Optimizing Polymer Performance in Municipal Water Treatment

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.

The Challenge

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 Solution

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. Recognizing the needs in this critical area of municipal water treatment, a manufacturer of liquid and dry polymer preparation systems offers a patented liquid and dry polymer blending system. Both offer high-energy, non-mechanical liquid-polymer activation and blending technology designed to effectively activate all types of liquid polymers. The technology produces more than six times the mixing energy per unit volume than some others. The three stages of operation are:

• Stage 1: Pressure drop across a specially designed variable-orifice water-control valve produces a high-velocity water jet, which travels at approximately 70 feet per second. The water jet is aimed directly at, and impinges on, the polymer as it enters the mixing chamber. At this point, the polymer coils up and is not susceptible to damage.
• Stage 2: In the concentric mixing chambers, the newly blended polymer recirculates multiple times for additional exposure to non-damaging turbulence, completing the blending process. The recirculation ensures that the polymer solution is present directly after the point of injection for an ideal activation and blending environment.
• Stage 3: The mixing energy then naturally diminishes in the concentric chambers, while the optimized flow path ensures optimum polymer performance by preventing the polymer from causing a disruption to the three-stage activation and blending process.

The three-stage method of operation ensures the patented system delivers high mixing energy without the use of mechanical impellers, which can cause polymer damage and gelling. Polymers that do not gel or get damaged maximize the treatment plant’s polymer investment by reducing actual polymer use. Additionally, the non-mechanical mixing chamber delivers a high degree of reliability. Also, the system’s injection-check valve has been designed with easy disassembly and inspection in mind. The technology is ideal for water-treatment applications because it utilizes a negative-pressure, blower-induced conveyance system to transport and disperse the dry polymer prior to the wetting process. Dispersing dry polymer prior to it coming into contact with the water or wastewater ensures effective polymer-particle wetting. The result is reduced mixing and hydration times, higher polymer performance, lower chemical costs and fewer moving parts, which results in increased reliability and reduced maintenance concerns. Other benefits include separation of polymer prior to wetting, which prevents polymer buildup at the volumetric feeder and plugging/clogging of the conveyance system. There are multiple hopper configurations. Hoppers have been designed for loading without the need for stairs or platforms, increasing operator safety, along with cleaner operation and a reduced chance of spills. And six standard control panels provide the flexibility to choose the features that best meet the operation’s requirements. Water treatment systems are only truly successful if they can consistently, efficiently and effectively perform as intended. Helping ensure the reliable performance of water treatment operations are the preparation systems that introduce the polymers that protect the coagulation and flocculation processes that are so crucial in municipal water-treatment systems.