DESIGN APPROACH. The design of skimming and pumping systems for classifying selectors must take into account three principal considerations:

Making Classifying Selectors Work for Foam Control in the Activated Sludge Process Denny Parker 1*, Steve Geary 2, Garr Jones 1, Lori McIntyre 1, Stuart Oppenheim 1, Vick Pedregon 3, Rod Pope 1, Tyler Richards 4, Christine Voigt 5, Gary Volpe 1, John Willis 1, Robert Witzgall 1 1 Brown and Caldwell, Walnut Creek, California. 2 Cobb County, Georgia. 3 El Paso Water Utilities, Texas. 4 Gwinnett County, Georgia. 5 Metropolitan Council of Environmental Services, St. Paul, Minnesota. * Email: DParker@brwncald.com. ABSTRACT Classifying selectors are used to control the population of foam causing organisms in activated sludge plants to prevent the development of nuisance foams. The term classifying selector refers to the physical mechanism by which it selects against these organisms; foam causing organisms are enriched into the solids in the foam and their rapid removal controls their population to low levels in the mixed liquor. Foam causing organisms are wasted first rather than the usual case where they accumulate on the surface of tanks and thereby are wasted last. The concept originated in South Africa where it was shown through pilot studies that placement of a flotation tank for foam removal prior to secondary clarifiers would eliminate foam causing organisms from the process. A variant of the concept called selective foam wasting was implemented at full-scale within a sludge reaeration tank at an Atlanta plant with excellent results; it was operated on a campaign basis when foam reached nuisance levels. Brown and Caldwell modified the concept to normally waste on a continuous basis and has since applied classifying selectors at seven plants. The design concepts, retrofit approaches and operating experience are reviewed. KEYWORDS: Activated sludge, classifying selector, foam, Nocardia, secondary clarifier. INTRODUCTION Nuisance foams have plagued activated sludge plants, particularly those operated with the higher solids residence times (SRTs) needed for nitrification or biological nutrient removal (BNR). Besides the method that is the subject of this paper, various schemes have been tried with varying degrees of success for foam control such as surface chlorination, RAS chlorination, anaerobic selectors, anoxic selectors, SRT control and organic polymer addition. Each method has had its successful applications but there seems to be no universally applicable method. Classifying selectors are used to control the population of foam causing organisms in activated sludge plants to prevent the development of nuisance foams. Brown and Caldwell coined the

term classifying selector in the early nineties to specifically reference the mechanism by which the process selects against foam causing organisms; foam causing organisms are enriched into the solids in the foam and their rapid removal controls their population to low levels in the mixed liquor. In essence, foam causing organisms are wasted first rather than the usual case where they accumulate on the surface of tanks and thereby are wasted last. Classifying selectors have proven effective in all the applications where a successful mechanical removal system has been implemented. Problems with applying classifying selectors have been primarily mechanical in nature due to challenges of removal of a difficult material from the aeration tanks (often in inconvenient locations), but the fundamental selection principle has proven sound. The origin of the classifying selector, as applied by Brown and Caldwell, derives originally from the pilot work of Pretorius and Laubscher (1987) which was extended to full-scale by Richards et al. (1990) for the City of Atlanta. Evaluation of Brown and Caldwell s first operational classifying selector design by Sacramento plant staff and parallel university research at benchscale (Pagilla et al. 1996) yielded better understanding and further refinements. Pretorius and Laubscher postulated that because of the physical properties of foam causing organisms, it should be possible to selectively separate them from the rest of the activated sludge and then waste them at a faster rate than they can grow. Their postulate is the fundamental basis of the biological selection function of the classifying selector. Pretorius and Laubscher used fine bubble aeration for separation in a flotation cell placed between the aeration tanks and secondary clarifiers. Pilot tests showed nearly complete elimination of foam causing organisms. Pretorius and Laubscher s concept is similar to what we apply today, except that selectors are incorporated into aerated channels or into the aeration basins themselves, eliminating the need for dedicated flotation basins and equipment. The first recorded attempt with this technique in the USA was in the late eighties at the City of Atlanta by Richards et al. (1990). It was reported that Nocardia filaments in this nitrifying plant were enriched into the foam and the practice of selective foam wasting was very successfully applied for foam control. An overflow device was placed directly in a sludge reaeration basin fed with return activated sludge. It was found that placement at this point resulted in higher concentrations of foam than would have resulted if removal took place from the aeration basin just prior to clarification. Brown and Caldwell s first operational classifying selector was retrofitted to an oxygen activated sludge (OAS) system in Sacramento, CA. Due to the typical construction used for the stages in OAS plants, foam trapping and enrichment is common and it is difficult to completely control Nocardia by any method. Methods of foam removal were provided at the ends of mixed liquor distribution channels through downward opening gates (a form of classifying selector), from the secondary clarifiers with rotating surface scrapers and by a classifying selector in an aerated return activated sludge (RAS) channel. It was intended that these methods were to be employed continuously rather than on a campaign basis. Parallel operation by splitting the two sides of the plant in half showed that the selectors at the end of the channels were more effective in Nocardia control than the one placed in the RAS channel (Pagilla et al. 1996). When all control units were placed in operation in 1993, Nocardia counts were 30 percent of those in the year 1991 (prior to the classifying selector implementation in the RAS channel). It should be noted that even with

