o&m -sludge dewatering plant

15
PANCHING WATER TREATMENT PLANT (DRAFT COPY R1) PROCESS PLANT O&M MANUAL VOL. 1 OF 3 1 | Page OPERATION AND MAINTENANCE MANUAL This document is a set of Operation & Maintenance Instruction Manuals for Panching Water Treatment Works and comprised the following SLUDGE DEWATERING PLANT & EQUIPMENT 1.0 INTRODUCTION The sludge dewatering plant system is designed to handle the sludge generated by the plant. The complete system includes a sludge cake handling system to transfer the dewatered sludge from the mechanical dewatering equipment into bins where it will be transported out from the building storage yard. The plant consists of the following equipments:- - Wash water recovery pumpset - Decanters - Submersible pumpset - Submersible mixers - Sludge scraper - Decanter feed pumps and holding tank feed pump 1.1 OPERATING PRINCIPLES The plant is designed for 24 hours continuous operation in automatic mode as programmed by the PLC at the respective plant. When required, the plant can also be operated under full manual mode by operator intervention.

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Page 1: O&M -Sludge Dewatering Plant

PANCHING WATER TREATMENT PLANT

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OPERATION AND MAINTENANCE MANUAL

This document is a set of Operation & Maintenance Instruction Manuals for

Panching Water Treatment Works and comprised the following

SLUDGE DEWATERING PLANT & EQUIPMENT

1.0 INTRODUCTION

The sludge dewatering plant system is designed to handle the sludge generated by the plant. The complete system

includes a sludge cake handling system to transfer the dewatered sludge from the mechanical dewatering

equipment into bins where it will be transported out from

the building storage yard.

The plant consists of the following equipments:-

- Wash water recovery pumpset

- Decanters - Submersible pumpset

- Submersible mixers - Sludge scraper

- Decanter feed pumps and holding tank feed pump

1.1 OPERATING PRINCIPLES

The plant is designed for 24 hours continuous operation in

automatic mode as programmed by the PLC at the respective plant. When required, the plant can also be

operated under full manual mode by operator intervention.

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1.2 TREATMENT PRINCIPLES

The plant is designed with the following treatment

processes to dewater the sludge prior to transfer out to the landfill:-

1.2.1 -Enhanced flocculation process. Polyelectrolyte is

dosed as a flocculent aid. The dosing points are at the pipeline entering into thickener tanks and

another at the pipeline entering decanters.

The dosing of polymer solution is to aid in sludge thickening and to enhance the dewatering process.

2.0 SLUDGE DEWATERING PLANT DESCRIPTION AND CONTROL PHILOSOPHY

2.1 DESCRIPTION

Settled sludge from the 10 sedimentation tanks is piped to 2 Nos. sludge balancing tanks (1 duty & 1 standby) while

filter backwash waste water is discharged into 2 Nos. wash-water recovery tanks (WWRT) (1 duty & 1 standby). One

WWRT is designed to hold the waste water from 2 filter backwashing. After the WWRT has collected 2 lots of

wastewater, the inlet penstock is closed and the content is settled. The supernatant is pumped back to the inlet of

aerator through a floating suction. The sediment from the WWRT is pumped to the sludge balancing tank. Settling

operation at WWRT is carried out batch-wise.

The sludge balancing tank (SBT) receives sludge transferred

from the WWRT as well as sludge and chemical waste discharged directly from the 10 sedimentation tanks and

chemical building respectively. Sludge particles are kept suspended in SBT by 2 submersible mixers. Sludge from

SBT is fed to the sludge thickener tank (STT) by 2 Nos. submersible thickener feed pumps (TFP). In operation,

either 1 or 2 feed pumps will run, depending on level at the

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SBT. When level is too low, both TFP will stop, but the submersible mixers are kept running.

