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Secondary Clarifier & RAS Pumps Controls
Control Strategies PIDs
Secondary Clarifiers and RAS Pump PID
 
Secondary Clarifiers Operation
If the secondary clarifier system is operating properly, the effluent from the clarifiers should have significantly less than 20 mg/L of suspended solids. The ability of the clarifiers to perform at this capacity, however, depends on a number of factors, including: 

   · Number of clarifiers in operation 
   · Clarifier solids loading rate (SLR) 
   · Clarifier surface overflow rate (SOR) 
   · Sludge volume index (SVI) 
   · Sludge blanket level
Targets
The initial targets for each of the secondary clarifiers are as follows:

Surface Overflow Rate

Average Annual Flow 500 gpd/ft2
Wet Weather Maximum Month 550 gpd/ft2
Peak Flow 1,094 gpd/ft2

Solids Loading Rate
Design Maximum Month Flow 22 lb/day/ft2
Control Variables
Number of Clarifiers in Operation
Objectionable odors, as well as rising sludge, may result if both clarifiers are used during low flow periods. This condition occurs when the hydraulic detention time in a clarifier is too long, allowing the sewage to turn septic. It may be necessary to take a clarifier out of service to prevent this occurrence.

Solids loading and overflow rates are the two major factors determining how well the clarifiers operate. Another factor is the detention time of the wastewater flow (minus RAS recycle) in the clarifiers.  A detention time of about 2.5 hours is desirable.  To calculate the HRT, Q + RAS should be used.  
Clarifier Solids Loading Rate
If the solids loading to the clarifiers become too high, the settling process is hindered because the high concentration of solids interferes with the settling rate of the individual particles. The solids loading is defined as the pounds of mixed liquor suspended solids applied per square foot of surface area per day. Generally, a solids loading of less than 25 lb/sq. ft/day is desirable.
Clarifier Overflow Rate
The clarifier overflow rate is expressed as gal/sq. ft/day and is calculated based on wastewater flows rate (excluding RAS flow). The secondary clarifier's overflow rate has an important effect on the efficiency of solid-liquid separation achieved in the clarifiers. Excessively high rates will carry over solids into the final effluent. What constitutes an "excessive" rate depends on the sludge volume index (SVI) and mixed liquor suspended solids (MLSS). Refer to the targets listed in this section for typical values.
Sludge Volume Index
The SVI indicates how well the mixed liquor can be expected to settle and compact in the secondary clarifiers. The SVI is calculated from the values obtained in the 30-minute settleability test performed on the aeration basin MLSS.
By definition, SVI is the volume in milliliters occupied by 1 gram of activated sludge after settling for 30 minutes. A high SVI indicates poor settleability, so an increase will decrease the ability of the clarifier to remove solids from the aeration basin effluent. Generally speaking, SVI levels at or below 150 are desirable.

If the SVI is below 150, the mixed liquor settles well and the plant is easier to operate. If the SVI is above 150, the RAS concentration will become lower, and a higher volume of sludge must be returned to maintain process control. At high SVIs (above 200) the sludge may not settle in the clarifiers fast enough to be returned to aeration, and some sludge may overflow the effluent weirs. However, if the SVI is less than 100, the mixed liquor may settle too fast and not have enough structure to trap smaller particles, resulting in "pin-floc" formation.

The measured SVI may vary during the day because of changes in the wastewater and sampling errors. A single SVI value of more than 200 does not necessarily indicate process problems unless it continues for most of the day.

A high SVI is caused by one of the two conditions:
   · Bulking sludge – sludge will not settle
   · Rising sludge – sludge settles, but later rises again

The SVI should be determined at least once a day, more often if high values are recorded.
Sludge Blanket Level
In a well-operating clarifier there will be a definite break between the clarified liquid and the settled sludge. The break indicates the top of the settled sludge, or depth of the sludge blanket (DOB). The DOB should be no greater than two feet from the bottom of the clarifier. If the depth of the blanket is greater than this, the turbulence created as the liquid overflows the weirs may sweep some of the sludge over the weirs and out with the effluent.

The exact sludge blanket level can be determined by using a sludge blanket finder. The sludge blanket level should be determined two to three times per day unless a history of consistent levels and reliable operation has been established. The sludge blanket should be measured at the same place each time at a distance of approximately 1/3rd radius away from the center.

Sludge blanket level is controlled by the RAS and WAS rates. Increasing or decreasing the RAS rate will quickly change the sludge blanket level. Increasing or decreasing the WAS rate will slowly change the blanket level. Drastic changes in RAS and WAS rates should be avoided because there is an optimum rate for both parameters. If a drastic change is made, more harm than good could result by overshooting the optimum rate.

These are general guidelines that should be followed initially. There are cases, however, when an increase in the RAS rate will cause the sludge blanket level to rise. If this happens, the RAS rate should be decreased along with an increased WAS rate to lower the sludge blanket level.

