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Symbols and Acronyms

safety valve   A valve that automatically opens when prescribed conditions, usually pressure, are exceeded in a pipeline or other closed receptacle containing liquids or gases. It prevents such conditions from being exceeded and causing damage.

Salmonella  A genus of aerobic, rod-shaped, usually motile bacteria that are pathogenic for man and other warm-blooded animals.

sampler  A device used with or without flow measurement to obtain a portion of liquid for analytical purposes. May be designed for taking single samples (grab), composite samples, continuous samples, or periodic samples.

sand filter  A bed of sand through which water is passed to remove fine suspended particles. Commonly used in tertiary wastewater treatment plants and sludge drying beds.

sanitary sewer  A sewer that carries liquid and waterborne wastes from residences, commercial buildings, industrial plants, and institutions together with minor quantities of ground, storm, and surface water that are not admitted intentionally. See also wastewater.

Sarcadina  Species of amoeba found in wastewater. Does not play a significant role in the activated-sludge process other than as an indication of start up or the passing of a toxic influence.

saturated air  Air containing all the water vapor that it is capable of holding at a given temperature and pressure.

saturated liquid  Liquid that contains at a given temperature as much of a solute as it can retain in the presence of an excess of that solute.

scraper  (1) Device used to remove solids from a clarifier to a sump. (2) Mechanism to remove dewatered solids from a belt filter press or conveyor.

screen  A device with openings, generally of uniform size, used to retain or remove suspended or floating solids in flow stream preventing them from passing a given point in a conduit. The screening element may consist of parallel bars, rods, wires, grating, wire mesh, or perforated plate.

screening  A preliminary treatment process that removes large suspended or floating solids from raw wastewater to prevent subsequent plugging of pipes or damage to pumps.

screenings  (1) Material removed from liquids by screens. (2) Broken rock, including the dust, of a size that will pass through a given screen depending on the character of the stone.

screenings grinder  A device for grinding, shredding, or macerating material removed from wastewater by screens.

screw-feed pump  A pump with either a horizontal or vertical cylindrical casing in which operates a runner with radial blades like those of a ship’s propeller. See also vertical screw pump.

scrubbing  Removal of suspended solids and undesirable gases from gaseous emissions.

scum  (1) The extraneous or foreign matter that rises to the surface of a liquid and forms a layer or film there. (2) A residue deposited on a container or channel at the water surface. (3) A mass of solid matter that floats on the surface.

scum baffle  A vertical baffle dipping below the surface of wastewater in a tank to prevent the passage of floating matter.

scum breaker  A device installed in a sludge digestion tank to break up scum.

scum chamber  A space provided in a sludge digestion tank for accumulated scum rising from the digestion unit.

scum collector  A mechanical device for skimming and removing scum from the surface of settling tanks.

scum removal  Separation of floating grease and oil from wastewater usually during preliminary or primary treatment.

scum trough  A trough placed in a primary sedimentation tank to intercept scum and convey it out of the tank.

Secchi disk  Tool to measure the clarity of the water.

secondary effluent  (1) The liquid portion of wastewater leaving secondary treatment. (2) An effluent that, with some exceptions, contains not more than 30 mg/L each (on a 30-day average basis) of BOD5 and suspended solids.

secondary sedimentation tank  A settling tank following secondary treatment designed to remove by gravity part of the suspended matter. Also called a secondary clarifier.

secondary treatment  (1) Generally, a level of treatment that produces secondary effluent. (2) Sometimes used interchangeably with the concept of biological wastewater treatment, particularly the activated-sludge process. Commonly applied to treatment that consists chiefly of clarification followed by a biological process with separate sludge collection and handling.

secondary voltage  The output or load-supplied voltage of a transformer or substation (see primary voltage).

second-stage BOD  That part of the oxygen demand associated with the biochemical oxidation of nitrogenous material. As the term implies, the oxidation of the nitrogenous materials usually does not start until a portion of the carbonaceous material has been oxidized during the first stage.

