Chapter 2:
On-Site
Sewage Treatment —
Old-Fashioned and Newfangled
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Chapter 2:
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When a community or individual is faced with the unfortunate requirement to make a decision about sewage treatment, the first problem is likely to be that the community or individual has thought as little as possible about the issue. The whole point of sewage treatment is to be imperceptible.
Proponents of different technologies make various impressive claims, often couched in technical jargon. So what follows is a plain-English discussion of how septic systems work.
On-site sewage treatment systems fall into two broad categories: (1) traditional systems, which send wastewater to a septic tank and then to a leaching field, and (2) alternative treatment systems (ATS), which pretreat effluent from the septic tank before discharging it into a leaching field or replace a traditional leaching field of stone or gravel with another high-density material, either natural (such as sand or peat) or synthetic. The intent of ATS is to permit the installation of septic systems with smaller leaching fields than required by traditional systems. ATS may also be recommended when the site has problematic soils, such as clay, or when the site has a high water table.
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Main components of a traditional residential septic
system. Graphic courtesy Onslow County, NC |
All claims to the contrary, Mother Nature really does know best. The goal of all treatment systems is to do almost as good a job as nature would do on its own in an unstressed environment. A simple, conventional septic system comes very close to matching Mother Nature:
In theory, a leaching system, sized in accordance with present codes, located in suitable soil conditions, maintained properly (septic tank routinely pumped, no toxic chemicals allowed to be discharged to the system, etc.) and utilized within water usage limits, should function properly indefinitely. CT Dept. of Health website, April 12, 2007
The main components of a traditional residential septic system are a tank, a pipe that connects the building to the tank, and a distribution box (or D-box) that divides the flow from the tank and channels it in approximately equal amounts into disposal lines in a leaching field (sometimes referred to as a “drainfield” or, with all technical flags flying, a “soil absorption system”).
Wastewater — from toilets, sinks, washers, and so on — flows from the house to the tank, then through the D-box to the leaching field. The flow is maintained by gravity unless the leaching field is higher than the tank; then a pump is required.
The wastewater includes a variety of organic and inorganic substances, which are treated in different ways in the septic tank and leaching field. The tank is primarily anaerobic (without oxygen). This is because as waste decomposes, the process rapidly uses up the available oxygen. The leaching field is primarily aerobic (with oxygen). But the leaching field also contains anaerobic bacteria, mainly contributed by the gut. These bacteria work to form a slimy lining called a biomat. The biomat contributes to the ongoing digestion of waste.
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The process of waste transformation begins in the tank. Graphic courtesy VA Dept. of Health |
The process of waste transformation begins in the tank, where organic solids settle to the bottom, forming a sludge, while greases, oils, toilet paper, and other lighter-than-water solids rise to create a scum at the top. The cloudy water in the so-called clear zone between the two is septic tank effluent. Anaerobic bacteria flourish in this oxygen-poor environment. Over a period of several days, these bacteria begin to decompose organic materials, breaking them down into gases and liquids. The gases (primarily methane and hydrogen sulfide) are discharged through the plumbing vent on the roof of the house. Organic nitrogen (N), excreted by people in the form of urea, dead cell material, amino acids, and proteins, is converted to ammonia (NH3).
Anaerobic microbes are smaller than aerobic microbes and work relatively slowly. They only partially treat the effluent before it flows into the leaching field. This first stage of the process, the separation of materials and partial treatment of the effluent, is called “primary treatment” and the partially treated effluent is “primary effluent.” As new wastewater enters the tank, the primary effluent overflows into the D-box, which distributes it into the leaching field.
Modern septic tanks usually are rectangular boxes made of concrete, fiberglass, or high-density polyethylene (HDPE). Newer tanks usually have two chambers, an arrangement that allows for more settling and a longer time for digestion of wastes. The effluent entering the second chamber is relatively clear. Here the finer particles can settle more readily. The size of the tank depends on projected flow, with the assumption that the average person will use 50 gallons per day (gpd) of water. In residences, flow is usually calculated according to the number of bedrooms. The typical residential tank holds from 1,000 to 1,500 gallons. Some jurisdictions may set a minimum capacity, such as 1,200 gpd.
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| Typical leaching field consists of a series of perforated,
four-inch-diameter PVC pipes buried in gravel-filled trenches.
Photo courtesy SD Department Environment and Natural Resources |
Proper soils are key. Before a system is installed, a soil percolation test is required. If wastewater cannot percolate through surrounding soils, sewage will back up. Clay and tight-silt soils prevent the necessary rate of percolation.
The system may function well for decades if properly maintained. The key is to pump the sludge out of the tank regularly. Requirements vary, but a pump-out every two to five years is commonly advised or required. Without pumping, sludge and scum accumulate, and the clear zone in a tank gradually is reduced. If it becomes too small, incoming wastewater will force effluent out of the tank before solids have sufficiently settled. If the guck is too thick by the outlet, it may block it or the solid material may be pushed through. With an excess of solids, the system becomes clogged. Effluent cannot percolate out of the trenches, and a backup may ensue, with gray water emerging at the surface of the soil or in the basement of the building. Flushing may falter.
