Introduction. Date: March 26, Wastewater Treatment Plant Capital Improvement Projects

Transcription

1 Rick Fuller Senior Project Manager Swinerton Management & Consulting 4055 Nelson Avenue Concord, CA Cell: To: Rich Davidson, City of Richmond Mary Phelps, City of Richmond Jim Good, Veolia Water Jon Whitfield, Veolia Water Steve MacLennan, Swinerton M&C Date: March 26, 2007 Re: Wastewater Treatment Plant Capital Improvement Projects Introduction The key unit processes at the Richmond wastewater treatment plant (WWTP) are shown in Figure 1. Capital improvement projects are recommended for several of these unit processes. In all, nine projects have been identified with a total estimated project cost of $17,430,000 (see Table 1 on the following page). A schedule for implementing these projects is provided in the Appendix. The cogeneration project, in this case using a fuel cell, has a cost of $5.5 million. Grant money from PG&E may be available to finance as much as one-half of this project. And it is the only project that will provide a direct benefit to Richmond in that it will reduce power consumption at the treatment plant by converting excess methane gas from the anaerobic digesters to electricity. Implementation of cogeneration at the Richmond WWTP needs to be a top priority for the City during 2007/2008. Figure 1: Richmond WWTP INNOVATION INTEGRITY EXPERIENCE Rev. A

2 The capital projects listed in Table 1 are not listed by priority. Rather, they follow the flow of wastewater through the treatment plant. But changing regulations may well result in one or more of these capital projects becoming a priority. Given that Richmond s National Pollutant Discharge Elimination System (NPDES) permit is up for renewal and that there is increasing pressure from the Environmental Protection Agency to minimize or eliminate blending events, there is a particular need at this time to focus on the performance of the primary clarifiers. In the sections that follow each of the nine recommended capital projects will be discussed in more detail. Table 1: WWTP Capital Projects Summary Capital Project Estimated Cost 1 Influent pumps $1,200,000 2 Enhanced primary clarification $375,000 3 Inclined plates for the primary clarifiers $425,000 4 Redirect primary clarifier surface skimmings from digester to dumpster $360,000 5 Boilers and heat exchangers for the anaerobic digesters $1,700,000 6 Cogeneration to utilize digester methane for energy savings $5,500,000 7 Gravity belt thickeners $750,000 8 Sludge dewatering facility $7,000,000 9 Stamford baffle for secondary clarifier no. 3 $120,000 Total: $17,430,000 Influent Pumps There are four Fairbanks-Morse influent pumps at the Richmond WWTP. Two are 125-HP units and two are 100-HP units. Combined, these pumps can produce 40 million gallons per day (MGD). Though the average daily flow to the treatment plant is approximately 8.0 MGD, during periods of precipitation, and a significant contribution from infiltration and inflow, the flow to the treatment plant can quickly increase to 40 MGD. When all four pumps are in service experience has shown that they are prone to tripping out. In addition, these pumps were installed in the 1950 s. All four pumps need to be replaced with new pumps. With the installation of new pumps we will also need to install new variable frequency drives (VFDs). Further, there may be a need to replace some of the wiring for these pumps. The installation of four new influent pumps, VFDs, and electrical improvements is estimated to cost $1,200,000. Enhanced Primary Clarification We don t know at this time how Richmond s NPDES permit will be modified upon its renewal later this year or next. But we can, and should, anticipate more stringent discharge requirements. And we can anticipate the need to reduce or eliminate blending events. Enhancing the performance of the primary clarifiers can be done through chemical addition. For this capital project we would need to install a concrete pad, chemical storage tanks, chemical feed pumps, and the associated piping. We would most likely feed ferric chloride at the head of the primary clarifiers to achieve chemical precipitation. The Swinerton Management & Consulting Rev. A 2

