1.1.1.1
The recommended treatment process,
Sequencing Batch Reactor (SBR), form the basis for the preliminary design of
the proposed sewage treatment works (STW).
The following unit processes will be provided upstream of the SBRs:
·
Fine
screening;
·
Grit
removal; and
·
SBR Feed
Pump Station.
1.1.1.2
Downstream of the SBR units there will
be:
·
An
Effluent Pump Station;
·
UV
disinfection facilities;
·
An
effluent twin rising main; and
·
Submarine
outfall.
1.1.1.3
In addition to the above unit
processes for liquid treatment, the works would include:
·
Waste
activated sludge holding tank;
·
Sludge
conditioning;
·
Mechanical
de-watering of conditioned sludge; and
·
Odour
control facilities.
2.1.1.1
The proposed sewage pumping station
(SPS) P2 will pump raw sewage to the SKW STW.
The pump station has two pumps (1 duty and 1 standby). Each pump will be
nominally rated at 41 l/s. Two 150 mm dia. rising mains will convey flow to the
Inlet Works of the SKW STW. Two 150-mm magnetic flow meters are located at P2
and will measure the influent flow to the SKW STW. There is also a pumped inflow from the Mo Tat Wan area.
3.1
Introduction
3.1.1.1
The inlet works will comprise,
mechanically-cleaned screens, grit removal, flow distribution, and pumping to
the sequencing batch reactors. The
inlet works will be housed in a building, which will be ventilated with
airflows being passed through a deodorization unit prior to discharge.
3.2.1.1
Flow enters the inlet works through
two 150 mm dia. rising mains from P2.
It flows by means of an open channel where it can be directed to the
screens by means of hand-pull slide gates. Two mechanically raked fine screens
(one duty and one standby) will be provided.
The screens will have a clear bar spacing of 6 mm. Each screen is sized
to handle the peak design flow. The
standby channel is connected to the influent channel. When the screen is duly
clogged, the flow will back up into the channel and overflow to the standby
screen. The failure of the duty screen
will sound a remote alarm but service will not be disrupted as the standby
screen will operate automatically.
3.2.1.2
The screenings volume expected would
be less than 0.4 m³/day on a peak day.
3.3.1.1
Taking into account of the relatively
small flows in off-peak periods on weekdays, a stirred type of grit removal
system (vortex type) is preferred to a horizontal flow type (detritor type)
since less organic solids are likely to be deposited at low flows.
3.3.1.2
A 1.85 m diameter grit separator would
be sufficient to handle peak flow and would remove grit particles (with
specific gravity of 2.65) of 0.2 mm diameter and above.
3.3.1.3
The proposed grit separator would have
a bypass arrangement to facilitate maintenance and grit would be pumped from
the unit to a screw classifier with cleaned grit being held in drums prior to
disposal.
3.3.1.4
The volume of grit expected would be
less than 0.2 m³/d on peak days.
3.4.1.1
The pump station will be designed to
deliver peak wet weather flow (PWWF), plus any plant liquors, to the SBRs. It will have a divided wet well. The divided wet well will have two
compartments each housing two submersible sewage pumps. The capacity of each compartment will be
sufficient to allow one compartment to be taken out of service without
impacting operations. There will be two
duty pumps and two stand-bys. On rising
level, the duty pumps will start sequentially.
If the level in the pump station continues to rise due to failure of a
duty pump, an alarm will be transmitted to the remote control center. After the alarm has sounded, the stand-by
pumps will start automatically so that service is not interrupted. The pumps will discharge to a common
pipeline, which will convey flow to the SBRs.
An automatic control valve will be provided from the pipeline to each
SBR. The control valves will be
operated through the SBR control system and will open and shut based on the SBR
cycle requirements. At least one valve
will be open at all times.
4.1.1.1
There will be three sequencing batch
reactors (SBRs). Under normal
conditions, all three SBRs will be in operation. However, the reactors are sized so that two units can accommodate
the design flow and still meet the effluent criteria.
4.1.1.2
There are several equipment suppliers
for SBRs. Each has slightly different modifications of the basic process but
all are capable of meeting the effluent requirements. For operational compatibility, the SBR equipment supplier should
be responsible for providing the PLC.
The actual installed equipment will vary depending on the supplier. The
following discussion provides a functional description of a typical system.
