Env Eng CE 3520 Final Exam

effects of oxygen demand on river water

(1) removal of oxygen by microorganisms during biodegradation (2) replenishment of oxygen through reaeration at the interface between the river and the atmosphere

What order is the rate of deoxygenation by organic waste?

1st order wrt L_t

Stream aeration and BOD have what type of relationship?

inverse

reaeration

rate at which oxygen is replenished and is assumed to be proportional to the difference between the actual DO in the river at any given location and the saturated values of DO

oxygen deficit

departure of the ambient DO from saturation

K_r = k_d at which point of the oxygen sag curve?

critical point

mass flux density

rate of mass transferred across the plane per unit time per unit area; can result from advection, diffusion/dispersion, or a combo of both

advection

movement of a compound along with flowing air or water; depends on concentration and flow velocity

diffusion

net effect of random molecular movement caused by the molecules’ thermal-kinetic energy; causes solutes to move from high to low concentrations

dispersion

(1) turbulent: mass transferred through mixing of turbulent eddies within the fluid (2) mechanical: variations in flow pathways taken by different fluid parcels that originate in nearby locations, dominant process in GW

Why does the contaminant move slower than GW itself?

sorption to soil particles; chemical will travel at a slow rate than V_a by a factor of R_f

purpose of water treatment

(1) potable: safe for human consumption, free of harmful elements (2) palatable: aesthetically acceptable (clear, no odor, no bad taste)

turbidity

measures the optical clarity or scattering and absorbance of light caused by suspended particles; WHO <5 NTU, US < 0.1 NTU; GW has <1 NTU

particles

larger than molecules, often not visible, can absorb toxic metals and organics

natural organic matter (NOM)

includes colloids and dissolved organic carbon, passes 0.45 filter

color is caused by

dissolved organic matter, natural metallic ions

• apparent color: unfiltered, caused by turbidity

• true color: filtered, caused by constituents

pathogens

microorganism that cause illness; includes viruses,bacteria, fungi, amoeba

indicator organism

used to monitor the microbial water quality; example: total coliform

maximum contaminant level goals

based on health and risk assessment info

maximum contaminant level

enforceable, also based on costs and technology availability

sorption

refers to either or both processes of adsorption of a chemical at the solid surface and/or absorption of a chemical into the volume (interior) of the solid

sorbate

substance transferred from the gas or liquid phase to solid

sorbent

solid material onto or into which the sorbate accumulates

sorption isotherm

relationship that describes the affinity of a compound for a solid in water or gas at a constant temperature

adsorbent types

granual activated carbon (GAC), powdered activated carbon (PAC), granular ferric hydroxide (GFH)

granular activated carbon (GAC)

removal of organic and inorganic constituents, SA=500-2500 m²/s

powdered activated carbon (PAC)

removal of seasonal taste and odor, SA = 800-2000 m²/s

granular ferric hydroxide (GFH)

used primarily for removing arsenic and selenium, SA = 250-300 m²/s

empty bed contact time (EBCT)

time required for one bed volume of influent to pass through the empty reactor

mass transfer zone (MTZ)

portion of bed in which the dissolved adsorbate concentration and the adsorption density change dramatically

breakthrough curve

eventually the leading edge of the MTZ reaches the outlet and the concentration of adsorbate in the effluent begins to increase significantly, which causes breakthrough

hardness

used to characterize a water that doesn’t lather well and leaves scale, sum of calcium and magnesium; units mg/L as CaCO₃

lime

CaO

soda ash

Na₂CO₃

coagulation

charge neutralization step that involves condition of colloidal matter with coagulants to destabilize particles

flocculation

aggregation of destabilized particles and formation of larger particles (flocs) for easy removal via settling or filtration

particle stability

tendency of suspended particles in water to remain separated and resist aggregation or settling

mechanism for coagulation and flocculation

compression of electrical double layer (EDL), charge neutralization, adsorption and interparticle bridging, precipitation and enmeshment

types of coagulants

inorganic metallic, prehydrolyzed metal salts, organic polymers, natural plant-based materials

Why do we add lime and soda ash?

the natural alkalinity of the water source isn’t sufficient and we need to maintain appropriate pH

granular (sand) filtration

solid-liquid separation process for removal of colloidal and suspended particles; final particle removal process (tertiary)

slow sand filtration

appropriate for rural communities due to simplicity, land availability, and low energy requirement; cycle of filtration and regeneration (top 1-2 cm of sand scraped off and cleaned)

rapid filtration

cycle of filtration (water flows down and particles trapped) and backwash (water flows up to flush out particles from bed)

sedimentation

process in which a majority of particles will settle by gravity in a reasonable time and be removed

discrete particle settling

occurs when particles are discrete and don’t interfere with one another; described by Stokes’ or Newton’s laws

When settling velocity is > OR what % of particles are removed?

100%

Why is water disinfected at almost the last stage?

economical, less formation of disinfection byproducts and disinfectant residuals

which free chlorine component is more effective for disinfection

HOCl (why pH<7 is desired)

lime removes

(H₂CO₃ or CO₂ + Ca CH + 2x Mg CH + Mg NCH + 30 mg/L CaCO₃) x 56/100

ash removes

NCH x (106/100)

C x t product

inactivation of microorganisms is controlled by concentration of disinfectant and time of contact; equal to dosage of disinfectant; higher value means more effective

A to B

chlorine reacts first with the reducing agents present and develops no measureable residual

B to C

chlorine continues to be added, reacting with ammonia and forming chloramines; chlorine residual begins to appear, but as combined residual, not free chlorine

C to D

with further chlorine addition past point C, chlorine begins oxidizing chloramines and other nitrogenous compounds, causing the chloramine residual to drop

after D

further chlorine addition results in accumulation of a stable free chlorine residual which is most effect for disinfection

What is the importance of the breakpoint chlorination curve?

helps water treatment operators identify optimal chlorine dosage to ensure effective disinfection; ensures chlorine residuals are in the effective “free chlorine” range to reliably eliminate pathogens

Chick’s law assumes the rate of disinfection reaction is what order?

pseudo first order

CH

= min(alkalinity, TH)