Dosing a drainfield puts less stress on wastewater treatment systems by encouraging a healthy population of microorganism and extending the life of the drainfield.
By Diane McDilda
Aesop’s tortoise figured out that quickness wasn’t the means to winning when the race was against the lead-footed hare. When it comes to septic effluent treatment, slow and measured doses—not speed—may prove the difference between successful water treatment and a failing system.
“Pressure dosing always has an advantage over gravity distribution, whether it is for septic tank effluent being applied to the soil, or for advanced treatment effluent being applied to the soil,” explains Mark Gross with Orenco Systems, Inc., an Oregon manufacturer of onsite water treatment systems. “Simultaneous activities like laundering, dish washing, and showering, can slug a gravity system with half or more daily flow in a matter of half an hour or so. In gravity-designed systems, these medium-to-large hydraulic loads flow in and concentrate the organic load heavily right at the front of the system, which initiates biological matting and leads to incomplete treatment.”
So, pressure outdoes gravity when it comes to effluent disposal, but let’s back up. The main stay of both gravity and pressure dosing systems is the septic tank, the first treatment process receiving waste from the house or business. Here in this airtight tank, water is held long enough that heavy solids can sink to the bottom, and oils and grease float to the top. A portion of the tank sludge is digested, but the remaining must eventually be removed. Effluent from the septic tank still needs additional treatment before merging with groundwater and surface water.
This is where the soil plays an essential role. Here in the moist, wet earth, a series of physical, chemical, and biological processes remove contaminants from the effluent. Naturally occurring bacteria enjoy digesting a bounty of septic effluent. Bacteria degrade nutrients and organic compounds, while suspended solids are absorbed by the soil.
In fact, below-grade food and lodging is so stimulating, that microorganisms at the front of the line or near the head of the pipe digest more than their share, leaving little for those further down the line. This first-come, first-served population proliferates and congeals into an obstructive mat, thereby hindering the development of a uniform population of bacteria along the length of the pipe. This mat can hinder flow, causing backups or propelling effluent further down the line; where an insufficient bacterial population cannot adequately treat the effluent before it traverses the drainfield.
Splitters or distribution boxes can also be used to evenly control effluent allocation and reduce the potential for the feast and famine scenario. But relying on gravity alone still limits the system’s ability to optimize bacterial loading and efficiently treat wastewater onsite.
Within this context, it becomes clear that pressure dosing helps eliminate many of the typical problems associated with leach field systems. By using relatively simple equipment, the disproportionate discharge can be eliminated and liquid food—or septic effluent—can be fed to the bacteria as a course of smaller meals throughout the day, rather than the equivalent of a holiday feast once or twice per day.
“Pressure dosing allows control of a dose spread throughout the full dispersal field, not limited to a small area; therefore, the dose per square inch is minimized,” says Gross. “It also controls the dose frequency, which helps minimize the hydraulic and organic loading per square inch of field, and controls the off, or rest time, to allow oxygenation to occur between cycles.”
The advantage of rest time is that it allows the pore spaces in the soil to fill with oxygen, further supporting a healthy aerobic bacterial population. Roxanne Groover is the Director of Education and Engineering for the Florida Onsite Wastewater Treatment Association in Lake Alfred, FL. “Dosing is part of the treatment of effluent disposal,” Groover says. “When you are dosing, wet soil is allowed to dry out. Oxygen can get into the dry soil and allows the bug population to be more effective and robust.”
Not only does oxygen penetrate the soil and encourage the proliferation of healthy aerobic bacteria, it discourages formation of the conventional slimy anaerobic layer that can clog a conventional system. By dosing a drainfield, less stress is put on system that not only encourages a healthy population of microorganism, but also extends the life of a drainfield.
What’s the Catch?
If pressure dosing is such an obvious solution, then why aren’t these systems installed with all septic tanks? The answer is that regulators rarely require these systems. While attitudes toward septic treatments are morphing as septic systems are increasingly seen as more long-term and viable methods of wastewater treatment, the importance of improving the effectiveness of wastewater treatment, as it impacts the environment, is often overlooked.
“Competitive pressures and overcoming old perceptions and attitudes regarding wastewater systems will limit a shift toward dosed systems in our area for the foreseeable future,” says Tony Mendes, president of the Nebraska On-site Waste Water Treatment Association. “We are a small state in the relative early stages of training and educating our professionals and our consumers.”
In Nebraska, onsite treatment largely comprises gravity systems with leach fields. Dosing systems are generally required only in two cases: when the dosing length exceeds 500 feet, or when a system produces in excess of 1,000 gallons per day. Considering the typical domestic dwelling produces upwards of 360 gallons per day, it’s likely pressure dosing would not be required for a private home. Even if dosing is required, a professional engineer must design the system.
Florida, on the other hand, has changed its requirements for low-pressure dosing systems, making them easier to install. “Florida now allows a master contractor to take a class, and then they can design a low-pressure distribution system. It used to be that systems were designed by engineers,” says Groover.
