D2 concepts

Backflow vs. backsiphonage

Backflow is any unwanted reversal of flow into a potable system. Backsiphonage is backflow caused specifically by subatmospheric or negative pressure.

Back pressure vs. backsiphonage

Back pressure occurs when the downstream pressure exceeds the potable-system pressure. Backsiphonage occurs when potable-system pressure falls below atmospheric or downstream pressure.

Air gap purpose

An air gap prevents backflow by providing a physical separation between the potable outlet and the nonpotable receiving point.

Alkalinity vs. pH

pH describes hydrogen-ion activity at a moment in time. Alkalinity describes the water's capacity to neutralize added acid.

Hardness vs. alkalinity

Hardness primarily reflects multivalent ions such as calcium and magnesium. Alkalinity reflects acid-neutralizing capacity, mainly bicarbonate, carbonate, and hydroxide.

Free, combined, and total chlorine

Total chlorine residual equals free chlorine residual plus combined chlorine residual.

Chlorine demand

Chlorine demand is the applied chlorine dose minus the chlorine residual after the specified contact time.

Breakpoint chlorination

Breakpoint chlorination adds enough chlorine to oxidize reducing compounds and combined chlorine so that additional dose produces a free chlorine residual.

CT concept

CT is disinfectant residual concentration multiplied by effective contact time. Required CT depends on organism, disinfectant, temperature, and pH.

Coagulation vs. flocculation

Coagulation destabilizes particles, usually through rapid mixing and chemical addition. Flocculation gently brings destabilized particles together into larger floc.

Sedimentation vs. filtration

Sedimentation removes settleable particles by gravity. Filtration removes particles as water passes through porous media or a membrane.

Adsorption vs. absorption

Adsorption occurs at a surface or interface. Absorption involves a substance being taken into the body or volume of another material.

Aerobic vs. anaerobic

Aerobic conditions contain available oxygen. Anaerobic conditions lack atmospheric or dissolved oxygen.

Acute vs. chronic health effects

Acute effects develop rapidly after exposure. Chronic effects develop slowly or after repeated or long-term exposure.

Static head

Static head is the vertical elevation difference between source and discharge water levels when no water is flowing.

Dynamic head

Dynamic head is the head condition while water is flowing and includes the effects of friction and other losses.

Pressure head

Pressure head expresses pressure as the equivalent height of a water column.

Velocity head

Velocity head is the energy per unit weight associated with water velocity.

Total dynamic head

Total dynamic head is the total head a pump must overcome while operating, including elevation, pressure, velocity, and friction components as applicable.

Head loss

Head loss is energy lost as water moves through pipe, fittings, valves, and equipment, primarily because of friction and turbulence.

Cavitation

Cavitation occurs when local pressure falls low enough for vapor bubbles to form and then collapse in a higher-pressure region.

Suction lift vs. suction head

Suction lift exists when the source water level is below the pump centerline. Suction head exists when the source water level is above it.

Gauge vs. absolute pressure

Gauge pressure is measured relative to atmospheric pressure. Absolute pressure is measured relative to a perfect vacuum.

Flow, velocity, and area

For steady incompressible flow, flow rate equals cross-sectional area multiplied by average velocity.

Detention time

Theoretical detention time equals basin volume divided by flow rate, using compatible units.

Surface loading rate

Surface loading rate equals flow divided by the basin's horizontal surface area.

Weir loading rate

Weir loading rate equals flow divided by the total effective weir length.

Filter loading rate

Filter loading rate equals flow divided by filter surface area.

Turbidity

Turbidity is an optical indication of light scattering caused by suspended and colloidal matter; it is not a direct measurement of particle mass.

Apparent vs. true color

Apparent color includes color from dissolved material and suspended matter. True color is measured after suspended matter is removed.

TDS vs. TSS

Total dissolved solids pass through the specified filter. Total suspended solids are retained by the filter under the test method.

Colloid

A colloid is small enough to remain suspended and typically requires destabilization before effective settling or filtration.

Zeta potential

Zeta potential represents the electrical condition around suspended particles and helps indicate how strongly they repel one another.

Jar testing

Jar testing compares coagulant type, dose, pH, and mixing conditions on a small scale before changing full-scale treatment.

Filter ripening

Filter ripening is the early part of a filter run when the media has not yet developed its best particle-removal performance.

Mudballs

Mudballs are accumulations of media and retained solids caused by inadequate backwashing or poor wash-water distribution.

Air binding in filters

Air binding occurs when dissolved air comes out of solution and blocks pore spaces, interfering with filtration and backwashing.

Disinfection vs. sterilization

Disinfection reduces or inactivates pathogenic organisms to a required level. Sterilization destroys all forms of microbial life.

Primary vs. secondary disinfection

Primary disinfection achieves required pathogen inactivation at treatment. Secondary disinfection maintains a residual in the distribution system.