3 addition of a polymer, in addition to an iron salt, will also need to be evaluated and has been shown to be very effective in combination with an iron salt. Chemically enhanced primary treatment, whereby wastewater is chemically coagulated before clarification, is the simplest enhancement that can be made to conventional primary clarification to increase treatment capacity. The use of chemicals allows a higher peak overflow rate during peak flow events while maintaining or increasing primary clarifier performance. With chemical precipitation, it is possible to remove 80 to 90 percent of the total suspended solids (TSS) including some colloidal particles, 50 to 80 percent of the biochemical oxygen demand (BOD), and 80 to 90 percent of the bacteria. Comparable removal values for well-designed and well-operated primary clarifiers without the addition of chemicals are 50 to 70 percent of the TSS, 25 to 40 percent of the BOD, and 25 to 75 percent of the bacteria. Enhanced primary clarification is estimated to cost $375,000 not including the annual cost of chemicals. Implementation of this option will require an amendment to the Veolia Water contract in order to compensate them for the unanticipated chemical use. Inclined Plates for the Primary Clarifiers Plates installed at an angle in a clarifier will significantly increase the settling area available within a given footprint (see Figure 2). It may be worthwhile to retrofit one primary clarifier with inclined plates and compare its performance under a variety of flow conditions to the standard clarifier. This retrofit could prove very beneficial in reducing the number of blending events. The estimated cost of this capital project, for both primary clarifiers, is $425,000. Figure 2: Inclined Plate Clarifier Remove Primary Clarifier Surface Skimming s from Digesters The Richmond WWTP has a mechanical barscreen to remove coarse material from the wastewater. The spacing between the bars is one inch. This spacing allows the passage of many floatable nonbiodegradable materials (plastics, for example) to the primary clarifiers. Currently, all skimming s from the primary clarifiers are pumped to the anaerobic digesters. This does not represent good engineering Swinerton Management & Consulting Rev. A 3

4 practice. The non-biodegradable materials take up space in the digester and reduce detention time. Instead of sending these materials to the digesters they need to be discharged to a container for disposal at a landfill. The estimated capital cost for this project is $360,000. Digester Boilers/Heat Exchangers The boilers and heat exchangers at the Richmond WWTP have reached the end of their useful life. They need to be replaced. Currently, the one in-service digester is operated at a temperature of approximately 75ºF rather than at the recommended temperature of 95ºF. In addition to replacing the boilers and heat exchangers there will likely be a need to replace some of the associated piping and valving. In order to maximize methane gas production and position the City to benefit from cogeneration, we need to make this a high-priority capital project. The estimated capital cost for replacing the boilers, heat exchangers, piping, and valving is $1,700,000. As stated above, the in-service digester is currently operating at approximately 75 F. This low temperature results in an improperly digested sludge that will be very odorous upon being pumped to the West County sludge drying beds where it is exposed to the atmosphere. Given the problems associated with cogeneration, including the need to reduce seawater intrusion and hydrogen sulfide concentrations in the digester, we need to proceed with the installation of new boilers and heat exchangers. This is a valid capital project and it needs to be at the top of our list for improvements needed at the treatment plant. Regardless of whether the boilers and heat exchangers are financed through a cogeneration project or through a direct capital improvement, the City will be using its bond money to implement this project so we should proceed with this immediately. In addition to the need for new boilers and heat exchangers we also need to replace the waste gas burners and the associated equipment which would include pressure regulators, gas meters, check valves, drip traps, and pressure relief and flame trap assemblies. Figure 3: Fuel Cell Cogeneration After completing the installation of new boilers and heat exchangers the City should proceed to implement a cogeneration project. Cogeneration can be accomplished using engines, microturbines, or fuel cells (see Figure 3). Veolia Water gave a proposal to Richmond in 2006 that recommended the use of an engine. The DER group also gave a proposal to the City in 2006 that recommended the use of a fuel cell. Richmond needs to implement some type of cogeneration at the WWTP. The City needs to turn its excess methane gas into electricity to reduce power consumption costs at the treatment plant. The Swinerton Management & Consulting Rev. A 4

5 estimated cost of installing a cogeneration project is $5,500,000 (for the fuel cell, the project cost for an engine would be much less). Up to one-half of this project would qualify for funding through PG&E. Gravity Belt Thickeners The use of dissolved air flotation (DAF) to thicken waste activated sludge (WAS) is inefficient given the technologies that exist today. WAS should be thickened on a gravity belt thickener (GBT) before being pumped to the anaerobic digester. The concentration of WAS is typically about ½ to 1% solids. A DAF unit will increase this concentration to about 3 percent. A GBT will increase the concentration of WAS to 5 or 6% prior to being pumped to the anaerobic digester. This reduces the volume of water that must be heated in the digester and increases the detention time in the digester which in turn maximizes methane gas production. The estimated cost of installing GBTs is $750,000. Figure 4: Totally Enclosed Gravity Belt Thickener for Outdoor Installation Stamford Baffle With the installation of a Stamford Baffle, which is also known as a Submerged Flow Deflection Baffle or an Upflow Baffle, in a clarifier, the clarifier s performance can be improved by up to 38%. Stamford Baffles eliminate the wall effect, whereby density currents flow up the wall of the clarifier, causing suspended solids to flow directly into the effluent trough. The baffles are attached to the wall of the clarifier and slope downward at a 45 angle, extending into the tank approximately 3 feet. The baffle re-directs the density currents away from the launder, toward the center of the tank, which increases efficiency by settling more solids. This is shown graphically on the following page in Figure 4. Swinerton Management & Consulting Rev. A 5

