels it can knock a person out before they notice the smell. The rotten-egg smell is a warning, but it is not a reliable measure of danger, because it can dull quickly on the nose.

• Treat the tank, the manhole and the gas store as confined spaces. Never enter the tank without a plan and a second person outside.
• Keep naked flames, cigarettes and sparks away from the digester, the gas pipe and the store.
• Check every joint and valve for leaks with soapy water, not with a flame.
• Keep children and animals away from the tank opening, and cover it when it is not being fed.
• Ventilate the gas store and the room where the burner stands.

Build a simple gas safety routine into the week. Check the pipes, the burner flame and the pressure once a week, and write down what you found. If the flame turns yellow or lifts off the burner, shut the gas off and call the builder.

Permits, quotes and the first six months

Get three written quotes from builders who have built poultry digesters before, not just household ones. Ask each one for the design, the materials, the expected gas output, the warranty and the first six months of support. A quote that does not mention feeding, dilution or digestate handling is only half a quote.

Ask the district assembly whether a building permit is needed for the tank and the gas store, and check with the Environmental Protection Agency whether an environmental permit applies to your flock. Ask the Energy Commission whether any licence or registration applies to the gas use. Requirements change, so check current rates and conditions with each body before you commit.

Plan for the first six months as a trial, not a finished project. The tank will take time to settle, the gas output will rise and fall, and you will learn your own mix. Keep a simple log of the feeding, the solids test, the gas readings and any problems. That log is what tells you whether the digester is paying its way.

• Written quotes from three builders, with design and output stated.
• Confirmation of the feeding routine you can keep every day.
• A solids test of your mix before the first load.
• District and EPA answers on file, with current rates noted.
• A safety plan posted at the tank, and a trained helper who knows it.
• A digestate plan showing which land or crop will take it.

When biogas is the wrong answer

A digester is not a status symbol. On a small flock, a few hundred birds on a family plot, the manure is better dried, bagged and sold or spread on the farm's own maize. On a flock that is seasonal, empty for long stretches, or about to be sold up, the digester will sit idle and cost you money. Build one only when the manure is going to arrive every day for years.

It is also the wrong answer if you do not yet have a steady flock. Our chicks arrive the day they hatch, and the first flock on a new site is always the one that teaches you what your manure really is. Wait until your house, your feed and your routine are stable before you spend on a digester. Our poultry business plan guide shows where a digester belongs in the full set of costs, and the break-even guide helps you test whether the gas and fertiliser income justify the spend.

If you are still choosing the size of the farm, our land guide shows how much space the houses and the yard take, and that affects where a tank can sit. Keep the tank well away from the houses and the boundary, because a leaking digester smells the same as a heap that is never moved. Our neighbour guide covers the odour and fly side of living next to a manure system.

Once you have the flock sorted, our layer day-old chicks page lists the breeds and box sizes we supply, and you can ask about current prices before you plan the flock size the digester has to match.

Questions farmers ask