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Aerobic systems are similar to conventional septic systems in that they both use natural processes to treat wastewater. But unlike septic anaerobic treatment, the aerobic treatment process requires oxygen. Aerobic treatment units, therefore, use a mechanism to inject and circulate air inside the treatment tank. This mechanism requires electricity to operate.
For this reason, aerobic systems cost more to operate and need more routine maintenance than most septic systems. However, when properly operated and maintained, aerobic systems can provide a high quality wastewater treatment alternative to septic systems.
Aerobic Treatment Units (ATUs) have proven to be a reliable technology when they are properly designed, constructed, and maintained. When properly monitored and maintained their performance is consistent.
ATUs may enable development or use of difficult sites. They can remedy existing malfunctioning systems and they can be a good option for homes in environmentally sensitive areas.
Aerobic systems treat wastewater using natural processes that require oxygen. Bacteria that thrive in oxygen-rich environments work to break down and digest the wastewater inside the aerobic treatment unit.
Like most onsite systems, aerobic systems treat the wastewater in stages. Sometimes the wastewater receives pretreatment before it enters the aerobic unit, and the treated wastewater leaving the unit requires additional treatment or disinfection before being returned to the environment.
Such a variety of designs exists for home aerobic units and systems that it is impossible to describe a typical system. Instead, it is more practical to discuss how some common design features of aerobic systems work and the different stages of aerobic treatment.
Some aerobic systems include a pretreatment step to reduce the amount of solids in the wastewater going into the aerobic unit. Solids include greases, oils, toilet paper, and other materials that are put down the drain or flushed into the system. Too much solid material can clog the unit and prevent effective treatment.
Some pretreatment methods include a septic tank, a primary settling compartment in the pretreatment unit, or a trash trap. Pretreatment is optional but can greatly improve a unit's performance.
The main function of the aerobic treatment unit is to collect and treat household wastewater, which includes all water from toilets, bathtubs, showers, sinks, and laundry. Aerobic units themselves come in many sizes and shapes-rectangular, conical, and some shapes that defy classification. There are two typical aerobic treatment designs: (1) suspended growth units and (2) attached growth units.
The process most aerobic units use to treat wastewater is referred to as suspended growth. These units include a main compartment called an aeration chamber in which air is mixed with the wastewater. Since most home aerobic units are buried underground like septic tanks, the air must be forced into the aeration chamber by an air blower or through liquid agitation.
The forced air mixes with wastewater in the aeration chamber, and the oxygen supports the growth of aerobic bacterial that digests the solids in the wastewater. This mixture of wastewater and oxygen is called the mixed liquor.
The treatment occurring in the mixed liquor is referred to as suspended growth because the bacterial grow as they are suspended in the liquid unattached to any surface.
Suspended Growth ATU (Norweco Aerobic System)
Unfortunately, the bacterial cannot digest all of the solids in the mixed liquor, and these solids eventually settle out as sludge. Many aerobic units include a secondary chamber called a settling chamber or clarifier where excess solids can settle. Other designs allow the sludge to accumulate at the bottom of the tank.
In aerobic units designed with a separate settling compartment, the sludge returns to the aeration chamber (either by gravity or by a pumping device). The sludge contains bacterial that also aid in the treatment process. Although, in theory, the aerobic treatment process should eventually be able to consume the sludge completely, in practice, the sludge does build up in most units and will need to be pumped out periodically so that solids don't clog the unit.
An alternative design for aerobic treatment is the attached growth system. These units treat wastewater by taking a surface made of material that the bacteria can attach to, and then exposing that surface alternately to wastewater and air. This is done either by rotating the surface in and out of the wastewater or by dosing the wastewater onto the surface. Pretreatment is required. The air needed for the process either is naturally present or is supplied mechanically.
Attached growth systems, such as trickling filters and rotating disks, are less common than suspended growth systems, but have certain advantages. For example, there is no need for mixing, and solids are less likely to be washed out of the system during periods of heavy household water use.
Attached Growth ATU (Bio-Microbics FAST System)
The way and the rate in which wastewater is received by and flows through the aerobic unit differs from design to design. Continuous flow designs simply allow the wastewater to flow through the unit at the same rate that it leaves the home. Other designs employ devices (such as pretreatment tanks, surge chambers, and baffles) to control the amount of the incoming flow. Batch process designs use pumps or siphons to control the amount of wastewater in the aeration tank and/or to discharge the treated wastewater in controlled amounts after a certain period.
Controlling the flow of wastewater helps to protect the treatment process. When too much wastewater is flushed into the system all at once, it can become overburdened, and the quality of treatment can suffer. The disadvantages to mechanical flow control devices are that, like all mechanical components, they need maintenance and run the risk of malfunctioning.
Some units have the disinfection process incorporated into the unit design (see figure 1). In some cases, disinfection may be the only treatment required of the wastewater from an aerobic unit before the water is released into the environment. Added costs for disinfectants, such as chlorine, should be taken into account with aerobic units.
Aerobic units should be large enough to allow enough time for the solids to settle and for the wastewater to be treated. The daily wastewater volume is usually determined by the number of bedrooms in the house. Flows in Arizona for individual homes are up to 140 gallons per day (gpd) per bedroom.
