| PRACTICAL
DESIGN AND INSTALLATION |
| |
|
| 4.1 |
A portable submersible pump should be provided in order
to allow flexibility in operation. This pump can be used
to recycle the plant effluent to maintain the STP in a
working condition when incoming flow is low. |
| |
|
| 4.2 |
A
general headroom of 3 m should be maintained for
enclosed or underground STP with artificial ventilation.
This may be reduced to 2.5 m at localized points,
e.g. under beams. |
 |
| |
|
| 4.3 |
For
enclosed STP, a minimum air volume of 14 m3
should be provided. Artificial ventilation should
be provided with not less than 10 air changes per
hour. Ventilation exhaust pipes should be carried
up to a height not less than 1 m above the roof
of the building at which the STP is located. |
|
| |
|
| 4.4 |
Access walkways of minimum 0.75 m clear width should be
maintained within the STP for access to all areas requiring
maintenance and operation. Walkways should have safety
rails (preferably stainless steel) and toe boards. Walkways
should be laid to a 1:25 crossfall to prevent ponding
if of solid construction and they should not be obstructed
by crossing pipework. |
| |
|
| 4.5 |
Where levels vary, staircases should be provided and not
catladders or step irons. |
| |
|
| 4.6 |
All tanks should be provided with stainless steel ladder
or step irons for access. Mild steel should not be used.
Step irons should be spaced at 300 mm c/c horizontally
and 250 mm c/c vertically. |
| |
|
| 4.7
|
If
a tank has to be covered, stainless steel or aluminium
alloy open mesh flooring should be used (aluminium should
however not be used in chlorination facilities located
in confined space). Open mesh flooring should be designed
for 5kPA uniformly distributed load. |
| |
|
| 4.8 |
Provision of an emergency by-pass to the STP is not allowed
under normal circumstances. |
| |
|
| 4.9
|
Fresh
water taps for hoses and handwashing should be provided
in the STP at convenient locations and should be shown
on the drawings. |
| |
|
| 4.10 |
Electrical distribution boards and control panels should
be installed in locations not liable to flooding and the
locations shall be shown in the drawings. Panels should
have a dehumidifying heater. |
| |
|
| 4.11 |
The control panel should incorporate a mimic/schematic
diagram and all labels should be in both English and Chinese.
All values, penstocks, pumps etc. should be appropriately
labelled.
|
| |
|
| 4.12 |
Hours run meters should be provided for all major equipment
to record the total hours of operation. They should be
of the non-resettable type. |
| |
|
| 4.13 |
Coarse screens should be provided to precede pumps. Fine
screens should be placed downstream of equalization tanks
equipped with air ejectors so as to minimize organic solids
content in the screenings. |
| |
|
| 4.14 |
Mechanical screens should be the static type or of the
type having the rakes moving in the bar openings and not
on the bar. Either manual or mechanical by-pass screens
should always be provided. The duty screen channel should
have a high level overflow into the standby screen channel
so that the flow diverts automatically through the standby
screen in case the duty one is blocked. |
| |
|
| 4.15 |
Excessive
screen channel width which results in shallow depth
of flow should be avoided. In case of small developments
where the smallest mechanical screen commercially available
is still too wide, a manual screen may be used.
Bars
of screen should be extended to the invert of the channel.
For manual screens, consideration should be given to:
- access
by operator;
- ease
of removal of screenings by operator, including provision
of suitable rake;
- draining
of screenings;
- bagging
and storage of screenings.
|
| |
|
| 4.16 |
A perforated/mesh type drain tray (of stainless steel
or plastic material) should be provided to allow the screenings
to drain prior to packaging. |
| |
|
| 4.17 |
Pump sumps (including equalization tanks) should
be designed to limit the pump starts to not more
than 10 times/hour. |
 |
| |
|
| 4.18 |
Electrical voltage for level sensors in pump sumps
should not exceed 24 V to avoid electrical hazard.
|
| |
|
| 4.19 |
High level alarms should be provided in all pump
sumps. This should be in the form of visual and
audio alarms connected to a suitable constantly
manned location. |
|
| |
|
| 4.20 |
Pumps should be carefully chosen to correspond to the
design duty point. Flow regulation tanks with control
weir should be used if only bigger pumps are available.
|
| |
|
| 4.21
|
Comminutors,
especially the enclosed type, are discouraged as they
often cause operational problems in the STP.
|
| |
|
| 4.22 |
Sedimentation
tanks should be designed either as rectangular horizontal
flow tank, or as square/circular upward flow tank. For
horizontal flow tank, the length to width ratio should
not be less than 2. The use of tanks with length to
width ratio between 1 and 2 should be avoided.