6 Figure 5: Stamford Baffle High solids carryover. For a variety of reasons, some clarifiers exhibit poor hydraulics. This is certainly the case with secondary clarifier no. 3. In such cases, a secondary clarifier without a Stamford Baffle allows solids to travel along the tank bottom, up the sidewall, and over the weirs. A secondary clarifier with a Stamford Baffle (portrayed as the red lines in the diagram) intercepts the solids and directs them back to the bottom of the tank where they are removed. Conclusion It should be understood that this document was prepared without direct input and participation from Veolia Water. But I have had many conversations with Jon Whitfield, Veolia Water Plant Manager, and know well his needs and concerns. So I believe I have identified those capital projects that will most beneficial to both Richmond and Veolia Water. Nonetheless, acceptance of this document by Richmond requires acceptance by Veolia Water. Therefore, before adopting the capital projects identified in this report, and their associated costs, Veolia Water needs to comment. Finally, the City needs to work with Veolia Water to implement a cogeneration system at the wastewater treatment plant. This is the single most important capital project the City can do that will directly result in lowering its cost of operation at the treatment plant. Excess methane gas is currently burned (flared) to the atmosphere. Methane is a valuable by-product of anaerobic digestion. The City needs to capture the excess methane and convert it to electricity. Swinerton Management & Consulting Rev. A 6

7 Appendix Swinerton Management & Consulting Rev. A 7

8 Figure 6: Fuel Cell Control Panel Swinerton Management & Consulting Rev. A 8

9 Figure 7: Fuel Cell Gas Cleaning System Swinerton Management & Consulting Rev. A 9

10 ID Task Name Duration Start Actual Start Finish Cost Predecessors 1 Richmond WWTP Capital Projects 990 days Mon 3/26/07 Mon 3/26/07 Fri 1/7/11 $17,430, Influent pumps 210 days Mon 3/26/07 NA Fri 1/11/08 $1,200, Electrical 120 days Mon 3/26/07 NA Fri 9/7/07 $300, VFDs 45 days Mon 9/10/07 NA Fri 11/9/07 $200, Pumps & motors 45 days Mon 11/12/07 NA Fri 1/11/08 $700, Digesters 255 days Mon 3/26/07 NA Fri 3/14/08 $1,700, Digester boilers & heat exchangers 120 days Mon 3/26/07 NA Fri 9/7/07 $1,000, Digester piping 60 days Mon 9/10/07 NA Fri 11/30/07 $250, Digester valving 30 days Mon 12/3/07 NA Fri 1/11/08 $400, Digester automation/scada 45 days Mon 1/14/08 NA Fri 3/14/08 $50, Cogeneration 450 days Mon 9/10/07 Mon 9/10/07 Fri 5/29/09 $5,500, Enhanced primary clarification 150 days Mon 5/4/09 Mon 5/4/09 Fri 11/27/09 $375, Primary clarifier inclined plates 150 days Mon 5/3/10 Mon 5/3/10 Fri 11/26/10 $425, Primay skimmings removal 160 days Mon 4/6/09 Mon 4/6/09 Fri 11/13/09 $360, Gravity belt thickeners 180 days Mon 5/3/10 Mon 5/3/10 Fri 1/7/11 $750, Sludge dewatering 730 days Mon 3/26/07 NA Fri 1/8/10 $7,000, Stamford baffle 60 days Mon 3/26/07 Mon 3/26/07 Fri 6/15/07 $120, Page 1