Sludge thickener tank (STT) concentrates the watery sludge

to at least 4% dry solids. The sludge rake runs continuously as long as there is water or sludge in STT (liquid level above

a pre-set level). Supernatant from the STT overflows and is diverted back to the WWRT. Solids carried over at STT

overflow outlet is manually monitored and checked. The

thickened sludge is transferred to the sludge holding tank (SHT) periodically by means of PC pumps under level

control at the SHT. The PC pump and sludge transfer pipe shall be flushed whenever the PC pump stops and the

flushed water is discharged directly into sludge holding tanks.

The sludge holding tank (SHT) supplies a steady stream of

sludge to the centrifuge decanters. Two PC pumps deliver sludge from a SHT to 3 units of decanters. Normally, only

one decanter runs during the day and afternoon shifts; but if sludge load is heavy, 2 decanters may be running.

The dewatered sludge cake from the decanter is collected in

a dedicated arm roll bin by operating a screw conveyor. The

collected dewatered solids are disposed out from the treatment works by trucks. The centrate from the decanter

is discharged to the surface drainage.

There is a polymer solution preparation system and dosing pumps at this sludge treatment plant. The dosing pumps

supply polymer solution to the STT feed to assist in sludge thickening and to the centrifuge decanter to enhance the

dewatering process.

2.2 WASH WATER RECOVERY TANK (WWRT)

2.2.1 FUNCTION

The WWRT serves as a holding & sedimentation tank for filter wash-water. It enables the supernatant to be returned

to inlet of the aerator and settled sludge to be transferred to SBT.

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2.2.2 OPERATION

For each WWRT, there is an “Auto/Manual” selection switch

on the SCADA screen to control the respective WWRP and STP. The position of the selection switch (on the SCADA

screen) will determine how the associated equipment response. Normally, the “Auto” position shall be selected

unless that WWRT is not available or under manual testing.

When a WWRT is selected to “Manual” at SCADA, start/stop

of the associated WWRP and STP shall be done on a fully manual basis via the SCADA or from the respective

start/stop push-buttons at the MCC. The PLC control programme is not available at the “Manual” mode.

When a WWRT is selected to “AUTO” at SCADA, the

operation of associated WWRP and STP will be automatically controlled base on current PLC control programme. The

selected WWRT will undergo an automatic “Filling”, “Settling” and “Disposal” sequence. The auto sequence will

be described later.

All WWRP and STP shall be provided with field mounted

emergency stop push-button stations.

The operation at WWRT is carried out batch-wise. There are 3 stages in this treatment process: (1) Filling with filter

wash-water, (2) Settling the waste water and (3) Disposal of the settled waste-water.

Under “Auto” mode, only 1 WWRT is selected to “Filling”

state. At the “Filling” state, the WWRT inlet penstock is open to receive filter wash-water. After receiving 2

continuous filter backwashes and the level electrode detects high water level, the inlet penstock of the second WWRT is

opened and the inlet penstock of the first WWRT is closed simultaneously. It will also trigger the "Settling" timer.

[Note: Ultrasonic level sensor shall be used to monitor the

water level of WWRT.)

If there is any fault at the actuators of inlet penstock or penstock status not in line with sequence, corresponding

alarms will be raised. Auto control sequence will still continue even when there is alarm.

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When the first WWRT enters the “Settling” stage and the second WWRT will go to the “Filling” state. Settling is purely

time base and the pre-set time can be adjusted from SCADA. Suggested settling time is 120 to 240 minutes. (to

be refined & checked during testing & commissioning). Most of the sludge solids in the filter wash water will settle under

gravity during the prescribed duration.

After the first WWRT has completed its “Settling”, it moves on to the “Disposal” state. In “Disposal” state, the dedicated

wash-water recovery pump (WWRP) will start and the discharge valve of the WWRP open. Status of WWRP & its

discharge valve are monitored, and alarm is raised when unusual condition arises. The supernatant from the WWRT

is recycled back to the inlet of the aerator. The recycle flow rate is monitored by a magnetic flow-meter & display on the

system. The recycle flow is manually pre-set by throttling a valve.

In case the recycle wash-water flow exceeds a pre-set high

flow value (SCADA adjustable) for more than 5 sec, an alarm “Recycle Flow High” will be raised. Pumping shall still

continue even in Hi Flow condition.

