Secondary Clarifier : A taken for granted Unit Operation

Over the past few years, we have been conducting a lot of surveys of existing wastewater treatment facilities. During our analysis of the existing infrastructure and operating issues related to it, we came one shocking revelation that secondary clarifier, one of the most important unit operation for biological process was totally neglected. From design to operation of point of view, it was just taken for granted. The results were drastic for the negligence!!! Despite having enough capability, the plants were not meeting the required treatment goals and clients were incurring unnecessary higher costs for facilities management.

The principle aim of any biological wastewater treatment process are twofold:

1. To convert organic pollution present in the wastewater to new biomass, CO2 and H20.
2. To separate this biomass effectively from the liquid stream and thus cause actual removal of organic pollution from the liquid phase.

Note that during biochemical oxidation process, the organics are converted to new bacterial cell, CO2 and H2O. So there is actually a phase transformation of organic pollutants from one form (actual organic chemicals) to another form (new biomass as C,H,N,P). Thus if the biomass which is commonly measured as MLSS/MLVSS is not separated from the liquid phase effectively then practically there is no organic pollution removal. And when this biomass is getting carried over in the treated effluent as TSS, it contributes to the BOD/COD values significantly.

Secondary clarifiers are aimed at separating the biomass and sending it back to the system in the form of RAS for the further processing of incoming wastewater using this settled biomass. So if there is a serious issue of biomass loss as TSS carryover in the secondary clarifier, then the biological process suffers from wash out of useful microbial strains responsible for the degradation of the pollutants present in the incoming wastewater.

As design of a secondary clarifier remain a domain of civil engineering professionals, various aspects related to SALR, central weir design, bottom slopes, scrapper mechanisms etc are well design and well looked after. However, We have observed that certain process related issues and operational matters are not overlooked within this standard design practice resulting in serious TSS carryover and floating sludge issues even for many well designed clarifiers.

We believe that most of the secondary clarifier designs in Indian subcontinent suffer due to following two major mistakes:

1. Single secondary clarifier for multiple aeration basins

The surveys indicated that it is a customary practice to provide single secondary clarifier for large plants with multiple aeration basins operated in parallel. The principle aim seem to be cost saving in civil and electromechanical costs however at the expense of biological process!!!!!

For large flow rates more than 5 MLD capacity, each aeration tank needs to be considered as a separate plant from microbial population point of view. The overall biological kinetics of each aeration tanks for such plants would differ significantly in terms of biological activity, MLSS/MLVSS, SVI and OUR. Combining their MLSS in a single secondary clarifier for settling operation will certainly transfer the bad attributes of one biological system to another and will change the overall population dynamics significantly.

Also from civil design point of view, it was observed that many clarifiers were designed with 2.5 peak factor for their incoming civil structures and thus bit oversized form hydraulic loading point of view. This had resulted in severe settling of MLSS in the inlet weir, channels causing loss of useful biomass and also causing problems of foul odor.

2. Excessive HRT at average flow rates

It was also observed that most of the industrial biological processes were designed with a secondary clarifier having more than 6 hours HRT at average flow rate. While industrial biological processes tend to have poor settling qualities of the MLSS, certainly having higher HRTs in the clarifier doesn’t help a lot in improving the settling quality and final treated effluent quality.

Imagine a condition when the plant is receiving less than 50% of the average design flow!!! The HRT in the clarifier will be just doubled under such condition. We found many clarifiers being operated at 12 to 24 hours HRT at actual plant hydraulic loading conditions.

This excessive HRT will develop anaerobic condition in the settled sludge blanket leading to the following issues:

• Death of live active biomass
• Excessive sludge floating which will increase TSS carryover and BOD/COD of the final effluent
• Severe deflocculation of MLSS causing pin flocs or dispersed growth resulting in fine particles carryover with treated effluent increasing its turbidity.

The overall result would be poor biological activity with higher effluent turbidity and higher BOD/COD then envisaged. 

We would suggest to my colleagues and clients a few points to consider while designing the clarifier and biological treatment plants so that problems associated with clarifiers can be avoided.

a) If possible provide separate clarifiers for each Aeration basin.

Indeed while this increases the CAPEX and OPEX in terms of land requirement, civil cost and associated electromechanical equipment and power consumption, separate clarifiers allows for efficient utilization of the secondary clarifier under partial load conditions. When the hydraulic load is lower, only one train of aeration tank and clarifier can be fed with the effluent and thus excessive HRTs in the clarifier can be avoided. Indeed it separates the two biomass systems and isolates the problem to one side of the plant so that it can be more easily addressed.

b) Operate Aeration Basins at close  to 2 ppm DO Levels

It is better to design aeration systems with provisions to maintain 2 ppm DO levels at the outlet of Aeration Tanks. It helps us to avoid depleted DO level conditions in the clarifier. This would prevent anoxic conditions within the sludge blanket and thus will help to avoid problems of floating sludge. For industrial effluents when it is imperative to operate at high MCRTs, this anoxic/anaerobic condition can trigger growth of filamentous bacteria especially M. Parvicella which can thrive under low DO conditions and HRTs long enough for them to proliferate in the clarifier.

c) Use Adequate HRT for clarifier and focus on developing good settleable MLSS by providing correct environmental/operating conditions within the Aeration tank.

One is inclined to think that the higher the HRT of clarifier, the better will be the solids separation. However such a general idea is not correct. Solids separation is mainly the function of qualitative composition of biomass developed in the Aeration basin which has a major effect on clarifier performance. Therefore trying to compensate poor microbiology with a longer HRT is not the optimum solution. Indeed Industrial effluents have their own physicochemical properties which hinder the optimum growth of good settling biomass but still focusing on developing good biomass is the best solution for better clarifier performance. A reasonable HRT of 4 hours is more than sufficient for good biomass with better settling properties.

Focus on optimum operating conditions of F/M ratio, DO Levels, Organic loading, optimum MCRT with the help of controlled sludge wasting and good biomass will be developed in the Aeration basin providing optimum clarifier performance. Dosing of microbial products can also play a role in addressing biomass issues associated with poor settling.

If required we can help you to assess the performance of biological systems including clarifiers for better performance and help you establish an optimum operating window for the plant allowing your plant to run at its maximum removal efficiency without making major changes to your existing plant and flow sheet.

Kindly contact us for further details.