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Secondary treatment and drip-irrigation systems successfully remedy a Denver-area shopping center’s environmental challenges.
Aspen Park Village Center, a 54,000-square-foot shopping mall and restaurant development on a 4.5-acre site located off of Interstate 285 about 30 miles southwest of Denver, is in an area that has made wastewater treatment problematic for decades. In mid-2006, the owner—amid emerging commercial growth in the region—realized that attempted periodic upgrades to conventional septic treatment were not working and consulted with an engineer to solve the mall’s waste management problem for good.
The result serves as a case study of how secondary onsite treatment and drip-irrigation systems can work effectively in commercial applications. In this case, high groundwater and soils that did not suit conventional septic leaching forced the owner to look for an alternative solution.
Engineering records for the development go as far back as the 1970s, indicating that conventional septic systems invariably had been used there, notes Engineer Drew Schneider of Inspectrum in Evergreen, CO. Schneider adds that the septic system—which was processing wastewater from both the shopping center and a high volume of grease, fats, and oils from its two restaurants—was modified four times, according to the records.
“The problem that the system had in the past was that the whole valley it sits in just has an amazing amount of groundwater that consistently sits within 2 to 6 feet of the surface,” Schneider says. In addition, the septic fields were consistently saturated.
Earlier soil testing had indicated that, in order to accommodate the daily maximum allowable 2,000 gallons, the development required four septic fields: two beneath grassy areas of the site and two others beneath the parking lot. “There were a number of septic tanks that they tried to bring together, and at one point they installed a transfer station so that when one half of the system became saturated they could pump it to the other half of the system,” says Schneider. “That was when they also installed a curtain drain and tried to get as much [septic treatment out of the site] as they could.”
A sense of urgency was added to the situation when a bank located next to the shopping center. The bank was to be serviced by the shopping center’s wastewater treatment system and experienced problems with the existing system’s ability to process the shopping center’s wastewater adequately. The old system backed up and the wastewater infiltrated the basement of the adjacent bank building. Even a check valve that was later installed on the system failed, causing another infiltration of the bank building’s basement and a costly cleanup.
Secondary Treatment Wins Out
It was at that point that the owner, Emerald Management of Boulder, CO, hired Schneider to solve the development’s wastewater problem once and for all.
Schneider did investigate the possibility of utilizing the onsite wastewater system from a new grocery store that was constructed next to the shopping center. This system, Schneider would discover, would be considered a regional wastewater treatment system, meaning it would serve nearby commercial properties. In 2006 the system was brought online and fresh water was pumped through it to test the capacity. When the system began pushing through only half of its designed capacity, Schneider reports, the site was saturated to within 2 feet of its parking lot. “We had filled out a formal application to tie into that system, but we were sent back a formal letter from the president of the wastewater district saying they were not allowing any out-of-district users to tie into the system,” he says. At that point, the focus turned to secondary treatment systems, which do not require as large of a septic field as conventional systems.
“I suggested that because of the type of effluent we’re disposing of, we should consider secondary treatment systems because it will clean up the effluent and allow dispersal of the effluent without overloading the soil in the area, and also because of the high groundwater—there was concern about groundwater contamination,” Schneider says.
After Schneider investigated several onsite systems with aerobic secondary treatment, the owner opted for a system from SeptiTech distributed in Colorado by RJF Engineering Inc. in Denver.
The system uses a three-compartment septic tank. Anaerobic decomposition (that which is not facilitated by oxygen) takes place in the first compartment as solids settle to the bottom and grease floats to the top. Aided by internal recirculating pumps, the wastewater is diverted from this tank and mixed with treated water in a second aerobic processing compartment, which contains the system’s unique component: biological filtering media consisting of tiny polystyrene beads. These beads are hydrophobic—meaning they do not absorb water molecules—and they make a perfect environment for bacterial growth. An air inlet from the outside draws oxygen into this chamber, accelerating the aerobic processing of the wastewater. From a reservoir in the bottom of the processing tank, this mixture of water is pumped up to a sprayer that douses the filtering media. Sewage inherently contains a large amount of bacteria that effectively consumes microorganisms, and the media feature plenty of surface area to allow this process to occur, purifying the wastewater. From there, the treated effluent goes into a dispersal compartment.
The manufacturer cites testing that indicates that effluent treated by the system contains far less than maximum accepted levels of pollutants in most jurisdictions [per NSF International/EPA’s Environmental Technology Verification (ETV) program]. Biochemical oxygen demand (BOD) reduction of at least 50% and nitrogen levels under 20 milligrams per liter are commonly required. At the Aspen Park Village Center, pollutants were measured at the following levels prior to and after treatment in the new system, respectively, after it had been online for three months: average BOD, 245 milligrams per liter versus 4.0 milligrams per liter (98.4% reduction); and total nitrogen of 83.8 milligrams per liter versus 4.2 milligrams per liter.
