NV Water/Wastewater Operator flashcards

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. Required CT rises as water temperature falls and depends on pH, disinfectant, and target pathogen log inactivation.

Chlorine demand

The difference between the chlorine dose applied and the chlorine residual remaining after a defined contact period. Demand must be satisfied before any free residual can persist in the finished water.

Four-log inactivation

Removal or inactivation of 99.99 percent of a target organism, meaning only one organism in 10,000 survives. The SWTR requires 4-log virus and 3-log Giardia treatment for surface water systems.

Combined filter effluent turbidity

Under the IESWTR, combined filter effluent must be at or below 0.3 NTU in at least 95 percent of monthly measurements and must never exceed 1 NTU at any time.

TTHM limit

The Stage 2 DBPR maximum for total trihalomethanes is 80 micrograms per liter, evaluated as a locational running annual average at each monitoring location in the distribution system.

Lead action level

The lead action level is 0.015 mg/L, assessed at the 90th percentile of first-draw samples from high-risk sites. Exceeding it triggers corrosion control evaluation and public education.

Detention time

Detention time equals tank or pipe volume divided by flow rate, expressed in consistent units. It describes how long water remains in a basin or pipeline at a given flow.

Minimum distribution pressure

Systems must maintain at least 20 psi at all points in the distribution system under normal operating conditions and during main breaks and fire flow events.

Free chlorine residual

The concentration of uncombined chlorine remaining after demand is satisfied. A detectable residual is maintained throughout distribution, commonly 0.2 mg/L minimum at the tap.

Coagulation and flocculation

Coagulation adds a chemical such as alum to neutralize particle surface charge and destabilize turbidity. Flocculation then gently mixes the water so destabilized particles collide and form settleable floc.

Jar test

A bench-scale procedure that doses raw water with coagulant at several concentrations and mixing speeds to identify the dose and pH that best remove turbidity before full-scale plant changes.

Backwash rate

The upward flow per unit filter area used to clean media, typically 15 to 20 gallons per minute per square foot. Excessive rates wash media out; low rates leave floc and shorten filter runs.

Disinfection byproducts

Compounds such as trihalomethanes and haloacetic acids formed when chlorine reacts with natural organic matter. Precursor removal and reduced contact time help control their formation.

Cross connection

Any actual or potential connection between a potable water system and a source of contamination. Backflow prevention devices and air gaps protect the distribution system from such connections.

Water hammer

A pressure surge caused by rapid changes in flow velocity, such as sudden valve closure. It can rupture pipes and damage fittings, so valves should be operated slowly.

Coliform rule

Total coliform bacteria are used as indicators of distribution system integrity. Presence requires repeat sampling, and E. coli presence triggers public notice and investigation of contamination sources.

BOD5

Biochemical oxygen demand measured over five days at 20 degrees Celsius. It estimates biodegradable organic matter, with typical untreated domestic wastewater near 200 to 250 mg/L.

Secondary treatment standards

Federal standards require 30-day average effluent BOD5 and TSS of 30 mg/L or less, with at least 85 percent removal of each, unless a stricter permit limit applies.

F/M ratio

Food-to-microorganism ratio equals pounds of BOD5 applied daily divided by pounds of MLSS under aeration. Low ratios favor nitrification; high ratios favor rapid BOD removal but poorer settling.

Mean cell residence time

MCRT, or sludge age, equals pounds of solids in the aeration basin divided by pounds of solids leaving daily in waste sludge and effluent. Longer MCRT supports nitrification.

Sludge volume index

SVI equals the 30-minute settled sludge volume in mL/L times 1000 divided by MLSS in mg/L. Values near 80 to 150 indicate good settling; above 200 indicates bulking.

Return activated sludge

Settled biomass returned from the secondary clarifier to the aeration basin to maintain the target MLSS. RAS rate is adjusted based on sludge settleability and clarifier blanket depth.

Nitrification

Biological oxidation of ammonia to nitrite and then nitrate by nitrifying bacteria. It consumes alkalinity and oxygen and requires longer solids retention time and warmer water.

Denitrification

Conversion of nitrate to nitrogen gas by bacteria in an anoxic zone with a carbon source. It recovers alkalinity and reduces total nitrogen, but can cause rising sludge if it occurs in the clarifier.

Infiltration

Groundwater that enters a sewer through defective pipes, joints, and manholes. It adds hydraulic load without organic load and is highest during wet weather and high groundwater periods.

Inflow

Direct stormwater or groundwater entries into a sewer, such as roof drains, foundation drains, and unsealed manhole covers. Inflow causes sharp peak flows during rain events.

Anaerobic digestion

Biological decomposition of sludge without oxygen, producing methane and carbon dioxide. Stable digesters hold pH near 6.8 to 7.4 and alkalinity above about 2000 mg/L as CaCO3.

Volatile solids reduction

The percentage of volatile solids destroyed during digestion, typically 40 to 60 percent in a well-operated anaerobic digester. It indicates digestion efficiency and sludge stabilization.

Fecal coliform limit

A common permit limit for disinfected effluent is a monthly geometric mean of 200 colony-forming units per 100 mL for fecal coliform, or 126 for E. coli, depending on the permit.

Preliminary treatment

Screening and grit removal that protect downstream equipment. Bar screens capture large debris, while grit chambers settle sand and heavy solids that would damage pumps and clog digesters.

Primary clarifier

A settling tank that removes settleable solids before biological treatment, typically achieving 50 to 65 percent TSS removal and 25 to 40 percent BOD5 removal with a hydraulic detention time of about 1.5 to 2.5 hours.

Dissolved oxygen control

Aeration basins typically maintain 1.5 to 3.0 mg/L dissolved oxygen for carbonaceous BOD removal. Higher levels waste energy; lower levels can cause filamentous bulking and incomplete treatment.