Wastewater Treatment Exam 2

Type 1 Settling

Discrete - low concentrations, particles do not interfere with each other during settling

Type 2 Settling

Flocculent - higher concentrations, particles accumulate during descent, increasing velocity

Type 3 Settling

Zone (Hindered) - much higher concentrations, particles have a fixed position relative to each other, settle as a zone

Type 4 Settling

Compression - very high concentrations, weight of particles causes more settllng in the lower depths, compress one another

How are CD (drag coeff olds number) related?

CD increases with increasing values of Re, depends on flow regime

Laminar Flow

Re < 1

Transitional Flow

1 < Re < 10^4

Turbulent Flow

Re > 10^4

When do you use Stokes Law in the sqrt(3.3g(Ss-1)d) form?

Assuming spherical particles and turbulent flow (CD = 0.4)

When do you use the Stokes Law in the ((ps-p)gd^2)/18u form?

Assuming spherical particles and laminar flow

Overflow Rate

Design parameter, the velocity required for a critical particle to reach the bottom of the tank

How does water become hard?

By passing through soil or ground that contains calcium or magnesium (ex: limestone)

Chemical Constituents of Hardness

Calcium and Magnesium associated with Bicarbonate and Carbonate

Significance of Alkalinity in lime-soda softening

Helps water resist changes to pH, if alkalinity is too low, pH may be significantly reduced

(Lime/Soda is added to increase the alkalinity)

Proper pH for removal of Ca

9-9.5

Proper pH for removal of Mg

11

How do these reactions ensure the proper pH?

When lime is added, it provides hydroxide ions, and when soda is added, it provides carbonate ions; these balance the reaction to keep the pH in the optimum range

Why can lime-soda softening not produce water completely free of hardness?

Process limitations: solubility of CaCO3 and Mg(OH)2, mixing time, contact time, etc.

Selective Calcium Removal

Removes only carbonate hardness caused by calcium

Excess Lime Softening

Removes carbonate hardness caused by calcium and magnesium

Lime-Soda Softening

Removes carbonate and non-carbonate hardness

Split Treatment

A portion of the water is treated with excess lime, then blended with the remaining portion of raw water by-passed

Purpose of Recarbonation

Decrease pH, stop the chemical reactions of softening

Why is the softening process good for removing constituents other than Ca and Mg?

Raises pH and adding carbonate/hydroxide ions allows more precipitation reactions to occur

Disinfection Mechanisms (5)

1. Damage to the cell wall

2. Alteration of cell permeability

3. Alteration of colloidal nature of the protoplasm

4. Alteration of the organism's DNA or RNA

5. Inhibition of enzyme activity

Disinfectants Examples

Chlorine, alcohols, heat, light, screens, gamma rays

Disinfection Process

Disinfection and filtration, inactivate pathogenic organisms, produces byproducts (THMs and haloacetic acids)

Chlorine Dissociation

At low pH, mainly HOCl form; at high pH, mostly OCl- form; pH = 7.5

Chlorine Speciation (forms)

HOCl, OCl-, CLO2, Cl2, NHCl2

Chlorine Demand

Difference between added chlorine and residual chlorine (portion lost due to reactions)

Free Residual Chlorine

Remaining after the chlorine demand has been satisfied (HOCl and OCl-)

Break Point Chlorination

The point at which adding more chlorine creates free residual

Disinfection byproducts

Trihalomethanes (THMs), formed from chlorine and organic matter