National Study for the Composition of Sewage Sludge
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Discharges of hazardous wastes to sewers can cause treatment processes to fail. This can result in unsatisfactory effluent being discharged to natural waters, or in incompletely digested sludge being pumped to lagoons where it can create a public health risk or bad odours.
Heavy metals and some intractable organic chemicals accumulate in the sludge. Sludges are used in domestic gardens, parks and reserves, dumped on landfills, in forests or spread on land. This may not always be environmentally sustainable. Run-off can pollute surface water, seepage can affect groundwater quality, and sludge can contaminate the soil and plants.
Sludges that achieve a prescribed standard in USA are termed `Biosolids' and are permitted to be applied to the land as a resource. NZWWA and other organisations are investigating whether overseas standards are acceptable for local soil and climate conditions; this study will assist that investigation.
Most councils that allow public use of sewage sludge rely on the Ministry of Health's recommendation that the sludge has aged sufficiently, to limit the effect of pathogens on humans. Very few councils know the environmental effects caused by the discharge of their sludge.
Many operators of sewage treatment plants in New Zealand have never tested their sludges for heavy metals or organic chemicals. This project will provide that information, cost-effectively.
It is intended to collect sludge samples from over the whole country, to be analysed for a full range of heavy metals, volatile organic chemicals, semi-volatile organic chemicals, polyaromatic hydrocarbons and halogenated organic chemicals, and some 'tentatively identified compounds'.
The samples will be tested using modern analytical equipment such as gas chromatograph/mass spectrometer (GC/MS) and inductively coupled plasma (ICP), which provide reliable, low-level results. The testing will be carried out by one laboratory so the results will be comparable.
A literature survey will compare these results with overseas sludges, and an attempt will be made to find cases of where the chemicals causing treatment failure have been identified and the levels that caused the failure.
The analytical results will produce a national benchmark for sewage sludge producers; they will be able to recognise any chemicals they have at relatively high concentration. By tracing the source they should be able to convince hazardous waste producers to implement a waste minimisation programme. This will reduce the environmental effects caused by sludge disposal. Also, the soluble fraction that is not taken up by the sludge will be found at a lower concentration in the sewage plant's final effluent.