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Symbols and Acronyms

raceway  Any channel for holding wires, cables, or bus bars that is designed expressly and solely for that purpose. Raceways may be of metal or insulating materials and the term includes rigid metal conduit, nonmetallic conduit, flexible metal conduit, and electrical metallic tubing. Raceways may be located beneath the floor or on or above the surface (refer to the National Electric Code for approved raceways).

rack  A device fixed in place and used to remove suspended or floating solids from wastewater. It is composed of parallel bars that are evenly spaced. See also screen.

radial flow  The direction of flow across a tank from center to periphery or vice versa.

radiation  (1) The emission and propagation of energy through space or through a material medium; also the energy so propagated. (2) The dispersion of energy by electromagnetic waves rather than by conduction and convection.

range  A measure of the variability of a quantity; the difference between the largest and smallest values in the sequence of values of the quantity.

rate  (1) The speed at which a chemical reaction occurs. (2) Flow volume per unit time. See also kinetics.

rate-of-flow controller  An automatic device that controls the rate of flow of a fluid.

rate-of-flow recorder  A recorder for registering the rate of flow of water; generally, used with a rapid sand filter.

raw sludge  Settled sludge promptly removed from sedimentation tanks before decomposition has much advanced.

raw wastewater  Wastewater before it receives any treatment.

reaction rate  The rate at which a chemical reaction progresses. See also kinetics, rate.

reactor  The container, vessel, or tank in which a chemical or biological reaction is carried out.

recalcining  Recovery of lime from water and wastewater treatment sludge.

recarbonation  (1) The process of introducing carbon dioxide as a final stage in the lime-soda ash softening process to convert carbonates to bicarbonates and thereby stabilize the solution against precipitation of carbonates. (2) The addition of carbon dioxide to the effluent of an advanced wastewater treatment ammonia air stripping process to lower the pH. (3) The diffusion of carbon dioxide gas through a liquid to replace the carbon dioxide removed by the addition of lime. (4) The diffusion of carbon dioxide gas through a liquid to render the liquid stable with respect to precipitation or dissolution of alkaline constituents.

receiving water  A river, lake, ocean, or other watercourse into which wastewater or treated effluent is discharged.

receptacle  A contact device installed at the outlet for connection of a single attachment plug. A single receptacle is a single contact device with no other contact device on the same yoke. A multiple receptacle is a single device containing two or more receptacles.

reciprocating pump  A type of displacement pump consisting essentially of a closed cylinder containing a piston or plunger as the displacing mechanism. Liquid is drawn into the cylinder through an inlet valve and forced out through an outlet valve. When the piston acts on the liquid in one end of the cylinder, the pump is termed single-action; when it acts in both ends, it is termed double-action.

recirculation  (1) In the wastewater field, the return of all or a portion of the effluent in a trickling filter to maintain a uniform high rate through the filter. Return of a portion of the effluent to maintain minimum flow is sometimes called recycling. (2) The return of effluent to the incoming flow. (3) The return of the effluent from a process, factory, or operation to the incoming flow to reduce the water intake. The incoming flow is called makeup water.

reclaimed wastewater  Wastewater used for some beneficial purpose usually after some degree of treatment.

recorder  (1) A device that makes a graph or other record of the stage, pressure, depth, velocity, or the movement or position of water-controlling devices, usually as a function of time. See also indicator. (2) The person who records the observational data.

recording gauge  An automatic instrument for measuring and recording graphically and continuously. Also called a register.

rectangular weir  A weir having a notch that is rectangular in shape.

recycle  (1) To return water after some type of treatment for further use; generally implies a closed system. (2) To recover useful values from segregated solid waste.

recycling  (1) An operation in which a substance is passed through the same series of processes, pipes, or vessels more than once. (2) The conversion of solid waste into usable materials or energy.

reduce  The opposite of oxidize. The action of a substance to decrease the positive valence of an ion.

reduction  The addition of electrons to a chemical entity decreasing its valence. See also oxidation.

refractory  Brick or similar material that lines a furnace or incinerator.

regeneration  (1) In ion exchange, the process of restoring an ion exchange material to the state used for adsorption. (2) The periodic restoration of exchange capacity of ion exchange media used in water ­treatment.

relative humidity  (1) The amount of water vapor in the air; expressed as a percentage of the maximum amount that the air could hold at the given temperature. (2) The ratio of the actual water vapor pressure to the saturation vapor pressure.

