|
The limits of geography can be a problem when it comes to wastewater. Here’s how one company came up with a solution.
It’s hard to forget the nickname of our smallest state, Rhode Island. But “Little Rhody” also indicates the trouble the state has when it comes to dealing with wastewater: a lack of space to treat or disperse effluent. As a result, those with homes and businesses within the state often find themselves in the middle of collisions between zoning and environmental issues. Such was the case with a development near the coast in Rhode Island.
 |
| The Xerxes Corp. often provides Walden's treatment tanks. |
A shopping center in Wickford Junction, RI, had a 63-acre parcel on which the flow and permeability of the groundwater had to be determined as well as the speed at which the nitrates would pass through the property.
This community designated nearly the whole area as a sole-source aquifer. When this was done, the nitrate maximum was reduced to 5 ppm from the EPA maximum of 10 ppm.
“As a result, being here in the Northeast, we have a lot of nitrates arriving simply through migration of birds and acid rain,” says Bob Cioe, owner and developer of Wickford Junction Shopping Center. “That made it very difficult for us to achieve such levels. “Because of the various factors involved, without the treatment system we chose, we would have been limited to about 100,000 square feet of commercial space. We ended up with 28,000 gallons per day, and we’re able to go up to 300,000 square feet of commercial shopping space now.”
The shopping center contains a 125,000-square-foot discount store, (including an in-store sub shop), a 24,000-square-foot office supply retailer and a 24,000-square-foot multi-tenant building with a restaurant, deli, medical office, medical building, dentist, physical therapist, jeweler, hairdresser, and real estate office. Even with this diversity, the automated wastewater system at Wickford Junction Shopping Center has worked well, according to Cioe.
Designed by Walden Inc., of Waterville, ME, the shopping center’s system includes a 12,000-gallon fiberglass equalization tank. Air pressure introduced to this does a good job of breaking up the solids. Four 10,000-gallon treatment tanks operate in series. A settling process occurs and then the top 12 inches of water is pumped into dosing chambers with a capacity of 11,000 gallons.
This allows the system to be flushed equally throughout a leach system measuring 345 feet in length. The leachate is virtually odorless and colorless, according to Cioe.
Four-by-8-by-18-inch flow diffusers with one foot of stone around them permit the final water released to enter the ground. Each distribution box handles a group of flow diffusers fed by the dosing chambers.
“If we didn’t have the dosing chambers, you’d have all the water starting with the distribution boxes filling up the system, and the liquid would never get down to the other end,” says Cioe. “Rather than going through the distribution boxes, we go through dosing chambers and it equalizes the water throughout the system. When it reaches its capacity, it goes through the associated flow diffusers.”
All of the tanks are buried underground in a 300-foot-wide area where the power company was granted an easement to the property. The leach field is also in that area. The control building has an 8-by-10 footprint and the entire system has a footprint of approximately 40-by-80-by-100 feet.
The motors are above ground or can be lifted out through the top, so there are no enclosed areas where work must be done. A control shed contains the power, the exerciser for the emergency power, the control panel, and three blowers pumping compressed air to the individual tanks.
“Pumps and pump stations were initially checked weekly, and water is now tested quarterly,” says Cioe. “For the first year and one half of operation, the water was tested monthly; but now, after almost 10 years of operation, that’s unnecessary.”
Once or twice all treatment was lost in this SBR. This occurred when the anchor department store stripped the floors with a chemical wax, which, instead of manifesting out, was drained into the system. “This killed all the bugs in our system,” says Cioe. “But within three weeks it was up again and running, and new bugs did not have to be introduced. Those that entered naturally were able to do the work.”
With a modem and telephone line in the system, if there is a problem that cannot be solved locally, Cioe calls up Walden and adjustments may be done by computer from Maine. “But that’s only happened once to us,” says Cioe.
“Other than routine replacement of pumps, we’ve had no problems with the Walden SBR system.
 |
| A combination of blower enclosures, locking covers, and a "dead front" MEMA panel allows for placement in public areas without fear of vandalism. |
Rebirth For An Old Technology
Sequencing batch reactor technology dates back to around the early 1900s, according Bob Johnson, president and owner of Walden, which specializes in SBR construction know-how for customers worldwide. Johnson thinks the SBR idea started in the UK but then fell out of favor due mainly to the cost of labor.
