Activated Sludge I

Centrifugal Blowers Characteristics

-A rotating impeller that provides a variable volume of air per revolution, depending on the output pressure required. -A relatively high operating speed, revolutions per minute (RPMs,) compared to positive displacement blowers. -A relatively larger unit capacity compared to positive displacement blowers.

Positive Displacement Blowers Characteristics

-A rotating set of lobes or rotors that provide a constant volume of air per revolution, within the operating pressure range of the blower, for each specific lobe design. - A relatively low operating speed, known as revolutions per minute (RPMs) compared to centrifugal blowers -A relatively smaller unit capacity compared to centrifugal blowers.

Equipment Is Designed To (Mechanical Aeration)

-Agitate the water surface in the aeration tank to cause spray and waves, which enhances the transfer of oxygen from the atmosphere into the wastewater. -Splash water into the air or entrain (mix) air into the water to enhance the transfer of oxygen into the wastewater. -Mix the incoming wastewater, the mixed liquor suspended solids, and the air bubbles in the aeration tank to enhance the aeration process.

conventional activated sludge treatment plant

-Because of its relatively low mixed liquor suspended solids (MLSS) concentration and its head-end loading, the process is most suitable for low-strength, domestic wastes with minimal peak load considerations. -operated so that the amount of food available for the organisms is limited, requiring the organisms to compete for the available food -A conventional process requires more tank volume and greater oxygen requirements per pound of BOD processed compared to a high rate process. Consequently the activated sludge plant itself is more expensive, but ancillary treatment processes are less expensive (such as sludge processing), and process control requirements are less demanding.

Positive Displacement Blowers Output Control Mechanisms

-Change the motor RPM for a motor directly connected to the blower. This can be done by physically replacing one motor with another or by changing the electrical frequency applied to the drive motor via a frequency control device, if a variable frequency drive (VFD) is used. -Adjust the speed using a magnetic drive coupling. -Change the pulley wheel (sheave) sizes for blowers connected to motors with drive belts. -Change the gear ratio for blowers connected to motors via a gearbox -Change the lobes or rotors in the blower.

Centrifugal Blowers (Output Control Mechanisms)

-Change the motor RPM for a motor directly connected to the blower. This can be done by physically replacing one motor with another or by changing the electrical frequency applied to the drive motor via a frequency control device, if a variable frequency drive (VFD) is used. -Adjust the speed using a magnetic drive coupling. -Change the pulley wheel sizes for blowers connected to motors with drive belts. -Change the gear ratio for blowers connected to motors via a gearbox. -Change the pressure requirements for the aeration system. Decreasing the pressure required will increase the output of the blower and increasing the pressure required will decrease the blower output. -Change the impeller size or design.

Waste Activated Sludge Rate

-Changing the rate at which sludge is wasted changes the nature of the organisms that predominate in the activated sludge plant. -The rate at which activated sludge is wasted needs to be managed in accordance with daily or seasonal variations in BOD loading (to properly balance the F/M ratio) and also to react to sudden or shock loads of BOD to the system.

Contact Stabilization

-Contact Stabilization presumes that organic matter (BOD) destruction is a two-step process in which: • BOD is first adsorbed by the microorganisms. • BOD is then metabolized by the microorganisms for energy and growth.

Kraus Process Key Process Design Parameters

-F/M Ratio= 0.3 - 0.8 #BOD/#MLVSS/day -Organic Loading (maximum)=70 #BOD/1,000 cubic feet/day -MLSS=1000 - 2500 mg/liter -Aeration Retention Time (minimum)= 4 hours -Solids Recycle Rate15% - 100%

Extended Aeration (Oxidation Ditch) Key Process Design Parameters

-F/M Ratio=0.05 - 0.1 #BOD/#MLVSS/day -Organic Loading (maximum) =15 #BOD/1,000 cubic feet/day -MLSS=3000 - 5000 mg/liter -Aeration Retention Time (minimum) =24 hours -Solids Recycle Rate=50% - 150%

Biological Nutrient Removal Processes: Bardenpho, Anaerobic/Anoxic/Oxidation (A2/O), Modified Ludzack-Ettinger (MLE) Key Process Design Parameters

-F/M Ratio=0.08 - 0.16 #BOD/#MLVSS/day -Organic Loading (maximum)=20 #BOD/1,000 cubic feet/day -MLSS=2000 - 5000 mg/liter -Aeration Retention Time (minimum)=12 hours -Clarifier Solids Recycle Rate=15% - 100% -Aeration Tank MLSS Recycle Rate=400%

