Water conservation and drainage blockages

Water conservation and drainage blockages

By Vollie Brink, Pr Eng
It is wonderful to see how much effort is going into water conservation. But water conservation should have been a top priority years ago, so why is it that all these so-called experts are only voicing their brilliant ideas after the fact?

Nowadays, even the newspapers are brimming with advice on how to use less water. A new standard is being developed on water conservation, even though we could not develop water regulations since 1977.

I was once asked to give a talk about civil engineering at a technical high school to give the pupils an idea of this kind of work, while other engineers had to talk about the pros and cons of all the other engineering careers.

I was the last speaker, which I was happy about because then I could say what I wanted to and nobody could repudiate me. I told the audience that we can forego all the other engineers and still survive, but we cannot do without the civil engineer and water.

You, Mr Plumber and Mr Engineer, play a crucial role in keeping our people alive.

For many years we have accepted water as a fait accompli and that it is plentifull. And then suddenly, the situation in Cape Town happened. This situation has opened many eyes, and many water engineers have predicted what we now have seen play out.

We have heard and read a lot about the water problems in the old mines around Johannesburg and how it is going to affect us all, and that places like Gold Reef City are going to be swamped — and then everything just went quiet.

Water engineering needs long-term highly skilled engineering planning and highly skilled budget planning to go with it. It also needs highly skilled engineers to design and manage the construction and highly skilled human resources to do the work.

I asked such a ‘highly skilled engineer’ why the forward planning was not done, and his answer was that it had been done, but not implemented. You can have wonderful planning but if it is not implemented, then it is not worth the paper it is written on and remains a pipe dream.

I attended a world conference in 1980 in Berlin, and I was invited to see the research that had been done by Professor Knaublaag of the Technical University of Berlin. The professor was doing research on flushing systems to see by how much he could reduce the water and still have an acceptable flushing of the effluent through the piping.

He had systems set up with glass piping and cameras down the piping to see what happens when the system is flushed with various flushes. He produced solids from a certain substance and texture to represent human solids and flushed it through the WC and configuration of piping to see to what extent he could reduce the flush water. In those days, we wondered why he was working on something so ‘unnecessary’, but he was a visionary and knew that water would become a problem.

Some European manufacturers have started making WCs that use less water, and most have reached the point where their system has two flushes: one for urine to flush with three litres and another for flushing solids with six litres; this seems to have become the norm.

Talks and efforts on how to minimise the flow rates of fixtures, such as the WC and shower and the various taps, have surfaced again. Most of these people see the solids and paper disappear from the WC pan, but they do not see what happens downstream from the WC, inside the drainage piping.

You need enough water and air inside the piping, and the piping must be at a specific minimum gradient to transport the effluent otherwise it will cause problems, which we call blockages.

Blockages are serious health risks, and two of the key elements of the National Building Regulations are to protect health and to ensure safety. The gradient of a drain or sewer pipe (note: there is a difference between the definition of a drain and a sewer) is critical to transport soil and waste from the various fixtures.

When the engineer designs the sewer for a township, he or she will start to calculate the invert level of the furthest stand or erf connection, which will be the depth of the stand connection and the beginning of the municipal sewer.

The engineer will calculate the depth of this connection to serve at least 80% of the erf and to start with a depth of 700mm, and then with a fall of 1 in 50 up to the proposed connection, followed by a drop of 450mm at the connection. This will allow any building on that erf to be connected to the municipal sewer.

In other words, if the drain on the site has a fall of 1 in 60, it will be able to flush the drain clean with clean flow velocity. A drain is the portion of piping that connects a building to the sewer connection on the boundary of the erf.

If the drain on the erf is at 1 in 60, which is the minimum allowed gradient as per SANS 10400-P and the deem-to-satisfy rules, and if the water/air ratio in the piping is correct, then there should never be blockages in the drain. If the drain is designed to carry the number of discharge units as per SANS 10400-P, then the air/water ratio should also be correct.

It must be noted that the tables as per SANS 10400-P were developed long before 1977 when the first National Building Regulations were promulgated, and in those days, the water flushed from a toilet was significantly more than what it is today, and the sewers were designed on a much higher water discharge from the erf than what we have today.

When we reduce the water flush from a toilet and the other fixtures, then we have less water with which to convey the contents in the drain and the sewer. If we adjust the water flush, then we must actually also change the gradient of the piping, which of course we cannot do.

You need enough water and air inside the piping and the piping must be at a specific minimum gradient to transport the effluent otherwise it will cause problems, which we call blockages.

You see Mr Plumber and Mr Engineer, you cannot only change the water discharge from the fixtures and ignore the rest of the system and think all will be well down the road.

I have become aware of problems with blockages in many old existing systems, and the main reason for this is that there is not enough water to allow what is called ‘clean flow velocity’. The existing systems have been designed against a quantity of water from the fixtures and gradients to suit, but the quantity of water has been reduced arbitrarily and without taking cognisance of the effect downstream.

You cannot flush the toilet and see how the solids disappear and think all is well. Some people just want to reduce the water flow rate of fixtures without understanding the consequences downstream.

There is one other element that could affect the drainage system and that is if grey water is extracted and used for irrigation and other purposes and not discharged into the drainage system after it has been used. I foresee that this will also have an effect on the flow in the drains and sewers.

We must study the entire system from beginning to end and realise what will be affected in-between. We cannot just sit around a table and arbitrarily decide what the new lower flow rate of the various fixtures must be. We will have to study the hydraulic impact on the existing infrastructure with less water in it and how to overcome the problems.

We will probably also have to consider and make provision for new rules for infrastructure, both new and existing.The question is also: will the new water-saving devices become compulsory or will it only be applicable to new projects?

New water-saving devices will require less water and discharge less water into the drainage system, and it will have to be regulated and policed; the question is, who will do this? Yes, we will have to implement water-saving measures and devices but we will have to do it carefully and intelligently. It must be part and parcel of the long-awaited water regulations.

However, I have found that some of the younger generation (please note, I say ‘some’ and not ‘all’) know better than the older generation and will know better how to solve the challenges that I have mentioned.

Just remember: if we have less water in the existing drains and sewers, we will have more blockages, but then again, it will generate more work for you …

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