As water costs rise, could rainwater harvesting provide cheaper water supplies for livestock farmers?
The advantages of being able to store rainwater go beyond making savings on purchased water. There are also savings to be made on the reduction in sewage costs. These are based on water supply costs and are between £1.20/cu m and £2.20/cu m.
Harvesting rainwater will also reduce the huge amount of water making its way into the slurry store. That means a smaller capacity store will suffice and there’s less slurry to spread too.
If you abstract from rivers or boreholes, rainwater harvesting could also allow you to reduce the volume used, cutting costs and bringing relief to water authorities.
Before embarking on rainwater harvesting, check that your existing supply is not being compromised by leaks and faulty taps. Annual rainfall levels in the UK range from 450mm to 1,500mm (18in to 60in). But even if your annual rainfall is a modest 500mm, amounts harvested can be substantial.
A typical grain store with a 20m x 30m roof has a roof area of 600sq m, but the speed with which water flows off the roof depends on the roof’s pitch. This run-off coefficient will dictate what size of filter you will need.
The potential collectable rainfall can easily be calculated – on this roof it would amount to 204cu m/year.
How clean does it need to be?
It depends what the water is going to be used for. Harvested water is likely to contain bird droppings, leaves and moss, but there is also a risk of contamination from asbestos or lead – this should be checked before a collection system is installed.
If the water is to be used for crop irrigation, there are standards set by food assurance schemes along with those set out by the World Health Organisation. Similarly, if the water is to be used for animal drinking, it’s worth checking that hygiene or farm assurance requirements are not being breached.
For some uses, such as in the dairy or for irrigating ready-to-eat crops, an Environmental Health Officer is required to test the water each year to ensure its purity.
There are several types of filter on the market. Some are self-cleaning and others are cartridge types that need to be replaced at regular intervals.
If you want water that is free of pathogens,ultra-violet radiation is effective at killing a wide range of bacteria, pathogens and viruses.
A UV system uses replaceable filters and units are priced on throughput – an 8 litre/min system will cost about £400.
If the tanks are above ground they need to be covered and insulated against frost. An underground store is more expensive, but provides weather protection and keeps the water cool, limiting bacterial growth.
All tanks must have an overflow arrangement which can be connected to a soakaway, storm drain or a sewage system.
Pumps are needed to either fill a header tank and/or increase the water pressure in a gravity system. Most rainwater harvesting systems will also have a mains water top-up arrangement which can be used in times of low rainfall.
Health and Safety
Harvested water may look clean but it shouldn’t be used for drinking, warns the HSE. It can cause Legionnaires disease or Weil’s disease – the latter spread by infected rat urine.
Find out more
You can find out more about rainwater harvesting from equipment suppliers at this year’s Livestock 2012 show on 4 and 5 September at the NEC.
Typical dairy farm example
A 200-cow herd uses 11,300cu m (2.49m gal) of water each year – 7,300cu m for drinking, 2,190cu m to cool the milk and a further 1,825cu m to wash the parlour.
A 3,000sq m roof area has a potential yield of 1,680cu m of rainwater. If you stored half of this water, it could save you about £400/year (assuming a mains water cost of £1/cu m. For a typical set up costing £4,000, that would mean a 10-year payback.