The needed size of an aerobic unit is often estimated the same way the size of a septic tank is estimated, by the number of bedrooms (not bathrooms) in the house. It is assumed that each person will use approximately 50 to 100 gallons of water per day, and that each bedroom can accommodate two people. When calculated this way, a three-bedroom house will require a unit with a capacity of 300 to 600 gallons per day.
Some health departments require that aerobic units be sized at least as large as a septic tank in case the aerobic unit malfunctions and oxygen doesn't mix with the wastewater. In such cases, the aerobic unit will work as a septic tank, which will, at least, provide partial treatment for the wastewater.
Lower temperatures tend to slow down most biological processes, and higher temperatures tend to speed them up. The aerobic process itself creates heat, which, along with the heat from the electrical components, may help to keep the treatment process active. However, cold weather can have adverse effects on the performance of aerobic units.
In one 1977 study of aerobic units, bulking of the sludge seemed to occur when the temperature of the mixed liquor fell below 15 degrees Celsius (59 degrees Fahrenheit). Problems can sometimes be avoided by insulating around the units.
Aerobic treatment systems are not accepted in all areas. Regulations for onsite systems can vary from state to state and county to county. A major reason that aerobic systems are not more widely used is concern about improper operation and maintenance by homeowners. Aerobic systems require regular maintenance, and abuse or neglect can easily lead to system failure.
A typical inspection should include removing the unit's cover and checking its general appearance. Check pipes and the inside of the aeration chamber and note the appearance of the wastewater inside the unit and its color and odor. If the unit includes a chlorinator, this too will need to be checked and may need cleaning. Samples may be taken of the mixed liquor from the aeration chamber, as well as the final treated wastewater. Check to see that all mechanical parts, alarms, and controls are in working order, and that solids are pumped from the system if needed.
It is important that mechanical components in aerobic systems receive regular inspection and maintenance. For example, air compressors sometimes need to be oiled, and vanes, filters, and seals may need to be replaced. Malfunctions are common during the first few months after installation. In most cases, homeowners do not have the expertise to inspect, repair, and maintain their own systems.
Most aerobic units have controls that can be switched on and off by the homeowner in case of emergency. Aerobic units also are required to have alarms to alert the homeowner of malfunctions. Depending on the design of the system, controls and alarms can be located either inside or outside the home, and alarms can be visible, audible, or both.
Homeowners should make sure that controls and alarms are always protected from corrosion, and that the aerobic unit is turned back on if here is a power outage or if it is turned off temporarily.
To assure homeowners are receiving a reputable ATU most states require the system to by approved by the NSF International (formerly the National Sanitation Foundation). NSF has tested aerobic units according to the requirements of ANSI/NSF Standard 40. NSF is a nonprofit organization devoted to the protection of the environment through the development of product standards, product evaluations, research, education and training. The American National Standards Institute (ANSI) is the recognized accredited in the U.S. for organizations that develop consumer standards and for those that provide independent product evaluations. NSF is accredited by ANSI for both of these areas of service. Aerobic units that satisfy the requirements of ANSI/NSF Standard 40 may carry the NSF mark.
If your unit carries the NSF approval, it will include the first two years of service visits with the purchase price and an option to renew2 the service contract after two years. It is a good idea for homeowners to renew their service agreements after two years or to find another service organization to take over the job.
In addition to routine maintenance, NSF requires service contractors to stock replacement parts for mechanical components and to be available for emergency servicing. Under the original two-year agreement, failed equipment is replaced at no additional cost to the homeowner.
The service contract may or may not cover such problems as damage from power failures, breaking or crushing of pipes leading to and from the system, flooding, fires, homeowner misuse, and other catastrophes beyond the control of the manufacturer.
Service visits will most likely be carried out by the dealer or another independent service organization that has an agreement with the manufacturer. In other cases, health departments will have maintenance management programs, such as sanitary districts, for aerobic systems in their area.
The first service visit should be scheduled immediately after the system is installed to make sure that everything is working correctly. The service contractor may also arrange a meeting with the homeowner to go over issue, such as proper operation, what to do in case of emergency, etc. With the first visit, the maintenance service contract should be issued to the homeowner.
The maintenance contract should include at least two service visits per year for the next two years. The number of visits and service performed will differ from; unit to unit and location to location depending on manufacturers' recommendations and local regulations.
It is a good idea for the homeowner or service provider to keep detailed records about the system and service visits. NSF-approved units are required to include a user's manual that describes such things as the manufacturer's recommendations for the unit, the system design, how to operate and maintain it, as well as how to tell if it is working properly. The state license, the date the system was installed, the type of disinfection, and any modifications to the system should also be recorded.
Other important information to keep on hand includes where to contact the owner if nobody is home, where to find a key to the system, and the schedule for service visits.
Homeowners should keep their own copies of all records and permits.
Some Advantages and Disadvantages are listed below.
Advantages:
· Can provide a high level of treatment.
· May extend the life of a leach field.
· May allow for a reduction in leach field size.
Disadvantages:
· Increased annual cost from continuously running air supply.
· Requires routine maintenance.
· Subject to upsets under sudden heavy loads (attached growth ATU's less susceptible) or when neglected.