“Someone used to sit waiting for time to go by so they could shut the air off, let effluent settle, and open the valve to decant it,” says Johnson. “After being out of favor a long while, the systems returned on a limited basis from time to time; but the labor intensiveness kept them out of favor. But the advent of the programmable logic controls (PLC), allowed SBR technology to really take off.”
PLCs are found everywhere from entry and car number tracking systems at the entrance to parking garages, to manufacturing processes in the food industry, to stoplights and automotive robotics. PLCs became a natural fit for SBR technology, relieving it of labor intensive aspects.
Different Drummer
With a 25-year background in contract and municipal wastewater treatment work, Johnson started Walden Inc. in the late 1980s. The name honors author Henry David Thoreau for his independent streak and for time spent in Maine.
SBR technology comes in handy in so-called “mixed-use” applications, including such locales as marinas, which have showers, toilets, and incidental water use on the land side, as well as vessels arriving from the water side to empty their holding tanks.
“How do you size those two different types of wastes?” asks Johnson. “What would be the best fit for them? We attack a problem in that way.”
Many engineering firms that do design work on housing developments have planning expertise, Johnson finds, but not as much background in wastewater management. “Likewise, those who work with municipal plants do not get involved with housing developments, so the knowledge base for the engineering firms comes from the manufacturer rather than their own design,” he says. “I came at things as if I were the end user; if I’m the maintenance guy, what would bother me the most and what would make my life easier?”
What evolved for Walden was a movement away from both galvanized pipes and steel tanks. Johnson wondered: Why not fiberglass instead of steel? Fiberglass, after all, is used for fuel tanks.
Johnson believes the main thing is finding the right equipment. He started his business subcontracting everything out. As the company grew, Johnson began to build his own fiberglass SBR systems, including his own molds and panel work. Now he builds the small proprietary parts and pieces and ship those to the tank manufacturers in five locations across the US.
Those are sent with a set of drawings. Johnson’s panel person in southern Maine is given the set of specs, and then Johnson builds some of the pumps and risers himself. “Essentially we have a 12-14 week lead time, all the parts and pieces show up at the site from four or five different places, and in a matter of days they have the assembly instructions for the final assembly and away they go.”
Perfecting The Union Of Parts And People
For their PLC systems, Walden uses AeroMat/Panasonic touch-screen systems, because the software worked well in writing a program easily accessible to the Web. Some utilities already have a SCATA system for pump stations or power plants, and Walden is often contacted regarding the ability to interface with them or send alarms out to pagers if there’s a problem.
“That is the beauty of the PLC,” says Johnson. “For example, currently we’re working on a project involving a drip-irrigation effluent disposal. This effluent must consequently be super-clean so it doesn’t plug up the emitters on the piping. On that particular system we’re placing sand filters on the effluent to provide failsafe filtration in case there is ever biological upset—a dumping of drain cleaner, for instance—where the process is killed. We’ve got things covered by knowing when to backwash that filter when it gets dirty.
“By writing in a few more lines of code, we’ll tell the setup: if the pressure on the backup sand filters has reached a threshold number and psi, then at the end of the next discharge cycle, the pump is to be turned on to backwash the filters, while the process is busy doing something else. This customer also requested a tie-in to a dial-out system to inform the operator exactly what’s going on. Everything is monitored all the time.”
Walden has found considerable talent in rural Maine. Soon after starting his business, when Johnson was realizing the limits to his own knowledge and abilities for writing PLC code, he met Chris Clark through one of his suppliers. Clark, who is president of Custom Controls Co. in Cumberland, ME, took Johnson’s 80 pages of code, threw them out, and then worked with Johnson—who simply stated exactly what he needed—to create some 15-20 pages of much more stable code for running his PLC system.
Clark has a sub-specialty in radios, including a recent huge project in Spain. Clark has a gift for translating through ladder logic code, exactly what Johnson needs to have happen with his SBR systems. Clark also builds the control panels for Walden using IEC, international standards for such electrical systems.
