The settleometer test yields a great deal of information about how well sludge will settle and compact, but it does not reveal the MLSS concentration. It would seem obvious that if the MLSS concentration increases, the settling rate would slow down. (That is; thicker mixed liquor should settle more slowly). To some extent, this holds true, but it is not always the case. The reason has to do with the fact that the type of microorganisms in the mixed liquor has more to do with how the sludge will settle and compact than the concentration does.
When an activated sludge process is operating well, it will primarily contain a mixture of simple round and rod shaped bacteria, an assortment of higher life forms known as protozoa, and a few long, hair-like filamentous bacteria that add strength to the bio-flocculated structure (known as floc). If the right conditions cannot be maintained in the system, this balance of microorganisms in the floc will change. For systems operating at very high F:M ratios and low SRTs, the organisms do not remain in the system long enough for the slower growing protozoa to appear.
For this reason, this condition is referred to as “young sludge”. The settling characteristics of young sludge are slow, and a cloudy supernatant, laden with large straggler floc particles is left behind.
If the system is operating at a very low F:M ratio and a high SRT, larger, slow growing organisms such as rotifers and sludge worms will begin to appear. This condition is often referred to as “old sludge”. Old sludge tends to settle very rapidly but leaves pin floc in the supernatant and a surface material known as “ashing”. Ashing appears just as though ashes were scattered on the surface of a settleometer test or secondary clarifier. One of the most common problems that arise in the activated sludge process is the proliferation of excessive amounts of filamentous type bacteria. The problem has many causes, including septic conditions, low D.O.conditions and operating the system at the wrong F:M ratio. The growth of excessive numbers of filamentous bacteria results in a floc that cannot separate from liquid due to the hair-like projections of the filaments.
This condition is know as “sludge bulking”, and it can lead to the total loss of the solids inventory in the treatment system as the sludge is washed out of the secondary clarifier.
The MLSS provides information regarding the concentration of solids and the settleometer shows how it will settle, but neither give the full picture. In order to analyze the settling characteristics at a given MLSS concentration, operators calculate a value known as the sludge volume index, or SVI. The SVI of mixed liquor is determined by knowing both the sludge’s settling characteristics and its MLSS concentration.
The SVI is an index of how well a sludge will settle at a given MLSS concentration. This means that it does not matter if the mixed liquor is at 1,500 mg/L or at 3,500 mg/ L when the settleometer test is performed, the setting characteristics can be quantified.
The SVI is most useful at identifying filamentous organism outbreaks, allowing operators to respond before the system is out of control. For most activated sludge treatment plants, a SVI range of 80 – 120 signals good treatment. SVI values of <80 indicate older, fast settling sludge and the need to waste solids, however, some SBR systems operate constantly in this range. SVI values over 150 almost always indicate a serious filamentous bacteria outbreak that must be dealt with before the entire solids inventor is lost from the system.
Understand that these ranges give a good indication of where most plants operate well and when most plants will get into trouble, but they are not hard and fast rules. This is because different systems have different secondary clarifiers. Shallow, poorly baffled secondary clarifiers do not respond well to bulking conditions, whereas deep, well baffled clarifiers can handle SVI values at or above 150 before loosing the sludge blanket to washout. If the equipment is available to measure the settleability and the MLSS concentration of a mixed liquor, the SVI value offers operators a powerful tool for assessing the condition of the biological process. When used by a conscientious operator that knows the history of their system, changes in
the system’s SVI value can even be used to predict filamentous bacteria induced sludge bulking problems and then measures can be taken to stop them early.