CHEM 321 - Module 2

water is

dipolar, facilitating interactions with other dipoles or charged molecules (ions)

the structure of water facilitates formation of

hydrogen bonds, leading to unusually high density and cohesiveness (high boiling point, surface tension), up to four H bonds can form

hydrogen bond

an attraction between a hydrogen atom that is covalently bonded to a highly electronegative atom (usually N, O, or F) and an electronegative atom with a lone pair of electrons

density of water

~ 1g/mL (highest at 4C, not freezing point thus water expands upon freezing, making ice less dense)

water is transparent to

visible and UV light - allows light for photosynthesis to penetrate water bodies to considerable depth

water is very polar

ability yp hydrate ions, good solvent for ions, most abundant/important solvent on earth

hydrogen bonding of water

high melting and high boiling points

water has a high heat capacity

temperature stabilization of organisms/geographical regions, water used for AC chillers, nuclear power plants, etc

water (aqueous) chemistry

is concerned with the chemical reactions and processes affecting the distribution and circulation of chemical species in natural waters

water as a polar solvent: Inorganic

salts that dissociate to anions and cations (CO3-, Ca2+. Cl-, Br-, Mg2+)

water as a polar solvent: organic

natural organic matter (decaying leaves), pollutants (pesticides, PAHs, endocrine disruptors)

water as a polar solvent: dissolved gasses

O2, CO2, SO2

water as a polar solvent: biological microorganisms

bacteria and viruses

point sources of pollution

comes from a single point (wastewater treatment plant, industry, easy to regulate)

nonpoint sources of pollution

land areas such as agricultural fields, roads and parking lots that generate surface runoff containing various types of contaminants

water characteristics: physical

temperature, colour, turbidity, taste and order, total solids

water characteristics: chemical

total dissolved solids, total organic carbon, chemical oxygen demand, biological oxygen demand, pH, alkalinity, hardness (minerals)

water characteristics: biological

indicator organisms for pathogens

physical characteristics: temperature

pH, density, dissolved gases

physical characteristics: tubidity

measure of water clarity, affected by suspended solids and coloured material

physical characteristics: colour

due to dissolved substances

taste and odor

organic compounds in drinking water: attributed to bacterial degradation of algae and algal waste products (algal blooms can cause these); inorganic compounds Ferrous (+2), manganous (+2), sulfide ions and chlorine

total solids

refer to material left in a dish after a water sample is evaporated and dried in an oven (103-105C)

dissolved solids

refer to material that pass through a standard glass-fibre filter (0.45um)

suspended solids

refer to a material retained by a standard glass-fibre filter

volatile solids

refer to material lost when dried solids are burnt organic material → CO2

fixed solids

refer to material retained when dried solids are burnt; inorganic materials that do not burn at 500C

salinity (dissolved solids)

measure of salts in water usually expressed in ppm (mg/L)

dissolved gases

such as O2 (for fish) and CO2 (for photosynthetic algae) are crucial for living species in water

factors affecting dissolved oxygen: temperature

the solubility of gases in water decreases with increasing temperature; dissolved O2 consumed by degradation of organic matter in water;→ many fish killed not from direct toxicity but O2 deficiency

temperature and DO of a lake

for approximately 2 weeks in the spring and fall, the typical lake is entirely mixed from top to bottom, all water at same temp

in the summer most lakes

with sufficient depth (greater than 30 feet) are stratified into three distinct temperature layers; epilimnion, metalimnion, and hypolimnion (top to bottom)

pE - electron activity

the degree to which a water is oxidizing or reducing depends on the solute species present and reflect the activity of the electron, e- (availability of electrons in solution)(analogous to the concept of pH)

biochemical Oxygen demand

the amount of oxygen required by microorganisms (mo) to oxidize dissolved organic matter in water samples

chemical oxygen demand

the amount of oxygen consumed as a result of oxidation (by dichromate, a powerful oxidizing agent) of dissolved organic matter (and other oxidizable components) in the water sample