sedimentation  (1) The process of subsidence and decomposition of suspended matter or other liquids by gravity. It is usually accomplished by reducing the velocity of the liquid below the point at which it can transport the suspended material. Also called settling. It may be enhanced by coagulation and flocculation. (2) Solid–liquid separation resulting from the application of an external force, usually settling in a clarifier under the force of gravity. It can be variously classed as discrete, flocculent, hindered, and zone sedimentation.

sedimentation tank  A basin or tank in which wastewater containing settleable solids is retained for removal of the suspended matter by gravity. Also called a sedimentation basin, settling basin, settling tank, or clarifier.

seed sludge  In biological treatment, the inoculation of the unit process with biologically active sludge resulting in acceleration of the initial stage of the process.

self-cleansing velocity  The minimum velocity necessary to keep solids in suspension in sewers, thus preventing their deposition and subsequent nuisance from stoppages and odors of decomposition.

separate sewer system  A sewer system carrying sanitary wastewater and other water-carried wastes from residences, commercial buildings, industrial plants, and institutions, as well as minor quantities of ground, storm, and surface water that are not intentionally admitted. See also combined sewer, wastewater.

septage  The sludge produced in individual on-site wastewater disposal systems such as septic tanks and cesspools.

septic  (1) Anaerobic. (2) Putrid, rotten, foul smelling; anaerobic.

septicity  A condition produced by growth of anaerobic organisms.

septic wastewater  Wastewater undergoing anaerobic decomposition.

service  Conductors and equipment for delivering energy from the electrical supply system to the wiring system of the premises served.

service charge  The rate charged by the utility for rendering service, usually used as a ready-to-serve charge.

service conductors  Supply conductors that extend from the street main or from transformers to the service equipment of the premises served.

service equipment  Necessary equipment usually consisting of a circuit breaker or switch and fuses and their accessories that is located near the point of entrance of the supply conductors to a building or other structure and intended to constitute the main control and means of cut-off of the electrical supply.

settleability  The tendency of suspended solids to settle.

settleability test  A determination of the settleability of solids in a suspension by measuring the volume of solids settled out of a measured volume of sample in a specified interval of time, usually reported in milliliters per liter (mL/L). Also called the Imhoff cone test.

settleable solids  (1) That matter in wastewater that will not stay in suspension during a preselected settling period, such as 1 hour, but settles to the bottom. (2) In the Imhoff cone test, the volume of matter that settles to the bottom of the cone in 1 hour. (3) Suspended solids that can be removed by conventional sedimentation.

settleometer  A 2-L or larger beaker used to conduct the settleability test.

settling  The process of subsidence and deposition of suspended matter carried by a liquid. It is usually accomplished by reducing the velocity of the liquid below the point at which it can transport the suspended material. Also called sedimentation.

settling tank  A tank or basin in which water, wastewater, or other liquid containing settleable solids is retained for a sufficient time, and in which the velocity of flow is sufficiently low to remove by gravity a part of the suspended matter. See also sedimentation tank.

settling time  Time necessary for the removal of suspended or colloidal substances by gravitational settling, aggregation, or precipitation.

settling velocity  Velocity at which subsidence and deposition of settleable suspended solids in wastewater will occur.

sheet flow  Flow in a relatively thin sheet of generally uniform thickness.

short-circuiting  A hydraulic condition occurring in parts of a tank where the time of travel is less than the flow-through time.

side contraction  The contraction of the nappe or reduction in width of water overflowing a weir brought about by the detachment of the sides of the nappe or jet of water passing over the sides of the weir.

side-water depth  The depth of water measured along a vertical exterior wall.

single-action pump  A reciprocating pump in which the suction inlet admits water to only one side of the plunger or piston and the discharge is intermittent.

single-stage digestion  Digestion limited to a single tank for the entire digestion period.

single-suction impeller  An impeller with one suction inlet.

skimming  (1) The process of diverting water from the surface of a stream or conduit by means of a shallow overflow. (2) The process of diverting water from any elevation in a reservoir by means of an outlet at a different elevation or by any other skimming device in order to obtain the most palatable drinking water. (3) The process of removing grease or scum from the surface of wastewater in a tank.

skimmings  Grease, solids, liquids, and scum skimmed from wastewater settling tanks.

