Dear experts,
Our clarifier faces a nuisance: serious sludge bulking. Since we started the system, sludge bulking appears often in our clarifier. During the first 3 months, a thick sludge layer appeared all over the clarifier's surface after each 4-5 days of operation (about 5-10 cm). Recently (6 months from the  start up), thick sludge layer appears almost everyday. The system operates during day time. In night time, there's no stream comming through the system (yet we still have the RAS pumps and air blowers  work). We suspect that there're 2 reasons causing this trouble: high sludge age and failure of clarifier design. If is was high sludge age, we can handle it. However, it's gonna be serious if it is the clarifier design.
So i want to ask if you guys can check if anything wrong with this clarifier:
Inf BOD = 400 mg/l
Flowrate = 5 m3/h (80 m3/d , working from 6 am to 10 pm)
MLSS = 2800 - 3000 mg/l
SV = 400 - 600 after 30 mins settling
water level in clarifier is 2.1 m
clarifier's length x width = 2.6 x 2.6
RAS pump's capacity is 30 m3/h, working based on 30 min cycle (runs 2 mins after each 28 mins stop)
There's no sludge scraper in the clarifier, only chamfer to collect sludge.
So please tell me if there is anything wrong with this clarifier design?

I agree with Sludgefather's conclusions. A further test of the floating sludge would be to stir it up and see if the sludge releases bubbles and then sinks which is a further indication of denitrification. Check the DO in the clarifier as another indication of denitrification.

Off the top of my head, I don't think that the overflow rate of the clarifier is too high, but the design may cause  sludge to accumulate and not be recycled effectively.

Finally, what are the discharge limits for the clarified effluent. If you have no NH₃-N limit, then a lower sludge age may prevent nitrification and denitrification. However, I would consider this a last resort.

grrun

Hi Cooper Harris,
I agree that the problem is not one of sludge bulking. It is more likely to be denitrification of septicity in the sludge in the clarifier.
You are running the sludge return pump for 2 mins every 28 minutes. This is obviously not frequent enough.
You could probably eliminate this problem of floating sludge by running the return sludge pump continuously, but continuous operation may not be necessary. However an increased frequency of running the sludge return pump is definitely needed.
Excluding any allowance for the capacity of the sloped area, (no angle of slope was given so I can't calculate the volume of that area), the straight walled section of the clarifier has a capacity of 14.2 m3, (equivalent to 2.8 hours retention time in that straight walled section of the clarifier). When the sloped floor section of the clarifier is taken into account the retention time is probably well in excess of 3 hours.
This sounds like a very long time for sludge to be sitting in the bottom of a clarifier, particularly if the effluent contains Nitrate, or if the DO in the effluent is low, or if the temperature of the water is about about 12 degrees C.
Hope that these comments are helpful.