The PCS monitors the filtration system parameters, equipment and instrumentation status. If an alarm condition is activated, it is indicated at the PCS and can be cleared at the PCS or the RTU.
Filtration Mode
The Filtration Mode sequence can be accomplished in manual (HAND) or automatic (AUTO) mode.
Local Control:
Manual or HAND operation requires manual initiation of each step of the filtration sequence. Each of the gates/valves has an OPEN / CLOSE / AUTO switch on the PCS. The Inlet Weir Gate and FE FCV are utilized in Filtration Mode. When the plant is OFF, and a filter is selected for ENABLED, the Inlet Weir Gate shall remain open. With no raw water being pumped to the plant, the water in the filter inlet channel will not flow into the filters. When a filter is selected for OUT OF SERVICE, the Inlet Weir Gate shall close (raise to up position), whether the plant is ON or OFF, preventing water from flowing into the filter. HAND operation of Filtration Mode is not routinely anticipated and will only be used during an emergency.
- To operate a filter in HAND mode, the operator shall first switch the filter to ENABLED, opening the Inlet Weir Gate (lowering to down position). The gate can be placed in AUTO or HAND.
- When the OPEN / CLOSE / AUTO switch is in the OPEN position on the FE FCV, the valve shall open slowly.
- When the FE FCV is placed OPEN by HAND, the FCV shall still be automatically controlled by the PCS using Filter Water Level, so as not to dewater or overflow the filter.
PCS Control:
Automatic operation is initiated once a filter is ENABLED and the PCS initiates a PLANT RUN sequence. During normal operation, the individual filters are controlled by the PCS. Each filter is either ENABLED or OUT OF SERVICE.
- Upon PLANT RUN, the ENABLED filters already have their Inlet Weir Gates open, allowing TW into the filters.
- The FE FCV shall also slowly open to position of approximately 5-10% and the PCS modulates the FE FCV to maintain a constant water level using the Filter Water Level instrument.
- The valve position (% open) is continuously monitored by the PCS.
- The PCS will continually modulate the FE FCV to maintain a constant water level (filter operating water level set point), and the IFE Turbidimeter will continually measure turbidity.
- The CFE Turbidimeter will also continually measure turbidity of the combined filter effluent.
- The Filter Media Headloss is continually measured by the PCS, showing headloss buildup across the filter media.
Backwash Mode
Backwash Sequence
The following sequence occurs on the initiation of backwash in either semiautomatic or automatic modes. The same steps should be taken in manual mode to complete a filter backwash.
- Close the filter inlet weir gate. Hold FE FCV and FTW FCV (assumes FTW is closed) in current position. Filter begins to drain.
- When filter level recedes to the top of the backwash trough, open the BWW FCV and close the FE FCV.
- When FE FCV valve is proven 100 percent closed, slowly open FTW FCV to begin draining the filter lower. Maintain the BWW FCV in its current open position.
- When filter level decreases to 6-inches above the media, close the FTW FCV.
- When the FTW FCV is proven 100 percent closed, start the backwash sequence 1 timer (delay prior to air wash). During this time, open the ALP Vent FCV.
- When the backwash sequence 1 timer times out, and the ALP Vent FCV is proven 100 percent open, start the air wash blower, open the ALP FCV and start the backwash sequence 2 timer (ALP Vent time).
- When the backwash sequence 2 timer times out, close the ALP Vent FCV, and keep the ALP FCV open. When the ALP Vent FCV is proven 100 percent closed, start the backwash sequence 3 timer (air wash duration).
- When the backwash sequence 3 timer times out, open the BWS FCV for the filter. The air wash blower is still operating at this point and ALP FCV is still open.
- When the BWS FCV is proven 100 percent open, start the backwash pump and the backwash sequence 4 timer (backwash pump delay).
- When the backwash sequence 4 timer times out, vary the pump speed to obtain flow equal to backwash flow set point 1 and start the backwash sequence 5 timer, initiating the combined air/water backwash.