relay  An electrical device that is designed to interpret input conditions in a prescribed manner and, after specified conditions are met, to respond to cause electrical operation or similar abrupt change in associated control circuits. The most common form of relay uses a coil and set of contacts. When current flows in the coil, contacts are opened or closed, depending on their arrangement. Relays are said to be normally open or normally closed.

relief valve  A valve that releases air from a pipeline automatically without loss of water, or introduces air into a line automatically if the internal pressure becomes less than that of the atmosphere.

removal efficiency  A measure of the effectiveness of a process in removing a constituent, such as BOD or TSS. Removal efficiency is calculated by subtracting the effluent value from the influent value and dividing it by the influent value. Multiply the answer by 100 to convert to a percentage.

repair  An element of maintenance, as distinguished from replacement or retirement.

replacement  Installation of new or alternate equipment in place of existing equipment for a variety of reasons, such as obsolescence, total disrepair, improvement, or modification.

replacement cost  (1) The actual or estimated cost of duplication with a property of equal utility and desirability. (2) The cost of replacing property.

residue  (1) The equilibrium quantity of a compound or element remaining in an organism after uptake and clearance. (2) The dry solid remaining after evaporation.

resistance  The property of an electrical circuit or device that opposes current flow, thereby causing conversion of electrical energy to heat or radiant energy.

respiration  Intake of oxygen and discharge of carbon dioxide as a result of biological oxidation.

retention time  The theoretical time required to displace the contents of a tank or unit at a given rate of discharge (volume divided by the rate of discharge). Also called detention time.

return sludge  Settled activated sludge returned to mix with incoming raw or primary settled wastewater. More commonly called return activated sludge.

reverse osmosis  An advanced method used in water and wastewater treatment that relies on a semipermeable membrane to separate the water from its impurities. An external force is used to reverse the normal osmotic flow resulting in movement of the water from a solution of higher solute concentration to one of lower concentration. Also called hyperfiltration.

revolving screen  A screen or rack in the form of a cylinder or a continuous belt that is revolved mechanically. The screenings are removed by water jets, automatic scrapers, or manually.

Reynolds’ number  A dimensionless quantity used to characterize the type of flow in a hydraulic structure where resistance to motion depends on the viscosity of the liquid in conjunction with inertia. It is equal to the ratio of inertial forces to viscous forces. The number is chiefly applicable to closed systems of flow, such as pipes or conduits where there is no free water surface, or to bodies fully immersed in the fluid so the free surface need not be considered.

riprap  Broken stone or boulders placed compactly or irregularly on dams, levees, dikes, or similar embankments for protection of earth surfaces against the action of waves or currents.

rising time  The time necessary for removal, by flotation, of suspended or aggregated colloidal substances.

rotary distributor  A movable distributor made up of horizontal arms that extend to the edge of the circular trickling filter bed, revolve about a central post, and distribute liquid over the bed through orifices in the arms. The jet action of the discharging liquid normally supplies the motive power. See also distributor.

rotary dryer  A long, slowly revolving, steel cylinder with its long axis slightly inclined, through which passes the material to be dried in hot air. The material passes through from inlet to outlet, tumbling about.

rotary pump  A type of displacement pump consisting essentially of elements rotating in a pump case that is closely fit. The rotation of these elements alternately draws in and discharges the water being pumped. Such pumps act with neither suction nor discharge valves, operate at almost any speed, and do not depend on centrifugal forces to lift the water.

rotary valve  A valve consisting of a casing more or less spherical in shape and a gate that turns on trunnions through 90 deg when opening or closing and having a cylindrical opening of the same diameter as that of the pipe it serves.

rotating biological contactor (RBC)  A device for wastewater treatment composed of large, closely spaced plastic discs that are rotated about a horizontal shaft. The discs alternately move through the wastewater and the air and develop a biological growth on their surfaces.

rotating distributor  A distributor consisting of rotating or reciprocating perforated pipes or troughs from which liquid is discharged in the form of a spray or in a thin sheet at uniform rates over the surface area to be wetted.

rotifer  Minute, multicellular aquatic animals with rotating cilia on the head and forked tails. Rotifers help stimulate microfloral activity and decomposition, enhance oxygen penetration, and recycle mineral nutrients.

roughing filter  A trickling filter used to remove an initial portion of the soluble BOD, usually about 50%, but not to provide complete removal.
roughness coefficient  A factor in many engineering equations for computing the average velocity of flow of water in a conduit or channel. It represents the effect of the roughness of the confining material on the energy losses in the flowing water.