If you are really into sewage, so to speak, there are scum sticks and sludge sticks for measuring whether the buildup is excessive. If you want to evaluate your own system, here’s a web address: http://www.co.thurston.wa.us/health/ehoss/pdf/StickTestBrochure.pdf. A learning-oriented Home Septic System Survey, developed by a high school science teacher to help students "guesstimate" when their leaching fields would fail, can be found at http://www.hvatoday.org/education/sound science/septicsurvey.htm.
Since 2000, Connecticut has required installation of a filter at the outlet of the tank to screen out particles of solids suspended in the effluent. The filter doesn’t catch everything but it is a good precaution. One of the chief sources of particulate matter in wastewater is lint from clothes washers. Using the required filter on the washer will prolong the life of your septic system.
Another practice that helps maintain the system is to use moderate amounts of water on a regular basis. Using excessive amounts of water (e.g., several loads of laundry in one morning) will force water in and out of the tank before the anaerobic bacteria have had a chance to do their work. Stopping water use for an extended period may cause tree roots to break into your system in search of water — they know where it is!
Local regulations and site conditions dictate the size of the leaching field and the required soil characteristics and percolation capacity. In Connecticut, the standard rule of the DEEP is that effluent must have 21 days of travel time before it reaches surface water or a well that supplies drinking water. A few feet of unsaturated soil usually suffices, but the distance will vary from site to site.
The purpose of the leaching field is to distribute septic tank effluent for further treatment by bacteria. This stage of the process is called “secondary treatment.” The materials in the effluent form the biomat, a layer of anaerobic bacteria and inorganic residues that builds up on the bottom and sides of the leaching trenches. The biomat is both a benefit and a problem. It continues anaerobic digestion begun in the septic tank. That’s the benefit. But if it becomes too thick or extensive, it will clog the system. That’s the problem.
The third stage of treatment, “tertiary treatment,” is the removal of disease-causing pathogens — dangerous viruses, bacteria, and other microbes. This removal takes place through natural processes in the leaching field and adjacent soil. In the typical public sewage treatment plant, tertiary treatment is done by disinfectants.
In a conventional well-maintained system, wastewater is transformed into clean water and harmless gas. The nitrogen in organic wastes goes through a three-step process:
(1) As mentioned above, anaerobic bacteria transform the nitrogen in urea and other organic materials into ammonia. Ammonia has quite a noticeable odor if you’re around babies’ diapers or a stable. The formula for ammonia is NH3, that is, one part nitrogen to three parts hydrogen (the hydrogen is present in water).
(2) The next step is called nitrification. This is the transformation of ammonia into nitrite and then nitrate by aerobic bacteria. Nitrites and nitrates are fairly harmless; in fact they are used as food preservatives. But they are plant nutrients, the growth ingredients in plant fertilizers. If the transformation process does not go beyond nitrification to denitrification, then nitrogen, which is a fast traveler, will get into groundwater and surface water, causing algae blooms and dead zones. In drinking water, too much nitrogen can cause health problems in infants and possibly in older individuals.
Nitrification is: NH3 (ammonia) + O2 (oxygen) ----> NO2 (nitrite) + NO3 (nitrate) + H2O
The hydrogen in ammonia is replaced by two oxygen atoms to form nitrite and three oxygen atoms to form nitrate. The displaced hydrogen links up with oxygen to form our friend water (H2O). Nitrification requires oxygen and aerobic bacteria. It cannot happen in an anaerobic environment. A leaching field must be well aerated.
(3) The last step in this sequence is called denitrification. Efficient, reliable denitrification is the ultimate goal of sewage treatment. It is frequently not attained.
Denitrification takes place in an anaerobic environment, with anaerobic bacteria doing the main work. Nitrites and nitrates, composed of nitrogen and oxygen, interact with hydrogen to produce nitrogen gas (N2) and water (H2O). The anaerobic bacteria mediate the process, feeding on the carbon in organic waste.
Here’s a formula: NO3- + 5/6 CH3OH ----> 1/2 N2 + 5/6 CO2 + 7/6 H2O + OH-
Here’s the translation: Nitrate (NO3) interacts with methanol (CH3OH), which consists of carbon, oxygen, and hydrogen — elements found in organic matter. The result is nitrogen gas (N2), carbon dioxide gas (CO2), water and a hydroxyl radical. This is the end game in the nitrogen cycle. The nitrogen escapes into the air and the cycle starts again.
Denitrification is the final task of anaerobic bacteria. That is why it is important to have a biomat. Until it builds up in your leaching field, the septic system is not fully functional.