the multiple classifying selector locations provided, Nocardia has been reduced but not completely eliminated from the system due the high level of trapping in the OAS reactors. Accumulation of foam in the plant s anaerobic digesters has been an ongoing problem. In 1997, the plant converted all of their digesters to a fixed cover design with a constant liquid surface. Provisions were made to remove foam laden digested sludge from the surface of the digester through an overflow standpipe. However, due to concerns of foam accumulation in the digester gas dome, the plant staff have been operating the digesters at a lower liquid level and removing digested sludge from the bottom of the digesters. As a result, accumulation of foam in the digesters remains a significant problem at this plant. Brown and Caldwell has found the optimum approach is one of continuous surface removal from the system, and to not let foam develop to nuisance levels. This requires continuous operation of the classifying selector regardless of the status of foam development. An elaboration of Pretorius and Laubscher s original postulate is that normal wasting is not sufficient to control foam causing organisms, because they concentrate on the surface of the tank and unless wasted into the effluent (an undesirable event), they will reside in the system longer than the average age of the mixed liquor. Without a classifying selector, some foam will always reside on the surface of aeration tanks and this situation is aggravated further by the presence of any structures that trap foam. The best applications for any control method is to minimize foam trapping to the extent possible. With a classifying selector, any foam causing organisms are preferentially removed first, due to their higher concentrations in the surface foam. This has the advantage of not only eliminating nuisance conditions but keeping foam causing organism counts low, which reduces the impact on downstream processes (e.g. a reduction of anaerobic digester foaming). Figure 1 shows the implications of not addressing foam control when retrofitting with fine bubble aeration. Fine bubble aeration tends to cause more foam to develop than any other type of aeration. When foam trapping occurred in this plant, nuisance accumulations of foam occurred throughout the plant and disrupted effluent quality. A classifying selector was subsequently added to this facility. Figure 2 shows the result of planning ahead for foam control in this plant (Utoy Creek) there is little accumulation of foam on the secondary clarifiers. This paper documents Brown and Caldwell s work taking research done in the laboratory, pilot and demonstration scale on selective foam wasting to the field, where refinements continue to occur. Figure 1. When this City of Atlanta BNR plant (in Georgia) was converted from mechanical aeration to fine bubble aeration, this foam buildup was the result. High effluent solids overloaded downstream filtration, causing filter bypassing and high effluent TP levels.

