Water Supply Week 11

total solids

residue on evaporation at 103C

measuring solids

1. place known value of sample (water + solids) in evaporating dish

2. allow water to evaporate (provide heat)

3. gooch crucible (with bottom holes) to separate suspended solids from dissolved solids

   1. place glass fiber filter on holes in crucible

   2. sample drawn through crucible with aid of vacuum

   3. suspended material is retained on filter

   4. dissolved fraction passed through filter

classification of solids

1. total solids = dissolved solids + suspended solids

   1. dissolved solids: salts, colloidal matter (i.e., not filterable)

   2. suspended solids: sand particles, silt (filterable)

   3. TS = DS + SS

2. total solids = volatilized solids + fixed solids

   1. volatile solids = solids that volatilized at elevated T

   2. fixed solids = solids that survived high-T treatment

   3. TS = VS + FS

3. Fixed solids (FS)

   

4. volatile solids (VS)

   1. VS = TS - FS

5. volatile suspended solids (VSS)

   1. determined by placing gooch crucible (with filtered content) in hot oven (600C) to burn organic fraction and weighed again

   2. loss in weight = VSS

Ex. Water sample has following laboratory measurements

• weight of crucible = 48.6212 g

• 100 ml of sample was placed on the crucible; after evaporation at 103oC, weight of crucible + content = 48.6432 g

• crucible (+ content) was placed in a 600oC furnace for 24 hours and subsequently cooled in a desiccator; weight of crucible + residue = 48.6300 g

Determine Total Solids, Volatile Solids, and Fixed Solids from the above measurements.

pathogens

• microorganisms that may potentially cause diseases/illnesses in humans

• infectious diseases such as typhoid and cholera can be transmitted by water

• common waterborne microbial pathogens in table below

• what do we do with pathogens? how do we measure presence?

common waterborne pathogens

indicator organism: E. coli

• measure bacteriological quality using indicator organism – species who presence indicates the likely presence of pathogens

• common indicator – Escherichia coli (E. coli), member of the coliform bacteria group; E. coli a.k.a. fecal coliforms that are found in digestive tracts of warm-blooded animals

• presence of coliforms indicates rather than proves, the likely presence of pathogens in a given sample; thus not definitive or suggestive

• E. coli is present in other animals, so seeing E. coli in water means animals deification likely contaminated the water, NOT because it always harms humans

measurement of fecal coliforms: (1) membrane filter technique (MF)

1. water sample filtered through sterile micropore filter

2. coliforms present are captured on filter

3. the filter placed in Petri dish with a culture medium that promote fecal coliforms growth but suppress other microbes

4. incubate for 24-h at 35oC

5. count number of shiny metallic red dots (each representing one fecal coliform colonies)

6. concentration of coliforms [coliforms/100 ml sample] reflects bacteriological quality

measurement of fecal coliforms: (2) most probable number (MPN)

1. coliforms metabolize lactose to produce gas and make lactose solution cloudy

2. gas produced and cloudy broth after incubation indicates presence of coliform

3. MPN used on very turbid, brackish, muddy water sample

MPN: record the number of positive responses with different amounts of water sample (potentially with microbes) and determine coliform concentrations using a standard scorecard

For the test results above (i.e., 5 +ve with 10 ml, 2 +ve with 1 ml, 0 +ve with 0.1 ml), the scorecard gives 49 coliforms / 100 mL

other water quality parameters / concerns

1. toxic to human and other aquatic organisms often at low/trace levels

2. exert a wide suite of toxicological effects

3. acute and chronic effects ➔ toxicology / ecotoxicology

4. can accumulate and transfer along the food web

5. addressing these contaminants effectively and comprehensively is challenging

heavy metals / trace elements

commonly monitored and regulated species:

arsenic (Ar), lead (Pb), selenium (Se), chromium (Cr), cadmium (Cd), cobalt (Co), nickel (Ni), antimony (Sb), mercury (Hg)

organic pollutants

- volatile organic compounds (VOCs)

- persistent organic pollutants (POPs)

limitations of standard water quality parameters

overlook new pollutants (micropollutants and emerging contaminants

e.g., perfluorinated chemicals such as PFOAs, PFOSs pharmaceuticals and personal care products pesticides (or “plant protection products” – PPP) (and many more….)

coagulation purpose

to remove colloids (<1 um) present in natural waters

not easily removed by sedimentation and filtration

stability of colloids

1. colloids are so small (dia. ~< 1 m) that they are very slow to settle out

2. colloids in natural waters carry negative surface charge

3. negative surface charge prevents formation of stable aggregates

4. how can we destabilize the colloids?

colloids surface charge

1. introduction of multi-valent ions can neutralize the negative surface charge of colloids, bringing down the repulsive forces

2. coagulation ➔ adding trivalent cations to reduce the energy barrier

coagulation mechanisms

a. charge neutralization

b. interparticle bridging

c. enmeshment in a precipitate (“sweep floc”)

charge neturalization

• destabilized particulates by adsorption of oppositely charged ions/polymers

• particulates in natural water are negatively charged (e.g., clays, humic acids, bacteria, etc.); pH ~ 6 to 8

• metal salts, and cationic organic polymers can encourage particulate aggregation through charge neutralization

• optimum coagulant dose occurs when the particle surface is only partially covered (< 50%)

• optimum dose generally increases with particulate surface are

polymeric coagulant and dose

• particle will coagulate when optimal amount of polymer has adsorbed to the particulate and neutralize its surface charge

• too much polymer ➔ particles remain +ve charged ➔ no aggregation

• low dose polymer ➔ sub-optimal coagulation

interparticle bridging

• large polymer molecules link up different particulates and form a large aggregate

• polymeric bridging is an adsorption phenomenon ➔ optimal bridging dose proportional to particulates present

• bridging occurs with nonionic polymers and high-molecular-weight (MW 105 – 107 g/mol), low-surface-charge polymers

• bridging efficiency = f(polymer size/length, charge, and charge density)

• polymer selection based on empirical testing

• dose effect on polymer bridging:

Precipitation and Enmeshment (or “Sweep Floc”)

• entrapment of particulate during aluminum (Al) and iron (Fe) precipitates formation (which occur at high metal dosages)

• steps:

  • 1. hydrolysis and polymerization of metal ions

  • 2. adsorption of hydrolysis products at interface

  • 3. charge neutralization

  • 4. at high metal ion dosages, nucleation of the precipitate occurs at the particulate surface, leading to growth of amorphous precipitate with the entrapment of particles in this amorphous structure

• entrapment mechanism dominates in pH 6 ~ 8 and applications of Al and Fe salts

sweep floc mechanism does not depend on type of particles (i.e., particles’ chemical makeup)

optimal pH range for sweep floc mechanism also consistent with solubility of Al3+ and Fe3+: