Water, Sewage & Effluent November December 2018 | Page 12

as hotels, have their own standards,
which is further complicated by the
regulations and standards of various
local authorities and others.
It is important to note that volume,
such as water storage, is related to the
number of people, and their resulting
and relative usage — and flow — is
related to how many fixtures are used
simultaneously.
I have been involved in the
adjudication of projects that had gone
wrong, and in most cases, it was
because the designer made the wrong
assumptions or based the design on
the wrong design norms.
is contrary to the national building
regulations (NBRs), which stipulate
that ‘one competent person’ designs
the complete system from the furthest
fixture, through the building, right up
to the municipal sewer connection.
And herein lies the problem. The fact
that the design is ‘split’ causes serious
problems in terms of responsibility
when problems occur and nobody
wants to take responsibility.
In many cases where there is this
split design of the internal and external
systems, the design is not cost-effective
and often, the two systems do not
match. The result is that the internal and
external systems are based on different
design criteria, and since the designer
of the external system does not know
what is going on inside the building,
the external designer provides piping
where it is not necessary.
Design assumptions
The design assumptions are the most
critical basis of any project. Any design
manual, or any literature on standards
and norms, will have a section titled
‘Definitions’, which are critical to the
design elements. For example, the
definition of the drainage piping inside
a building is called a ‘discharge pipe’.
The definition for the drainage piping
from the building up to the municipal
sewer connection is called a ‘drain
pipe’, and from the connection with the
municipality, it is called a ‘sewer’. The
design criteria for a discharge pipe,
drain, and sewer are all different.
A set of so-called ‘discharge units’
for each fixture is used to size the
internal building discharge piping, and
the external drain from the building up
to the sewer connection, from where
the engineer bases the design on a
flow rate of litres per second.
The problem with the discharge
unit method, is that the relevant table
indicates the maximum number of
discharge units per pipe diameter
per gradient and not the minimum.
The minimum is essential, though, as
it must allow at least a minimum flow
rate of 0.7m/s to allow for clean flow
velocity to convey the effluent.
Another problem with the flow
table is that non-engineers select a
larger pipe diameter, since this larger
diameter pipe can be laid at less of a
gradient. The fault here is that this looks
only at the maximum number discharge
units that the pipe can convey, instead
of looking at the minimum flow rate for
clean flow velocity.
The solution?
It was found that where one engineer
designs the internal and external
systems, this can be rationalised with
significant cost savings. Historically,
this is relevant to sanitary drainage,
domestic water, and fire water
systems, and the savings were always
in the millions of rands.
I have seen a project where the
internal piping connection to the
external system was 100mm internal
diameter — which is the correct
minimum diameter required in terms
of the NBR — but the external system
piping that it was connected to, had
a 100mm outside diameter. The result
was that the two systems’ piping did
not match up, which caused serious
and costly problems, and will continue
to cause problems for the remainder of
the life of the system.
The fact that the design
is ‘split’ causes serious
problems in terms of
responsibility when
problems occur and
nobody wants to take
responsibility.
It is essential that the internal and
external systems are designed by one
engineer, with one responsibility, and
based on one set of design criteria and
assumptions.
When the drainage is one single
drain connecting the building to the
sewer connection, then the NBR
design criteria is relevant, but when
it is a drainage system connecting
a number of outlets to the municipal
connection, such as in a hospital or a
townhouse complex, then it should be
designed as per the design criteria of
a township sewer, as it is basically an
extension of the municipal services
and requires a significantly higher
competence to design and construct.
Hospital sanitary
drainage and water supply
There are no specific design norms
and standards for hospital sanitary
drainage and there are no water
regulations in the NBR.
The building control office (BCO)
does not require water drawings
for ‘any building’, except for the fire
water installation (but excluding fire
sprinklers). There are presently no
SANS standards for fire sprinklers. It is
These various ways of calculating
the flow rate is the cause of many
misunderstandings and incorrect
assumptions. Added to this is the
fact that in most cases, the internal
building drainage system is designed
by one designer and the external
drainage system by another. This
10
The challenges
The barrier between the user and possible horrible contamination is a mere 75mm of water
trap in the S-bend.
Water Sewage & Effluent November/December 2018
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