4.1.1.3
SBRs operate on a batch cycle. The
cycle time is based on the normal fluctuations of the wastewater flow. At the SKW STW, the flows will be governed
by P2, which will pump at the peak flow rate intermittently. The daily accumulation of flow will be equal
to the average dry weather flow (ADWF), which will essentially govern the
operating time of P2. The collection
system will be a dedicated sewage collection system and will not include much
stormwater. The average daily flow will
increase during the rainy season due to increased groundwater infiltration and
inadvertent stormwater inflows.
Consequently, the operating time for P2 will be longer during the rainy
season but the peak flow will be fixed by the pump capacity.
4.1.1.4
The SBRs have been sized to operate on
a four-hour cycle during the dry season.
If the operating time for P2 increases substantially during the rainy
season, the SBRs will automatically shift to a three-hour storm cycle. In the extreme case, it is possible that
direct inflow to P2 could cause the pumps to run continuously. In this event, the SBRs will automatically
shift to an emergency two-hour cycle time and sound a remote alarm. The remote alarm would alert operators of an
unusual condition that should be investigated but would not disrupt the STW
operations. Typical cycle times and
their applications are shown in the following table:
Table
1.1 Typical Cycles for Sequencing
Batch Reactors
|
|
|
Normal Cycle
(minutes)
|
Stormwater Cycle
(minutes)
|
Emergency Cycle
(minutes)
|
|
Fill-Aerate
|
120
|
90
|
60
|
|
Fill-Settle
|
60
|
60
|
30
|
|
Decant
|
60
|
30
|
30
|
|
Total
Cycle
|
240
|
180
|
120
|
|
Theoretical
Cycles/Day
|
6
|
8
|
12
|
|
Aeration
Time/Day
|
720
|
720
|
720
|
4.1.1.5
For a 3-SBR system, the cycle for each
SBR unit is staggered so that only one SBR is decanting at any time.
4.1.1.6
The cycle time is automatically
adjusted by the PLC that controls the SBR operation. During the fill-aerate cycle, the level in the SBR is measured
and the rate of fill is measured. Depending on the rate of fill, the PLC will
decide when the inlet valve should be closed and/or how long that particular
cycle will be.
4.1.1.7
The SKW STW has been sized for a cycle
time of 4 hours at ADWF. The system
also has sufficient hydraulic capacity to handle the peak flow without loss of
treatment efficiency.
4.1.1.8
The SBR units will be divided into
three zones. Zone 1 (the first zone) is
the selector zone. A small portion of
MLSS is recycled to the selector zone and is mixed with the influent flow. The selector provides a high initial F/MLVSS
that favours the growth of readily settleable microorganisms and eliminates
filamentous organisms. The resulting
MLSS will typically have an SVI of 120 or less. Zone 2 is a small aerated zone that precedes the main aeration
zone (Zone 3). It is baffled off from
the main aeration zone (Zone 3).
4.1.1.9
The main aeration zone (Zone 3)
typically occupies 80% of the SBR unit.
It operates in a complete mix mode.
Aeration will be provided by variable speed blowers through fine bubble
diffuser systems controlled both by D.O. probes in the aeration zones which
feed into the PLC which also controls flows, level, decanting operations etc.
to provide a fully integrated operational system.
4.1.1.10
At the completion of the aeration
cycle, the MLSS is allowed to settle.
At the completion of the settle period, the decanter removes clear
supernatant from the surface of the SBR.
The decanting mechanism is sized to provide a continuous, constant flow
during the decant period.
4.2.1.1
The SBR is conservatively designed to
allow for operation with one unit out of operation. The process is designed to operate at loadings consistent with
extended aeration. Typically extended
aeration plants will provide very low BOD (less than 10mg/L).
4.2.1.2
The SBRs are designed to meet the
specified discharge limits, and also to produce minimal quantities of well
stabilized sludge. The key operational
parameter is the solids retention time (SRT).
The design is based on a high SRT which minimizes sludge
production. Under normal operations,
three SBRs will be in service and will operate on a 30 day SRT. The projected
waste sludge production will be about 184 kg TSS/Day.
4.2.1.3
If one SBR is taken out of service,
the discharge limitations can still be met with the two remaining units. The smaller volume available for treatment
will require reducing the SRT to 20 days.
