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High COD in treated effluent

High COD in treated effluent

Hi All
For last 15 days we are facing high COD of our treated effluent and we have have aatken all known actions but its not restoring. Urgently need advice. Below are details:
We are operating an activated sludge waste water treatment plant. This an aerobic system involving deepshaft technonolgy. This reactor is designed to handle 24 te/day COD load
1- We have reduced bioreactor feed @ 13 tes COD Load from 20 Tes/day
MLSS = 6500 ppm
BIOreactor DO value is increased to 6 ppm from 2 ppm
Bioreactor delta T is lowered to 4C from 13 Celsius. Bio reactor temp is maintained @ 34C.
Flow of RAS ( return activated sludge ) = 225 m3/h @ 1.0 % solids concentration
Nutrients reserves in treated effluent are NH3 = 3 ppm, PO4 = 1 ppm
SOUR test value is 7-8
There are clarifiers but sludge settlig in these has also been deteriorated
There is unusually high amount of foaming in bio reactor surface.
Note that our feed effluent is mostly of organic nature with some aromatics also
We dont have fresh BOD5 results.however it was 0.6COD four years back

During normal conditions,we used to see 150- 250 ppm COD in our treated effluent which is now increased to 1500 ppm and these results are consistent.Apparently,there is no change in feed effluent comoposition but we are quiute sure of this.

Kindly tell what may be the reasons for this upset and what remedial actions we should take?

Best Regards

RE: High COD in treated effluent


It seems that the clarifier(s) may be upset from young sludge (low SRT) and possibly insufficient F/M. It may be actually made worse by the excessive D.O. If you sample the MLSS for nocardia or similar filamentatious microbes, what is present?

Is the system using a full MLE, Bardenpho, or other process with anoxic zones, an SBR with anoxic phases, or just extended air?

Depending upon the influent and method of aeration, the system may be acting as much as a DAF thus carrying over solids rather than subjecting them to digestion.

Other parameters which might be informative are the RAS flow as a percentage of plant flow, surfactant level, and FOG level?

Also, 34°C is reaching a point of reduced biological activity. Even though the SOUR tests high, the microbes may not be happy.

Sorry for all the questions, but more information is needed.

RE: High COD in treated effluent

Thanks Watthour

At the moment we have no facility and experience to judge microbiology of organisms, however we are trying to get it checked by some expert. In this regard, pl give your opinion

We have extended air system which goes directly into bioreactor from a compressor.
What is "DAF" ?am sorry i dont understand this term.
Presently RAS flow is almost double the feed flow.We are a petrochemical plant and we normally dont have surfactants in our feed effluent.What is FOG?

Please note that with same RAS flows,MLSS levels and reactor temp, we have been succefully operating this bioreactor facility for last 17 years and it produced on spec product water.we used to operate at F/M 0.7 - 0.8.
This problem just encountered for last 15 days when vioreactor outlet COD jumed to 1500 ppm from 200.

Please suggest some tests or wayforward

Best Regards

RE: High COD in treated effluent

To understand what is happening, you really need to know the BOD content, influent and effluent. You may have non-biodegradable COD.

There is no reason to increase the DO to 6 mg/L. The excess aeration may be causing the foaming.

Have you always had the MLSS at 6,000 mg/L? That level appears to be on the high side. Once the MLSS increases above 5,000, the high solids are difficult to settle in the clarifier. What is the design value?

What is the sludge level in the clarifiers?

RE: High COD in treated effluent

Thnaks Bimr
We normally used to operate MLSS ~6000 ppm. Design data is ~5600 ppmNormally our DO remains 2-3 ppm when effluent COD is normal.
With this increase in effluent COD,DO level is increasaed to 6+ ppm although aeration rate is as such fixed-no change.Pl tell activated sludge process is not capable to handle non bio-degradeable (in organi ) COD? It means if our BOD < COD (influent), then our bioreactor will not perfrom.This is what i understand, pl tell if i am right or wrong?Pl note that apparently there is no change in influent composition and we have been geetin normal COD results with same influnet and same bio reactor
Also suggest what actions we now need to revive.

Best Regards

RE: High COD in treated effluent

You need the BOD results to understand what is happening. There may be COD that is non-biodegradable. You will not know without the test. COD is reduced through the process only if the COD is soluble and biodegradeable.

As a start, reduce the aeration so that you still have good mixing and a basin with a minimum of 2 mg/L oxygen. The SOUR test shows you do not need the aeration.

The low value on the SOUR test indicates that there may have been an incident with something toxic to the process.

Typically activated sludge systems operate within a range of 2-4% concentration (v/v) based on a centrifuge analysis. Systems with aeration tank concentrations > 4% may begin to experience a slow settling rate due to a high concentration of biomass. To correct, slowly (over several days), increase the sludge wasting rate to reduce the aeration tank biomass. Continue to decrease the aeration tank biomass until the desired settling rate is achieved.

Typically domestic activated sludge systems operate within a range of 2-4% concentration (v/v) based on a centrifuge analysis. Systems with aeration tank concentrations > 4% may begin to experience a slow settling rate due to a high concentration of biomass.

Would you explain this statement: "Bioreactor delta T is lowered to 4C from 13 Celsius. Bio reactor temp is maintained @ 34C."

Note that the maximum temperature that you can expect good settling in the clarifier is approximately 36C.

Redo the SOUR test daily.

Slow increase the reactor feed.
Do you have a daily log of influent and effluent results?

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