Figure 2. The Utoy Creek plant (another City of Atlanta BNR plant) included a classifying selector when it was converted. It was placed on the mixed liquor channel ahead of the secondary clarifiers. As a result, foam accumulation in the secondary clarifiers has been minimal and no problems have been experienced with effluent filtration. DESIGN APPROACH The design of skimming and pumping systems for classifying selectors must take into account three principal considerations: The system should be capable of removing a limited amount of liquid continuously, or almost continuously, from the top layer of the channel or aeration basin in which it is located. The foam causing organisms of interest for the most part are concentrated into the surface foam and in the floatable material in the liquid to be pumped. The pump and its inlet system must be designed for complete removal of the skimmed material. Skimming System The liquid removed from the process via the classifying selector system must be considered part of the activated sludge wasting system. In most designs, the system must remove liquid from a layer of liquid on the surface of the aeration tank or an aerated channel that is no more than 1 to 3 cm deep. Since the surface environment is often subject to considerable turbulence caused by mixing and subject to level variations caused by changes in flow through the reactor, the design of the skimmer must perform in the face of several considerations that tend to increase flow across a simple weir to avoid seriously compromising the accuracy and control of the activated sludge wasting system. In most applications the amount of liquid removed from the reactor or channel is necessarily small. It is therefore often desirable to position the skimming system at locations where forward momentum in the reactor or channel will force the floating material to a central location. Locations of choice include the exit end of aeration basins or aerated channels or where physical features such as an entrance to a draft tube or inverted siphon create natural barriers. To

augment the effectiveness of the skimming system, surface baffles are often placed to encourage movement of material at the surface of the liquid toward the foam removal weir(s). Pump Inlet System The liquid must discharge to a collection system that is designed to remove floatable material and convey it efficiently to where it is combined with the remainder of the waste activated sludge (in most cases the foam removal station is not the sole location of wasting). If the collection system can be drained to the disposal system by gravity, then the operation is easily accomplished. However, in most instances, the material must be pumped. The sump for location of the pump inlets must be specifically designed to concentrate floating materials so that as the liquid level in the pump sump is lowered, the pump will remove material on the liquid surface and pump it to the plant wasting system. The design of the skimming sump must be integrated with the control of the pumping equipment, which must be operated to break suction rather than allow a vortex to develop and inhibit the removal of floating materials. To prevent the vortex from forming, a pocket-type sump is used similar to that recommended in the Hydraulic Institute Intake Pump Standard (1998), which has been found to work well. Pump Selection Depending upon the specifics of the application, the type of pump can vary from several types: submersible solids handling, column type solids handling, positive displacement diaphragm type, self-priming or air-lift. The important ingredients are capacity, ability to pass liquid with entrained air without damage, and to be able to break suction in the sump without damage. Capacity. The central concern is pump capacity. The pump must be selected to provide a capacity greater than the flow over the foam removal weir in the skimming system during all times except peak flow through the reactor in order to routinely pump the skimming sump down, break suction and remove the material floating on the surface of the sump contents. If the selection fails to take this consideration into account adequately, the skimming system will fail because the material of concern, the foam causing organisms, will only concentrate in the sump, eventually resulting in a serious foaming problem. Control. The pump operational strategy, therefore, is simple: cycle the pump to start when the liquid level in the skimming sump is relatively high and operate the pumping system until the sump is empty. Control devices must be selected with care, since many are unable to accurately detect levels when the liquid has a significant quantity of entrained air. Displacer type devices are not recommended. Some ultrasonic and capacitance type devices are well suited for the purpose. If the pump is a centrifugal type, decreasing motor amps or closure of the discharge check valve can be used for termination of operation when the sump empties. CASE EXAMPLES Table 1 shows the Brown and Caldwell s designs of classifying selectors that are already operational (others are in design or construction). Most of these plants are retrofit situations with differing requirements, so a variety of designs have resulted without identification of a single standard.