Auto-stopping of WWRP is by level electrode. When the low

limit contact is activated, the WWRP stops and the discharge valve is close.

If WWRP fails before the water reaching low level, the “No

WWRP Available” alarm will be raised and the control sequence will be suspended. Operator interference is

required to rectify and normalise the situation. [One unit of WWRP is dedicated to each WWRT, there is no standby

unit.]

Under normal condition when the WWRP has stopped, the duty submersible sludge transfer pump (STP) will

automatically start run until the low-low limit contact (of

level electrode) is activated. Status of the STP is monitored. The stand-by STP will run if the duty pump were to fail.

After the STP has auto stopped (when WWRT has been

emptied), “Disposal” stage is completed and the WWRT is put to “Stand-by” (inlet penstock continues to remain

close). It will remain in this state until the other WWRT is

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fully filled (water reaches high limit contact of level electrode).

2.2.3 Process Interlocks & Alarms / Failure Conditions:

A. The WWRP is dedicated to one unit of WWRT. The 2

STP from the same WWRT may run together

automatically (by manually switching both STP to 'Available to start').

B. Control sequence will normally continue even when

there is an active alarm (except as in (C) below).

C. Process control sequence is suspended with alarm raised when the WWRP or both the STP fail together.

In this case, manual intervention is required.

D. WWRT is not available when : (a) Inlet penstock or valve is not “remote” selected

at actuator; or (b) The associated WWRP or both STP are not

available, fail/fault.

E. Alarm to be raised when :

(a) Any pump, valve or penstock fails during operation.

(b) Alarm is raised when both duty & standby pump fail.

(c) Alarm is raised when both the WWRT are NOT available.

2.3 SLUDGE BALANCING TANK (SBT)

2.3.1 FUNCTION

Sludge Balancing Tank (SBT) acts as a buffer tank for

storage of sludge from clarifier, chemical waste from chemical building and sludge transferred from WWRT before

it is fed to the thickener tank. This serves to even out the sludge feed rate to the 2 sludge thickening tanks.

Dimension of a SBT = 9.00mW x 10.685mL x 2.80m Depth

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2.3.2 OPERATION

When a SBT is put to “Service” from the SCADA screen, the

associated inlet penstock will open. When the SBT is put to “Standby” or “Out of Service”, its inlet penstock will be

closed. It is possible that while a SBT is “Out of Service” and yet its associated submersible thickener feed pump

(TFP) and submersible mixers (SMX) are still running on

“Manual”

For each SBT, there is an “Auto/Manual” selection switch on the SCADA screen to control the respective TFP and SMX.

The position of the selection switch (on the SCADA screen) will determine how the associated equipment response.

Normally, the “Auto” position is selected unless that SBT is not available or under manual testing.

When a SBT is selected to “Manual” at SCADA, start/stop of

the associated TFP and SMX shall be done on a fully manual basis via the SCADA or from the respective start/stop push-

buttons at the MCC. The PLC control programme is not available at the “Manual” mode.

When a SBT is selected to “AUTO” at SCADA, the operation of associated TFP and SMX will be automatically controlled

base on the current PLC control programme. The PLC control programme is basically a level based program to

start and stop the TFP and SMX. Further, whenever a TFP runs, the duty selected Thickener Polymer Dosing Pump

(TPDP) shall be auto started and the polymer solution dosing valve shall open.

The polymer solution will be dosed to the common TFP

outlet. When two TFP on a same line are running, the control system shall automatically double the polymer

dosage (double the TPDP running speed).

All TFP and SMX shall be provided with field mounted

emergency stop push-button stations.

Operation at a SBT is essentially based on level status within the tank. The primary control element in each SBT is

the level electrodes. Each SBT feeds into a dedicated sludge thickener tank (STT); SBT#1 feeds STT#1 and SBT#2

feeding to STT#2; cross over operation is not possible.