“When one of my panels goes out—no matter where in the world that might be—if I have an IEC contact for 30 amps, it doesn’t matter who the manufacturer is: Any 30-amp contactor will fit into the system into the same space,” says Johnson. “I’ve tried to think of everything so that if something happened to me things would still keep running even when it comes to replacement parts anywhere in the world.”
 |
| The use of IEC components allows for direct replacement of parts from any manufacturer's line. |
Tanks For A New Use
On its systems, Walden uses pumps made by Goulds Pumps, of Seneca Falls, NY. The blowers on which the pumps rely are manufactured by M-D Pneumatics, whose parent company is the Burr Ridge, IL-based Tuthill Corp. Minnesota-based Xerxes Corp., primarily a manufacturer of fuel tanks, provides Walden with water treatment tanks. Xerxes is used to dealing with stringent requirements when it comes to fit and finish, according to Johnson. Walden ended up using a tank smooth-walled inside for easy cleanup and one completely enclosed so that odors can be controlled at the point of exit.
Johnson has also made various pieces of molds for the aeration manifold supports. Anything coming in contact with the water is fiberglass, PVC, or stainless steel. “In theory, other than doing routine maintenance, there should be no reason for taking out a tank,” says Johnson. “You get grit building up no matter how tight your system is. But my experience has been that that is so insignificant and the tank only needs to be taken down if you absolutely have to.”
Putting The System Together
Walden tanks typically are from 8 to 10 feet in diameter, with the longest being 65 feet. Tanks measuring 12 feet in diameter can be used, if necessary. A 10-foot tank handles 35,000 gallons. An 8-foot tank contains 15,000 to 20,000 gallons, while 12-foot tanks hold 50,000 gallons. Workers are able to go inside tanks to clean and service them as needed.
On a Walden project, there are typically two tanks. One is an equalization tank capable of holding 40%-50% of the daily flow. The fresh effluent enters this first tank at a variable rate. A transfer pump fills the tank until it reaches the batch elevation float, and the SBR shuts off the pump. This is the beginning of the process.
During that time, if tank two is ready to begin accepting effluent, that will begin as well. They’ll always be out of phase on sequence as they should be, but in the event that both tanks are processing, the equalization is holding flow until the next tank is ready to take.
“What this gives you is perfect control and repeatability with every batch,” says Johnson. “You are processing the same amount, and it’s taking you the same amount of time to do it, and you have perfect quiescent conditions in which to withdraw your liquid.”
In contrast, Johnson says, with a standard small flow-through plant, many of these are visited weekly or every other week by a contract operator, who has to visit the site and decide what has occurred and what needs to be changed in the process.
“If that operator guesses wrong or adjusts wrong and the process goes out of balance, it’s out of balance until the next time he comes back,” says Johnson. “Then the guess has to be made again and adjustments must be made and he has to hope he interpreted properly.
“With waste treatment, if you start losing solids, effectively what you’ve done is decreased your sludge age, decreasing the quality of the settleability and you continue to lose solids. It’s a constant solid loss; your treatment goes down, along with effluent quality, and it takes a long time to bring all of that back.
“Whereas with a batch reactor—designed on a 288-minute cycle—giving you five cycles per day per tank, which is adjustable, what you end up with is going through the steps of loading the tank, aerating the tank, settling the tank and finally decanting.”
During the settling period there is no outside influence or anything to disrupt the settleability of the contents of the tank. Depending on the quality of the sludge, the settling time can be increased. Many of the common issues can be controlled.
SBRs are relatively unaffected by foam overflow, because the suction for the decant is always 6–8 inches below the liquid level. Fluid is always being drawn from the clearest zone.
“Xerxes has been very accommodating in making internal modifications for my design,” says Johnson. If a project requires the tank to be double-walled, this requirement is not a problem for them.
Johnson feels what makes Walden exceptional is its variety of components. And the company tries to make its systems as easy as possible to maintain.
