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| موضوع: طرق معالجة الصرف الصحى للمنازل والتجمعات السكنية بيولوجيا/HOME AEROBIC WASTEWATER TREATMENT: AN ALTERNATIVE TO SEPTIC SYSTEMS السبت مارس 10, 2012 5:59 am | |
| HOME AEROBIC WASTEWATER TREATMENT: AN ALTERNATIVE TO SEPTIC SYSTEMS
BY
GENERAL.DR
BAHAA BADR
TECHNOLAB EL-BAHAA GROUP
In these communities, central sewerage systems are often not cost-effective, so many homeowners rely on septic systems or other systems that treat and dispose of household wastewater onsite.
Some homes for which septic systems are not a good option rely on individual home aerobic systems for wastewater treatment.
Aerobic systems are similar to 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.
Why choose aerobic treatment?
This is due, in part, to the widespread use of septic systems.
Septic tank-soil absorption systems are relatively inexpensive and are easy to maintain.
They are the most common onsite wastewater treatment systems used in rural areas. However, there are many households for which a septic system may not be the best wastewater treatment option.
For example, septic systems are not suitable for every lot.
In fact, approximately two-thirds of all the land area in the U.S. is estimated to be unsuitable for the installation of septic systems.
Some homes may not have enough land area or appropriate soil conditions to accommodate the soil absorption drainfield.
In some communities, the water table is too high to allow the drainfield to give adequate treatment to the wastewater before it is returned to the groundwater.
Other site-related concerns include homes located on wooded lots or on lots close to a body of water.
Homeowners in wooded areas may not want to clear enough land to install a septic tank and drainfield, and the wastewater treated by a septic system is often not of high enough quality to be discharged very close to a body of water.
But one of the most common reasons that aerobic wastewater treatment units are chosen by communities is to replace failing septic systems.
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HOW AEROBIC TREATMENT WORKS
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 exist 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.
PRETREATMENT
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 treatment unit, or a trash trap.
Pretreatment is optional but can greatly improve a unit’s performance.
AEROBIC TREATMENT UNITS
The main function of the aerobic 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.
Suspended 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 a compressor.
The forced air mixes with wastewater in the aeration chamber, and the oxygen supports the growth of aerobic bacteria that digest 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 bacteria grow as they are suspended in the liquid unattached to any surface.
Unfortunately, the bacteria 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 bacteria 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 at least once a year so that solids don’t clog the unit.
Attached Growth Units
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 is either 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.
Flow Design
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 of time.
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, --------------------------------------------------------------------------------------------------
HOW AEROBIC TREATMENT WORKS
soil or directly to a body of water.
Your health department is familiar with local regulations and the treatment options that are best in your area and for your property.
Soil absorption fields (or drainfields) are the most common method of final treatment used for septic systems.
If an aerobic system is being used in place of a septic system or to replace a failing septic system, a drainfield may not be an option.
However, an aerobic unit can sometimes help to prolong the life of a drainfield.
The amount of dissolved oxygen contained in wastewater from an aerobic unit can help the growth of microorganisms that treat the wastewater in the soil, and can help prevent the pores in the soil from clogging.
However, when aerobic units malfunction, they can release solids that can clog the drainfield, which may cancel out any potential benefits.
Evapotranspiration beds are a less common method of final treatment and use vegetation and evaporation to naturally treat the wastewater.
Drip irrigation is another less commonly used method to treat and dispose of wastewater.
Sand filters are sometimes used to treat the wastewater from aerobic units.
The wastewater is pumped evenly over several layers of sand and gravel, which are located either above or below ground.
As with soil treatment systems, the purification process is aided by bacteria that occur naturally in the sand.
Disinfection is another method of treatment commonly used with aerobic units.
Some units have the disinfection process incorporated into the unit design.
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.
One disadvantage of this method is the added cost of the disinfectants, such as chlorine.
OTHER DESIGN CONSIDERATIONS
Controls and Alarms
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 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.
Homeowners can help their system’s performance by conserving water.
Leaking faucets and running toilets should be repaired, and washing machines and dishwashers should be used only when full.
Installing water saving devices in toilets, faucets, and showers can reduce water use by up to 50 percent.
Also, homeowners should try to space out activities requiring heavy water use (like laundry) to avoid overloading their systems.
FINAL TREATMENT AND DISPOSAL
Although properly operated and maintained aerobic units are very effective, the wastewater leaving the units is not ready to be returned to the environment and must receive final treatment or disinfection.
Methods for final treatment include discharge to a soil absorption field, a sand filter, or an evapotranspiration bed.
Sometimes, the wastewater receives disinfection before being discharged to the 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 there is a power outage or if it is turned off temporarily.
Size Aerobic units should be large enough to allow enough time for the solids to settle and for the wastewater to be treated.
The size of most units range from 300 to 1,500 gallons per day, but local regulations often require that the unit be at least large enough to handle 500 gallons of wastewater per day.
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.
Temperature 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 study of aerobic units, there were problems when the temperature of the wastewater inside some of the units fell below 15 degrees Celsius (59 degrees Farenheit).
Problems can sometimes be avoided by insulating around the units.
Your health department should know whether aerobic systems are suitable for your area.
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