Decentralizing wastewater treatment
Centralized wastewater management has been the norm in municipal engineering circles for more than 100 years. Based on the “Pipe it away first, then think about what comes next” philosophy, centralized management is the structure of choice in most cities and counties.
That may be changing. Small communities in Wisconsin have found conventional systems to be hugely expensive and have begun to investigate decentralized concepts. The decentralized concept is based on a simple premise: Wastewater should be treated (and reused, if possible) as close to where it is generated as is practical.
That philosophy allows local governments to circumvent one of the major disadvantages of the conventional, centralized management system — huge investments in an extensive collection system that does nothing more than move pollution from place to place. (The phrase “decentralized management” is used here, but it is somewhat of a misnomer, because, while facilities are decentralized, management may be handled by a central entity.)
For example, recognizing that conventional strategies are unaffordable in many of its client communities, the Midwest Assistance Program is creating workshops with the Missouri Department of Natural Resources to train engineers in decentralized management strategies. In Texas, planning studies conducted for the Texas Water Development Board have shown that decentralized systems are a far more cost-effective way to provide service to many colonias in the counties along the Mexican border.
In the Northeast, Gloucester, Mass., was faced with extending service to North Gloucester. Its engineers determined that using decentralized treatment methods would be a far more cost-effective solution than extending the city’s centralized system into the area.
Overcoming opposition Opposition to the concept of decentralized management is strong, primarily because operators, engineers and regulators are naturally reluctant to abandon the familiar. In Austin, Texas, however, education and experience are helping to overcome that reluctance.
Austin has determined that much of the new development in its Drinking Water Protection Zone will be served using decentralized concepts. Such strategies had been urged on the city for many years by local environmentalists, especially for wastewater service in the environmentally sensitive Texas Hill Country watersheds.
Those watersheds drain into Lake Austin, source of the city’s water supply, and recharge the Edwards Aquifer. The aquifer, the drinking water source for 40,000 or so suburban residents, discharges at Barton Springs, a natural swimming pool that provides a cool oasis in the heart of Austin. Both the pool and the aquifer provide habitat for endangered species. Spring discharge also feeds into the city’s drinking water supply. Extending or creating centralized wastewater systems in those watersheds created concern about pollution — directly from leaking mains and lift station malfunctions and indirectly because of the increased density typically engendered by conventional sewer systems.
Austin began looking into decentralization in 1993. Because of environmental concerns, the city council directed the Water & Wastewater Utility to investigate options to centralized sewer service in the watersheds contributing to Barton Springs. That directive led to development of the Decentralized Wastewater Management Project in 1994. Based in large part on the findings of that project, the city determined that it would greatly limit and perhaps forego future extensions of its centralized collection system into the sensitive watersheds, instead employing the decentralized concept.
“Decentralized management is not our experience,” says Randy Goss, director of the utility. “We’re not the experts in that. We’re on a learning curve. There are questions about the legal and regulatory aspects of this approach, especially as it regards public and private ownership issues. It’s not that those can’t be overcome, but they require that we generate good policies and procedures to ensure that the issues are addressed. Also, we must be careful to assure that we are providing protection of public health and the environment equal to that provided by our traditional systems.”
That statement highlights both the promise and the problem of moving to decentralized management concepts. Its proponents argue that decentralized management is inherently more friendly to public health and the environment than conventional management strategies.
Environmental advantages Centralization causes large flows to be concentrated through one pipe, lift station or treatment plant. Any mishap, therefore, could have extensive consequences.
Recently, for example, a lift station in Austin’s treatment system failed. The failure went unnoticed for several hours, during which raw sewage was dumped into a creek that fed into the Edwards Aquifer’s recharge zone. Resulting cryptosporidium contamination sickened as many as 1,300 residents.
In a decentralized concept system, flows at any point remain small, so an accident would cause less environmental damage or threat to public health.
