Introduction to Environmental Engineering and Science (3rd ed.) Chapter 6. Water Quality Control

Municipal Water and Wastewater Systems

Drinking Water Standard Primary Standards: enforceable and based on health-related criteria: Maximum Contaminant Levels (MCLs) 1) chemical standards, 2) radionuclides, 3) microbiological standards (Table 6.1) Secondary Standards: nonenforceable and intended to protect welfare (Table 6.2)

Schematic of a typical surface water treatment plant Figure: 06-02

Schematic of a typical groundwater treatment plant Figure: 06-03

Forces acting on a particle settling in water FB = vpρg Figure: 06-04 FG = vpρpg FD = 3πμvsdp

Sedimentation: gravitational settling Stokes’ law vs = g(ρp-ρ)dp2/18μ (6.5) only valid at laminar region (Re < 1) Rectangular settling basin (Figure 6.5) hp = vsθ = vsVb/Q (6.6) Critical settling velocity (vo) vo = h/θ =hQ/Vb = hQ/hAb = Q/Ab (6.7) (Example 6.1)

Trajectory of a particle settling Figure: 06-05 To design a clarifier to remove all particles of a size dp at a flow rate Q, Surface area, Ab = 18Qμ/g(ρp-ρ)dp2 (6.8)

Coagulation and flocculation Coagulation: a chemical treatment process that destabilizes particles (sticky) Coagulant: alum, Al2(SO4)3·18H2O Al2(SO4)3·18H2O + 6HCO3- → 2Al(OH)3(S) + 6CO2 + 18H2O + 3SO42- Al(OH)3: a light, fluffy floc that adsorbs destabilized particles on its surface as it settles. Rapid mix/coagulation: less than one-half min. Flocculation: gentle agitation for approximately one-half hour

Flocculation and sedimentation tank Figure: 06-06

Filtration & Disinfection Filtration: removing small particles from water Primary disinfection: to kill any pathogens in the water. Dinfectant: hypochlorous acid (HOCl) and hypochlorite (OCl-) Secondary (or residual) disinfection: to prevent pathogen regrowth in the water during the period before it is used. Disinfectants: chloroamines (NH2Cl, NHCl2, NCl3)

A disadvantage of free chlorine disinfection Formation of halogenated disinfectant byproducts (DBPs), such as trihalomethanes (THMs) and haloacetic acids (HAAs) Alternative disinfectants: ClO2, O3 The rate of inactivation of pathogens: Chick-Watson model r(N) = dN/dt = -kCnN

Hardness Water hardness: the presence of multivalent cations, mostly Ca2+ and Mg2+ (unit: mg/L as CaCO3) Temperature increase results in precipitation of CaCO3 and Mg(OH)2 Equivalent weight (eq)= molecular weight / n Normal concentration = eq/L = N mN = meq/L = (mg/L)/eq 1 meq/L = 50 mg/L as CaCO3 (Example 6.4) Hardness classification: Table 6.4

Alkalinity Alkalinity = (HCO3-) + (CO32-) + (OH-) - (H+) Unit: meq/L or mg/L as CaCO3 Alkalinity (meq/L) = (HCO3-) for 6 ≤ pH ≤ 8.5 Total hardness = carbonate hardness + noncarbonate hardness Example 6.6

Hardness & Alkalinity (Example 6.6) Figure: 06-09

Softening Lime-soda ash process Add hydrated lime (Ca(OH)2) to increase pH above 10: CaCO3 and Mg(OH)2 precipitation Add soda ash (NasCO3): promote CaCO3 precipitation Recarbonation by adding CO2(g) Ion-exchange process using ion-exchange resin (must be regenerated!)

Size ranges and types of contaminants Figure: 06-11

Schematic of a typical wastewater treatment facility Figure: 06-12

Primary and Secondary Treatments Primary treatment: removal of 50-65% suspended solids and 25-40% BOD (detention time: 1.5-3 hrs) Secondary (biological) treatment Suspended growth treatment: a) activated sludge, b) membrane bioreactor, c) aeration lagoons and oxidation ponds Attached treatment process: a) trickling filters, b) rotating biological contactors (RBC)

Activated sludge wastewater treatment process Figure: 06-15

An oxidation pond Figure: 06-17

A trickling filter Figure: 06-18

Rotating biological reactor Figure: 06-19

Sludge treatment: anaerobic digestion Figure: 06-20

Schematic of a two-stage anaerobic digester Figure: 06-21

Nutrient removal Nitrogen removal: nitrification/denitrification NH4+ + 2O2 → NO3- + 2H+ + H2O 2 NO3- + organic matter → N2 + CO2 + H2O Phosphorous removal: by adding coagulant alum (Al2(SO4)3) or lime (Ca(OH)2) Al2(SO4)3 + 2PO43- → 2AlPO4(S) + 3SO42-

Hazardous waste A listing of substances and industrial process (Code of Federal Regulations) (Table 6.9) Any of the following characteristics Ignitable: vapor ignites at low temperature (<60oC) Corrosive: pH <2 or pH >12.5 Reactive: unstable under normal conditions Toxic: Toxicity Characteristics Leaching Procedure (TCLP) test

Hierarchy of priorities in hazardous waste management Figure: 06-23

The Superfund remedial process Figure: 06-24

Hazardous waste treatment technologies: Remediation technologies Treatment technologies vs. hazardous wastes (Table 6.11) Chemical, biological and physical wastewater treatment processes Thermal destruction processes Fixation/stabilization

Physical treatment Sedimentation Adsorption: granular activated carbon (GAC) Aeration Reverse osmosis Ion exchange Electrodialysis Monitored natural attenuation (MNA)

Vacuum filter Figure: 06-25

Air stripping tower Figure: 06-26

Chemical treatment Neutralization: lime (Ca(OH)2) or acid Chemical precipitation: lime, caustic soda (NaOH) Chemical reduction-oxidation: reductants/oxidants UV radiation/oxidation: AOPs O3 + hv + H2O → H2O2 + O2 H2O2 + hv → 2OH·

Neutralization Figure: 06-27

Chemical Precipitation of metals vs. pH Figure: 06-28

Destruction of chlorinated organics using O3 with or without UV Figure: 06-29

Biological treatment: Aqueous waste treatment

Rotary kiln hazardous waster incinerator Waste incineration: Rotary kiln hazardous waster incinerator Figure: 06-31

Land disposal Landfills Surface impoundments Waste piles Injection wells Land treatment facilities Salt domes or salt bed formation Underground mines or caves Concrete vaults or bunkers

Landfill Figure: 06-32

Hazardous waste injection well Figure: 06-33