Introduction To Activated Sludge Study Guide

Define denitrification

A biological process where bacteria convert nitrate (NO3-) and nitrite (NO2-) to nitrogen gas (N2) under anoxic conditions.

Define nitrification

A biological process where nitrifying bacteria convert nitrogen in the form of ammonia (NH3) into nitrite (NO2-) and nitrate (NO3-) under aerobic conditions.

Define aerobic (oxic) [O2]

A condition in which free and dissolved oxygen is available in an aqueous environment

Define Anaerobic [Ø].

A condition in which free, dissolved, and combined oxygen is unavailable in an aqueous environment.

Define Anoxic [NO2, NO3, SO4]

A condition in which oxygen is only available in a combined form such as nitrate (NO3-), nitrite (NO2-), or sulfate (SO4) in an aqueous environment.

Define diffusers

A diffuser is a perforated membrane, porous disc, or other device used for discharging air into the aeration basins.

Define sludge volume index (SVI)

A numerical expression of the settling characteristics of activated sludge in the final clarifier. SVI is expressed as the ratio of the volume in milliliters of activated sludge settled from a 1,000-mL sample in 30 minutes divided by the concentration of mixed liquor in milligrams per liter multiplied by 1,000. A good settling sludge (textbook value) is 100, but can commonly be between 80-150.

Define clarification

A process to reduce the concentration of suspended matter in water. In the activated sludge treatment process, the removal of suspended solids from wastewater is usually through gravity separation in a clarifier.

Define 30 minute settling test

A sample of mixed liquor is taken as it exits the aeration tank. It is mixed, and then allowed to settle for 30 minutes in a 1000 mL beaker or cylinder. This test shows the sludge settling characteristics and the clarity of the water on top of the sludge. It reflects the performance of the secondary clarifier and can be used to help diagnose clarifier settling problems.

Define secondary treatment

A term to describe the biological treatment of wastewater. Activated sludge is a type of secondary treatment. Secondary treatment provides a high level of removal of biodegradable organic pollutants to protect receiving water quality that clarification alone cannot provide.

Contact Stabilization

A two tank process of activated sludge treatment. The first tank (contact tank) has a short detention time, followed by clarification. The settled sludge is pumped to a second tank (re-aeration tank) with a much longer detention time. The advantages of contact stabilization are less total tank volume required than that needed for conventional processes, and it reduces the potential loss of MLSS through washout. Contact stabilization may be applicable when stringent effluent limits are not required.

Define weir(s)

A weir is a level control structure (often v-notched) in a final clarifier used to provide a uniform effluent flow.

List common operational problems that can occur in the activated sludge process.

A. Aeration basin low dissolved oxygen B. Clarifier settling C. Foaming D. Loss of nitrification

List the basic components of an activated sludge system.

A. Aeration tank B. Blowers and diffusers or mechanical aerators C. Clarifiers D. WAS/RAS pumps

Describe common indicator organisms one would see when looking under a microscope at a sample of activated sludge.

A. Amoeba B. Flagellates C. Free swimming ciliate D. Crawling ciliate E. Single stalked ciliate F. Community stalked ciliate G. Rotifer H. Filamentous organism

Discuss the methods of controlling dissolved oxygen levels in diffused air systems.

A. By controlling air valves B. By controlling the blower output such as using VFDs C. By increasing or decreasing the number of blowers in operation D. Cleaning or replacing diffusers E. Changing the number of diffusers F. Process control (ex. MLSS levels)

Discuss the methods of controlling dissolved oxygen levels in mechanical aeration systems.

A. By increasing or decreasing the aerator speed by using VFDs B. By increasing or decreasing the aerator submergence by adjusting the tank water level C. By increasing or decreasing the number of aerators in operation D. Process control (ex. MLSS levels)

List the maintenance considerations for a diffused aeration system.

A. Centrifugal Blowers 1. Unusual noise or vibrations 2. Lubrication of blowers and motors 3. Check and lubricate couplings 4. Check discharge pressure and temperature 5. Check filters and obstructions 6. Check amperage meter

List the maintenance considerations for diffusers

A. Check surface aeration patterns for uneven distribution B. Check air line pressure reading C. Check and purge moisture as needed D. Drain, inspect, and clean the aeration tanks annually All maintenance and repairs should be documented.