Table 1. Activated sludge plants with classifying selectors in operation. Plant/location MMF a capacity, ML/d Aeration type: aeration basin/channels Selector location/ installed date Selector wasting location SCRSD, Sacramento, CA 675 Oxygen/ coarse bubble ML distribution channel/1991 RAS channel/1992 WAS line, WAS goes to Dissolved Air Flotation (DAF) Appleton, WI 85 Fine bubble/ coarse bubble ML distribution channel/1993 Alternative WAS wasting system; goes to DAF President Street, Savannah, GA 101 Fine bubble/ no channels Aeration tank/1995 Combined with WAS and primary sludge South Cobb, Cobb County, GA 150 Fine bubble/ coarse bubble AB effluent collection channel/1997 Waste mixed liquor line, WML goes to DAFs Metro Plant, MCES, St. Paul, MN 304/ Fine bubble/ 1211 b coarse bubble AB Zone 1 (RAS Reaeration) /1998 End of AB Passes 2,3,4 (ML) /1998 Combined with WAS ahead of DAFs Haskell R.St., El Paso, TX 109 Fine bubble/ coarse bubble RAS channel/1999 Combined with WAS, goes to DAFs Utoy Creek, Atlanta, GA 165 Fine bubble/ coarse bubble ML distribution channel/1999 a maximum month flow b pilot test on one quarter of plant; design/construction for remainder continues Combined with WAS ahead of thickening centrifuges The primary evidence used in this paper for assessment of nuisance foam control success is photographic. However, when results of microscopic analysis are available, these results are reported. In the cases that follow, organism identification was by morphological means using methods presented by Jenkins et al. (1993). It is recognized that genetic testing has demonstrated that foaming properties attributed to a single organism Nocardia (or Gordona) amerae are shared by a larger set of organisms with similar foam causing abilities (Soddell et al., 1997).

Haskell R. Street Plant, El Paso, Texas The City of El Paso s Haskell R. Street plant was converted from oxygen activated sludge to fine bubble aeration with an anoxic selector for bulking control. Due to contractor problems, the classifying selector implementation lagged behind the rest of the construction by a few months and as a result the new aeration tanks and secondary clarifiers were plagued with floating sludge as shown in Figure 3. This problem caused violation of the 20 mg/l monthly average TSS requirement of the City s discharge permit in January, 1999. An investigation (Kinnear et al., 2000) showed that the problem was due to a combination of Nocardia and slight supersaturation of dissolved gas, owing to the deeper than normal aeration tanks employed (6.7 m). The Nocardia originated in the sludge transferred from the oxygen reactors to the new aeration tanks, when seeding the new process. Prior to the availability of the classifying selector, the problem was resolved by several methods including vacuum removal of foam from the aeration tanks, chlorination, reduction of activated sludge solids retention time and polymer addition. The most effective interim measure was polymer addition to the return sludge line. A classifying selector placed on the aerated RAS channel is the only point of WAS wasting and has resulted in almost complete elimination of foam from the system as shown in Figure 4. Figure 3. The City of El Paso s Haskell R. Street plant showing severe foam problem after startup of new aeration tanks prior to installation of classifying selector. Figure 4. The City of El Paso s Haskell R. Street plant showing classifying selector on return sludge channel (left), aeration tanks (center) and secondary clarifier (right). The aeration tank photo (January 2001) shows the plant was nearly clear of foam.

This case illustrates some of the issues encountered when retrofitting an existing plant. While it was possible to eliminate most foam trapping points, we could not completely convert to aerated channels between the new aeration tanks and secondary clarifiers. This left several pipeline sections and boxes that became foam trapping points. Classifying selectors were provided at two locations, in the RAS channel and in the aeration tank. Each aeration basin is furnished with a submersible sump pump, located at the effluent end of the basin. The pumps are designed to remove any accumulated foam and transfer the material to the waste activated sludge sump. There it is combined with any foam removed from the RAS aerated channel. The aeration basin foam pumping system is rarely used, since it was found to be more effective to waste the foam causing organisms in combination with the waste activated sludge from the surface of the RAS channel. Figure 5 shows the classifying selector on the RAS channel. Under the normal mode of operation, this is the total point of wasting of WAS and foam. A downward opening gate is used to control the amount wasted from the system. The gate is manually controlled and is generally maintained in a constant position. The downward opening gate ensures that there is a differential level between the RAS channel and the level within the sump. Because this is the WAS station and sludge is wasted continuously, sump level is maintained at a constant level by automatically controlling the waste activated sludge variable speed controller. Samples were taken for microscopic analysis during the foam problem period and then compared to a final sample after the classifying selector was put in place. Table 2 shows the results of the quantitative analyses using the intersection method. The data show that the Nocardia count after selector installation is only 2 to 6 percent of the levels prior to selector installation (the first eight samples). The plant normally had experienced seasonal outbreaks at this seasonal occurrence which have also been virtually eliminated and at other times foam is absent. The plant also now typically produces effluent SS ranging from 5 to 10 mg/l. Figure 5. Section of classifying selector station at Haskell R. Street WWTP.