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Phosphorus in freshwater systems can cause excessive algae growth. Graphic courtesy town of Branford, CT |
Recently, the EPA and the DEEP have also been giving attention to phosphorus removal. Phosphorus is the companion nutrient to nitrogen. Generally, phosphorus is more important in algae growth in fresh water, and nitrogen is more responsible for algae blooms in salt water. Phosphorus in wastewater was once thought not to be much of a problem. Phosphorus does not travel as readily as nitrogen. It binds to metal compounds in the soil. It is taken up by plants. It may be digested by microorganisms. Nevertheless, it does get into water. One common route is by the erosion of sediment into a stream or pond. Less well known but increasingly a matter of concern is the release of phosphorus by soils that are saturated with phosphorus and cannot hold more.
Once phosphorus is in water it causes major problems, such as the green scum that covers so many ponds. Under that scum is dead, oxygen-deprived water. Phosphorus can also contaminate drinking water. The damaging effect of excess phosphorus is the reason for the bans and recommendations against phosphorus in detergents.
The fate of phosphorus in a well-maintained conventional system depends on numerous variables, especially soil characteristics. Generally, the less saturated the soil, the more likely that phosphorus will be removed from the wastewater.
Very few ATS address phosphorus removal.
In a well-functioning conventional septic system, the action of soils and bacteria prevents pathogens and other pollutants from contaminating groundwater.
Most of the pathogens (both bacteria and viruses) that cause diseases are adapted to the anaerobic environment of the human digestive system; exposure to oxygen and aerobic microorganisms in the leaching field and surrounding soil will destroy them.
Pathogens and toxins (such as discarded pharmaceuticals, solvents, and machine oils) are also reduced by physical straining as effluent passes through soils or other receiving material. But there are limits to the capacity of a septic system to deal with contaminants. Garbage disposals also stress the system. Be careful what you pour down the drain. The bacteria down there are your friends.
Advanced, engineered sewage treatment is not as effective as nature in tertiary treatment — that is, destroying pathogens. Sewage facilities that discharge to surface water generally are required to disinfect the effluent. (This process is sometimes called, charmingly, “polishing.”) Common disinfectants are chlorine, ozone, and ultraviolet light. Some AT systems that discharge on-site also provide polishing by a disinfectant or the addition of a filter before the final discharge of the effluent.
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Leaching fields may fail for a variety of reasons. Graphic courtesy Public Health Dept., King County, WA |
Leaching fields may fail for a variety of reasons. If not well aerated, the aerobic bacteria die off, normal digestion of waste is halted, and the all-important step of nitrification doesn’t happen. The system can flood out, with water displacing air. This can be a problem if there is too small a separation between the bottom of the leaching field and groundwater or if heavy rains or excess wastewater overload the system.
Alternatively, cessation of use deprives the anaerobic bacteria in the biomat of food. Nearby trees may send out roots in search of water, roots that can break into the system. The interruption of the system may be signaled by a sinister gurgling in the pipes. By and large, however, low wastewater flows are less of a threat in a natural system, which is open to restoration of water and organisms from surrounding soils, than in closed ATS, where bacteria are isolated in a box.
Other causes of failure include improper sizing or installation of a system and installation in soil that lacks sufficient hydraulic conductivity, or permeability. The effluent can’t get through the soil, stagnates in the leaching field, and eventually backs up or erupts to the surface.
Various toxins, harsh chemical cleaners, paint, grease, fats, and solvents kill off bacteria. Digestion of waste slows or halts, thus clogging the field and leading to contamination of groundwater. Covering a leaching field with an impervious surface, such as paving, a patio, or a building, will damage it by preventing oxygen from penetrating the soil. A driveway, playground, or tennis court can compact the soil, with the same result.
The effectiveness of traditional septic systems may be increased by pumping air into the tank or the leaching field. Higher oxygen levels in the tank will cause anaerobic bacteria to die and be replaced by more active aerobic ones. The infiltration capacity of a failed leaching field can also be restored by pumping air into it. If this is done at regular intervals, the periodic aeration will engender alternating aerobic/anaerobic conditions that increase the amount of nitrogen removed from wastewater.
Sometimes, therefore, a failing conventional system can be revived relatively inexpensively. ATS are more complex and more delicate. This is one reason why many experts feel that ATS should be used only where there is sufficient area for creation of a conventional septic system in case of failure.
Many kinds of alternative sewage systems have been devised in order to permit development of land parcels where lack of space and environmental concerns preclude the installation of conventional septic systems. ATS can be used in individual residences, clusters of homes, condominiums, schools, recreational facilities, restaurants, or other businesses. These alternative systems are proposed more frequently as good land (meaning land that is relatively easy to develop) becomes scarcer.
ATS are attractive in many cases as an alternative to a public sewer and waste treatment system. The community may fear that sewer service will open the door to unwanted development. Or that it is too expensive. In some places, such as western Massachusetts, large volumes of wastewater are being transported from that area to coastal treatment plants. As a result, not enough water is left for local recharge of aquifers. So the EPA has been encouraging more use of AT systems, which typically discharge into the ground.