COD vs BOD

BOD is fast and reliable for wastewater treatment operation and monitoring; COD does not distinguish between biodegradable and non-biodegradable organic matter (typically used for industrial wastewaters)

total organic carbon TOC

nonfiltered samples; important parameter for both drinking water and wastewater treatment

dissolved organic carbon (DOC)

total carbon - inorganic carbon; DOC analyzer (some analyzers bubble nitrogen to remove CO2 and all carbonate species

biological mircoorganisms - pathogenic

bacteria (vibrio cholera, salmonella, legionella, pathogenic E. coli), protozoa (giardia, cryptosporidium), viruses (poliovirus, hepatitis, rotavirus), Helminths (intestinal roundworm, hookworm, whipworm)

biological microorganisms - non-pathogenic

algae and fungi, non-pathogenic bacteria

pathogenic microogranisms - infectious diseases

fecal-oral route, disease passed from feces of a host to the mouth of another, usually indirect route via contaminated water and food

difficulties with routine testing of pathogens in water

present in low numbers, limited survival time, numerous pathogens to analyze, time and cost prohibitive

indicator organisms

are a basic monitoring tool used to determine the presence of pathogens in natural and treated wayers

ideal characteristics of indicator organisms

present when pathogens are in the water, absent in non-contaminated water, present in higher numbers than pathogens in contaminated water, higher survival in water than pathogens, easy to analyze

total coliform bacteria

are present in polluted and non-polluted soil and water. Their presence indicates a possible existence of waterborne pathogens. Total coliform refers to all coliform bacteria from feces, soil, or other origin

fecal coliform bacteria

are a sub-group of total coliform group and appear in intestines and feces of people and warm blooded animals. Their presence indicates possible recent fecal contamination

photosynthesis

atmospheric CO2 is transferred to biosphere (plants) by photosynthesis and convert it to biomass

respiration

animals also produce CO2 to the atmosphere by respiration

biodegradation

plants and living organisms die and become humic carbon in dirt and soil

Atmospheric CO2 is

adsorbed to surface water and oceans to form bicarbonate and carbonate

carbon cycle: minerals and rocks

carbonates in water in the presence of Ca2+ can precipitate to form CaCO3 (limestone) and other rocks. limestone can also dissolve to form carbonates in water

fossil fuels

human economic activities (burning of carbon-containing fuels) also contribute to the release of CO2 to the atmosphere

why is the ocean pH <8.70

temperature gradients (arctic ocean) and stratification, deposition of acidic compounds, presence of other buffers (dissolution of silica), non-equilibrium: ocean not well-mixed

alkalinity

is a measure of the capacity of water to neutralize a strong acid; on natural waters HCO3-, CO3(2-), OH-, borates, ammonia, phosphates, organic bases, organic matter

total alkalinity

is the number of H+ equivalents required to titrate 1L of water to pH ~4, methyl orange indicator changes colour at pH of 4

most important species in the environment

CO2, HCO3-, CO3 (2-)

total alkalinity =

2 [CO3(2-)] + [HCO3-] + [OH-] - [H+]

alkalinity units

expressed as mg/L of CaCO3 (MW = 100 g/mol), each CO3 ion has the capacity to neutralize 2 H+ or 2 equivalents

alkalinity and water treatment

stability on distribution system (100-120 mg/L as CaCO3)

too little alkalinity in water

wide changes in pH is not good for the distribution system, corrosive environment (acidic conditions), depletion of oxidants (protect against pathogens)

too much alkalinity in water

may contribute to scaling

raw water needs to be

treated to specific standards set by agencies such as Health Canada (guidelines) and the Environmental Protection Agency in the USA (regulation)

purification of drinking water

removal of pathogens, turbidity, color, taste and odor, organic matter, and other contaminants; safe for human consumption

purification of waste water

removal of BOD and COD, pathogens, nutrients, organic matter, contaminants; safe to return to rivers and lakes (does not affect wildlife and minimize ecological impacts)

Sources of raw water: freshwater

requires conventional treatment; surface waters - rivers, lakes; groundwater - wells

Sources for raw water; alternative sources

requires advanced treatment; ocean water, saline groundwater, wastewater

conventional drinking water treatment

screens → flocculation/coagulation → sedimentation → filtration → disinfection → distribution

screens

keep out fish, mussels, and large particles from coming into the facility

flocculation

coagulants (+, with positive charge), are added to water to help bind and neutralize particles and organic matter (-, with negative charge) to create “flocs”

