The activated sludge process is the most widely used biological water and wastewater treatment. As is well known, by its means, suspended-growth microorganisms are applied to breakdown wastes. Aeration, the mixing of air and a liquid, is the means used to speed the reactions involved.
Aeration methods include spraying the liquid into the air; diffusing air into the liquid or agitating the liquid to promote surface adsorption of air.
An extended-aeration system includes capabilities for aeration & mixing, settling, return of activated sludge and solids removal.
In this process the wastewater is in contact with air. In this method, the produced sludge settle and part of it is returned to the aeration tank. The key parameters that distinguish extensive aeration from other activated sludge processes are the hydraulic retention time of approximately 24 hours and the solids retention time (SRT) in the aeration basin. Aeration can be provided with surface-mounted air vents or underwater diffusers. Surface fans may be mounted horizontally, mounted, or floated vertically. Submerged diffusers may be fixed or suspended. The aeration systems may be switched on or off or modulated to regulate dissolved oxygen in the basin so that they can biologically remove organic matter.
BOD must be at a rate sufficient for the organisms to use the oxygen in water or wastewater while stabilizing decomposable organic matter under aerobic conditions. In decomposition, organic matter serves as food for the bacteria and energy results from its oxidation.
An extended aeration process is sometimes referred to as a “total oxidation process,” which means all the primary treated or influent BOD is converted to CO2.
The system utilizes a longer sludge age than other aerobic systems. Sludge age, also known as SRT (Solids Retention Time) or MCRT (Mean Cell Residence Time), defines the operating characteristics of any aerobic biological treatment system. A longer sludge age dramatically lowers effluent BOD and ammonia levels.
In this process, the structure of organic matter changes and is generally converted to water and carbon dioxide.
Another parameter which should be considered is the sludge volume index (SVI) from which the sludge volume is accumulated and deduced.
Sludge Volume Index (SVI) is an extremely useful parameter to measure in a wastewater treatment process. In simple terms, SVI is the result of a mathematical calculation.
SVI gives a more accurate picture of the sludge settling characteristics than settle ability or MLSS alone.
SVI can indicate changes occurring in the activated sludge treatment process. By trending SVI data over a period of time, operators are able to prevent. The best SVI for each plant will be different problems.
EAAS process includes the following equipment:
• Transfer pump
• Sludge return pump
• Diffusers and blowers
• Chemicals for disinfection can also be considered
Operating conditions for EAAS process:
• (HRT) hydraulic retention time between 20-30 hours
• (SRT) Sludge age between 20-40 days
• MLSS between 3000 and 600 mg/l
Advantages of EAAS process:
• Ease of installation and operation
• High efficiency
• Can handle shock loads of flow and pressure
• has lower sludge production than other activated sludge processes
• need low space
• the sludge is digested during the process
Disadvantages of EAAS process:
• High cost
• High energy consumption
• produce high volume sludge
EAAS units are suitable for the following applications:
• wastewater flow is low
• The quality of the input wastewater is intermittent
• Limits on sludge production.
Comparison of Activated sludge systems: