DC Water 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 contact time in minutes. Required CT varies with disinfectant, pH, temperature, and target organism.
- Chlorine demand
- The difference between the chlorine dose applied and the residual remaining after a specified contact time. It reflects consumption by organics, ammonia, and other reducing substances.
- Jar test purpose
- A bench-scale test that mixes raw water with varied coagulant doses and pH to identify the combination producing the lowest settled turbidity before full-scale adjustment.
- Filter loading rate
- Flow through a filter divided by its surface area, usually expressed in gpm/ft². It determines hydraulic stress on the media and affects particle penetration.
- Backwash bed expansion
- The percentage increase in filter media depth during backwash, typically 15 to 25 percent. It is controlled by backwash flow rate and varies with water temperature.
- Turbidity units
- Turbidity is measured in nephelometric turbidity units (NTU). Combined filter effluent is commonly limited to 0.3 NTU in 95 percent of monthly samples and never above 1 NTU.
- Log removal
- A logarithmic expression of pathogen reduction: 1-log is 90 percent, 2-log is 99 percent, 3-log is 99.9 percent, and 4-log is 99.99 percent removal or inactivation.
- Coagulation versus flocculation
- Coagulation neutralizes particle charges so they can stick together; flocculation is gentle mixing that builds those particles into larger, settleable flocs.
- Pressure head conversion
- One foot of water column equals 0.433 psi, and one psi equals 2.31 feet of head. These conversions are used constantly for pressure and elevation calculations.
- Hazen-Williams equation
- A head loss formula for full pipes: hf = 10.44 × L × Q^1.85 divided by (C^1.85 × d^4.87) in US customary units, where C is pipe roughness.
- Minimum distribution pressure
- Normal operating pressure in a distribution system is generally kept at or above 20 psi. Lower pressure raises contamination risk and triggers investigation and public notice.
- Air gap backflow protection
- A physical separation between a potable water outlet and the flood level of a receiving vessel. It provides the highest level of cross-connection protection.
- Reduced-pressure-zone assembly
- A testable backflow preventer with two check valves and a relief valve between them. It protects against both backpressure and backsiphonage in high-hazard connections.
- Water age
- The average time water spends in storage or piping. High water age depletes disinfectant residual and promotes disinfection byproduct formation and nitrification.
- Main disinfection dose
- New or repaired water mains are commonly disinfected with 25 to 50 mg/L free chlorine for 24 hours, followed by flushing and bacteriological sampling before service.
- Minimum night flow
- The lowest system flow, usually overnight. Elevated minimum night flow relative to normal use suggests leakage or unauthorized consumption in the distribution network.
- Pipe flow equation
- Flow equals area times velocity, Q = A × V. For a full circular pipe, area is 0.7854 times the diameter squared, using consistent units.
- BOD5 definition
- Biochemical oxygen demand measured over five days at 20 degrees Celsius. It estimates biodegradable organic matter consumed by microorganisms in a wastewater sample.
- Solids retention time
- Also called sludge age: the mass of solids in the aeration basin divided by the mass of solids leaving the system per day, expressed in days.
- Food-to-microorganism ratio
- Pounds of BOD applied per day divided by pounds of mixed liquor suspended solids under aeration. It indicates organic loading relative to the biomass available.
- Sludge volume index
- Settled sludge volume in mL/L after 30 minutes times 1,000, divided by MLSS in mg/L. Values near 80 to 150 indicate good settling.
- Return activated sludge rate
- The RAS flow needed to maintain basin MLSS, calculated as influent flow times MLSS divided by (RAS concentration minus MLSS), with consistent units.
- Secondary clarifier loading
- Solids loading rate equals (influent flow plus return flow) times MLSS, divided by clarifier surface area. Excess loading causes solids carryover in the effluent.
- Nitrification oxygen demand
- Oxidizing ammonia to nitrate consumes about 4.57 mg of oxygen per mg of nitrogen. This demand adds to aeration requirements beyond carbonaceous BOD.
- Secondary treatment limits
- Common regulatory limits are 30 mg/L BOD and 30 mg/L total suspended solids, with at least 85 percent removal of both, based on monthly averages.
- Hydraulic detention time
- Tank volume divided by flow rate, expressed in hours or days. It describes how long wastewater remains in a treatment unit at a given flow.
- Manning's equation
- Velocity equals (1.486/n) times hydraulic radius to the two-thirds power times slope to the one-half power in US customary units, where n is roughness.
- Self-cleansing velocity
- A minimum gravity sewer velocity of about 2 feet per second at design flow. Below it, grit and solids settle and can cause blockages.
- Maximum sewer velocity
- Gravity sewers are usually designed to keep velocity at or below about 10 feet per second to limit abrasion, scour, and release of hydrogen sulfide.
- Infiltration versus inflow
- Infiltration is groundwater entering through pipe defects and joints. Inflow is direct stormwater entry through connections such as roof drains, sumps, and manhole covers.
- Surcharging
- A condition where flow exceeds gravity pipe capacity, causing the hydraulic grade line to rise above the pipe crown and potentially back up into basements.
- Confined space atmosphere
- Entry requires testing for oxygen (19.5 to 23.5 percent), flammable gas, and hydrogen sulfide before entry and continuously during occupancy, with ventilation as needed.