Bypasses, leaks and overflows are far less likely to occur in a decentralized system because more fail-safe treatment technologies are employed, and lift stations are reduced in number or eliminated altogether. Additionally, septic tanks intercept solids at or near the point of wastewater generation.
The decentralized collection system consists of small diameter effluent sewers containing no openings for overflows. Since the collection system feeds into multiple treatment centers, it consists of short runs of small pipes, reducing the potential for infiltration and exfiltration.
Additionally, system construction causes little environmental disturbance. The small collection system pipes are installed at shallow depths and thus can be routed more flexibly than can large pipes. Large interceptor mains, which typically run in creek bottoms, are unnecessary, so riparian environments are not torn up to install, repair or upgrade sewer lines.
Environmental disturbance is eliminated over the long term as well because existing lines do not need to be torn up to upgrade capacity. System expansion instead involves adding new treatment centers rather than routing more flow from farther and farther away to existing centers.
The management angle As with any wastewater treatment system, management is a key component in efforts to ensure that the benefits of decentralized treatment are realized. That means that the system must be designed using technologies amenable to small-scale deployment and that management arrangements be well-planned. As with centralized systems, management styles may be varied and are largely a matter of local preference.
In Austin, for example, Craig Bell, the utility’s manager of integrated water resources planning, echoes Goss’s proceed-slowly approach. “We’re interested in directly managing cluster systems,” he says. “We want to encourage cluster systems because we think collective facilities could be better and more readily managed. Having the city provide operations and maintenance should be a carrot for the developer to use cluster systems.”
Cris Guzman, supervising engineer for Austin’s decentralized wastewater management project, adds, “We’re going to leave individual on-site systems in the hands of the local health department for the time being. We think that trying to own, operate or maintain on-lot facilities is more than we want to tackle right off the bat.”
That is possible with the decentralized concept because it is not a one-size-fits-all approach. Whichever system best suits the characteristics of a given development can be employed. Some neighborhoods, for instance, might employ on-lot systems, while others might use a collective (cluster) treatment system for a group of lots, a commercial center or a subdivision.
Austin officials point out that the real challenge in creating a workable decentralized system is coming up with a management system that effectively addresses deployment of the collection and treatment hardware. “We’re looking at all kinds of options,” Guzman says. “We could train city crews to do everything, we could contract with private firms to do it all, or we could do some of both.”
Ensuring safe disposal As with any wastewater system, safe disposal procedures are paramount in decentralized facilities. A number of factors, including topography, soil conditions, development density (existing or desired) and type of land use, help determine the most practical scale for the treatment and disposal system. In places like Austin, where long-term water supply availability is a critical issue, whether and how the effluent can be reused is another critical issue.
In fact, with a decentralized system, reuse could become far more cost-effective because effluent is available throughout the service area. That availability vastly decreases the cost of reclaimed water distribution. With appropriate safeguards, reclaimed water can be used for nonpotable demands such as landscape irrigation, toilet flush supply and cooling tower makeup.
Because of the fact that they are spread out, decentralized facilities must ensure safe disposal methods for their effluent. (The same problem that occurs at one central facility would be magnified were it to occur at two, three or four facilities at the same time.) Consequently, decentralized facilities must use technologies that can be effectively managed when deployed on a small scale.
Decentralized management would be an operations and maintenance nightmare if it were executed using the activated sludge package plants that are mainstream technology. The inherent instability of that process means that it must be watched carefully — a much easier proposition in one central plant. The activated sludge process involves few “trophic” levels of organisms. (A trophic level is a step in the food chain. In other words, if Organism A feeds on Organism B, Organism A is up one trophic level.)
The few trophic levels of organisms involved in the activated sludge process live in concentrations far higher than would be found in nature. Because it is “unnatural,” the activated sludge process is inherently unstable. That instability, easily controlled by vigilant observation in a centralized facility, would pose problems for management that is responsible for several smaller facilities.
