Some historians say one of the first recorded examples of water treatment occurred more than 4,000 years ago when ancient Egyptians used almonds to coat the inside of storage vessels to help clarify river water. Around 77 A.D., the Romans became the first to use alum (a colloquial name for “hydrated potassium aluminum sulfate”) as a coagulant during water treatment. By the mid-1700s, alum was regularly being used as a coagulant in municipal water treatment activities in England.
Almost from the dawn of civilization, people have known the dangers of untreated water—and those concerns remain relevant. The challenge has been to implement treatment systems that result in water and wastewater that will not harm users.
Wet or dry polymer preparation systems, which aid in the critical coagulation and flocculation stages of municipal water treatment, can help water treatment facilities efficiently deliver clean water for the public and the environment.
One of the main challenges in municipal water treatment is finding a way to reduce the water’s turbidity to acceptably low levels. Turbidity is a measurement that determines the amount of cloudiness or haziness that can be found in untreated water. Turbidity is caused by the presence of microscopic particles—usually bacteria, dirt, pathogens, metal ions, phosphates, plant materials or minuscule pieces of crushed rock or stone—that are generally invisible to the naked eye.
Countries around the world set standards for turbidity levels in drinking water. In the U.S., systems that use conventional or direct-filtration methods for water treatment cannot produce water with turbidity levels at the plant outlet that are higher than 1.0 nephelometric turbidity units (NTU). The World Health Organization has determined that the turbidity level of drinking water should not be more than 5 NTU at any time and should ideally be below 1 NTU.
The inherent problem in attempting to decrease the turbidity of municipal drinking or wastewater is that the microscopic particles are so light in weight that they either will settle very slowly over time (as measured in hours, days or weeks), or will not settle at all. This means the water treater must rely on different methods to remove the particles in a timely and efficient manner.
One of the more popular methods is known as coagulation and flocculation. Coagulation and flocculation are chemical processes wherein the particles in water are coaxed into forming larger clusters, known as flocs, which make them easier to remove through a settling or filtration process. Coagulation neutralizes the electrical charges of the suspended particles, allowing them to form a mass that will settle or be trapped in a filter. Flocculation is a gentle agitation of the water that encourages the floating particles to gather into masses that can be filtered or will settle.
This clumping process is driven by the chemical reagents, or coagulants/flocculants, that allow the suspended particles to attach to each other to form larger clusters that can be easier to remove from the water. Two common types of chemical reagents used in municipal water-treatment systems are:
- Ferric chloride: Another name for iron(III) chloride, ferric chloride breaks down when dissolved in water, which gives it the ability to sufficiently form suspended solid particles into flocs.
- Alum: Alum is an astringent compound that allows the negatively charged suspended particles to clump together so they can be more easily removed from settling basins or trapped during filtration.
A potential roadblock to successful removal of the clumped particles is that flocs formed through the use of the alum and ferric chloride can be fragile. Any agitation of the water in the settling or filtration processes can break apart the flocs. When this happens, the floc-removal processes can be compromised, leading to water that does not meet the standards for purity and turbidity. This also means that the water will need to be re-treated, which is costly and time-consuming.
To combat floc failure, polymers are injected into the water during the coagulation stage. These polymers strengthen the flocs, allowing them to withstand the rougher treatment during the filtration or settling processes. The ultimate challenge in these circumstances is to employ a polymer preparation technology that can adequately introduce the polymers to the municipal water-treatment process.
The treatment plant operator has two polymer choices: wet or dry. In both cases, polymer preparation systems can deliver the injection rates that are required and handle the growing number of polymers that are now available, some of which are exceptionally delicate and must be handled with extreme care.