The trend toward substituting ATS for conventional sewers and conventional in-ground treatment has not generally included adequate safeguards to be sure that the systems function as promised. In Connecticut, clean water has benefitted from a legal ban on ATS in drinking-water watersheds and a practice, until recently, of not permitting ATS for single-family residences. Therefore, some of the most critical questions about performance have been sidestepped. As of late 2010 only about 60 ATS had been permitted. Now hundreds are in the works.
Most of the newer alternative technologies aim to remove much of the nitrogen from wastewater before the effluent reaches groundwater or the property line. As in conventional systems, the process requires both aerobic and anaerobic bacteria.
The nitrification/denitrification processes involve a balancing act. As in conventional systems, if the aerobic bacteria do their nitrification too well, they will not leave enough organic material as food for the anaerobic bacteria. Therefore, denitrification will be impeded. On the other hand, if the aerobic bacteria are weakened or killed, say, by low temperatures, they cannot oxidize the ammonia in the waste, and the all-important first step of nitrification will not happen. Therefore, many alternative systems actively oxygenate sewage in order to encourage growth of aerobic bacteria.
The various alternative systems have at least one of the two following features:
(1) The key feature in some systems is pretreatment of effluent from the septic tank before its discharge into a leaching field or, rarely in Connecticut, surface water. The object of pretreatment is reducing levels of organic matter and other wastewater constituents, such as fecal coliform, cleaners, and degreasers, as well as so-called “emerging contaminants of concern,” meaning personal care products and pharmaceuticals. The pretreatment typically is in a closed-box arrangement, a mini-version of the standard municipal treatment plant. It depends on pumps, air blowers, and other mechanisms to work properly. It also depends on careful maintenance to support or replace the necessary bacteria.
(2) The key feature in some ATS is that traditional trenches of gravel or stone (where secondary treatment occurs) are replaced with beds of sand, peat, plastics, or other high-density materials that require less area for leaching.
These features may be combined and assembled in many sizes and configurations. There are ready-made commercial products, custom-designed systems, and combinations of the two. They may be installed either above ground, providing they are properly protected and insulated, or underground, providing there is access to all mechanical parts.
The bacteria in a sewage-treatment facility do best on regular meals in moderate portions. Most households and commercial users have times of peak use that will overwhelm the system. Therefore, ATS frequently depend on pumps that move the wastewater in timed doses from the septic tank to a treatment tank.
Typically, the pump in an alternative system is housed in a watertight tank or chamber. The pump tank usually is underground, but it has an opening that protrudes above the surface, providing access for maintenance and repairs. The pump operates on a schedule set by the user, discharging effluent in controlled amounts, or doses. For instance, the pump may be set to run for a certain period of time at a certain time of day, or it may be directed to operate for X number of minutes every Y number of hours.
The storage capacity of a pump tank is a critical design consideration. The tank must be large enough to hold effluent accumulated during peak periods of use. Normally, the larger the tank, the greater the cost, so there is a temptation to undersize. Moreover, wastewater generation may be underestimated, especially with respect to future usage.
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| Pump tanks are normally fitted
with safety controls. Graphic courtesy VA Dept. of Health |
To deal with variations in flow, pump tanks are normally fitted with safety controls. For example, a float system will prevent the pump from turning on if the wastewater in the chamber falls below a certain level. On the other hand, the chamber also should have a float and overflow alarm to let the operator know if the system's capacity has been exceeded or if it is malfunctioning.
Dosing techniques may be used to send effluent directly to a leaching field or other medium, or to other units (in some cases to a second septic tank or settling chamber), or to a separate compartment for further treatment prior to discharge. This further treatment, in advanced systems, depends on installation and maintenance of environments for microbial growth.
In theory, pumped doses improve on steady-flow gravity systems by distributing the effluent more evenly and giving the receiving system time to rest between applications. The effluent stays in the system longer and is more thoroughly treated. In practice, the system may work well or badly. The size may be wrong, the pumps may malfunction, the alarms may fail, the alarms may go off but no one hears them, the owner may turn off the system, or the pattern of use may be just too challenging. This last problem is common in schools and vacation homes, where periods of nonuse are followed by periods of high use. Not only is it difficult to size the system but also the bacteria alternate between starving and drowning.
Natural and synthetic media are used in ATS for basic mechanical filtering and for surfaces on which microorganisms can grow and digest organic waste. Natural-media filters generally are constructed on-site, while synthetic ones are manufactured off-site or assembled on-site from modular components. In some cases, disinfecting processes are available for tertiary treatment.
Synthetic filters typically weigh less and need less space than natural materials. The disadvantage of synthetic filters is that they are usually not as dense as natural materials, not as friendly to bacterial growth, and more susceptible to temperature variations. Also, they are relatively new and their lifespans are not known.