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Sludge solids are prevented from settling on SBT tank floor

by two submersible mixers which are running perpetually as long as that SBT is put to “Auto” and the water level is

above preset. (The said SBT can be put “Out Of Service”)

Level electrode function: L1 : Longest electrode, serves as reference (100mm ftf)

L2 : both SMX stop below this level (350mm ftf) L3 : both SMX start to run & both TFP stop (400mm ftf)

L4 : First TFP auto starts on reaching this (1200mm ftf) L5 : Second TFP auto runs on reaching this (2000mm ftf)

L6 : SBT Overflow alarm (2900mm ftf)

ftf = from the tank floor

Capacity of each TFP = 35 m3/hr @ 10m

Plan area of each SBT = 9,000 x 10,685 mm SBT tank floor level = IL 31.00m

Normal TWL at SBT = TWL 33.80m Estimated TFP run time = 30 minutes

When a TFP runs, the associated motorised butterfly valve

shall open. When the TFP stops, the discharge valve will close.

The TFP can run as long as the dedicated sludge thickener

tank (STT) is put to “Service”. Operation of TFP is not affected by level status or sludge density / concentration at

the STT; it is only dependent on level at the SBT. Either one or two TFP may run, depending on level at the SBT.

The operation of SMX is only dependent on the level at SBT.

Both SMX will run or stop together. The spare unit is an off

line item supplied loose.

Both the TFP and the SMX may be started/stopped “manually” from the SCADA control screen.

If the duty TFP fails before the pre-set level is reached, the

stand-by TFP auto starts & continues to run until the pre-set level is reached. If the stand-by TFP is not available or fails

after running for a while, “No TFP Available” alarm will be raised and control system is suspended. Operator

interference is required to normalise the fault condition.

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2.3.3 Process Interlocks & Alarms / Failure

Conditions:

A. One SBT will serve only one dedicated STT. No cross

over is permissible.

B. One or both TFP may run simultaneously, depending

on level control at the SBT.

C. When a TFP runs, the duty selected thickener polymer dosing pump shall be running and the corresponding

thickener polymer dosing valve shall open.

D. Control action is still active even when there is an active alarm.

E. SBT is not available when :

(a) Inlet penstock is not “remote” selected at actuator; or

(b) Both the TFP or both SMX are not available, fail/fault.

F. Alarm to be raised when : (a) Any pump, valve or penstock fails during

operation. (b) Alarm is raised when one SMX or both duty &

standby pump fail. (c) Alarm is raised when both the SBT are NOT

available. (d) When a TFP has started and none of TPDP is

running, alarm “Thickener Polymer Dosing Fail” shall raised. System is allowed to continue.

2.4 SLUDGE THICKENER TANK (STT)

2.4.1 FUNCTION

The Sludge Thickener Tank (STT) increases the solids

concentration of sludge before it is pumped to the holding tank. The clear supernatant overflows the weir and is

returned to WWRT by gravity. The STT can also act as sludge storage tank in case the sludge holding tank is full or

temporary out of service.

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The STT is dedicated to receive sludge from only one SBT

and the thickened sludge will only be pumped to one of the 2 sludge holding tank (SHT).

Diameter of one Sludge Thickening Tank = 25.00m

diameter x 3.50m depth

Volume of sludge, 1.0m depth in STT = 750 cu meter.

2.4.2 OPERATION

For each STT, there is an “Auto/Manual” selection switch on the SCADA screen to interlock with the TFP. The position of

selector switch (on SCADA screen) will determine how the

TFP response. Normally the “Auto” position is to be selected unless that STT is not available or under manual testing.

When the STT is put to “Auto” from SCADA, the sludge

scrapper or rake will run continuously; and the 2 corresponding TFP will receive a start permissible signal;

i.e. it is free to start & stop depending on level at the corresponding SBT.

When a STT is put to “Manual”, the sludge scrapper will

retain its last status before the switch-over (i.e. scrapper will be running if it was running before or the scrapper will

be stop if it was not running earlier) and the corresponding TFP is NOT start permissive (i.e. the TFP will not auto

start/stop with level changes at the corresponding SBT.).

However, the TFP can still be manually started if so wishes.