Complete Mix Key Process Design Parameters

-F/M Ratio=0.2 - 0.5 #BOD/#MLVSS/day -Organic Loading (maximum) =40 #BOD/1,000 cubic feet/day -MLSS=1000 - 3000 mg/liter -Aeration Retention Time (minimum)=6 hours -Solids Recycle Rate=15% - 100%

Contact Stabilization Key Process Design Parameters

-F/M Ratio=0.2 - 0.6 #BOD/#MLVSS/day -Organic Loading (maximum)=.60 #BOD/1,000 cubic feet/day -MLSS= 1000 - 3000 mg/liter -Aeration Retention Time (minimum)=5 hours -Solids Recycle Rate=50% - 150%

Biochemical Oxygen Demand (BOD)

-Food is known as BOD. -BOD is the measure of oxygen demand in the incoming wastewater. -A strong wastewater will have a high demand. A weak wastewater will have a lower demand. -BOD is the measure of how much oxygen it will take to stabilize the waste (or food) that is in the wastewater.

Influent BOD and changing waste characteristics

-High BOD levels can cause an organic overload. -A change in influent temperature or pH can change biological reaction rates and the normal equilibrium of wastewater constituents, potentially changing the availability of food for organisms or the rate at which they can process it. Biological activity decreases considerably when temperatures drop below 5 degrees Celsius. -Introduction of toxic substances in the wastewater influent can kill or inhibit organisms.

High Rate Process Characteristics

-High F/M ratio (0.4 to 1.5). -Short sludge age (0.5 - 5 days). -High organism growth rate. -1 to 3 hour hydraulic retention time. -Lower than desirable effluent quality (because some food passes through the process without being stabilized by the organisms).

Activated Sludge Plants

-High Rate (Modified Aeration) -Conventional -Extended Aeration

Equipment Can Consist of (mechanical aeration)

-Horizontally-oriented paddlewheels or rotating brushes -Vertically-oriented mixers or turbines

Activated Sludge Process Components

-Influent Pretreatment (preliminary) -Primary Treatment (clarifiers) - Aeration Basin -Return Activated Sludge - Waste Activated Sludge -Secondary Clarifiers -Solids Handling

Dissolved Oxygen Level

-Low DO levels in the aeration tank can inhibit the activity of desired organisms and allow undesirable (i.e., facultative and filamentous) organisms to thrive. Too much air, and the solids will not floc (clump together) or settle properly. The desired goal is 2.0 mg/l measured at the end of the aeration tank for conventional secondary treatment (nitrification). - Changes in the organic strength (amount of food) in the influent will cause changes in the oxygen requirements for the activated sludge system. -The rate at which oxygen transfers from air to water is dependent on the temperature of the liquid in the aeration tank and the size of the air bubbles. The transfer rate of oxygen increases as the temperature of the liquid and size of the air bubbles decreases, and vice versa. This means that finer bubbles and colder water results in optimal saturation of dissolved oxygen. The solubility of oxygen in water is temperature dependent and about twice as much dissolves at 0 degrees C than 20 degrees C

extended aeration treatment plant characteristics

-Low F/M ratio (0.05 to 0.15). -A relatively long sludge age, generally more than 10 days and often 20 to 30 days. -A zero net organism growth rate. -A relatively long hydraulic retention time of 18 to 24 hours.

When wastewater is added to activated sludge

-Microorganisms feed and grow on waste particles in the wastewater. -As organisms grow and reproduce, waste is removed and wastewater is partially cleaned. -Organisms need a balance of food (BOD) and oxygen. BOD is inherent in the wastewater and oxygen is added by aeration equipment. - The balance of food to organism mass is known as F/M ratio, food to microorganism ratio. An appropriate F/M ratio is necessary to obtain proper performance from the activated sludge process.

Characteristics of conventional activated sludge treatment plant

-Moderate F/M ratio (0.2 to 0.5) #BOD/#MLVSS/day -Organic Loading (maximum)... 40#BOD/1000 cubic feet/day -MLSS=1000-3000 mg/liter -Aeration Retention Time (minimum)= 6 hours -Solids recycle rate=15%-100% -A mid-range sludge age of approximately 3.5 - 10 days. -A slow organism growth rate. -Hydraulic retention time of 6 to 8 hours. -A good quality effluent because any available food is processed by the organisms.

