Dr. Martin T. AuerMTU Department of Civil & Environmental Engineering

Water Treatment

ChargeNeutralization

Bridging

negatively-chargedparticle

negatively-chargedparticle

hydroxo-metalliccomplex

floc

Particle Settling Velocities

Particle Diameter (mm) Velocity (m/s)

Sand 1.0 2x10-1

Fine sand 0.1 1x10-2

Silt 0.01 1x10-4

Clay 0.001 1x10-6

Source: Vesilind & Morgan

sorption

Chlorine Distribution

Drinking Water Process Train

Equilibrium Chemistry

Iron and Manganese

Hardness

The most commonly used adsorbent is granular activated carbon (GAC). These irregular particles, 0.2-5 mm in diameter, are a char of carbon material (wood or coal). They are ‘activated’ or made more porous by exposure to steam at high temperature. Activated carbon has 1000 m2 of adsorbing surface area per gram (~ 1 teaspoon) or equivalent to that of a 40 acre farm in one handful!

Source: Sontheimer et al. 1988

Source:Chemviron Carbon

Source: Millenium Inorganic Chemicals

Granular Activated Carbon

Organic chemicals are typically removed from a water supply prior to distribution through the process of adsorption:

the physical-chemical attraction of a solid material for a chemical in solution.

In adsorption, the chemical being adsorbed is termed the adsorbate and the solid to which it sorbs is the adsorbent.

Influentstream

Effluentstream

The Adsorption Process

For adsorption to be effective, the chemical must sorb strongly. Poorly soluble (hydrophobic) compounds (e.g. the components of gasoline) adsorb more strongly than highly soluble (hydrophilic) compounds (e.g. table salt).

0

2

4

6

0 2 4 6 8

Time (d)

Co

nc

entr

ati

on

(mg

/L)

Poorly sorbed

Strongly sorbed

… tendency to sorb

In drinking water treatment, adsorption with GAC is accomplished using a packed bed column. The untreated water is introduced at the top of the column and trickles down through the GAC. Contaminants are removed en route and clean water emerges at the bottom of the column. In application, columns 6 feet in diameter and 30 feet in height are not uncommon.

GAC columns

BedCarbon

… application in water treatment

Water flows thru the column and contaminants are adsorbed. With time, the GAC becomes saturated (sorption capacity is reached) and contaminants exit the bed (breakthrough). The exhausted carbon must then be replaced.

Cin

Ceq

Cout

exhaustion

breakthrough

… column operation

Asbestos/Arsenic/Metals Removal

Sorption with Ferric sulfate: asbestos, arsenic, cadmium, chromium, copper, lead, mercury, molybdenum, selenium, silver

Coagulation with Alum: asbestos, nickel, uranium

Reverse Osmosis

Ultrafiltration

Ultrafiltration

300 µm ID500 µm OD

polypropylene fiber

Ultrafiltration

0.2 µm nominal pore size

Ultrafiltration Technology

epoxy seal

per

mea

te f

lowraw

waterin

Ultrafiltration

banks of fiber bundles

Ultrafiltration

contaminants

backwashing

Home Water Treatment

Reverse osmosis unit (salt)

Softening byion exchange(hardness)

Home Water Treatment

Three step process:• sieve and bottom filter – rust, sand, turbidity • activated carbon filter – chlorine, color and SOCs• ion exchange resin – metals

Bottled Water: $8 /gallon

Tap Mount: $0.25 / gallon

Municipal: $0.0015 / gallon

Achieves 99.99% removal of Giardiaand Cryptosporidium cysts, but does not remove all pathogenic organisms.