In centralized plants that use the activated sludge process, any problem, accident, equipment failure, large flow variation or untimely maintenance can lead to a process breakdown in fairly short order. Because the process provides no physical barrier to its passage, poorly treated effluent is released when that happens. Once the process goes off-track, it usually takes hours, and often days, for it to re-establish itself.
In a decentralized facility, use of intermittent sand filters, which can upgrade septic tank effluent to advanced secondary or even tertiary levels, would be appropriate. A sand filter is “like a refrigerator; you don’t worry about what it’s doing day to day. It just runs,” says Harold Ball, an engineer with Sutherlin, Ore.-based Orenco Systems, which manufactures components for on-site, small-scale wastewater systems.
Many trophic levels of organisms live on and in a sand filter bed, and the media grains provide a huge surface area, so the process is inherently more stable than an activated sludge process. Clogging of the filter bed occurs gradually, allowing an operator great latitude in responding to incipient failure before it reaches the point at which treatment may degrade significantly.
When properly designed and operated, a filter bed can operate for several years before maintenance is required. When that time comes, the filter bed can be restored within a few hours.
Saving money Decentralized concept systems can provide cost savings by addressing development when, where and as it occurs. In contrast, centralized collection systems and the treatment plants they feed typically are planned to serve 20 years of growth. Even some of the neighborhood portions of the collection system generally are built well in advance of a real need for service, since developers must usually agree to complete all utilities as part of the development approval process.
The Austin area may be on the verge of becoming a hotbed of decentralized concept development. Taking the city’s lead, officials representing a large development have proposed using an array of cluster systems to route effluent for irrigation reuse to maintain common areas. Additionally, the suburban city of Rollingwood, which had been negotiating with Austin for centralized sewer service, has recently changed course and expects to maintain the majority of its area on a decentralized system.
The technology shows promise, but getting city and county officials to turn their backs on years of tradition will not be easy. However, as more local governments take a hard look at the idea, the decentralized concept may begin to make financial, operational and environmental sense.
The success of the ventures currently under way depends on those cities working through the legal and regulatory issues, formulating the policies and procedures, and becoming conversant with the technologies that support the decentralized concept. That success will help determine the future of decentralization.
David Venhuizen is an Austin, Texas-based professional engineer. His practice focuses exclusively on decentralized wastewater system planning, design and management.
Ron Leitao, acting director of public works for Pawtucket, R.I., was facing what was arguably the most challenging problem in the city’s 112-year history. A video had shown significant cracks running along the crown of the 24-inch sewer line that runs directly below the highway. And the city had already spent hundreds of thousands of dollars to excavate the concrete road base and repair breaks in the aging clay pipe.
“The pipe had become so badly deformed that its vertical diameter had shrunk to just 18 inches in some places,” says Richard Chiodini, chief engineer for Providence, R.I.-based Siegmund & Associates, which was handling the repair job for the city. To further complicate matters, inspectors found that a 15-inch plastic pipe, placed inside one section of the deformed sewer in an earlier repair effort, also was cracked.
“It looked like we had two choices, neither one good,” Leitao says. “We could dig up the road and replace the sewer, at a high cost and with major disruption to traffic in this busy residential and commercial area. Or we could face a future of very expensive repairs.”
The engineers had another idea. “We concluded that the 120-foot length of 15-inch plastic pipe would need to be repaired using open cut construction,” says Chiodini. “For the remainder of the damaged pipe, however, we recommended a cured-in-place repair method that could be accomplished without digging up the four-lane road and disrupting traffic in the area.”
On a per-foot basis, the low bid for the open cut repairs was more than 13 times that of the low bid for cured-in-place repairs because it included the cost of replacing the roadway.
For the cured-in-place repairs, the city decided on a process invented by Memphis, Tenn.-based Insituform. In the process, a customized felt tube is impregnated with a liquid thermosetting resin. The tube is then inverted (turned inside out) in the deteriorated pipe from an existing manhole. Water pressure propels the tube through the sewer so that it can navigate bends and offsets along the way.
