Looking for a more cost-effective product to replace some of the increasingly expensive fertiliser used on your crops? Then you are far from alone, according to Anglian Water Services‘ Biosolids Team.
Some 400 farmer customers mean there is no shortage of demand for recycled sewage sludge from the firm’s newly commissioned £28m plant at King’s Lynn, Norfolk, communications manager Mr Burch told Farmers Weekly at a press briefing last week. “In fact there’s a waiting list.”
A tour of the works showed that turning human waste into a product that may safely be applied to farmland has come a long way since sewage sludge dumping at sea was banned in 1998, and its untreated use on land growing food crops was phased out a year later.
A typical 20t/ha dressing of the latest enhanced nutri-bio product supplied nutrients, mainly phosphate, nitrogen, and sulphur, worth an estimated £450/ha (at present fertiliser prices) in the two years after application.
Used once every four years it made an ideal dressing initially for winter oilseed rape.
“The price has gone up, but at £60/ha delivered and spread we still clearly don’t charge enough!”
The main downside is that nutri-bio contains almost no potash. The firm is exploring the possibility of incorporating some but not for agriculture. “It would make it very expensive,” he explained.
The key technological advance at the new plant, one of four in Anglian Water’s £100m expansion programme and sited beside an existing sewage works, is new form of Monsal’s enzymatic hydrolysis.
This process relies on heat to kill harmful pathogens. But the King’s Lynn plant is the first to employ steam as well as hot water to avoid forming vivianite, an iron phosphate compound that can interfere with subsequent processing.
The steam also eliminates salmonella and kills 99.9999% of pathogens, explained technical manager Steve Riches. “The figure for the old [unenhanced] product was 99%.”
Thereafter treatment is based on conventional anaerobic digestion for a minimum of 12 days during which valuable biogas, mainly methane, is created. This is collected and burned by two CHP boilers providing the hot water and steam plus electricity to power the whole operation.
The digested sludge is aerated to destroy the potentially harmful anaerobic bacteria and then passed through dewatering centrifuges to make the 23% dry matter nutri-bio for delivery to farms.
Extensive computer controls are employed to ensure consistent product quality.
Although only 2% of the organic matter applied to land comes from sewage system biosolids, its use is tightly controlled, stressed Mr Burch.
Indeed the Safe Sludge Matrix, the voluntary agreement between all interested stakeholders, was likely to be made compulsory within a few months, he believed.
Countering concerns that such products might contain soil-damaging heavy metals, Mr Riches said levels of industrial contamination by elements such as chromium and zinc had fallen considerably over the years. “That’s why Severn Trent Water recently closed its sludge incineration plant in Birmingham.“
AWS has nine spreaders, each with GPS allowing them to be tracked centrally from headquarters in Cambridge. That allowed public complaints about smell, which often turned out to be triggered by other manure spreading activities, to be justifiably countered, explained Mr Burch.