Discuss factors that influence the flow rates of return activated sludge (RAS)

A. Clarifier Sludge Blanket B. RAS Concentration C. Final Clarifier Solids Loading Rate (SLR) D. Denitrification

List the maintenance considerations for a mechanical aeration system.

A. Consult the O&M manual for the lubrication needs of the motor, gear box, shaft and others B. Inspect aerators C. Check for unusual vibration All maintenance and repairs should be documented.

Compare the maintenance requirements of fine bubble to coarse bubble diffuser systems.

A. Course Bubble Aeration Systems 1. Aeration basins should be drained annually 2. Remove excess settled solids that have accumulated 3. Clean diffusers and piping assemblies as needed 4. Inspect all hardware and components 5. Repair, replace, and tighten components as needed 6. Refill aeration tank following startup procedures

Describe the environmental conditions necessary to support the growth of nitrifying bacteria. Nitrifying bacteria convert ammonia to nitrate. They work best under the following environmental conditions:

A. Dissolved oxygen greater than 1.0 mg/L B. pH between 7.0 and 8.5 C. Alkalinity greater than 50 mg/L D. Temperature between 50-85 ºF (10-30 ºC)

Describe the environmental conditions needed for the growth of denitrifying bacteria. Denitrifying bacteria convert nitrite and nitrate to nitrogen gas. They work best under the following environmental conditions:

A. Dissolved oxygen less than 0.2 mg/L B. pH between 7.0 and 8.5 C. Adequate organic matter (BOD) D. Temperature between 50-85 ºF (10-30 ºC)

Describe other variations of the activated sludge process

A. Step Feed B. Contact Stabilization C. Complete Mix D. Sequencing Batch Reactors (SBRs)

The major factors that influence energy consumption in a diffused aeration system are:

A. Type of aeration equipment 1. Fine bubble diffuser 2. Coarse bubble diffuser B. Placement of diffusers on tank floor 1. Full tank floor coverage 2. Non-full tank floor coverage (i.e. side tank wall(s) or center of tank placement) C. System operating pressure D. Oxygen transfer efficiency (following ASCE standard test protocol). Oxygen transfer efficiency is a function of bubble size and diffuser placement. E. Diffuser maintenance requirements

List common process control tests used for operating an activated sludge treatment plant

A. sight and smell B. dissolved oxygen/pH/temperature C. 30 minute settling test and settling curve D. sludge volume index (SVI) E. sludge age F. F:M ratio H. return activated sludge (RAS) flow and concentration I. waste activated sludge (WAS) and concentration J. mixed liquor suspended solids (MLSS of MLVSS) K.clarifier sludge depths I. microscope

Describe the activated sludge process

Activated sludge is a biological process that utilizes microorganisms to convert organic and certain inorganic matter from wastewater into cell mass. The activated sludge is then separated from the liquid by clarification. The settled sludge is either returned (RAS) or wasted (WAS). Activated sludge is commonly used as a wastewater treatment process because it is an effective and versatile treatment process and capable of a high degree of treatment.

Define activated sludge

Activated sludge is a mixture of bacteria, fungi, protozoa and rotifers maintained in suspension by aeration and mixing

Sludge Age

Activated sludge is recycled back through the aeration basins by returning settled sludge in the final clarifiers and thus remains in the activated sludge system for a number of days. For effective treatment, a certain sludge age is desired for the type of activated sludge system. For conventional activated sludge, a sludge age of 3-10 days is typical. For extended aeration activated sludge, older sludge ages of 15-30 days are common. F/M ratio and sludge age are inversely related (1 divided by the sludge age approximates the F/M ratio). The older the sludge, the lower the F/M ratio; conversely, the younger the sludge, the higher the F/M ratio.

Discuss the importance of routine preventative maintenance of aeration basins and clarifiers.

Aeration basins and clarifiers should be emptied on a regular basis to: A. Perform a detailed inspection of the structure, valves, and control gates B. Clean out grit and settled solids C. Maintain equipment and piping.

Temperature

All biological and chemical reactions are affected by temperature. Microorganisms growth and reaction rates are slow at cold temperatures and much faster at warmer temperatures. Most microorganisms do best under moderate temperatures (10-25ºC). Aeration basin temperatures should be routinely measured and recorded.