11 ID Task Name 1 Richmond WWTP Capital Projects '07 Q2 '07 Q3 '07 Q4 '07 Q1 '08 Q2 '08 Q3 '08 Q4 '08 Q1 '09 Q2 '09 Q3 '09 Q4 '09 Q1 '10 Q2 '10 Q3 '10 Q4 '10 Q1 '11 e MarApr a Jun Jul u e Oct o e Jan e MarApr a Jun Jul u e Oct o e Jan e MarApr a Jun Jul u e Oct o e Jan e MarApr a Jun Jul u e Oct o e Jan e 2 Influent pumps 3 Electrical 4 VFDs 5 Pumps & motors 6 Digesters 7 Digester boilers & heat exchangers 8 Digester piping 9 Digester valving 10 Digester automation/scada 11 Cogeneration 12 Enhanced primary clarification 13 Primary clarifier inclined plates 14 Primay skimmings removal 15 Gravity belt thickeners 16 Sludge dewatering 17 Stamford baffle Project: WWTP_CIP Date: Mon 3/26/07 Task Split Progress Milestone Summary Project Summary External Tasks External Milestone Deadline Page 1

12 USFilter to Supply Primary Wastewater Treatment Technology for Alberta's Capital City WARRENDALE, Pa., August 30, 2005 The City of Edmonton, Alberta, Canada, has chosen USFilter, a part of Siemens Water Technologies, to provide primary wastewater treatment technology for an upgrade and expansion of its Gold Bar Wastewater Treatment Plant (WWTP). USFilter will supply Zimpro inclined plate separators and Envirex HS730 Loop Chain and scraper systems for the plant s new, enhanced primary treatment process. As part of the Enhanced Primary Treatment project, USFilter provided 32 Envirex longitudinal loop chain and scraper collector systems with four cross collectors. The collectors will be installed into a new rectangular clarification tank, providing the city with additional treatment capacity. Typically, fiberglass flights mounted on two parallel strands of collector chain scrape the settled solids along the tank floor to sludge hoppers. On the return run, the flights can skim the surface and concentrate the floating material at a scum removal device. Faced with more than 100 mechanisms to monitor and adjust, reduction of maintenance was an important issue for the wastewater treatment staff, said Bill Selle, product manager for USFilter. With the collector systems light weight components, such as the nonmetallic chain and fiberglass flights, they are designed to make maintenance tasks as easy and convenient as possible. In addition, within four new basins, 192 Zimpro incline plate separators will provide dual use either as primary clarifiers prior to biological treatment or as stormwater clarification. The 158-mgd system is the first installation by USFilter of inclined plate separators for primary clarification of wastewater. Typically a drinking water treatment product with 2-inch-spaced inclined plates, the Gold Bar WWTP separators will use inclined plates with increased spacing to accommodate a higher concentration of wastewater solids. During normal flow conditions, the separators can be used as primary clarification prior to the biological treatment, helping improve the efficiency of existing equipment. Secondarily, the separators can be used to treat stormwater peaks, which are then blended with effluent from the conventional treatment train before discharge. This is an interesting application for our plate separators, said Mike Mayer, technical director for USFilter. For the same amount of treatment, conventional clarifiers would take up to 10 times the space of our separators. This is critical because without the use of inclined plate separators, there would not have been enough space available at the site to accomplish the desired treatment. The separator system will provide the customer with flexibility and major space savings. USFilter partnered with mechanical subcontractor Schendel Mechanical Contracting, Ltd.; prime contractor Sure-Form Construction Ltd.;

13 and project consultant Stantec Consulting, Ltd., all of Edmonton, Alberta, Canada. Funding for the project was provided under the terms of the Infrastructure Canada-Alberta Program Agreement. USFilter is represented locally by Mequipco Sales, Ltd., Calgary, Alberta, and Canada. As part of the ongoing process upgrade at the Gold Bar WWTP, USFilter will also provide chain and scraper components to retrofit existing clarification basins at the plant. Components and hardware for 45 longitudinal collectors and 10 cross collectors will be supplied.