The sludge blanket level is very sensitive to changes in process dynamics and changes in flow. It is also very easy to measure and can, therefore, be used as a good indicator of whether the plant is operating correctly.
 
Non-controllable Variables
Influent Flow
The plant influent flow is dictated by the influent rate into the Preliminary Treatment Building from the raw sewage force mains. This flow is weather dependent and will affect the solids loading rate, surface overflow rate, and SVI.
 
Calculations
Solids Loading Rate
Surface Overflow Rate
Sludge Volume Index
Sodium Hypochlorite Usage
 
Scum Pumps
Secondary Clarifier Scum Washdown Valves Control Modes
MODE DESCRIPTION
Remote - Auto The Secondary Clarifier Scum Washdown Valve is automatically opened for 15 seconds every time the secondary clarifier rake arm approaches the secondary clarifier scum beach to flush scum to the scum pit..
Remote - Manual The Secondary Clarifier X Scum Washdown Valve is manually operated through the PCS.
 
Secondary Clarifier Scum Pumps Control Modes
MODE DESCRIPTION
Remote - Manual The Secondary Clarifier Scum Pump X START-STOP control is manually operated through the PCS.
Local- Manual The Secondary Clarifier Scum Pump X is manually operated through the MCC-2 in the Blower Building.
 
 
RAS Pumping Operation 
Targets
Typical RAS flow ranges are from 25% to 75% of plant influent flow.
 
RAS Pumping Control Modes
MODEDESCRIPTION
Influent Flow / RAS Flow Ratio Control ModeControl of the RAS flow to the Aeration Basins from the respective Secondary Clarifiers is automatically adjusted to maintain an operator-entered ratio between the plant influent flow and the RAS flow.
RAS withdrawal ratio between the Secondary Clarifiers is also adjusted to maintain the operator-entered ratio set point.
Remote Auto Control ModeThe RAS is automatically adjusted to maintain an operator-entered RAS flow set point (mgd).
Remote Manual ModeThe RAS Pump(s) speed is manually adjusted through the PCS.
Local Manual ModeThe RAS Pump(s) speed is manually adjusted on the pump drives.
 
Control Variables
Return Activated Sludge Flow Rate
MLSS that settles in the secondary clarifiers must be pumped back to the aeration basins, with a portion being wasted. The approximate rate at which sludge should be re-circulated depends upon the RAS concentration, the sludge blanket level, and the desired clarifier solids loading rate.

There are two basic approaches that can be used to control the RAS flow rate. These approaches are based on the following:

   · Controlling the RAS flow rate independently from the influent flow, i.e., as an operator input constant. 
   · Controlling the RAS flow rate as a constant percentage of the influent flow.

The advantages of the constant RAS flow approach are the following: 

   · Simplicity 
   · Maximum solids loading on the clarifier occurs at the initial start of peak flow periods. 
   · Requires less operational time.

The advantages of the constant percentage RAS flow are the following:

   · Variations in the MLSS concentration are reduced and the F/M varies less. 
   · The MLSS will remain in the clarifier for shorter periods, which may reduce the possibility of denitrification in the
     clarifier.

The Wenatchee WWTP is designed so the operator has the flexibility to operate the RAS flow rate in either of the two modes.
Non-controllable Variables
Non-controllable variables include the plant influent flows, BOD and suspended solids loads.
Calculations
Sodium Hypochlorite Usage
If the plant experiences sludge bulking problems related to growth of filamentous organisms in the mixed liquor, sodium hypochlorite can be added to the RAS to reduce filamentous organisms. Caution should be used in adding hypochlorite to RAS, to avoid over-dosing and killing desirable organisms in the mixed liquor. See the Chemical Addition section of this manual for more detail on the sodium hypochlorite system.

The equation for calculating the sodium hypochlorite dose is:

      CL2 = Dose x [VAB x NAB x MLSS x 8.34]

Where:

CL2 = Pounds of CL2 required per day
Dose = Desired Hypochlorite dose (typical range is 0.005 to 0.015)
VAB = Volume of one Aeration Basin, MG
NAB = Number of Aeration Basins on line
MLSS = mixed liquor suspended solids, mg/L

An example of how to calculate the chlorine dose is shown below:

                                       Dose =   0.010   lb CL2/lb MLSS
                               VAB (each) =   0.54 MG
      Number of Basins in Operation =   2
                  Aeration Basin MLSS =   3111 mg/L


Calculate the desired chlorine dose:

   CL2= 0.010 x [0.54 x 2 x 3111 x 8.34]
   CL2 = 280 lbs CL2 per day

Calculate the required dose rate in gal/hr and gal/min.
 
   Lbs CL2 per Gallon = 1.25 lb/gal
   So 280 lb CL2 per day = 280/1.25 = 224 gal/day = 9.34 gal/hr = 0.16 gal/min

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Last Updated: 9/16/2013 9:35:08 AM
Version 4.0.1