The Cabinet Secretary Richard Lochhead announced last week that a formal review of the legislation and guidance in relation to sewage sludge use on land has been commissioned following a number of public complaints last year.

Sadly, the regulations and recommendations on spreading sewage sludge have not always been strictly adhered to.

Sewage sludge is claimed to be a natural by-product of the biological waste water treatment process. Waste water and storm-water enter the sewage system and flow into sewage works, where the solids are separated from the liquid wastes through settling. This sludge is then further treated to produce a bio-solids product using a number of complex treatments such as thickening, dewatering, digestion, drying and lime pasteurisation.

These separated processed solids - sewage sludge - are rich in organic matter, slow release nitrogen and phosphorous nutrients together with valuable trace elements and are an ideal soil conditioner owing to their humus forming and fertilising properties. They also contain numerous hazardous materials that includes everything flushed into the sewer system such as household, medical, chemical and industrial waste.

The amount of sewage sludge that Scotland produces annually is expressed as tonnes of dry solids (TDS) - the total mass of the sludge material less the included water - and is just over 122,000 TDS.

About 49,000 TDS is spread annually on Scottish farmland as enhanced sewage sludge, and 7,000 TDS as conventionally treated. In addition, 24,000 TDS is used for land reclamation, 40,000 TDS is incinerated, and just over 1,000 TDS goes to landfill.

Guidelines agreed between ADAS, Water UK representing the 14 UK Water and Sewage Operators, and the British Retail Consortium (BRC) representing the major retailers - the "ADAS safe Sludge Matrix" - effectively banned the use of untreated sewage sludge on agricultural land for food production from 31st December 1999.

Under these guidelines, sludge must be treated to either "conventional" or "enhanced" treated standards before recycling to agriculture. Conventionally or enhanced treatment standards refer to the level of pathogen destruction in the end product.

Conventional treatments are used to significantly reduce the odour and at least a 99 per cent reduction in its microbiological content. The most common form of treatment is anaerobic digestion, where sludge is digested at a temperature of 35 degrees centigrade for a minimum of 12 days, followed by a further period of maturation.

Enhanced treatment is a term used to describe treatment processes capable of virtually eliminating any pathogens which may be present in the original sludge. Enhanced treated sludge must demonstrate a 99.9999 per cent reduction in E. coli and be free from pathogens such as Salmonella sp..

The surface spreading of conventionally treated sludge on grazed grassland was banned from the end of December 1998. Conventionally treated sludge can only be applied to grazed grassland where it is deep injected into the soil. The regulations require that there will be no grazing or harvesting within three weeks of application.

There is also unease in the grain trade about the use of these bio-solids and growers are best advised to consult with their merchant as some end-users, such as maltsters, are reluctant to accept these treatments.

Waste-water comes from a variety of sources, including urban run-off, and contain heavy metals such as lead, mercury, molybdenum, cadmium, and thallium - a toxin hazardous to humans in small doses that gained notoriety as the "poisoner's poison" and the "inheritance powder".

Treated sewage sludge can only be used on agricultural land under strictly controlled conditions. It is an integral part of Scottish Water's quality control processes to ensure that sludge and soils are analysed prior to application. The analysis will consider nitrogen, phosphate, potassium and pH, plus a full suite of potentially toxic elements in both sludge and in soils to check that the limit on the concentration of metals in the soil will not be exceeded by spreading the sludge.

Don't let all these regulations and safety precautions lull you into a false sense of security. While certain sanitation processes do decrease some health risks, persistent organic pollutants such as PCBs (polychlorinated biphenyls) and endocrine disrupters - many of which are carcinogens - are not filtered out and persist in the soil.

In addition, the US Food and Drug Administration recently expressed concern that triclosan and other antibacterial compounds used in soaps and body washes, toothpastes and cosmetics, also found in sewage sludge, could be contributing to antibiotic resistance.

Perhaps the time has come to consider playing safe and incinerating all of it.