Poor Settling in Wastewater Digesters

Published on by in Technology

Our facility is a 2 MGD Sequential Batch Reactor Activated Sludge Wastewater Treatment Facility.

We waste our solids to a sludge anoxic holding tank from the SBR tanks when decanting. When the sludge anoxic tank level reaches the set point, it begins to feed one of our digesters.
When the digester reaches a high level, it begins to feed back into the sludge anoxic and it continuously circulates between the two tanks until we isolate the digester and cut off the air to decant the clear.

It has operated that way for years but now it is not settling well or at all.

There are occasions of popup sludge or no clear.

Our SBR profile has not changed and nothing in our lab testing has shown any changes. 

What could be causing poor settling and can we solve it?

Taxonomy

5 Answers

  1. You should certainly look at the biomass under microscope to determine if filamentous bacteria are the cause as well as determining the floc structure.  What DO level is maintained in the digester?  What is the TSS concentration in digester?  Rising sludge usually is associated with denitrification which should not occur in aerobic conditions, but if the DO is low and the TSS is high, anoxia may be contributing.

  2. Before the reason for poor sludge settleability can be pinpointed, a diluted settleometer test must be run. A simple old-sludge condition, with just too much sludge in the system, will show greatly improved settling in the diluted test. However, a bulking situation, even though it shows some improved settling, will not show large improvement.The required pressure is a decrease in the total system sludge mass. Increased wasting is required to accomplish that objective. This problem is very common. If it is reduced, the concentration gets so low that effective settling is impossible.Normally, one would try to reduce the MCRT, but this requires wasting and a reduced MLSS and it seldom works in this case. It may be worth trying to grow some filaments, such as low DO filaments, that would slow the settling and improve the capture of solids. Maintaining a DO between 0.1 and 0.5 in the aeration tank will usually allow low DO filaments to grow. Be careful

  3. I suggest to introduce periodic microscopic observations of the sludge, either of the SBR activated sludge and anaerobic digester sludge.  A simple phase contrast microscope, 100 x and 1000 x magnifications, can allow, even to non specialised operators, to evaluate whether the activated sludge to digest changed, containg filamentous bacteria (proliferating and coming from the SBR) that hinder the solid/liquid separation. Problems can rise if Gram positive filamentous bacteria are present, such as Microthrix parvicella and Nocardioforms. They are  quite "robust" not easy to go to anaerobic lysis. Such simple microscopic observations can also conduct you to exclude such phenomena, looking for other ways of investigation, starting from solid mass balances (do the digesters work always in the same conditions in terms of solid loads, retention time, etc.).

    1 Comment

    1. I agree with Valter Tandoi, phase contrast microscopy will help.  I normally jump from 100 to 4-500 times because 1000 times needs to be looked at without a cover slip and under oil immersion, which needs a dry film, whereas the 100 & 500 will use a cover slip.  If you don't know what is normal, then it is difficult to assess what is abnormal.  Do you measure the oxygen during the aeration stage, although this in itself may be mis-leading.  Nocardioforms are easy to identify because they are highly branched, and although some are in the bulk, the majority will be found at the surface of the sludge.  There are a whole range of filamentous, which can cause problems.  If it is anaerobic, then lots of free and flocced spiral bacteria will be present and the dominant filamentous bacteria may be Beggiatoa.  Beggiatoa are large filaments and will be seen moving (gliding motion) and in the event of anaerobic conditions, other sulphur filaments may be present.  Other filaments, which may be present are the mentioned Microthrix, also Sphaerotilus, both of which thrive in the presence of iron or manganese.  

  4. One of the culprits that can cause such a condition is over digested sludge in the form of "pin-floc" that requires additional polymers to enhance settling. This can be caused by too much aspirated oxygen in the aeration process used in decanting the recycle. Review any possible changes in the air system such as temperature--increased solubility means more O2 aspirated in the transfer and available for enhanced digestion.