Equalization Tank in Wastewater Treatment Plant

What is Equalization Tank in Wastewater Treatment Plant

Equalization Tank

Effluent from the collection tank comes to the equalization tank in wastewater treatment. The main function is to act as buffer. To collect the incoming raw effluent that comes at widely fluctuating rates and position to the rest of the ETP at steady ( Average ) flow rate. During the peak hours ETP comes at high flow rate. The equalization tank stores this effluent and lets it out during  the non peak time when there is no /little incoming effluent. The inlet pipe of equalization tank carries filtered effluent from cooling tower. The effluent NPSH lifting pumps move the effluent to the EC SKID.

  • The delivery pipe takes the effluent to the EC SKID.
  • The coarse bubble diffusers are short length of tubes that have holes at regular spacing .they release large bubbles in the tank to lightly aerate the effluent and also to agitate the mix continuously.
  • Compressed air from blower s through this air supply pipe line. This may be rigid pipe line.
  • inlet pipe line 2.collection tank  3.raw effluent lift sump . 4. delivery pipe 5.coarse bubble diffuser 6.air supply pipe line.

Equalization Tank Operation And Maintenance

  • Keep air mixing on at all times.
  • Ensure that the air flow /mixing is uniform over the entire floor of the tank. Adjust the placement of diffusers and the air- flow rate needed.
  • Keep the equalization tank in wastewater treatment nearly empty before the expected peak load hours (otherwise it will over flow)
  • Check and clean clogged diffusers at regular intervals.
  • Manually evacuate settled much/sediments at least once in a year.

Equalization Tank Trouble Shooting

1Insufficient mixing/aerationPoor maintenance of diffuser cleaning and blower maintenance
2Excessive odorInsufficient air supply, poor maintenance and operation
3Insufficient capacity to handle peak flowsConstant outflow pumping, do not maintained.
4Usable capacity reduced due to solids accumulationPoor maintenance.

Construction of coarse bubble diffusers  in equalization tank.

  • If keeps the raw effluent aerated thereby avoiding septicity  and suppressing odor generation.
  • If keeps said in suspension and prevents setting of said in the tank ,thereby reducing frequency
  • Of manual  cleaning of the tank.
  • If membrane diffusers are used ,they will fail frequently, due to the repeated cycles expansion  and contraction caused by fluctuating water levels in the equalization tank. there fore only coarse bubble diffusers must be used in the equalization tank in wastewater treatment.

Equalization Tank Maintenance

  • switch between the main and standby pump (every 4 hrs approximately).
  • Check oil in the pump every day. Top up if necessary.
  • Check motor to pump alignment after every dismantling operation.
  • Check condition of coupling and replace damaged parts immediately.
  • Check for vibration and tighten the anchor bolts and other fasteners.
  • Check condition of bearing ,oil seals, mechanical seal and replaced if necessary.
  • Completely drain outoil and replace afresh as per manufactures recommendation.
  • Always keep safety guard in its proper position.
  • Follow the LOTO safety principle while performing maintenance activities.
Equalization Tank Maintenance
Equalization Tank

Maine use of Equalization tank

  • To reduce the temperature in Equalization tank
  • For homogeneous mixing
  • To give the optimum dosing of HCl for neutralizing
  • To increase the Bacteria growth in FBBR
  • To reduce the insoluble BOD and COD in the System
  • To avoided the solids cloaking in the air distribution system in Equalization tank
  • To increase the air volume in equalization tank in wastewater treatment
  • To remove the odor in Equalization tank due to anaerobic reaction
  • To increase the efficiency of Biological system
  • To reduce the current and electrode consumption in EC system

List of civil tank dimension

Collection tank  (1.5 m* 1.5 m* 1.5 m).

Equalization tank in wastewater treatment (3.6 m* 3.6* 2.4m).

Biological tanks for Activated Sludge Process  (3.0* 3.0* 2.4m).

Flash mixer (0.9*0.9* 1.2m).