- When the backwash sequence 5 timer times out, stop the backwash blower, close the ALP FCV, increase speed of the backwash pump to obtain flow equal to backwash flow set point 2, and start the backwash sequence 6 timer (water only backwash duration).
- When the backwash sequence 6 timer times out, close the BWS FCV for the filter.
- When the BWS FCV is proven almost 100 percent closed, stop the backwash pump. When the backwash pump is stopped, close the BWW FCV.
- When the BWW FCV is proven 100 percent closed, start the rest sequence timer (backwash sequence 7 timer).
- When the rest sequence timer times out, open the filter inlet weir gate and the water level in the filter will increase. If backwashing occurs when plant is not operating, PCS shall “hold” the filter in this backwash step until plant is running, then proceed with the next step.
- When filter water level increases to operating level set point, slowly open the FTW FCV and start the FTW timer (backwash sequence 8 timer).
- When the FTW timer times out, and if filtered water turbidity is below the high-level set point (0.25 NTU initially), close the FTW FCV and slowly open the FE FCV and proceed with normal filter operation. If filtered water turbidity is above the high level when the FTW timer times out, close all filter control valves for that filter and signal a backwash failure.
The table below shows the initial values for the backwash cycle time setpoints.
| Backwash Sequence Timer Step | Description | Initial Set Points |
| 1 | Delay prior to air wash | 30 sec |
| 2 | Air wash vent open | 15 sec |
| 3 | Air wash duration | 1 min |
| 4 | Delay for Backwash Pump to ramp up to speed | 30 sec |
| 5 | Combined air/water backwash duration | 1 min |
| 6 | Water only backwash duration | 15 min |
| 7 | Rest duration after backwash | 1 min |
| 8 | FTW duration1 | 30 min |
| 1. Operator adjustable. Duration is dependent on water flow rate and number of filters online. |
FTW time is highly dependent on the water flow rate coming into the filter influent channel and the number of filters online. The FTW cycle allows backwash water to drain and replaces that volume with chemically treated water from the CACs. During this process, FTW continues until particles begin to accumulate on the media surfaces to “ripen” the filter, and the effluent turbidity reaches an operator entered setpoint and the filter returns to normal operation. Each filter contains approximately 17,000 gallons of water at the design 13-foot operating level. This volume of “clean” backwash water must be flushed out during the FTW process. The higher the flow rate per filter (e.g. 1,200 gpm total, 2 filters operating, 1 in FTW, equals 400 gpm per filter), the quicker the turbidity will decrease to the set point value during FTW.
If at any time during the backwash sequence a control device such as a valve or backwash pump fails to respond to the command of the backwash sequence within a preset time, a FAIL alarm will be indicated for that filter. Manual reset will restart the backwash sequence at the step that failed. If corrective action cannot be taken to restart the backwash sequence for the filter being backwashed, a manual reset will clear the backwash sequence.
Each automatically controlled backwash pump/blower is monitored for failure. Failure detection is developed by monitoring the pumps/blower that are in automatic and the automatic start signals issued by the PCS. If the control system does not receive an ON status input within a preset time after a pump/blower is commanded to start, the respective failure alarm is initiated. If a lead pump fails or is not enabled for PCS control, start the lag pump if it is in automatic. If it is not in automatic, the system failure alarm will be initiated.
Monitor the position of the respective filter gates/valves and alarm after an appropriate time delay if a gate/valve is not in the correct position for the current automatic mode.
The Backwash Mode sequence can be accomplished in manual (HAND), semiautomatic, or automatic (AUTO) mode.