14 BAFFLE BASICS... A BRIEF TUTORIAL "Density currents" and "density current baffles" have become common terms in the lexicon of clarifier design over the past several years, with the baffle becoming a standard element in clarifier design. Here are some "Baffle Basics" to keep in mind when designing the baffle into the next project. 1. Density currents form in all activated sludge clarifiers and cause short-circuiting of the main clarification volume of the tank. This increases effluent suspended solids and reduces the tanks actual hydraulic capacity. In fact, the hydraulic performance of all center-fed clarifiers is the same, regardless of the process type, i.e. activated sludge, trickling filter, etc. In each case the magnitude of the density currents will vary depending upon flow, the configuration of the tank, the configuration of the feedwell and the operation of the clarifier. 2. The principal cause of density currents in center-feed tanks is the downward flow of the dense input mixed liquor at the feedwell, although temperature differentials and internal tank hydraulics may also introduce currents which disrupt settling. Because the hydraulic flow within the tank tends to spiral slowly from the center outward toward the walls, the currents acting within the tank contribute to the movement of solids toward the weirs. 3. Density currents affect deep tanks as well as shallow tanks. In deep tanks, the input liquor cascades downward over a greater distance and may reach significant velocity. Tanks with 12 to 15 foot SWD commonly exhibit short-circuiting, as do those with less SWD. Density currents also occur irrespective of tank diameter, with short-circuiting common in both 25 foot and 200 foot diameter clarifiers. 4. Density currents tend to move along the top of the blanket, where MLSS are in the 2000 range, and carry the lighter solids to the outer wall and upward toward the weir. If there is insufficient distance between the bottom of the inlet feedwell and the top of the blanket, the currents may be sufficient to cause scouring of the tank. 5. The action of the Density Current Baffle is two-fold: 1) it directly impedes the current (and solids) flowing up the tank wall and diverts it back into the center of the tank, and 2) it produces a hydraulic barrier around the tip of the baffle which causes those currents in the vicinity of

15 the baffle to flow around and toward the tank center without ever reaching the wall. 6. The Stamford, or Inclined, Baffle offers several advantages over the McKinney, or Horizontal, Baffle: 1) because it is inclined away from the weir, the Stamford Baffle keeps the lighter solids at a greater distance from the weir overflow current, 2) the incline allows solids which build up on the upper surface of the baffle to slide off periodically... the horizontal baffle is known to collect solids and cause maintenance problems, 3) the inclined design is easier to retrofit in an existing tank than is a horizontal design. 7. Tanks with internal launders also require density current baffles. Although the bottom of the launder does act as part of the baffle, solids emerging from beneath the launder are easily drawn into the localized current of the overflow weir. A baffle cantilevered out and downward from the lower inboard comer of the launder will deflect solids away from the weir and back into the main clarification volume of the tank, just as in the case of a wall mounted baffle.

16 8. Baffle size varies as a function of tank diameter, depth and configuration. For inboard launders, we recommend a 24" to 36 long baffle mounted at the lower inboard corner of the launder and inclined at a 45 angle. Larger baffles may be required in larger tanks. For outboard launders and dual weir configurations, we recommend a baffle mounted to the tank wall at a 45 angle. The size of this baffle can range from 24" to as large as 68". 9. The proper depth of the baffle in the tank is a function of tank depth and the height of the blanket. Ideally, the baffle should be positioned such that the lowest point of the baffle will be in the clear zone above the blanket. This usually means that the baffle should be mounted three to four feet below the weir. This height can vary somewhat to avoid pipes, supports and other obstructions in the tank, but the bottom of the baffle should be positioned low enough to be effective, yet as far above the blanket as possible. It may be necessary to reduce the inclination angle of the baffle to obtain the desired horizontal projection and keep the baffle above the blanket. 10. At a minimum, the baffle must be designed to withstand the upward buoyancy force of the liquid in the tank. This force varies as a function of the density of the mixed liquor. Assuming that the liquid has the density of water, this force can be 3000 pounds or more under an eightfoot baffle section. Some form of venting is required to allow gas that can collect beneath the baffle to escape. These conditions require a rugged baffle that is firmly attached to the tank wall. Thin baffles, and those that

17 simply cantilever from the wall without benefit of a triangular support bracket, cannot withstand the forces at work in the tank. 11. Density current baffles are most effective when the clarifier is operating in the average to peak flow ranges. TSS reductions in the range of 25% to 35% are routinely reported under average flow conditions, and 40% to 50% improvements are not uncommon at peak flow. Clarifiers that are operated below design flow may not benefit from the use of a Density Current Baffle.