Nitrification

Nitrification is biological oxidation of ammonia to nitrite and then nitrate; in chloraminated systems it can consume disinfectant residual and affect water quality.

Corrosion vs. scaling

Corrosion dissolves or deteriorates materials. Scaling deposits mineral material on surfaces.

Langelier Saturation Index

LSI estimates whether water is undersaturated, saturated, or supersaturated with respect to calcium carbonate; it is an indicator, not a direct corrosion-rate measurement.

Cathodic protection

Cathodic protection reduces corrosion by making the protected metal act as the cathode of an electrochemical cell.

Galvanic corrosion

Galvanic corrosion can occur when dissimilar metals are electrically connected in an electrolyte; the more active metal becomes anodic and corrodes preferentially.

Anode and cathode

Oxidation occurs at the anode. Reduction occurs at the cathode.

Oxidation and reduction

Oxidation is loss of electrons; reduction is gain of electrons.

ORP

Oxidation-reduction potential indicates a solution's tendency to accept or donate electrons under the measured conditions.

Ion exchange

Ion exchange reversibly swaps ions in water with ions held on a resin.

Softening

Water softening removes or reduces hardness-causing ions, primarily calcium and magnesium.

Membrane pressure hierarchy

In general, microfiltration and ultrafiltration use lower pressure than nanofiltration and reverse osmosis.

Reverse osmosis

Reverse osmosis applies pressure greater than osmotic pressure to move water through a semipermeable membrane while rejecting many dissolved constituents.

Recovery and reject flow

Membrane feed flow divides into permeate flow and concentrate or reject flow. Recovery is the fraction of feed converted to permeate.

Cross-connection

A cross-connection is an actual or potential connection between a potable system and a nonpotable source.

Direct vs. indirect cross-connection

A direct cross-connection is subject to back pressure or backsiphonage. An indirect connection is generally subject only to backsiphonage.

MCL vs. MCLG

An MCL is an enforceable drinking-water limit. An MCLG is a nonenforceable public-health goal.

MRDL vs. MRDLG

An MRDL is an enforceable maximum disinfectant residual level. An MRDLG is the corresponding nonenforceable health goal.

Grab vs. composite sample

A grab sample represents one place and time. A composite sample combines multiple samples over time, flow, or location.

Accuracy vs. precision

Accuracy is closeness to the true value. Precision is closeness of repeated measurements to one another.

Calibration

Calibration compares and adjusts an instrument against a known standard so its readings correspond to accepted values.

Preventive vs. corrective maintenance

Preventive maintenance is scheduled to reduce failure risk. Corrective maintenance repairs equipment after a defect or failure is found.

Lockout vs. blockout

Lockout controls hazardous energy sources. Blockout physically prevents unexpected movement of equipment or components.

Confined space

A confined space is large enough to enter, has limited entry or exit, and is not designed for continuous occupancy; additional hazards may make it permit-required.

Water hammer

Water hammer is a pressure surge caused by a rapid change in flow velocity, such as sudden valve closure or pump stoppage.

Specific gravity

Specific gravity is the ratio of a substance's density to the density of the reference substance, usually water for liquids.

Specific weight vs. density

Density is mass per unit volume. Specific weight is weight per unit volume.

ppm and mg/L in water

For dilute water with density near 1.0 kg/L, 1 mg/L is approximately equal to 1 ppm by mass.

Million gallons and acre-feet

One million gallons is approximately 3.07 acre-feet; one acre-foot is approximately 325,851 gallons.

Demand patterns

Average demand describes typical use; maximum-day and peak-hour demands are used to size facilities for higher short-term requirements.

Dead end

A dead-end main has no downstream looped connection, so water age and sediment accumulation can be greater than in a well-looped system.

Pressure zone

A pressure zone is an area operated within a selected pressure range based on elevation, storage, pumping, and pressure-control facilities.

Unaccounted-for water

Unaccounted-for or nonrevenue water can include real losses, apparent losses, authorized unbilled use, and meter or accounting errors, depending on the accounting method.

Water audit

A water audit accounts for water entering and leaving a system to identify real losses, apparent losses, and data-quality problems.

Valve exercising

Valve exercising confirms operability, reduces seizure risk, and documents position and condition before an emergency.

Hydrant flushing

Flushing can remove accumulated sediment, restore disinfectant residual, and verify flow, but must be planned to control pressure, discharge, and water quality.

Reservoir turnover

Turnover describes replacement and mixing of stored water; poor turnover can increase water age and contribute to residual loss or stratification.

Stratification

Stratification is the formation of water layers with different temperatures or densities, which can limit mixing.

Eutrophication

Eutrophication is nutrient enrichment that promotes excessive biological growth and can create taste, odor, oxygen, and treatment problems.