Local Control:
Manual or HAND operation requires manual initiation of each step of the backwash sequence. The Inlet Weir Gate, BWS and BWW FCV’s are utilized in Backwash Mode. Each of the valves has an OPEN / CLOSE / AUTO switch on the PCS. When a filter is placed OUT OF SERVICE manually, the Inlet Weir Gate will close (raise into up position). The BWW valve can be manually placed in OPEN, which will drain the filter to the level of the BWW trough. The FTW valve can be manually placed in OPEN to drain the filter water level to just above the media level. The invert of the FTW piping and air release valve are set to avoid accidental dewatering (or siphoning) of the filter in either HAND or AUTO mode. HAND operation of Backwash Mode is not routinely anticipated and will only be used during an emergency.
- To initiate an Air Wash, the Backwash Air Blower can be manually started and the Filter ALP FCV can also be manually placed in OPEN to route air to the filter.
- To initiate a water backwash, a Backwash Pump can be manually started and the Filter BWS FCV can also be manually placed in OPEN to route BWS water to the filter.
- The Backwash Pump speed can be adjusted on the MCC for the proper flow rate.
- A combined air/water backwash can be initiated with this sequence as desired.
PCS Control:
Automatic Mode
During normal operation, backwash is initiated automatically by one of four conditions previously discussed, and the Backwash sequence is accomplished automatically. All gates/valves for an ENABLED filter must be in AUTO. The detailed Backwash sequence was described previously in this section.
Semiautomatic Mode
Semiautomatic mode begins with the manual initiation of a backwash cycle on a particular filter, but the backwash sequence is accomplished automatically.
- A START Backwash switch on the PCS is used for semiautomatic initiation of a Backwash sequence in each filter.
- A HOLD Backwash switch on the PCS is used for manual delaying of a backwash cycle in each filter.
- A RESUME Backwash switch on the PCS is used for resuming a backwash cycle in each filter after a delay.
- When a backwash cycle fails, a manual Backwash RESET switch on the PCS is used to restart the cycle at the step that failed.
- If backwash again fails, a manual STOP Backwash switch on the PCS is used to cancel the backwash cycle for that filter.
Filter-to-Waste
The FTW Mode sequence can be accomplished in manual (HAND) or automatic (AUTO) mode.
Local Control:
Manual or HAND operation requires manual initiation of each step of the FTW sequence, and may be used if operators feel a filter is not performing. Instead of FTW, operators would likely manually initiate a backwash. Each of the gates/valves has an OPEN / CLOSE / AUTO switch on the PCS. The Inlet Weir Gate and FTW FCV are utilized in FTW Mode. When the OPEN / CLOSE / AUTO switch is in the OPEN position, both of these shall open. When the FTW FCV is placed OPEN by HAND, the FCV will still be automatically controlled by the PCS using Filter Water Level, so as not to dewater or overflow the filter. HAND operation of FTW Mode is not routinely anticipated and will only be used during an emergency.
PCS Control:
During automatic Filtration Mode, the PCS will continually modulate the FE FCV to maintain a constant water level, and the IFE Turbidimeter will continually measure turbidity. On an IFE HIGH ALARM for turbidity, initiate 0-5 minute time delay cycle continuing to keep the FE FCV operating. If turbidity is still high after the time delay, open the FTW FCV and close the FE FCV and initiate 0-5 minute time delay cycle for FTW. On IFE HIGH-HIGH ALARM turbidity level, open FTW FCV and close FE FCV and initiate 0-5 minute time delay cycle for FTW. Alarm HIGH and HIGH-HIGH turbidity levels at PCS. If turbidity drops below HIGH ALARM level, close FTW FCV and open FE FCV. Alarm resets are manual. If FTW timer times out, initiate backwash. Allow only one filter to backwash at any one time. A maximum % open setpoint is entered to control rate of flow through the filter.
PCS / HMI Configuration:
- Status Display:
- Filter ENABLED / OUT OF SERVICE switch indication (each).
- Filter Water Level indication (each).
- Inlet Weir Gate switch position OPEN / CLOSED / AUTO (each).
- Filter Media Headloss indication (each).
- FE FCV position % OPEN / CLOSED / AUTO.
- BWS FCV switch position OPEN / CLOSED / AUTO (each).
- BWW FCV switch position OPEN / CLOSED / AUTO (each).