18 Density Currents In Activated Sludge Clarifiers here are 22,000 waste treatment plants in the United States, each processing over 1 MGD. Two-thirds approximately 15,000 are municipal plants, while the remainder are industrial. It has been estimated that some 40 percent of these plants are operating at or beyond permit levels. There are numerous reasons for this, including design problems, operating problems, and increased loading. Density Currents In activated sludge plants, we can attribute some of the problem to the presence of density currents in the clarifiers. We know that density currents form in all activated sludge clarifiers, regardless of configuration. We also know that these currents have a significant negative impact on clarifier performance. Density currents create short circuits, increase effluent solids and reduce hydraulic capacity. Figure 1 depicts a typical centerfeed well and external launders. Density currents form in this tank when the denser input from the feedwell plunges to the bottom of the tank and then moves across the bottom just above the blanket until it reaches the outer wall. It then travels up the tank wall toward the effluent launder. As it courses along the bottom of the tank, the density currents pick up lighter solids which have been deposited and carry them up the wall to the effluent, in effect short-circuiting the main clarification volume of the tank. The net effect is an increase in TSS and a decrease in the apparent hydraulic capacity of the tank. Density currents do not move in waves. Rather, their action is jet-like and random. They have been known to bounce off the bottom of the tank, and ricochet off the sides. Potential solutions to the density current problem include modifications to the feedwell where the problem appears to begin, placement of the weirs as far from these currents as possible, and the use of in-tank baffles as a physical barrier to these currents. Virtually every form of feedwell modification has been attempted over the years. While several have been successful in specific configurations, no one modification has proven transportable for general use in clarifier design. Numerous design standards recognize the effect of density currents and suggest that weirs be placed as far from them as possible. Here again, numerous configurations have been examined, including the use of duel inboard weirs located several feet from the tank wall. No single design has proven effective in and of itself in eliminating the effect of density currents, and some have even exacerbated the problem. In-Tank Baffles Earle Schaller In-tank baffles have proven to be the most effective defense against density currents. Baffles have reduced TSS by as much as 50 percent, restored hydraulic capacity, provided a significant reduction in BOD, and reduced the downstream costs of treatment in terms of chlorine and other chemicals. Three types of baffles have been employed and each has been successful to almost the same degree in managing the impact of these currents on clarifier performance. The ring baffle is a cylinder that sits within the clarifier and has a diameter approximately half that of the tank itself. If the OCTOBER 1995 It s Better To Be Baffled! Figure 1. Density Currents in an Activated Sludge Clarifier Figure 2. Clarifier with Inclined Stamford Baffle tank has a 100-foot diameter, the ring baffle has a 50-foot diameter. The baffle cylinder reaches from just above the blanket to just below the surface of liquid in the tank. The baffle is suspended from the clarifier mechanism and rotates with it. While ring baffles have proven effective in dissipating density currents, they are more complex and more expensive to implement than the other baffle types. Moreover, the ring baffle places additional load on the clarifier mechanism and imposes added maintenance complications. Since it is no more effective than the other baffles, it is not generally found in use. It should also be noted that the use of the ring baffle causes some problems with sludge removal balance where riser pipes are used. The horizontal baffle, sometimes known as the McKinney or Lincoln baffle, is formed by extending the bottom of the inboard launder eighteen to twenty-four inches into the tank. This baffle has proven effective in stopping the solids carried by the density currents from reaching the weir. It is generally poured in place during the initial construction of the tank and is not easily retrofitted. The negative aspect of the horizontal baffle is the fact that sludge collects on the upper horizontal surface and requires considerable maintenance. Note that the baffle extends inboard of the weir and does not stop at the inboard edge of the launder trough. This is important in ensuring that the solids carried by the density currents are redirected towards the central clarification volume of the tank and are not allowed to simply drift upward toward the weir. FLORIDA WATER RESOURCES JOURNAL 33