Complete Mix

An activated sludge process where the contents of the entire aeration tank are rapidly mixed to provide a uniform distribution of food (BOD), microorganisms, and dissolved oxygen (see figure 1.3.3.3). An advantage is the ability to handle surges in loading. This process is used when a high quality effluent is not required. Complete mix is most commonly used in industrial activated sludge plants.

Define bulking

An activated sludge that does not settle well and may overflow the weirs of the final clarifiers resulting in excess suspended solids in the effluent. It is usually caused by filamentous organisms.

List the items to include in a maintenance schedule for final clarifiers

An operator should consult the O&M Manual for the preventative maintenance schedule. Daily observations should be performed such as checking for oil leaks, unusual vibrations or noises, scum collection, weirs, and floating solids. All maintenance and repairs should be documented.

Compare the maintenance requirements of fine bubble to coarse bubble diffuser systems

B. Fine Bubble Aeration Systems 1. Aeration basins should be drained annually 2. Drain aeration basin and leave air on 3. Remove excess settled solids that have accumulated 4. With air on, hose off and wash each diffuser with clean water 5. With air off, if needed scrub each diffuser with either a soft bristle brush or rag. 6. Turn air back on and repeat hosing procedure for each diffuser 7. Inspect all hardware and components 8. Repair, replace, and tighten components as needed 9. Refill aeration tank following startup procedures

List the maintenance considerations for a diffused aeration system.

B. Positive Displacement Blowers 1. Unusual noise or vibrations 2. Lubrication of blowers and motors 3. Check and lubricate couplings 4. Check and exercise pressure relief valve 5. Check discharge pressure and temperature 6. Check filters and obstructions 7. Check blower seals 8. Check drive belt alignment and tension All maintenance and repairs should be documented.

pH

Biological and chemical reactions are affected by pH. Most microorganisms do well in a pH environment between 6.0-9.0. Acidic (low pH) or alkaline (high pH) conditions can adversely affect microorganism growth and survival. Operators measure both influent pH and aeration basin pH to ensure proper plant pH conditions

List what to look for when inspecting clarifier weirs.

Clarifier weirs should be inspected daily for: A. Level weirs- even flow uniformly across the weirs B. Algae or debris plugging v-notches C. Condition of the weirs D. Effluent quality

Define floc

Clusters of microorganisms and solid particles that form in the activated sludge process and settle in the final clarifier.

Oxygen

Conventional activated sludge is an aerobic process. Many bacteria in the activated sludge process need free oxygen (O2) to convert food into energy for their growth. For optimal performance, it is very important for an operator to be sure enough oxygen is being provided in the aeration tanks for the microorganisms (typically 1.0-3.0 mg/L). Aeration basin dissolved oxygen concentrations (milligrams per liter) are measured continuously in many plants to ensure adequate oxygen is available.

Describe conventional (plug flow) activated sludge process.

Conventional plug flow activated sludge is a process in which influent and returned activated sludge enters at the head of the aeration tank and travels through the tank at a constant rate to the point of discharge. The sludge age is generally less than 15 days, usually best between 3-10 days.

Describe a diffused aeration system

Diffused aeration is a method of introducing air at the maximum possible submergence. Oxygen is transferred into the liquid as the bubbles rise through the water to the surface. Because the retention time of the air bubbles is maximized, oxygen transfer is greater. The most common types of diffusers generate fine or coarse bubbles.

Describe the extended aeration activated sludge process.

Extended aeration uses conventional plug flow patterns, however, the aeration tanks are larger to provide for over 15 hours of hydraulic retention times (HRT). The sludge age is typically greater than 15 days, usually best between 15-30 days. This results in a highly treated effluent, and less WAS produced. The oxidation ditch is a variation of the extended aeration process.

Define filaments

Filamentous organisms are a group of thread-like organisms that, when in excess, can impair the settling of activated sludge and create a bulking condition in the final clarifier

Define food to microorganism ratio (F:M or F/M)

Food to microorganism ratio (F:M or F/M) is the amount of food (BOD5) provided to the microorganisms (MLVSS or MLSS) in the aeration basins. F/M is determined by dividing the pounds of influent BOD5 by the pounds of mixed liquor volatile suspended solids or mixed liquor suspended solids under aeration.