slake  To become mixed with water so that a true chemical combination takes place, as in the slaking of lime.

slimes  (1) Substances of a viscous organic nature, usually formed from microbiological growth, that attach themselves to other objects forming a coating. (2) The coating of biomass (humus, schmutzdecke, sluff) that accumulates in trickling filters or sand filters and periodically sloughs away to be collected in clarifiers. See also biofilm.

slope  (1) The inclination of gradient from the horizontal of a line or surface. The degree of inclination is usually expressed as a ratio such as 1;25, indicating unit rise in 25 units of horizontal distance; or in a decimal fraction (0.04); degrees (2 deg 18 min); or percent (4%). (2) Inclination of the invert of a conduit expressed as a decimal or as feet (meters) per stated length measured horizontally in feet. (3) In plumbing, the inclination of a conduit, usually expressed in inches per foot (meter) length of pipe.

sloughing  The disattachment of slime and solids accumulated on the media of trickling filters and RBCs in contact areas. Sloughed material usually is removed subsequently in clarifiers. See also slimes.

slow sand filter  A filter for the purification of water in which water without previous treatment is passed downward through a filtering medium consisting of a layer of sand from 24- to 40-in. (0.6- to 1-m) thick. The filtrate is removed by an underdrainage system and the filter is cleaned by scraping off the clogged sand and eventually replacing it. It is characterized by a slow rate of filtration, commonly 3 to 6 mgd/ac (28 to 56 ML/ha·d) of filter area. Its effectiveness depends on the biological mat (or schmutzdecke) that forms in the top few millimeters.

sludge  (1) The accumulated solids separated from liquids during the treatment process that have not undegone a stabilization process. (2) The removed material resulting from chemical treatment, coagulation, flocculation, sedimentation, flotation, or biological oxidation of water or wastewater. (3) Any solid material containing large amounts of entrained water collected during water or wastewater treatment. See also activated sludge, settleable solids.

sludge age  Average residence time of suspended solids in a biological treatment system equal to the total weight of suspended solids in the system divided by the total weight of suspended solids leaving the systems.

sludge blanket  Accumulation of sludge hydrodynamically suspended within an enclosed body of water or wastewater.

sludge boil  An upwelling of water and sludge deposits caused by release of decomposition gases in the sludge deposits.

sludge circulation  The overturning of sludge in sludge digestion tanks by mechanical or hydraulic means or by the use of gas recirculation to disperse scum layers and promote digestion.

sludge collector  A mechanical device for scraping the sludge on the bottom of a settling tank to a sump from which it can be drawn.

sludge concentration  Any process of reducing the water content of sludge leaving the sludge in a fluid condition.

sludge density index (SDI)  A measure of the degree of compaction of a sludge after settling in a graduated container, expressed as mL/g. The sludge volume index (SVI) is the reciprocal of the sludge density index.

sludge dryer  A device for removing a large percentage of moisture from sludge or screenings by heat.

sludge drying  The process of removing a large percentage of moisture from sludge by drainage or evaporation by any method.

sludge-gas holder  A tank used to store gas collected from sludge digestion tanks for the purpose of stabilizing the flow of gas to the burners, maintaining a nearly constant pressure, and supplying gas during periods when the digestion tanks are temporarily out-of-service or when gas production is low.

sludge-gas utilization  Using the gas produced by the anaerobic digestion of sludge for beneficial purposes such as heating sludge, mixing sludge, drying sludge, heating buildings, incineration, or fueling engines.

sludge pressing  The process of dewatering sludge by subjecting it to pressure, usually within a cloth fabric through which the water passes and in which the solids are retained.

sludge reaeration  The continuous aeration of sludge after its initial aeration for the purpose of improving or maintaining its condition.

sludge reduction  The reduction in quantity and change in character of sludge as the result of digestion.

sludge solids  Dissolved and suspended solids in sludge.

sludge thickener  A tank or other equipment designed to concentrate wastewater sludges.

sludge thickening  The increase in solids concentration of sludge in a sedimentation tank, DAF, gravity thickener, centrifuge or gravity belt thickener.

