Safe Drinking Water Act (SDWA)

• Enacted by Congress in 1974, the SDWA has been amended several times since.
• Establishes drinking water standards for all public water systems throughout the United States.
• SDWA standards apply to water used for drinking, cooking, and washing, aiming to safeguard public health and welfare.
• Stream standards protect surface water quality and aquatic ecosystems.
• Ensures public water supplies are safe by setting limits on the amounts of various substances present.

Drinking Water Standards

• The EPA has established two types of drinking water standards: primary and secondary, based on public health research and scientific judgment.

Primary Standards

• Primary standards are legally enforceable and protect public health by setting maximum permissible levels for potentially harmful substances in water.

Secondary Standards

• Secondary standards are not enforceable by law.
• They offer guidelines regarding the aesthetic aspects of drinking water, such as color and odor, which do not pose a health risk.

MCLs

• Primary standards are typically specified as maximum contaminant levels (MCLs).
• MCLs represent the highest allowable concentration of a substance in public drinking water supplies.
• These are enforceable limits.

Treatment Technique (TT) Requirements

• Treatment technique (TT) requirements are set for contaminants that are difficult or costly to measure.
• TT refers to a specific treatment process, such as filtration, needed to remove certain contaminants from drinking water.

MCLGs

• The EPA also sets maximum contaminant level goals (MCLGs).
• MCLG that does not have adverse health effects; however, it is a goal and not an enforceable standard

Contaminant Categories

• Primary contaminants include organic chemicals, inorganic chemicals, microorganisms, turbidity, and radionuclides.
• Exceeding MCLs for most substances doesn't pose an immediate health threat, except for some microorganisms and nitrate.
• Long-term exposure to drinking water exceeding standards can be harmful.

Organic Chemicals

• Volatile organic compounds (VOCs) are organic compounds that easily evaporate and participate in smog-forming photochemical reactions.
• Even small concentrations of organic chemicals can have public health significance, expressed in mg/L, where 1 mg/L equals one part per million.

Inorganic Chemicals

• Several non-carbon-containing inorganic substances, including heavy metals, are significant for public health.
• MCLs for most inorganics are the same as the MCLGs.
• The MCLG for lead is zero, with lead pipes no longer allowed in public water systems in the United States.

Lead and Copper Rule

• Treatment techniques have been set for lead and copper.
• The presence of lead and copper in drinking water usually comes from corroded plumbing materials.

Microorganisms

• The Total coliform group of bacteria may indicate the possible presence of pathogenic organisms.
• EPA set maximum limits and testing protocols via the total coliform rule (TCR).
• The GWR (Ground Water Rule), along with the total coliform rule, prevents pathogenic microorganisms from entering community water supplies cost-effectively.

Turbidity

• Turbidity is the presence of suspended particles like clay, silt, and plankton in water.
• While not required for groundwater sources, turbidity testing is necessary for surface water sources.

Radionuclides

• Exposure to ionizing radiation in drinking water may increase the risk of cancer and other health issues.
• There are presently no enforceable federal regulations for radon amounts in drinking water.
• However, the EPA has proposed regulations for communities that use ground- or surface water.
• The proposed MCLG for radon in drinking water is zero.

Secondary Standards

• A range of concentrations is established for substances that affect mostly the esthetic qualities of drinking water, such as taste, odor, and color.
• These do not have a direct effect on public health.
• The secondary standard for Fluoride is set at 2.0 mg/L to prevent dental fluorosis in children.

Sampling Procedures

• The sampling frequency requirement varies by contaminant group and individual contaminant
• It also depend on the population served and whether surface or ground water is used.
• The sampling frequencies can range from once every 4 h (for turbidity) to once every 9 years (for asbestos).
• Drinking water samples are fully flushed
• The exception to this is monitoring for copper and lead, for which a first draw sample is required at the consumer's taps, where contamination is more likely to occur

Record Keeping and Reporting

• Records should include the name, date, and place of sampling; the name of the technician who took the sample; the type of sample; and the place, date, method, and results of the analysis.

Public Notification

• Public water systems should submit reports to consumers and an appropriate local regulatory agency.
• Public notification is required to advise consumers of the potential health hazards and to educate them about the importance of adequate financing and support for drinking water systems.

Water Treatment

• Clarification usually involves coagulation-flocculation, sedimentation, and filtration.

Sedimentation

• The impurities in water may be either dissolved or suspended.
• Under quiescent conditions, when flow velocities and turbulence are minimal, sedimentation allows particles denser than water to settle.
• The accumulated solids at the bottom is sludge.
• The tank may be called a sedimentation tank, a settling tank, or a clarifier.

Particle settling rate

• The speed at which suspended particles settle toward the bottom of a tank depends on particle size and density, as well as the water's temperature and viscosity.
• Larger, heavier particles settle faster than smaller or lighter ones.
• Buoyancy and friction oppose the downward force of gravity.