Two of the more common natural materials used in ATS are sand and peat.
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Sand filters provide secondary treatment of effluent. Photo courtesy Public Health Dept., King County, WA |
Sand filters provide secondary treatment of effluent. Sand may receive effluent from a septic tank or from a secondary chamber in which bacterial treatment has occurred. A sand filter not only strains the effluent but also serves as a fixed medium for bacterial growth and, thus, for secondary treatment of the waste. Aerobic bacteria should grow in the upper regions, while anaerobic bacteria grow below. Sand also provides even temperature; it does not freeze if buried more than four feet deep and protected from influxes of cold air. A filter of unsaturated sand (or soil) will remove most pathogens and deactivate viruses if it is a few feet in thickness.
Sand filters are produced in many sizes and configurations, and they have been used in many applications, ranging from single-family homes to large commercial establishments and small communities.
Sand filters may be bottomless, in which case the filter amounts to a leaching field, or they may be contained. An enclosed sand filter is typically a box (often made of concrete or treated wood) with an impervious lining, containing washed sand, 24-36 inches deep, over a gravel underdrain; vents allow the passage of air to the top of the filter. The filter may either be buried, usually with a layer of pea gravel between the top of the filter and the soil that covers it, or open to the atmosphere, though commonly with a wooden top for protective and aesthetic purposes. The structure resembles a play sandbox.
The sand itself must meet particular specifications (varying from place to place) of size, uniformity, and fineness.
Sand can be used in a single-pass treatment, where the effluent is delivered in intermittent doses, or in recirculating systems, where some of the effluent is redirected to the sand for a second pass through the system.
Sand filters are relatively inexpensive to build; materials for constructing them usually can be obtained locally. Maintenance, however, is required. If a septic tank is used for primary treatment, the tank must be pumped out periodically. To prevent clogging of the sand filter's surface, the upper layer of an open sand filter may need to be raked occasionally. The upper layer may also have to be removed and replaced from time to time. In the case of buried filters, occasional flushing of the effluent distribution piping at the top of the filter is needed to assure relatively even distribution and treatment of effluent across the unit.
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Preconstructed modular peat filter systems are marketed by a number of
companies. Photo courtesy VA Dept. of Health |
Preconstructed modular peat filter systems are marketed by a number of companies. The modules contain compacted peat or peat fiber and piping in a solid box. The units can be installed with minimal site preparation. They are designed to drain directly into the soil or into an underdrain for recirculation, discharge into another treatment unit, or disposal in a small leaching field.
Peat acts much like a sponge, absorbing effluent and dispersing it in all directions. Peat fibers are highly porous and thus have a large surface area, providing a suitable environment for sewage-eating microorganisms. Peat also is acidic and therefore kills some pathogenic bacteria. Fungi in the peat may also produce antibiotics and other organic compounds that attack pathogens.
Another feature of peat is its ability to hold water. This ability may extend the life of microbes during periods when the system is not being used. The moisture also helps keep the temperature of the peat relatively constant. This moisture-holding ability can also be a problem, however, especially in warm climates, where the peat may become mushy.
Peat beds may have to be replaced anywhere from five to 14 years after installation, while sand filters have an indefinitely long life if well protected.
Warning: Not just any kind of peat can be used. The peat must be specially dried, have a specific fiber and moisture content, and possess a specified degree of decomposition. Horticultural peat marketed at garden-supply centers is not suitable.
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An above-grade leach field
consisting of a mound of fill and aggregate.
Graphic courtesy U.S. EPA Risk Management Research |
On some small sites, it is possible to take a low-tech approach by creating an above-grade leaching field consisting of a mound of fill and aggregate. Effluent from the septic tank is dispersed to the mound by gravity or by pumping. The pump typically is housed in a separate chamber or vault. Partially clarified effluent from the septic tank is fed by gravity to the pump chamber (sometimes called the dosing chamber), then pumped in controlled amounts up to the mound. When built on slopes, mounds parallel the contours of the land and are much longer along the contours than they are wide. Ditches or curtain drains may have to be installed on the uphill side to prevent surface water from penetrating the mound. Mound systems are built over soil that cannot accept septic tank effluent; all treatment within a mound system must take place within the mound.
One objection to mound systems is that they look funny. Neighbors may not consider a mound system in a front yard to be an aesthetic asset.
Raised-bed systems are similar to mound systems but are constructed over soil in which at least some treatment of the septic tank effluent can occur.
The three principal AT processes on the market are (1) fixed-film (or fixed-medium), (2) suspended-growth (or culture), and (3) membrane bioreactors (MBR). The first two encourage the growth of aerobic bacteria in order to increase nitrification; the third operates as a superfine filter. Packaged systems may combine the different processes.
