WATER TREAMENT PROCESS
PRESENT FACILITIES AND OPERATION
Patoka Lake Regional Water Treatment Plant treats water in a series of steps. The first step utilizes chemicals known as coagulants to react with suspended particles in the water to promote their removal prior to filtration. Filters are then used as a final step in the process to remove a very high degree of any remaining solid particles not removed in the clarification processes.
The following is a brief and simplified description of the various treatment processes utilized within the Patoka Lake Regional Water Treatment Facility.
LAKE WATER IS PUMPED TO THE PLANT
Untreated water is supplied to the water treatment plant from an intake structure located in the Patoka Lake. From this station the water is pumped at demand related controlled rates into the water treatment plant. The water flows into an aerator, which aids in the oxidation and removal of some organic compounds.
FLASH MIXING
The raw water enters a high-energy mixing chamber, where it is treated with several chemicals for a variety of purposes. Chlorine is added for primary disinfection. This chemical is used to destroy or inactivate potentially harmful bacteria and other microorganisms which may be present in the water.
Hydrated Lime is added to control the pH (acidity-alkalinity), and to produce a palatable, non-corrosive water.
Potassium Permanganate is added as an oxidant to remove Manganese and reduce taste and odor from several naturally occurring organic formations such as algae growth.
Powdered activated carbon may also be used as an absorbent for taste and odor control or to remove a wide variety of chemicals that may accidentally contaminate the water supply. Carbon in this form absorbs volatile chemicals in the same way as charcoal absorbs charcoal lighter fluid used in a backyard grill. The carbon then settles to the bottom of the sedimentation basin, where it is removed with the sludge and discharged.
COAGULATION AND FLOCCULATION
Also in the flash mixing chamber, chemical coagulants are added to the water to aid in the removal of fine suspended solids. Since these solids, by themselves, are too small to be removed from the water by gravitational settling within a practical period of time, the coagulant is added to increase particle size and weight, thus improving solids removal during the clarification and filtration processes. The chemicals used for this purpose in Patoka Water’s Treatment plant is liquid Aluminum Sulfate, commonly called alum.
The alum reacts with the natural alkalinity of the water to form a gelatinous precipitate called Aluminum Hydroxide. As the water passes through the distribution laterals into the clarification basins, a mixing action takes place and this gelatinous precipitate entraps the fine suspended solids and forms larger and denser masses called “floc”.
A polymer is also added in this step to neutralize the natural negative electrical charges of the solids in the water. The charge neutralization further reduces the tendency for the solids to stay in suspension.
SEDIMENTATION AND CLARIFICATION
The water then flows into a mixing and sedimentation basin which provides extended time for chemical reaction as well as clarification by gravity settling.
The water is first gently mixed. This mixing moves the water around the secondary floc particles and brings more suspended solids into contact with the floc, enhancing floc growth. As the floc grows, the particles become large enough in size to settle to the bottom of the sedimentation basin. A solids blanket is maintained in these basins through which the floc in the incoming water is removed by a solids contact filtering action. This step clarifies the water 95 to 98%. This clarified water then passes up and into an effluent trough and out of the clarification basins.
The excess accumulation of solids is deposited in the center of the basins. This accumulation of solids is now called “sludge”. The sludge is concentrated, and then siphoned from the sludge hoppers into tanks, where the sludge is thickened further and removed to sand drying beds.
FILTRATION
After the treatment processes of coagulation and clarification, the clarified water flows through dual media filter beds to remove a very high degree of any remaining solids. Prior to filtration, over 98% of the impurities have been removed by the preceding treatment steps.
Filters provide a total filtration capacity of eight (8) million gallons per day. This allows the plant to operate at full capacity even with one filter out of service.
The filter beds are constructed of layers of anthracite coal, filter sand and supportive gravel. Twenty inches of anthracite having an approximate diameter of 1.0 mm rest over ten inches of fine sand with an average diameter of 0.5 mm.
As the water passes down and through the filters, the remaining suspended solids become entrapped in the interstices of the filter media. At routine intervals, the filters are cleaned of the entrapped solids by forcing clear, fully treated water in the reverse direction at high velocities. This reverse flow is called backwashing and is required to remove the build-up of captured solids within the upper levels of the filter media. This water and the solids it carries is then discharged into sludge holding ponds.
FINISHED WATER STORAGE AND PUMPING
Following filtration, the water is treated with a final control dosage of chlorine to adjust the residual disinfection concentration.
Flouride is added to reduce the insistence of tooth decay in children who drink Patoka’s water.
The treated water is stored in an underground reservoir called a “Clearwell”. The clearwell has a capacity of 1.2 million gallons.
From the clearwell, water is pumped directly into the distribution system with three high service pumps having a total of 750 horsepower.
FINISHED WATER DISTRIBUTION
By both weight and volume, the delivery of water is, by far, the largest transportation operation in the United States. A complex network of pipes, booster pumping stations, tanks and reservoirs is necessary to move the water from the treatment facilities to the customer. Patoka’s distribution system contains about 850 miles of pipe ranging in size from 3-30 inches.
In each of the pressure areas, there is at least one tank. The height of the water in the tank or reservoir determines the pressure in the area. These facilities ensure a relatively uniform pressure in the supply system with sufficient reserve storage for high demand periods, and an adequate supply of water for emergencies. Slight variations in pressure cannot be avoided. For example, in the early evening hours as people return from work, the demand of water may increase. As a result, the water level in storage tanks is lowered and the pressure drops slightly. During the night, when industrial activity is reduced and people are sleeping, pumps continue to work, and the tanks and reservoirs are refilled to provide maximum storage for the next day.