Clay beats PVC for green drainage
The use of clay pipes for drainage systems has many advantages for sustainable construction, says Edward Naylor, chief executive of clay products manufacturer Naylor Drainage
For years the construction industry enthusiastically embraced PVCu, despite indications that it was environmentally damaging. As far back as 1996, Greenpeace warned that "the production, use and disposal of PVC... results in the creation and release of large amounts of toxic chemicals". Even then, Greenpeace highlighted clay as an environmentally preferable material for a range of reasons.
Clay pipes have been proven over hundreds and even thousands of years in service.
For design purposes, clay can be assumed to have an infinite life. In an era where the rate of sewer replacement and renovation implies that the average sewer will be required to last for an average 350 years, this ought to be an increasingly important criteria.
The clay drainage industry uses clays and shales as its raw material. These are entirely natural and no harmful additives or admixtures are required to aid production or to improve the properties of the finished product. The clays and shales used by all UK clay pipe producers are in abundant supply locally and will satisfy the needs of the industry well into the next millennium. Also, clay is a raw material which finds little use in other industrial applications, and is consequently not subject to market fluctuations.
Many clays are extracted from open-cast sites, during the process of coal extraction. Typically, each has a working area below ten hectares and is invariably situated close to the production site.
In general, such sites are in areas designated as scrub land, and producers work closely with national and local agencies when reinstating the land after extraction.
Extraction is governed by the guidelines laid down in the Environmental Code for the Heavy Clay Industry, prepared and published by the British Ceramic Confederation.
Energy analyses of the manufacture of clay drainage originated in response to environmental pressures, including the need to conserve finite material resources and concern about climate changes that could result from the increasing release of heat into the Earth's atmosphere. The UK clay-pipe industry is the most advanced in the world, and has pioneered many major improvements in both quality and energy use.
As a consequence, the total energy requirement for the production of a clay drainage system compares very favourably with all systems produced from alternative materials including PVCu, concrete and cast iron. The energy used in clay production is significantly less than all other materials.
The most energy-intensive is spun cast iron, which requires more than nine times the amount of energy compared with clay. This demonstrates why this is an expensive alternative and should only be used in special circumstances.
Waste products generated by clay pipe production are minimal and all raw clay scrap is returned for reuse. In addition, most of the fired scrap is ground up and fed back into the clay mix in the form of grog. This is an essential element of the mix. It improves dimensional accuracy and performance. And it speeds up drying and firing, which cuts energy consumption.
As they have inherently high structural strength and rigidity, clay pipes can often be laid directly on a trimmed trench bottom, with minimal or no bedding, thus significantly reducing the use of scarce and expensive aggregates and minimising site excavation.
Broken or obsolete clay pipes are an inert material that can be returned to the earth and will have absolutely no detrimental impact in the short or long term. Better still, any waste can be ground down to provide an excellent inert hard-core or aggregate, so helping to reduce pressure on the extraction of limited resource natural aggregate.
This treatment is entirely consistent with the aims of the Department of the Environment's revised MPG6 guidance notes to local authorities, which encourage the use of secondary materials rather than virgin aggregates.
Clay pipes have long been the preferred option in a broad range of aggressive conditions, and can carry all effluents acceptable to WwTWs. They are unaffected by the presences of hydrogen sulphide in the pipeline and sulphates in the ground.
At temperatures found in public sewers in the UK, they are resistant to damage from effluents and groundwater at pH2 to 12. Couplings are chiefly made from polypropylene, which is temperature resistant, and will not distort, soften or embrittle, even when subject to temperatures well above those experienced in normal service.
As a result of the ability of clay drainage to withstand such aggressive conditions, it is eminently suitable to use on reclaimed industrial or commercial development sites. Clayware has excellent resistance to abrasion, and is unaffected by pipe-cleaning operations, particularly rodding and high-pressure jetting. In practice, therefore, a clay drainage system can be considered to have unlimited life.
Clay is also less vulnerable to poor site practice than plastic materials. In particular, bedding is critical to PVCu and other plastic pipes as the backfill forms an integral part of the structural integrity of the pipeline.
A plastic pipeline has little inherent strength and will flatten under relatively light loads, unless it is fully and carefully surrounded by a bed of single-sized pea gravel or similar material. As a result of this, significant quantities of imported aggregate are needed for PVCu and other plastic drainage systems.
In short, manufacturers of clay drainage lay claim to a fundamental environmental advantage, embracing factors such as a much reduced energy requirement in
production, less need for bedding aggregate, resistance to heat and chemicals, and a considerably longer service life than that of competitive materials.