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In a fixed-film (fixed-medium) method, effluent from the septic tank goes to a
treatment chamber containing permeable
materials. Graphic courtesy VA Dept. of Health |
In a recirculating system, some of the
effluent is recycled through the system for additional treatmentnt. Graphic courtesy U.S. Environmental Protection Agency |
In a fixed-film method, effluent from the septic tank goes to a treatment chamber that contains permeable materials, such as rock, sand, peat, foam, textile, or other natural or synthetic medium. The medium must offer large surface areas for supporting microorganisms, which form a biomat. Natural or forced ventilation supports the growth of aerobic bacteria.
Trickling filter systems distribute the effluent over the surface of a stationary medium in the treatment chamber, usually in timed doses, by spraying from one or more nozzles or by means of perforated pipes. After reaching the base of the treatment chamber, the treated effluent is dispersed by plastic pipes directly into the leaching field. In some cases, the effluent may receive some form of disinfection before discharge. In a recirculating system, some of the effluent (perhaps 50 percent or more), along with the excess biomass (microorganisms) sloughed from the filter medium, is recycled through the system for additional treatment. Filtered effluent may also be sent to a clarifier tank for additional treatment by settling. During quiescent periods, the remaining solids and biomass settle out of the effluent, which then is recycled in doses to the top of the treatment chamber, or reactor, for a second pass through the medium. As new effluent enters the clarifier, compensating volumes of treated wastewater are discharged into the leaching field. The sludge that forms at the bottom of the clarifier may be pumped out periodically and trucked off site or returned to the septic tank for eventual removal.
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RBC systems locate the medium (typically rotating plastic disks)
within the effluent. Graphic courtesy Milton Beychok |
In the suspended-growth process, effluent is circulated rapidly within a chamber that has an aerobic environment. Aerobic conditions may be enhanced by pumping air into the chamber or by mechanical agitation. Vigorous mixing of the effluent causes the bacteria to remain in suspension, where they digest biodegradable compounds.
Suspended-growth systems usually are marketed, delivered, and installed as self-contained modules or packages, often referred to as “aerobic treatment units” (ATUs). Suspended-growth systems generally can be designed to operate in smaller spaces than fixed-film systems treating an equal volume of wastewater, but they generally do not do as well adjusting to marked changes in flow or removing organic material and suspended solids. Incomplete settling of solids is the problem.
The suspended-growth and fixed-film processes may be employed together, either in a dual-process system or in a series known as a “continuous-flow-suspended-growth aerobic system” (CFSGAS). The combination systems are used for treating high-strength wastewaters. They have several technical names but “high-biomass system” is the most descriptive. They can be loaded at higher rates than can either component by itself, but the EPA recommends that they be installed only if management services are available on-site.
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Continuous flow (CF) systems usually include a
partition in the treatment chamber to separate the turbulent aeration area from
the quiescent settling zone. Animation courtesy VA Dept. of Health |
Activated sludge technology is a common variation on the suspended-growth system. This approach combines aerobic and anaerobic treatments, either in a single vessel, called a “sequencing batch reactor” (SBR), or in linked aeration chambers and clarifiers. Different activated sludge systems have different components, but all operate in alternating sequences. Treatment begins with forced mixing of effluent with air (“activation” of the sludge). Aerobic bacteria go to work transforming ammonia into nitrite (nitrification). Then mixing stops. During this quiescent period, solids settle out of the effluent and form a sludge at the bottom of the chamber, while grease, oils, and other floating materials rise to the surface, where they can be skimmed off. The sludge creates a suitable environment in which anaerobic bacteria can continue treatment of the sewage by changing the nitrate into nitrogen gas (denitrification). Settling tanks may also be equipped with scrapers that continuously drive sludge toward a hopper at the base of the chamber, from which it can be pumped for further treatment.
The aeration-settling sequence may be repeated several times before a finished batch of sewage is pumped out of the system and a new one is admitted. In an intermittent-flow (IF) system, two units may be operated in parallel, with one open for intake while the other goes through its cycles. A continuous-flow (CF) system operates, as its name implies, without interruption in intake. Such systems usually include a partition in the treatment chamber to separate the turbulent aeration area from the quiescent settling zone.
Sequencing systems require precise timing to work right.
The use of fine-filter membranes can significantly improve the performance of suspended-growth methods of sewage treatment. MBR systems are usually expensive to purchase, and, as with all ATS, require regular maintenance, actually more than average. The reward is that the MBR method is likely to produce higher quality water in the end.
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Membranes serve as super
clarifiers, filtering microscopic solids and even individual bacteria. Graphic courtesy King County, WA, Dept. of Health |
Membranes do not work on digestion of waste. They serve as super clarifiers, filtering microscopic solids and even individual bacteria. They basically provide “polishing.”
Fine-filter membranes, with microscopic pores, may be immersed in an aeration tank or stand as a separate unit. The effluent may be pumped through the membranes at relatively low pressure or pulled through them by vacuum force.