Hot water is then circulated through the tube, curing the resin and forming a jointless and corrosion-resistant pipe. Later, a robotically controlled cutting device is used to restore service laterals. The entire process is usually completed in a day.
City officials had never used the cured-in-place method, so there was some initial skepticism about how successful lining an 800-foot section of pipe would be. However, the project was completed in several segments over a seven-day period, with little interruption of business. (The two center lanes of the four-lane road were closed temporarily, with one lane kept open in each direction at all times.)
Video inspections of the finished product confirmed the project’s success. “For a very affordable cost, we essentially got a brand-new sewer and eliminated the danger of a collapse that would have cost dramatically more to repair,” Leitao says.
Pawtucket officials were so pleased that they doubled the size of the original project to 1,600 feet. Plans are under way to use the process to reline a sewer that runs beneath McCoy Stadium, home of the Pawtucket Red Sox baseball team.
The 1994 National Water Quality Inventory report to Congress stated that storm sewer runoff caused about 46 percent of major water quality impairment. As a result, EPA proposed Phase II of its storm water regulatory program. Now EPA, acting on the recommendations of an advisory committee, is working to combine Phases I and II into one stormwater regulatory program.
As a result of the unified program, all communities of 50,000 people or more, or communities with 1,000 people or more per square mile, will be required to implement public awareness campaigns to prevent dumping of oil, paint and other pollutants into municipal separate storm sewer systems. Nearly 3,500 communities will be affected along with many state and federal facilities, universities and correctional institutions.
The new set of rules will require a program to reduce discharges to the “maximum extent practical” in order to protect water quality. To comply with those regulations, many communities are attempting to prevent contamination of storm water systems by putting warning messages at each drain. That is most often accomplished by marking the curb or the drain frame.
Stenciling currently is the most common method of curb marking, but it presents problems even as it helps reduce contamination. For example, paint can smear, making the warning message difficult to read. And stenciling can even contribute to pollution because, as the paint flakes off over time, it drops into the storm drain.
In fact, one Minnesota publication on storm drain stenciling instructs volunteers to: * avoid dripping or spilling paint into the drain; * check with the county on how to dispose of paint cans in order to comply with hazardous waste disposal regulations; * beware of exposure to fumes; and * avoid using aerosol paint on windy days.
Most of the other warning methods involve fastening or taping preprinted aluminum, ceramic or plastic markers near the drain. Glue-down markers range in size from 2 1/2-inch diameter circles to large rectangular markers of virtually any size.
In new-construction situations, some communities stamp the message into the sidewalk. That creates a permanent message, but it often is difficult to read as the area fills with dirt and debris.
In evaluating the various options, city officials should answer a number of questions, including: * Will the warning message be easy to spot and read for many years? * Can the warning message be scratched off the marker surface by foot traffic, vehicles or street sweepers? * Will the adhesive permanently bond to the curb or inlet? * Can the base material be pulled or bent away from the curb and become a potential hazard that could trip pedestrians? * Are the markers easy to install (potentially by volunteers)? and * How long will the marker material and message last?
Installation procedures vary dramatically from product to product. One instruction booklet for stenciling estimates that a crew of six students and one adult can paint three to four curbs per hour (not including travel time).
At the other end of the spectrum, a das Curb Marker can be installed by one person in one to two minutes. (The marker has been used since 1983 to mark buried cables, pipelines and valves, and it has been used to mark storm drains for several years.) Where no curbs or paved surfaces exist, markers may be glued directly to the metal inlet if there is room. If there is not enough area to glue or stencil a warning message to the metal inlet, a marker post may be installed behind the storm drain.
A product with a message that cannot be scratched off allows for more precise lettering on the marker and can include more dramatic graphics. Using bright colors and precise printing also will permit use of a smaller marker. If a combination of stencils, glue-down markers and posts are used, the graphics and message should be uniform.