Table 2. Nocardia counts at the Haskell R. Street plant. Date Intersections/g Date Intersections/g March 17, 1999 2.55 x 10 6 April 21, 1999 1.82 x 10 6 March 18, 1999 2.00 x 10 6 April 29, 1999 1.65 x 10 6 March 19, 1999 2.24 x 10 6 May 21, 1999 0.91 x 10 6 April 14, 1999 1.43 x 10 6 July 2, 1999 1.77 x 10 6 January 18, 2001 a 0.57 x 10 5 a After classifying selector was in operation. Utoy Creek Water Reclamation Center, Atlanta, Georgia The City of Atlanta s Utoy Creek Water Reclamation Center (WRC) has six aerated basins with fine bubble diffusers; these are preceded with anaerobic reactors for biological phosphorus removal. The reactors and aeration basins were designed with an open water surface into and through the serpentine path and out of the tanks. The open water surface allows foam to pass into and out of the reactors. A minimum of a 1.8 m wide opening at the water surface was used as the design criteria, to ensure foam was not trapped or bridged across an opening. Figure 6 shows modifications to create an opening for foam travel between passes in an existing multipass tank. All the mixed liquor is transported from the aeration basins to the secondary clarifies via an aerated channel. The aerated channel was selected as the classifying selector location so that nuisance causing organisms could be wasted from the mixed liquor at a single location. A baffle was located in the channel as shown in Figure 2 to trap the foam and divert the foam to a side outlet weir gate in the channel wall. Water spray was provided to assist in pushing the foam over the weir gate. Figure 7 shows the section view of the classifying selector station including mixed liquor channel, sump and pumping station. The foam flows over the weir gate into a steep sided sump with additional water sprays. The weir gate is automated such that the gate adjusts with the changing liquid level in channel. The weir gate maintains a 50 mm gap to minimize mixed liquor flowing into the sump. The weir gate controls are set with a high gain to minimize the gate movement and resulting wear. The foam in the sump is pumped with two double acting diaphragm pumps. The water sprays in the sump are activated when the pumps are on. The pumps are operated from a timer, since level monitoring on foam levels is marginally accurate. The pumps are timed to pump the sump dry on each cycle. Precise control on the timer is not required as the double acting diaphragm pumps have the advantage that they can run dry without damaging the pump. However pumping at high pressures can require frequent replacement of the diaphragms.

A high level of foam control has been obtained as demonstrated by the minimum amount of foam on the final clarifier shown in Figure 2 as well as the condition of the aeration tanks shown in Figure 7. This is the condition of the plant the majority of the time. The foam removal system and the fine bubble aeration system were started in mid January 2000. Table 3 shows the qualitative microscopic analyses and shows that overall, Nocardia control has been very effective. It took about a month to resolve several mechanical and instrumentation issues (the suction pipe to the foam pumps had been clogged with concrete and the weir gate actuator had two wires landed incorrectly). This inability to waste probably caused the high Nocardia counts early in 2000. In mid April 2000, all the primary sedimentation tanks were taken off line for rehabilitation. The mixed liquor was increased (from about 2,500 to 4,500 mg/l) to account for the increased organic load. This change probably resulted in the slightly higher abundance of Nocardia in April 2000. The rehabilitated primary sedimentation tanks and additional aeration tanks were put back on line in October 2000. The mixed liquor was reduced (from over 4,000 mg/l down to close to 2,000 mg/l) to account for the reduced organic load. This transition may have resulted in the somewhat higher abundance of Nocardia in October 2000. Figure 6. Utoy Creek s anaerobic reactor on the left and aeration basin on the right.

