How Your Aerobic System Works
A complete guide to understanding the six-stage treatment process that keeps your system running efficiently.
A typical aerobic system consists of a trash tank, treatment unit (aeration chamber, air supply, clarifier), disinfection, pump tank, and spray field. Each component must perform its intended purpose to achieve the desired effluent quality.
Aerobic treatment systems consist of several processes that work together to provide a high-quality effluent: removal of solids (trash), aeration, clarification, and sludge return.
The Treatment Process
Six interconnected stages ensure wastewater is properly treated before reaching your spray field.
Trash Tank
Heavy solids settle to the bottom while lighter materials float to the surface.
Aeration
Air pumped into the chamber allows microbes to convert waste into harmless substances.
Clarification
Microbial cells, cell waste, and dead cells are removed from the effluent.
Sludge Return
Settled solids return to the aeration chamber to maintain healthy microbe populations.
Disinfection
Chlorinators or UV light systems destroy disease-causing organisms.
Pump Tank & Controls
Stores and delivers treated effluent to the spray field at controlled intervals.
Trash Tank
Wastewater leaves the house through a pipe and enters the trash tank. This is where the heavier solids will settle to the bottom while the lighter materials will float to the surface.
Potential Issues
- ▸ Tank integrity
- ▸ Lid integrity
- ▸ Failure of seals around risers, etc.
Aeration Process
The wastewater flows from the trash tank into the aerobic treatment unit. Air is pumped into the aeration chamber of the unit, and the wastewater remains there for a relatively long period to allow the microbes to convert the waste into less harmful substances: water, carbon dioxide, and new cells.
Aerobic Systems rely on a continuous supply of oxygen to keep the microbes treating the wastewater healthy. Oxygen can be supplied to the wastewater in several ways depending on the brand of system, but can include aerators, compressors or blowers.
Tracking air pressure can significantly affect the longevity of your system.
High Air Pressure
- • More frequent pump-outs
- • High solids build-up in pump tank
- • Premature air pump failure
Low Air Pressure
- • Bad odor
- • Untreated wastewater sprayed onto lawn
Wastewater Treatment Microbes
The aerobic treatment process involves two types of microbes living together:
Aerobic microbes
which use free oxygen dissolved in wastewater
Facultative microbes
which can grow in environments that have free oxygen and those that do not.
These two types of microorganisms can decompose a broad range of materials.
Clarification
Step 3 of the six-stage aerobic treatment process.
After the aeration process, the effluent now moves into a clarifier, which removes the microbial cells, cell waste, and dead cells from the wastewater. Clarifiers can be oriented vertically or horizontally.
Potential Issues
- ▸ High water usage can cause a buildup of solids, either in top of the clarifier or deep in the chamber, leading to excessive solids being carried over into the pump tank.
Clarifier Types
Vertical clarifiers
The wastewater enters through the bottom of the chamber, and flows upwards, toward the outlet.
Horizontal clarifiers
The wastewater flows laterally from the inlet to the outlet.
Active filtration
Another approach to clarification is active filtration. Some aerobic treatment unit products incorporate filtration media to filter the biomass from the effluent before it is discharged. In these approaches, the effluent filters through a porous material or settles in a plate-settling process. One type uses socks to filter the effluent leaving the aerobic chamber.
How Settling Works
During the clarification process, some materials will float to the surface of the clarifier. These floating materials can be skimmed from the surface of the clarifier.
In all clarifiers, the waste particles must be able to settle to the bottom of the tank. For a particle to settle, the downward forces must be greater than the upward forces.
Several factors affect the settling process:
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Gravity pulls the material downward.
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Density of the materials determines the rate at which it falls. This is because gravity pulls down the material at a speed based on the material's weight.
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Buoyancy pushes the materials upward. The water is resisting the object down through it and is actually pushing it up based on the amount of water being displaced.
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Water flow speed in the system is a crucial factor in the design of a clarifier. Water flow carries suspended particles along with the water. When the water is moving too fast, fewer particles can settle to the bottom.
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Turbulence can also interfere with the downward movement of particles.
Sludge Return
The solids that settle from the effluent in the clarifier should be returned to a previous component. Usually, the settled material, or sludge from the clarifier, is returned to the treatment tank via a sloped clarifier bottom.
Sludge return is essential to the treatment process.
With sludge return, the microbes in the sludge breakdown additional wastewater and reproduce, increasing the population to replace dying microbes.
Several systems use a compartmented tank to automatically feed the sludge back to the aeration chamber. In these systems, the rolling motion of the effluent in the aeration chamber will entrain the sludge back into the digestion process.
Other systems that use a separate tank for a clarifier may have a pump to return solids, either to the trash or treatment tank. Additionally, returning the solids to the trash tank may help remove more nitrogen from the system.
Return Process Methods
Vertical settling chambers
Most vertical settling chambers have a passive return process. Solids/sludge pass through the bottom of the clarifier and return to the aeration process.
Horizontal clarifiers
Horizontal clarifiers typically have a separate chamber where solids accumulate and require an active return process.
Disinfection
Step 5 — reducing pathogens before the effluent reaches the spray field.
Some pretreatment devices include a disinfection unit as part of the treatment system. In the disinfection process, disease-causing organisms are destroyed or inactivated. This process reduces the concentration of pathogens to an acceptable level.
Important: The wastewater is only disinfected, not sterilized (free of all life).
Before the wastewater enters the disinfection stage, it must be pretreated to remove significant amounts of organic matter and solids. If these are not removed first, they can impede the disinfection process.
Disinfection Methods
Chlorinators (Tablet & Liquid) and UV light systems serve as secondary treatment devices. Tablet Chlorinators are most common.
Tablet Chlorinators
Tablet Chlorinators typically consist of a long narrow tube that holds tablets, which acts as a dispenser for the tablets into the effluent below.
Pump Tank and Controls
Pump systems are used in many ways in on-site wastewater treatment systems. For example, they may be used to store and deliver effluent to a spray field or other soil dispersal area.
Pump tanks may also be used earlier in the treatment train to deliver wastewater uphill to a treatment unit. In these applications, the tank may serve as a flow equalization tank.
The pump system consists of a pump tank, pump, discharge assembly, controller, and associated electrical components.
Control Panel Functions
Pump systems should include a controller to sense water level in the tank, and dose the field when required, or set off alarms if needed. The control panel can be simple or complex depending on the functions it must perform.
Examples of controller functions:
- • Automatically turning the pump on and off with a manual override.
- • Sounding an alarm to indicate problems.
- • Providing a means of monitoring the system (meters/counters).
- • Electronic monitoring and automatic notifications.
- • Monitoring the air system and tracking air pressure levels to extend compressor's lifespan and ensure proper treatment.
Understanding Aerobic Treatment
How aerobic systems achieve superior treatment results
Aerobic treatment units are pre-secondary treatment devices. They can remove substantial amounts of BOD & TSS that are not eliminated by simple sedimentation. The aerobic process also breaks down dissolved solids and ammonia and reduces the number of pathogens in waste.
These processes may occur either in separate chambers of a single tank or in a series of tanks.
Comparison to Conventional Systems
Conventional onsite wastewater treatment systems depend on the soil and the organisms in it to treat wastewater. Aerobic treatment units, by contrast, pretreat the wastewater before it reaches the soil, resulting in higher quality effluent.
Questions About Your System?
Understanding how your aerobic system works is the first step to proper maintenance. We're here to help keep your system running efficiently.