Water Treatment Systems

Water Treatment Systems

note: the outline for this section is sparse since the material is covered very well in the text.

Surface Water Plants

Rapid sand filtration

Lime-soda softening plants

Groundwater Plants

Gas stripping and chlorination

Softening: lime-soda or ion exchange

Flow Sheets: diagrams of unit operations and processes in plants – see handouts

Unit operations: physical treatment

Unit processes: chemical or biological treatments

Coagulation and Softening

Coagulation: Chemical treatment that turns small particles of color, turbidity, and bacteria into larger particles or flocs. Technically, this process applies to removal of colloids, but often applied to removal of ions as well (i.e., precipitation).

Colloid stability: particles are small and negatively charged

Colloid destabilization: add non-toxic, insoluble (in neutral pH range) cations to reduce electrostatic forces to the point where van der Waal’s forces (intermolecular attraction) dominate.

The most common coagulants are Al3+ and Fe3+

Aluminum: dry or liquid Alum – Al2(SO4)3 · 14H2O

Each mole of alum uses 6 moles of alkalinity and produces 6 moles of carbon dioxide; shifts carbonate equilibrium and decreases pH. As long as sufficient alkalinity and CO2 allowed to evolve, pH not drastically reduced.

Iron: sulfate salt (Ferric Sulfate) or chloride salt (Ferric Chloride)

Coagulant aids:

pH adjusters

activated silica

clay

polymers

Softening: Goal is to reduce hardness, which is the sum of all polyvalent cations. Primary contributors are Ca2+ and Mg2+. Expressed as mg/L CaCO3.

carbonate hardness vs. non-carbonate hardness

Lime-soda softening: direct application of the law of mass action

·  free acids neutralized

·  pH raised to precipitate CaCO3

·  if necessary, raise pH further to remove Mg(OH)2

·  if necessary, CO32- added to precipitate non-carbonate hardness

Mixing and Flocculation

Mixing: distribute treatment chemicals - fast

Flocculation: slow mixing to encourage particle growth

degree of mixing in either process is measured by velocity gradient, which is related to shear in the fluid, and is a function of power input per unit volume

Rapid Mix: quickly and thoroughly disperse chemicals – the most important physical factor affecting coagulant efficiency.

Flocculation: bring particles into contact with one another, building particles that readily settle – most important factor affecting particle removal efficiency.

Design mixing and flocculation tanks using power considerations

Sedimentation (settling) and Filtration

Settling tanks comprised of four zones:

inlet

settling

outlet

sludge storage

Settling efficiency a function of settling velocity and overflow rate

But, must filter water through porous media because settling never 100% efficient

slow sand filters

rapid sand filters

dual media filters

multimedia filters

Filter Hydraulics: based on porous media theory

Disinfection: destruction of pathogens to some tolerable level (vs. sterilization)

organisms of concern: bacteria, viruses, amebic cysts

disinfection is a complex rate process dependent on:

·  physio-chemistry of disinfectant

·  cyto-chemical nature and physical state of pathogens

·  interactions between the above factors

·  environmental factors (pH, temperature, electrolytes, interfereing substances

viruses generally more resistant that bacteria

types of disinfectants:

·  oxidizing agents

·  cations of heavy metals

·  organic compounds

·  gaseous agents

·  physical agents

disinfection kinetics: generally a first order reaction, but first order equation does not do a good job for later times in the process

Chick’s Law

Chlorination: most widely used (cheap, effective, forms resu\idual)

* powerful oxidizer: oxidizes enzymes of cells vital to their metaboloc processes

chorine gas: dissolved into water, dissociates to HOCl, which in turn dissociates to OCl- - these are the active compounds (lowers pH)

hypochlorate salts: also dissociate to yield OCl- (raises pH)

disinfecting action is very complex, and empirical equations are used to relate concentration of disinfectant and contact time; these are of the form:

Cntc = K, where C = concentration of free chlorine at to, tc = time required to kill certain percentage of microorganisms, n = constant = f(disinfectant), K = constant = f(microorganism type, environmental factors)

Chlorine-ammonia reactions: form chloramines, which are also oxidizers (but less effective than free chlorine)

Dose-demand-residual relationship: Figure 3-44

Chlorine Dioxide:

·  more powerful than Cl2

·  easily removed using areation

·  does not react with ammonia

·  forms stable residual

·  more expensive

·  adverse health effects

Ozonation:

·  unstable, with half life = 20-30 min in distilled water

·  powerful oxidant

·  does not form THMs

·  must combine with method that forms residual

UV Radiation:

·  effective for viruses and bacteria

·  leaves no residual

Adsorption

·  particles physically or chemically attached to media

·  example: activated carbon

Water Plant Waste Treatment

solid/liquid (sludge) wastes

liquid wastes (ion exchange, reverse osmosis)

gas wastes (air stripping)

Sludge Management process

·  minimize generation

·  chemically recover precipitates

·  volume reduction

·  disposal

Sludge Production and Characteristics

·  presedimentation

·  sludge from softening process

·  sludge from coagulation process

* perform mass balance on settling basin to obtain sludge production rates

Sludge minimization

·  direct filtration

·  chemical coagulants

·  optimize dosage

Sludge treatment

·  function of ultimate disposal method

·  conditioning

·  thickening

·  dewatering