Chapter 8
Denitrification
(1) The anoxic biological reduction of nitrate nitrogen to nitrogen gas. (2) The removal of some nitrogen from a system. (3) An anoxic process that occurs when nitrite or nitrate ions are reduced to nitrogen gas and nitrogen bubbles are formed as a result of this process. The bubbles attach to the biological floc and float the floc to the surface of the secondary clarifiers. This condition is often the cause of rising sludge observed in secondary clarifiers or gravity thickeners.
Absorption
(1) The taking in or soaking up of one substance into the body of another by molecular or chemical action (as tree roots absorb dissolved nutrients in the soil). (2) The gathering of a gas, liquid, or dissolved substance on the surface or interface zone of another material.
Activated Sludge Process
A biological wastewater treatment process that 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.
Sludge Volume Index (SVI)
A calculation that indicates the tendency of activated sludge solids (aerated solids) to thicken or to become concentrated during the sedimentation/thickening process. SVI is calculated in the following manner: (1) allow a mixed liquor sample from the aeration basin to settle for 30 minutes; (2) determine the suspended solids concentration for a sample of the same mixed liquor; (3) calculate SVI by dividing the measured (or observed) wet volume (mL/L) of the settled sludge by the dry weight concentration of MLSS in grams/L. SVI, mL/gm = (Settled Sludge Volume/Samp;e Volume, mL/L) / (Suspended Solids Concentration, mg/L) x 1,000 mg/gram
Composite Sample
A composite sample is a collection of individual samples obtained at regular intervals, usually every one or two hours during a 24-hour time span. Each individual sample is combined with the others in proportion to the rate of flow when the sample was collected. Equal volume individual samples also may be collected at intervals after a specific volume of flow passes the sampling point or after sample and is analyzed to determine the average conditions during the sampling period.
Anoxic
A condition in which the aquatic environment does not contain dissolved oxygen, which is called an oxygen deficient condition. Generally refers to an environment in which chemically bound oxygen, such as in nitrate, is present. The term is similar to anaerobic.
Septic
A condition produced by anaerobic bacteria. If severe, the sludge produces hydrogen sulfide, turns black, gives off foul odors, contains little or no dissolved oxygen, and the wastewater has a high oxygen demand.
Diffuser
A device (porous plate, tube, bag) used to break the air stream from the blower system into fine bubbles in an aeration tank or reactor.
Diffused-Air Aeration
A diffused-air activated sludge plant takes air, compresses it, and then discharges the air below the water surface of the aerator through some type of air diffusion device.
Secchi Disk
A flat, white disk lowered into the water by a rope until it is just barely visible. At this point, the depth of the disk from the water surface is the recorded Secchi disk transparency.
Protozoa
A group of motile, microscopic organisms (usually single-celled and aerobic) that sometimes cluster into colonies and generally consume bacteria as an energy source.
Manifold
A large pipe to which the ends of a series of smaller pipes are connected. Also called a Header
Header
A large pipe to which the ends of a series of smaller pipes are connected. Also called a manifold
Polymer
A long-chain molecule formed by the union of many monomers (molecules of lower molecular weight). Polymers are used with other chemical coagulants to aid in binding small suspended particles to larger chemical flocs for their removal from water.
Biomass
A mass or clump of organic material consisting of living organisms feeding on the wastes in wastewater, dead organisms, and other debris.
Sludge Age
A measure of the length of time a particle of suspended solids has been retained in the activated sludge process. Sludge Age, days= (Suspended Solids Under Aeration, lbs) / (Suspended Solids Added, lbs/day)
Endogenous Respiration
A situation in which living organisms oxidize some of their own cellular mass instead of new organic matter they absorb or absorb from their environment.
Air Lift Pump
A special type of pump consisting of a vertical riser pipe submerged in the wastewater or sludge to be pumped. Compressed air is injected into a tail piece at the bottom of the pipe. Fine air bubbles mixed with the wastewater or sludge to form a mixture lighter than the surrounding water, which causes the mixture to rise in the discharge pipe to the outlet.
Burping
A term used to describe what happens when billowing solids are swept by the water up and out over the effluent weirs in the discharged effluent. Billowing solids result when the settling tank sludge blanket becomes too deep (occupies too much volume in the bottom of the tank.)
Batch Process
A treatment process in which a tank or reactor is filled, the wastewater (or other solution) is treated or a chemical solution is prepared, and the tank is emptied. The tank may then be filled and the process repeated. Batch processes are also used to cleanse, stabilize, or condition chemical solutions for use in industrial manufacturing and treatment processes.
Stabilized Waste
A waste that has been treated or decomposed to the extent that, if discharged or released, its rate and state of decomposition would be such that the waste would not cause a nuisance or odors in the receiving water.