This will result in an increased sludge production of about 195
kg/TSS/Day.
5.1.1.1
The hydraulic level of the effluent
from the SBR units will be such that the flow must be pumped to the
outfall. Consequently a pumping station
will be provided for the final effluent with a sump volume that will also
provide balancing capacity for decanted flows from the SBR units.
6.1.1.1
The effluent of the SKW STW will be
disinfected using UV radiation. Two
disinfection units (one duty and one stand-by) will be provided. The two units will be installed in series
along the effluent pump station rising main.
The UV system is of the closed reactor type with multi-wave high intensity
UV lamps with a broad spectrum of wavelength.
6.1.1.2
In the UV disinfection process, the
radiation penetrates the cell wall of the bacteria/microorganism and is
absorbed by the cellular materials, hence damage is not only on the DNA but
also on the RNA, proteins, enzymes, and other bio-molecules of the cells. As such and because of the high photon
density generated by the lamps, a greater number of molecules can be affected
simultaneously over a very short exposure time. Reactivation is therefore impossible as the deactivation is total
and permanent.
6.1.1.3
The UV disinfection system will be
tied into the operation of the effluent pumps and will have a capacity equal to
the firm pumping capacity of the effluent pump station. A UV intensity monitoring transmitter will
be used to monitor the disinfection process.
At low disinfection levels, an alarm will be initiated and the system
will automatically change over to the standby disinfection unit.
6.1.1.4
The exact type of UV disinfection
system should be determined at the detailed design stage.
7.1.1.1
Waste activated sludge be stored and
thickened on site prior to de-watering.
Surplus sludge thickening would be achieved by using two aerobic holding
tanks. The sludge storage tanks will be
fitted with coarse bubble diffusers that will keep the solids completely mixed
and with sufficient oxygen so that the sludge is aerobic at all times. Periodically, the aeration system will be
shut down for a short period of time to allow the solids to settle and
thicken. After settling, a decant
devise similar to that used in the SBRs will skim the water from the surface
and discharge it by gravity to a recycle pump station. The recycle pump station will pump the
liquid back to the SBRs.
7.1.1.2
The sludge will be dewatered
weekly. The Preliminary design is based
on providing two centrifuges.
Initially, when the sludge production is low, a single unit may be
sufficient for dewatering all the sludge generated. As sludge production is increased, the second centrifuge would be
required or the operating time of the duty unit could be extended.
=
7.1.1.3
Sludge at about 0.5 to 1.5 % solids
concentration would be withdrawn from the sludge storage tanks by two
progressing cavity feed pumps. The feed
pumps would deliver the sludge to the centrifuges. Polymer would be added in-line to condition the sludge before
dewatering.
7.1.1.4
There are a number of polymers that
could be used for sludge dewatering.
They may be delivered in either liquid or powder form. At small installations such as SKW STW,
liquid polymers are preferable as they simplify the polymer preparation. Nevertheless, a polymer preparation system
that can handle dry or liquid polymers should be provided so that the
operations staff can have flexibility in selecting polymers. The typical polymer preparation system mixes
polymer with dilution water to achieve a concentration suitable for the dosing
pumps. Three dosing pumps will be
provided (two duty and one stand-by).
Each of the duty dosing pumps will be isolated with a centrifuge and
sludge feed pump. The dosing pumps will
pump polymer to the sludge feed line.
The polymer will be further diluted in-line before reaching the sludge
line to provide more efficient usage of polymer. The approximate dosage of polymer will be about 10 kilograms per
tonne of sludge solids. The amount of
polymer consumed would be about 10 to 20 kg per week.
7.1.1.5
The sludge dewatering operation will
generate wastewater (centrate) that must be returned to the SBRs for further
treatment. Centrate will discharge from
the centrifuges by gravity and flow to the recycle pump station where it will
be pumped to the SBRs.
7.1.1.6
The exact type of the sludge
dewatering process should be reviewed at the detailed design stage.
7.1.1.7
The sludge cake from the sludge
dewatering operation would have a solids concentration of at least 30%. The weight of the wet sludge would increase
from an average of about 2.7 tonnes per week to a design value of about 4.3
tonnes per week. The corresponding
volume of sludge cake that must be disposed at design is about 6 cubic meters
per week.