Ultrafine (UF) and microfine (MF) membranes typically remove particulates of less than 1 micron in size. A micron (µm) is a billionth of a meter. By comparison, a human hair is about 100 µm in diameter. Membranes have to be protected from hair and other stringy or abrasive waste by fine screens with openings of 3 millimeters (mm) or less, i.e., three one-thousandths of a meter.
Membranes typically are assembled in modules or compartmentalized cells. The main types of module are the tubular membrane system and the plate and frame or flat-sheet membrane system. In both cases, the modules are densely packed to provide the largest possible surface area in the smallest possible overall area. For example, a single unit may contain thousands of hollow membrane fibers with microscopic pores.
Tubular membrane systems, as the name implies, are made up of thousands of tiny tubes with microscopic pores. Hollow fibers, for example, may be used for the membrane material. Tubular filtration is especially suitable where levels of solids are high, liquids are viscous, and cleaning and maintenance are important issues.
Plate and frame membranes are divided into spiral and pillow-shaped membranes. Spiral-wound membranes provide a higher packing density than tubular ones and may be more economical in high-flow applications.
There are several advantages to MBR systems over competitors. Since membrane filtration clarifies effluent faster than settling, MBR systems can operate with smaller tanks — 30 to 50 percent less in volume — than those of other systems. That means membrane systems can be located alongside buildings and in utility buildings or basements.
Compared with other technologies, membrane systems should result in relatively less sludge for a given volume of effluent, thus lowering disposal costs. Membrane systems also seem to be better at dealing with low temperatures and uneven wastewater flows (as at schools or vacation areas).
But membranes must be cleaned regularly to prevent fouling and consequent loss of permeability. Compartmentalized cells may be cleaned individually while other membrane units remain in service. Different manufacturers use different cleaning processes, including chemicals (chlorine, for example), air flushing, forward and backward flushing with water, and, in some cases, a combination of these methods. Depending on the manufacturer and the particular system, cleaning of some sort may be recommended at intervals of once a week to once a year.
To our knowledge, the best-performing AT system in Connecticut is a GE/Zenon system in Georgetown in Fairfield County. Using a ZeeWeed filter, this system is doing well at nitrogen removal and is meeting health criteria as well. But again we emphasize that buying a particular brand does not automatically confer success. Looking back several years (before GE bought Zenon), one of the most unsuccessful installations was a Zenon system at the Joel Barlow School, not far from the Georgetown sewage treatment plant. A brand name does not guarantee good performance. The product must be properly designed, installed, and maintained.
Traditional septic systems need more care and maintenance than they usually get. ATS need much more care and maintenance. In Connecticut, few regulators, vendors, operators, or owners appear to be prepared to provide that care and maintenance.
In Minnesota, officials did a study entitled NERCC Individual Alternative Wastewater Treatment Systems: Pollutant Removal in 2003 and Long-term Performance. (NERCC stands for Northeast Regional Correction Center, where the study was performed.) Sand and peat were used as receiving materials. The authors report, “The systems were checked and monitored regularly (typically weekly). When problems occurred, they were usually discovered in a timely fashion and corrected within a few days, limiting reduced treatment performance of the systems” [emphasis added].
The range of problems that can affect ATS includes design and construction flaws, loss of power (whether deliberate or accidental), floods, lightning, cold, and, probably most important, homeowner abuse. When we have asked designers and vendors why they are not more willing to guarantee performance, one reasonable answer is that they can’t give treatment guarantees when users aren't giving them guarantees as to what will be entering the system. Homeowners and home cleaners are not accustomed to considering the effects of harsh cleaners, pharmaceuticals, solvents, polishing products, and so forth on processes underground that they can’t see. Even if the original owner understands the system, subsequent owners may not.
In Connecticut and Massachusetts, as well as in Minnesota, experts compare buying and maintaining an AT system to buying and maintaining a car. An O/M (Operation/Maintenance) manual should come with the system, and someone should be checking whether it is being followed and whether the system is working.
Here are some of the checks proposed by the Minnesota experts for their sand and peat systems:
(1) Septic Tank and Pump Tank
(2) Control Panel and Controls
(3) Sand, Peat, Textile Filters
ATS are complex systems with numerous mechanical and electrical components that may fail. The work they are supposed to do involves biochemical reactions that are influenced by thousands of variables.
It appears that with frequent monitoring and maintenance, the best-quality ATS will perform well, though not necessarily as well as the buyer or regulator may expect. The technology in some products has improved to the point that facilities can be required to meet the environmental standards appropriate for discharge to surface water. This means strict limits on nitrogen, phosphorus, and pathogens.
It is not clear, however, that Connecticut is willing to impose such standards. The proposed standards are loose for the Old Saybrook Decentralized Wastewater Management District (the first district in Connecticut where residential ATS will be installed in large numbers). Elsewhere, individual permits vary in numerous details. Nonreporting and violations, or exceedances, of permit requirements are tolerated in many cases.
