When we talk about water treatments, regardless of whether the origin of the water is marine, continental, or treated waste, we cannot ignore the importance of the potential problems caused by the presence of colloidal particles. These are largely responsible for the colour, turbidity, taste, and odour of water. Moreover, their small size (between 10−3 and 1 μm) means that they cannot be naturally filtered out. The origin of colloids is remarkably diverse:
- Microorganisms (bacteria, viruses, and algae).
- Minerals (colloidal clays, silts, metal salts, and silica, among other compounds).
- Organic (such as surfactants, colourants, and humic or fulvic acids).
Without proper pre-treatment systems, these small, non-dissolved particles present in feed water will find their way to reverse osmosis (RO) membranes, settling on their surface and thus causing a fouling problem.
As a consequence of this fouling (as well as other factors such as age and temperature, etc.), membranes lose their most important characteristics over their normal operating period, especially in terms of the flow rate and the selectivity to salts. Therefore, depending on the membrane operating conditions, there may be a decrease in the flow rate of up to 10–12% per year. Thus, the passage of salts through the membrane increases by around 10–15% per year. This causes an increase in the feed pressure, a reduction in flow, and a worsening of the quality of the permeate (higher conductivity). Depending on the type of fouling, recovering a contaminated membrane through cleaning is often complex and so it is always better to stop the passage of the causative elements with adequate physical and chemical treatment before feeding the water into the trains.
Colloidal particles have a negative electrical charge in natural waters which results in a repulsion phenomenon that prevents them from joining to form larger particles. Therefore, we must neutralise these negative charges by adding positive charges that destabilise them so that the forces of attraction exceed those of repulsion. This leads to agglomeration of the particles—a phenomenon known as coagulation.
The chemical reaction of coagulation occurs in three stages:
- Neutralisation of the negative charges of the colloidal particles by adding a coagulant (chemical reagent) with positive ions.
- Coagulant reaction and the formation of positively charged colloidal hydrated oxide flocs which attract negatively charged colloidal impurities.
- Adsorption (linked bonding or surface bonding) of impurities by the flocs.
When the reagent is solubilised in water it releases positive ions with a sufficient charge density to attract the colloidal particles and neutralise their charge.
The masses formed by the aggregation of destabilised colloidal particles (flocs) have a gelatinous texture and have dimensions in the order of tens to hundreds of microns. These flocs have an appreciable sedimentation rate and can therefore be separated from the water by decantation.
The type and amount of coagulant, pH, stirring speed of the mixture, coagulation period, and water temperature are just a few of the many factors that influence the coagulation process.