CE 547 - Coagulation.pps

Coagulation
CE 547
Overview
Turbidity in surface waters is caused by colloidal clay particles. Color in water is caused by colloidal forms of Fe, Mn, pounds.
Colloidal Particles
Difficult to settle
Pass through small pores of conventional filters
How to remove colloidal particles?
By aggregation (making them bigger sizes)
Why aggregation is difficult?
Small size of particles
Physical and electrical forces
How to aggregate?
Use of chemical agents
Chemical Coagulation
It is the process of destroying the stabilizing forces and causing aggregation
Aggregation occurs in two steps:
1. reduction or elimination of the inter-particle forces responsible for stability (by addition of chemicals)
2. collision due to
molecular motion
mechanical mixing (using rapid or flash mixing for very short time, less than 1 minute)
After destabilization, gentle mixing is provided to increase the rate of particle collision without breaking the aggregates or flocs (this process is called ulation)
Stability of Particulates
Colloidal particulates remain in suspension for very long periods due to their stability (is it possible to give such particles sufficient time to settle?)
Particulate Characteristics
1. Size
colloidal materials
upper limit of ≈ 1 m
lower limit of ≈ 5 nm
suspended solids
larger than ≈ m
Particles larger than 5 nm are in suspension
2. Nature of solid-water interface
Hydrophobic
Well defined interface
Low affinity for water molecules
Thermodynamically unstable and will aggregate over time (irreversible)
Generally anic
Hydrophilic (Fig )
Lack of clear interface
anic matter such as proteins
Can aggregate (but reversible)
Mechanisms of Stability
The main mechanism of particle stability is the electrical repulsion
Presence of adsorbed water molecules (this will provide barrier to essful collision)
How electrical charges exist at particle surfaces? There are three principle ways:
1. Crystal Imperfections
Silicon atoms in crystals can be replaced by atoms