Chapter 8: Water Treatment Flashcards
Physical characteristics of water (6)
Color, turbidity, temperature, particulars, taste, odor
Characteristics of groundwater (5)
- Constant composition
- High mineral content (Fe, Mn)
- Low turbidity
- Low or no DO
- High hardness
Characteristics of surface water (5)
- Variable composition
- Low mineral content
- High turbidity
- DO present
- Low hardness
Colloids
Discrete particles that are separated by a dispersal medium (fluid). (EX: milk, fog, butter, blood, paint)
Properties of colloids
- Negative electrostatic surface charges
- 1 nm to 1,000 nm (1µm)
- High surface to volume ratios
- Called “stable ” when colloidal particles remain in suspension without coagulation and flocculation.
Rapid sand filtration/conventional treatment steps (3)
- Pre-sedimentation (removes easily settleable material)
- Pre-chlorination (Cl added at rapid mix stage)
- Direct filtration (coagulation, flocculation, filtration, disinfection)
Coagulation (rapid mixing)
Destabilization and initial coalescing of colloidal pollutant particles. Particles stick to each other.
Flocculation (slow mixing)
Formation of loosely attached larger aggregates of pollutant particles (flocs) via the addition of coagulants.
How does coagulation and flocculation occur? (3)
- Brownian motion of particles (particles < 0.5 µm)
- Stirring
- Differential settlement
Coagulants for water treatment (6)
Alum or ferric iron salts, ferric chloride, polymers, slaked lime, ferric sulfate, ferrous sulfate
Sedimentation
Gravity separates particles from water.
Reynold’s Number
Ratio of inertial forces to viscous forces
Type 1 sedimentation
Particles that settle discretely at a constant velocity. Occurs in dilute suspensions
Type II Sedimentation
Particles that flocculate during sedimentation. Changing size and settling velocity due to flocculation.
Type III/Zone, Sedimentation
High concentration of particles (>1000 mg/L) settle as a mass, a distinct clear zone and sludge zone are present.
Type IV/Compression Settling
Setting of particles that are of such concentration that the settling can occur only by compression of the mass.
Charge neutralization
Coagulation by reducing the energy barrier height through adding positive charge counterions as coagulant to the negatively charged colloidal particles.
Sweep coagulation
Add high concentration of Fe or Al salts so that they form a cloud of metal hydroxide and settle down sweeping the colloidal particles downward
Sand size
500 µm
Size of particles passing through sand
0.15d
Size of soil
1-100 µm
Size of bacteria
<300 µm
Size of viruses
0.005-0.01 µm
Size of flocs
100-2000 µm
Slow sand filters
Removal by physical straining using fine sand and biological degradation. Clean by scraping top layers.
Low loading rates (0.05-2 m/s)
Rapid granular filtration
Coarser grain size (0.4 < d < 1) with higher loading rates (5-15 m/s). Like sedimentation/coagulation/flocculation
Backwashing
Forcing water/air up from the clear well back through the filter to release trapped particles.
Disinfectants (3)
Chlorine, ozone, UV light
CT concept
Combination of disinfectant residual concentration, C (mg/L) and the effective disinfection contact time, T (in minutes) to determine the required dose of a disinfectant for effective pathogen reduction.
Infiltration
Groundwater that enters sewer systems through cracks and/or leaks in the sewer pipes
Inflow
Stormwater that enters into sanitary sewer systems at a point of direct connection to the system
Municipal wastewater treatment stages (4)
- Preliminary treatment: removes materials that can cause operational problems (sand, grit, debris, etc.)
- Primary Treatment: removes mainly suspended solids
- Secondary Treatment: remove remaining organic suspended solid and organic dissolved solids
- Tertiary Treatment: Further removal of nutrients, suspended solids, dissolved organics and microorganisms
Equalization
Reduced difference between night time flow and morning high flow.
In-line equalization
Dampen variations in both concentration and flow rate
Off-line equalization
Dampens flow rate but has a smaller effect on concentration
Dispersed growth
When microorganisms are in wastewater (EX: Activated sludge, Biological nutrient removal (BNR), Oxidation ditches/ponds, Aerated lagoons, stabilization ponds)
Fixed growth
When microorganisms are fixed on surfaces (EX: Trickling filters, Rotating Biological Contactors (RBCs))
Activated sludge
Process in which a mixture of wastewater and microorganisms is agitated and aerated.
Aeration tank
Contains mixed liquor (suspended solids), the combination of influent wastewater and return (recycled) activated sludge.
Trickling filters
Not a true filtering or sieving process, as material (rock or plastic) only provides a surface on which bacteria to grow. Less energy is needed, but has a poorer effluent quality.