Oxidation and removal of soluble or suspended solids

-Oxidation and removal of soluble or suspended solids is the result of the activated sludge process in waste treatment. -This treatment takes place in a few hours in an aeration tank.

Importance of Proper management of RAS

-RAS provides a source of organisms that is returned to the activated sludge process as required. -By changing the RAS rate, the operator can control the concentration of organisms in the aeration tank in response to the food supply present in the incoming wastewater. This allows him to maintain the proper F/M ratio for good system performance. -Increasing the RAS rate, decreases the hydraulic detention time in the aeration basin. Decreasing the RAS rate, increases the hydraulic detention time. -The well-being of the aerobic organisms deteriorates as long as they remain in the secondary clarifier. If the sludge remains in the clarifier too long, the aerobic organisms will die for lack of oxygen. -Caution has to be taken when increasing RAS rates because hydraulic overloading of the activated sludge system can occur. Increasing the RAS rates increases the volume in the aeration tank and in turn decreases the hydraulic detention time.

Detriments of a high rate plant include

-Relatively high BOD and suspended solids in the effluent. -High sludge production rate. -Requires more operator attention and monitoring of process parameters. -More easily upset than other processes.

Mechanical aerators

-Simple mechanical aeration devices either splash water into the air or entrain (mix) air into the wastewater so that oxygen can be absorbed into the water. -Typically: 1. Use an electric motor for power 2. May be of a fixed-location design (i.e., platform-mounted) or a floating design. 3. Use a submerged or partially submerged impeller or splashing device

Solids Handling: Return and Waste Sludge

-Solids handling is an important function in operating an activated sludge plant - sludge from the clarifier is either returned to the aeration tank or disposed of. -The proper balance between WAS and RAS is required to provide the proper amount of organisms for the aeration tank in response to the incoming wastewater characteristics.

Swing Header

-Sometimes these air lines are equipped with swivel joints that allow them to be pulled out of the aeration tank without disassembling the air line -If they do not have swivel joints, they are sometimes referred to as fixed headers. The fixed headers may also be located on the side of the aerated tank wall or in another location, such as on a rotating arm.

Stabilized soluble or suspended solids occur

-Stabilized soluble or suspended solids occur when organisms partially oxidize solids -Organism activity forms carbon dioxide, water, sulfate, and nitrate compounds. -Remaining solids are changed to a form that can be settled and removed as sludge during sedimentation.

Disadvantages of fine bubble diffusers

-Susceptibility to clogging due to dirty air on the inside or biological growth on the outside. -Their limited air handling capacity compared to other types of diffusers. -Relatively greater capital cost compared to other types of diffusers.

Intermittent Cycle Extended Aeration System (ICEAS)

-The Intermittent Cycle Extended Aeration System (ICEAS) operates with a continuous influent flow and a periodic discharge. -uses an inlet baffle to separate the inlet zone from the reactor zone, allowing semi-batch treatment with minimal disruption of clarification.

Air Piping Systems

-The air piping system consists of pipes, valves, and metering devices that deliver air from the blowers to the diffusers in the aeration tanks. -Air mains are the large arteries of the piping system that transfer the bulk of the air from the blowers to the smaller distribution pipes. -Air headers are smaller distribution pipes that transfer air from the mains usually to several air branch lines connected to the header. -The air lines that distribute the air from an air header to the air diffusers are sometimes referred to as headers also, if they supply air to a number of diffusers mounted on branch air lines.

Mixed Liquor Volatile Suspended Solids (MLVSS)

-The amount (mg/L) of organic or volatile suspended solids in the mixed liquor of an aeration tank. This volatile portion is used as a measure or indication of the microorganisms present. - Organism mass -Normal ratio= 80% (percent volatile) or better

Mixed Liquor Suspended Solids (MLSS)

-The amount (mg/L) of suspended solids in the mixed liquor of an aeration tank. -The overall concentration of suspended solids in an aeration tank. This consists mostly of microorganisms and non-biodegradable suspended solids.

benefits of a conventional activated sludge treatment plant

-The high quality of the effluent. -Its ability to absorb some shock loads.

aerobes (or aerobic organisms)

-The most effective and quickest decomposition of wastes is achieved by organisms that thrive in an oxygen-rich environment. - they require the presence of molecular oxygen, O2, to survive. -Require a proper dissolved oxygen level (molecular oxygen, O2). -Produce little to no odor. -Efficiently oxidize waste. -Grow relatively quickly