19 The inclined, or Stamford, baffle is simply the horizontal baffle inclined at an angle sufficient to allow built-up sludge to fall away. The Stamford baffle has proven extremely effective and is the most popular baffle in use today. The baffle can be fabricated from numerous materials and is ideally suited to both new and retrofit installations. In Figure 2, the inclined baffle is mounted directly to the tank wall and inclined at an angle of 30 to 60 degrees. The baffle is normally located approximately three feet below the level of the scum baffle. In tanks with internal launders, the baffle is cantilevered from the lower inboard corner of the launder trough. The Connecticut Experience The Stamford baffle was first implemented at the Stamford, CT WWTP. The Stamford plant supports a large urban population and is located on the shores of the Long Island Sound. Stamford had two 130-foot secondary clarifiers. The plant had never operated within permit levels and had a significant odor problem. Dye tests and other studies indicated that density currents were a significant contributing factor to the plant s problems. A prototype baffle was fabricated of plywood and was installed in one of the two tanks. This resulted in a 38 percent improvement in TSS. Within 60 days the plant was operating within permit levels. NEFCO, Inc. later replaced the wooden prototype with the first full scale fiberglass density current baffles which were installed in each of the two 130-foot clarifiers. Another baffle was installed in the new third clarifier, which was constructed in All the work carried out at the Stamford plant was done in conjunction with the Connecticut DEP, which called the baffle the most cost-effective improvement in clarifier performance today. Connecticut DEP lobbied the state legislature to institute a municipal grant program enabling municipalities to improve clarifier performance by installing baffles and other devices. Connecticut now requires that a baffle be installed as part of all new and upgrade projects. Other states are expected to follow this lead and mandate the installation of baffles. Connecticut now has more than fifteen plants equipped with density current baffles. According to the State DEP the baffles improve plant performance by 35 to 40 percent under average flow conditions and as much as 50 percent during peak flow. The Greater Lawrence plant in Lawrence, Massachusetts, installed density current baffles in all four of its 165-foot diameter secondary clarifiers in 1993 and experienced even greater improvements than those at Stamford. In mid-1994, the city of Phoenix, Arizona, installed baffles in four of ten 140-foot diameter primary clarifiers. This underscores the fact that density currents form in primary as well as secondary clarifiers and that the baffle is equally effective in dealing with these currents at the primary treatment level. Baffles are currently being installed in four of the 80-foot square tanks at the Calumet WWTP, which serves much of Chicago. Here in Florida, density current baffles are currently being installed at the Northside WWTP in Lakeland and are planned for several other locations. Design Considerations There is a relationship between tank diameter and the optimum width, D, of the baffle, where D is measured as the horizontal distance from the wall to the outer edge of the baffle. D generally ranges from 18 to 48 inches. The actual length of the baffle, of course, depends upon the inclination angle. In the case of an inboard launder configuration, the bottom of the launder trough contributes to the baffle action. Here, a smaller baffle can be used. Note, however, that this baffle extends out beyond the weir to ensure that solids are not simply allowed to drift upward. In the case of dual weirs, solids are often deposited in the space between the outer weir and the wall and then flow over the outer weir. Here the baffle is fastened directly to the tank wall as though in an external launder configuration. The baffle is typically described as a series of brackets attached to the wall at two-foot intervals and then covered with panels of aluminum, pvc, stainless steel, or fiberglass. The brackets themselves may be made of any of these materials. While this is a typical baffle configuration, it is expensive to fabricate and labor intensive to install. It involves considerable material handling, and alignment can be tedious and time-consuming. Proper fastening of the baffle is critical, however. The force of gasses trapped beneath the baffle, and the hydraulic pressure on the baffle when the tank is being filled can be sufficient to tear the baffle from the tank if it is not adequately secured. The Stamford Baffle is comprised of eight foot baffle modules. Each module is a one-piece unit which incorporates the baffle panel, bracket, and mounting and stiffening flanges. The module is molded of corrosion-resistant, uv protected fiberglass. The modules interlock to form a rigid structure, once installed. There are a number of criteria to consider when developing specifications for the baffle. It must fit the curvature of the tank with no large gaps between the baffle and the tank wall. The width of the baffle should be scaled to the tank diameter and range from 18 to 48 inches. Too small a baffle would be ineffective. Too large a baffle would interfere with the clarification action of the tank. The baffle should always extend beyond the weir. The baffle should be inclined at an angle of 30 to 60 degrees to facilitate self-cleaning of sludge. The baffle should be supported adequately to ensure that it is held rigidly in place and has a professional, wellengineered appearance. Provision should be made for venting gases and air that can become trapped beneath the baffle. To accomplish this, NEFCO molds vents directly into the mounting flanges. There are three materials generally considered appropriate for baffle construction: stainless steel, aluminum and fiberglass. Stainless steel is an expensive material for use in this application. Aluminum is subject to corrosion in many waste environments. Fiberglass is the ideal material: it can be molded to shape; it can withstand years of being submerged; and finally, it can be formulated to withstand the most corrosive environments, a factor which is often encountered in industrial waste. Earle Schaller is president of NEFCO, Incorporated, Palm Beach Gardens. 34 FLORIDA WATER RESOURCES JOURNAL OCTOBER 1995

20