Combined deck

Total solids (TS) = ?

TS = TSS + TDS

total solids = Total suspended solids + total dissolved solids

TVS

Total volatile solids

VSS

Volatile suspended solids

Nephelometric turbidity unit (NTU)

Measure of TSS based on light scan instead of filtration. Stronger the scattered light = higher turbidity value in NTU.

Is there a correlation/relationship between NTU and TSS?

No.

TOC

Total organic carbon

TIC

Total inorganic carbon

DOC

Dissolved organic carbon

NDOC

Non-dissolved organic carbon

VOC

Volatile organic carbon

NPOC

Non-volatile organic carbon

NOM

Natural organic matter

DOM

Dissolved organic matter

POM

Particulate organic matter

POC

Purgeable organic carbon

TOC = ? (3 eqns)

DOC + NDOC

TC - TIC

VOC or POC + NPOC

Natural organic matter (NOM)

Formed via chem transformations (e.g., polymerization, photolysis) of smaller macromolecules (e.g., protein, lignin, polysaccharides). Heterogeneous and doesn’t have a well-defined chem structure.

Humic substances

TC Measurement

TIC Measurement

no combustion

TOC = ? (equation)

TOC = TC - TIC

Total organic carbon = total carbon - total inorganic carbon

NPOC measurement

POC = ?

POC = TOC - NPOC

DOC = ?

DC - IC

DOC includes?

VOC & NPOC.

Typical DOC ranges?

Suspended solids examples

• Inorganic minerals: e.g., clays (i.e., aluminosilicates), iron (hydr)oxides

• sand, silt, clay

Why don’t suspended solids settle?

1. Smaller particle = slower settling rate (Stoke’s law)

2. Suspended particles’ surface is usually negatively charged = repel each other and won’t settle

Stoke’s law

Origin of surface charge

1. isomorphic substitution

2. acid-base reactions at the surface

3. adsorption of Natural Organic Matter (NOM) on the surface

Isomorphic substitution

Common in clays. Oxidation state of Al = +3, Si = +4. When Al ion takes the place of Si ion, solid acquires a net charge of -1.

pH_pzc or pH_zpc

Point of zero charge or zero point of charge. This is the pH at which the surface charge is 0.

Why do acid-base reactions at the surface prevent suspended particles from settling?

When solution pH is < pH_zpc, the surface is positively charged. The lower the pH, the more positive the surface charge is. When the solution pH > pH_zpc, the surface is negatively charged. The higher the pH, the more negative the surface charge is (pKa).

Electric double layer - colloid suspensions should have no net charge (electroneutrality), but a particle’s surface is mostly negatively charged. What happens for electroneutrality to be obeyed?

Charge on the particles must be counterbalanced (counterions). E.g., negatively charged particulates, adsorbed cations form a fixed layer called Helmholtz or Stern layer. Concentration of cations decr as distance from charged particle grows until equilibrium is reached. Layer extending from outer edge of Helmholtz layer to bulk solution where charge is 0 = diffuse layer. Together, this is the electric double layer (EDL)

Zeta potential

Potential that exists at the plane of shear between the bulk liquid & an envelope of water that moves with the particle.

Calculated from the measurements of electrophoretic mobility. Determines the extent of electrostatic forces of repulsion between charged particles.

Zeta potential for particles in natural water

Typically -20 to -40 mV.

Electric double layer repulsion

Repulsion when 2 double layers (of similar sign) start to overlap

Van der Waals attraction

Attraction force arises from electron motion within molecules.

DLVO (Derjaguin, Landau, Verwey, Overbeek) Theory

Aggregation and stability of colloids. Total force = repulsion + attraction. Because of repulsion, particles won’t get closer to each to aggregate → suspended in solution.

Suspended solids are ___ and ___ particles.

inorganic (e.g., clays) and organic (e.g., bacteria, algae)

Why do suspended solids not settle in the solution? (leads to turbidity - NTU)

Small size & surface charge (usually negative)

Suspended solids can do what?

1. reduce clarify of water to unacceptable levels (i.e. cause turbidity) and impart colour to water (aesthetics).