Nitrification
An aerobic process in which bacteria change the ammonia and organic nitrogen in wastewater into oxidized nitrogen (Usually nitrate). The second-stage BOD is sometimes referred to as the "nitrogenous BOD" (first-stage BOD is called the "carbonaceous BOD")
Cathodic Protection
An electrical system for prevention of rust, corrosion, and pitting of metal surfaces that are in contact with water, wastewater, or soil. A low-voltage current is made to flow through a liquid or a soil in contact with the metal in such a manner that the external electromotive force renders the metal structure cathodic. This concentrates corrosion on auxiliary anodic parts, which are deliberately allowed to corrode instead of letting the structure corrode.
Turbidity Meter
An instrument for measuring and comparing the turbidity of liquids by passing light through them and determining how much light is reflected by the particles in the liquid. The normal measuring range is 0 to 100 and is expressed as nephelometric turbidity units (NTUs). Also called a turbidimeter.
Anaerobes
Bacteria that do not need dissolved oxygen (DO) to survive.
Obligate Aerobes
Bacteria that must have atmospheric or dissolved molecular oxygen to live and reproduce.
Aerobes
Bacteria that must have dissolved oxygen (DO) to survive. Aerobes are aerobic bacteria.
Bulking
Clouds of billowing sludge that occur throughout secondary clarifiers and sludge thickeners when the sludge does not settle properly. In the activated sludge process, bulking is usually caused by filamentous bacteria or bound water.
Floc
Clumps of bacteria and particles that have come together and formed a cluster. Found in aeration tanks, secondary clarifiers, and chemical precipitation processes.
Contact Stabilization
Contact stabilization is a modification of the conventional activated sludge process. In contact stabilization, two aeration tanks are used. One tank is for separate deaeration of the return sludge for at least four hours before it is permitted to flow into the other aeration tank to be mixed with the primary effluent requiring treatment. The process may also occur in one long tank.
Coning
Development of a cone-shaped flow of liquid, like a whirlpool, through sludge. This can occur in a sludge hopper during sludge withdrawal when the sludge becomes too thick. Part of the sludge remains in place while liquid rather than sludge flows out of the hopper. Also called coring
Facultative Bacteria
Facultative Bacteria can use either dissolved oxygen or oxygen obtained from food materials such as sulfate or nitrate ions. In other words, facultative bacteria can live under aerobic, anoxic, or anaerobic conditions.
Food/Microorganism (F/M) Ratio
Food to microorganism ratio is a measure of food provided to bacteria in an aeration tank. F/M=BOD, lbs/day / MLVSS, lbs
Bacterial Culture
In the case of activated sludge, the bacterial culture refers to the group of bacteria classified as Aerobes and Facultative Bacteria, which covers a wide range of organisms. Most treatment processes in the United States grow facultative bacteria that use the carbonaceous (carbon compounds) BOD. Facultative bacteria can live when oxygen resources are low. When nitrification is required, the nitrifying organisms are Obligate aerobes (require oxygen) and must have at least 0.5 mg/L of dissolved oxygen throughout the whole system to function properly.
Zoogleal Mass
Jelly-like masses of bacteria found in both the trickling filter and activated sludge processes. These masses may be formed for or function as the protection against predators and for storage of food supplies.
Supernatant
Liquid removed from settled sludge. Supernatant commonly refers to the liquid between the sludge on the bottom and the scum on the surface of an anaerobic digester. This liquid is usually returned to the influent wt well or to the primary clarifier.
MCRT
Mean Cell Residence Time. An expression of the average time (days) that a microorganism will spend in the activated sludge process. MCRT, days = (Total Suspended Solids in Activated Sludge Process, lbs) / (Total Suspended Solids Removed From Process, lbs/day) Note: Operators at different plants calculate the Total Suspended Solids (TSS) in the Activated Sludge Process, lbs, by three different methods: (1) TSS in the Aeration Basin or Reactor Zone, lbs (2) TSS in the Aeration Basin and Secondary Clarifier, lbs (3) TSS in the Aeration Basin and Secondary Clarifier Sludge Blanket, lbs These three different methods make it difficult to compare MCRTs in days among different plants unless everyone uses the same method
Aeration Liquor
Mixed liquor. The contents of the aeration tank, including living organisms and material carried into the tank by either untreated wastewater or primary effluent.
Dissolved Oxygen
Molecular oxygen dissolved in water or wastewater, usually abbreviated DO.
Filamentous Organisms
Organisms that grow in a thread or filamentous form. Common types are Thiothrix and Actinomycetes. A common cause of sludge bulking in the activated sludge process.
Oxidation
Oxidation is the addition of oxygen, removal of hydrogen, or the removal of electrons from an element or compound; in the environment and a wastewater treatment processes, organic matter is oxidized to more stable substances. The opposite of Reduction.
