APPENDIX 2.2  SEWAGE TREATMENT WORKS

 

1.                              Introduction

 

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.                              Interface with Sewerage Works

 

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.                              Inlet Works (Including SBR Feed Pump Station)

 

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                          Fine Screens

 

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                          Grit Removal

 

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                          Sequencing Batch Reactor Pump Station

 

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.                              Biological Treatment Process

 

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                          Impacts on Effluent Quality

 

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.                              Effluent Pump Station

 

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.                              UV Disinfection

 

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.                              Sludge Treatment

 

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.                              Odour Control

 

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.                              Chemical Selection and Storage On-site

 

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.                          Instrumentation Control Aspects and Telemetry

 

10.1                      Introduction

 

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                      Control and Monitoring

 

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                      Telemetry

 

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.