sludge utilization  The use of sludges resulting from industrial wastewater treatment as soil builders and fertilizer admixtures.

sludge volume index (SVI)  The ratio of the volume (in milliliters) of sludge settled from a 1000-mL sample in 30 minutes to the concentration of mixed liquor (in milligrams per liter [mg/L]) multiplied by 1000.

slurry  A thick, watery mud or any substance resembling it, such as lime slurry.

soda ash  A common name for commercial sodium carbonate (Na2CO3).

sodium bisulfite (NaHSO3)  A salt used for reducing chlorine residuals; a strong reducing agent; typically found in white powder or granular form in strengths up to 44%. At a strength of 38%, 1.46 parts will consume 1 part of chlorine residual.

sodium carbonate (Na2CO3)  A salt used in water treatment to increase the alkalinity or pH of water or to neutralize acidity. Also called soda ash.

sodium hydroxide (NaOH)  A strong caustic chemical used in treatment processes to neutralize acidity, increase alkalinity, or raise the pH value. Also known as caustic soda, sodium hydrate, lye, and white caustic.

sodium hypochlorite (NaOCl)  A water solution of sodium hydroxide and chlorine in which sodium hypochlorite is the essential ingredient.

sodium metabisulfite (Na2S2O5)  A cream-colored powder used to conserve chlorine residual; 1.34 parts of Na2S2O5 will consume 1 part of chlorine residual.

soil absorption capacity  In subsurface effluent disposal, the ability of the soil to absorb water. See soil absorption test.

soil absorption test  A test for determining the suitability of an area for subsoil effluent disposal by measuring the rate at which the undisturbed soil will absorb water per unit of surface.

soil horizon  A layer or section of the soil profile, more or less well defined, occupying a position approximately parallel to the soil surface, and having characteristics that have been produced through the operation of soil-building processes.

soil infiltration rate  The maximum rate at which a soil, in a given condition at a given time, can absorb water.

soil porosity  The percentage of the soil (or rock) volume that is not occupied by solid particles, including all pore space filled with air and water. The total porosity may be calculated from the following formula: percent pore space 5 (1 - volume weight/specific gravity) × 100.

solids disposal  Any process for the ultimate disposal of solid wastes or sludges by incineration, landfilling, soil conditioning, or other means.

solids inventory  Amount of sludge in the treatment system typically expressed as kilogram (tons). Inventory of plant solids should be tracked through the use of a mass balance set of calculations.

solids loading  Amount of solids applied to a treatment process per unit time per unit volume.

solids retention time (SRT)  The average time of retention of suspended solids in a biological waste treatment system, equal to the total weight of suspended solids leaving the system, per unit time.

sparger  An air diffuser designed to give large bubbles, used singly or in combination with mechanical aeration devices.

species  A subdivision of a genus having members differing from other members of the same genus in minor details.

specific gravity  The ratio of the mass of a body to the mass of an equal volume of water at a specific temperature, typically 20 °C (68 °F).

specific oxygen uptake rate  Measures the microbial activity in a biological system expressed in mg O2/g·h of VSS. Also called respiration rate.

specific resistance  The relative resistence a sludge offers to the draining of its liquid component.

specific speed  A speed or velocity of revolution, expressed in revolutions per minute (rpm), at which the runner of a given type or turbine would operate if it were so reduced in size and proportion that it would develop 1 hp under a 1-ft head. The quantity is used in determining the proper type and character of turbine to install at a hydroelectric power plant under given conditions.

spiral-air flow diffusion  A method of diffusing air in the grit chamber or aeration tank of the activated-sludge process where, by means of properly designed baffles and the proper location of diffusers, a spiral helical movement is given to the air and the tank liquor.

splitter box  (1) A division box that splits the incoming flow into two or more streams. (2) A device for splitting and directing discharge from the head box to two separate points of application.

spray aerator  An aerator consisting of a pressure nozzle through which water is propelled into the air in a fine spray.

spray irrigation  A method of land treatment for disposing of some organic wastewaters by spraying them, usually from pipes equipped with fixed or moving spray nozzles. See also land application.