Organisms

-The organisms are the workers in the activated sludge treatment process. They use the wastes as food and an energy source for survival and for reproduction

Dissolved Oxygen Levels

-The organisms in an activated sludge plant need a satisfactory level of dissolved oxygen (DO) to function efficiently. -Low DO in the aeration tank will reduce the activity of aerobes. -Low DO levels may promote the growth of filamentous and other unwanted organisms. -High DO in the aeration tank will promote the growth of organisms responsible for pin floc formation, which adversely impacts sludge settling. (causes sludge bluking)

Filters

-The primary purpose of filters is therefore to keep the diffusers clean by removing dust and dirt before air is compressed and transferred to the diffusers. In addition filters: 1. Prevent large objects from entering the piping and possibly damaging the blowers. 2. When constructed with integral silencers, they control the noise inherent in the operation of blower

Preliminary Treatment (Primary Treatment)

-a physical stage which may consist of coarse screen, raw influent pumping, static fine screening, grit removal, and selector tanks. -The raw wastewater enters from the collection system into the coarse screening process where large objects are removed.

Activated Sludge

-aerated sewage containing aerobic microorganisms that help to break it down -consists of sludge particles, teeming with living organisms, produced in either raw or settled wastewater by the growth of organisms (which include bacteria) in aeration tanks where dissolved oxygen is present

Mean Cell Resident Time (MCRT) (aka Solids Retention Time (SRT)

-an average measure of how long the microorganisms remain in contact with the substrate (food source). - Changing the MCRT changes the population dynamics of the organisms in the activated sludge process. -MCRT can impact BOD removal. -MCRT can impact the clarifier operation and solids settleability. -MCRT can impact the oxygen requirements of the activated sludge process. -MCRT can impact sludge production. An older sludge will produce less and vice versa.

Fine Bubble Diffusers

-available in a range of designs including plates, tubes, and domes. Ceramic is a common material of construction for fine bubble dome diffusers. -The principle benefit of fine bubble diffusers is the relatively high oxygen transfer efficiency compared to other types of diffusers, meaning the plant requires less energy to run. Fine bubble diffusers have a nominal oxygen transfer efficiency of approximately 6-15%, -Fine bubble diffusers provide substantial transfer of oxygen to the water. Also, please remember the water temperature discussion from Unit 1. Colder water contains more dissolved oxygen, which combined with fine bubble diffusers creates optimal saturation of dissolved oxygen.

facultative organisms

-can utilize either molecular oxygen or oxygen bound in inorganic compounds, such as nitrate, NO3 -organisms grow in either an aerobic or an anaerobic (no oxygen) environment -Are less efficient organisms for waste processing than aerobes. -Produce foul-smelling products of decomposition and incomplete reactions when oxygen is scarce. -Grow somewhat more slowly than aerobes.

Medium Bubble Diffusers

-have oxygen transfer efficiencies between that of fine and coarse bubble diffusers, approximately 5-12%. -Maintenance requirements are less rigorous than for fine bubble diffusers; however, care must be taken to minimize the impact from dirt and dust in the pressurized air. -A common medium bubble diffuser design employs a synthetic (i.e., nylon or Dacron) sock clamped to a plastic diffusion tube. The synthetic sock material further diffuses the coarse bubbles released by the plastic tube, somewhat enhancing the oxygen transfer by reducing the size of the bubbles released by the diffuser

extended aeration treatment plant

-operated so that the amount of food introduced into the process is not sufficient to support net organism growth (i.e. organisms die at a rate equal to their growth rate). In this mode, the organisms must obtain some of their food by breaking down their own cellular material. -The chief detriment of extended aeration is the expense of building an extended aeration plant (because of the large size of the tank needed to provide the requisite hydraulic retention time of 18 to 24 hours.)

Diffused Aeration Systems

-the most common type of aeration systems used in the activated sludge process. These systems utilize pressurized air supplied to a galley of distribution devices (diffusers) submerged in the aeration tank -Blowers are the devices that provide the pressurized air. -Air piping transports the pressurized air from the blowers to the diffusers. -Diffusers are the devices that distribute the air within the aeration tank. -Diffused aeration systems utilize diffusers, blowers, transfer piping, and other associated equipment to transfer oxygen into the wastewater. -Placement of the diffusers in the aeration tank can optimize the contact time between bubbles and wastewater.