HRSCC (3.6 * 3.3 * 2.4m).

Filter  Feed  Tank ((3.0 *3.0*2.4 m).

Sand filter (dia 1.8 m* 2.1 m).

Sludge thickener tank ( dia 1.5 m * 2.4 m).


  • Ph
  • Total dissloved solids(tds)
  • Color
  • Temperature
  • Dissloved oxygen (do)
  • Mixed liquor suspended solids (mlss)
  • Constant feed flow (biological process)
  • Total suspented solids(tss)
  • Bod
  • Cod
  • Nurients requirment
  • Pva content

Sampling points

  • Equlization tank
  • Biological process
  • Ec feed
  • Ec out
  • Sand filter out
  • Post areation tank

Test equipments

  • tds meter
  • ph meter.
  • Turbidity meter.
  • Cod skid.
  • Spectro photo meter.
  • Magnetic stirrer
  • Weighing balance
  • Jar test (flacculator)
  • Distillation unit (dm water preparation)
  • Titration equipments (pipette, buratte, conical flask, chemicals).

Internal Test

  • Equlization tank: raw efrfluent
  • Test : ph, tds, tss,hardness, frc,cod,bod,color,turbidity, pva content,
  • Biological process:
  • Test : ph,do, turbidity, cod, bod, mlss, sv- 30, svi, frc,temperaure.
  • Ec feed (biological outlet ):
  • Test : ph, tds, turbidity,color, cod , hardness, tss.
  • Sand filter feed (ec out)
  • Test :ph, cod, tss, turbidity,tds, color.
  • Post areation ( sandfilter  out)
  • Test : tds,cod, ph, bod, tss, color, do,temperature, turbidity.
  • Filter press : dry sludge
  • Test : ph, moisture content , densidy.

External Test

Three months once

  • sampling point :equliation tank, biological outlet, ec out, post atreation out : test – cod, ph, tss,tds,bod
  • Filter press (dry sludge ); test : moisture content, volatile matter, toxicity, density, ph.

Emfm meter postion

  • Equlization tank (bio feed )
  • Ec feed
  • psf out (ec out)

Log sheets

  • Elctro coagulation sheet(daily log sheet)
  • Etp labanalysis.
  • Management checking list( lab report).
  • Maintenace activities.
  • Weekly report (qty treated record).
  • cost analysis report (monthly report).
  • Etp maintenance chart

Monitoring and control

  • hree months once external lab test  report (all parameter -pollution board requirements)
  • Every day sludge genration record.
  • Monthly sludge genration record.
  • Yearly sludge storage record.
  • 6 months once air stock monitaring analysis report(bioler, power house(dg ), etp in and out area,sludge storage area).
  • Yeraly environmental statement record.
  • yearly once external lab test report (dry slduge)
  • Slf (secured land filling) diagram
  • Etp plant layout(pi&pid ).
  • mass balace chart.
  • Msds of all etp handling materils.
  • safe operation :
  • Without authorized person do not change in the Electrical panel
  • All the pumps ,motors and Blowers give ground EARTH connection
  • When cleaning process of filter press and EC skid use safety dress.
  • When cleaning process switch off the EC skid power.
  • Always follow the instruction manual of equalization tank in wastewater treatment process


  • Before handling ETP chemical ,Must wear safety equipments like  gloves,  shoes, mask, etc.
  • Must identified Hazardous ,non hazardous chemical list.
  • Every one Follow 5 ‘S RULES.
  • Must identified high electrical power (Danger symbol )equipments.
  • EC process must wear shoes, gloves and mask.
  • Must Identified All civil tank dimension.
  • Maintenance period of equalization tank in wastewater treatment must follow LOTO procedure.

About Engr. Kh. Mashiur Rahman

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  2. Online Garments Academy

    Nice article.

  3. Ma Thae Nan Dar Soe

    Air blowers for Equalisation tank and aeration tanks

  4. Thank for your sharing us in this useful report
    Causes of colour change in clarifier

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