Mass-Balance lysis. The sludge pumping rate mass-balance analysis around either the settling tank or the appropriate limits for Definition sketch for suspended mass balance for retum shdge control: (a)secondary darifier and the two mass-balance on tank analyses are illustrated on Fig. 7-7. Assuming the sludge-blanket level in the settling tank remains constant and that the solids in the effluent from the tiling tar eligible the mass balance around the settling tank shown on Fig. 7-7a is as follow The required ras pumping rate can also be estimated by performing a mass balance around the aeration tank(see Fig 7-7b). The solids entering the tank will equal the solids leaving the tank if new cell growth can be considered negligible. Under conditions such as high organic loadings, this assumption may be incorrect. Solids enter the aeration tank in the return sludge and in the influent to the secondary process. Sludge Wasting, To maintain a given SRT, the excess activated sludge produced each day must be wasted. The most common practice is to waste sludge from the return sludge line because rAs is more concentrated and requires smaller waste sludge pumps. The waste sludge can be discharged to the primary sedimentation tanks for co-thickening, to thickening tanks, or to other sludge -thickening facilities. An alternative method of wasting sometimes used is withdrawing mixed liquor directly from the aeration tank or the aeration tank effluent pipe where the concentration of solids is uniform. The waste mixed liquor can then be discharged to a sludge-thickening tank or to the primary sedimentation tanks where it mixes and settles with the untreated primary sludge Oxygen Uptake Rates. Microorganisms in the activated-sludge process use oxygen as they consume the substrate. The rate at which they use oxygen, known as the oxygen uptake rate(OUR), is a measure of the biological activity and loading on the aeration tank. Values for the OUR are obtained by performing a series of DO measurements over a period of time, and the measured results are conventionally reposed as 2/L. min or mg O2/Lh. Oxygen uptake is most valuable for plant operations when combined with VSS data The combination of OUR with MLVsS yields a value termed the specific oxygen uptake rate(SOUR) or respiration rate. The SOUR is a measure of the amount of oxygen used by microorganisms and is reported as mg O2/g MLVSS h. It has been shown that the mixed liquor SOUR and the final effluent COD can be correlated thereby allowing predictions of final effluent quality to be made during transient loading conditions Changes in SOUR values may also be used to assess the presence of toxic or inhibitory substances in the influent wastewater Microscopic Observations. Routine microscopic observations provide valuable monitoring information about the condition of the microbial population in the activated-sludge process. Specific information gathered includes changes in floc size and density the status of filamentous organism growth in the floc the presence of Nocardia bacteria, and the type and abundance of higher life-forms such as protozoans and rotifers. Changes in these characteristics can provide an indication of the changes in the wastewater characteristics or of an operational problem. a decrease in the protozoan population may be indicative of DO limitations, operation at a lower SRT inhibitory substances in the wastewater. Early detection of filamentous or Nocardia growth will allow time for corrective action to be taken to minimize potential problems associated with excessive growth of these organisms. Procedures may be followed to identify the specific type of filamentous organism, which may help identify an opeating or design condition that encourages their growth Jenkins et al., 1993) Operational Problems The most common problems encountered in the operation of an activated-sludge plant are bulking sludge, rising sludge, and Nocardia foam. Because few plants have escaped these problems, it is appropriate to discuss their nature and methods for their control 7-107-10 Mass-Balance Analysis. The return sludge pumping rate may also be determined by making a mass-balance analysis around either the settling tank or the aeration tank. The appropriate limits for the two mass-balance analyses are illustrated on Fig. 7-7. Assuming the sludge-blanket level in the settling tank remains constant and that the solids in the effluent from the settling tank are negligible, the mass balance around the settling tank shown on Fig. 7-7a is as follows: The required RAS pumping rate can also be estimated by performing a mass balance around the aeration tank (see Fig. 7-7b). The solids entering the tank will equal the solids leaving the tank if new cell growth can be considered negligible. Under conditions such as high organic loadings, this assumption may be incorrect. Solids enter the aeration tank in the return sludge and in the influent to the secondary process. Sludge Wasting. To maintain a given SRT, the excess activated sludge produced each day must be wasted. The most common practice is to waste sludge from the return sludge line because RAS is more concentrated and requires smaller waste sludge pumps. The waste sludge can be discharged to the primary sedimentation tanks for co-thickening, to thickening tanks, or to other sludge-thickening facilities. An alternative method of wasting sometimes used is withdrawing mixed liquor directly from the aeration tank or the aeration tank effluent pipe where the concentration of solids is uniform. The waste mixed liquor can then be discharged to a sludge-thickening tank or to the primary sedimentation tanks where it mixes and settles with the untreated primary sludge. Oxygen Uptake Rates. Microorganisms in the activated-sludge process use oxygen as they consume the substrate. The rate at which they use oxygen, known as the oxygen uptake rate (OUR), is a measure of the biological activity and loading on the aeration tank. Values for the OUR are obtained by performing a series of DO measurements over a period of time, and the measured results are conventionally reposed as mg O2/L.min or mg O2/L.h. Oxygen uptake is most valuable for plant operations when combined with VSS data. The combination of OUR with MLVSS yields a value termed the specific oxygen uptake rate (SOUR) or respiration rate. The SOUR is a measure of the amount of oxygen used by microorganisms and is reported as mg O2/g MLVSS.h. It has been shown that the mixed liquor SOUR and the final effluent COD can be correlated, thereby allowing predictions of final effluent quality to be made during transient loading conditions. Changes in SOUR values may also be used to assess the presence of toxic or inhibitory substances in the influent wastewater. Microscopic Observations. Routine microscopic observations provide valuable monitoring information about the condition of the microbial population in the activated-sludge process. Specific information gathered includes changes in floc size and density the status of filamentous organism growth in the floc, the presence of Nocardia bacteria, and the type and abundance of higher life-forms such as protozoans and rotifers. Changes in these characteristics can provide an indication of the changes in the wastewater characteristics or of an operational problem. A decrease in the protozoan population may be indicative of DO limitations, operation at a lower SRT inhibitory substances in the wastewater. Early detection of filamentous or Nocardia growth will allow time for corrective action to be taken to minimize potential problems associated with excessive growth of these organisms. Procedures may be followed to identify the specific type of filamentous organism, which may help identify an opeating or design condition that encourages their growth (Jenkins et al., 1993). Operational Problems The most common problems encountered in the operation of an activated-sludge plant are bulking sludge, rising sludge, and Nocardia foam. Because few plants have escaped these problems, it is appropriate to discuss their nature and methods for their control. Fig. 7-7 Definition sketch for suspended mass balance for return sludge control: (a)secondary clarifier and (b)aeration tank