MIXED LIQUOR CHANNEL BAFFLE WEIR GATE PUMP Figure 7. A section of the classifying selector station at Utoy Creek. The selector is placed adjacent to an aerated channel feeding the secondary clarifiers. Table 3. Nocardia assessment at the Utoy Creek Water Reclamation Center. Date Abundance Date Abundance Date Abundance 1/31/00 5, abundant 4/07/00 1, few 6/13/00 0, none 2/09/00 4, very common 4/10/00 2, some 7/10/00 1, few 3/06/00 4, very common 4/13/00 1, few 9/28/00 2, some 3/22/00 1, few 4/18/00 1, few 10/19/00 3, common 3/38/00 1, few 4/25/00 3, common 3/31/00 1, few 5/18/00 1, few President Street Water Quality Control Plant, Savannah, Georgia The City of Savannah s President Street Water Quality Control Plant had two oxidation ditches equipped with brush aerators. The ditches were converted to conventional aeration basins with

fine bubble diffusers. The existing piping from the ditches to the secondary clarifiers was retained. It was recognized that the exit conditions from the ditches would serve as a trap for collecting foam unless provisions were made to mitigate this. A classifying selector system was installed in each basin. The foam removal system, as shown in Figure 8, consisted of a slotted pipe and a cantilevered sump pump. The design was intended to ensure that the flow of foam through the slotted pipe was less than the flow capacity of the pump. This allowed a hydraulic fall of the foam into the pipe. As the liquid level in the pipe lowered, the pump s suction head MOTOR PUMP DISCHARGE MAX. WL MIN. WL SLOTTED PUMP SUCTION PUMP Figure 8. Section view of classifying selector installed at Savannah s President Street Plant. decreased and the pump discharge declined. Conversely, as the level rose in the pipe, the suction head increased and the pump discharge increased. This system allowed equilibrium to be reached between the foam flowing into the pipe and the foam being pumped. This was Brown and Caldwell s first in-basin and has not worked as well as other installations; the small slot openings (51 mm) interfered with passage of stringy materials that had gotten past the bar screens. The bypassed screenings blocked the openings, preventing foam from flowing freely into the slotted pipe. The absence of a sump also prevented fluidization of the foam before it was pumped.

South Cobb, Cobb County, Georgia The South Cobb classifying selector was set up with a fixed weir (consisting of a single 51 mm wide notch) to divert the surface mixed liquor layer and foam to the foam wasting sump. The station was placed in a mixed liquor collection channel at the effluent end of the aeration basins. Figure 9 shows a drawing of the station. The weir crest is submerged at all plant flow conditions. When foam wasting is desired, the top-suction cantilever pump is started and the level in the wet well is drawn down, providing the needed hydraulic differential to draw mixed liquor and foam through the notch. Surface sprays, the direction of flow in the channel and the baffle divert floating material to the weir for wasting. The top suction pump design allows the pump to convey the foam and break suction routinely without starving the pump; any air entering the pump volute escapes from the top when displaced by flow entering the wet well. The pump discharges into a common waste mixed liquor line upstream of the flow meter so that the wasted foam is included in the totalized waste flow and in the SRT calculation. The heaviest demand on the wasting system occurred during startup of the new air-activated sludge reactors with seed material from the previous oxygen activated sludge system. The foam wasting system was able to remove 150 to 200 mm of foam on the aeration basins within the first day-and-a-half of operation. Now the plant uses the system only on a campaign basis when required to clean the basin of foam. In this their operational concept is close to the selective foam wasting procedure originally developed at the City of Atlanta. Figure 10 shows the classifying selector and the foam free condition of the aeration basins and secondary clarifiers. Figure 9. Section view of classifying selector at Cobb County (1 in. = 25.4 mm).

