Secondary Wastewater Treatment...

Wastewater leaving the primary settling tank has lost about 40 to 60 % of the suspended matter contained in the sewage, but it still has a high BOD. The BOD must be reduced further to ensure there is little adverse effect on the dissolved oxygen concentration in the receiving water course. Secondary treatment processes are designed specifically to reduce the oxygen demand of wastewaters. EPA defines a secondary treatment process as one that reduces the BOD and suspended solids levels to below 30 mg / L over a monthly average. Most secondary treatment processes use microbial action to reduce the oxygen demand of the waste. That is, the microorganisms within the process use oxygen - demanding materials as nutrients and energy for growth.

Components of a Secondary Treatment Process...

Components of a secondary treatment process can be listed as follows ; (1) Microorganisms, (2) O2 supply (aeration), (3) Wastewater and (4) Mixing to bring all the other components together.

Classification of Secondary Treatment Process...

Classification of secondary treatment processes is often based on the nature of microbial growth. Organisms can be suspended in wastewater, or they can be attached to an inert surface.

Suspended Growth Processes : Activated Sludge (most common) ; (1) Conventional (tapered aeration), (2) Step aeration, (3) Contact stabilization, (4) Extended aeration, (5) High purity oxygen and (6) Oxidation ditch.

Attached Growth Processes : Attached growth processes can be listed as follows ; (1) Trickling filter, (2) Rotating disks and (3) Biological towers.

Lagoons : Lagoons can be listed as follows ; (1) Mechanically aerated and (2) Waste stabilization ponds.


"Process Flow - Chart - 1"...

"Process Flow - Chart - 2"...

Process Definition of the Activated Sludge Process...

The activated sludge process is a suspended - culture system that has been in use since the early 1900s. The process derives its name from the fact that settled sludge containing living, or active, microorganisms is returned to the reactor to increase the available biomass and speed up the reactions. It may be either a completely mixed or plug - flow process. The process is aerobic, with oxygen being supplied by dissolution from entrained air. Activated sludge processes consist of a tank within which the biological reaction occurs, a settling tank, a recycle pumping system, and an aeration system.


Process Modifications...

Three basic classifications of the activated sludge process, defined by the process loading range or the organic matter available to the microorganisms, are high, medium, and low.

"Process Loadings"...

Within these 3 ranges, the shape and number of aeration tanks can be modified to vary the flow pattern.

Conventional Activated Sludge Process - Plug Flow...

In a conventional activated sludge process, raw or settled sewage flows into a large, long concrete tank along with a mixed population of microorganisms (the activated sludge). The mixture (mixed liquor) is agitated by blowing air through it. Aeration also provides O2 to make the biological degradation of the wastes occur at a faster rate. After about 6 to 8 hours of aeration, the mixture flows into a large settling tank where the biomass slowly settles out of suspension and the supernatent is collected, disinfected, and, usually, discharged into the nearest water course. Because the microorganism populations are growing, some activated is removed or "wasted" from the process. The rest is recycled to the aeration tank. In this configuration, the oxygen demand is highest near the inlet of the tank and decreases as the mixture flows through the tank. Therefore, when designing the aeration system, the engineer should make sure more oxygen is supplied near the inlet end of the tank. More air diffusers should be placed near the inlet.

Wastewater is routed through a series of channels constructed in the aeration basin. Wastewater flows through as a plug and is treated as it winds its way through the tank. As the wastewater goes through the system, BOD and organics concentration are greatly reduced. Variations to this method include: adding return sludge and / or in decreasing amounts at various locations along length of the tank; wastewater BOD is reduced as it passes through tank, and air requirements and number of bacteria required also decrease accordingly.


"Plug Flow"...

Completely Mixed Activated Sludge Process...

Wastewater may be immediately mixed throughout the entire contents of the aeration basin (mixed with oxygen and bacteria). This is the most common method used today. Since the wastewater is completely mixed with bacteria and oxygen, the volatile suspended solids concentration and oxygen demand are the same throughout the tank.

"Completely Mixed"...

Step Aeration...

To even out the BOD loading to the aeration tank and, thereby, even out the air loading along the length of the tank, wastewater may flow into the tank at several locations along the length. This is called step aeration. The BOD is still highest where the waste is injected but the concentrations are much less than in the conventional system. The air diffusers can then be evenly spaced making the design much simpler.

"Step Aeration"...

Contact Stabilization...

In this process, wastewater is contacted with the microorganisms for a much shorter time (1 to 2 hours as compared to 6 to 8 hours for conventional treatment). After settling, the activated sludge is pumped to a reaeration tank where the microorganisms metabolize the nutrients they have extracted from the waste. Although the configuration is slightly less efficient than the conventional activated sludge process, it is more stable when subjected to large variations in flow or BOD loading.

Microorganisms consume organics in the contact tank. Raw wastewater flows into the contact tank where it is aerated and mixed with bacteria. Soluble materials pass through bacterial cell walls, while insoluble materials stick to the outside. Solids settle out later and are wasted from the system or returned to a stabilization tank. Microbes digest organics in the stabilization tank, and are then recycled back to the contact tank, because they need more food. Detention time is minimized, so the size of the contact tank can be smaller. Volume requirements for the stabilization tank are also smaller because the basin receives only concentrated return sludge, there is no incoming raw wastewater. Often no primary clarifier before the contact tank due to the rapid uptake of soluble and insoluble food.

"Contact - Stabilization"...

High Purity Oxygen...