Waste Activated Sludge (WAS)

-the sludge removed from the system to prevent buildup of excessive solids in the aeration tank and in the activated sludge process. -This sludge is removed from the clarifier and disposed of at the plant's sludge disposal facility and is not reused in the process - can be used to control the amount of MLSS in an aeration tank -By decreasing the WAS rate, the MLSS Concentration will Increase and vice versa

Return Activated Sludge (RAS)

-the sludge settled in the clarifier that is returned to the aeration tank

High Rate Process

-used when the discharge quality needs to be greater than that of raw wastewater or wastewater that has undergone only primary treatment, but not as great as that obtained by a conventional activated sludge process. -A high rate activated sludge treatment plant is operated so that the amount of food available exceeds the capacity of the organisms to stabilize it -The chief benefit of a high rate plant is the rapid uptake of food from the wastewater into the organism biomass

Variables that impact plant operation

1. Influent BOD and changing waste charasteristics 2. waste activated sludge rate 3. Dissolved Oxygen Level

Reasons for modifications of the conventional activated sludge process

1. Operational Benefits 2. Site Characteristics 3. Economic and Labor Benefits

Primary Sludge

1. the solids remaining after sewage treatment has been completed 2. the mass of solids

Activated Sludge Process

A biological wastewater treatment process which speeds up the decomposition of wastes in the wastewater being treated. Activated Sludge is added to wastewater and the mixture (mixed liquor) is aerated and agitated. After some time in the aeration tank, the activated sludge is allowed to settle out by sedimentation and is disposed of (wasted) or reused (returned to the aeration tank) as needed, the remaining wastewater then undergoes more treatment.

Biological Nutrient Removal Processes: Bardenpho, Anaerobic/Anoxic/Oxidation (A2/O), Modified Ludzack-Ettinger (MLE) System Configuration

A simple four-tank Bardenpho process may be configured as follows: • Influent wastewater, return sludge from the clarifier, and nitrified mixed liquor from the effluent end of the first aeration zone are introduced into the first anoxic tank. • Effluent from the first anoxic tank discharges to an aerobic tank where nitrification occurs. • Effluent from the nitrifying tank discharges to a second anoxic tank where denitrification occurs. o Nitrate-nitrogen is converted to nitrogen gas. • Effluent from the denitrification tank discharges to a second aerobic tank. o Second tank raises DO level of the wastewater before clarification process begins. • To obtain phosphorus removal, as well as nitrogen removal, an anaerobic tank is typically added to the front of the treatment train.

Types of Aeration

A. Contact Stabilization B. Kraus Process C. Step-Feed Aeration D. Complete Mix E. Extended Aeration (Oxidation Ditch) F. Biological Nutrient Removal Processes: Bardenpho, Anaerobic/Anoxic/Oxidation(A2/O), Modified Ludzack-Ettinger (MLE)

Activated Sludge Process Control

An activated sludge plant requires influent water quality testing and activated sludge process testing to insure proper treatment

Site Characteristics

Another common reason for modification of the process is to provide a treatment system that is suitable to the available site conditions. For example, extended aeration systems, especially oxidation ditch configurations, require more space than conventional systems. Alternately, where space is limited, a pure oxygen system or a complete mix configuration would be more suitable

Submerged Turbine Aerators

Are commonly-used hybrid mechanical aerators that also incorporate: -Submerged impellers. -Diffused air that is dispersed beneath the aerator. -Draft tube to induce a more effective flow pattern for transferring oxygen to the wastewater.

benefits and detriments of extended aeration

Benefits: -The stability of the process. -Less demanding operational requirements. -High quality of the effluent (because complete oxidation of food occurs). -Low sludge production rate. -Often nitrification will occur if the sludge age is on the longer end of the range. Detriment: -expense of building an extended aeration plan because of the large size of the tank needed to requisite hydraulic retention time of 18 to 24 hours

Blowers

Blowers for aeration systems, which are sometimes referred to as process air compressors, are generally either a positive displacement type or a centrifugal type.

Coarse Bubble Diffusers

Coarse bubble diffusers are available in the widest range of designs. To keep the cost down, coarse bubble diffusers are commonly made of plastic. The benefits of coarse bubble diffusers are: -Relatively low cost. -Greater air handling capacity compared to other types of diffusers. -Less maintenance concerns compared to other types of diffusers. -The principal disadvantage of coarse bubble diffusers is the relatively low nominal oxygen transfer efficiency of approximately 4-8%.