stabilization pond  A type of oxidation pond in which biological oxidation of organic matter is effected by natural or artificially accelerated transfer of oxygen to the water from air.

staged digestion  The progressive digestion of waste in two or more tanks arranged in series, usually divided into primary digestion with mixed contents and secondary digestion where quiescent conditions prevail and supernatant liquor is collected.

staged treatment  (1) Any treatment in which similar processes are used in series or stages. (2) In the activated-sludge process, two or more stages consisting of a clarifying stage and a biological stage, or two biological stages. (3) In anaerobic digestion, an operation in which sludge is completely mixed in the first tank and pumped to a second tank for separation of the supernatant liquor from the solids.

staged trickling filter  A series of trickling filters through which wastewater passes successively with or without intermediate sedimentation.

stale wastewater  Wastewater containing little or no oxygen, but as yet free from putrefaction. See also septic wastewater.

stalked ciliates  Small, one-celled organisms possessing cilia (hair-like projections used for feeding) that are not motile. They develop at lower prey densities, long SRTs, and low F:M ratios.

Standard Methods  (1) An assembly of analytical techniques and descriptions commonly accepted in water and wastewater treatment (Standard Methods for the Examination of Water and Wastewater) published jointly by the American Public Health Association, the American Water Works Association, and the Water Environment Federation. (2) Validated methods published by professional organizations and agencies covering specific fields or procedures. These include, among others, the American Public Health Association, American Public Works Association, American Society of Civil Engineers, American Society of Mechanical Engineers, American Society for Testing and Materials, American Water Works Association, U.S. Bureau of Standards, U.S. Standards Institute (formerly American Standards Association), U.S. Public Health Service, Water Environment Federation, and U.S. Environmental Protection Agency.

standard pressure  Atmospheric pressure at sea level under standard conditions.

static head  Vertical distance between the free level of the source of supply and the point of free discharge or the level of the free surface.

static level  (1) The elevation of the water table or pressure surface when it is not influenced by pumping or other forms of extraction from the groundwater. (2) The level of elevation to which the top of a column of water would rise, if afforded the opportunity to do so, from an artesian aquifer, basin, or conduit under pressure. Also called hydrostatic level.

static suction head  The vertical distance from the source of supply, when its level is above the pump, to the center line of the pump.

static suction lift  The vertical distance between the center of the suction end of a pump and the free surface of the liquid being pumped. Static lift does not include friction losses in the suction pipes. Static suction head includes lift and friction losses.

steady flow  (1) A flow in which the rate, or quantity of water passing a given point per unit of time, remains constant. (2) Flow in which the velocity vector does not change in either magnitude or direction with respect to time at any point or section.

steady nonuniform flow  A flow in which the quantity of water flowing per unit of time remains constant at every point along the conduit, but the velocity varies along the conduit because of a change in the hydraulic characteristics.

step aeration  A procedure for adding increments of settled wastewater along the line of flow in the aeration tanks of an activated-sludge plant. Also called step feed.

stoichiometric  Pertaining to or involving substances that are in the exact proportions required for a given reaction.

straggler floc  Large (6-mm or larger) floc particles that have poor settling characteristics.

submerged weir  A weir that, when in use, has a water level on the downstream side at an elevation equal to, or higher than, the weir crest. The rate of discharge is affected by the tailwater. Also called a drowned weir.

submergence  (1) The condition of a weir when the elevation of the water surface on the downstream side is equal to or higher than that of the weir crest. (2) The ratio, expressed as a percentage, of the height of the water surface downstream from a weir above the weir crest to the height of the water surface upstream above the weir crest. The distances upstream or downstream from the crest at which such elevations are measured are important, but have not been standardized. (3) In water power engineering, the ratio of tailwater elevation to the headwater elevation when both are higher than the crest. The overflow crest of the structure is the datum of reference. The distances upstream or downstream from the crest at which headwater and tailwater elevations are measured are important, but have not been standardized. (4) The depth of flooding over a pump suction inlet.

substation  An assembly of equipment for switching and/or changing or regulating the voltage electrical supply.

