| DESIGN
PARAMETERS |
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| 3.1
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The
designer should give a reasonable estimate of population
and a detailed breakdown of the total flow rate in the
calculation.
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| 3.2
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The
basic design parameters for residential populations shall
be taken as follows:
- 0.30
to 0.46m3/h/d (cu.m. per head per day)
depending on types of development.
- 55g
BOD/h/d
- 55g
SS/h/d
For
other types of population, reference should be made
to Civil Engineering Manual Vol. 6 and Water Supplies
Department Provisional Standing Order 1309, relevant
data of which has been attached as Appendix
2. If service trades (shops, restaurants etc.) occur
in the development, the flows and loads of the trades
shall also be included as per Appendix
2.
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| 3.3 |
The design peak flow arriving at the STP as a proportion
of dry weather flow (DWF) shall be taken as:
- 6
DWF for population equal to or under 1 000
- 4
DWF for population over 1 000 but not less than that
based on 1 000 population.
Either
the STP can be designed for the above peak flow rate
or it can be designed to cater for a peak flow of 3
DWF, excess flow over 3 DWF being equalized in an equalization
tank. In the latter case the feed pumps must be sized
accurately to avoid excessive peak flow rate production.
Equalization
tanks should be designed to hold the excess flow for
a period of two hours. Only the tank volume above the
duty pump cut-in level should be considered as effective
equalization volume. Air ejectors should be provided
to prevent septicity of sewage.
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| 3.4 |
Primary sedimentation tanks shall be designed for:
- maximum
surface loading of 40 m3/m2/d
at peak flow,
- minimum
retention time of 2 hours at peak flow,
- maximum
weir loading of 250 m3/m/d at peak flow.
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| 3.5
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Final
sedimentation tanks shall be designed for:
- maximum
surface loading of 35 m3/m2/d at peak flow if they
are preceded by Rotating Biological Contactor (RBC)
or biological filters,
- maximum
surface loading of 22 m3/m2/d at peak flow if they
are preceded by the extended aeration process,
- minimum
retention time of 2 hours at peak flow.
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| 3.6 |
The reduction of total BOD by different stages of the
primary treatment processes should be taken as follow:
-
-
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| fine
screen (max. opening 2 mm) |
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7.5% |
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| primary
sedimentation |
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30% |
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| fine
screen and primary sedimentation |
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30% |
The
net BOD load going into the subsequent biological processes
should be computed accordingly.
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| 3.7
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Biological
processes should be designed to the following organic
loading:
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10
g total BOD/m2/d (i.e. per sq.m. of RBC
surface) entering into the RBC or as recommended
by manufacturer, whichever is smaller, |
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0.07
kg total BOD/kg MLSS/d and MLSS in the range between
2 000 and 3 500 mg/L, |
- Plastic
biological filter (Biofilter)
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0.5
kg total BOD/m3/d (i.e. per cu.m. of
media volume) or as recommended by manufacturer,
whichever is smaller. |
- For
other biological treatment methods, such as
sequential batch reactor, contact stabilization
and other patented processes etc., organic loadings
proposed in the design should be justified by
supporting documents such as relevant literature,
local monitoring results and catalogues.
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| 3.8 |
Chlorination tanks should be designed for minimum 30 minutes
contact time at peak flow.
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| 3.9 |
Sludge quantities generated from the processes should
include:
- solids
removed from primary sedimentation, which should be
taken as 70% of the influent SS.
- sludge
yield from biological processes, which should be taken
as 0.5 kg solids/kg BOD removed when preceded by primary
sedimentation, or 1 kg solids/kg BOD removed when
not preceded by primary sedimentation. In the latter
case no inclusion of primary sludge is necessary.
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| 3.10
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Solid
concentration of sedimentation tank sludge shall be taken
as:
- 1%
for extended aeration sludge,
- 2%
for RBC, biological filter and primary sludge.
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