Another modification of the activated sludge process is the use of pure oxygen instead of air for aeration. Oxygen can be transferred to the process at a much faster rate and the reduction of BOD occurs at a much faster rate - about twice as fast as conventional. Therefore, aerations tanks can be made smaller - a 3 h detention time as compared to 6 hours. The tank must be covered to conserve the O2 which is expensive to generate. The cost savings of a smaller treatment plant may be offset by higher operating costs. Used often where land costs are high.

Extended Aeration...

With this variation of the activated sludge process, the aeration time is increased to about 24 hours. The major advantage of this configuration is that the rate at which waste activated sludge is removed from the process approaches zero. The treatment efficiency decreases, however, and the power costs for aeration are higher. Primary treatment is usually eliminated. These are typically designed for small flows (less than 1 MGD or 0.04 m3 / s) and can be prefabricated.

Used to treat industrial wastewater containning soluble organics that need longer detention times. This is the same as complete mix, with just a longer aeration. Advantage - long detention time in the aeration tank; provides equalization to absorb sudden/temporary shock loads. Less sludge is generally produced because some of the bacteria are digested in the aeration tank. One of the simpler modifications to operate.

Other Modifications (Less Common)...

Oxidation ditch : Similar to plug flow but uses a circulator aeration basin.
Step feed : Microbes gradually feed organics in a step feed mode at multiple points along the tank plug flow.
Tapered aeration : Air flow rate to the aeration basin may be tapered along the length of the basin.
Kraus process : The anaerobically digested sludge and digester supernatant may be added to the return sludge, thus improving settling of the floc. A portion of the return sludge flow from the secondary clarifier is mixed with the anaerobically digested sludge and the digester supernatant before being combined with the return sludge stream andrecycled back to the aeration basin. Denser anaerobically digested sludge settles rapidly and is added to the lighter secondary sludge to add weight and improve settling.

"Oxidation Ditch - Kraus Process"...

Modification Use
Conventional Low-strength domestic waste, susceptible to shock loads
Complete-mix General application, resistant to shock loads, surface aerators
Step-aeration General application to wide range of wastes
Modified-aeration Intermediate degree of treatment where cell tissue in the effluent is not objectionable
Contact-stabilization Expansion of existing systems, package plants, flexible
Extended-aeration Small communities, package plants, flexible, surface aerators
Kraus process Low-nitrogen, high strength wastes
High-rate aeration Use with turbine aerators to transfer oxygen and control the floc size, generals application
Pure-oxygen General application, use where limited space is available, requires inexpensive oxygen source, turbine or surface aerator


Click here for detailed info about "Activated Sludge Process Kinetics"...

Design Considerations...

In the design of the activated sludge process, consideration must be given to ; (1) selection of the reactor type, (2) loading criteria, (3) sludge production, (4) oxygen requirements and transfer, (5) nutrient requirements, (6) control of filamentous organisms and (7) effluent characteristics.

"Design Parameters"...

Selection of the Reactor Type...

One of the main steps in the design of any biological process is the selection of the type of reactor or reactors to be used in the treatment process. Operational factors that are involved include ; (1) reaction kinetics governing the treatment process, (2) oxygen transfer requirements, (3) wastewater characteristics, (4) environmental conditions and (5) construction, operation, and maintenance costs considered in conjuction with the secondary settling facilities.

Loading Criteria...

Both empirical and rational parameters have been proposed for the design and control of the activated sludge process. The two most commonly used parameters are ;

"Food - to - microorganism ratio"...

"Mean cell - residence time"...


Package Activated - Sludge Treatment Plant...

Commercially available prefabricated treatment plants, known as package plants, are often used for the treatment of wastewater for individual properties and small communities, especially for resort units, hotels, motels, etc. Although package plants are available in capacities up to 3,800 m3/day, they are used most commonly for wastewater flows in the range from 20 to 2,000 m3/day. Properly sized, operated, and maintained, these plants can usually provide satisfactory treatment for small wastewater flows.

Design and Operational Considerations...

(1)Hydraulic shock loads, (2) very large fluctuations in both flow and BOD5 loading, (3) very small flows that make the design of self - cleansing conduits and channels, (4) adequate or positive sludge return, requiring provisions for a recirculation rate of up to 3 : 1, for extended aeration systems to meet all normal conditions, (5) adequate provision for scum and grease removable from final clarifier, (6) denitrification in final clarifier, (7) inadequate removal and improper provision for handling and disposing of waste sludge, (8) adequate control of MLSS in the aeration tank, (9) adequate antifoaming measures, (10) large and rapid temperature change, (11) adequate control of air supply rate and (12) adequate design under organic and solids loadings.


Extended Aeration...

A typical example of a commercially available extended aeration activated - sludge package plant is shown below. In general, primary clarification is not employed in extended aeration package plants. to avoid the accumulation of solids, the aeration system should provide sufficient agitation to keep the solids in suspension. To ensure optimum performance under field conditions, it is recommended that the maximum organic loading, expressed in terms of F : M ratio (food to microorganism ratio), be in the range from 0.05 to 0.15 kg BOD5/kg MLVSS.day.


(2) Tank, (3) manhole, (4) cover, (5) baffle, (6) concave baffle, (7) support baffle, (8) screen baffle, (9) discharge pumps, (10) transfer pumps, (11) sludge return pump, (12) aeration pump, (13) float - sludge return, (14) float - discharge, (15) float - discharge alarm, (16) float - discharge shut off, (17) float - dual aeration, (18) float - high water alarm and (19) floating discharge assembly.

Another critical area of concern is the design of the secondary settling tank and related facilities. It is recommended that the overflow rate at the design peak hourly flowrate be limited to 24 to 32 m3/m2.day. Although air lift pumps have been used for returning waste sludge, they are undesirable in this application because the rate of return cannot be adjusted easily or reliably.

Design Criteria...

"Design Criteria for PTP"...