Impact of Raw Water Temperature and Alkalinity on Coagulation

In Quebec, many filtration plants produce drinking water from surface water (e.g., the St. Lawrence River, the Rivière des Milles Îles, etc.).

The most difficult undesirable substances to remove from natural waters are those that have a very small size (colloidal particles) and those that are dissolved (organic matter that causes water discolouration and trihalomethanes (THMs)). These substances usually carry a negative electrical charge that prevents the particles from agglomerating with each other to form larger particles (flocs) and facilitate their removal by sedimentation and filtration.

Coagulation is the process of destabilizing these negative charges of the particles. It is a question of neutralizing their electrostatic charges of repulsion to allow them to agglomerate. When an inorganic aluminum-based coagulant is added, the metal ion Al3+ is hydrolyzed to form a precipitate (floc or pinfloc) of aluminum hydroxide as well as hydrogen H+ ions. These hydrogen ions (H+) released are responsible for the acidification of treated water. In the case of alum, our Ultra C-210, is not pre-hydrolyzed, so it must look for 3 OH- ions in the water to be treated to form an aluminum hydroxide Al(OH)_3. Thus, the release of 3 H+ ions is responsible for the significant acidification of the treated water and the consumption of the initial alkanity of the raw water. It will be necessary to adjust the pH of water treated with caustic soda or other alkaline product in order to meet the pH discharge standards.

The coagulation reaction is a kinetic reaction, i.e. the temperature of the water partly dictates the rate of reaction; the rate of reaction is therefore slowed down by a colder water temperature.

In hot water, if the coagulant is injected at a location where the agitation allows rapid dispersion in the raw water, then the coagulation reaction will take place in just a few seconds.

However, in cold water (temperature of only 2-3°C), as is the case in winter for many filtration plants in Quebec that source their water from rivers or streams, the coagulation reaction is significantly slowed down.

There are other NSF-approved aluminum-based coagulants on the market that are ‘pre-hydrolyzed’ such as Ultra C-221.

This means that, due to the chemical nature of these coagulants, which already contain a certain amount of aluminum hydroxides (OH-), these pre-hydrolyzed coagulants will consume less alkalinity in the water to be treated and will therefore affect the pH less. This can be a wise choice for raw water that starts with low alkalinity.

Also, because these coagulants are pre-hydrolyzed, they react much faster in cold water.

Trust AQUASAN’s technical team. We have extensive technical expertise in water treatment. By considering the characteristics of your raw water and your treatment system, we will choose the most relevant coagulant for your treatment and the one that will allow you to have the lowest operating costs.

Our goal is to offer you an innovative and easy-to-use chemical program that performs well and meets your performance objectives. We have unrivalled training and technical know-how and we know how to provide creative solutions that ensure the maintenance of treated water quality.

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