Rivers Alliance has been collecting data on ATS performance in order to ascertain what we can expect if the installation of ATS becomes commonplace. One tentative conclusion is that performance probably will meet permit requirements only about 40 to 70 percent of the time. This means that if the state is issuing permits with the idea of maintaining high water quality, the state is probably missing that goal by a significant margin.
These shortcomings in the overall performance of ATS are documented in one of the few peer-reviewed studies of performance. In the summer of 2006, the journal Small Flows Quarterly published an analysis of the performance of 200 AT facilities by Susan Peterson, Ph.D. These facilities had permits from the Massachusetts Department of Environmental Protection to treat wastewater flows less than one million gallons per day (gpd). The limit for total nitrogen in the effluent was 10 mg/l (milligrams per liter). To quote from the summary of the findings: “Of the 90 facilities whose discharge data were reviewed (up to 41 data points from 2001 to 2004 for each facility), only seven percent (six facilities) always met the 10 mg/l TN [total nitrogen] permit requirement. Overall, the best TN removal was for membrane bioreactor facilities, of which 73 percent met the 10 mg/l permit standard more than 80 percent of the time, as did 69 percent of the rotating biological contactors, 67 percent of the sequencing batch reactors, and 56 percent of the activated sludge [systems].”
The author concluded that the likelihood of success was far higher when the facility was designed, installed, and operated by the same party or a single team. Subsequently, when testifying in an AT permit hearing in Connecticut, the author noted that for a facility she managed in a multi-unit building she was constantly on hand checking and adjusting. She pointed out that under her contract, payment was tied to performance.
Incidentally, the best-forming product by trade name in Massachusetts has a poor record in Connecticut, more evidence that one cannot count on a regulatory approach that relies on brand certification alone.
In the Massachusetts Health Department's ATS testing center in Barnstable County (Cape Cod), the results have been similar to those reported by Peterson. In a study of 297 single-family systems and 50 multifamily systems, the facilities were in compliance 60 percent of the time for the latter and 69 percent for the former. The total nitrogen considered acceptable was higher than in the Peterson study — 19 mg/l versus 10 mg/l. (We prefer the 10 mg/l standard.) This measure was on the basis of median values (one-half of tests higher, one-half of tests lower.) One concern with using this measure is that it does not reflect potentially serious problems when exceedances spike. (Barnstable County did look at the mean average as a secondary indicator.)
In Connecticut ATS reports, we have seen spikes of e. coli bacteria and nitrogen that cry out for someone to fix the system.
Other problems reported in Connecticut and with facilities in nearby states are high water, insecure connections, improperly installed electrical components, flawed materials, die-offs of bacteria, improper sizing, changes in water quality (such as changes in pH, i.e., acidity versus alkalinity), breakdown of blowers, clogged outlets, highly variable usage (as in schools), and so forth. Murphy’s Law rules. What can wrong will go wrong. Most (not all) problems can be fixed relatively easily, but someone has to be there to fix them.
One of the points of conflict in Connecticut is how often to monitor and report ATS performance. The state has fairly strict standards for wastewater discharges into surface water, and it requires monthly tests and reports. Up to now, the DEEP has also required monthly testing and reporting for ATS that discharge into the ground. However, in the proposed Decentralized Wastewater Management Districts and discussion of statewide regulations, much looser monitoring and reporting are contemplated. Also, almost no one is calling for one of the most obvious criteria of quality control: the monitoring should be done by or checked by independent testers.
Advocates for high-quality water tend to want frequent testing and strict enforcement. Monthly reports seem reasonable. The testing should be by independent testers on an unannounced schedule. Penalties should apply promptly when a system is out of compliance.
The DEEP has invested in an online reporting system that will be a great improvement over the reams of paper now filed in the reporting system. Old Saybrook is considering real-time monitoring of systems but apparently only for whether a system is turned off or on, not for whether it is doing its job.
References: See the Endnotes of the ATS Law and Order chapter.
Additional References:
"Operation and Maintenance of a Subsurface Sewage Disposal System," Connecticut Department of Public Health Sewage Program; retrieved on April 12, 2007 from http://www.dph.state.ct.us/BRS/sewage/sewage_operation.htm
"Groundwater and You," Housatonic Valley Association; retrieved on Jan. 11, 2007 from http://www.hvatoday.org/publications/Groundwater%20&%20You.pdf
"Design Manual (part 1) Subsurface Sewage Disposal Systems for Households and Small Commercial Buildings," http://www.dph.state.ct.us/BRS/sewage/desman1.pdf)
"Drain Cleaners," Matt Brown, Karla Embleton, Lora Risk, Keith Sullivan; retrieved on April 12, 2007 from http://www.purdue.edu/dp/envirosoft/housewaste/house/draincl.htm
"Home Septic System Survey Sound Science Student Page," Housatonic Valley Association; retrieved on July 23, 2011 from http://www.hvatoday.org/education/sound%20science/septicsurvey.htm
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