The DPI bioenergy project involves converting the raw organic material in dairy effluent to create methane, which can then be used as a feedstock to produce electricity or as a gas to heat water or cool milk. US-based researchers have found that the excretion of a single cow can produce about 100 watts of power.

Methane has not been associated with green and clean power, primarily because research has found that it is at least 20 times better at trapping heat in the atmosphere than carbon dioxide. However scientists have claimed that controlling methane emissions from animals such as cows will be hugely important in preventing climate change, particularly in Australia where the agricultural sector is such a substantial part of the national economy.

“In the face of worsening climate change, it is vitally important that we explore low cost options for the capture and use of methane as a source of energy as opposed to a harmful greenhouse gas which would otherwise go into the atmosphere,” says DPI Effluent Specialist Barrie Bradshaw.

The DPI researchers have found that effluent routinely captured on a 500-cow dairy farm can produce methane equivalent to 4,000 tonnes of carbon dioxide per year. In part, this is because producing 1 kilowatt hour (kWh) of methane-fired electricity generates 30 times less carbon dioxide equivalent of greenhouse gasses compared with coal-fired electricity generation.

Piloting manure-able energy

The DPI researchers have set up a trial site at Ellinbank, where a six-week study treated a total of 53,000 litres of effluent. The effluent was transferred to a digester, which uses the beneficial attributes of anaerobic bacteria to break down the raw organic solids in the effluent to produce methane, water and other nutrients.

The trial has demonstrated potential, says Project Leader Richard Williams. When the pilot, which only took a portion of the effluent produced on the farm, was scaled up, the results showed that the 440-cow Ellinbank farm could create 42,000 litres of methane per day and this could be converted into 120 kWh of electricity each day, or enough to power 60 homes a day.

More advanced testing will focus on making the anaerobic digester more cost-effective, since the initial work suggested that the farm would require a methane digester system that costs somewhere around $400,000.

Mr Williams notes that the $400,000 digester would actually be going hungry because it could use more organic waste than is currently being generated. With an annual saving of $15,000, which includes the benefits in water savings and proposed carbon tax, the payback period would amount to 26 years.

“The challenge is making it five years or less,” Mr Williams says.

“A lot of the proposals that we have for anaerobic digesters involve lots of steel and lots of concrete tanks, but we think we can build a methane digester by using a hole in the ground and a plastic cover. There’s some work happening on effluent ponds in dairies and covering those ponds to capture methane.”

Mr Williams believes that the key to creating an effective low-cost digester may lay in changing the proportions of effluent ponds. Specifically, the group are looking at changing the shape of the hole from a short rectangle (20 x 40 m), which he notes is quite difficult to cover, to making the pond quite narrow (3 x 60 m).

He adds that if the cost of systems for converting methane into electricity could be reduced to $100,000 or less, the technology may become viable for producing on-farm electricity, and reduce both direct costs and the annual carbon footprint of the Victorian dairy industry.

Cow powering a wide range of applications

The research has found that there are a range of different energy generation options available to deal with the methane produced, and that using the effluent to generate electricity may not be the best option.

“Once we’ve got methane - that is a ready-usable energy source. We could use it to power hot water or we could use it to put central heating into the farm house,” Mr Williams says, adding that it could also be used to cool the milk using methane-powered refrigeration.

“We know that when we convert methane to electricity we only end up with 30-33 per cent of the energy we started with as electricity. If we reclaim some of the waste heat as hot water we can take that efficiency up to 60-70 per cent.”

“But when we convert electricity to the cooling of milk, the coefficient of performance through refrigeration is around 2.5:1 to 3:1. So if we go from methane gas to electricity to cold milk, we should end up with the energy in methane equalling the heat energy removed from the milk. We are still looking for information on the efficiency of gas-powered refrigeration units to see if that might be a better alternative,” Mr Williams says.

“At the moment we know the potential of our effluent, we’ve got ideas on how we can use it and in the next three to four years we’ll be looking at trying to make it a reality on our farm here at Ellinbank. Because we can do things with research funds that don’t impact on the profitability of the farm, we can make sure that it works and have demonstrated it here and then encourage farmers to take it up as it becomes a viable option for them.”

“One of the big advantages we see from the production of electricity from methane on-farm is on single wire earth return lines where farmers are struggling to get enough electricity from the grid. If they’re producing some on-farm that’s that much less that they need to take from the grid, which should lighten the grid load, meaning that the grid may not need upgrading in the immediate term.”

Reducing the environmental footprint

The project has the potential to reduce the environmental footprint of farms in a range of other ways.

One of the most useful by-products of the digestion process is water, which settles at the top of the pond, which was reused on the farm. The pilot found that reuse of 10 ML of water each year resulted in an annual saving of $1,000.

Since the digestion process focuses on mainly carbon, the nutrients in the organic matter are conserved and reduced to more soluble and biologically-available forms, creating a quick-release fertiliser. A dairy generating 30,000 litres per day of wastewater would be creating a benefit of 9 kg of nitrogen per day.

In the United States, cow-powered farms also compress and dry out the leftover solid effluent, using the fluid as a fertiliser and the solids as bedding for the herd and compost. The project also helps to prevent groundwater pollution, a common side-effect of manure storage.

Challenges in producing methane, manure and milk

In the next year, the research group will focus on designing the new digester in an effort to demonstrate the cost-effectiveness of the system. This will primarily involve calculating the dimensions of the trench to ensure all solids are captured.

Mr Williams notes that such modelling “will allow us to determine how much methane we can produce and how much pond length we would need to make a methane digester that didn’t have solid effluent overflowing”.

If the group can model more cost-effective digesters, and make the technology more accessible for drought-affected farmers, both the agricultural sector and the national economy can profit from a wide range of benefits, including generating renewable energy, potent fertilisers and reusing a considerable amount of water. While it may not be the ‘cleanest’ energy around, cow power is certainly one of the greener options available to farmers.