The rainwater taken in must also be spread each spring and this imposes an additional, ongoing cost.
However, it is possible to exclude rainfall and the costs that go with it by covering a lagoon with a waterproof cover. The cover will keep rain out of the lagoon, eliminating the wasted capacity and reducing spreading costs.
Depending on how it is installed and used, a lagoon cover can also reduce the release of odour — and greenhouse gases — and retain nitrogen.
Earlier this year, I featured an open lagoon built by Padraig Ryan of A Lagoons Ltd, based in Tipperary. Padraig also builds covered lagoons and when I was with him he showed me a covered lagoon, which he had built in the county.
On the continent, covered lagoons are in widespread use on both dairy and pig units. Here, they have mostly been used on pig units, but there are a number in place now on dairy farms.
COST BENEFIT
Depending on local climate, a lagoon will typically take in 1m of rainfall per square metre each year. This means that one third of the capacity of a 3m deep lagoon must be set aside for rainfall. Therefore, covering a lagoon makes an additional 33% of capacity available for slurry storage.
I asked Padraig Ryan about the cost of covering a typical lagoon.
The construction cost of a 1m gallon, lined, open lagoon is approximately €52,000. One third of this cost, which is attributable to holding rainfall, is €17,000. The same lagoon built with a cover and agitator would cost €80,000 to €83,000, some €30,000 extra.
The open lagoon would hold 660,000 gallons of slurry; the covered one 1m gallons. There is also a saving due to a reduction in spreading costs.
A cover would be expected to have a working life of at least 25 years, Padraig said.
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