2. be infectious agents (e.g., viruses, bacteria, protozoa), and/or;

3. reduce disinfection efficiency

4. have toxic compounds adsorbed to their external surfaces

How can suspended solids be removed?

Coagulation & flocculation

Coagulation

Process of destabilizing particles

Flocculation

Process of particle aggregation & growth which occurs due to destabilization.

Coagulant

Chemical that destabilizes particles

Flocculant

Chemical that is added to enhance flocculation process. Usually a polymer or other organic compound.

Coagulation - particle destabilization mechanisms

1. compression of double layer

2. adsorption & charge neutralization

3. enmeshment in precipitates

4. adsorption & interparticle bridging

Particle destabilization mechanisms - compression of double layer

Addition of salts increase ionic strength in the solution → double layer compression → more interaction between particles → more collision and attachment.

Repulsive interaction between similar colloidal particles decreases, decreasing the energy barrier and allowing particles to aggregate.

This is the main mechanism for alluvial deposition at river mouth.

Particle destabilization mechanisms - adsorption & charge neutralization

If right amount of chemicals (metal salts, cationic organic polymers for negatively charged particles) is added → net charge on the particles = 0 → no repulsion → more collision and attachment.

If the adsorbed species carries the opposite sign as the colloid, the particle can be destabilized.

Particle destabilization mechanisms - enmeshment in precipitates

Inorganic salts (Al and Fe salts) are added to form precipitates (flocs). Precipitates are formed on the surface of the particles initially present in the solution → particles are entrapped in the flocs.

This is the dominant mechanisms in most water treatment applications where 6 < pH < 8; Al and Fe salts used at concentrations exceeding saturation.

Particle destabilization mechanisms - enmeshment in precipitates: sweep flocculation

Inorganic salts (Al and Fe salts) are added to form precipitates (flocs). Precipitates are formed on the surface of the particles initially present in the solution → particles entrapped in flocs

Particle destabilization mechanisms - adsorption & interparticle bridging

Linking of particles by polymer chain segments adsorbed to more than 1 particle. Polymers can adhere to surface of colloids & it’s possible for these to “join” together to form a “chain”. These “chains” of joined colloids/polymers can be removed from solution through gravity separation and/or filtration. Commonly non-ionic & anionic polymers (in the context of destabilizing negatively charged particles).

Coagulants used in water treatment

• metal salts: alum, iron salts, polyaluminum chloride

• polymers

Why are metal salts most commonly used as a type of coagulant?

High availability and cheap cost

Polymers as coagulants

Synthetic coagulating agents are widely available. Cationic, anionic, and nonionic polymers have all been found to provide excellent results in different situations. These agents are usually more costly than alum or iron salts but much smaller dosages are required. Polymers don’t produce voluminous, gelatinous flocs as their inorganic counterparts do.

Destabilization by Al(III) and Fe(III) - 3 mechanisms

1. compression of double layer: add salt → increase ionic strength.

2. Charge neutralization: cationic species adsorb on particle surface 0> charge neutralization. But if too much coagulant added = charge reversal

3. Enmeshment in precipitates: appropriate coagulant dosage & appropriate pH = formation of Al(OH)_3(s) and Fe(OH)_3(s) = enmesh suspended solids (sweep floc coagulation)

Destabilization by specific adsorption and charge neutralization occurs with a small dosage of coagulants due to low concentration of particles. However, # of particles is too small to be flocculated & settled. Sweep coagulation (large coagulant dosage) can remove particles.

Settable flocs are mainly formed at “zone 4 of high dosing conditions”

Destabilization by specific adsorption and charge neutralization occurs with a relatively small dosage of coagulants. Some particles are flocculated and settled (zone 2), but particle removal efficiency is still low due to a lack of particles in water.

When more coagulant is added, particles re-stabilize (too positive) and turbidity increases (zone 3). As coagulant addition is increased further, sweep coagulation occurs (zone 4).

Deal with this irl.

Destabilization by specific adsorption & charge neutralization occurs and most of particles are removed (zone 2). Low coagulant dose can form settable flocs.

