Disinfection

INTRODUCTION TO WATER TREATMENT & ABSTRACTION

DISINFECTION

A brief history
Chlorination
Alternative methods of disinfection
The process animated

The first permanent Chlorination plants were installed in England in 1905 in an attempt to reduce the outbreaks of Typhoid & Cholera,

The whole of London's water supply was being chlorinated by 1936.
The last major outbreak of Typhoid was in Croydon in 1937 this was attributed to somebody carrying the bacteria & not from the water supply.

The introduction of chlorination plants also helped reduce the occurrences of other bacteriological & virus outbreaks such as Dysentery, Meningitis, & Polio.

Chlorination

Chlorine is used primarily to kill micro organisms, the Volume of chlorine required varies greatly depending on the source & quality of the water to be treated. For example water from a river will require much higher dose rates than water taken from a good quality borehole source.

Another factor to consider is the network that the final water is being distributed, to for example in a large town where the water will be used fairly quickly the chlorine residual in the water will probably not need to be as high when compared to a rural area where the water will need to be distributed over a large area & consumption will not be so great.

The amount of chlorine required is called DEMAND & is generally measured in mg/l
(milligrams per litre) or ppm (parts per million)

There are three main types of chlorination these are:-
Simple chlorination
Breakpoint chlorination
Super chlorination

Simple chlorination will normally be used on sites where water quality is very good & the bacteria content is very low.
This is normally limited to good quality borehole sources or later on in a treatment process when rechlorination is required.

Breakpoint chlorination occurs when the chlorine in the water reacts with other chemicals already present in the water for example, ammonia will reduce the effectiveness of chlorine, after the chlorine has neutralised the ammonia the chlorine that remains is used for the disinfection process. Obviously any change to water quality can have a large effect on the chlorine demand & it is important that the contact time is maintained to ensure the process is complete before water is pumped to service reservoirs.

The best method for controlling this process is with a process loop that constantly monitors the chlorine residual present at various points in the treatment process.

Super chlorination is most commonly used when the water being treated will have a very high bacteria content generally this will be river sources or where some form of pollution has occurred. Dose rates in treatment works requiring super chlorination can be very high.

Alternatives methods of disinfection

Ozone is probably the most effective method of disinfection & is particularly good at removing cryptospuridium.
While ozone is good for disinfection it has disadvantages primarily cost & safety. Another disadvantage is that there will be no chlorine residual present after the water has been treated so chlorine will need to be added before water can be pumped to the consumer.

Ultraviolet Light is a potent disinfectant, A water flow is passed over a stream of light emitted from a UV lamp. The radiation produced from the light is absorbed by any micro organisms in the water which changes their basic structure & leads to them being killed.

The main disadvantage of ultraviolet light is that there will be no chlorine residual present after the water has been treated so chlorine will need to be added before water can be pumped to the consumer.

The process animated

The animation below shows two process control loops
The first controls the amount of chlorine or sodium hypochlorite that is introduced at the start of the disinfection process.

Chlorine is added to achieve a higher level than is required at the end of the process. This is to take account of the amount of chlorine that will be used to remove all the bacteria & undesirable elements that are present in the water & ensure that there will be enough chlorine residual left when the water is pumped to the consumer.
The water can take several hours to clear the contact tank this is important as it allows the disinfection process time to complete before the water leaves the treatment works.

The second process control loop that takes place is the addition of Sulphur dioxide or sodium bisulphate to remove the excess chlorine from the water.
this final residual is calculated to ensure residual is present as the water moves through any service reservoirs & pipework on route to the customers tap.