Diffusers

Diffusers distribute air into the aeration tank to dissolve oxygen into the wastewater and to enhance mixing. Three general classifications of diffusers are in common use. -Fine bubble diffusers -Medium bubble diffusers -Coarse bubble diffusers As the names suggest, the differences between the classifications is related to the size of the air bubbles they produce.

Extended Aeration (Oxidation Ditch)

Extended aeration is often used for small treatment facilities requiring a simple process, in the form of a package treatment plant. It is also used for larger treatment plants in the form of oxidation ditches. Principal benefits of extended aeration modifications include reduced sludge handling and lower power requirements. • A long aeration time (hydraulic loading) and low organic loading characterize this process. • Primary clarification is often eliminated. • Dissolved oxygen (DO) is introduced at intermittently spaced aerators and the DO concentration may be allowed to decrease significantly between aerators. • The ditch configuration and the mixing energy applied are designed to maintain a velocity of approximately one foot per second, in order to keep solids in suspension.

Basic Operating Principles of the Sequencing Batch Reactor (SBR)

Following are the basic overview principles of the SBR: • The SBR is a fill-and-draw activated sludge system. • Wastewater enters a partially filled reactor containing biomass. • When the required operating liquid level is reached, influent flow to the reactor stops and a specified, timed treatment sequence begins. • Because the influent flow to each reactor is not continuous, at least two reactors are necessary to accommodate a system with continuous influent flow.

Extended Aeration (Oxidation Ditch) System Configurations

In this example of extended aeration, an oxidation ditch configuration is presented as a representative of the extended aeration modification. Many other configurations are possible. The principal differences between an oxidation ditch and a conventional activated sludge system are: • An oxidation ditch is configured as a ring with continuous flow around the ring, which is induced by aerators. • A clarifier may be located within the annular space of the ditch to save on construction costs and the amount of land required. • Oxidation ditch rings may be interconnected at the ends in order to produce a long, continuous loop.

Kraus Process

Kraus Process is used to treat wastewater that is deficient in nitrogen. It is also used when activated sludge has poor settling characteristics. This modification is most applicable for treatment facilities receiving wastewater that is high in carbohydrates. The process uses a reaeration tank that is similar to the contact stabilization process, with some important modifications: • Not all of the return sludge is reaerated; some is returned without being retreated. • Digester supernatant and digester sludge is also added to the reaeration tank. • Retention time in the reaeration tank is approximately 24 hours. Ammonia nitrogen in the digester sludge and supernatant is converted to nitrate nitrogen in the reaeration tank. Effluent from the reaeration tank is mixed with the return sludge to correct the nitrogen deficiency in the influent wastewater. Also, the concentration of inert solids from the digester, when mixed with the mixed liquor, improves the settleability of the mixed liquor.

Operational Benefits

Modifications to the process can be made in an existing system or in the design process of a new system. Potential operational benefits of modifying the conventional activated sludge system include: • Increasing organic loading. • Providing additional nutrients required for proper treatment. • Accommodating flow rate or organic loading that varies seasonally. • Achieving nutrient removal.

Incoming Wastewater Testing

Monitoring of influent wastewater is an important component of operating an activated sludge plant. -Influent BOD testing will allow monitoring of F/M ratio and plant organic loading. -The water quality parameters of pH and alkalinity and the presence of toxic substances can impact plant operation, because of their impact on the health of the organisms. -The incoming wastewater flow rate will impact the organic loading on the plant.

primary clarifier, Equalization Basin, Pre-settling Basin

Pollutants that are dissolved or are very fine and remain suspended or floating in the wastewater aren't removed effectively by gravity settling. When the wastewater enters a sedimentation tank, it slows down and the suspended solids gradually sink to the bottom. -The floating solids are usually removed with a mechanical skimmer or is manually removed.