Dispersal and System Reliability
Despite the danger of soil saturation and groundwater infiltration, Emerald management was concerned about irrigating greenery on the site. Here, too, the owner faced two problems. “In the area, we have moderately dry summers on and off—we’re in a drought cycle,” notes Schneider. Additionally, the property is located near a commercial well that feeds a large cistern, but the state had begun to prohibit use of this water supply for irrigation several years earlier. Schneider recommended using a drip-irrigation system for treated effluent dispersal. “You’re taking all of the effluent and putting it into soil that can accept effluent,” he says. “It takes care of the irrigation problem of having to irrigate the green belts around the sides and front of the commercial property, which [the owner] was constantly battling with the dry, arid climate of Colorado.”
The treated water is periodically time-dosed from the system to a disposal field, ensuring absorption by the soil while preventing saturation. A subsurface drip-irrigation system from Geoflow was chosen that uses tubing that is similar to a garden “soaker hose,” except that the tubing is located several inches underground. Because the tubing is located underground, the soil more readily absorbs the treated effluent and the time-dosing makes the soil less susceptible to saturation. The tubing also has filters that prevent clogging, and it is able to retain flexibility in subfreezing temperatures.
Bob Frishman, RJF Engineering owner, says that the local climate is a consideration for such a system. “High elevations do create some challenges,” he says, adding that Aspen Park Village Center is at an elevation of 8,500 feet. “The challenges are, one, that there’s less oxygen in the air; these kinds of treatment systems rely on bacterial treatment. This system is aerobic; it creates somewhat of a challenge because there’s less oxygen, so you need a good system that will provide a lot of oxygen. The other challenge is the cold temperatures. Normally, at higher elevations, you get much lower temperatures, so that tends to inhibit the bacterial action.”
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Noting that the system creates its own heat through the biological sewage breakdown and no internal heat source is needed, Frishman focused on keeping the heat from escaping. “The heat is generated within the process; you just have to make sure that you don’t lose the heat,” he says. “We actually insulate around our tanks with 2-inch foam to keep the temperature up.”
The system also is designed to prevent problems that would occur from a high volume of grease, fats, and oils. “Typically, restaurants are a difficult application because restaurants create a lot of grease, and grease inhibits the bacterial action,” notes Frishman. “If [the media] got all covered with grease, that would interrupt the biological action. There is some toleration of grease in the process; the bacteria can work on grease, but when it becomes excessive, it causes problems.” Schneider’s design thus called for regular maintenance of grease traps located upstream from the treatment system. These traps capture the grease from the restaurants in the development and allow the remainder of the wastewater to enter the treatment system.
Given the malfunctioning of the previously used conventional septic system, the owner benefits from continuous monitoring and zero maintenance of the new system. Frishman notes that a programmable logic controller (PLC) sends an audible message that includes the installation name, the model number and the problem. In the case of the Aspen Park Village Center system, the system manufacturer, Frishman, and the owner receive alerts on their cell phones. The owner is not responsible for any required maintenance.
Frishman points out that the PLC sends signals indicating two general types of problems: a power failure or a system malfunction. For system malfunctions, when floats inside the processing tank detect either a high water level or low water level, a switch indicates which condition exists and sends a signal to the PLC. Frishman adds: “With the pumps, what we do with the control panel is we monitor the amount of current going to the pumps. Normally, with a pump, if you have low current, it means that the pump either isn’t pumping or you have an impeller that’s loose. In other words, you’re not pumping any liquid; so therefore, you’re not generating a lot of energy or power, so the amps tend to be low. Let’s say you have an impeller pump that’s jammed. Then the motor tries to really work that impeller and the amps go very high—in that sense, we’re monitoring either low flow or high flow.”
Frishman cites some common causes behind low or high flows. First, a tank rupture might cause a large ingress of water. “Let’s say there’s a leak somewhere in the system because it’s really close to the parking lot,” says Frishman. “When it rains, the parking lot develops a lot of water. And let’s say there’s a leak inside one of the tanks—in fact, two of the tanks are right [beneath the driveway] and trucks drive over them. Let’s say there’s a leak and it starts putting through a lot of water—we know that. Or, high flow would be, let’s say, a stuck toilet running all day—toilets put out a lot of water.” Low flows “would be something like, after the septic waste has gone into the tank, the tank itself is leaking. It’s not getting to the treatment system. That would be low flow.
“We also have a modem in the control panel, where we can dial in and download such information as running time, amount of alarms since last time, pump number of cycles, elapsed time of pumps running, number of high-low indications, et cetera,” says Frishman.
With the system in place, the development is not experiencing excessive soil saturation, and system backups affecting the neighboring bank are a thing of the past. Schneider says the lesson from this project is that secondary treatment can answer the challenge of less-than-ideal sites for commercial wastewater treatment.
“A lot of municipalities and jurisdictions allow you to reduce your separation distances with secondary treatment systems,” he says. “From the standpoint of longevity, because your effluent is highly oxygenated and cleaner when going into your dispersal system, in addition to making your harder soils easier to work with, they can increase the longevity of whatever dispersal system you use. From an environmental consciousness standpoint, you’re one step from turning your wastewater into drinking water; you’re preventing your groundwater from getting contaminated.”
Communications specialist Don Talend resides in West Dundee, IL.
OW - March/April 2007 |
