Bedding-in a use for sludge

Lecturer DJ Newport at the University of East London reports on research into new forms of sustainable manufactured aggregate

Research work undertaken at the University of East London has produced new forms of sustainable manufactured aggregate, which gives a backfill material that has many advantages over natural materials. A matrix of sewage sludge, sewage sludge ash and clay is used as a near standard blend for aggregate production, thus providing the possibility for certain water companies to recycle waste materials that would normally go to landfill.

The geotechnical properties of the aggregate make it an ideal material for the laying of plastic pipes, allowing better support and stability, contributing to the longevity of pipelines. The safe installation and long-term performance of plastic pipe systems is now recognised partially as a function of the excavation and reinstatement techniques adopted by the installer.

Historic evidence has shown that poor laying technique has led to premature failure of pipe systems. The construction of the excavation depth, width and backfill material all influence the performance of the finished pipeline. A great deal of work has been undertaken to assess the best specification for the laying of underground plastic pipe systems.

In the UK the Water Industry Sewers and Water Mains Committee has published information to assist in the structural design of buried plastic pipe systems. However, the successful installation of plastic pipe systems requires a great deal of care in the placing and compaction of backfill, which necessitates competent and dedicated supervision and quality control.

One of the crucial decisions required to ensure effective construction is the choice of bed and surround material. Experience has shown the wrong material can lead to pipeline failure, therefore many installers are using imported granular materials to prevent pipe damage or poor compaction. The consequence of this use of imported material is significant in terms of the need for either quarried or sea dredged material. This requirement impacts on the environmental costs of transportation and disposal of waste material.

The UK government has for some time been concerned about the sustainability of mineral extraction and carefully monitors the amount of aggregate that is used and its sources. In addition, it is also acutely aware of how the aggregate extraction industry is viewed by the public. It has therefore undertaken research to assess how much the public is willing to pay to reduce significantly the environmental effects of quarrying.

This research shows the public is willing to pay to reduce the environmental impact of mineral extractions and therefore the government is considering how taxation can be used to encourage the development of sustainable alternatives. This has also led the government to promote the use of secondary and recycled aggregates through bodies such as WRAP.

This minimises waste to landfill and reduces the need to obtain quarried natural aggregate. All these elements have led the University of East London to examine how a new form of artificial aggregate can be manufactured, which is both sustainable and gives better geotechnical properties.

This new form of aggregate has substantial benefits for use in the construction of pipelines. The pilot plant is situated at the Manufactured Aggregate Research Centre (MARC), University of East London and has been designed using state-of-the-art technology. It is currently a static plant and a mimic of the RTAL plant at Tilbury. Manufactured aggregates have been produced for more than 70 years from such materials as clay, shale, slate and slag, and more recently from pulverised fuel ash (PFA). This facility enables large-scale feasibility testing to be carried out on aggregates designed and manufactured using a wide variety of waste materials.

Current developments in the science and technology of thermal processing now enable aggregates to be designed and manufactured from various combinations of resource materials, such as:

  • residues from bio-degradable materials – sewage and paper sludges, etc,
  • municipal solid wastes,
  • industrial and by-product ashes – sewage sludge ash, PFA, etc,
  • reclaimed argillaceous and granular materials,
  • demolition and construction-derived wastes.

The pilot plant process allows waste materials to be used in the manufacture of commercially viable aggregates. Many waste materials contain combustible components, which provide some of the energy required for kilning and reduce the density of the finished product, improving the overall efficiency of the process. Other wastes contain materials that can be expanded by heat (bloating) to reduce density. This process can transform wastes, which are difficult or extremely costly to manage into useful, safe and valuable aggregates.

In order to satisfy European construction product specifications, the MARC facility has been designed to manufacture aggregates for high-performance applications especially to satisfy legislation for ecotoxicity. The processing technology enables aggregate to be designed and produced to meet specific market applications, thus allowing natural aggregates to be substituted by
manufactured aggregates of superior technical performance at a competitive cost.
This project aims to significantly reduce the disposal of both hazardous and non-hazardous wastes to landfill. By using materials perceived as wastes as resources for aggregate manufacture, this project has demonstrated a sustainable waste management policy in support of Waste Strategy
2000 by:

  • reducing waste to landfill and developing new waste management techniques,
  • conserving non-renewable resources,
  • conserving the natural environment,
  • lowering construction costs.

The facilities provided by this project consist of:
  • a portable state-of-the-art integrated bench-top mixer, pelletiser and Trefoil kiln based at MARC for feasibility studies, including initial raw material assessment and mix optimisation,
  • a static pilot plant at MARC which can be operated in separate stages or continuously.

The gas-fired high-temperature (more than 1,100°C) rotary Trefoil kiln is at the heart of the facility. The name Trefoil relates to the internal shape of the kiln, which unlike conventional cylindrical kilns, is similar to a three-leaf clover.
This shape, when rotated, allows a bed of pellets to gently cascade from leaf to leaf with every revolution of the kiln to ensure even distribution and mixing. The super alloy steel lining of the kiln wall is shaped to allow a greater proportion of the radiant heat emitted from the wall to be directed back to the bed of pellets, resulting in an even temperature throughout and more efficient heating. Insulating fibres surround the kiln wall to further minimise heat loss.
The resulting low thermal mass gives this lightweight kiln excellent temperature control permitting rapid heat-up and cool-down times. This is not possible with traditional kilns lined with refractory bricks. The research to date has attracted interest from water companies including Anglian and Thames.

The research now hopes to focus on the possibility of constructing a portable system for producing manufactured aggregate insitu from sewage sludge, sludge ash and trench overburden. This benefits the environment because it reduces the need to deposit large amounts of waste trench spoil to landfill, it reduces transportation requirements, therefore reducing emissions.

The research enables compliance with HAUC code of practice regarding trench re-instatement because the spherical nature of the aggregate
provides an excellent material for levelling and compaction. It could also be considered to benefit local authority recycling initiatives. The work undertaken at the University of East London has shown manufactured artificial aggregates cannot only replace but in some respects are superior in behaviour to natural materials.
The importance of ensuring plastic pipelines are properly installed is paramount to the longevity of the asset. The use of manufactured artificial materials will not only
support effective construction, but will reduce the need to
use precious natural resources and is more sustainable and effective than the use of other recycled materials. For the wastewater utilities, the
added advantage is the raw materials needed to manufacture the aggregate are available from existing processes within the industry

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