7.1.1.8
De-watered
sludge cake would drop into wheeled skips with lids, located directly below the
centrifuges, for subsequent transport to the Sok Kwu Wan Transfer Facility.
Currently it is proposed that the skips could be towed by an electric powered
forklift truck which could also be used to transport conditioning chemicals
etc. and would not require fuel supplies to be kept locally.
8.1.1.1
The EIA
appraisal of odour impacts has confirmed that deodourisation is necessary on
the ventilation flows from the inlet works.
8.1.1.2
The
channels, screens, grit removal chamber and classifier, SBR Feed Pump Station
and Recycle Pump Station would be enclosed within the Inlet Works.
8.1.1.3
It is
proposed that the combined ventilation flows be passed through a deodourisation
facility with about 99.5 % removal efficiency.
The exact type of deodourisers should be determined at the detailed
design stage.
8.1.1.4
See Section 3 for details of air
quality assessment results.
9.1.1.1
The main
chemical to be used at the STW is polymer to aid in sludge dewatering. The
quality and forms for delivery vary with suppliers.
9.1.1.2
Polymers
are available both in dry and liquid forms.
For small installations, it is preferable to use liquid polymer to avoid
the mixing problems associated with dry polymers. The liquid is delivered and
stored in small drums.
9.1.1.3
Given the relatively isolated area,
deliveries should be scheduled monthly.
10.1.1.1
The incoming sewage flow rate and its
pH would be monitored at the inlet to the SBRs. Within the SBR, tank levels and dissolved oxygen levels would be
monitored and used to automatically control the SBR process. The dissolved oxygen probes would be mounted
such that the changes in level in the SBR tank did not affect the measurement.
10.1.1.2
The final effluent flow rate would be
monitored prior to the Effluent Pump Station.
10.1.1.3
Level, flow and process parameters
appropriate to the waste activated sludge withdrawal, chemical preparation and
dosing, and subsequent centrifuge de-watering processes
would be measured.
10.2.1.1
The control systems would be designed
to provide a reliable method for the safe and efficient operation of the
plant. Use would be made of both
electronic and hard-wired control systems; in addition back-up systems would be
provided where considered necessary.
10.2.1.2
A number of the process and plant
items would be provided with integral automatic control system by the plant
suppliers. Such items would include the
SBR process; the sludge dewatering plant; the hydropneumatic set; and the
standby generator. Other plant would be
automatically controlled from a control section of the relevant switchboard on
time, level, flow or other process parameters as appropriate. Chemical dosing would be controlled automatically
on a flow proportional basis, with the dosing rate being adjusted manually.
10.2.1.3
The operational status and process
parameters would be monitored and displayed on a p.c. based visual display unit
and held in the p.c. memory for analysis and evaluation. All plant status and fault alarm digital
signals, and instrumentation analogue signals, would be gathered by an on-site
SCADA system. It would be possible to
exercise supervisory control of plant over the SCADA
system from the p.c. console.
10.2.1.4
In addition, an auto-sampler and
on-line monitoring device should be provided.
10.2.1.5
CCTV and burglary alarm should also be
provided within the STW.
10.3.1.1
DSD has advised that the SKW STW would
not be continually manned, and a regional
plant would be the operations and maintenance base.,
and that Aberdeen STW would be the operations and maintenance base for Yung
Shue Wan STW. According
to DSD, the master stations for monitoring the operational status and process
parameters of the plant at SKWSTW and associated pumping stations should be
installed at Cheung Chau STW. In
addition, the
design should be such that it should be able to monitor the operational status
and process parameters of the plant at SKW STW and associated pumping stations
from DSD’s controlling station at Stonecutters Island STW via the existing LAN
and server at Stonecutters Island STW.
10.3.1.2
To enable the operational status and
process parameters at the SKW STW to be monitored at the regional plant, a
dedicated private wire telemetry link would be provided. All plant fault and alarm digital signals
generate, together with all process critical analogue signals from
instrumentation, would be transmitted to the regional plant where they would be
displayed on a p.c. based visual display unit and held in the p.c. memory for
analysis and evaluation.
10.3.1.3
In addition to the private wire
telemetry link, there would be a public telephone (PSTN) line for voice
communication, with handsets being provided as necessary.