Presentation on theme: "Often water is prescreened, treated with ozone, or pre-chlorinated before entering the coagulation basin (depending on the quality of the water). Coagulation."— Presentation transcript:
Often water is prescreened, treated with ozone, or pre-chlorinated before entering the coagulation basin (depending on the quality of the water). Coagulation is a process that causes small suspended particles to attract to one another and form larger particles. Source raw water entering the coagulation basin is monitored for turbidity (cloudiness), temperature, alkalinity, and visual appearance by operators using sensors to determine the amount of coagulant chemical dosages, and tank detention times.
The next step is flocculation: a process that follows coagulation that uses gentle stirring to bring particles together. Particles reacting to the coagulation chemicals previously added are subjected to typically three speeds of stirring, starting with a rapid flash mix and progressing to a gentle stirring speed, allowing particles to collide with one another, increasing their density so that they can more effectively fall out of suspension in the next treatment step: sedimentation
Sedimentation is the process that reduces the velocity of the water so suspended material in the water can settle out by gravity Most of the filtration is completed by coagulation/flocculation and sedimentation before the water even reaches the filtration process, which comes next!
The water is then sent through a slow sand filtration process using filter media comprised of anthracite coal, sand, and gravel-the water entering from the top, traveling downward through the coal, then the sand, filtering out through the gravel and out through effluent collection tunnels at the bottom Periodically, these sand filtration basins must be back-flushed in order to remove layers of debris that are filtered out of the water and gather in a kind of sludge on the top. This filter cleaning flush-water is sent back to the beginning of the treatment process.
The water is then disinfected through the introduction of chemical; either chlorine, chloramines, or chlorine- dioxide. The disinfection process must meet the minimum regulatory removal and inactivation requirements. Effective disinfection will eliminate harmful biological contaminants such as Giardia, Viruses, Cryptosporidium, and Protozoa. The water purveyor is also responsible for the removal of potentially harmful disinfectant byproducts such as Trihalomethanes (THMs) and Haloacidic Acids. This can be achieved by the addition of neutralizing chemicals such as ammonia. All effective disinfection leaves a residual in the water that keeps it disinfected as the water travels outward through the distribution system and out to customers.