 
|
| |
|
| 4.23 |
Upward flow sedimentation tanks should be provided with
minimum 60o hopper wall slopes to facilitate
easy sludge collection and removal. |
| |
|
| 4.24 |
Horizontal flow sedimentation tanks should be provided
with a mechanical scraper system on a nominal floor slope,
discharging to a sludge hopper. Twin tanks should preferably
be used. If single tank is used, the scraper should be
of the type that allows the underwater parts to be maintained
without the need of shutting down the tank. |
| |
|
| 4.25 |
Adjustable weirs should be used for sedimentation tanks
to enable fine levelling.
V-notch weirs should be used where wide flow variation
occurs. The side-wall height should be not less than 1
m in order to avoid sludge carryover. The layout should
be such that the inlet arrangement never becomes submerged
in sludge. |
| |
|
| 4.26 |
A chamber should be provided at the final sedimentation
tank outlet after the weir for installation of recirculation
pump in the event that the actual initial flow is much
lower than the design flow (see 4.1 above). |
| |
|
| 4.27 |
Sludge in sedimentation tanks should be removed as frequently
as possible, at least once per day by a suitable sludge
withdrawal device such as submersible sludge pumps, airlifts
or valves. A scum removal device should also be provided.
The sludge and scum removal device should have independent
adjustable control. Inspection trough should be provided
at the discharge end for inspecting the quality of sludge
withdrawn so that adjustment to the withdrawal system
may be made accordingly. |
| |
|
| 4.28 |
Chlorination facility should include a flash mixing
device followed by a plug flow contact tank. Baffles
should be provided in chlorination tanks to prevent
short-circuiting. Lateral baffling should be used
for shallow tanks and up-and-over baffling should
be used for deep tanks. Dechlorination facility
may be required in situations where the residual
chlorine poses hazard to fisheries or shellfisheries.
|
 |
| |
|
| 4.29
|
When
tertiary treatment is required, disinfection (chlorination,
ultra-violet radiation, ozonation, etc.) should
be placed after tertiary treatment. |
| |
|
| 4.30 |
Sludge storage tanks should be provided with aerators
when they are located in enclosed plant rooms to
maintain the aerobic condition of the sludge. However,
sludge storage tanks should preferably be ventilated
separately to the exterior. Decanting facilities
should be fitted to sludge storage tanks. Supernatant
should be returned to the equalization tank. |
|
| |
|
| 4.31 |
Sludge pumps should be designed with positive suction
head to minimize the possibility of clogging.
|
| |
|
| 4.32 |
The diameter of the sludge draw-off pipework should be
of a minimum diameter of 80 mm in order to prevent pipe
blockages. |
| |
|
| 4.33
|
A
sludge dewatering machine capable of attaining 30% w/w
dry solids content for landfill disposal and a sludge
tank for emergency storage of 14 days sludge volume
should be provided. For small STP serving less than
100 population, it may be acceptable to provide only
a sludge storage tank for wet disposal provided that:
- a
storage tank capable of holding 60 days sludge volume
be provided,
- vehicular
access is maintained for desludging tankers,
- written
confirmation is provided from a desludging contractor
undertaking to carry out the work and specifying the
ultimate disposal site for the wet sludge.
|
| |
|
| 4.34 |
An automatic flow measuring device with non-resettable
type totalizer should be provided at suitable location
for measuring the flow for STP serving a population of
more than 50. |
| |
|
| 4.35
|
An
easily accessible sampling point should be provided for
taking samples of the treated effluent. |
| |
|
| 4.36 |
A
minimum lighting intensity of 300 lux should be provided
in the STP. Lights should be located where they are accessible
for maintenance and replacement. |
|