This article was written by Scott Landes, president of Repnet, a Bloomington, Minn.-based company that manufactures marking products.
Sturgeon Bay, Wis., is located on Wisconsin’s spectacular Door County Peninsula, which has a very shallow and fractured bedrock. That bedrock represented a costly barrier for Sturgeon Bay Utilities, an electric, water and wastewater utility with 4,500 piped-in wastewater customers (all within the Sturgeon Bay city limits) and 8,000 electric customers.
The utility not only processes wastewater from its customers, but also accepts about 1.5 million gallons annually of trucked-in wastewater and biosolids from throughout the peninsula. It was using a single-stage mesophilic anaerobic digestion process for sludge stabilization. The digested biosolids (Class B) were either applied to land as liquid or after being dewatered. The shallow bedrock made it difficult to find suitable land application site for Class B sludge, which requires a minimum 36-inch depth to bedrock.
In 1996, Sturgeon Bay applied biosolids on 230 acres of farmland at a total application cost of $97,475. However, in Wisconsin and other areas where winter turns the ground to ice, winter application is not allowed in order to prevent environmental problems caused by spring run-off.
In response to that concern, the Wisconsin Department of Natural Resources (DNR) issued a requirement that wastewater plants using land application had to have a biosolids storage capacity of 180 days by Oct. 1, 1998. However, Sturgeon Bay’s existing plant storage capacity for dewatered biosolids stood at 30 days. “We were faced with spending $300,000 for a storage building plus associated operational costs over the next 20 years,” says Utility Manager Scott Adams. On-site storage also is a problem because several residences and a marina are located within 500 feet of the treatment facility, so it was necessary to address concerns about odors and the problems of public perception.
The utility hired Neenah, Wis.-based McMahon Associates, an engineering firm, to perform an evaluation of sludge storage options. Adams had heard about temperature-phased anaerobic digestion (TPAD) and asked the firm to evaluate it in their study.
TPAD uses two anaerobic digesters operating in series, the first at a thermophilic temperature of 131 degrees F and the second at a mesophilic temperature of 95 degrees F. Reported advantages of TPAD, based on laboratory tests performed on a 50-50 mixture of primary and secondary sludge, include: * Class A pathogen reduction; * an 18 percent increase in volatile solids reduction with an accompanying increase in gas production; * reduced foaming; and * comparable odors to mesophilic digestion.
Additionally, TPAD can achieve the same volatile solids reduction in half the retention time of single-stage mesophilic digestion.
The 180-day storage requirement only applies to those who recycle sludge through land application or site reclamation projects. If Sturgeon Bay could produce an “Exceptional Quality” Class A biosolid through TPAD, the utility would not longer have to provide that storage. Those who accept the sludge would provide their own storage.
A survey conducted by the utility suggested that adequate demand existed for Class A biosolids on the part of nurseries, landscapers and residents. So Sturgeon Bay became the second utility in the nation to adopt the TPAD concept. (A utility in Newton, Iowa, was the first.)
The existing digestion process at Sturgeon Bay Utilities consisted of a primary digester maintained at 98 degrees F followed by an unmixed, unheated secondary digester. Each is 50 feet in diameter, has a 24-foot sidewater depth and holds 375,000 gallons of biosolids. Sturgeon Bay facility operator, Midwest Contract Operations, performed nearly all of the refitting tasks, including electrical, plumbing and control process adaptations. Only the stainless steel work inside the secondary digester was contracted out.
To convert the existing system to TPAD, the primary digester was designated as the thermophilic digester, and the secondary digester was designated as the mesophilic digester. A nozzle mixing system and external heat exchanger were installed to mix and heat the contents of the mesophilic digester.
A sludge-to-sludge heat exchanger also was installed to recover heat from the thermophilic biosolids. Recovery eliminates the need for cooling the mesophilic digester to 98 degrees F and reduces the energy required to maintain the thermophilic digester temperatures.
