Turbidity: top Key Parameter in Water Disinfection

Turbidity is the cloudiness caused by fine suspended matter in the water. It affects the appearance of water. The main contributors to turbidity in water are clay, silt, fine organic matter and microscopic organisms, such as algae. Turbidity can be reduced to almost undetectable levels by means of turbidity removal filter, also called sediment filters for water.

Turbidity of water is a key parameter of its suitability for water disinfection. These are the four main reasons:

• Suspended solids are an excellent base for bacteria growth
• Harmful pollutants such as heavy metals and pesticides are easily absorbed by suspended solids
• Suspended solids represent a risk for the main water distribution systems, pumps and other equipment where tend to deposit and block pipes and nozzles. Deposits on heat-exchange elements hamper heat transmission
• Turbidity has an impact on the aesthetic acceptability of water. Consumers relate the appearance of water to its safety and turbid or coloured water is considered unsafe to drink

2. Water treatment before water disinfection

In case of oxidation by means of Chlorine or Ozone, which are the most common water disinfection methods, upstream treatment must be used to:

• Reduce oxidant demand, particularly from TOC, allowing higher oxidant concentration to be achieved with less chemical dosing as well as lower by-product formation
• Provide a consistent water quality what simplifies the oxidant dosing
• Reduce turbidity of water facilitating the effect of disinfection chemicals and UV

2.1 Turbidity impact on bacteria growth and water disinfection effectiveness

Suspended particles in water act as a protective shield for micro-organisms and provide an excellent substrate for bacteria growth. In general turbidity should be kept below 1 NTU for effective disinfection. The lower the turbidity of the water at the time of chlorination the more effective chlorination will be and if chlorination occurs at turbidities higher than 1 NTU, the effectiveness of the chlorination process should be validated.

2.2 Turbidity impact on pipes and equipment

The major problem with turbid water is that the matter it contains can remain in suspension for a long time. This is particularly true for colloidal suspended solids (< 1 μm ) that can stain clothes, foster corrosion in plumbing fittings, block irrigation spray nozzles, contribute to a build-up of sludge in drippers and pipes and reduce the efficiency of water-softening units.

3. Conventional vs modern turbidity removal filters

Conventional turbidity removal filters using sand as main filtration media are usually uncapable of delivering an acceptable water quality. These results can be improved by means of coagulation and flocculation, which will enhance the filtrate quality providing a higher micro-organisms removal. However coagulation and flocculation adds complexity to a process that deliver good results with modern sediment filters for water.

Adding extra filtration stages can also improve the results. Even though its primary function is not filtering, the use of GAC (Granular Activated Carbon) after the sand filters can also improve the final water quality, providing a lower and more stable Chlorine demand in water.

Modern filtration media, based on activated alumino-silicate glass, can deliver a higher water quality, with at least 50% lower chlorine oxidation demand than an equivalent sand filter.

However the highest degree of water quality can be achieved with membrane filtration, in particular ultrafiltration membranes. With a pore size smaller than 0.1 micron ultrafiltration delivers a 99.99% bacteria and virus removal.

4. Turbidity limits in water disinfection

According to the Australian Drinking Water Guidelines are:

• < 5 NTU is not noticeable in a glass
• < 0.2 NTU is the target for effective filtration of Cryptosporidium and Giardia
• < 1 NTU is the target for effective disinfection
• WHO guidelines recommend < 0.1 NTU for effective disinfection, however this value is quite impractical and difficult to achieve with standard sediment filters for water

Particles smaller than 1 μm as they do not scatter light and are thus, not detected using turbidity measurements.

5. Conclusion: filtration is key to effective water disinfection

Caused by clay, silt, fine organic matter and microscopic micro-organisms, turbidity of water is a key parameter of its suitability for water disinfection.

Conventional turbidity removal filters using sand as main filtration media are usually uncapable of delivering an acceptable water quality.

Modern filtration media, based on activated alumino-silicate glass, can deliver a higher water quality, with at least 50% lower chlorine oxidation demand than an equivalent sand filter.

However the highest degree of water quality can be achieved with membrane filtration, in particular ultrafiltration membranes. With a pore size smaller than 0.1 micron ultrafiltration delivers a 99.99% bacteria and virus removal.