Kildeopsporing af tungmetaller i afløbssystemer

Summary

Tracing of heavy metal sources to sewage sludge conta-mination - an investigation of sewer sludge with special attention to mercury contamination from older domestic heating stations

Five sewer catchment areas

This report deals primarily with mercury contamination of sewers and sewage sludge. Field investigations have been carried out in five sewer catchment areas. Common to four of the five catchment areas is, that the mercury contents in the sewage sludge on the municipal waste water treatment plants receiving sewage from these areas are high. The fifth catchment area represents the background level as specific contamination sources are not present.

Mercury pressure holding devices

The four sewer systems contaminated with mercury receive sewage water from existing or former domestic heating stations being equip-ped with special pressure holding devices. Each of these devices con-tain several hundred kilograms of mercury in a U-bend tube. On all four domesting heating stations mercury have been found in the inter-nal sewer systems. This is ascribed to mercury blow-outs from the U-bend tubes caused by pressure instabilities.

Samples of sewer sludge

Sampling of sludge settled in the main sewers downstream the four domestic heating stations have been conducted. A special sampling method has been developed. The method is based on the utilization of a new generation of a combined high pressure sewer cleanser and sludge suction vehicle. The sluice water is filtrated through 2 mesh-sizes. The sewer sludge and other solids are kept in a holding tank and the filtrated sluice water reused. Samples of the retained sludge have been analysed for mercury and the results made possible for an easy tracing of the mercury contamination from the four domestic heating stations downstream to the waste water treatment plants.

Contamination sources

It is concluded that mercury trapped in the internal sewer systems on the four domestic heating stations is responsible for the high mercury content of the sewage sludge on the four municipal waste water treatment plants. It is likely that the same contamination situation prevails on other Danish heating stations having been equip-ped with the mercury containing U-bend pressure tubes. Approx. 300 of such stations are still in operation or not fully removed or closed down.

Pilot project

It is anticipated that the mercury contamination of the sewage sludge can be resolved by a combined action comprising the stopping of the mercury outlets from the heating stations and the cleansing of the sewer pipelines containing the most mercury contaminated sediments. It is advantageous using the new special sludge suction vehicle. However, further documentation needs to be established by means of a pilot project.

Source tracing

The method used for taking samples combined with the sampling strategi described in the reports annex E is effective and well suited for tracing heavy metal sources contaminating sewage sludge. Further, it may be an advantage to integrate this tracing method in the preven-tive cleasning and maintenance of the sewer systems.