Particle Interaction

• The nature of sedimentation also depends on the concentration of suspended particles and their tendency to interact.
• In a dilute suspension, where settling occurs without interference, the process is called free settling or discrete settling.

Hindered Settling

• As concentration increases, particles interact, interfering with movement.
• This is sometimes called hindered settling.

Detention Time

• Detention time describes how long water will remain in the settling tank.
• Settling tanks facilitate slow, continuous flow from inlet to outlet, enabling quiescent settling of suspended particles.
• Water remains in the tank for a few hours before reaching the outlet.

Detention Time Equation

• The theoretical detention time can be calculated:
  • TD = V/Q , where TD is detention time.
  • V is the volume of water in the tank.
  • Q is the average inflow rate.
• Usually is pressed in hours
• A minimum of 3 hours is required to ensure sufficient settling

Overflow Rate

• Overflow rate, or surface loading rate (SLR), affects settling tank design and operation.
• It indicates water volume passing over the clarifier's surface per unit area.
• Critical for determining a sedimentation tank's size and efficiency.
• With a lower SLR allowing more time for particles to settle and improving efficiency

Overflow Rate Equation

• Overflow rate can be computed as
  • V₀=Q/Aₛ ​​, where
  • V₀ = overflow rate in m³/m²⋅d
  • Q = average inflow rate in m³/d
  • Aₛ = tank surface area (top view) in m²

General Flow Pattern

• Clarified water is skimmed from the surface over weirs into an effluent channel.
• All suspended particles with a settling velocity faster than (overflow rate) reach the sludge layer at the tank's bottom.

Settling Tank Design

• Detention time and overflow rate must be specified to determine dimensions.
• Tanks may be rectangular or circular.
• Side water depth (SWD) is the actual water depth.

Tank Walls

• The height of a tank wall is typically about 0.45 m, or 1.5 ft, above the SWD, known as freeboard.
• This also prevents splashing

Coagulation and Flocculation

• Colloids can settle out of suspension with the help of coagulation.
• Coagulation and flocculation are important for removing suspended particles and impurities from water and wastewater.
• They are distinct treatment steps

Coagulation

• Purpose: to neutralize charges on suspended particles (colloids) so they come together.
• Coagulants, such as alum, ferric chloride, or polyaluminum chloride (PAC), are added to water
• These neutralize negative charges on particles, so they clump together into micro-flocs.
• The micro-flocs are still too small to settle easily.
• Coagulants are rapidly mixed in the water, then slowly stirred before sedimentation.

Flocculation

• Flocculation enhances the size of the clumps (flocs) formed during coagulation for effective settling.
• Water gets gently mixed to encourage connections between micro-flocs.
• Polymers may be added to assist the binding of flocs.
• Resulting in larger, heavier flocs that settle more easily or can be filtered

Filtration

• Filtration is often needed even with chemical coagulation and flocculation,
• Clarification through gravity settling/dissolved air floatation does not remove all impurities.
• Around 5% of non-settleable floc particles may remain, causing turbidity.
• Remaining flocs can cause noticeable turbidity and may shield microorganisms from the disinfection process.

Filtration Importance

• For crystal-clear, potable water meeting SDWA's 0.5 NTU requirement (via MCL for turbidity), an extra water treatment step (filtration) is needed.

Filtration Defined

• Filtration is removing suspended particles as water passes through a layer or bed of porous granular material like sand, trapping the particles within pore spaces.

Filter Media

• Early filters used very fine sand, known as slow sand filters.
• Slow sand filters take up a considerable amount of land area because of slow filtration rates.
• Regular manual scraping of the sand surface is necessary when clogged with suspended particles.
• They are effective and inexpensive to operate.

Rapid Filters

• Rapid filters have generally replaced slow sand filters in modern plants.
• Water flows 30x faster than it flows through slow sand filters.
• Cleaning is done more frequently by reversing the water flow through the bed and backwashing, not manual scraping.
• Filtration occurs as water flows downward under gravity.
• Clean water is forced upward during backwashing, expanding the filter bed and carrying away impurities

Disinfection Defined

• The unit processes including coagulation, sedimentation, and filtration compose clarification.
• Clarification removes microorganisms from water by removing suspended solids.
• It is not sufficient to ensure the complete removal of viruses or pathogenic bacteria.
• Potable water must be crystal clear but also free of disease-causing microorganisms.
• Therefore the final part is disinfection - the most important process for preventing the spread of waterborne diseases.

Chlorination

• Chlorination involves adding chlorine or chlorine compounds to drinking water for disinfection purposes.
• Molecular chlorine (Cl₂) appears as a greenish-yellow gas under standard conditions

Alternatives to Chlorination

• Ozone (03)
• Ultraviolet Radiation

Overview of Water Treatment Process

• Coagulation: Chemicals are added, neutralizing charges and forming microflocs.
• Flocculation: Through gentle mixing, microflocs combine into form larger flocs.
• Sedimentation: Flocs settle.
• Filtration: Clearwater is run through a filter.
• Disinfection: The water has been disinfected for safety.