Activated Sludge Process Testing

Process variables impacting the performance of the activated sludge process: -the mass of organisms in the aeration tank or activated sludge concentration -the sludge age, the dissolved oxygen concentration -the proper distribution of flow to parallel treatment units -management of return activated sludge (RAS) and waste activated sludge (WAS)

Aeration

Purpose: 1) to dissolve oxygen into the wastewater in the aeration tank 2) to intermix the mixed liquor suspended solids in the aeration tank with the incoming wastewater. Methods: Mechanical and Diffused

Suspended solids (SS)

Suspended solids (SS) in the aeration system can be calculated with just the solids in the aeration tank, or with solids in both the aeration tank and the secondary clarifier

A2/O (anaerobic, anoxic and aerobic, or oxic)

The A2/O (anaerobic, anoxic and aerobic, or oxic) process adds an anaerobic tank to the head of the MLE process and returns clarifier solids to this tank rather than the anoxic tank. The A2/O process will provide phosphorus as well as nitrogen removal.

MLE (Modified Ludzak Ettinger)

The MLE (Modified Ludzak Ettinger) process uses only the first two tanks in the four-stage Bardenpho process: the anoxic tank followed by the aeration tank. Both clarifier solids and aeration tank MLSS are returned to the anoxic tank.

Economic and Labor Benefits

The energy and labor requirements of each system are determinants in choosing the best modification. A smaller municipality may not be able to sustain some of the more labor and energy intensive systems. For example, oxidation ditches are usually low energy, low labor systems. Pure oxygen systems, however, require greater amounts of energy and labor.

Step-Feed Aeration Key Process Design Parameters

The following range of process design parameters is permissible for a step feed activated sludge process: -F/M Ratio=0.2 - 0.5 #BOD/#MLVSS/day -Organic Loading (maximum)=40 #BOD/1,000 cubic feet/day -MLSS=1000 - 3000 mg/liter -Aeration Retention Time (minimum)=6 hours -Solids Recycle Rate=15% - 100%

Activated Sludge Concentration

The mass of microorganisms is a component to be monitored and adjusted for good plant operation. -The Mixed Liquor Volatile Suspended Solids (MLVSS) concentration is a measure of the concentration of organisms present in the mixed liquor. The test for MLVSS first requires the determination of the Mixed Liquor Suspended Solids (MLSS) concentration. The MLVSS is a fractional percentage of the MLSS.

Biological Nutrient Removal Processes: Bardenpho, Anaerobic/Anoxic/Oxidation (A2/O), Modified Ludzack-Ettinger (MLE) Key Process Design Parameters (Typical Design)

The successful operation of a biological nutrient removal (BNR) system is dependent upon several process parameters, including: • Temperature, which affects microbial reaction rates. • pH and alkalinity, which must be controlled to prevent inhibition of microbial reactions. • Dissolved oxygen concentration, which affects reaction rates and creates inhibitions. • Mixing, this ensures uniform reactor conditions. • Return of activated sludge and internal nitrate recycling, which provides necessary microorganisms and nitrate for the denitrification process in the anoxic zones.

Difference between SBRs and continuous Activated Sludge

The unit processes of SBR and conventional activated sludge systems are the same. However, these processes occur over time in the SBR; in the conventional system the unit processes occur simultaneously.

Biological Nutrient Removal Processes: Bardenpho, Anaerobic/Anoxic/Oxidation (A2/O), Modified Ludzack-Ettinger (MLE)

These process modifications to the conventional activated sludge system are made to enhance the removal of nutrients from wastewater. A simple Bardenpho process is presented here to represent the range of Biological Nutrient Removal Processes. • Ammonia nitrogen, which is present in raw municipal wastewater, is converted to nitrate nitrogen during normal activated sludge treatment. • Anoxic (containing no residual dissolved oxygen) tanks or zones are added to the conventional activated sludge process train to convert the nitrates to nitrogen gas. • In the anoxic zones, facultative bacteria strip oxygen from the nitrates since oxygen is not available in dissolved form in the wastewater.

Complete Mix

This modification is used to simulate a completely-mixed reactor tank in which conditions within the tank are the same throughout the tank. The benefits of this modification include greater volumetric loading, a more stable microbial population, more efficient aeration, and tolerance of shock loads. • Both primary effluent and return sludge are distributed uniformly along the length of the aeration tank. • Mixed liquor is removed uniformly from the length of the aeration tank. • Aerators are located uniformly along the aeration tank.

Step-Feed Aeration

This process modification is used to provide a more uniform distribution of oxygen demand throughout the aeration tank. It is particularly beneficial when dealing with variable shock loads. • Primary effluent is piped to two or more locations along the length of the aeration tank. • Distributed loading minimizes any decreases in dissolved oxygen concentration along the length of the aeration tank. • The percent of primary effluent distributed to each location can be varied to optimize process performance.