Gross doesn’t believe that pressure systems are necessarily harder to design than gravity flow. “The calculations required for pressure dosing design are actually less complicated than calculations required for a properly designed gravity system.” Gross adds that often times gravity system designs are two-dimensional representations that only include a series of tanks, a distribution box, and a drainfield when they should address grades, slopes, flows, ground surface, changes in elevations through the systems components, and fitting the system on the lot.
Break It Down
Leachfield design is based on the hydraulic conductivity of the soil, the distance between the drainage lines and either an impermeable zone or groundwater table, and changes in elevation. While clay has not routinely been considered a viable soil for adsorption zones, the use of pressure dosing has allowed for the installation of systems within otherwise unfeasible areas.
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Photo: POK |
| Depending on the soil and allowable depths, trenches can range 8–18 inches. |
From the pump tank, the distribution system includes a supply transport line that conveys the water to the actual distribution manifold. Depending on the soil and allowable depths, trenches for effluent treatment can range from as shallow as 8 inches to as deep as 18 inches. The shallower depth allows easy movement of oxygen from the surface to the subsurface, improving the environment for aerobic bacteria.
Trenches are approximately 6–12 inches in width. The laterals laid in the trenches are typically made up of rigid PVC pipe 1–2 inches in diameter and perforated with holes ranging from one-eighth to one-quarter of an inch in diameter. The pipes are surrounded by washed stone, gravel, or synthetic media. Depending on local regulations, the distance, between the bottom of the trench and the wet season groundwater elevation or other restrictive layer, can range from 24 to 60 inches.
With smaller pipes installed at shallower depths, there is less disturbance of the natural environment. This limited destruction means less of an impact on natural fauna living in the soil. “The biologically active zone, 12 to 18 inches or less in depth, allows oxygen to transfer into the soil. In undisturbed native soils, there is a large percentage of bugs and worms that create holes, which in turn helps aerate the soil,” explains Paulette Hache co-owner of the Limnoterra Group, a products distributor in Canada.
Hache’s group provides the equipment associated with low-pressure dosing of drip systems, which uses drip fields primarily for disposal rather than effluent treatment. This means supplemental treatment beyond the septic tank is required prior to discharge. Additional treatment often comes in the form of a prepackaged treatment plant with a rotating biological contactor. The drip lines are installed as shallow as 4 inches deep, and can be used to supply irrigation to water-dependent businesses such as golf courses.
Pump tanks are made of concrete, fiberglass, or polyethylene. Rather than pumping from a separate compartment, a screened pump vault can be installed within the septic tank. The decision to install a single or dual component system should be based on needed storage volume and pump rate.
How Much and When?
Dosing can be demand based (where the effluent is discharged after a certain volume is reached), or time based (which allows flow to the adsorption field at certain time intervals). Engineers and designers must be aware of local regulatory requirements, as they can constitute regulated intervals or volumes.
The EPA is anticipating that states will begin considering more progressive technology and implementing performance-based design requirements that take into account the total load being discharged into a water body. However, until the push is greater or the incentive more lucrative, prescriptive or specific requirements will still implemented; and while these regulations can be satisfactory it’s a one-size-that-may-or-may-not- fit-all answer to a complex question.
“Both our state law and regulations are very prescriptive,” says Craig Sanders, environmental program supervisor for the Jefferson County Department of Health and Environment. “It is hoped that this can change in the future, but, for now, most local Colorado codes are descriptive rather than performance based.”
“Research has repeatedly shown that small and frequent doses leads to better treatment,” says Gross. “However, if the regulations require the minimum dose size to be one-fourth to one-half of the daily design flow, many of the advantages of timed pressure dosing are eliminated.” This is of particular concern when the daily design flow is two to four times greater than the actual flow, meaning the regulated dosing will result in a large dose once or twice a day.
According to Gross, as part of the design of the system, effective and site-specific dosing can be determined. “Generally, the process is to determine the required soil absorption area and its location. This is done by a soil and site investigation. After that, the number and size of orifices in the pressurized distribution is determined. That will allow the designer to calculate the minimum required flow to pressurize the system.
“Using such site conditions as elevation change and depth of burial of particular components, the inputs to the pump calculations are provided. Research and field performance have shown that setting the timer to deliver approximately one dose per hour to the drainfield will most often work quite well.”
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Photo: POK |
| A trencher installs the drip tubing, approximately 8 inches below the surface. |
Dosing is controlled through a timer programmed to cycle a pump on and off for set periods of time. The scheduling ensures that peak flows will be released uniformly over the course of a day. This can be particularly important when appliances are used, such as washing machines that discharge upwards of 40 to 60 gallons per load.
“In our regulatory environment with prescriptive regulations, we are required to base our calculations upon prescribed ‘design flows’ from the regulations,” says Goss. “A conservatively safe operation results by sizing the timing and the dose volumes for these flows, setting the timer to deliver 24 even doses per day, and sizing the surge volume in the dose tank to accommodate the design flows.”