Food To Microorganism Ratio

For microbiological health and effective treatment, the microorganisms (mixed liquor suspended solids) under aeration should be maintained at a certain level for the amount of food (influent BOD) coming into the plant. This is known as the food to microorganism ratio. For conventional activated sludge, the F/M ratio is usually between 0.2-0.5. For extended aeration systems, such as package plants and oxidation ditches, the F/M ratio should be between 0.03-0.10.

Define hydraulic load

Hydraulic load is the flow entering the plant, measured in million gallons per day (MGD)

Discuss the significance of denitrification occuring in a final clarifier

In the absence of oxygen, a sludge blanket that is too thick and remains in the clarifier too long can denitrify. Nitrates in the sludge will be converted to nitrogen gas. The release of nitrogen gas will cause small gas bubbles that will be observed at the clarifier surface. Clumps of sludge may also rise to the surface

Step Feed

In the step-feed process, primary effluent is added at two or more points along the length of the aeration tank. This configuration spreads out the organic load and evens out the oxygen uptake rate throughout the length of the basin. This configuration also allows for better control in handling shock loads and lower MLSS to the secondary clarifiers.

Flow

Incoming wastewater must flow through a treatment plant at a rate that allows microorganisms sufficient time to consume the incoming food and to settle properly. High flows can shorten the time necessary for the full treatment of wastewater. Extremely high flows can wash microorganisms out of the plant through the final clarifier.

Toxicity

Incoming wastewater to a treatment plant may at times contain materials or compounds that are toxic to microorganisms. Depending on the concentration of toxic material, microorganisms could be destroyed or their metabolic rates affected, thus impairing the wastewater treatment plant efficiency.

Food

Incoming wastewater to a treatment plant provides the food that microorganisms need for their growth and reproduction. This food is mostly organic material. The more soluble the organic material is, the more easily microorganisms can use it. Since the amount and type of organic loading in the treatment plant affects the growth of the microorganisms, influent total BOD and soluble BOD are measurements an operator can make to determine the amount and type of incoming food for the microorganisms

Describe a mechanical aeration system

Mechanical aeration is a method that forces oxygen and surface water down into the liquid with a mechanical mixing device. The most common types of mechanical aerators utilize paddles or discs, spray or turbine mechanisms.

Nutrients

Microorganisms need trace nutrients such as nitrogen, phosphorus, and some metals for their metabolism. Most incoming wastewater to a treatment plant, especially domestic sewage, contains an abundance of these trace nutrients. The ratio of BOD5 to nitrogen (N) to phosphorus (P) should be at least 100:5:1. Influent wastewater can be measured to determine this nutrient ratio.

Define slime bulking

Nutrient deficiency causes stressful conditions for bacteria. Nutrient deficient bacteria are unable to produce proper cell walls and as reaction to stress will produce excess amounts of a slimey, fat (lipid) layer instead of a normal cell wall. Excess organic acids can also cause stress on bacteria and can increase slime bulking. Slime bulking affects sludge settling.

Old Sludge

Old sludge consists of sludge in which the sludge age is too high to be most effective in a particular activated sludge process. Dark brown foam and a somewhat greasy or scummy appearance is an indicator of old sludge. Settling in the clarifier is rapid, but pin floc can be present in the effluent and the effluent is hazy. Old sludge is often associated with a low F/M ratio. To correct for old sludge, it is necessary to increase wasting rates and return less sludge to the aeration basin. This will reduce the amount of solids under aeration, increase the F/M ratio and decrease the sludge age

Define organic loading and organic overload

Organic loading is the amount of biodegradable material that exerts an oxygen demand on the biological treatment process. The organic strength of the wastewater is usually measured as biochemical oxygen demand (BOD) in milligrams per liter (mg/L). An organic overload is an event which significantly increases the organic loading (BOD) to the aeration basin above normal influent organic loading conditions.

Define Protozoa

Protozoa are single celled microscopic organisms that require oxygen and food (bacteria) for growth and reproduction. Protoza include amoeba, flagellates and ciliates.

Describe the types of protozoa and organisms commonly found in activated sludge and observable under a microscope.