Coagulants

may be polymers, aluminum or iron based chemicals: aluminum sulfate, ferric chloride

flocculation mechanism

negatively charge particles attract and tightly bind to the particle surface, forming the stern layer

flocculation - beyond the stern layer

the net negative charge and electric field of the particle can attract additional positive ions but are more loosely associated with the surface (diffuse ion layer)

flocculation - the electrical double layer

consists of the stern layer and the surrounding diffuse layer of counterions

flocculation: particles experience two competing forces

electrostatic repulsion (from overlapping electrical double layers) and van der Waals attraction

Flocculation - net energy

is the sum of attractive and repulsive forces; net energy 1 - an energy barrier prevents particles rom sticking together

flocculation: coagulants are added to

increase the ions dissolved in solution (ionic strength) which will compress the electric double layer reducing the repulsion forces (Curve 1 → Curve 2) and energy barrier is eliminated (net energy 2). particles will attract each other (van der Waals) to form small flocs and settle down

sedimentation

large flocs are allowed to settle at the bottom of tanks. clear water is collected at the top of the tank

filtration

small particles and pathogens are filtered out. Filter media can be sand, anthracite, or activated carbon; filters need to be cleaned every ~60 hours of operation

Activated carbon filter

adsorbs organic compounds and are typically used to remove organic matter and trace contaminants (pesticides, taste & odor compounds), used as filer media or a finishing step after filtration

disinfection

chemical disinfectants and UV light are applied to water to inactivate pathogens (bacteria, viruses protozoa) after added, water is sent to storage tank to allow disinfectants to react and inactivate pathogens

Commo chemical disinfectants

chlorine (HOCl), combined chlorine ( ammonia + chlorine = NH2Cl, NHCl, NCl3), chlorine dioxide (ClO2), ozone (O3)

inactivation goal

99.9% protozoa and 99.99% viruses

pathogen inactivation is dependent on

disinfection concentration and contact time with the pathogen

Chlorine (HOCl)

an acid (pKa = 7.50, stronger oxidant than OCl-, however ineffective against cryptosporidium parvum (protozoa), not stable at high concentrations so produced on site by Cl2(g) + H2O = HOCl + H+ + Cl-

Combined chlorine

series of reactions between chlroine and ammonia: NH3 + HOCl = NH2Cl, NH2Cl + HOCl = NHCl2, NHCl2 + HOCl = NCl3; mostly used as a residual disinfectant for the distribution system

UV light

200-400 nm, disrupts DNA in micoorganisms and prevent from replicating, lamps centered on UV-C (254nm) need about 10s, effective against cryptosporidium parvum

distribution

water may travel through for up to 5 days, typical disinfectants are chlorine or combined chlorine ~ 1-4 mg/L as Cl2 known as chlorine residual

Chlorine residual and sustained water pressure

protects treated water against recontamination in the distrivution system

water disinfection

is used to inactivate pathogens and provide safe drinking water

disinfection by-products

are always in your drinking water (usually at ug/L levels)

regulated DBPs

trihalomethanes, haloacetic acids, nitrosamines, chlorite, bromate

non-regulated DBPs

are 1-2 orders of magnitude more cyto- (cell death) and genotoxic (DNA damage)

Drinking water DBPs - health risks

increased risk in the incidence of bladder cancer and DBP exposure; low birth weight and preterm delivery

tradeoffs of disinfection: acute risks

pathogens that cause immediate waterborne diseases

tradeoffs of disinfection: chronic risks

long term exposure to low level concentration of DBPs

Alternative sources of water: Ocean/saline waters

limited access to freshwater, close to ocean of salty groundwater, desalination is energy-intensive therefore expensive

desalination treatment process

screens → microfiltration → reverse osmosis → disinfection → distribution

Membrane technology

purify water by moving through a membrane with small pores (microscopic +uniform), rejected components go to waste stream, permeable components go to product stream, need significant amount of pressure (~ energy = $$) to drive water across the membrane

draw backs of membrane filtration

membranes can foul, that means it can no longer filter water (they need to be cleaned with disinfectants) . react with disinfectants and may degrade, about 1/3 of water is wasted, neutral compounds can pass through (DBPs)

Alternative source waters: wastewater effluents

various degrees of reuse (ag, potable, industrial), public acceptance is a big issue (more education), reuse is also an option to desalination, reuse practiced in places that are in-land