Reduction
Reduction is the addition of hydrogen, removal of oxygen, or the addition of electrons to an element or compound. Under anaerobic conditions (no dissolved oxygen present), sulfur compounds are reduced to odor-producing hydrogen sulfide (H2S) and other compounds. In the treatment of metal finishing wastewater, hexavalent chromium (Cr^6+) is reduced to the trivalent form (Cr^3+). The opposite of Oxidation.
Aliquot
Representative portion of a sample. Often, an equally divided portion of a sample.
Rising Sludge
Rising sludge occurs in the secondary clarifiers of activated sludge plants when the sludge settles to the bottom of the clarifier, is compacted, and then starts to rise to the surface, usually as a result of denitrification, or anaerobic biological activity that produces carbon dioxide and/or methane.
Flights
Scraper boards, made from redwood or other rot-resistant woods or plastic, used to collect and move settled sludge or floating scum.
Seizing or Seize Up
Seizing occurs when an engine overheats and a part expands to the point where the engine will not run. Also called freezing.
Activated Sludge
Sludge particles produced in raw or settled wastewater (primary effluent) by the growth of organisms (including zoogleal bacteria) in aeration tanks in the presence of dissolved oxygen. The term "activated" comes from the fact that the particles are teeming with bacteria, fungi, and protozoa. Activated sludge is different from primary sludge in that the sludge particles contain many living organisms that can feed on the incoming wastewater.
Step-Feed Aeration
Step-feed aeration is a modification of the conventional activated sludge process. In Step-feed aeration, primary effluent enters the aeration tank at several points along the length of the tank, rather than at the beginning or head of the tank and flowing through the entire tank in a plug flow mode.
Mixed Liquor Suspended Solids (MLSS)
The amount (mg/L) of suspended solids in the mixed liquor of an aeration tank.
Mixed Liquor Volatile Suspended Solids (MLVSS)
The amount (mg/L) of suspended solids in the mixed liquor of an aeration tank.
Shock Load (Activated Sludge)
The arrival at a plant of a waste that is toxic to organisms in sufficient quantity or strength to cause operating problems. Possible problems include odors and bulking sludge, which will result in a high loss of solids from the the secondary clarifiers into the plant effluent and a biological process upset that may require several days to a week to recover. Organic or hydraulic overloads also can cause a shock load.
Aerobic Digestion
The breakdown of wastes by microorganisms in the presence of dissolved oxygen. This digestion process may be used to treat only waste activated sludge, or trickling filter sludge and primary (raw) sludge, or waste sludge from activated sludge treatment plants designed without primary settling. The sludge to be treated is placed in a large aerated tank where aerobic microorganisms decompose the organic matter in the sludge. This is an extension of the activated sludge process.
Coagulation
The clumping together of very fine particles into larger particles (floc) caused by the use of chemicals (coagulants). The chemicals neutralize the electrical charges of the fine particles, allowing them to come closer and form larger clumps. This clumping together makes it easier to separate the solids from the water by settling, skimming, draining, or filtering.
Agglomeration
The growing or coming together of small scattered particles into larger flocs or particles, which settle rapidly
Lineal
The length in one direction of a line. For example, a board 12 feet long has 12 lineal feet in its length.
Solids Concentration
The solids in the aeration tank that carry microorganisms that feed on wastewater. Expressed as milligrams per liter of mixed liquor volatile suspended solids (MLVSS, mg/L).
Volute
The spiral-shaped casing that surrounds a pump, blower, or turbine impeller and collects the liquid or gas discharged by the impeller.
Mechanical Aeration
The use of machinery to mix air and water so that oxygen can be absorbed into the water. Some examples are: paddle wheels, mixers, or rotating brushes to agitate the surface of an aeration tank; pumps to create fountains; and pumps to discharge water down a series of steps forming falls or cascades.
Sludge Density Index (SDI)
This calculation is used in a way similar to the Sludge Volume Index (SVI) to indicate the settleability of a sludge in a secondary clarifier or effluent. The weight in grams of one milliliter of sludge after settling for 30 minutes. SDI = 100/SVI.
TOC
Total Organic Carbon. TOC measures the amount of organic carbon in water.
Pinpoint Floc
Very small floc (the size of a pin point) that does not settle out of the water in a sedimentation basin or clarifier.
Microorganisms
Very small organisms that can be seen only through a microscope. Some microorganisms use the wastes in wastewater for food and thus remove or alter much of the undesirable matter.
Mixed Liquor
When the activated sludge in an aeration tank is mixed with primary effluent or the raw wastewater and return sludge, the mixture is then referred to as mixed liquor as long as it is in the aeration tank. Mixed liquor also may refer to the contents of mixed aerobic or anaerobic digesters.
Aeration Tank
the tank where raw or settled wastewater is mixed with return sludge and aerated. The same as aeration bay, aerator, or reactor.