The state DNR has approved the TPAD process at Sturgeon Bay for achieving a Class A pathogen reduction. Now, Adams says, “We are looking for ways to enhance the value of the Class A sludge by mixing it with topsoil. We have done some preliminary work and will give it another trial sometime in the near future. It is a business development project — to build the market.”
By distributing the Class A biosolids, Sturgeon Bay has eliminated the need for building 180 days of storage capacity. Additionally, the city now has available to it land application sites that were once restricted because of shallow bedrock.
The utility calculates a payback of two years or less on its investment. “The project came in comfortably under budget,” Adams says.
(The TPAD conversion project won the 1998 Grand Prize for Excellence in Environmental Engineering (Small Projects) from the American Academy of Environmental Engineers, Washington, D.C.)
This article was written by Howard Hinterthuer, a Cedarburg, Wis.-based freelance writer specializing in design-, engineering- and construction-related topics.
In communities with combined sewer overflows (CSOs), the clarifier often is identified as the weakest link in the chain of treatment processes. However, since no two plants are alike, optimizing clarifier performance is not always easy.
The primary treatment, the effect of rag and grease removal, and the influent pumping system all affect clarifier performance. Aeration systems also can have an effect on settling in terms of floc formation and heat transfer.
Perhaps the most important consideration in determining a clarifier’s capacity is the even distribution of flow to each clarifier. Most systems that feed multiple clarifiers from the same influent channel often do not work well, especially when a single clarifier is out of service. An up-flow, overflow distribution box, if properly sized, will provide the best flow distribution.
Critical factors A side-water depth of less than 12 feet often is cited as a performance-limiting factor; however, depth, in itself, usually is not critical. The currents that develop within any clarifier are much more important. In fact, one 10-foot clarifier that was tested by the American Society of Civil Engineers’ Clarifier Research Testing Committee out-performed deeper clarifiers, producing an acceptable effluent at overflow rates exceeding 2,000 gallons per square foot per day. Additionally, many people believe circular clarifiers to be better than rectangular clarifiers. Again, however, the ASCE-CRTC showed that rectangular clarifiers, including the shallow clarifier mentioned above, can out-perform circular clarifiers.
The inlets and outlets are the most important design factors to consider. The inlet should be designed to distribute the flow evenly and to enhance flocculation. In rectangular clarifiers, an influent port similar to that used by most Los Angeles County and city plants can help accomplish those objectives.
In circular clarifiers, many designs have incorporated an energy-dissipating inlet (EDI) device. However, although EDIs may enhance flocculation and evenly distribute the flow, they have been shown, in at least one instance, to reduce clarifier performance.
Newer clarifier designs feature larger and deeper centerwells, although studies have shown them to be largely ineffective when they exceed 0.25 times the clarifier diameter. While the centerwell depth is a more elusive factor, it appears that 0.5 times the clarifier depth is a good size.
Clarifier effluent launders probably are the most critical components in enhancing or diminishing clarifier performance. In circular clarifiers, they should be placed far enough from the sidewalls to minimize the effect of the density current, and they should be shallow enough to avoid interfering with the outward flow. In rectangular clarifiers, the best alignment is parallel to the flow, covering at least 30 percent of the tank length.
Optimizing an existing clarifier Several actions can improve the performance of existing clarifiers. Those actions include: controlling the flows to and from the clarifiers, controlling sludge blanket depths, removing scum and algae, and reducing the effect of the density currents.
Good flow measurement is necessary to control both influent flow and return sludge flow. Measuring the individual clarifier effluent flows often is easy to accomplish with simple weirs or flumes, while the return activated sludge flows usually can be measured accurately with some type of strap-on Doppler-principle flow meter.
