What is Turbidity
Turbidity is a widely used water parameter that is representative of water quality. It expresses, in a quick and clear manner, the concentration of suspended solids and a few other specific types of contaminants present in a water sample. In general, water is said to be turbid when the quantity of contaminants (TSS and others) is high, and their presence affects the clarity and transparency of the water.
Expressed in nephelometric units, the turbidity of water can range from 0 to 1000 NTU.
- In an aside, NTUs are Nephelometric Units and represent the ability of light to pass through a water sample. To learn more about nephelometry, we suggest you read this article: nephelometric units and nephelometry.
Typically, in Canada, the standard for turbidity in drinking water is that it should not exceed 0.3 NTU, but the goal should be to reach 0.1 NTU. For example, a water sample with 5 NTU is visibly turbid and when it reaches 25 NTU, the water becomes blackish.
Sources of Turbidity
There are 4 main sources of contamination that can promote turbidity in water. They are suspended matter, dyes, humic acids, and coloured dissolved organic matter.
Suspended Solids
As mentioned above, turbidity is, in part, caused by suspended solids in a water sample. For this reason, it is important to define suspended solids more precisely.
Simply put, suspended solids are any materials that have a low enough mass to allow them to stagnate between floating or sinking.
They fall into three main categories:
- Mineral particles: They are particles coming from minerals like iron. The mineral particles are responsible for the catalysis of oxidation and hydrolysis reactions in a humid environment.
- Organic debris: These are debris that are biodegradable under natural conditions. One can think of food, paper, or any other residue at risk of decomposition. In the long term, the presence of this debris can cause fermentation and the emission of contaminating gases. Moreover, their decomposition can promote the appearance of pathogens.
- Microorganisms: This category includes all living organisms, visible only under the microscope, but with a minimum size of approximately 2 microns. Microorganisms are also divided into three categories (bacteria, yeast & protozoa). These contaminants participate in the decomposition of organic matter.
The type of suspended solids found in water varies greatly. You can find waters with only one of the three types of suspended solids, or you can find waters with all three. It depends on many factors such as the surrounding flora, fauna, or soil type. Obviously, human activities near water bodies can also have a significant impact on the composition of suspended solids present in a water body.
Secondly, it is important to note that the type of suspended solids will have an impact on the risks associated with their presence and the types of effective treatment to remove them.
Dyes and Colouring Agents
Whether they are dyes used for textiles or for the food sector, their presence tends to reduce the transparency of the water and thus decreases the ability of light to pass through the water. These contaminants can pose significant risks to human health and to the survival of surrounding life forms (flora and fauna). This is because most dyes are aromatic compounds and their degradation produces by-products such as aromatic amines or benzene-sulfonic acids, which are known to alter the DNA of cells, resulting in their death.
Humic Acids
These acids, which are not well understood, come from humus found in soils. Characterized as an assembly of aromatic polymers of varying acidity and reactivity. As they are naturally formed in humus, these acids are naturally propagated.
Although there is no evidence that they are hazardous to consumption or equipment, their presence is legislated by most governments as these acids are precursors to carcinogenic and mutagenic disinfection by-products.
Dissolved organic matter chromophores (coloured)
They represent the portion of dissolved organic carbons that absorb light in the blue and UV part of the electromagnetic spectrum. The presence of these contaminants gives the water a colour similar to that of tea. Besides giving an unattractive colour to the water, these contaminants do not have any extreme risks associated with their consumption.
What causes turbidity in water
Since turbidity is caused by the presence of suspended solids and some of the contaminants discussed above, it is necessary to understand where these come from in order to understand where turbidity comes from and what causes it.
How do materials dissolve in water
The dissolution of any material (solute) is carried out thanks to the interaction and the forces of attraction which present between the solute and the solution. This means that when the force of attraction between the solute and the solution is stronger than the force of tension between the ions forming the solute, they break up and join the solution. Take for example the addition of NaCl to a sample of H2O to form salt water. In short, the covalent bonds between Na & Cl being less strong than the force of attraction between Na & H2O, these covalent bonds will be broken and NaCl will go to bind ionically to the water molecules. It is due to these reactions that various materials can dissolve in water.