substrate  (1) Substances used by organisms in liquid suspension. (2) The liquor in which activated sludge or other matter is kept in suspension.

suction head  (1) The head at the inlet to a pump. (2) The head below atmospheric pressure in a piping system.

suction lift  The vertical distance from the liquid surface in an open tank or reservoir to the center line of a pump drawing from the tank or reservoir and set higher than the liquid surface.

suctoreans  Ciliates that are stalked in the adult stage and have rigid tentacles to catch prey.

sulfate-reducing bacteria  Bacteria capable of assimilating oxygen from sulfate compounds, reducing them to sulfides. See also sulfur bacteria.

sulfur bacteria  Bacteria capable of using dissolved sulfur compounds in their growth; bacteria deriving energy from sulfur or sulfur compounds.

sulfur cycle  A graphical presentation of the conservation of matter in nature showing the chemical transformation of sulfur through various stages of decomposition and assimilation. The various chemical forms of sulfur as it moves among living and nonliving matter is used to illustrate general biological principles that are applicable to wastewater and sludge treatment.

sump  A tank or pit that receives drainage and stores it temporarily, and from which the discharge is pumped or ejected.

sump pump  A mechanism used for removing water or wastewater from a sump or wet well; it may be energized by air, water, steam, or electric motor. Ejectors and submerged centrifugal pumps, either float or manually controlled, are often used for the purpose.

supernatant  (1) The liquid remaining above a sediment or precipitate after sedimentation. (2) The most liquid stratum in a sludge digester.

supersaturation  (1) An unstable condition of a vapor in which its density is greater than that normally in equilibrium under the given conditions. (2) The condition existing in a given space when it contains more water vapor than is needed to cause saturation; that is, when its temperature is below that required for condensation to take place. This condition probably can occur only when water or ice is immediately present, and when the space contains no dust or condensation nuclei. (3) An unstable condition of a solution in which it contains a solute at a concentration exceeding saturation.

suppressed weir  A weir with one or both sides flush with the channel of approach. This prevents contraction of the nappe adjacent to the flush side. The suppression may occur on one end or both ends.

surface aeration  The absorption of air through the surface of a liquid.

surface overflow rate  A design criterion used for sizing clarifiers; typically expressed as the flow volume per unit amount of clarifier space (m3/m2·s [gpd/sq ft]).

  A surface-active agent, such as ABS or LAS, that concentrates at interfaces, forms micelles, increases solution, lowers surface tension, increases adsorption, and may decrease flocculation.

surge  (1) A momentary increase in flow (in an open conduit) or pressure (in a closed conduit) that passes longitudinally along the conduit, usually because of sudden changes in velocity or quantity. (2) Any periodic, usually abrupt, change in flow, temperature, pH, concentration, or similar factor.

surge suppressor  A device used in connection with automatic control of pumps to minimize surges in a pipeline.

surge tank  A tank or chamber located at or near a hydroelectric powerhouse and connected with the penstock above the turbine. When the flow of water delivered to the turbine is suddenly decreased, the tank absorbs the water that is held back and cushions the increased pressure on the penstock caused by the rapid deceleration of the water flowing in it; also, when the flow delivered to the turbine is suddenly increased, the tank supplies the increased quantity of water required until the flow in the penstock has been accelerated sufficiently. Also used in connection with pumping systems.

suspended matter  (1) Solids in suspension in water, wastewater, or effluent. (2) Solids in suspension that can be readily removed by standard filtering procedures in a laboratory.

suspended solids  (1) Insoluble solids that either float on the surface of, or are in suspension in, water, wastewater, or other liquids. (2) Solid organic or inorganic particles (colloidal, dispersed, coagulated, or flocculated) physically held in suspension by agitation or flow. (3) The quantity of material removed from wastewater in a laboratory test, as prescribed in Standard Methods and referred to as nonfilterable residue.

switchboard  A large panel or assembly of panels on which switches, overcurrent, and/or other protective devices such as buses and instruments are mounted. Switchboards are generally accessible from the rear as well as from the front and are not intended to be installed in cabinets.
synergism  Interaction between two entities producing an effect greater than a simple additive one. See also antagonism.