DESIGN : DEAR MR PLUMBER
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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 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 … PA
Vollie Brink
Vollie Brink is one of the industry ’ s longest serving wet service engineers . He continues to serve on SABS committees and has been involved in the Green Building Council Star rating system . Brink continues to consult for various organisations while enjoying a well-earned retirement .
July 2018 Volume 24 I Number 5