Last year we visited wastewater treatment facilities within both municipal and industrial sectors. Most plants we visited had issues with secondary clarifier operation. Looking closely at the problem it was found that inherent design issues relating to the clarifier were so dominating and affecting the final discharge BOD/COD/TSS values so drastically that one would think that the biological system was not performing at all!

While the basic aims of any aerobic biological wastewater treatment are:-

1) To convert the organic pollution present in the effluent into new biomass, CO2,H2O 

2) To separate this biomass from the liquid stream efficiently so that actual removal of organic pollution from the liquid phase is achieved. It seemed to me that most of the plants’ clarifier designers had not understood the second part of the aims well enough resulting in poorly performing plants.

As Secondary clarifiers are basically the domain of a civil engineer, the important civil engineering related aspects like correct Surface Area Overflow Rate, efficient central pier design, bottom slope, efficient effluent collection launder were well designed and put in the correct manner for the clarifier. However, still most of the clarifiers had been observing severe floating sludge, TSS carryover, deflocculating issues which were obvious due to certain aspects of clarifier operation and performance completely missed during the design process.

We believe that most of the secondary clarifiers designed in India are suffering from two principal mistakes related to its design

1)  Single Secondary clarifier for multiple Aeration basins

Most of the plants are designed with two or three parallel aeration basins but single secondary clarifier for all the reactors. For large flows greater than 10 MLD, one has to understand from microbiological perspective that each Aeration Tank is a separate plant in terms of microbial activity and thus associated kinetics, MLSS/MLVSS ratio and also settling properties of the MLSS.

It is very difficult to maintain completely ideal operating conditions for both aeration tanks at such large flows and thus there exists a variation in terms of F/M ratio, DO Levels, mixing conditions, toxic shock loads, etc. and thus both reactors exhibit a significant difference in microbial activity and act as separate plants.

Combining these two streams for settling in a single clarifier will certainly transfer the bad attributes of one aeration tank to the second aeration tank and thus entirely change the population of both aeration tanks and thus removal kinetics and settling properties.

Worst part of all is that the single clarifier is designed with 2.5 peak factor for its hydraulic loading and thus when the hydraulic load coming to the plant is lower, it is very difficult to keep the MLSS in suspension during transit to the clarifier and partial settling of MLSS happens within the effluent carrying channels.

Furthermore, most of the industrial effluent treatment plants are designed with 4-6 hours or longer HRT. Imagine the conditions at partial load when the plant is receiving half of the design hydraulic load.

This almost doubles the HRT in the clarifier and causes severe conditions within sludge blanket. Moreover most of the plants are operated under DO levels of < 2ppm and thus when the HRT of the clarifier is too high, it causes severe DO depletion within the sludge blanket causing anaerobic/anoxic conditions which kills significant amount of biomass in the clarifier itself.

Eventually due to noxious gases developing within the sludge blanket, this dead biomass floats to the surface of the clarifier which is termed as floating sludge conditions in the clarifier. This floating sludge increases the TSS carryover and each gram of TSS contributes at least 0.8g of BOD to the treated effluent.

When high TSS carryover is happening in the clarifier, the BOD values are shooting up and at times causing violation of discharge limits while actually the aeration system has effectively removed the soluble BOD to desired levels.

Also high HRT causes excessive thickening of the sludge and associated sludge deflocculating conditions leading to dispersed growth and/or pin floc conditions which further deteriorate the settling properties of the MLSS in the clarifier and thus associated TSS carryover and higher BOD values.

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