Metropolitan Council Environmental Services Metro Wastewater Treatment Plant, St. Paul, Minnesota As a pilot project in 1996, a quarter of the nitrifying activated sludge system at the large Metro WWTP (St. Paul, MN) was modified to biologically remove phosphorus (Bio-P). The modifications to four of the 16 aeration tanks included division of each 4-pass basin into twelve zones, five of which were separated by concrete overflow/underflow baffles. It quickly became apparent that installation of the baffle walls, conversion from coarse bubble to fine pore diffusion and a submerged aeration tank outlet resulted in an accumulation of nuisance foam, which were later identified as Nocardia and Microthrix parvicella by Council staff. To combat this it proved necessary to add classifying selectors to each aeration tank. Figure 10. A classifying selector placed in the aeration basin effluent collection channel keeps both the aeration basin and secondary clarifiers free of foam in this nitrifying activated sludge plant at Cobb County. Council engineering and maintenance staffs were instrumental in the development of alternative concepts and details to address foam control. As a test, a standpipe surface collection system was installed in the four modified aeration tanks and consisted of PVC piping graduated in size from 400 mm diameter at the surface to 152 mm at the tank floor. The piping was connected to an existing tank drainage sump and pumped by existing piston pumps, originally installed to remove heavy sludge from the tank floor when draining the tank. Standpipes were installed at the beginning of passes 1 and 3, and the ends of passes 2 and 4. In pass 1, where the liquid level

varies 160 mm, 51 mm wide vertical slots were cut into a PVC collar and were adjustable in height to control the flow (Figure 11). Fixed elevations were provided at the remaining locations. Figure 11 shows the Zone 1 configuration under normal conditions when the classifying selector is not in operation. The system operates intermittently, as required, to address seasonal nuisance organism outbreaks. The goal of operating the system is to prevent the foam from overtopping the walls. Most of the year the depth of the foam seems to stabilize and as it does not cause problems, no skimming is conducted. Usage has been primarily in the spring and fall, and has been effective in controlling the foam when the pumps function properly. The available piston pumps were not the best application for foam pumping. Operation of this system has proved to be very labor intensive, requiring close monitoring of the skimmer elevation and pump operation, but when in balance it has been effective in foam control. Operations at this site are similar to the original selective foam wasting procedure originally employed at Atlanta. The remaining 12 aeration tanks are scheduled for Bio-P conversion starting in 2001. A new and improved foam wasting system will be installed in all of the aeration tanks as shown in Figure 12). Key components include: 1. The primary collection point will be Zone 1, which is always in a return activated sludge (RAS) reaeration mode. Experience has demonstrated that if the foam is controlled in Zone 1, there is little or no problem in the remaining zones. Disproportionately, more foam is formed at higher sludge concentrations of the RAS and as long as it is removed at this point, it does form in the downstream passes. 2. A 1.2 m diameter fiberglass sump structure will be installed in Zone 1 to provide storage capacity for flexibility in pump operation. One of the design criteria was specific to the collection sump, which requires an operating level at least 1.2 m below the liquid level in the tank. This elevation difference results in the foam liquefying, which facilitates pumping. 3. Zone 1 will have a manually adjustable, slotted weir collector similar to the existing, which will also be screened to deflect floating debris. 4. Fixed elevation standpipes, similar to the existing ones, will be provided in the remaining 3 passes. The elevation will be set higher than that in Pass 1 to enable foam collection under higher liquid level elevations. 5. Surface baffles were considered to direct the foam to the collection system, but since the baffles at the end of the zone contained the foam within, and since the existing pilot system was effective without additional surface baffles, the new system design does not include any additional baffles. 6. The existing piston pumps now used for foam pumping will be replaced with positive displacement rotary lobe type pumps.

