Cow Power
Colder milk, better fertiliser
The generator uses biogas to generate electricity which is either used onsite or exported into the network.
As the cows are milked their milk is cooled rapidly by two plate heat exchangers, one inside the milking shed and one outside the building. The second heat exchanger uses ice-cold water from ice slurry made at off-peak times by a machine normally used in the food processing industry and stored in a large tank. The faster the milk is cooled the better its quality, as bacterial counts are lower and the milk foams less. Milk can be kept on the farm for longer at a cooler temperature meaning fewer tanker visits are needed to collect it. This is an added benefit with the present rise in transport fuel costs.
The contents of the biodigester (digestate) flow out into a pit alongside one containing the fresh material, and are used as fertiliser on the farm. Testing shows that it is a more successful fertiliser than the original material and is a healthier, less odorous substance to spread around the farm. It has a neutral pH, compared with the initial effluent which is acidic, and a much reduced pathogen count.
"We constantly have the digestate tested and can see that the N:P:K [nitrogen:phosphorous:potassium] values are unchanged. They are actually stabilised as the material passes through the system and are not lost through leaching or evaporating," says Mr Bywater.
At the moment all of the digestate is spread on the farm but it could be used on neighbouring farms. Mr Bywater has several ideas on how this product could be used, including processing it to extract the nutrients that cause unwanted enrichment of waterways and making the remainder safe for discharge. This is ideal for areas where the ground is waterlogged for long periods, preventing the spreading of manure slurry onto paddocks.
The solid contents of the biodigester flow out into a pit for use as fertiliser.
Mr Bywater is also convinced that the digestate could be used as a food source for farming algae to produce bio-oil. This would replace the mineral diesel presently used in the generator and make the system more independent of fossil fuels. "We have all the raw materials, such as nutrients, sunlight and carbon dioxide, and could use them to grow algae in large plastic tubes or 'photobioreactors'," he says.
The present plant could produce more biogas if more manure was collected. Mr Bywater hopes this will happen if Landcorp decides to build a concrete feed-pad nearby – not only would the animals get better nutrition and produce more milk, they would also leave more of their droppings behind for BioGenCool to process into biogas.
Natural Systems Ltd is looking for investors and people interested in setting up similar systems. There has been strong interest from farmers in Southland who often deal with waterlogged soils and find it hard to dispose of dairy waste. An experienced dairying practitioner in Australia is also keen to find out more and the company has been in contact with the renewable energy centre at Newcastle University in the United Kingdom, which is contemplating installing a biodigester on its farm.
