The presence of dioxins in sewage sludge has been known for some time, but has generally been assumed to be due to trade discharges. But experiments by a group at the University of Ume\'e5 led by Professor Cristoffer Rappe, a leading dioxin researcher, suggest that they are formed from organochlorines present in sewage by biological processes.
In 1989, the Department of the Environment (DoE) issued a report which confirmed that traces of polychlorinated dioxins and furans are commonly present in sludge, even in samples taken from essentially domestic drainage areas (ENDS Report 173, pp 18-19). Preliminary studies suggested a typical dioxin concentration of 150-200ng/kg of dry solids, expressed as the toxic equivalent (TEQ) of the most toxic of the dioxins, 2,3,7,8-TCDD. The dominant isomer was the less toxic octachlorodibenzodioxin (OCDD).
According to Professor Rappe, typical levels of OCDD in European sewage sludges are 10,000-50,000ng/kg of dry solids, translating to a TEQ of 10-50ng/kg for OCDD alone. At these levels, he believes, "the use of sewage sludge as a fertiliser should be avoided if the land is used to raise cattle."
Any further restrictions on sludge spreading on agricultural land could have serious implications for the water industry. Dumping of sewage sludge at sea is to end by 1998, and the industry is planning to divert a large proportion of the sludge to agricultural land. Sludge generation is also expected to increase by about 30% over the next 15 years as the 1991 EC Directive on urban waste water treatment is implemented, putting additional strains on disposal routes.
In the UK, no limits have been set for dioxins in sludge intended for agricultural uses, although a limit of 100ng/kg TEQ is in force in Germany.
A DoE study which is nearing completion will provide more data on dioxin levels in sludge and recommend steps to control their release into the environment. But it seems unlikely that the study will take into account Professor Rappe's findings that primary formation of dioxins could make a significant contribution to their overall levels in sludge.
In the most revealing experiment, micromolar concentrations of C13-labelled pentachlorophenol were added to sewage sludge. After one week at room temperature, during which the samples were kept in the dark to prevent possible photochemical dioxin formation, Professor Rappe found that "significant levels" of C13-labelled OCDD were formed.
Further work is needed to quantify the rate of the reaction and to assess the proportion of OCDD in sludge that may be formed by this route. It is also not clear whether the dioxin is formed by a chemical or biological route, although Professor Rappe speculates that a peroxidase enzyme system could be involved. This is supported by his finding that dioxins and furans are also formed in composting processes - though here the source of chlorine is not clear.
If a biological mechanism is proven it is unlikely to be restricted to sewage sludge. Professor Rappe believes that dioxins could also be formed at all stages of sewage treatment and even within sewers. If further studies confirm this, it could signal the need for stricter control on discharges of potential dioxin precursors, in particular chlorinated phenols.