ACTIVATED SLUDGE: PART 3 OF 3 PARTS
"The Problems and Solutions"

Suspended growth biological processes, like activated sludge, possess more operational parameters and variables than fixed growth biological process like trickling filters. It follows reason therefore, that most operators feel that the suspended growth biological treatment processes have more problems! I like to put a more positive spin on this observation by stating that you have more variables to "correct problems" rather than opportunities "for creating problems"! When an undesirable treatment process effluent is being produced, it becomes the task of the operator to identify the condition or conditions that are producing this undesirable effluent. In some cases we may perceive that a problem may exist, when in fact one does not. An illustration of this is a process that consistently producing an effluent of 10 mg/L of BOD and suspended solids out of the clarifiers. But the aeration basins are producing swings in the dissolved oxygen concentration, with no filamentous microbe increase, or other impact. The operator feels that he (she) must therefore stabilize the D.O. concentration at a precise concentration (a value between 2 to 3 mg/l). In many cases this may not be necessary with the current hydraulic and organic loading rates. But if on the other hand, the process is operating well in converting BOD and suspended solids, but impacting the downstream disinfection process by going in and out of nitrification, then some corrective action may be necessary.

We have previously defined the different types or variations of activated sludge and their operating parameters under which they are controlled. If your or process has been optimized, as we discussed earlier, then deviations in the process effluent parameters or operational problems, are easier to identify and analyze. The subsequent corrective actions that you will need to take are also easier to identify.Generally speaking, we find that many of the sources of the problems are related to the aeration basins. Higher or lower than desired dissolved oxygen concentrations, mixing, food to microorganisms ratios (F/M), increasing or decreasing values in the mixed liquor's suspended solids, etc. impact the operation of the clarifier(s) that follows your aeration basin(s). Remember, that solving an operational problem is made much easier when your approach to solving a problem is methodical in nature.

"Bulking sludge is usually caused by an abundance of filamentous microorganisms. These filaments are stringy masses of filamentous microorganisms that do not compact well, nor do they settle well in a secondary clarifier. An increasing sludge volume index is a very strong indication that there is a growth of filamentous microorganisms occurring. (Remember that the sludge volume index is the volume in ml that the weight of one gram of microorganisms occupies. ) An analogy is that the high sludge volume index (SVI) reflects one gram of feathers, and the low sludge volume index (SVI) would be one gram of lead. Obviously the feathers, while weighing the same amount as the lead, has a greater volume. The bulking sludge, does not settle well and is usually carried out of the secondary clarifier.

Rising sludge is usually the result of the formation of gases in the sludge blanket. Rising sludge looks like clumps of solids ranging in size from that of a dime to that of larger "blocks" or "clumps". If the sludge is held in the tank too long, denitrification may occur, which produces nitrogen gas bubbles in the sludge blanket. These gas bubbles cause the sludge to breakaway in clumps from the sludge blanket (at the bottom of the clarifier) and rise to the surface.

Nitrification is the conversion of ammonia into nitrates. Ammonia is converted into nitrites by nitrosomonas bacteria, and the nitrites are subsequently converted into nitrates by nitrobacter bacteria. This conversion of ammonia into nitrates may or may not occur completely. It is not unusual for a treatment plant to have negative impacts by partial or incomplete nitrification on the downstream disinfection process. Partial or incomplete nitrification does not substantially affect the removal of suspended solids or BOD in the process, but it does greatly impact the efficiency of the downstream disinfection process. A "fully nitrified" effluent is very clear, almost swimming pool like, as our example to the left.

If the goals of your activated sludge process do not include the task to "nitrify", then a low solids retention time (Mean Cell Residence Time) value, in days, will be selected. If the raw wastewater entering the aeration basins increases in temperature, the activated sludge process may try to enter into nitrification. (Nitrification and its impacts will be the focus of several operator notebooks in the coming year due to its major impact upon the disinfection process, and due to the fact that it is not well understood by most operators.)

Aeration basin foam is usually caused by filamentous microorganisms such as Nocardia sp. The presence of fats, greases, and oils; warmer water temperatures; and usually higher MCRTs, are conditions which are usually associated with the filamentous growth. My personal experience has shown that with a pretreatment ordinance that restricts the amount of oils and greases that are put into the treatment plant; and proper scum removal in the primary sedimentation tanks by the scum mechanisms, reduction or even prevention of the filamentous foaming is possible.

I have had some success of controlling the foaming by directing the foam into a fiberglass box, located in one area of the aeration basin, by the use of the process water sprays. I then spray the foam captured in the box with a five to ten percent concentration of sodium hypochlorite using a chemical metering pump designed for chlorine applications. The box is used for concentrating the foam, and for also controlling the hypochlorite solution safely. This higher chlorine concentration, in a very fine mist, has been shown to kill the filamentous microbes that exist in the foam. It has a negligible effect on the mixed liquor microorganisms, and is a reasonably cost-effective corrective measure. As always, it is important to try and limit, or control, the conditions under which the filamentous foaming is created, rather than having to reduce or control the foam by corrective actions later. Some plants have also skimmed the foam off the top of the tanks and then disposed of the foam in such a manner that it does not return to their aeration basins to "re-infect" them.

Ashing is the term used to describe very fine sludge particles which may exist on a secondary clarifiers, and in some extreme cases, even on the aeration basins. Ashing is generally caused by having an excessively high Mean Cell Resident Time. This is a common problem in many extended aeration processes. It is corrected by reducing the MCRT time to a level that the ashing does not occur.

For proper settling of the activated sludge in the clarifiers, an operator has to rely on the bioflocculation of the sludge particles. Excessive air applied in the aeration basins prevents the formation of a biological floc that will settle well in the clarifiers. Generally speaking when 2 to 3 mg/L of dissolved oxygen is held across the aeration basins, over-aeration affects do not occur. Operators must balance the mechanical effects of aeration with those of the resulting dissolved oxygen concentration, in order to create a biological floc that will settle.

It is not unusual for clarifiers to experience some solids wash-out at particular times of the day. During low influent wastewater flow rates, which typically occur between the hours of 1 a.m. and 6 a.m., there is usually an accumulation of a biological floc that is suspended just below the weirs and launders of the clarifier. The increasing flow rates occurring in the following morning carries the accumulation of solids out of the sedimentation tank. Another type of solids wash-out occurs when high hydraulic loading rates occur such as during heavy rain events.

A conscientious sampling and lab testing program will provide valuable information, not only for the routine control of the activated sludge process, but also for the times that troubleshooting steps are required. Mixed liquor volatile suspended solids (MLVSS), sludge volume index (SVI), dissolved oxygen concentrations (D.O.), microscopic examinations of the activated sludge; and the recording of the return activated sludge (RAS) flow rates, quantity of activated sludge wasted (WAS), secondary clarifier sludge levels, influent and effluent biochemical oxygen demand (BOD) and suspended solids (SS) values are all important control and troubleshooting indicators. If your intention is to nitrify (remove ammonia) then you must also sample and analyze the process for ammonia, nitrites, and nitrates values throughout the process (establish the nitrogen profile).

When all of these values are trended (or graphed), they provided "visual clues" as to changes in the process. This is especially true when there is a change in the process which occurs at the end of one month and continues into the beginning of another; or when there are increasing or decreasing values which are not readily apparent when you look at the tabular data.

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