Operating data from wastewater treatment plants has generated some useful operational control strategies. Some of these control methods include:
By definition, the SVI is the volume of settled sludge in milliliters occupied by 1 gram of dry sludge solids after 30 minutes of settling in a 1000 ml graduated cylinder or a settleometer.
A liter of mix liquor sample is collected at or near the outlet of the aeration
tank, settled for 30 minutes in a 1 liter graduated cylinder, and the volume
occupied by the sludge is reported in milliliters.
The SVI is computed by dividing the result of the settling test in ml/liter by the MLSS concentration in mg/L in the aeration tank times 1000.
The common range for an SVI at a conventional activated sludge plant should be between 50 and 150. Optimum SVI must be determined for each experimentally.
Sludge Density Index is used like the SVI to determine sludge settling characteristics and return sludge pumping rates. SDI is computed by:
The common operational range for SDI is 1.0 - 2.5
The SVI and SDI indexes relate the weight of sludge to the volume that the
sludge occupies and attempts to show how well the activated sludge separates
from the mix liquor. Sludges with a low SVI (hig SDI) have good settling
and compaction characteristics.
The common range for sludge age for a conventional activated sludge plant is between 3 and 15 days. For extended aeration activated sludge plants the range is between about 15 and 30 days. Generally during the winter months, higher sludge ages are required to maintain a sufficient biological mass. In the summer time, biological activity increases and lower sludge ages normally produce a higher quality effluent. Thus, the sludge age should be adjusted at least twice a year to accommodate seasonal variations.
The operator must realize, however, that the optimum sludge age may not fall in the common ranges given here. This is due to the fact that the waste characteristics, process design, flexibility in operation, and process control equipment are different for all facilities. The operator, by trial and error, can find the optimum sludge age for that particular plant and specific conditions.
A low sludge age tends to produce a light, fluffy, buoyant type of sludge particle commonly referred to as straggler floc, which settles slowly in a final clarifier. This will be witnessed in a clarifier when these buoyant, fluffy sludge particles are being pulled over the weirs even though the effluent may be crystal clear.
A high sludge age or too many solids in the system tends to produce
a darker, more granular type of sludge particle, commonly called pin floc,
which settles too fast in a final clarifier. Pin floc is observed
as many fine tiny floc particles coming over the final clarifier weirs
leaving a very turbid effluent.
Mean Cell Residence Time (MCRT)
Another operational approach for solids control, like the sludge age, is the mean cell residence time (MCRT) or Solids Retention Time (SRT). This parameter is a refinement of the sludge age and takes into consideration the total solids inventory in the secondary or biological system.
Again, the desired MCRT for a given plant must be found experimentally just like the sludge age.
The MCRT is calculated as:
The F/M ratio is calculated as follows:
*The mixed liquor volatile suspended solids (MLVSS) may be a more accurate estimate of the mass of microorganisms than MLSS.
When the operator finds the optimum MLSS concentration for each plant, he attempts to maintain this value by adjusting the sludge wasting rate.
One rule of thumb for activated sludge systems is that for every pound of BOD removed in the secondary system a half a pound of new solids is generated through reproduction of the organisms and addition of new organisms from the influent wastes. So, the operator tries to waste the proper amount of solids to keep his selected optimum mix liquor concentration constant.
If the MLSS concentration is above the desired concentration, wasting of the excess solids will have to be started or increased.
If the MLSS concentration is below the desired concentration level, wasting should be decreased or stopped.
Operators should keep in mind that in most cases it is better to waste continuously
over 24 hrs/day, seven days a week than to waste intermittently. Drastic
changes in sludge wasting rates are undesirable. Increases or decreases
in wasting should be made gradually, i.e., 20 - 25 percent per day.
Constant RAS Flow Rate Control.
Settling the RAS at a constant flow rate that is independent of the aeration
tank influent wastewater flow rate results in a continuously varying MLSS
concentration that will be at a minimum during peak influent flows and a maximum
during minimum influent flows. This occurs because the MLSS are flowing
into the clarifier at a lower rate during peak flow when being removed at
a constant rate. Similarly, at minimum influent flow rates, the MLSS
are being returned to the aeration tank at a higher rate than are flowing
into the clarifier. The aeration tank and the secondary clarifier must
be looked at as a system where the MLSS are stored in the aeration tank during
minimum wastewater flow and then transferred to the clarifier as the wastewater
flows initially increase. In essence, the clarifier acts as a storage
reservoir for the MLSS, and the clarifier has a constantly changing depth
of sludge blanket as the MLSS moves from the aeration tank to the clarifier
and vice versa. The advantage of using this approach is simplicity,
because it minimizes the amount of effort for control. It is also especially
advantageous for small plants because of limited flexibility.
