Things down the drain (Part 1)

Things down the drain (Part 1)

By Benjamin Brits

Having spoken with several plumbers in the industry, a few common questions arose around drainage.

Drainage is something that most people do not even think about; yet, it is vital, as drainage allows the removal of waste and wastewater, creating a safer and healthier environment for us to live in.

Looking back over history, we can all appreciate the need for proper drainage systems. If you had to imagine yourself as a settler in a new village in ancient times and you happened to be one of the less fortunate, you would typically find yourself at the foot of a hill. Now, you may not think that is such a bad thing.

Well, the more well-to-do you were, the higher up the hill you would be, and on top of the hill would be the people who are considered our affluent citizens in today’s times. The reality is that in those days, there was no such thing as a drain, a sewer, or a stormwater pipe.

The common way to dispose of your waste was through a hole in your floor; this included old food, urine, and faeces. This would then pile up under the houses, attracting all sorts of pests and, of course, was a perfect breeding ground for diseases. Now, when the rains came, the natural thing happened: all the waste would be caught up in the stormwater and flow down the hill, around and then potentially into the houses below. You can see how this would be a ghastly way to live, especially for the poor at the bottom of the hill. Unfortunately, South Africa still sees these sorts of living conditions in the informal settlements and rural areas.

Fortunately, in those days, it didn’t take too long before channelling methods were developed to at least control where and how the waste was handled, albeit still open systems, as people would regularly become gravely ill and die, due to these unsanitary circumstances. It is amazing to know that the materials used (short of a few that have been discovered to be deadly) can still be found in current drainage and sewer systems. These materials were terracotta, clay, lead, and bronze; the Egyptians were fortunate to discover copper as early as 500BCE.

Getting back to 2019, we answer some of the questions posed to Plumbing Africa pertaining to how things work nowadays. Please note that references are made to specific standards, but not all the details can be included in this article, as it covers too vast an amount of information.

What are the correct standards for above-ground and below-ground piping and installation?

Firstly, it is always best practice to apply all South African National Standards (SANS) regulations pertaining to installations. Although some will dispute that not all standards are compulsory, which is indeed true if they are not part of regulations, standards have been established not only for the sake of best practice, but also to meet the requirements of the consumer in terms of product performance and, most importantly, safety.

A sanitary drainage system design and installation by a plumber must comply with SANS10400-P and comply with the deem-to-satisfy rules. These rules must be closely followed to comply with the regulations that specify the performance of the system; that is, the system needs to function as it is supposed to.

There are only seven regulations and SANS10400-P, P2, specifies the performance. A rational design (by an engineer) must comply with the regulations. It is important that you as the plumber know the rules and their application to meet the overall correct functioning of the system. When a learner plumber is tested for registration as per SANS10400-A, their knowledge of the theory as per SANS10400-P is also tested.

It is stated in SANS10400-A that the National Building Regulations (NBR) is not a design manual, but this document contains all the details on drainage; however, as you would know, it sometimes needs clarification and the correct interpretations to be applied correctly.The NBR also has a list of other related standards that need to be studied, such as piping and fixtures. It also lists the standards for underground and above-ground piping. The above-ground types of plastic piping must be ultraviolet (UV) resistant, which is essential and often gets neglected in installations. Not using the correct material is a sure way to get the installation failed. You cannot use below-ground plastic pipes above ground because below-ground pipes do not get manufactured with UV-resistant stabilisers. This essentially means that when exposed to sunlight, these below-ground pipes will not last the required length of time, and they will become brittle, crack, or burst.

The installation of above-ground piping is specified in detail in SANS10400-P and the most important part is the fixing of the piping in a stable position with saddles, pipe clamps, brackets, supports, hangers, and the like, but it must always allow for expansion and thermal movement. The best option is to use pre-manufactured systems that have been designed to consider this.

SANS1200 is a civil engineering standard for construction, but valuable for the installation of below-ground piping and the related elements such as the construction of manholes and inspection or cleaning chambers. It also describes the excavation, the pipe bedding, the laying of the piping, and the material required around and above for backfilling. Both SANS10400-P and SANS1200 also specify the testing requirements.

What is the correct method of calculating the flow of waste?

A plumber is allowed to carry out a deemed-to-satisfy-rule design (DTSR), and there are a number of tables in SANS10400-P that indicate how to size the various types of pipe, such as a drain (which shall not have a gradient less then 1 in 60 in the case of the DTSR design), and to size the stack pipe and branch pipes.

One of the tables indicates the number of discharge units (DU) per fixture, such as one DU for a washbasin and eight DUs for a water closet (WC). The design of a drainage pipe relates to DUs and not litres per second as you will find in water system designs. This system is considered ‘old’ and originated many years ago but worked well in the case of a DTSR design for simple building systems.

The existing tables are outdated and render an over-design; however, it is still within safe design parameters. The original tables were based on a toilet flushing volume of 12–13 litres per flush; this volume has been significantly reduced to between three and six litres. Today, there are systems that flush with two litres.

The critical element of a drainage system is not only the flushing of the WC and the volume of water to flush the bowl clean, but the volume of water required to flush the effluent down the piping and carry it away to the municipal sewers, and further. The DUs relate to the volume of water to flush the effluent down the drain and then through the municipal sewer — this has not yet been addressed and therefore not revised to suit current requirements.

Ultimately, the answer to this question is that you as the plumber are obliged to comply with SANS10400-P as per the latest deem-to-satisfy rules, to size the various pipes or have a rational design done by the engineer, which is not necessary for a house.

In the case of a complex building, it would be better to do a rational design by a registered engineer who can then calculate the pipe sizes in terms of hydraulic design principles, and the same for the municipal sewers.

If you analyse the pipe diameters of most of the non-complex buildings, then you will find that most of the pipe systems consist of common sizes between 50mm and 100mm. The 50mm is mostly for the wastewater and the 100mm is the minimum for soil piping. It is recommended to work according to the present tables until a formal revision is carried out. 

Part 2 continued in the next issue.
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