There is no “In service” /”Stand-by” /”Out of Service” selection for the STT; the only available selection is “Auto”

or “Manual”.

The sludge feed to STT and discharge from a STT is not controlled by status at the STT itself. When the TFP runs,

supernatant from the STT will overflow and return to the WWRT by gravity. When the SHT feed pump (SHTFP) runs,

the concentrated sludge will be transferred to the sludge holding tank.

The only equipment that is controlled from the STT is the

sludge scrapper drive.

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Sequential time base control or level control is not

applicable to STT system.

2.4.3 Process Interlocks & Alarms / Failure

Conditions:

The “Auto”/”Manual” selection state of a STT is interlocked

to start permissive-ness of the corresponding TFP as described earlier.

Whenever a Sludge scrapper fails or trips, an alarm must be

raised. Sludge scrapper shall be provided with a remote mounted emergency stop button station.

If the torque switch on the sludge scraper gearbox is

activated, Alarm “Sludge Scraper Tripped on Over-Torque” is raised.

2.5 HOLDING TANK FEED PUMPS AND SLUDGE HOLDING TANK

2.5.1 FUNCTION

A Sludge Holding Tank Feed Pump (SHTFP) transfers the concentrated sludge from the bottom of a STT to the sludge

holding tank (SHT). There are 2 sets of dedicated STT, SHTFP and SHT; cross-over operation between these 2 sets

of equipment is not possible.

The SHT stores and provides a steady stream of thicken sludge of fairly uniform solids concentration for feeding to

the centrifuge decanters. The 2 submersible mixers (SSMX) at each SHT prevent the sludge particles from settling.

Dimension of a SHT: 7.50mL x 7.00mW x 2.35mD

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2.5.2 OPERATION

For each SHT, there is an “Auto/Manual” selection switch on the SCADA screen to interlock with the SHTFP and SSMX.

When “Auto” position is selected, the SHTFP can auto start/stop depending on the thickened sludge level in STT

and sludge level at SHT, while the auto start/stop of SSMX depends on the sludge level at SHT. [Note: Thickened

sludge level in STT to be monitored via sludge level probe.]

The operation of SHTFP is independent of the status at the

respective STT; and there is hardwired safety interlock at each SHTFP motor starting circuit to protect the pump. In

case of dry running, a temperature switch embedded in the pump body will send a signal to trip the affected pump.

When pump discharge pressure is too high, the common pressure switch will stop the pump.

When selected to “Manual”, SHTFP and SSMX can only be

started or stopped manually. The hard-wired interlock on SHTFP will still function, but control from level electrodes

will be disabled.

When a SHTFP stops, the sludge delivery line and pump

shall be auto flushed before the pump is turned off. While the pump is still running, the flushing water inlet valve is

open and the flushed water is discharged into SHT. After a time delay of several secs (adjustable) the common

motorised pump suction valve is closed. Then the SHTFP runs for a further pre-set time (120 to 600s, adjustable)

before the SHTFP is stopped. The flushing water inlet valve is closed after the pump has stopped.

In case there is a valve actuator failure during the pump

flushing sub-routine, the SHTFP will stop and the respective valve failure alarm is raised. Operator attention is required

to check & reset.

There are 2 SHTFP in a system, when a SHTFP fails during

an auto running operation, the stand-by unit shall auto start and a “SHTFP Failure” alarm raised. Operator intervention is

required on the failed pump – to be repaired & manually flushed before returning to standby.

The suggested level control set points#1 at sludge holding

tank (SHT) for auto control are given below. These level

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set-points for electrode (measured from tank floor) are still subject to re-confirmation after site testing /

commissioning.

L1 0.050 m Reference level electrode L2 0.300 m Stop SSMX & start both SHTFP

L3 0.400 m Start SSMX L4 0.700 m Start one SHTFP

L5 2.300 m Stop all SHTFP

L6 2.350 m SHT overflow alarm is raised

2.5.3 Process Interlocks & Alarms / Failure

Conditions:

The “Auto”/”Manual” selected state of a SHT will either enable or disable SHT level control of the corresponding

SHTFP and SSMX.

