Brickworks could make use of sludge

Sewage sludge can be used as a partial substitute for clay in bricks. Dr Glynn Skerratt of Staffordshire University looks at the process and asks why it has not yet taken off in the UK.

The use of sewage sludge as a building material in the UK has not yet taken off on a major scale. But the potential for brick manufacturers to save money is huge as up to 30% sludge can be used without a reduction in brick quality. The water in liquid sludge can also save the plant operator money, and dried sludge pellets can also be used to help fire the kilns which heat the bricks.

An original patent covering the incorporation of raw sewage sludge into clay bricks dates back to 1889. Thomas Shaw’s invention covered the use of sludge as an addition to bricks, tiles, blocks and other building materials. The patent went on to claim that the fired products had valuable qualities such as hardness and durability, and the colour was far superior to that of ordinary bricks made from clay.

The idea does not seem to have taken off until the 1980s, when scientists at Maryland University in the US did some small-scale research into a new brick-making process with sludge as an ingredient. Several other scientists have now performed such studies, with similar results for the amounts of sludge that can be successfully used. The UK government’s Department of the Environment (DoE) also produced a leaflet in 1995, explaining the potential benefits of sludge use in terms of energy efficiency.

Another advantage is that heavy metals contained in the sludge are immobilised in the fired matrix of the brick. This is likely to be of interest for authorities looking to dispose of large quantities of sludge, as the EC’s directives on disposal are becoming increasingly strict with regard to the heavy metal content of sludge spread on land. Organic matter contained in the bricks is fully oxidised and any pathogens are destroyed.

Wet sludge (30-50% water) can be used in most brick production processes, with a typical brickworks producing 27M bricks per year able to save 7Ml/yr of water.

Structural concerns

The amount of sludge needs to be limited as addition of more than 40% sludge leads to poor bonding and extrusion characteristics. But repeated research has shown that bricks containing 30% sludge (fired at 1,000°C, for 20hrs) will meet US building standards.

If more than 30% sludge is used problems are commonly encountered with shrinkage, due to destruction of the high organic content. This leads to the formation of cracks. Compressive strength is also reduced and water absorption increased. Porosity is increased by the burning of organic matter, leaving tiny spaces in the brick matrix. Up to a point this may be an advantage, as increased porosity reduces susceptibility to frost damage. But if porosity is too high water absorption can become a problem.

Dried sludge can also be used, not just for firing the kiln but in the brick itself. This is likely to be an important consideration in terms of transport costs. Studies of dried sludge application are limited, with work by Professor Ed Stentiford (1984) showing that 15-20% could be used in conjunction with pulverised fuel ash (PFA) and china clay. This resulted in a brick with improved porosity and a reduced drying time compared with standard clay bricks at 1,050°C. The sludge pellets did, however, require pre-soaking with water before mixing with the PFA.

Commercial reality

Full-scale commercial production of bricks containing sludge has begun in other countries. One of the best known examples is the Port Elizabeth brickworks in South Africa, where ‘sludge bricks’ have been made in compliance with South African Building Standards (SABS) since 1979.

At Port Elizabeth sludge is used not just in the bricks, but also as fuel in the boilers which dry the bricks before firing. The Port Elizabeth plant is relatively isolated, so the odours generated by the drying process are not a problem for local residents. In theory brickworks in the UK could be modified to contain odours, but this would be expensive and the owner would need to be keen to take on the responsibility.

This is just one of several hurdles to overcome if large-scale production is to occur in the UK. Even if the technique is environmentally preferable, it will not be put into practice if it is more expensive than other legally-approved options. Despite the escalating landfill tax, landfill costs in the UK are still low in comparison with many other European countries.

There needs to be a greater incentive to try alternative disposal options. At present, disposal to agricultural land and incineration are the two options most highly favoured by water companies in the UK. These are not without environmental concerns in terms of sludge safety and atmospheric pollution. But disposal on land as a soil conditioner is viewed as a sustainable route in the long-term, and incineration means the water company needs not rely on a third party.

Despite the attraction of land spreading and incineration, water companies should not give up hope on brick manufacture as a disposal option. Some brickworks in the UK are now looking at the inclusion of sewage sludge incinerator ash. If a large STW producing sludge is located near a brickworks, then it would be foolish not to take this into account when looking for a disposal route.

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