NC Water/Wastewater Operator flashcards
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32 cards. Read the term and the definition here. The deck at the top of this page is optional.
- CT value
- The product of disinfectant residual concentration in mg/L and effective contact time in minutes, used to demonstrate the required inactivation of Giardia and viruses.
- Jar test
- A bench-scale procedure that mixes raw water with different coagulant doses and pH values to find the dose giving the lowest filtered turbidity.
- Alum alkalinity demand
- Each mg/L of alum applied consumes roughly 0.45 to 0.5 mg/L of alkalinity as CaCO3, so low-alkalinity waters may need lime addition.
- Filter loading rate
- Flow through a filter in gallons per minute divided by the filter surface area in square feet; rapid sand filters typically operate at 2 to 4 gpm/ft².
- Combined filter effluent turbidity
- Regulatory limit of 0.3 NTU or less in at least 95 percent of monthly samples, and never more than 1 NTU, under the surface water treatment rule.
- Chlorine demand
- The amount of chlorine consumed by microorganisms, ammonia, and other reducing substances during contact; dose equals demand plus the residual you want to maintain.
- Pounds formula
- Pounds of chemical = concentration in mg/L times 8.34 times volume in million gallons; used to convert a dose into the weight of chemical needed.
- Langelier Saturation Index
- A calculated index that predicts calcium carbonate behavior: positive values indicate scale-forming water, negative values indicate corrosive water, and near zero is balanced.
- Head to pressure conversion
- Pressure in psi equals feet of head multiplied by 0.433; conversely, divide psi by 0.433 to find feet of head.
- Hazen-Williams C factor
- A roughness coefficient in the Hazen-Williams head loss formula; higher values mean smoother pipe and less friction loss for the same flow.
- Water hammer
- A pressure surge caused by a rapid change in flow velocity, such as fast valve closure; prevented with slow closure, surge tanks, and air valves.
- Minimum distribution pressure
- North Carolina requires at least 20 psi at all points in the distribution system, while normal service pressures are typically 35 to 60 psi.
- Backflow prevention methods
- Air gaps provide the highest protection, followed by reduced-pressure zone assemblies and double check valves; selection depends on the degree of hazard.
- Pipe volume formula
- Gallons in a pipe equal 0.785 times the diameter in feet squared times the length in feet times 7.48 gallons per cubic foot.
- Main flushing velocity
- Flushing should move water through mains at 2.5 feet per second or faster to scour and carry away sediment and stale water.
- Elevated tank pressure
- Each foot of water elevation produces 0.433 psi, so a tank 100 feet above a service point delivers about 43.3 psi.
- Food-to-microorganism ratio
- Pounds of BOD5 applied per day divided by pounds of MLSS under aeration; conventional activated sludge runs about 0.2 to 0.5.
- Sludge volume index
- Settled sludge volume in mL/L after 30 minutes times 1000, divided by MLSS in mg/L; values of 80 to 150 mL/g indicate good settling.
- Mean cell residence time
- The pounds of solids in the aeration system divided by the pounds of solids leaving each day; also called solids retention time.
- Surface overflow rate
- Clarifier flow in gallons per day divided by clarifier surface area in square feet; a key design and performance measure for settling tanks.
- Nitrification alkalinity demand
- Oxidizing ammonia consumes about 7.14 mg/L of alkalinity as CaCO3 per mg/L of ammonia nitrogen, so alkalinity can limit nitrification.
- Aeration basin dissolved oxygen
- Operators typically maintain about 2 mg/L of dissolved oxygen to support carbon oxidation and nitrification without wasting blower energy.
- Mesophilic digester temperature
- Anaerobic digesters in the mesophilic range are held near 95 degrees Fahrenheit, usually between 90 and 100 degrees, for stable digestion.
- Wasting rate control
- Increasing waste activated sludge lowers mean cell residence time and MLSS; decreasing wasting raises both and can overload clarifiers.
- Manning's equation
- Velocity equals 1.486 divided by roughness n, times hydraulic radius to the two-thirds power, times slope to the one-half power, in English units.
- Minimum sewer velocity
- Gravity sewers should flow at 2 feet per second or more at design flow to keep solids moving and prevent deposition.
- Maximum sewer velocity
- Flow above about 10 to 15 feet per second can erode pipe walls and manholes, so steep grades need drop structures or energy dissipation.
- Infiltration versus inflow
- Infiltration is groundwater that enters through defective pipes and joints; inflow is storm water entering through manholes, roofs, and direct connections.
- Confined space oxygen range
- Acceptable entry oxygen is 19.5 to 23.5 percent; below or above this range requires ventilation and reassessment before entry.
- Hydrogen sulfide exposure limit
- The permissible exposure limit for hydrogen sulfide is 10 ppm; it is immediately dangerous to life at 100 ppm and deadens smell at high levels.
- Eight-inch sewer minimum slope
- North Carolina requires at least 0.4 percent grade (0.004 ft/ft) for 8-inch gravity sewer; larger diameters may use flatter slopes.
- Lift station wet well
- A wet well stores wastewater so pumps can cycle within their run and rest times; undersized wells cause short cycling and premature pump wear.