DISINFECTION
Definition and theories
Water disinfection is the removal, deactivation, or killing of pathogenic microorganisms. Microorganisms
are destroyed or deactivated, resulting in the termination of growth and reproduction.
The purpose of disinfection is to kill organisms present in the water that cause disease to consumers,
destroys fecal and intestinal bacteria.
Some protozoa and viruses can survive even though it undergoes disinfection process such as
cryptosporidium.
TYPES OF DISINFECTANT
Physical Treatment – By boiling water, you are disinfecting it, also known as boil water order.
Irradiation – consists of passing water that should be clear through a thin film over a quartz enclosed
mercury vapor lamp, considered as UV radiation treatment. No chemicals are needed, no odors and no
possibility of overdosage.
Metal Ions - Disinfection at low concentrations employed is slow and silver is costly at practicable
concentrations.
Oxidants – ozone, halogens
CHLORINATION
final treatment process after clarification and filtration, process of adding the element chlorine to water
as a method of water purification to make it fit for human consumption as drinking water.
WHAT IS CHLORINE
process of adding the element chlorine to water as a method of water purification to make it fit for
human consumption as drinking water.
FORMS OF CHLORINE
Chlorine gas, calcium hypochlorite, sodium hypochlorite (define mo)
TYPES OF CHLORINATION
Pre-chlorination – we apply chlorine to the water before filtration
Break-point Chlorination – in this process, the amount of chlorine added to the water until such demand
is satisfied.
Super Chlorination – we then add large doses of chlorine to the water
Ammonia-Chlorine Treatment – chloramination means applying joint form of ammonia and chlorine
Post Chlorination – the final step of water trreatment. It happens before the water leaves treatment
plant and chlorine is applied after the process of filtration.
UV DISINFECTION
an extremely effective way to combat microbial contamination in water. However, microbes have to be
exposed to UV-C light in the proper amount in order to effectively disinfect the water.
UV disinfection technology can be used for drinking water disinfection, process water disinfection,
wastewater disinfection, and surface disinfection and destroys the DNA of microorganisms which leaves
them dead and unable to grow further.
OZONATION
The treatment of water with ozone has a wide range of applications, as it is efficient for disinfection as
well as for the degradation of organic and inorganic pollutants.
Ozonation has been successfully applied for water disinfection and can kill most bacteria viruses and
protozoa. It is a suitable process to degrade organic pollutants. Sophisticated generators consuming
high-amounts of electricity are required to produce ozone.
UV Disinfection system requires low capital and operating cost and provides an excellent ease of
maintenance and installation. The required contact time is less than 10 seconds. Compared to
Chlorination that demands lower capital costs, relatively low operating cost and when it comes to ease
of installation and maintenance, UV Disinfection system performs better. The contact time required is
20 to 30 minutes. Ozonation disinfection system requires high operating and capital cost compared to
the two other system. Its installation is complex and has poor ease of maintenance. The required
contact time is 10 to 20 minutes.
SEDIMENTATION
Theory
In the design of an ideal sedimentation tank, one of the controlling parameters is the settling
velocity ( vs ) of the particle to be removed. For the purpose of discussion and illustration, the settling properties of particles are categorized into four classes: (1) discrete particle settling, (2) flocculant settling, (3) hindered settling, and (4) compression settling. By convention these categories
have been labeled Type I, Type II, Type III, and Type IV settling, respectively. In actual settling
tanks, it is not uncommon to see all of these types of settling. The value of separating the discus-
sion into these categories is that it provides a means of understanding the relationship between
variables in the design of the sedimentation basin.
TYPE 1 SEDIMENTATION
Type I sedimentation is characterized by particles that settle discretely at a constant settling
velocity. They settle as individual particles and do not flocculate during settling. Examples of
these particles are sand and grit (a mixture of abrasive particles that may include sand, broken
glass, etc.). Generally speaking, the only applications of Type I settling are during presedimentation for sand removal prior to coagulation in a potable water plant, in settling of sand particles
during cleaning of rapid sand filters, and in grit chambers.
Stokes’ Law. When particles settle discretely, the particle settling velocity can be calculated,
and the basin can be designed to remove a specific size particle. In 1687, Sir Isaac Newton showed
that a particle falling in a quiescent fluid accelerates until the frictional resistance, or drag, on the
particle is equal to the gravitational force of the particle ( Figure 10-1 ) (Newton, 1687). The three
forces are defined as follows: