It’s difficult to give you a definitive answer, as there are all kinds of elements that could be contributing to this phenomenon:
– presence of filamentous bacteria,
– inadequate or malfunctioning degassing process in the clarifier
– denitrification in the clarifier
– sludge recirculation rate is too high (excessive load at inlet)
– sludge production rate is too high
– long sludge age
It is important to get a comprehensive overview of the installation: analyses, operational settings… that will help you pinpoint the problem.
I second the comments made by yvart_pascal.
But I also recommend you take a look at the results obtained with a product now being used in France called Speed-O-Clar.
Low flow capacity treatment plants can experience sludge bulking for a number of reasons but I suggest that the top two be confirmed by you and your staff:
#1 – To many solids – you have a sludge inventory that is to large and these solids are hindered in settling in your clarifier due to low settling rates. To verify you can run a simple Hindered Settling test. Get 3 2 L beakers or settleometers if you use this test for sludge quality evaluation. In the first fill to the 2 L line with 100% of the MLSS basin. In the second 2 L beaker put in 50% of the volume (1 L) of either primary effluent or raw influent and then 1 L of MLSS, in the third 25% of primary effluent or influent and then 75% of the MLSS. Stir each and then allow them to settle – Measure the settled solids in each beaker at 5, 15, 30 and then 60 min.
If you plot the settling rates by the mL of solids over time the three decreases proportionally as you have diluted the MLSS (from none to 25/75) then I would suggest your MLSS is too high and by reducing the MLSS concentration (total solids under aeration) will improve your bulking situation. IF the solids settled is not proportional – that there is less clear supernant in the 50/50 and the 25/75 beakers than you could have filamentous bacteria being the source of your bulking issue. A waxy and large volume of foam on your aeration basin could also confirm this filamentous (nocardia sp.). The solution is to reduce the branching of this bacteria – reducing MLSS can help put an applied HypoChloride into the volute of the RAS pumps is more effective.
Hope this helps
This video explains how the product was developed.
The Speed-O-Clar solution is marketed by Densiline, and for more info on the product see below. It is easy to implement and looks like an effective solution. This prompts the question of why this type of system is not more widespread.