Adapting systems used to provide clean drinking water almost eliminates methane from dairy effluent. Anne Lee reports.
Careful science and a pinch of good fortune have resulted in one of the biggest breakthroughs to date for New Zealand dairy farmers searching for ways to cut methane emissions.
Lincoln University scientists, working with Ravensdown on effluent technology which clarifies effluent, have found the same drinking water treatment additive it uses almost eliminates methane emissions from effluent ponds.
It’s resulted in a new system that allows the additive to be mixed “in-line” as the effluent flows to the effluent pond.
Methane emissions from effluent ponds make up about 4-5% of a dairy farm’s total methane emissions.
The ability to cut that out almost altogether is significant given most other practical, ready to go options result in a drop in milk production.
It’s also significant in terms of helping farmers get to the looming target set by the Government of cutting biogenic methane emissions by 10% from 2017 levels by 2030.
Lincoln University emeritus professor Keith Cameron and Lincoln University professor Hong Di had been carrying out further studies into the ClearTech effluent system they helped develop when they made the methane reduction discovery.
That system uses an additive, iron sulphate, commonly used in treating drinking water to improve water quality.
The scientists were checking for any unintended consequences of using the additive, looking for any negative outcomes.
Instead, they found a major positive and have been working to understand the greenhouse gas (GHG) benefits and what’s behind them further.
A peer-reviewed paper was published in the Journal of Soils and Sediments earlier this year.
It detailed the experiments carried out at Lincoln University and the results that showed methane emissions could be reduced by 99.9% when iron sulphate was mixed with the effluent.
Initially the studies collecting gases from treated effluent were carried out in small experimental-sized columns.
After the discovery that methane emissions dropped by about 95% and some further testing, the decision was made to scale up the experimental design.
That meant the installation of 100,000 litre tanks at the Lincoln University Dairy Research Farm (LURDF).
Keith says there was some nervousness given scaling up biological system experiments can see a drop in efficacy.
“It’s often called going into the ‘valley of death’,” Di says.
But in this case the reverse happened and the reductions were even greater at up to 99.9% less methane emitted.
They also found the effect on methane reductions continued for two months after the last dose of iron sulphate, boding well for New Zealand dairy farming systems where seasonal dry-off means no effluent is added to the pond over winter months.
Based on their research and in conjunction with the scientists, Ravensdown has developed a new system, EcoPond which enables the iron sulphate additive to be mixed with effluent “in-line” as it flows from the farm dairy and yard to the effluent pond.
The EcoPond system differs from ClearTech in that it doesn’t include a clarification tank where the effluent is mixed with the additive to allow the more solid portion of effluent to settle out leaving clarified water which can then be recycled and used in dairy yard wash down.
Because the EcoPond system mixes with the effluent in-line it can be readily retrofitted into most existing effluent systems.
Cutting out the water clarification part of the effluent treatment process also means the system will be much cheaper with fewer tanks, less ground-work and plumbing.
ClearTech product manager Carl Ahlfeld says the EcoPond system will likely have a capital cost of about a third to half the price of the clarification system.
It will use similar technology to monitor the process of mixing the iron sulphate with the effluent to ensure the right dosage is maintained to accommodate variations in effluent pond sizes and effluent volumes as well as seasonal weather variations and effluent characteristics.
A storage tank will be included to contain the iron sulphate and its volumes will be monitored automatically and co-ordinated with deliveries so the additive never runs out.
On top of the capital cost farmers will have an annual cost for the additive which, based on current pricing for the ClearTech system could be about $8000 for a 550-cow farm milking twice-a-day for 270-days.
At carbon prices of $65/tonne carbon dioxide equivalent (CO₂-e) the cost of the additive is likely to be more than buying carbon credits. It’s worth noting though that it is unclear how farming emissions will be treated in an emissions trading scheme.
It’s also fair to note that buying carbon credits to offset emissions isn’t going to help the planet.
Keith says the ClearTech system gives farmers the same methane reductions as the EcoPond.
Di and Keith’s large-scale study at the LURDF found that along with the reductions in methane emissions, carbon dioxide (CO₂) emissions from the effluent were halved and hydrogen sulphide emissions were also reduced. There was a small increase in nitrous oxide emissions but it was very small at less than 3% of the total CO₂-e GHG emissions.
To put the methane emissions reductions in perspective in terms of potential reductions, nationally Di and Keith’s paper points out that based on Ministry of Primary Industries (MPI) data from 2014 more than 9700 dairy farms have effluent ponds and the average pond volume is 1745 cubic metres with average storage at 86 days.
If the iron sulphate treatment system was installed to treat each of those ponds and, at a conservative estimate of a 90% reduction in methane emissions, the total methane reduction would be 622,890t CO₂-e.
That’s more than $40.4 million worth of carbon at the current $65/t CO₂-e NZ price.
The system has been installed at the LURDF to treat the farm’s effluent and allow further studies with another commercial-scale system being installed on a Canterbury dairy farm.
The system is expected to be commercially available to farmers next year.
How it works – the science
The iron sulphate added to the effluent works in two ways to reduce the production of emission in effluent ponds.
It boosts the growth of other naturally occurring bacteria – iron-reducing bacteria and sulphate-reducing bacteria – in the effluent pond and both of these naturally outcompete and inhibit the methanogens responsible for producing methane.
It also keeps the redox potential (a measure of the effluent pond’s aeration or oxygen status) at a level where the methanogenesis reaction can’t occur.
There’s some complex chemistry involved in understanding how the iron sulphate stops the breakdown of organic matter before the methanogenesis step of that breakdown but it’s using nature’s own processes.
Under normal anaerobic conditions in an effluent pond the first step of anaerobic digestion is hydrolysis where complex organic matter including carbohydrates, proteins and fats are broken down into soluble organic molecules such as sugars, amino acids and fatty acids.
The next step is acidogenesis where the soluble organic molecules are broken down to volatile fatty acids.
The third step is acetogenesis where the volatile fatty acids are broken down into acetic acids, hydrogen gas and carbon dioxide.
When the iron sulphate has been added at the correct dosage the next stage of anaerobic digestion – methanogenesis doesn’t occur.
That’s because the methanogens that carry out that reaction are inhibited by the other reducing bacteria and the sulphide produced from iron sulphate.
The redox potential, described as Eh and measured in millivolts (mV) is maintained in the effluent at a level that’s higher than where methanogenesis occurs.
Keith says the “in-line” mixing of the iron sulphate with the effluent still gives the other benefits they’d found with the in-tank system of ClearTech in that Escherichia coli (E. coli) numbers are dramatically reduced, ammonia emissions are cut and phosphorus leaching losses on free-draining soils where the treated effluent is applied will be cut by about 90%.