- FTW FCV position % OPEN / CLOSED / AUTO (each).
- ALP FCV switch position OPEN / CLOSED / AUTO (each).
- TW Turbidity indication.
- IFE Turbidity indication (each).
- Filter Run Time (each).
- Backwash Step indication (each).
- CFE Turbidity indication.
- CFE Flow indication.
- Air Blower local panel switch position HAND / OFF / AUTO.
- ALP Vent FCV switch position OPEN / CLOSED / AUTO.
- BWS Pump local panel switch position HAND / OFF / AUTO (each).
- BWS Pump VFD speed indication (each).
- BWS Flow indication.
- BWS Pressure indication.
- Operator Entries:
- TW Turbidity high value set point.
- TW Turbidity high-high value set point.
- Filter ENABLED / OUT OF SERVICE switch (each).
- Filter Water Level high set point (each).
- Filter Operating Water Level set point (each).
- Filter Water Level low set point (each).
- Inlet Weir Gate switch OPEN / CLOSE / AUTO (each).
- Filter Media Headloss differential pressure high set point (each).Filter Level Alarm setpoints HIGH-HIGH, HIGH, LOW, LOW-LOW (each).
- FE FCV switch OPEN / CLOSED / AUTO (each).
- BWS FCV switch OPEN / CLOSED / AUTO (each).
- BWW FCV switch OPEN / CLOSED / AUTO (each).
- FTW FCV switch OPEN / CLOSED / AUTO (each).
- ALP FCV switch OPEN / CLOSED / AUTO (each).
- FTW Time set point (each).
- IFE Turbidity high value set point (each).
- IFE Turbidity high-high value set point (each).
- Filter Run Time max value.
- Initiate Backwash (each).
- Pause Backwash.
- Backwash return to run.
- Exit Backwash.
- Filter level to begin backwash cycle.
- Filter level to close FTW FCV.
- Air Wash Time Delay set point.
- ALP Vent Time set point.
- Air Wash Time set point.
- Backwash Pump Delay Initiate Timer set point.
- Backwash flow setpoint (Air/Water).
- Combined Air/Water Backwash Time set point.
- Backwash flow setpoint (Water only).
- Water Backwash Time set point.
- Filter Rest Time set point.
- FTW Time set point.
- FTW Turbidity set point (end BW sequence) (each).CFE Turbidity high value set point.
- CFE Turbidity high-high value set point.
- ALP Vent FCV switch OPEN / CLOSED / AUTO.
- Alarms:
- Filter Water Level HIGH-HIGH ALARM (each).
- Filter Water Level HIGH ALARM (each).
- Filter Water Level LOW ALARM (each).
- Filter Water Level LOW-LOW ALARM (each).
- Inlet Weir Gate FAIL (each).
- IFE Turbidity HIGH (each).
- IFE Turbidity HIGH-HIGH (each).
- FE FCV FAIL (each).
- BWS FCV FAIL (each).
- BWW FCV FAIL (each).
- FTW FCV FAIL (each).
- ALP FCV FAIL (each).
- CFE Turbidity HIGH ALARM.
- CFE Turbidity HIGH-HIGH ALARM.
- CFE Flow setpoint HIGH-HIGH ALARM.
- CFE Flow setpoint HIGH ALARM.
- CFE Flow setpoint LOW ALARM.
- CFE Flow setpoint LOW-LOW ALARM.
- Air Blower FAIL.
- BWS Pump FAIL (each).
- BWS Pump HIGH PRESSURE ALARM (each).
- Historical Trending:
- Filter Water Level (each).
- Filter Media Headloss (each).
- Filter IFE Turbidity (each).
- FE FCV position (each).
- FTW FCV position (each).
- Filter Run Time (each).
- Unit Filter Run Volume (each).
- Filter Rate (each).
- Chlorine Residual indication.
- CFE Turbidity.
- CFE Flow.
- BWS Flow.
- BWS Pressure.