As coagulant dosage increases further, particles are re-stabilized with a positive charge (zone 3). Sweep coagulation occurs in zone 4 with higher coagulant addition.

Deal with this irl.

Sweep coagulation & specific adsorption/neutralization regions merge due to # of colloidal particles present in water. Coagulant dosage required to neutralize particles coincides with the onset of sweep coagulation precipitates.

Processes associated with polymeric coagulants

Factors affecting coagulant dose

Turbidity, initial pH & ALK, NOM, algae

How does initial pH and ALK affect coagulant dose?

pH can decrease significantly after coagulant addition (when ALK is low), decreasing overall coagulation efficiency → pH adjustment may be required

Rapid mixing synonyms

Flash mixing, initial mixing

Residence time and purpose of rapid mixing

Seconds, to disperse chemicals (esp. coagulants) quickly & uniformly in raw water. Coagulants dissolve instantaneously and destabilize particles. Critical for alum and ferric chloride coagulants

Mixing mechanisms for rapid mixing

In-line mech mixing, in-line static mixing, diffusion mixing by pressurized water jets, mechanical mixing.

Mixing energy - Camp equation

In-line static mixing equation, advantages and disadvantages

Mixing energy is described by what equation in diffusion mixing by pressurized jets?

Camp equation

Pump equation (for pump diffusion)

Turbine impellers

Paddle impellers

Propeller impellers

Flocculation

Physical process of bringing about interparticle contact.

3 types of flocculation

perikinetic, orthokinetic, differential settling

Perkikinetic flocculation (brownian diffusion)

random motion of particles caused by their continuous bombardment by surrounding water molecules

Orthokinetic flocculation (hydraulic transport)

Fluid shear causes velocity gradients in either laminar or turbulent flow field. Particles follow motion of suspending fluid resulting in interparticle contacts.

Differential settling flocculation

Produces vertical transport of particles resulting in collisions

Orthokinetic flocculation is used in water treatment, and the mixing intensity is controlled by?

Determining G or Gt for different types of flocculation design. Floc particles are redistributed by induced mixing; however, the intensity of mixing should be such that the floc particles will grow and not break up.

Particle collision & attachment results in?

Reduction of total number of particles & incr in average particle size

Assumptions for collision frequency function

Collision frequency function - brownian motion

Collision frequency function - differential sedimentation

Collision frequency function - fluid shear

Total collision frequency

Water treatment is mostly…?

Orthokinetic (fluid shear)

Why is water treatment mostly orthokinetic (fluid shear)?

Fluid shear causes velocity gradients in either laminar or turbulent flow field. Particles follow the motion of suspending fluid, resulting in interparticle contacts.

What is mostly orthokinetic water treatment driven by? What do we need to do to increase the number of contacts between particles suspended in water?

Depends on size & number of particles but primarily driven by velocity gradient G. Incr contact by gentle & prolonged agitation (aggregation with minimal breakup).

Degree of flocculation depends on?

Floc characteristics, velocity gradient (G), G x t (related to total # collisions), temp

Power required for agitation (for flocculation) of water may be imparted by?

Mechanical, hydraulic, pneumatic flocculators

What kind of flocculation is typically used?

Mechanical flocculators - types

Horizontal shaft paddle, vertical shaft paddle/turbine

Mechanical flocculators description

Compartments are separated by baffled walls. Each baffle has orifices 4-6 inches in diameter.

Drag on paddle wheels

Hydraulic flocculators

Mixing provided by hydraulic means. Simple and inexpensive, used in small plants & developing nations.

Designing flocculation basins - what can it be characterized by?

Coagulation/flocculation can be characterized by Camp # (Gt)

Sedimentation (i.e., settling)

Particle separation by gravity → remove majority of settleable particles & flocs by gravitational settling, maximizing performance of downstream units.

where is sedimentation used in oil sands tailing ponds?

Tailings are a mix of water, sand, silt, clay, residual bitumen. By-product of hot water treatment process used to separate oil from sand and clay. Settling basins enable process water to be separated & recycled back into extraction process. Synthetic polymers used as coagulant to speed sedimentation.