NC Phase W flashcards

31 cards. Read the term and the definition here. The deck at the top of this page is optional.

Coagulation optimum pH

Alum works best around pH 5.5-6.5, while ferric salts function roughly at pH 4.5-6.0. Jar testing confirms the dose and pH where turbidity removal is best.

Alkalinity consumed by alum

About 0.45 mg/L of alkalinity as CaCO3 is destroyed per mg/L of alum applied. If alkalinity falls too low, floc formation fails and lime must be added.

Velocity gradient G

G measures mixing intensity in coagulation and flocculation. Rapid mix uses roughly 250-1000 s^-1 for seconds; flocculation tapers from about 75 down to 10 s^-1 over 15-45 minutes.

Overflow rate

Overflow rate equals flow divided by basin surface area and represents the settling velocity of the slowest particle fully removed. Larger area lowers overflow rate and improves removal.

CT concept

CT is disinfectant residual concentration in mg/L multiplied by contact time in minutes. Required CT rises with higher pH, lower temperature, and greater target log inactivation.

Chlorine demand

Chlorine demand is the difference between the applied dose and the residual measured after the contact period. It reflects consumption by organics, ammonia, and reduced compounds.

Lime-soda softening

Lime removes carbonate hardness and soda ash removes non-carbonate hardness. Practical softening stops around 50-80 mg/L as CaCO3 because of solubility limits.

Filter effluent turbidity

Combined filter effluent is typically held to 0.3 NTU or less, and individual filters must stay below 1 NTU in at least 95 percent of monthly measurements.

Minimum distribution pressure

Residential service should maintain at least 20 psi at ground level under peak demand. Normal pressure is 40-80 psi, and above 100 psi needs pressure reduction.

Hazen-Williams C factor

The C factor describes pipe roughness. New cement-lined ductile iron is about 130-150, while old unlined cast iron drops to 80-100, raising head loss.

Water hammer surge

Sudden velocity change creates a pressure surge equal to wave speed times velocity change divided by gravity. Slow valve closure limits the surge magnitude.

Hydrant flow test

A flow test records static pressure, residual pressure while flowing, and pitot pressure at the nozzle. These values estimate available fire flow at a chosen residual such as 20 psi.

Pressure per foot of head

One foot of water column produces about 0.433 psi, and one meter produces about 9.8 kPa. Tank elevation therefore sets the pressure available in the system.

Coliform monitoring

Small systems collect at least five routine coliform samples monthly, scaled upward with population. Any positive sample triggers repeat sampling and investigation for E. coli.

Distribution chlorine residual

A detectable disinfectant residual must persist throughout the distribution system. Operators measure free chlorine at representative points to confirm protection against regrowth.

Peak wet-weather flow

Collection systems are sized for peak wet-weather flow, often 2-4 times average dry-weather flow. Infiltration and inflow are estimated separately and added to the design flow.

Self-cleansing velocity

Sanitary sewers should reach at least 0.6 m/s at design flow to keep solids moving. Velocities above roughly 3 m/s risk scour and pipe damage.

Manning equation

Gravity sewer capacity follows Q = (1/n) A R^(2/3) S^(1/2). Roughness n is about 0.013 for concrete and clay and about 0.012 for PVC.

Sewer minimum slope

Minimum slope is whatever achieves self-cleansing velocity at design flow, not a fixed percentage. A common starting point is 0.4-0.5 percent for 200 mm pipe.

Manhole spacing

Manholes are placed at every change in grade, size, or alignment and at intervals of roughly 90-150 m. They provide access for cleaning and inspection.

Hydrogen sulfide corrosion

H2S forms in anaerobic slime above the waterline and oxidizes to sulfuric acid on concrete. Ventilation and chemical dosing control odors and crown corrosion.

Inflow and infiltration

I/I is storm water and groundwater entering the sewer. Smoke testing, dye testing, and CCTV locate defects, and reducing I/I lowers treatment cost and overflow risk.

BOD5 test conditions

The BOD5 test incubates a seeded sample at 20 C for five days with a nitrification inhibitor. Depletion must fall within the valid range for the result to count.

F/M ratio

Food-to-microorganism ratio is BOD mass applied per day divided by MLSS mass. Conventional activated sludge runs about 0.2-0.5 kg BOD per kg MLSS per day.

Sludge volume index

SVI equals settled sludge volume in mL/L after 30 minutes times 1000 divided by MLSS in mg/L. Values of 80-150 settle well; above 200 indicates bulking.

Nitrification requirements

Nitrifiers need long solids retention time, dissolved oxygen above 2 mg/L, temperature above 10 C, and about 7.1 mg/L alkalinity as CaCO3 per mg/L ammonia oxidized.

Denitrification carbon demand

Reducing one mg of nitrate nitrogen consumes about 2.86 mg of BOD. An anoxic zone with recycled nitrified mixed liquor and a carbon source drives the reaction.

Secondary clarifier loading

Activated sludge clarifiers are checked on both overflow rate and solids loading. Typical overflow rate is 16-32 m3/m2-day and solids loading 4-6 kg/m2-h.

Effluent dechlorination

Sulfur dioxide or sulfite salts neutralize excess chlorine after disinfection. Dechlorination protects aquatic life in the receiving stream from chlorine toxicity.

Anaerobic digestion

Mesophilic digestion runs at 30-38 C and thermophilic at 50-57 C. Volatile solids destruction is 40-60 percent and biogas is roughly 60-65 percent methane.

Biosolids classes

Class A biosolids meet strict pathogen limits and allow broad land application. Class B meets lesser limits but requires site access restrictions and harvesting setbacks.