Protozoa are single-celled microscopic organisms, several hundred times larger than bacteria. It is the protozoa we observe under a microscope since bacteria are actually too small to see. There are four types of protozoa commonly found in activated sludge systems. They are identified by their method of movement within the wastewater environment. The four types are amoebae, ciliates (free-swimming and stalked), flagellates and suctoreans. Rotifers are multi-celled (metazoa) organisms also commonly found in activated sludge systems. The relative predominance of these protozoa is commonly associated with the age of the activated sludge.

Constant MLSS

Provided the influent loadings are constant, the operator maintains a relatively constant solids inventory (MLSS level) in the aeration basins for a desired level of treatment. The range of MLSS is typically between 1000-4000 mg/L.

Define short-circuiting

Short circuiting is an uneven flow distribution in a wastewater tank. Density currents occur in some parts of a tank and the wastewater travel time (detention time) is less than in other parts of the tank

Define sludge age

Sludge age is the theoretical length of time a particle of activated sludge stays in the treatment plant, measured in days. In an activated sludge plant, sludge age is the amount (lbs) of mixed liquor suspended solids divided by the suspended solids, or excess cell mass, withdrawn from the system per day (lbs per day of waste activated sludge).

Define straggler floc

Small, light, and fluffy floc particles with poor settling characteristics, usually indicative of a younger sludge and/or low MLSS levels.

Explain how solids are generated in an aeration basin, and the consequences to the operation if excess solids are not removed (wasted).

Solids are generated by microorganism growth and reproduction. The influent BOD supplies the food for the growth and reproduction. As microorganisms' populations multiply, excess solids (microorganisms) must be removed (wasted). If excess solids are not removed, the mixed liquor suspended solids (MLSS) and sludge age will increase and process efficiency will be lowered. Sludge settling rates are affected. Eventually, if excess solids do not get wasted, they can overflow the clarifier weirs and into the receiving water.

A. Clarifier Sludge Blanket

Solids settle and concentrate in the final clarifiers forming a sludge blanket. The sludge blanket can increase or decrease depending on the RAS flow rate. The proper RAS flow rate allows for a desired sludge blanket.

Sequencing Batch Reactors (SBRs)

The Sequencing Batch Reactor (SBR) is a modification of the activated sludge process that treats wastewater in batches as opposed to a continual or flow-through basis. In most facilities the use of an SBR requires at least two reactor vessels so that wastewater can be accepted at all times. There are a great number of variations based on proprietary mechanisms and processes in SBR treatment, but all systems have a sequence of at least four steps that need to occur for the proper operation of the systems. The four treatment steps need to include: 1. Fill 2. React 3. Settle 4. Decant/Sludge waste SBRs that remove nutrients may have additional steps in the sequence. SBRs provide secondary treatment within a smaller footprint because final clairfiers are not used in the process, but the process requires a more sophisticated control system.

Define waste activated sludge (WAS)

The activated sludge (excess biomass or cell mass) removed from the secondary treatment process. For most treatment plants, this will be a portion of the Return Activated Sludge (RAS) flow stream.

Describe the environmental factors that influence the health and growth of microorganisms

The activated sludge wastewater treatment process must operate under proper environmental conditions to support a healthy, growing population of microorganisms. The operator must monitor the activated sludge process to ensure the right environmental conditions are being provided for the microorganisms. Efficient wastewater treatment plant performance will then be achieved.

Describe the purpose of the aeration system

The aeration system in the activated sludge provides oxygen to the microorganisms and mixes the contents of the aeration basins. The mixing brings the wastewater pollutants into contact with the microorganisms to treat the wastewater and reduce the pollutants.

Discuss energy usage in an activated sludge process

The aeration system of an activated sludge uses the largest percentage (60%+) of the energy in the treatment process. Wastewater pumping is another large energy user (12%) at a wastewater plant. Energy usage can be reduced with cost savings by having energy efficient aeration systems, blowers, motors, and pumps.

Define mixed liquor volatile suspended solids (MLVSS)

The amount of organic or volatile suspended solids in an aeration tank, expressed in mg/L. This volatile portion is used as a measure of the microorganisms present in the aeration tank. Total pounds of MLVSS in an aeration tank can be calculated by multiplying the concentration of MLVSS (mg/L) in the aeration tank by the tank volume (MG), and then multiplying the product by 8.34 (lbs/gal).

