Digested sewage turned into fertiliser granules

19 February 1999

Digested sewage turned into fertiliser granules

By Brian Lovelidge

GRANULES made from digested sewage sludge with similar spreading characteristics inorganic fertiliser have been launched by Southern Water.

A new £20m plant is producing 9000t/year and five further plants are to be built along the south coast over the next 10 years, producing a total of 45,000t/year.

The 92% dry matter granules, processed at 400C, were developed to make digested sewage more attractive to farmers. The move follows bans on dumping raw sewage at sea from Jan 1 and use on farms from Dec 31 this year.

"In essence we have had to become fertiliser manufacturers," declares Peter Soulsby, SWs recycling manager. "The granules are designed to mimic fertiliser, with a set specification and very low levels of bacteria and viruses."

The British Retail Consortium has sanctioned granule use for all agricultural and horticultural field crops because the pathogen content is so low, maker SW says.

"Granules come in the advanced treated category and currently are the only product of this type to be accepted by retailers," explains Gordon Hickman, of ADAS, who runs SWs product trials.

He expects the granules to produce similar crop responses and soil benefits to digested cake. In trials on SWs demonstration farm near Horsham, West Sussex, and at ADAS Bridgets that boosted yield by more than its nutrient content alone.

However, high temperature granule production means much of the freely available nitrogen is driven off. Only 5-7% of the granules nitrogen is available in the first year against 15-20% for cake, explains Mr Hickman.

The £10/t cost of the granules, which includes delivery and spreading, is based on fertiliser value.

Application rate is up to 3.5t/ha (1.4t/acre). &#42

Nutrient value of 3.5t/ha

granule dressing (kg/ha)

Year 1 Year 2

Nitrogen 15 13

Phosphate 75 37

Potash 2 1

Sulphur 42 21

What users think

Soil conditioning claims for Bestway Granules are confirmed by Alastair Fitzgerald who grows about 405ha (1000 acres) of combinable crops on light sandy loam over chalk near Salisbury, Wilts.

Mr Fitzgerald uses the Wessex Water product at 2.5t/ha (1t/acre). Apart from providing nutrients at a very competitive price they have improved his soil structure and significantly boosted yields and grain quality, he claims.

He has also been able to reduce applications of sulphur and copper, as both are provided by the granules.

Hants farmer Julian Lewis, intends to use SW granules instead of the digested cake he is trying for the first time this season. The treated 30ha (74 acres) of winter cereals on clay cap seem to have greater vigour than the untreated, he asserts.

"These sewage products provide nutrient at lower cost, which is important when were having to farm at minimal cost," he says.

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