Earth Banked Slurry Lagoons – Think Before You Dig!

Published 1 June 09

Earth banked slurry lagoons - think before you dig!

Around 75% of dairy farmers do not have sufficient slurry storage to comply with new Nitrate Vulnerable Zone regulations. But there are ways to increase storage capacity on a budget, as DairyCo's Hugh Black reports.

Many thousands of dairy farmers are facing massive investment over the next three years, to meet new environmental regulations. In January this year Nitrate Vulnerable Zones were extended across about 70% of England, bringing stringent slurry storage and spreading requirements.

From 1 January 2012, all dairy farmers in NVZ areas must be able to store slurry for five months of the year, from 1 October to 1 March, and will not be able to spread slurry during winter closed periods of up to five months.

This would leave many farmers having to significantly increase their slurry storage - a difficult and costly operation, said Gillian Preece, a consultant with the Dairy Group. But, speaking at a recent DairyCo event in Gloucestershire, she said that there were ways to reduce the storage requirement, and to minimise capital outlay.

The first step was to work out how much storage would be required to meet the five-month regulation, she said. The easiest way was to use DairyCo's Dairy Wizard, which calculated storage requirement based on cow numbers, the dairy system, yard area and rainfall.

It was also important to understand the difference between slurry, which needed to be stored, and dirty water and farmyard manure, which could be spread throughout the closed period, she added.

All effluent from weeping walls, silage clamps and loafing yards counted as slurry, as did yard and parlour washings where the area had not been scraped. But runoff and washings from cleaner, scraped yards qualified as dirty water, which could be stored and spread separately. Rainwater from roofs and clean concrete areas was clean water, and could pass through normal drainage systems.

Any solid manure - whether from straw yards or a separator - would be regarded as farmyard manure as long as it was stackable and did not have liquid run-off, said Mrs Preece. This too could be stored and spread separately.

"Once you've worked out your storage requirement, start thinking about whether there is anything in the slurry that you can take out." It was easier and cheaper to redirect clean and dirty water than to create more storage for it, she added. Farmers could also examine ways to reduce water use, although some may need to keep enough water in the slurry to pass easily through umbilical systems.

Typical dairy washings averaged 30 litres per cow per day, and a 25x40m roof with 311mm of rainfall from October to February, the five month storage period, would produce 311 cubic metres of water - a significant amount if it ended up in the slurry store.

Using a slurry separator was another good way to decrease the amount of storage required, as it reduced slurry volume by 15-20%, said Mrs Preece. It also produced solids which could be spread during the closed period, and more liquid slurry, which would be easier and quicker to pump out and would not cause contamination of grass swards.

However, many farms would still require extra storage to meet the NVZ regulations, and while some could add height to a tin store, others would need a completely new solution.

The cheapest option for those with suitable clay soils would be an earth banked lagoon, said Nigel Belton from Acorus. Typical construction costs averaged £6/ m3 - although this would rise sharply if imported clay or a liner were needed. Soil needed to have a20-30% clay content and be tested for impermeability. Farmers would have to take a number of samples to ensure the site was suitable. "Just digging a hole in the ground is not the way to do it these days."

The lagoon required at least 1m of impermeable soil beneath its base, and its construction must be carefully planned to ensure structural stability, he added. Farmers should use competent contractors working within Health and Safety guidelines, and check for tree roots or utilities in the planned excavation site.

Planning permission would normally be required with a typical charge of £170 per 0.1ha of the site, said Mr Belton. Even those within Permitted Development Rights would need to be notified to the Local Planning Authority.

Banks needed to be carefully constructed and rolled to avoid slippage, with a maximum gradient of 1:2.5 on the inside and 1:2 on the outside. The topsoil should be used to seal the banks, and 750mm freeboard from the liquid to the top of the bank was essential, he added. Fencing, and access for tractors to fill or empty the lagoon, also needed to considered, and farmers had to inform the Environment Agency 14 days before bringing new lagoons into use.

Case study

Calvin and Rachele Pugh, who hosted the DairyCo event at Court Farm, Kempley, reckoned they would need 120,000 gallons more slurry storage to meet the NVZ requirements. They had already diverted clean water into drains, installed a separator, and had been quoted £50,000 for a new tin store.

They considered creating an earth banked lagoon as a cheaper option, and siting it in the middle of the farm, to allow access to outlying pasture using the umbilical spreader. "If we are going to go down the route of increasing storage we decided we might as well put it where we can spread it," said Calvin. They might also install more storage than strictly necessary, so that they could spread when they wanted, to suit the farming system.

Key Points

Calculate storage requirement.
Think before you dig - do you need more storage or are there other options?
Separate clean water, dirty water, slurry and manure.
Lagoons require planning permission and careful construction.
Clay content of 20-30% is essential for earth lined lagoons.

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