- As a small aside, water is recognized as a universal solvent since its chemical composition means that it can associate with a very wide variety of molecules.
This is how materials get into the water or in contact with it. Although most contaminants that cause turbidity in water are spread in much the same way, by dissolving in water. Dyes and colourants, on the other hand, are generally of human origin. In fact, it is mainly due to the food and textile sectors that these contaminants are found in water.
What Are the Problems Associated with Turbid Water
The problems caused by high turbidity are various. One can obviously think of the aesthetics of the water. Indeed, a very turbid water will present an unattractive colour, a particular taste and may also give off odours. It is for these reasons, among others, that the turbidity limit for drinking water varies between 0.1 and 0.3 NTU. In other words, turbid water is not always safe to drink.
- It should be noted that the risks associated with drinking turbid water vary depending on the type of contaminant.
Risks related to the environment
Turbid waters can also be hazardous in their natural state. Whether turbidity is caused naturally or by human activities, its presence impacts the amount of light that passes through water bodies.
This decrease in light will have an impact on photosynthesis. Since photosynthesis is responsible for a good percentage of the dissolved oxygen found in water naturally, the decrease in photosynthesis will therefore have an impact on the amount of dissolved oxygen.
Briefly, dissolved oxygen in water is used for the survival of all living organisms in the water. This means that the absence of dissolved oxygen prevents aquatic life from remaining there.
- For technical purposes, to learn more about the impact of dissolved oxygen depletion in a water body, consult this article: BOD/COD reduction: a versatile solution
In addition, as discussed above, some contaminants can cause significant risks when they degrade, and genotoxic by-products are found in the water.
Risks related to equipment
Typically, the risks associated with equipment are relatively low. But in general, turbid water can have an impact on water disinfection. One example is chlorination, which in turbid waters will require more chlorine to complete the treatment.
In addition, during a treatment with UV irradiation, the particles causing turbidity in the water can block the UV waves and thus protect the bacteria present in the water.
This means that the presence of turbidity tends to influence the effectiveness of water disinfection treatments.
Effective turbidity treatment technologies
As we saw at the beginning of this article, turbidity can be caused by several types of contaminants. Each contaminant can have a different impact on the water and its hazardousness, but they are generally all removed by the same or similar technologies.
Pretreatment Steps
Generally speaking, before proceeding with the filtration of contaminants causing turbidity, it is advisable to carry out pretreatment steps to facilitate their extraction. This can be done by flocculation or coagulation.
Coagulation/flocculation
The coagulation/flocculation step consists of a chemical pretreatment during which polymers and coagulant can be added to the water. At the same time, it is common to see a pH adjustment performed to promote precipitation reactions. In short, in this way, the negatively charged colloidal particles become destabilized and it is through this destabilization that the particles agglomerate through chemical interaction.
Typically, aluminum salts or iron salts are the main coagulants used. As for polymers, they are used as flocculants.
- To learn more about precipitation reactions, coagulation, and flocculation, we suggest you visit this article: precipitation reactions.
Filtration
Different types of filtrations can be an interesting choice for turbidity reduction. However, it is important to understand that it is not enough to use any type of filter for any level of turbidity. In fact, the higher the turbidity level, the coarser the filtration must start in order to avoid premature clogging of the filtering membranes. For example, reverse osmosis membranes should not be used to treat water with turbidity higher than 1 NTU.
In short, all types of filtrations can be used for turbidity reduction. To better understand the differences between the types of filtrations, here are some articles that may help you.
- Types of filtrations
- Reverse osmosis explained
- Understanding the molecular weight cut-off of a membrane
Turbidity, an amalgam of contaminants
As we have seen, the causes of water turbidity are varied and the methods of reducing it are relatively simple. This is why water turbidity is the parameter of choice for monitoring the effectiveness of a water treatment system. Although it is a good indicator of the effectiveness of a water treatment system, turbidity is not closely related to other riskier types of contaminants such as dissolved solids, bacteria, viruses or other contaminants that are not considered turbidity.
In other words, ideally you want to reduce dissolved solids, suspended solids and any other contaminants that need to be removed as needed, but monitor turbidity to stay on top of undesirable fluctuations in your equipment.