CLEANER IMAGE FOR SLUDGE
CLEANER IMAGE FOR SLUDGE
Supermarkets and water companies have agreed a single set of guidelines that could help clean up the soiled image of sewage sludge, writes Lucy Stephenson.
THERES no reason to turn your nose up at sewage sludge these days. The water companies can now treat it so that its appearance, smell, and bacterial content are innocuous. One Essex cereal farmer says it saves him £20/ha in fertiliser costs, as well as adding organic matter to his sandy clay loam soil.
Consumers and supermarkets have in the past regarded application of sewage sludge to food crops with distaste. Despite a mass of guidelines – the UK Governments interpretation of Council Directive 86/278/EEC – the Sludge Use In Agriculture Regulations, and the Code of Practice for Agricultural Use of Sewage Sludge, concerns over unpleasant smells and health risks were still being voiced. The supermarkets were coming up with protocols of their own and these were often more stringent than the Government guidelines, according to Steve Dudman, senior consultant with ADAS.
So ADAS mediated with the British Retail Consortium and Water UK to arrive at a set of guidelines that satisfy supermarkets, water companies and farmers, known as the sludge matrix. The new guidelines are likely to be incorporated into the sludge code of practice, which is being updated, says Mr Dudman.
Richard Cullen first had sludge applied on his 160ha (395 acres) farm near Kelvedon, Essex, seven years ago. Anglian Water deliver and contract apply it: "I just have to sit down at my desk for longer," he says.
Growing about 110ha (271 acres) of winter wheat and 50ha (123 acres) of break crops including oilseed rape, peas, beans and borage, Mr Cullen views the sludge guidelines as he does ACCS – "just another way to demonstrate that everything is done properly and safely".
The new guidelines mean that raw or untreated sludges can no longer be applied to grass for grazing or silage, maize silage, vegetables including potatoes, or salad, fruit and horticultural crops. Raw sludge will be allowed for combinable and animal feed crops, but unless the crops are processed, only until the end of this year. Treated sludges – liquids and digested cakes – can be used on combinable crops and animal feed crops including silage. The only type of sludge recommended for use on all crops, including fruit and salad crops, is advanced treated sludges such as thermally dried digested granules.
Digested cake costs more to produce than liquid sludge, but Peter Garnett, biosolids recycling manager for Anglian Water Services, says they can recoup the costs because cake has less bulk; it can be transported by one lorry compared with five for the equivalent amount of liquid.
Mr Garnett says that the move away from injected liquid sludge has advantages for the farmer. Digested cake is faster to apply than injected liquids, which hold up farming operations for a week or so. Because the ground isnt waterlogged after cake application, says Mr Cullen, subsequent operations will be much easier to perform.
Cake can be delivered, stored on-farm and spread over a period, and can also be applied more flexibly within the rotation. On Mr Cullens farm, sludge was injected after oilseed rape and before first wheat. Now, because application is faster, it can be applied before winter rape is drilled.
The composition of sludge isnt very variable, says Mr Garnett. The main cause of variation in the past has been from industrial sources which Anglian Water now controls with a trade effluent programme.
The sludge is analysed, and more precise applications of nitrogen and phosphorous are becoming possible. This means that Mr Cullen can have more confidence in the product, particularly as he farms in a nitrate vulnerable zone.
Typically, digested cake will contain about 7.5kg/t nitrogen and 9.1kg/t phosphorous. The 250kg/ha limit of nitrogen is reached if cake is applied at 33t/ha. Of this, 4.1t will be organic matter. About 25kg nitrogen will be available to the next crop, worth about £5.75/ha, calculates Mr Dudman, and about 150kg phosphorous will be available, worth £37.5/ha.
If the soil phosphorous index is 3 or above, sludge should only be applied every three to four years, points out Mr Garnett. Anglian Water currently charges £1/t for digested cake to be delivered and spread.
Says Mr Cullen: "I believe its cost-effective – my savings on bought-in nutrients are £20/ha."
Last year Anglian Water recycled 80% of its sludge to agriculture. The company hopes that more farmers will see sludge as an option – this year it hopes to increase this to 90%, to include the 8% it is no longer allowed to dump out at sea.
lFurther information from Dr Brian Chambers, principal research consultant, ADAS Gleadthorpe, on 01623 844331.
Nutrient Total Available Approximate value of
available nutrient
Nitrogen 250kg 25kg @23p/t, £5.75/ha
Phosphorous 300kg 150kg @25p/t, £37.5/ha
Sulphur 50kg
Magnesium 20kg
Organic matter 4.1t/ha
Crop Raw or untreated Treated Advanced treated
type sludges sludges sludges
eg. digested cakes eg. thermally and liquids dried granules
Fruit 8 8 4
Salads 8 8 4
Vegetables 8 8 4
12-month harvest
interval applicable
Horticulture 8 8 4
Combinable crops 4 4 4
& animal feed crops Target end
date 31/12/99
Grass grazing 8 8 4
Deep inject and/or
ploughed in early
Grass silage 8 4 4
Maize silage 8 4 4
NB. All tick applications must comply with the current DoE Code of Practice and certain additional crop and harvest interval restrictions.
Source: ADAS
Crop Raw or untreated Treated Advanced treated
type sludges sludges sludges
eg. digested cakes eg. thermally and liquids dried granules
Fruit 8 8 4
Salads 8 8 4
Vegetables 8 8 4
12-month harvest
interval applicable
Horticulture 8 8 4
Combinable crops 4 4 4
& animal feed crops Target end
date 31/12/99
Grass grazing 8 8 4
Deep inject and/or
ploughed in early
Grass silage 8 4 4
Maize silage 8 4 4
NB. All tick applications must comply with the current DoE Code of Practice and certain additional crop and harvest interval restrictions.
Source: ADAS