What Causes Bulking Sludge Problem

rising sludge bulking problemBulking Sludge is a major problem that can cause serious operational issues to the management of wastewater treatment plant. Basically with this condition around, it will be very difficult in order to get a good separation of sludge and water and this will lead to carry over of solids to the discharge side and clog up the final polishing filter.

A bulking sludge is a condition defined by solids with poor settling characteristic (which are either slow or unable to settle at all and will just float on top) and this can be observed by the high SV test result. As a general guide, the SV test will indicate the volume of settled solids after 30 minutes time period. Another characteristic which we can use to refer to the bulking sludge problem is the poor compactibility of the sludge in which there are water or gas trapped in between the solid floc and thus leads to the sludge having a low density and it won’t agglomerate well together in a compact matter. Both sludge characteristics are the main reasons that will affect quality of wastewater discharge.

Growth of filamentous bacteria is the main cause that leads to poor settling characteristic of the waste sludge. Although presence of these microorganisms can help towards efficient removal and breakdown of organic matter, they have weak floc forming behavior and sludge mass containing these bacteria will be slow to settle. Although presence of large number of filamentous bacteria is generally blamed and identified as the main cause, there could be other microorganisms that could also lead to the same condition which are the growth of acid-favoring fungi which predominates due to the low nitrogen content of the feed water and acidic condition in the pond. There are no specific names towards naming of these microorganisms but they are all grouped under the slime producing genera.

Lack of macronutrients and imbalances in terms of F/M ratio can also induce growth of these unwanted microorganisms and in order to solve the sludge bulking issue, the best approach is to look into the low pH problem and ensuring that the F/M ratio is correct. Both methods are considered the more viable and workable solution without involving extra cost because it basically involves monitoring and ensuring the process control is correct. Sometimes depending on time availability and whether both options are applied correctly, another last option will involve the costly method of bacteria seeding which can be troublesome as it involves stripping and reloading of the whole system.

On sludge compactability issue, so far one of the main causes that lead to the sludge having low density and floats to the surface is the entrapment of gas coming from denitrification. The sludge here is usually called rising sludge and apparently it will float to the surface after some time left after good settling. Unfortunately there is not much you can do when it comes to this but generally the approach to tackle this issue is by adjusting to increase the sludge take off rate (desludging rate) and also decreasing the incoming wastewater flow to the aeration pond. All in all, with tight process control and monitoring, the sludge bulking issue can actually be prevented from happening.

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