Section 20 - Grit Removal

20.01 General Overview

The purpose of providing grit removal is to protect downstream mechanical devices from accelerated wear due to abrasion. In addition, hydrogen sulfide will be stripped from the waste water which is ventilated to the air treatment system.

The aerated grit chambers are designed to be constructed in three phases. During Phase I, two grit chambers with a central grit pumping gallery were constructed. In Phase II, two additional chambers were constructed however, only one of the two chambers was equipped for operation. In Phase III, the final chamber will receive the pumps and other equipment necessary for operation. Each grit chamber is 45 feet long, 10 feet wide and 10 feet deep, and has three grit bays to collect the settled grit. The flow is introduced into the chamber at right angles to the chamber length to impart a spiral flow pattern. The grit chambers have coarse bubble diffusers located above the grit bays which will induce a vertical roll into the waste water perpendicular to the general flow pattern. Positive displacement blowers supply air to the diffusers. The velocity of the roll governs the particle size of a specific gravity that will be removed. By adjusting the airflow supplied by the blower(s), it will be possible to cause the grit to settle and keep the organic material in suspension. During periods of low influent flow, hydrogen sulfide will be stripped from the wastewater by aeration.

The settled grit collected in the grit bays is removed from the chamber with grit pumps, one for each bay. The grit pumps for each chamber pump the grit slurry to two cyclone degritters. Through centrifugal force, the cyclones separate the heavier particles of grit and solids from the bulk of the liquid. The underflow from the degritters is then discharged to a grit classifier, which washes and separates any remaining organic material contained in the grit. The liquid from the degritters and classifiers is returned to the wastewater upstream of the grit chambers.

Specific design features of the grit chambers include:

• Detectors to monitor for dangerous concentrations of explosive gases.
• Ventilation of odorous air to the air treatment system.
• Flow splitting achieved with manually adjustable vertical slots at the entrance to each grit chamber.

The design criteria for the Grit Removal System Phase II are presented below.

20.03 Computer Assisted Control Strategies

20.03.a. General Overview

The Marine Park Facility has available three grit chambers, with a total of four chambers planned after Phase III is completed. The operator will select one, two or three chambers to be on line. The grit removal system consists of:

• Grit Chambers (3)
• Grit Blowers (2)
• Grit Pumps (9)
• Cyclone Degritters(6)
• Grit Classifiers (3)

Raw wastewater enters one or more grit chambers that provide up to 2.5 minutes of detention time. As grit settles out it is withdrawn from the grit chambers by the nine grit pumps. If desired, air is injected into the "grit sump " to prevent plugging of the pump. Grit is pumped to one of the six cyclone degritters, where heavier material is removed. The degritted water is returned to the headworks. The grit is then classified by one or more grit classifier(s) prior to discharge into a rolloff dumpster for final disposal.

20.03.b. Control Strategies

Operation:

1. A gas sensor in each grit chamber continuously monitors for the presence of combustible gases. When the level of combustible gases reaches the alarm level as set on the alarm module located on LCP-01, the alarm beacon and alarm horn are activated. Pressing the horn silence button silences the alarm horn. The alarm condition is also monitored by the PLC/SCS system. The alarm condition is reset at the alarm module. When the level of combustible gas reaches the warning level as set on the alarm module, an input to the PLC is activated to notify the operator of the conditions through the SCS. Alarms also trigger the autodialer system as described elsewhere.

20.03.b.2. Grit Blowers

Operation:

1. One of two grit blowers is placed in REMOTE position (after first checking that discharge valves are open). Whichever blower is placed in REMOTE position first becomes the lead blower, with the second blower functioning as the standby blower should the lead blower fail. A Lead Select button on the Low Pressure Blower SCADA screen allows the operator to alternate lead blowers as necessary. Selecting a new lead blower initiates a countdown timer, which delays the stopping of the previous lead blower for 30 seconds. The SCS monitors which blower is on line and informs the operator in the event of a failure.

20.03.b.3. Grit Pumps

Operation:

1. When in remote, the pump connected to the first grit bay operates continuously. The other two pumps connected to the second and third bays are controlled by a recycle timer with an adjustable ON/OFF setpoint.

20.03.b.4. Grit Classifiers

Operation:

1. When in REMOTE position, the classifier is started when the grit pump is discharging to it turns on. When the grit pump turns off, the classifier will continue to run for a set time delay.

20.04 Operating Strategies

20.04.a. General Operating Strategy

The first blower placed in REMOTE mode will function as the LEAD and start immediately, the second blower placed in remote functions as a standby and will come on should the first blower fail. The LEAD blower may be changed by operations through the SCS CRT.

When in REMOTE, the pump connected to the first grit bay operates continuously. The other two pumps connected to the second and third grit bays are controlled by an alternator and a re-cycle timer with an adjustable ON/OFF setpoint.

When selected to REMOTE, the conveyor is started after a set time delay following initiation of a bar screen cleaning cycle. It continues to operate after the bar screen stops for a set time delay. The setpoints and progress of both the on delay and off delay timers can be viewed through the SCS CRTs.

When in REMOTE, the classifier is started when the grit pump discharging to it turns on. When the grit pump turns off, the classifier will continue to run for a set time delay.

The air flow rate to each chamber and each diffuser assembly is manually adjustable to provide uniform aeration of the waste water.

A gas sensor in the grit chamber (#3 only) continuously monitors for the presence of combustible gases. When the level of combustible gases reaches the alarm level as set on the alarm module located on LCP-01, the alarm beacon and alarm horn are activated. The alarm horn is silenced by pressing the horn silence button. The alarm condition is also monitored by the PLC/SCS system. The alarm condition is reset at the alarm module. When the level of combustible gas reaches the warning level as set on the alarm module, an input to the PLC is activated to notify the operator of the conditions through the SCS.

20.04.b. Normal Operating Conditions

Note: Normal plant operations will use the PLC and the SCS CRT to select equipment operating modes, and monitor equipment status. Air flow rate, as mentioned above, is manually adjusted at the air diffuser headers.

20.04.c. Alternate Operating Conditions

Blank.

20.04.d. Startup/Shutdown Conditions

• Grit Chambers
• Grit Blowers
• Grit Pumps
• Grit Cyclones
• Grit Classifiers

20.04.e. Key Control Data and 4 Level Alarm Table

Blank.

20.04.f. Contingency Plans

A Contingency Plan is written to handle an unpredictable occurrence that has a reasonable chance of happening. Most Contingency Plans deal with situations where having a written procedure can help prevent loss of a unit process, handle a process upset or respond to a hazardous situation. The following is a list of Contingency Plans developed for the Grit Removal System. The plans are kept in the Contingency Plan Notebook located in the Operator Laboratory and the Shift Supervisor Office.

• Response to hazardous gas level.
• Disruption of normal disposal service of grit storage bins: Long term storage of grit, or no disposal service.

20.04.g. Standard Operating Procedures

Standard Operating Procedures are written to handle operator activities performed on a regular but not necessarily daily basis. Most SOPs deal with potentially hazardous activities making it important that a written, step-by-step procedure be available. The following is a list of SOPs developed for the Grit Removal System. The procedures are kept in the SOP Notebook located in the Operator Laboratory and the Shift Supervisor Office.

• Entering the Grit Channels

20.05. Troubleshooting

20.06. Safety

20.06.a. Confined Spaces

20.06.a.1. Grit Chambers

20.07.a. Sampling Schedule

Not used.