Figure 11. Metro WWTP s four pass aeration tanks with biological phosphorus removal. In the photo on the right, the pass with the most foam has the pilot classifying selector. The photo on the left shows the slotted weir arrangement that was adjustable by hand. Figure 12. Typical aeration tank classifying selector locations at the Metro Plant. SUMMARY AND CONCLUSIONS Foam causing organisms have created nuisance conditions in a wide range of activated sludge plants. Control of the problem has been difficult and heretofore no means has been universally successful at all plants. Only recently applied in the USA, classifying selectors are used to control the population of foam causing organisms to low levels in activated sludge plants to prevent the development of nuisance foams. The term classifying selector refers to the biological mechanism by which it selects against foam causing organisms; foam causing organisms are enriched into the solids in the foam and their rapid removal controls their population to low levels in the mixed liquor. Brown and Caldwell normally applies the concept to operate on a continuous basis. Since 1990 we have applied classifying selectors at 7 operational installations and a number of others in

design or construction. Mechanical design has been key to the successful implementation of the concept and several challenges have been overcome. Refinements continue as operating experience is gained. Classifying selectors have been successfully applied in a number of different situations, often in retrofit applications. The most difficult situation to address is where there is a high degree of foam trapping due to the original construction of the oxidation reactors. In these cases, system complexity increases as multiple removal points are necessary and foam control becomes a major operational focus. OAS plants present a special challenge in terms of foam trapping, since no practical way has been found to retrofit classifying selectors within the oxidation reactors. In the single OAS case studied, significant improvements in nuisance foam control have been gained, but the problem of nuisance foams has not been eliminated. The other extreme is the most desirable application, which is where a free water surface can be maintained from the aeration tanks on through distribution channels up to the clarifiers. In these cases, provision of a single classifying selector station has been proven highly successful, whether it is placed on aerated mixed liquor collection or distribution channels or in an aerated return sludge channel. Moreover, for more recent designs, operation has proven simple and relatively trouble-free. When processing nuisance foams, the materials should be sent to thickening systems that minimize the return of seeding organisms to the process. While no thickening systems eliminate the return of seed organisms, dissolved air flotation thickening and centrifugal thickening are superior to gravity thickening. ACKNOWLEDGEMENTS The authors gratefully acknowledge the cooperation of staffs of the wastewater treatment plants included in this survey and applaud their resolve to be the pioneers in adopting this relatively new Nocardia control technology. We also acknowledge the assistance of the individuals who did the microscopic analyses reported in this paper: Dr. Michael Richard of the Sear-Brown Group for the City of El Paso and Dr. Mesut Sezgin for the City of Atlanta. REFERENCES Jenkins, D.; Richard, M. G.; Daigger, G. T. (1993) Manual on the Causes and Control of Activated Sludge Foaming, 2nd ed.; Lewis Publishers: Chelsea, Michigan. Kinnear, D.; Tackman, T.; Bratby, J.; Muirhead, W.; Oppenheim, S.; Parker, D. (2000) Overcoming Difficulties Converting from Pure-Oxygen to Air Activated Sludge at El Paso, RX Haskell R. Street Wastewater Treatment Plant. CD ROM Proceedings of the 2000 Water Environment Federation Plant Operations & Maintenance Conference; Portland, Oregon. Hydraulic Institute (1998) Pump Intake Design; ANSI/HI 9.8; Parsippany, New Jersey. Pagilla, K. R.; Jenkins, D.; Kido, W. H. (1996) Nocardia Control in Activated Sludge by Classifying Selectors. Water Environ. Res., 68 (2), 235-239. Pretorius, W. A.; Laubscher, C. J. P. (1987) Control of Biological Scum in Activated Sludge Plants by Means of Selective Flotation. Wat. Sci. Tech., 19 (5/6), 1003-1011.

Richards, T.; Nungesser, P.; Jones, C. (1990) Solution of Nocardia Foaming Problems. Research Journal WPCF, 62 (7), 915-919. Sodell, J. A.; Seviour, R. J.; Blackall, L. L.; Hugenholtz, P. (1997) New Foam-Forming Nocardiofoams Found in Activated Sludge. Proceedings of the Second International Conference on Microorganisms in Activated Sludge and Biofilm Processes; Jenkins, D., Hermanowicz, S. W., Eds.; International Association on Water Quality: London; 285-292.