SEWAGE SLUDGE HAS ITS RISKS
Financially, sewage sludge
can be an attractive source
of nutrients. But growers
need to consider
restrictions on use and
Louise Impey reports
MOST sewage sludge is delivered and applied free of charge, making it an attractive proposition for growers concerned about the farm balance sheet.
But the crop limitations and implications of long-term use on heavy metal concentrations in soils must be appreciated, says Steve McGrath of IACR-Rothamsted.
"The main value of sewage sludge is its nitrogen and phosphate. Most of the potassium is lost in the treatment process, though there can be a lime value, especially in hard water areas. Magnesium and sulphur are also present, albeit in smaller quantities."
The extra organic matter can also improve soil structure. "Certain soils, such as heavy clays and light sands, will benefit. Increasing organic matter content improves water holding capacity, although that is difficult to value."
One potential downside to consider is heavy metal content. "This has come down dramatically in the past 10-20 years," Dr McGrath acknowledges. "A good example of this is cadmium. The source was identified and tackled and we have seen cadmium levels come down from 350ppm to 1-1.5ppm.
"The same applies to most other heavy metals, so that a modern sludge contains the lowest possible levels. It would take 40-50 years application before growers hit the accepted thresholds, but they will be reached eventually."
The problem with heavy metals is not crop uptake, as they bind in the top layers of the soil, but their effect on soil microbes and sustainability, Dr McGrath points out. "Sludges can be used for a number of years without damage, but growers have to control applications and know when to stop.
"Soil sampling has to be done every five years, so that there is no chance of exceeding the safe limits."
The other two areas which remain a concern are the risk to human health from pathogens and organic pollutants, which can be present in sludges.
Dr McGrath dismisses fears about organic contaminants. "They are present, but they are either well bound in the soil or rapidly broken down. There is no uptake by crops so they do not pose a risk to consumers."
He believes pathogens are potentially more serious, but says sewage applied within the Department of the Environments Code of Practice is safe.
"There are livestock exclusion times and crop restrictions to protect public health. Also, most of the sludges applied are treated, which reduces the concentration of pathogens, as well as eliminating the odour."
USE CONTROLS FOR SAFETY
GUIDELINES governing the use of sewage sludge ensure risks to soils, crops and operator and public health are minimised, says Chris Rowlands, sludge planning manager at Severn Trent Water.
Most sludge is applied to grassland – both after first cut and in mid-summer – and to stubbles at harvest, he says. It is also used in the spring when seed-beds are being prepared for maize.
"There are restrictions on use. Sludge cant be applied to crops that are eaten raw, such as fruit and salad, and it has to be used before potatoes are planted."
He adds that growers have a choice between a liquid digested sludge and a cake. "The nitrogen is more readily available in liquid format, so that is preferred for silage land. The cake, which releases the nitrogen more slowly, tends to be used on cereals."
He also highlights the fact that the amount of untreated sludge being used on farmland has reduce dramatically. "At Severn Trent, only 2% of the total output is used untreated. That will have stopped altogether in three years."
Before any type of sewage sludge can be applied a number of checks are done, warns Dr Rowlands. "First, we have to make sure the area is suitable. We need to know where the watercourses are and ensure there is no risk of run-off.
"Having satisfied the environmental concerns, we then have to sample the soil. Sewage sludge cant be applied if it is too acid, or where heavy metal concentrations are already high."
The third stage is to calculate the application rate. "This depends on the nitrogen requirements, which in turn depends on the crop being grown and the soil analysis," continues Dr Rowlands.
"Last, the sludge is applied by trained, approved contractors and the details recorded. The farmer has a printout which tells him how much has gone on, together with the nutrient content and the nutrient availability."
Sludge applied to grassland has a three-week grazing and harvesting exclusion period, to prevent any possible risk from pathogens. "The three-week rule is sound, especially for salmonella. And even more controls apply to untreated sludge; it has to be injected or ploughed in."
Dr Rowlands appreciates that consumers have some anxieties over the use of sewage sludge. But he believes investment in research and sludge treatment has helped.
"Modern, treated sludges are as clean as they possibly can be and their use is tightly regulated and regularly reviewed," he stresses. "A great deal of money is being invested to ensure they remain safe and suitable for farmland.
Pipeline to profit or pathway to problems? Water companies want to get shot of sewage sludge – but growers should consider the risks to farmland and crop marketing before taking the plunge.
• Cost effective nutrient source, but some concerns.
• Contains N + P, some S + Mg.
• Organic matter can improve structure of lighter soils.
• Small heavy metals risk.
• Organic pollutants low risk.
• Pathogen risks avoided by treatment guidelines.