Kraus Process System Configuration

To modify the conventional activated sludge system into a Kraus process system, the following changes are made: • The reaeration tank is added. • Recycling and reaeration of the digester supernatant and solids occurs. • A portion of the return sludge is reaerated.

Complete Mix System Configuration

To modify the conventional activated sludge system into a complete mix system, the following changes are made: • Primary effluent is distributed to multiple locations throughout the aeration tank. • The flexibility of operation is enhanced by varying the amount of primary effluent distributed to each location. • Baffling of the aeration tank creates multiple mixing zones coincident with the distribution points.

Contact Stabilization System Configuration

To modify the conventional activated sludge system into a contact stabilization system, the following changes are made: • Primary effluent is introduced into the Contact Zone. • A stabilization tank is added to stabilize the adsorbed organics. • Overall oxygen demand is split between contact tank and stabilization tank. • A benefit of this process modification is the ability to perform well under high flow, wet weather conditions.

Step-Feed Aeration System Configuration

To modify the conventional activated sludge system into a step-feed aeration system, the following changes are made: • Primary effluent is distributed to multiple locations in the aeration tank. • Baffling of the aeration tank creates multiple mixing zones coincident with the distribution points. • Operational flexibility is achieved by varying the amount of primary effluent distributed to each area.

Flow Splitting Where Sufficient Loadings Exist

When duplicate plant units exist, accurate flow splitting is important to achieve consistent operation and effluent quality. -A hydraulic imbalance can cause an organic overload and overcome the design capability of a specific unit. -Unbalanced flows and organic load can impact the operation of aeration tanks and clarifiers. -Unbalanced flows and organic load can affect the activated sludge process by impacting sludge volumes, sludge age, and F/M ratio. -Uneven flows may cause overfeeding or starvation that may upset the activated sludge process.

Inflow/Outflow Charasteristics

both influent and effluent characteristics occur periodically in SBR which is a batch activated sludge process; therefore the continuous treatment plant periodically transitions from one reactor to another

Anoxic

containing very little residual dissolved oxygen

Diffused Aeration

disperses compressed air to provide both oxygen to and the mixing of wastewater.

secondary clarifier operation

the physical process of removing microorganisms and solids from treated wastewater. The purpose of the clarifier is to produce a clear effluent suitable for discharge, to remove excess organisms from the activated sludge system, and to provide a source of organisms to return to the activated sludge process as required.

Adsorption

the process in which the colloidal and particulate organic content of the wastewater becomes attached to the microorganisms

Contact Tank

used for aeration of the mixed liquor. • Influent or primary clarifier effluent is added to the contact tank. • While in the contact tank, colloidal and particulate organic matter (BOD) is adsorbed onto the microorganisms. • Residence time in the contact tank is approximately 30 to 90 minutes. • After leaving the contact tank, the mixed liquor is settled in the secondary clarifier and the MLSS containing the biomass is returned to the stabilization tank. • The treatment cycle then restarts.

Metering devices

used to assist in balancing the air distribution between several air lines, or to measure the air flow from a blower or through a particular pipe. An orifice plate with up stream and downstream pressure gauges is a commonly-used metering device for air piping.

Valves

used to isolate blowers, headers, or other areas of the air piping based on system performance requirements or to perform maintenance. Valves may also be used to balance the air flow between several air lines or to throttle centrifugal blowers. Throttling a blower creates a pressure drop that changes the output of the blower. This would be done to reduce the aeration capacity of the system, if required.

Stabilization Tank

used to reaerate return sludge prior to mixing it with the primary effluent. • While in the stabilization tank, most of the organic material that was adsorbed by the microorganisms in the contact tank is metabolized. • Residence time in the stabilization tank is approximately 4 to 6 hours.

Mechanical Aeration

uses devices to either splash water into the air or mix air into the wastewater.

Parameters of the SBR

• Multiple processing stages occur in one tank. o SBRs use specified time durations for each treatment process. • Multiple processing cycles occur each day. o Typically, from two to six complete treatment cycles will be completed in each reactor each day. • Batch processing of wastewater occurs. o A normal SBR system operates in the batch mode, wherein the influent flow is cyclic and each batch is processed through the entire treatment process. • SBR modifications provide continuous processing of wastewater. o A modification of the normal SBR process allows for continuous influent flow without sacrificing the benefits of the SBR process.