It is increasingly difficult to dispose of the sludge produced at sewage treatment plants. Limits on heavy metals and organic micro-pollutants are continually being reduced, leading to a reduction in the amount that can be used on arable land. As a result, burning sludge is becoming more attractive. A study was carried out in Sweden at a CHP plant in Linkoping to evaluate and compare methods of sludge disposal. A city produces enough sludge to provide approximately 2 per cent of the fuel needed to power the city.
The high humidity of the sludge means that extra fuel must be added, making the process expensive. However, the phosphorous contained in the sewage sludge can not be reused, and must be disposed of, together with the heavy metals in the ash.
One solution is to hydrolyse sludge. It is divided into three parts, organic sludge, inorganic sludge, and organic dissolved material. Between 50-60 per cent of the organic material can be separated as clean organic material, with a heavy metal reduction of 90 per cent.
Several trials have been carried out on the properties of hydrolysed organic sludge with respect to incineration. These trials gave a heat value of between 6-10 MJ/kg. All the various types of organic sludge have a moisture percentage of approximately 50 per cent. Lower values were obtained for sludge hydrolised in a more acidic environment.
At one trial carried out in Sweden, organic sludge was removed from a treatment plant in Helsingborg, and transported to a CHP in Linkoping, where it was combined with household waste, the organic sludge forming approximately 5-10 per cent of the waste content.
It was not possible to prove whether or not the organic sludge affected the incineration with such a small volume. All that could be established was that it was possible to incinerate a combination of household waste and sewage sludge.
Another trial was carried out with a combination of wood chips, bark and organic sludge at the same plant. Combustion was carried out on a moving grate, the boiler producing a steam pressure of 60 bar at a temperature of 475°C. The nominal boiler power is 60 MWe. The mixture was 20 per cent organic sludge, 40 per cent bark, and 40 per cent wood chips.
The fuel used in this test had a higher energy value than sludge mixed with household waste, and required a higher consumption of urea, used to decrease NOx exhaust. This probably resulted from either a higher nitrogen content in the fuel, or the wrong temperature in the boiler during the test.
The findings from these tests showed the following results:
Organic sludge increased the level of SO2 produced.
The heat value of organic sludge is higher than that of wood chip and bark.
Sewage sludge is a viable fuel source when mixed with other fuels, although more work is required to determine the best proportion for the mixture.
As a result, the tests demonstrated that sewage sludge – a renewable biofuel – is viable as a replacement for a small proportion of fossil fuel use.
TablesTable 1. Heat value from sewage sludge tests