State of odour capture

By Tristan Wiggill

Waste water traps, though unsung and rarely spoken of, are critical to health and safety, and form an integral part of modern sanitation systems.

Waste water traps are designed to retain a small body of waste water from the system to which they are attached. By forming this liquid barrier, unpleasant odours and bacteria are stopped from travelling back down/up the trap and into one’s home or building.

Similarly, foul-smelling gases and harmful pathogens from larger municipal sewers are prevented from entering household drainage systems with intercepting traps. Well-designed and well-constructed interceptors can quickly remove foul matter of residential or municipal sewers.

Waste water traps are manufactured in rubber, PVC (white and black), polypropylene, brass, and stainless steel. They are classified as either P-Traps, Q-Traps, or S-Traps, depending on the shapes they take.

Rubber and PVC waste water traps are mostly used in inland applications, while polypropylene is preferred in the coastal areas in South Africa. Brass, meanwhile, is typically used for applications where chemicals are used, such as in laboratories.

“The future of local waste water trap manufacturing looks positive.”

“Stainless steel traps are strong, corrosion resistant, and generally lower in maintenance than any other material,” says Shaun Coombes, co-owner of Alltrap Engineering.

He adds that most building codes require separators to be vented back through the inlet plumbing and to a roof vent. “In almost all cases, odour problems are caused by improper venting of the building’s plumbing system.”


One of the advantages of modern traps constructed from plastic materials is the ease with which they can be dismantled for cleaning.

Traps should be self-cleaning, that is to say, they should be designed so that their walls are scoured by the discharging water. The interior surface of the trap should be smooth so that water flow is not obstructed, thereby enabling self-cleansing to take place.

An access door should be provided to assist with cleaning the trap, which ideally should be made from a non-absorbent material.

Good traps should provide enough water seal (around 50mm) with a large surface area. The depth of a trap seal depends on the intended use of the pipe, and varies from 25mm to 75mm deep.

If the trap seal is lost or damaged, odours from sanitary appliances will enter the home or building. Therefore, the water seal in the trap must be inspected and maintained in all circumstances.

For multiple storey applications, a reseal trap is used. Due to multiple floor installations having their plumbing in-line with one downpipe, the reseal ‘bubble’ breaks the suction and ensures water is left in the seal.

Some of the side effects of damaged trap seals include:

  • Evaporation
  • Capillary attraction
  • Momentum loss
  • Leakage
  • Wavering out
  • Compression of back pressure
  • Induced siphonage
  • Self-siphonage.


The general, functional requirements of SANS 50858-1 states that “The design of the separator system shall ensure that separated light liquid cannot be discharged either accidentally or in an uncontrolled way, for example by syphoning. The design shall also ensure that any separated and retained light liquid is not disturbed.”

Capillary attraction is a rare occurrence, which happens in S-Traps when a piece of porous material is caught over the bend of the trap. The material then absorbs water and deposits it down the waste discharge pipe.

Momentum is caused by a sudden discharge of water. Due to velocity, water is discharged and it shoots around the trap bend and goes down the waste pipe, leaving no seal.

Leakage is most often caused by a fault in the trap or plumbing installation. Water would be evident on the floor beneath the trap.

Wavering out is caused by the effect of the wind, which passes over the top of the ventilation pipe, resulting in pressure fluctuations.

Compression in back-pressure usually occurs in high-rise buildings. When water is discharged down to the main discharge stack, air is compressed at the base of the stack.

A waste pipe connected to the stack in the pressure zone may have the seal of the trap lost by the compressed air forcing out the water. Detergent foam further increases the risk of compression.

Induced siphonage is caused by discharge of water from another sanitary appliance connected to the same discharge pipe.

In either a vertical or horizontal main waste pipe, as water flows down the pipe and passes the branch pipe connected to it, it draws air from it, thus creating a partial vacuum and subsequently, siphonage of the trap takes place.

Self-siphonage is caused by appliances, such as wash basins and other small appliances, due to their curved shape, and is caused by a moving ‘plug’ of water in the waste pipe. A partial vacuum is created at the outlet of the trap, thus causing siphonage to take place.

When the trap is not being used, the rate of water evaporation depends on the relative humidity of the air in the room. The rate is approximately 2.55mm per week, so a 25mm seal would last for 10 weeks.

  • Dean Cane, operations manager at Modern Plumbing, says that while all waste water traps are pretty easy to install, better quality traps generally have fewer leaks. He believes the telescopic pipes could be a bit longer, though. “The bottom part of the trap is now plastic and is easily broken. We have no problem with the flexi traps. In general, the consistency of waste water trap manufacture in South Africa is pretty good.”
  • He insists traps must have the correct water seals, as some have been found to be too shallow.


Compliance of waste water traps in South Africa is governed by parts one and two of SANS 132, which covers the requirements for ‘P’, ‘S’, and dip-tube bottle waste traps moulded from suitable butyl or EPDM rubber compounds and designed for use in domestic applications, including baths, sinks, showers, and other fitments.

“Each and every application can differ and, therefore, the supply of the correct unit is vital. Getting the correct information from the client is of utmost importance,” continues Coombes. “Many clients get confused between oil separators and grease traps. A grease trap is for the hospitality industry, that is, for cooking fats, and an oil separator is for the motor industry, meaning, motor oils.”

Working out specific flow rates will confirm the sizing of the units required for both oil/water separators and grease traps.

He says the difference between locally manufactured traps and imported ones predominantly lies in the pricing thereof.

“Some imported products are of superior quality and meet ISO standards, but are not as affordable as locally manufactured units. The lead time for locally manufactured units is also a lot shorter than the imported units,” Coombes explains.


Modern plastic waste plumbing (apart from WC’s), normally use either 32mm (for hand basins) or 40mm (for sinks, baths, showers, dishwashers, washing machines) pipe and fittings (including traps).

He says the future of local waste water trap manufacturing looks positive. “It seems the councils throughout South Africa are clamping down and becoming quite strict when it comes to the disposal of oils and greases into the country’s waste water systems. This creates more business for local manufacturers, as clients are faced with fines if they do not comply,” he explains.

While the external diameter of pipework can vary between manufacturers and may not be interchangeable, various clamps and couplings are used to avoid problems. Pipe can also be rolled to the correct sized outside diameter.

As with most local products, the challenge lies in manufacturing a quality product with all applicable accreditations for a competitive price against the imports.

“Dutton Plastics strives to supply high-quality products with the added benefit of the universal design, giving our customers the advantage of stocking fewer products with more function,” says regional sales manager, Charlene Joubert.

The resources and raw material available to the African market, although superior to imported products, come at a higher cost, she says. “If supplied through a credible manufacturer, customer support, after-sales service, and product training offer huge advantages to not only the merchant and plumber, but also to the end user.

“We are being challenged by cheap imports. Our products are being copied and some of these are plagued with quality issues,” she says.

“As a local manufacturer, we guarantee our products with the right backup and service. And we are ISO-certified.”

Joubert says local waste water trap manufacturing companies are making further investments in terms of capital expenditure as well as upgrading existing systems and machinery to offer world-class quality products.

“The benefit a number of international markets had over the local manufacturers for a number of years was their output volumes. That is changing with the arrival of new machinery, multicavity moulds, and South African production expertise, which is increasing annually,” she concludes.

* All images courtesy of Dutton Plastics

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