Constant Percentage RAS Flow Rate Control
The second approach to controlling RAS flow rate requires a programmed method
for maintaining a constant percentage of the aeration tank influent wastewater
flow rate. The program may consist of an automatic flow measurement
device, a programmed system, or frequent manual adjustments. The programmed
method is designed to keep the MLSS more constant through high and low flow
Comparison of Both RAS Control Approaches
The advantages of the constant RAS flow approach are as follows:
The advantages of the constant percentage RAS flow are the following:
The most significant disadvantage of the second approach is that the clarifier is subjected to maximum solids loading when the clarifier contains the maximum amount of sludge. This may result in solids washout with the effluent.
In general, it appears that most activated sludge operations perform
well and require less attention when the constant RAS flow rate approach
is used. Activated sludge plants with flows of 10 mgd or less are
often subject to large hydraulic surges, and performance of these plants
will benefit the most from the use of this approach.
Though these operating parameters are widely used, the details of the operating procedure will vary at different activated sludge plants, depending on the type of facilities available, strength and character of the wastewater, temperatures, requirements of the receiving waters, etc. The best operating procedure for each plant must be determined by experience. Some guidelines that may be applied to a conventional activated sludge plant are:
For an activated sludge process to achieve optimum plant efficiency
the final clarification unit must effectively separate the biological solids
from the mix liquor. If these solids are not separated properly and
removed from the clarifier in a relatively short period of time, operating
problems will result, causing an increased load on the receiving waters
and a decline in plant efficiency. The most important function of
the final clarifier is to maintain the wastewater quality produced by the
Final clarifiers should be designed with rapid sludge withdrawal systems
to inhibit the tendency of the sludge to become anaerobic if not removed
Photo Credit: Virginia Department of Health
|Weirs should be of the saw-tooth type to allow for better weir overflow and flow distribution. Weirs should be level and fee from scum and algae to prevent short-circuiting within the clarifier. The tanks should be sufficiently baffled to reduce velocities and to disperse the flow evenly to reduce short-circuiting. Short-circuiting will tend to increase the solids losses over the clarifier weirs. Also, final clarifiers should include some type of surface skimming device to remove floating solids and scum. Final clarifiers should be designed with a hydraulic detention time from 2 - 2 hours.|
Operational Problems with Final Clarifiers
The operator must keep in mind that many operating problems in the final clarifier can be associated with operating problems in the preceding processes, i.e. mainly the aeration system.
The nitrates that are formed in the aeration tank then flow into the final settling tank where quiescent settling and solids removal will take place. If the dissolved oxygen levels are sufficiently low in the settling tank and there is some organic matter available, denitrificaton will take place.
Rising sludge is caused by denitrification in which nitrites and nitrates
in the wastewater are reduced to nitrogen gas. Denitrification occurs
in the sludge layer in the secondary clarifier when conditions become anaerobic
or nearly anaerobic. As the nitrogen gas accumulates, the sludge mass
becomes buoyant and rises or floats to the surface. Rising sludge can
easily be differentiated from a bulking sludge by noting the presence
of small gas bubbles attached to the floating solids and by microscopic examination.
This problem can be overcome by increasing the removal rate of the sludge
from sludge-collecting mechanism, by regulation of the flows (loading) and
monitoring of the dissolved oxygen levels in the final settling tank.
The problem of bulking is not easy to deal with since its causes cannot always be identified. However, a careful review of the operating records with respect to pH, loading, and aeration tank DO, MLSS, etc. is always useful in attempting to develop relationships between poor operating conditions and bulking. Careful records and trending as well as a close control over operating conditions and a knowledge of inputs into the wastewater system is useful.
When bulking of activated sludge is caused by overloading, prechlorination to reduce the load on the aeration process has been tried with some success. Prechlorination of the primary tank influent to produce a residual of about 0.1 mg/L in the primary tank effluent is used. Prechlorination of the primary tank influent is particularly useful in controlling septicity. Chlorination of the return activated sludge can control filamentous bulking. The point of application should be where the return sludge will be in contact with the chlorine solution for about one minute before the sludge is mixed with the aerator influent. The chlorine dose is varied according to the variations in the sludge volume index and may be estimated as follows:
SVI = Sludge Volume Index
F = Return sludge rate in million gallons per day
W = Suspended solids in return sludge in mg/L
Chlorine dosages can better be determined by trial and error. In general, chlorination of a bulking sludge will reduce the sludge volume index, thus the dose is reduced daily until bulking is corrected. In some plants intermittent chlorination will maintain a low sludge volume index, and in others continuous chlorination of the return sludge has proven more satisfactory. Generally, when chlorination of the return sludge is started, the turbidity of the plant effluent will increase, but after a few days of operation the turbidity will again decrease to that of normal conditions.
Extensive wasting of the biological sludge to reduce the filamentous organisms also has proven to be somewhat effective in alternating a bulking situation.
The operator must realize that these measures are only temporary steps
to alleviate bulking and that the problem may reappear if the cause is
not identified and corrected.