1- pH Scale

pH is a measure of the acidity or basicity of water based solution.

pH is defined as the negative logarithm to the base 10 to the hydrogen ion activity which is approximately equal to the hydrogen ion concentration for the dilute solutions.

pH=-log[H+]

The pH scale is a logarithmic scale that usually runs from 1 to 14. Acids have a pH less than 7 while bases have a pH greater than 7. Distilled water is considered to be neutral; because it has a pH equals 7. It is neither an acid nor a base, but is the reference point for acids and bases.

The neutrality of pure water:

The chemical formula for water usually is written as H2O, but another way to consider the formula is HOH, where a positively charged hydrogen ion H+ is bonded to a negatively charged hydroxide ion OH-. This means water has properties of both an acid and a base, where the properties essentially cancel each other out.

H+ + (OH) - = HOH = H2O = water

Drinking water pH:

Although the pH of pure water is 7, drinking water and natural water exhibits a pH range because it contains dissolved minerals and gases. Surface waters typically range from pH 6.5 to 8.5 while groundwater ranges from pH 6 to 8.5.

Water with a pH less than 6.5 is considered acidic. This water typically is corrosive and soft. It may contain metal ions, such as copper, iron, lead, manganese and zinc. The metal ions may be toxic, may produce a metallic taste, and can stain fixtures and fabrics. The low pH can damage metal pipes and fixtures.

Water with a pH higher than 8.5 is considered basic or alkaline. This water often is hard water, containing ions that can form scale deposits in pipes and contribute an alkali taste.

There is no regulation for the pH of the drinking water. It is classified as a secondary drinking water contaminant whose impact is considered aesthetic.

pH Treatment:

The pH of water is controlled by the carbon dioxide–bicarbonate–carbonate equilibrium system. An increased carbon dioxide concentration will therefore lower pH, whereas a decrease will cause it to rise. Temperature will also affect the equilibrium and the pH. In pure water, a decrease in pH of about 0.45 occurs as the temperature is raised by 25 °C.

If the water is acidic (lower than 7), lime, soda ash, or sodium hydroxide can be added to raise the pH during water purification processes. For highly acidic waters, forced draft degasifiers can be an effective way to raise the pH, by stripping dissolved carbon dioxide from the water.

Acid (carbonic acid, hydrochloric acid or sulfuric acid) may be added to alkaline waters in some circumstances to lower the pH.

2- Turbidity (العكارة)

Turbidity is the cloudiness or haziness of a fluid caused by individual particles (suspended solids) that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of water quality.

Fluids can contain suspended solid matter consisting of particles of many different sizes. While some suspended material will be large enough and heavy enough to settle rapidly to the bottom of the container if a liquid sample is left to stand (the settable solids), very small particles will settle only very slowly or not at all if the sample is regularly agitated or the particles are colloidal. These small solid particles cause the liquid to appear turbid.

Turbidity of water can be measured using nephelometer, and units of turbidity from a calibrated nephelometer are called Nephelometric Turbidity Units (NTU).

Water Standards:

For the drinking water: According to WHO, Turbidity must not exceed 5 NTU, and ideally below 1 NTU.

For other usages, it may be greater than 5 and it’s different from place to another.

Turbidity Treatment:

Turbidity is commonly treated using either a settling or filtration process. Depending on the application, chemical reagents will be dosed into the wastewater stream to increase the effectiveness of the settling or filtration process. Potable water treatment and municipal wastewater plants often remove turbidity with a combination of sand filtration, settling tanks and clarifiers.

3- Fouling (Membrane fouling)

When the water passes through a membrane, there are some solutes or particles accumulate at the membrane which decreases its efficiency. This is called membrane fouling.

Membrane fouling can cause severe flux decline and affect the quality of the water produced. Severe fouling may require intense chemical cleaning or membrane replacement. This increases the operating costs of a treatment plant.

Fouling can be treated using chemical treatments or mechanical means to clean the membranes.