Define mixed liquor suspended solids (MLSS)

The amount of suspended solids in an aeration tank, expressed in milligrams per liter (mg/L). MLSS consists mostly of microorganisms and non-biodegradable suspended matter. Total pounds of MLSS in an aeration tank can be calculated by multiplying the concentration of MLSS (mg/L) in the aeration tank by the tank volume (MG), and then multiplying the product by 8.34 (lbs/gal).

Describe the characteristics of healthy acitvated sludge

The color of healthy activated sludge is tan to brown. It would have an earthy odor. During a 30 minute settling test, the settled sludge volume would be 200-300 mL/L. The SVI would be 80-150. The supernatant would be clear with little or no floc particles. Sludge age for conventional systems would be 3-10 days and 15-30 days for extended aeration systems

Describe the affect of waste activated sludge concentration on desired wasting rates

The concentration of WAS has a direct bearing on how much to waste and the volume wasted. On a volume basis, a thicker waste activated sludge (high WAS concentration) will require less amount of wasting than a thinner waste activated sludge (low WAS concentration).

Describe the conditions that favor the growth of filamentous organisms in the activated sludge process.

The growth of filamentous organisms can occur due to the following conditions: A. Low dissolved oxygen B. Low food to mircoorganism (F/M) ratio C. Low pH D. High sulfides E. Nutrient deficiency F. Excessive grease

Describe the role microorganisms have in the activated sludge process

The principle role microorganisms have in the activated sludge process is to convert dissolved and particulate organic matter, measured as biochemical oxygen demand (BOD), into cell mass. In a conventional activated sludge process, microorganisms use oxygen to break down organic matter (food) for their growth and survival. Over time and as wastewater moves through the aeration basin, food (BOD) decreases with a resultant increase in cell mass (MLSS concentration).

C. Final Clarifier Solids Loading Rate (SLR)

The rate at which the activated sludge is returned from the final clarifiers to the aeration basins, along with the influent flow, effects the flow of solids into the clarifiers. Aeration basin mixed liquor suspended solids must have sufficient time to settle and be returned or wasted in the activated sludge system. Clarifiers are designed for certain solids loading rates that should not be exceeded.

Define return activated sludge (RAS)

The settled activated sludge (biomass) that is collected in a secondary clarifier and returned to the secondary treatment process to mix with incoming wastewater. This returns a concentrated population of microorganisms back into the aeration basin.

Define sludge blanket

The sludge blanket is the layer of solids on the bottom of the clarifier.

Define variable frequency drive (VFD)

Variable Frequency Drive (VFD) is a system for regulating the rotational speed of an alternating current (AC) electric motor by controlling the frequency of the electric power supplied to the motor.

B. RAS Concentration

Varying the RAS flow rate will affect the concentration and detention time of clarified solids. Adjusting the RAS pumping rate allows the return of more or less concentrated solids while also increasing or decreasing the depth of the sludge blanket. RAS flow rates can be paced off influent flow rates.

Define pin floc

Very fine floc particles with poor settling characteristics, usually indicative of an old sludge (high MLSS levels).

Define washout

Washout is the loss of biomass from the final clarifiers due to high flows

Discuss the importance of wasting sludge on a regular basis

Wasting sludge is the most important operational process control of the activated sludge process. By wasting sludge on a consistent basis, preferably daily, the biomass within the aeration tank will remain healthy and at a consistent MLSS level

D. Denitrification

When RAS flow rates are too low, thick sludge blankets in the final clarifier can result. The operator will see gas bubbles (from nitrogen gas) and rising/floating sludge clumps on the clarifier surface.

Young Sludge

Young sludge consists of sludge which has not yet reached a high enough sludge age to be most effective in a particular activated sludge process. Billowing whitish foam is an indicator that the sludge age is too low. Young sludge will often have poor settling characteristics in the clarifier, and can leave straggler floc in the clarifier effluent. Young sludge is often associated with a high F/M. To correct for young sludge it is necessary to decrease wasting rates. This will increase the amount of solids under aeration, reduce the F/M ratio, and increase the sludge age

Discuss the transfer of oxygen into wastewater.

diffused aeration system or a mechanical aeration system. Transfer efficiency depends on the contact time between the bubble and the liquid, the size of the bubble, and the turbulence of the liquid. Longer contact time, smaller bubbles and more turbulence of the liquid creates greater transfer efficiencies.