Along with this, they’ve tried to make it as unobtrusive at a jobsite; they are almost completely buried underground. For aeration, positive displacement blowers by M-D Pneumatics are used. “I can’t say enough good things about these,” says Johnson. “They’ll take an incredible amount of abuse and lack of maintenance, yet they keep on running.”
The single biggest problem with some of the larger SBR technologies involves the decanter mechanism. Typically, one tank is being used for all of the processing, with sometimes those uses being aeration, clarification, and decant. The decanter, which draws off the clarified liquid at the end of the process, is susceptible to first flush of solids.
“Under aeration, if it’s not properly designed, solids are entering into that pickup somehow,” says Johnson. “Different larger manufacturers therefore are doing things like physically lifting the decanter out of the water with hydraulics during aeration or containing a floating decant, which is actually a giant turn-screw that clamshells shut so things can’t get into it. There are countless variations of that.
“As I looked at the problem, I came up with a simple modification to a standard Goulds sewage pump, whereby the pump would be stationary. We did some modifications to the intake and now we are able to place a hose on there and have a floating decanter on a slide-rail system. The check valve I use has a half pound of pressure against the face of it, so during aeration that is positively sealed to exclude the solids.”
For a small plant in the size range Walden typically handles, less than 100,000 gallons, this is a tremendous accomplishment, says Johnson: being able to shut the suction off at the proper time to insure there is a decent effluent quality. At the sludge-water interface, a half-inch mesh stainless steel screen holds back any inorganics as liquid flows into the main area for treatment. This keeps pumps from clogging up, along with filtering out feminine hygiene products.
Because this chamber is so large and there is so much turbulence, the maintenance involved can be done quite infrequently. “There are few, if any, problems with the plugging of screens there,” says Johnson. “It’s a huge storage chamber for inorganics, keeping maintenance costs down. Some plants require workers to be in there on a daily basis with a rake or a dip net, trying to keep these things out. I have plants which have actually never been pumped out. In mine, the inorganics are able to simply float in there, not causing any harm.
“When people see smaller plants, they tend to relate them to municipal facilities and the concerns about inorganics and how fast they build up, as well as how often they’ll have to be removed. But in my experience, that is a minor issue in small package plants: it’s usually a tight collection system unless the system is old. You know where everything is coming from and you know what to expect.”
The positive displacement blowers aerate, adding air to the mix to help the organisms to respire, and at the same time provide mixing. The diffusers lie on the bottom of the tank, on the centerline. Therefore, what occurs is a double roll or mixing coming up from the middle of the tank, creating a roll to both sides of the tank. This action increases the oxygen transfer rate.
 |
| Walden's use of noncorrosive materials minimizes maintenance requirements. |
When All Is Done
At the end of the settle-and-decant cycle, the system is now at less than one on dissolvedoxygen. Therefore, when the system goes into time zero, the bugs have been starved of both food and oxygen for a period of time. Fresh raw sewage pumped in at that point supplies the carbon source for the bugs to do the final conversion from nitrite to nitrate.
“Subsequently, nutrient removal on a SBR does not involve a lot of sludge and chemical management, because everything needed is there,” says Johnson. “Grounds-maintenance workers do not spend an inordinate amount of time attempting to get the process right; it’s inherently stable due to the sequencing process. I could have built any type of system when we first started out. I went with this one because it works, it’s largely foolproof, and it’s that much less labor-intensive. Ours also saves projects when both zoning and environmental issues collide, such as what happened with our project in Rhode Island.”
In the end, Johnson knew he had a winning idea and a workable solution: “It was this whole transition of ‘OK, I have this idea and this is what I’m going to do.’ I started out by going from everything in-house to having a variety of people who all do their own things extremely well.
“But by letting go of my ego in being ‘super-manufacturer’ to letting those who do their jobs well take part, the best things have worked out; we have a much better program, the program is bullet-proof, I have reliable help who can make on-the-fly changes without a second thought.
“I do march to a different drummer. Life is short, we all have to work for a living—but why not have some fun at the same time? Solving a dilemma and pleasing someone along the way is a good thing.”
Thoreau would likely be pleased with that philosophy.
Pete Hildebrandt is a writer specializing in science and engineering topics.
OW - November/December 2006 |
