It is estimated that in the United States about 34 billion gallons of wastewater is processed every day. What are the steps in the treatment of municipal water? There are 5 important steps in treating municipal water: chemical addition, coagulation and flocculation, sedimentation and clarification, filtration, and disinfection. Let’s look at this process in further detail.
Steps in the Municipal Water Treatment Process
The process for treating municipal water includes 5 crucial steps that ensure clean and safe water.
Step 1: Chemical Addition
Chemical addition is the process in which a chemical is added that reacts along with the natural alkalinity to form an insoluble precipitate. The chemicals help make the suspended particles floating in the water clump together to form a gelatinous particle called a floc, which is usually larger and heavier than a single particulate.
There is a variety of different chemicals that can be used in this process. These chemicals are called coagulants. Probably the most popular chemical used is aluminum sulfate or alum. A few other coagulants are ferrous sulfate, sodium aluminate, ferric chloride, and manufactured chemicals, which are compounds called polymers. The polymers are classified as cationic polymers, anionic polymers, and nonionic polymers. No matter which coagulant or combination of coagulants are used, they must be mixed extremely well with the water to form a heavier floc.
Step 2: Coagulation and Flocculation
Coagulation and flocculation is where chemicals with a positive charge are added to the water. These chemicals neutralize the negative charge of dirt and other dissolved particles in the water. Particles bind with these chemicals, forming floc, similar to the process in the chemical addition step. The treatment unit where coagulation and flocculation are performed is called the “flocculator.”
Step 3: Sedimentation and Clarification
After the flocculation process is complete, the water will then head over to the center of the clarifier or sedimentation basin for sedimentation and clarification. The water will make its way from the center of the clarifier to the saw tooth weir at the perimeter of the unit. The large floc of particles are allowed to settle out to the bottom of the clarifier. A rake is continuously traveling across the bottom of the clarifier to scrape the floc to the middle of the unit. To pull the settled sludge out of the clarifier and send it into a sedimentation or disposal pond, pumps are used. With this process, the majority of suspended material can be removed prior to clarification. This avoids overloading the filters and allowing more water to be filtered before the filters are required to be backwashed.
Step 4: Filtration
During the filtration process, clarified water enters the filters from above then is collected in a drain system at the bottom of the filter unit. Filters are made up different materials or media, such as sand or gravel. Granular activated carbon is becoming the media of choice in many conventional plants because it provides mechanical filtration of particulate matter and it removes organic compounds which are often associated with taste and odor problems.
Step 5: Disinfection
After the filtration process, the water is clear and as clean as it can get but there may still be bacteria and viruses present. To destroy the viruses and bacteria, the disinfection process begins. In the United States, chlorination is commonly used for disinfection. Chlorine gas, chlorine dioxide, and hypochlorite are just a few of the different forms that chlorine comes in. Chlorine is added to the water in an amount that will ensure all microorganisms are destroyed. Chlorine levels are continuously and very carefully monitored by the water plants because enough chlorine must be added to ensure the water is disinfected but also to avoid excess that can cause taste and odor problems when delivered to the customer.
Types of Filtration Systems Used for Treating Municipal Water
A process that removes particles of a certain size from liquids is called liquid filtration. Filtration systems vary based on the condition of the water coming into the system and the required purity of the water to be reused after being filtered. Particle filtration and membrane filtration are the two primary types of filtration municipal water treatment systems.
Particle Filtration
Particle filtration is a system using either mechanical or physical means to separate solids from liquids. For treatment of contaminated wastewater, particle filtration is commonly one of the first steps. Particle filtration takes place early in the treatment process because it is designed to remove solids measuring larger than one Micron. Cartridge filtration, bag filtration, and self-cleaning filters are the three common filter types for particle filtration.
Cartridge Filtration is meant to trap particles, and even chemicals, using cartridge filters. These filters utilize pleated fabric or another type of screen. Typically separated into two categories, cartridge filters are either surface filters or depth filters. Surface filters retain particles on the surface of the liquid while depth cartridge filters use a thick media to create a twisted path that retains particles.
Bag filtration is a great option for smaller systems and applications where minimizing waste is an important factor. Bag filters are in the shape of an elongated bag where wastewater travels into the bag and solid particles from the water are caught, allowing only clean water to flow through the pores of the bag. Depending on the different sized pores of the bag, particles of various sizes can be caught, depending on the system's needs. Multi-bag filtration allows for a higher flow rate within the system.
Self-cleaning filters set themselves apart from other filters because of their ability to clean themselves, which is considered one of the most useful aspects of these filters. In high demand, these filters are enabled to be customized with a number of different sizes and materials. Self-cleaning filters are ideal within systems that cannot be shut down for cleaning purposes. To remove debris, self-cleaning filters typically utilize mechanical processes or backwashing.
Membrane Filtration
When particle filtration alone is not sufficient for water reuse in municipal water treatment, membrane filtration is typically the go to process. When the highest water quality is required, membrane filtration systems are the most used. Reverse osmosis, ultrafiltration, and microfiltration are the common types of membrane filtration.
The reverse osmosis process helps remove and reduce the presence of very small organic particles. Reverse osmosis offers the highest level of filtration, helping filter out contaminants other systems may not even be able to filter.
Where water is pushed through small pores of a filter between 5 and 100 nanometers, ultrafiltration is used. This type of filtration is performed under low pressure. The system helps remove silt, high molecular weight organic material, and even water borne pathogens like viruses. At times, chemicals can also be added to the backwash of the filter to provide additional aid in the cleaning process. Ultrafiltration and microfiltration are commonly used prior to reverse osmosis treatment.
The Importance of the Municipal Water Treatment Plants Process
Chemical addition, coagulation and flocculation, sedimentation and clarification, filtration, and disinfection are the 5 steps in the liquid filtration process for cleaning municipal water. The filtration process is important for many reasons: it allows for water companies to reduce water, diminish possible chemical consumption, and lower their water use. If you need filtration solutions for your municipal water treatment plant, check out Commercial Filtration Supply’s filtration systems and filters today.
