Rack: Evenly spaced parallel metal bars or rods located in the influent channel to remove rags, rocks, and cans from wastewater.

Radio frequency interference (RFI): EMI in electronic equipment caused by radio frequencies, ranging typically from 10 kHz (104 Hz) to 1000 MHz (109 Hz or 1 GHz).

RAS: Return activated sludge - settled activated sludge that is collected in the secondary clarifier and returned to the aeration basin to mix with incoming raw settled wastewater.

RASVSS: Return Activated Sludge Volatile Suspended Solids.

RBC: Rotating biological contactor - an attached culture wastewater treatment system..

Reagent: A pure chemical substance that is used to make new products or is used in chemical tests to measure, detect, or examine other substances.

Recycle: The use of water or wastewater within (internally) a facility before it is discharged to a treatment system.

REDOX: Biological reductions/oxidations. These reactions usually require enzymes to mediate the electron transfer. The sediment in the bottom of a lake, sludge in a sewerage works or septic tank will have a very low redox potential and will likely be devoid of any oxygen. This sludge or waste water will have a very high concentration of reductive anaerobic bacteria, indeed the bulk of the organic matter may in fact be bacteria. As the concentration of oxygen increases the oxidation potential of the water will increase. A low redox potential or small amount of oxygen is toxic to anaerobic bacteria, therefore as the concentration of oxygen and redox potential increases the bacterial population changes from reductive anaerobic bacteria to oxidative aerobic bacteria. Measurement of REDOX potential is also referred to as ORP.

Reducing agent: Any substance, such as the base metal (iron) or the sulfide ion that will readily donate (give up) electrons. The opposite of an oxidizing agent. Refractory materials: Material difficult to remove entirely from wastewater such as nutrients, color, taste, and odor-producing substances and some toxic materials. Residual shrinkage: The amount of shrinkage remaining in a fabric after it has undergone all fabric weaving, washing and heat setting steps. Respiration: The energy producing process of breathing, by which an organism supplies its cells with oxygen and relieves itself of carbon dioxide. A type of heterotrophic metabolism that uses oxygen in which 38 moles of ATP are derived from the oxidation of 1 mole of glucose, yielding 380,000 cal. (An additional 308,000 cal is lost as heat.) Rhizosphere: Soil surrounding plant roots. Retentate: Substance retained in the upstream side of a filter. RF (radio frequency) welding: Utilizes specific bands of radio frequency waves which are directed through specially constructed tooling to form localized melting/joining of certain dielectric thermoplastic materials. Can be used to form hermetic seals. Also known as high frequency or dielectric welding. R/O unit: Reverse Osmosis Unit for water purification in small aquariums and miniature yard-ponds, utilizes a membrane under pressure to filter dissolved solids and pollutants from the water. Two different filter membranes can be used: the CTA (cellulose triacetate) membrane is less expensive, but only works with chlorinated water and removes 50-70% of nitrates, and the TFC membrane, which is more expensive, removes 95% of nitrates, but is ruined by chlorine. R/O wastes water and a system that cleans 100 gallons a day will cost from $400 to $600 with membrane replacement adding to the cost. A unit that handles 140 gallons a day will cost above $700.00.

RR: Respiration rate - the weight of oxygen utilized by the total weight of MLSS in a given time.

Runoff: Water running down slopes rather than sinking in (again, result of poor humus content) Ex. erosion due to deforestation

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