Activated sludge is a biological wastewater treatment that speeds waste decomposition. It is added to wastewater and the mixture, referred to as “mixed liquor,” is aerated and agitated. Subsequently held in a tank, the mixture settles out by sedimentation and is disposed of, i.e., as they say, “wasted,” or returned to the aeration tank as needed. The remaining wastewater then undergoes further treatment.
The activated-sludge-containing aeration tank, by adding air to the wastewater, “freshens” it, keeping solids in suspension. Activated sludge is different from primary sludge in that it contains many living organisms, which can feed on the wastewater. The air added to the mixed liquor aids the microorganisms active in the wastewater treatment.
To the perceptive operator, visual and olfactory indicators of treatment progress can be very informative.
Of interest are foam color and type found on the aeration tank surface, mixed-liquor color, sludge types rising to the final-clarifier water surface and other observations indicative of process status. Other key “data points” come from monitoring instruments and the changes operators make to maintain or improve conditions.
Visual and data-point observations are especially helpful in revealing long-term activated-sludge wasting requirements. "Long-term" is emphasized because, unlike what might be the case in other process industries, these indicators normally won’t change rapidly and are indicative of process conditions that have prevailed over a period of days and may be the culmination of trends that have persisted weeks or even months.
Indicators such as foam qualities or color don't change quickly without a major upset. Dark brown, scummy foam covering an aeration basin won’t appear overnight, and it won't be corrected quickly either. That dark foam is the result of operational adjustments made — or not made — over the previous weeks or months.
Olfactory indications
Strong odor typically doesn’t follow from an activated sludge process. Only when strongly odorous materials enter the plant in the raw wastewater or if dissolved oxygen in the aeration basin persists at zero for a term of hours will strong odors be noticed coming from an aeration basin.
The odor normally associated with an aeration basin has been described as “earthy.” If you’re wondering what that’s like, pick up a handful of rich, moist earth and you will know. Some others say it smells like a fresh plowed field. Poets have gone even further… well, at any rate, if, on the other hand, you smell rotten eggs, i.e., hydrogen sulfide, this is a sure sign that aeration rates are too low. In this event, blower discharge rates or mechanical aeration should be increased immediately.
A good inspection, following checking for odors, would include inspecting the entire aeration tank surface for turbulence. This is most important for operators in diffused aeration treatment plants, where air is compressed and released near the bottom of the aeration basin.
Though experience rules, the amount of surface turbulence is indicative of mixing capacity throughout the aeration tank. More action at the surface indicates more mixing.
Noting dead spots on the surface will tend to tell you if mixing is consistent. This is especially noticeable at oxidation ditch plants when only one rotor is running. A small amount (an inch or two) of separation is usually not a problem under these circumstances as long as you maintain dissolved oxygen levels 1.0 to 3.0 Mg/L.
Watch for blowouts
Another common problem with diffused aeration systems is blowouts, which occur when diffusers or headers are damaged and release large amounts of air at a particular location. Seeing excessive amounts of large bubbles come to the surface indicates a blowout. Blowouts should be repaired quickly. They lead to poor oxygen transfer, lower dissolved oxygen and mixing rates and energy wasted to compress air.
Foam and scum types, if any, on the aeration tank-surface, and to a lesser extent, mixed-liquor color gives you a clue to how well the process is working.
In a conventional process, only modest accumulations of light-colored, crisp-appearing foam are present on the aeration-tank surface when discharging excellent final effluent. The operator should note carefully process conditions at that time so as to more easily maintain them.
As with aeration-basin foam, in general, dark-brown mixed-liquor means sludge is old. A light-brown to sand-colored tan mixed-liquor indicates a sludge that is fairly young. Given indications of old sludge, wasting may need to be increased, assuming other control tests corroborate the observation.
A secondary or final-clarifier settling tank or sedimentation basin holds wastewater for a period of time during which the heavier solids settle to the bottom and the lighter materials float to the water surface. Solids include impurities plus the zoological bacteria.
On-going assessment
If the final effluent is clear or improving day-to-day, operators will stick with what works. However, if effluent appears turbid or contains noticeable solids, or if the clarifier sludge blanket is rising toward the surface, trouble may be just around the comer.
At times, fluffy, almost transparent sludge particles 1/8- to 1/4-inch in diameter may rise to the clarifier surface near the outlet weirs. This condition is usually worse in a shallow clarifier and may be especially noticeable at high-return sludge-flow rates. When a straggler floe in an effluent that is otherwise clear appears, increase sludge age.
This type of straggler floc usually if found at relatively low mixed liquor-solids concentrations and is usually worse during early morning hours. It may be reduced by cutting back on sludge wasting rates 10% to 20% to increase sludge age. Return sludge and air discharge rates may also need to be adjusted, depending on results calculated from other control tests.
Certain general steps for visual and olfactory observations of activated sludge have been described above. If you have specific activated sludge or other wastewater queries, please submit a question.