An equipment (pump, mixer & valve) failure alarm shall be raised whenever such an event arises. Except for SHTFP,

the failure of the duty unit shall not cause the standby unit to run.

Before a SHTFP stops on auto, the pump and the sludge delivery piping shall be auto-flushed as described.

2.6 DECANTER FEED PUMPS AND SCREW CONYEVOR

2.6.1 FUNCTION

The decanter feed pump (DFP) transfers the sludge from the sludge holding tank to the centrifuge decanter (CD) for

dewatering. Dewatered sludge as discharged from decanter is transported by a screw conveyor (SC) to a collecting bin.

There are 4 decanter feed pumps and 3 centrifuge decanters in this system. One DFP shall run to supply the

feed to one decanter. Each decanter will have its dedicated screw conveyor to transport the sludge cake to an arm-roll

bin.

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When any CD or DFP runs, the duty selected decanter polymer dosing pump (DPDP) will auto start and the

corresponding polymer inlet valve to CD will open.

Normally only one centrifuge decanter shall be running, the other 2 units act as standby. However when the qualities of

raw water deteriorates and sludge volume increases, two decanters may be operated in parallel (duty cycle).

The sludge cake collected in the arm roll bin shall be

transferred and stored at sludge disposal yard periodically by the arm roll truck.

2.6.2 OPERATION

2.6.2.1 Selecting available equipment to duty

The suction pipeline is arranged in such that sludge holding

tank No.1 (SHT #1) supplies feed to DFP#1 and DFP#2; while SHT#2 serves DFP#3 and DFP#4. Individual

discharge pipeline of all DFP will join to a common 'ring main' and then equally distributes the feed to each CD. The

provision of 'ring main' allows duty-cycle operation of the decanter feed pumps (DFP) and centrifuge decanters (CD).

When only one unit centrifuge decanter need to be run, any

available DFP and CD may be selected for the duty.

Operator has to ensure the suction and delivery manual valves are open and then the duty DFP and CD are selected

to “Auto” at the LCP or SCADA. The motorised suction valve of the duty selected DFP and the motorised inlet valve to

the duty selected CD will auto open. The sludge cake screw conveyor (SC) of the duty selected CD shall be put to

“Auto” as well, to enable remote control by the PLC.

When two decanters are required in operation simultaneously, any 2 available DFP may be selected

manually for the duty while the 2 duty CD units are automatically selected by PLC (based on available running

hours). The setting of running hour is adjustable at SCADA/PLC. The default running hour setting shall be 8

hours per day for each CD. Total running hours for all three

CD is 8 x 3 = 24 hours.

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When 2 sets of DFP and CD are running simultaneously, both DFP supplies feed to a common discharge line (ring

main) and then equally distribute into both CD. The respective SC of both duty CD will automatically start via

PLC remote control.

2.6.2.2 Operation of centrifuge decanter & related equipment.

After the selected DFP, CD and SC are switched to “Auto”

on the LCP and the correct isolation valves are open, the centrifuge decanter may be manually initiated to start from

the decanter local control panel – by pressing a push-button on the decanter LCP control touch-screen or remote

initiation from SCADA. The decanter will start and undergo a normal starting sequence as described later. The

motorised valves, feed pump and screw conveyor will be opened and started automatically at the correct sequence.

Similarly, the decanter will go through an auto stop sequence when the “Stop” switch is activated.

When the decanter is switched to “Manual” position, the standard decanter starting and decanter stopping

sequences will be both deactivated. The associated motorised valves, feed pump and screw conveyor must be

manually started or stopped at the proper sequence.

A low level condition (L2) at SHT (sludge holding tank) shall to trigger the centrifuge decanter connected to this SHT to

undergo a normal stop sequence on Auto.

2.6.2.3 Centrifuge Decanter standard starting sequence

When a decanter is initiated to start, the standard

centrifuge decanter starting sequence is activated. A typical

decanter starting sequence is as follows:

(1) Decanter start initiated from the LCP. (after checking

manual valve position)