American Water College 2010

Transcription

1 Vocabulary Activated Sludge (Part 1) Activated Sludge Sludge particles produced in raw or settled wastewater (primary effluent) by the growth of organisms (including zoogleal bacteria) in aeration tanks in the presence of dissolved oxygen. The term activated comes from the fact that the particles are teaming with bacteria, fungi, and protozoa. Activated sludge is different from primary sludge in that the sludge particles contain many living organisms which can feed on the incoming wastewater. Agglomeration The growing or coming together of small scattered particles into larger flocks or particles which settle rapidly. Anoxic Oxygen deficient or lacking sufficient oxygen. Biomass A mass or clump of organic material consisting of living organisms feeding on the wastes in wastewater, dead organisms and other debris. Bulking Clouds of billowing sludge that occur throughout secondary clarifiers and sludge thickeners when the sludge does not settle properly. In the activated sludge process, bulking is usually caused by filamentous bacteria or bound water. Contact Stabilization Contact stabilization is a modification of the conventional activated sludge process. In contact stabilization, two aeration tanks are used. One tank is for separate reaeration of the return sludge for at least four hours before it is permitted to flow into the other aeration tank to be mixed with the primary effluent requiring treatment. The process may also occur in one long tank. Diffuser A device (porous plate, tube, bag) used to break the air stream from the blower system into fine bubbles in an aeration tank or reactor. Dissolved Oxygen (DO) Molecular (atmospheric) oxygen dissolved in water or wastewater, usually abbreviated DO

2 Facultative Facultative bacteria can use either dissolved molecular oxygen or oxygen obtained from food materials such as sulfate or nitrate ions. In other words, facultative bacteria can live under aerobic or anaerobic conditions. Floc Clumps of bacteria and particles or coagulants and impurities that have come together and formed a cluster. Found in aeration tanks, secondary clarifiers and chemical precipitation processes. Food/Microorganism Ratio (F/M) Ratio used for process control of activated sludge plants. BOD load in pounds/day divided by the MLVSS in pounds. Mechanical Aeration The use of machinery to mix air and water so that oxygen can be absorbed into the water. Some examples are: paddle wheels, mixers, or rotating brushes to agitate the surface of an aeration tank; pumps to create fountains; and pumps to discharge water down a series of steps forming falls or cascades. Mixed Liquor Suspended Solids Suspended solids in the mixed liquor of an aeration tank. Mixed Liquor Volatile Suspended Solids The organic or volatile suspended solids in the mixed liquor of an aeration tank. This volatile portion is used as a measure or indication of the microorganisms present. Nitrification An aerobic process in which bacteria change the ammonia and organic nitrogen in wastewater into oxidized nitrogen (usually nitrate). The second-stage BOD is sometimes referred to as the nitrogenous BOD (first-stage BOD is called the carbonaceous BOD ). Denitrification An anoxic process that occurs when nitrite or nitrate ions are reduced to nitrogen gas and nitrogen bubbles are formed as a result of this process. The bubbles attach to the biological flock in the activated sludge process and float the flock to the surface of the secondary clarifiers. This condition is often the cause of rising sludge observed in secondary clarifiers or gravity thickeners. Oxidation Oxidation is the addition of oxygen, removal of hydrogen, or the removal of electrons from an element or compound. In wastewater treatment, organic matter is oxidized to more stable substances