Controlling the sludge blankets requires effective activated sludge process control, ranging from reducing mixed liquor levels to taking advantage of step-feed or contact-stabilization modes of operation. It also requires dedicated operator attention to field conditions through consistent manual or automatic sludge blanket level monitoring. Effective removal of surface scum is easiest with a rectangular clarifier scraper/skimmer mechanism. In a circular clarifier, scum removal can be accomplished most effectively by using a job-built or manufactured scum baffle.
Algae control in circular clarifiers often is quite demanding, although the algae brush devices as well as job-built chlorine solution feed systems have practically eliminated the need for additional labor. However, there are no known automated methods for controlling the algae growth on rectangular clarifiers.
Reducing the effect of the density currents is much more challenging and generally requires a thorough clarifier evaluation that includes locating and measuring the various currents in the clarifiers. Depending on the type of sludge collection device, some variation of an in-tank baffle can help reduce the currents in rectangular
clarifiers. In circular clarifiers, two types of perimeter baffles are available: a horizontal shelf baffle and a Crosby sloped baffle. The latter baffle is generally sloped at 45 degrees from the horizontal, although flatter and steeper versions also have been used.
The horizontal shelf baffle and the Crosby sloped baffle are both effective in re-directing much of the current that flows upward along the perimeter wall of a rectangular clarifier. However, neither reduces the density currents. Moreover, the horizontal shelf baffle always accumulates solids deposits on its top surface. The sludge from those deposits then becomes anaerobic and floats to the surface at the effluent weir.
Certain types of circular clarifiers have responded well to a Crosby cylindrical baffle, which has proven effective in plants in Franklin, N.H., and Atlanta.
Optimizing clarifier performance is an activity that involves design, operations and management, and even administrative activities. Although there are several standard, off-the-shelf solutions available, the unique performance characteristics of the different clarifiers at wastewater treatment plants generally require solutions that are tailored to a plant’s unique needs. The benefit of a thorough clarifier evaluation followed by specific operational and design modifications often can increase the capacity of a secondary clarifier system significantly.
For more information, see “Retrofitting POTWs” (EPA/625/6-89/020, July 1989) or “Hydraulic Considerations That Affect Secondary Clarifier Performance” (EPA Technology Transfer document, March 1980.)
This article was written by John Esler, president of CPE Services, an Albany, N.Y., engineering firm. He may be contacted at [email protected]
Changing pumps has helped Philadelphia solve a clogging problem in its water pollution control plants. The installation of recessed impeller pumps has reduced plant downtime and eased maintenance.
Philadelphia’s Northeast, Southwest and Southeast Water Pollution Control Plants have a combined average throughput of 530 mgd of storm and sanitary flows. Raw sewage coming into the plants is screened to remove trash that is especially prevalent during storms. Sewage then flows to grit tanks, primary sedimentation tanks, aeration tanks and final sedimentation tanks.
Sludge collects at the bottom of the primary and final sedimentation tanks, while clear effluent is removed from the top. The sludge is then pumped into anaerobic digesters.
Previously, non-clog pumps were used to circulate sludge in the digesters. However, “mopheads,” masses of stringy material that accumulate in the digesters, caused frequent clogging of the pumps. Mechanics were forced to shut the pumps down, disassemble them and clean the casing, an unpleasant job that took several hours. Consequently, maintaining the pumps was practically a full-time task.
To stop the clogging, plant engineers recommended switching to recessed impeller pumps. One Torus series recessed impeller pump from Hayward Gordon, Buffalo, N.Y., was installed at each plant for a one-year trial.
The new pumps include a semi-open impeller recessed into the rear section of the volute and out of the path of the solids being pumped.
The impeller uses about 15 percent of the fluid being pumped to create a torus or vortex that adds energy to the pump’s fluid and solids, very similar in action to one-half of a fluid coupling.
Following the trial period, plant managers decided to replace all the existing digester non-clog pumps in both plants with the new pumps. The plants use the belt-driven — as opposed to direct-drive — models, and the digesters can now readily accommodate different concentrations of solids by switching to different pulleys and belts.