Janet Murray is the Environmental Health Supervisor for the Randolph County Health Department, and the president of Missouri Smallflows Organization, an organization comprising regulators, installers, soil scientists, and designers, who work on large and small wastewater systems. Murray admits that, in Missouri lagoons, these systems are more prevalent than pressure dosing, but she promotes the belief that any septic systems installed should include a dosing system.
“Say you have 100 gallons going into a septic tank,” says Murray. “That 100 gallons in is going to equal 100 gallons out into the drainfield. The first line is going to get hit and it will be very wet. The volume may never make it down to the end of the trench. With a timer, it’s set to dose out a specific amount to the field; it’s timed so the field can accept it.”
The timer not only allows for controlled pumping of the effluent, making sure that sufficient volume is available for pumping, but it also controls the time between dosings. Simpler dosing timers generally installed for residential units include simple on and off settings. More complicated timers may be needed for businesses that experience significant variations from day to day. In this case, settings would be based on the average daily flow over a period of time, not on a daily peak flow.
With programmed dosing comes the need to monitor water use. If excess effluent enters the pump tank, an alarm is triggered notifying the home or business owner that water usage needs to be reduced. If the system is relatively flexible, if the hydraulic conductivity of the adsorption area can handle occasional excess or the tank is sized sufficiently to store it, an override may be programmed, allowing excess demand on the septic system and continued water use.
An added advantage from monitoring water levels in the pump tank is that it allows users to identify leaks and continuing demands on the system. Even a slow leak can be identified, as it could add up to more than 1,000 gallons of excess and unused water going to the septic system, pump tank, and drainfield every day.
Other benefits of using a pump to move effluent to its next treatment phase is that it can pump water to a higher or lower elevation that would prove difficult if left to gravity alone. “One of the advantages of pressure distribution is that it is compatible to pumping to a location on a site which isn’t downhill,” says Ed Church, of Church Onsite Wastewater Consultants, LLC in Lakewood, CO.
Even though septic systems with either gravity or pressurized distribution systems cost less than most new cars, they aren’t quite as rewarding to show off to friends and neighbors. This makes price a large factor when it’s time to choose a system. Low-pressure dosing is one of the least expensive unconventional systems to install and operate. The Texas Agricultural Extension Service estimates installation of a low-pressure system to be on the order of $3,000 to $10,000, relative to the size of the house and soil types for the distribution system. The difference in capital cost between a gravity and pressurized system runs about $1,200. Even within that cost there is flexibility.
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Photo: POK |
| From the septic tank, the effluent flows to pump tanks and then is dispersed into an imported soil drip field. |
Church points to the pump when it comes to price versus performance, and explains that an inexpensive pump will cost about $250 and a very good pump about $350. “It’s not a significant difference, but people will still pick the cheaper pump,” he says. “This is an important issue, particularly when the potential for a mechanical component to malfunction can be seen as a disadvantage when deciding whether or not to install a gravity or pressurized system.”
Other disadvantages include lack of service during a power outage, and the perception that burdensome operation and maintenance is required on a pressure dosing system. As with a gravity system, the septic tank should be pumped out every two to three years, or an alternate schedule based on tank size and loading. Distribution lines should also be flushed every five years to remove sediment. With a pressure dosing system, the pump and alarm system should be inspected annually.
While damage from excess solids may not be seen immediately, over time they will overflow the tank, damaging the equipment, and, possibly, the drainfield. Rain from roofs or other impermeable areas, should be directed away from the drainfield so that oxygen, not water, is allowed to fill the void spaces during times septic effluent isn’t being discharged.
The Texas Agricultural Extension Service calculates the annual maintenance cost to be $125, assuming the tank is pumped every three years and the pump replaced every five years.
Going Retrofit
For gravity systems that have failed, pressure dosing may be the remedy. When anaerobic conditions have resulted in significant amounts of fouling, timed releases may adequately encourage the growth of aerobic bacteria. Gross sites two examples of leachfields being rehabilitated by dosing.
One included an aging sand filter. Large doses were applied and distributed via gravity over splash plates. The filter became clogged every few months, incurring maintenance costs and impacting the system’s ability to meet its discharge limits. When the gravity distribution system was replaced with a timer-controlled pressurized system, the filter no longer clogged, and discharge criteria were met.
The second case was a soil adsorption system being deluged with large doses. As the system flooded, water and septic tank effluent would pond on the surface. After the dose volume was reduced and timed to deliver periodically throughout the day, the problem was alleviated.
Onsite treatment systems, in general, are becoming more acceptable, but mindset and bad experiences still pose a problem. “Septic tanks usually equate with small houses and trailers, not $2 million homes,” says Groover. “Politicians are often unaware of what’s available and how conventional systems really operate.” While onsite systems are less expensive than larger central plants, they don’t encourage the population growth and increasing tax base usually desired by municipal entities.
The general consensus is that well operating systems represent the benefits of onsite water treatment. “Some systems are designed and installed well and operate well,” says Hache. “Some don’t and it makes it difficult on everyone when systems aren’t installed right.”
Diane McDilda is a writer living in Gainesville, FL.
OW - March/April 2008 |
