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Water Treatment Process

Head Tank
Raw water for the Howell Water Treatment Plant is drawn from six wells. The water is pumped to the water plant through an 18-inch transmission line. The water enters the plant and goes into a Head Tank equipped with a multiple tray aerator. Aeration is necessary for the removal of taste and odor causing substances such as carbon dioxide and hydrogen sulfide gases, which might interfere with the softening and treatment processes. The Head Tank is positioned higher than the cone to supply pressure to lift the lime blanket inside the cone.

 

Claricone Clarifier
Water is directed from the Head Tank to the Claricone clarifier. Water enters the clarifier at the mixing zone located in the bottom of the Claricone. Lime is added to the water in this zone where it reacts with the calcium and the magnesium in the water to form precipitates.

 

The Claricone utilizes its unique shape and helical flow path to provide a smooth transition from rapid mixing to gentle mixing, as particle move upwards to the solids contact zone of the cone. After sufficient time, particles adhere to each other and grow into larger particles, or, floc, which is prone to settle in water. This process of floc formation is called flocculation. To aid in the flocculation of particles, an anionic polymer is added in the mixing zone of the cone.

 

Clarification of water is accomplished by the seperation of suspended solids from water by gravity. The clarification zone of the cone is located directly above the solids contact zone. The clarified water moves upward slowly, eventually passing over the effluent weir to the cone outlet and on the to filters for further treatment.

 

Water Softening
The water plant incorporates a single stage lime softening operation to remove hardness. Hardness is caused by the presence of dissolved bivalent and polyvalent metal ions, predominantly calcium and magnesium. The water plant utilizes conventional lime softening treatment where raw water passing through the Claricone, is subjected to lime addition, mixing, and clarification.

 

Sulfuric acid and polyphosphate are added to control pH and stabilize the softend water.

 

Filtration
Filtration is a process which water is passed through a porous medium to remove suspended matter present in the water or generated from the chemical reactions occuring in the softening process. The suspended matter consists of silt, clay, colloids, and micro-organisms ranging in size from one millimicron to fifty microns.

 

The Howell water filtration system consists of four cell dual media gravity filter with combination air/water backwash. Each of the 4 cells is 12' by 14' and under normal flow conditions 2.7 gallons per minute of water passes through each square foot of the surface. The filter media is comprised of 20 inches of anthracite (coal), and 10 inches of sand. The filters are individually backwashed every 50-75 hours with both water and air. An air wash rate of 2 cfm/sf and a liquid backwash rate of 15 gpm/sf are used concurrently for approximately 12 minutes, then continued with water only for and additional 12 minutes. From the filters, water goes to a clearwell and then on to the 620,000 gallon ground storage reservoir.

 

Chlorination
Pre-chlorination and post-chlorination are used to insure the safety of the water supply. Pre-chlorination yeilds benefits by improving coagulation in the Claricone treatment process, suppressing decomposition of organic matter in the sludge, and aiding filter operation by controlling slime formation in the filters. Post-chlorination is used primarily for disinfection of the water. This is added both in the clearwell and the high service discharge as water enters the distribution system.

 

Fluoridation
Hydrofluosilicic acid provides the source of fluoride, which is beneficial in the prevention of tooth decay in children. A fluoride concentration of 1.1-1.2 mg/l is usually desirable in the plant tap. The City of Howell's groundwater does have some natural fluoride in it. Generally, it measures about 0.42 mg/l in the raw water before the addition of hydrofluosilicic acid.

 

Sludge Handling
Lime sludge is drawn from the Claricone as a by product of the softening process. This sludge is pumped to the sludge dewatering beds located at the wastewater treatment plant via sludge pumps and forced main. Dewatered lime sludge is annually removed from the beds for application to agricultural land for pH control.

 

Ground Storage Reservoir
From the clearwell inside the water plant, water flows into a 620,000 gallon ground storage tank as a reserve for city consumption and fire control.

 

High Service Pumps
Treated water is pumped to the distribution system by high service pumps. There are four pumps available for this, each with the ability to pump 1100 gallons of water per minute. These pumps also fill the north tower storage tank. The cycle throughout the day as demand dictates. When these pumps are on they maintain system pressure. Average system pressure is 50-55 psi.

 

North Elevated Storage Tank
This 300,000 gallon elevated tank is located at the entrance to the City Park. This tower was constructed in 1959. The tank supplies system pressure when the high service pumps are off.

 

Standby Power
The water plant can be run in emergency situations due to a loss of electricity. A 250kW diesel generator can run the entire water plant, Department of Public Works compound, and the high service pumps the fill the north tower and maintains system pressure.