3 Sludge Age A measure of the length of time a particle of suspended solids has been retained in the activated sludge process. Sludge Volume Index This is a calculation which indicates the tendency of activated sludge solids (aerated solids) to thicken or to become concentrated during the sedimentation/thickening process. SVI is calculated in the following manner: (1) allow a mixed liquor sample from the aeration basin to settle for thirty minutes; (2) determine the suspended solids concentration for a sample of the same mixed liquor; (3) calculate SVI by dividing the measured (or observed) wet volume (ml/l) of the settled sludge by the dry weight concentration of MLSS in grams/l. Step Feed Aeration Step-feed aeration is a modification of the conventional activated sludge process. In step aeration, primary effluent enters the aeration tank at several points along the length of the tank, rather than all of the primary effluent entering at the beginning or head of the tank and flowing through the entire tank in a plug flow mode. Zoogleal Mass Jelly-like masses of bacteria found in both the trickling filter and activated sludge processes. These masses may be formed for or function as the protection against predators and for storage of food supplies Purpose of Activated Sludge Process To remove dissolved and suspended colloidal organic waste from wastewater. Description of Process The activated sludge process uses microorganisms to break down organic waste. The conditions are controlled to encourage the growth of beneficial bacteria to speed the process of decomposition. Oxygen is supplied to a tank, where these microorganisms feed on the waste in the water. After a period of time in the aeration portion of the process, the Activated Sludge is sent to a clarifier where the sludge and water are separated. Once settled in the secondary clarifier, a portion of the Activated Sludge is returned to the aeration basin while some is sent to the solids handling facility as waste. Operators take special care to maintain an appropriate population of healthy microorganisms in the aeration basin to handle the incoming organic load. Water Quality Indicator Typical Removal Efficiencies Typical Removal Efficiency Suspended Solids 85% - 95% Biochemical Oxygen Demand (BOD) 85% - 98%

4 Preliminary Treatment Supernatant Typical Activated Sludge Treatment Plant WAS Primary Primary Secondary RAS Sludge Dewatering Aeration Basin Secondary Conventional Process 1. Main objective is to convert dissolved and suspended solids into settleable solids while slower rate processes stress oxidation. 2. Food to Microorganism ratio (F/M) is normally used to control the process 3. Sludge wasting is the method used to control the F/M 4. Normal F/M is lb BOD/day/lb MLVSS 5. Oxygen must be maintained in aeration basin to keep process aerobic and to favor beneficial organisms Low DO will lead to a. Odor problems b. Die-off of aerobic organisms c. Filamentous organisms (don t settle well cause bulking) 6. Too much DO will lead to pinpoint floc which won t settle in secondary clarifier 7. Normal DO range 2 mg/l to 4 mg/l 8. High concentrations of acids or bases will kill microorganisms 9. Normal Sludge age is days High Rate Process 1. Sludge age normally days 2. High F/M 3. Easily upset by shock loads Extended Aeration (Oxidation) 1. Sludge age greater than 10 days 2. Low F/M lb BOD/day/lb MLVSS 3. Very stable process CCB

5 Aeration Tank (Mixed Liquor) Extended Aeration Package Plants Package Plants Extended Aeration Double Compartment Package Plant 1. Normally operated as extended aeration plants 2. Sludge age between days 3. MLVSS 2,000 6,000 mg/l 4. Low F/M lb BOD/day/lb MLVSS

6 Contact Stabilization Triple Compartment Package Plant Aeration Tank (Mixed Liquor) Contact Stabilization Package Plants RAS WAS 1. Normally operated as extended aeration plants 2. Sludge age between days 3. MLVSS 2,000 6,000 mg/l 4. Low F/M lb BOD/day/lb MLVSS 5. Aerobic Digestion is part of this process Aerobic Supernatant sludge

7 WAS RAS Oxidation Ditch Oxidation Ditch 1. Variation of activated sludge process Oxidation Ditch 2. Normally operated in the extended aeration mode Final Disinfection 3. Raw wastewater flows directly into mixed liquor (no primary settling) 4. Usually some form of preliminary treatment (racks, bar screens, comminution) 5. Rotors act as aeration device and maintain flow between fps to prevent solids from settling 6. DO level is controlled by depth of water which determines rotor brush depth 7. DO levels are lowest just prior to rotors (maintain mg/l at this point) 8. MLVSS normally 2,000 6,000 mg/l 9. If ditch develops a crisp, white foam reduce sludge wasting rate 10. If ditch develops a thick, dark foam increase sludge wasting rate 11. Allow several days for process to stabilize after making a process change 12. Mixed Liquor should have a medium to dark brown color