Origin of trace metals in STPs and in sewage sludge

The amounts of incoming trace metals to sewage treatment plants (STPs) in European cities vary a great deal depending on the age and technical standard of the infrastructure, but also on the general way society is organized. In the present context, we are going to focus on the metal levels in sewage sludge, and these are determined not only by the total amount of incoming metals with the raw sewage. In summary, the main factors influencing the levels of trace metals in sewage sludge are (Landner et al., 2000):

(1) the sources of incoming sewage and, in particular, the relative importance of industrial inputs, street run-off and domestic sewage;

(2) the physico-chemistry of the drinking water, influencing the corrosion rate of tap water pipes and, thus, the levels of copper and zinc in domestic sewage;

(3) the success of exisiting emission reduction strategies (e.g. reducing industrial inputs);

(4) the size of the treatment plant;

(5) the sludge treatment process itself.

In Stockholm , for instance, a range of emission management initiatives have achieved an important decrease in the metal levels entering the STPs over the last 25 years, especially with regard to copper. The decreasing tendency for copper is, however, not necessarily reflected in the copper concentrations measured in the sewage sludges because metal levels, expressed as sludge dry matter, depend to a large extent on sludge treatment practices. For example, the anaerobic digestion of a sludge will reduce its content of organic matter - in most cases - by 30 to 50%. This will automatically entail variability in sewage sludge data. For comparison purposes, it is therefore recommended to set sludge limits for metals in direct relation to the macronutrient phosphorous and thus express sludge limit values in mg metal/ kg P instead of mg metal/ kg sludge dry matter (Delbeke and Landner, 2000).

An approximate picture of the relative contribution of different sources to the metal loading of the STPs - and hence to the sewage sludges - in a city like Stockholm, with minor additions from industrial sources, can be obtained from Table 3.11. Especially in older parts of the city, there is a combined sewer system for stormwater and domestic sewage, which means that metals released e.g. in the street environment or from roofing and building materials will be transported to the STP together with the domestic sewage.

One of the important sources of metals in the incoming water to the STPs in Stockholm as well as in many other cities obviously is the tap water pipes and other parts of the plumbing system in houses, especially when these are made of copper. Therefore, many attempts have been made to quantify the rate of metal release and explain the exact mechanisms behind this release, in order to find appropriate counter measures. Recently, Linder and Taxen (2002) have made an in-depth examination of the problem by means of literature research and equilibrium calculations, based on the situation (water characteristics and service conditions, etc.) existing in Sweden, see below.

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