PRELIMINARY WATER TREATMENT

WATER SOURCES

HYDROLOGIC CYCLE: The circulation of water from the oceans where about 6 feet each year is evaporated into the atmosphere. Clouds are formed; precipitation falls on land; develops surface water supplies in lakes and rivers; percolates into the ground to recharge groundwater supplies; and it all eventually ends up back in the oceans.

GROUND WATER AND WELLS: Wells and springs. Groundwaters are characterized by lower color levels, freedom from microbial contamination, higher hardness than surface waters, higher dissolved solids and gases values, and less variability in water quality parameters. "Safe yield" is the quantity and/or withdrawal rate of water that does not cause land subsidence, drastic lowering of the water table, or saltwater intrusion. Hardness is the result of water percolating through mineral deposits containing such elements as calcium and magnesium. Usually requires minimum treatment.

SURFACE WATERS: Rivers and streams, lakes and reservoirs. Surface waters are usually higher in turbidities, suspended solids, possess some color, are lower in hardness, have microbial contamination concerns, and have greater changes in water quality parameters than groundwaters. For rivers and streams must know and address silting, ice, and upstream pollution problems. For lakes and reservoirs must know and address watershed uses in agricultural, recreation, and domestic uses; stratification (layers of different temp. water) and reservoir and lake "turnover" each fall; algae with taste and odor problems and algal blooms. Reclaimed water is used for non-food crop irrigation, irrigation of parks, golf courses, wetlands, industrial reuse. Lakes and reservoirs: Natural factors affecting water quality are fires; geology; plant and vegetation type, growth; climate, and wind. Man-made factors are agricultural activities; domestic and industrial discharges; recreational uses.

PRELIMINARY TREATMENT

WATER TREATMENT:

1) SCREENING/RACKING: Removes the sticks, stones, and other debris that are in the supply water. Screens are cleaned either manually or automatically. Debris hauled away. Purpose is to protect the facilities. Screens/racks are categorized by their opening (spacing): Fine racks are 1/16" to 1/2" inch opening (spacing); Medium 1/2" to 1"; Coarse racks have 1 1/14" to 3" or 4" openings. The images at the left are of a "climbing bar screen." You can see the screen's bars in the second photo.

Materials in the raw water to be treated may also be removed by a "rotating drum screen." The raw water enters the center of the drum screen. When it becomes time to clean the screen's mesh, the drum rotates, and sprays blow the debris off the screen onto a chute in the center of the drum. The chute conveys the material of to a basket for removal. Note the fish removed by the rotating drum screen at the left.... add some lemon and butter, and we have lunch! (Ok, bad joke.... bad joke!)

Microstraining is when very small particles like algae are strained from the process stream. One has approx. 160,000 openings per square inch. (The picture to the left illustrates the type of material removed by the coarse bars of the climber screen shown above.)

2) SAND REMOVAL/PRESEDIMENTATION: Sand is very abrasive to equipment such as sedimentation flights and pumps. Sand traps, sedimentation impoundments, cyclone degriters are used to remove the sand. (At the left we show a sand basin which is essentially a small clarifier than allows for the sand to settle from the incoming water stream.)

3) AERATION: Removes gases such as hydrogen sulfide (H2S), methane (CH4), and carbon dioxide (CO2). Aeration followed by a sufficient detention time then filtration decreases the amount of iron and manganese in the water. Reduces organic gases and volatile materials that can cause taste and odor problems. Do NOT exceed dissolved oxygen (DO) levels above 2 to 4 mg/L or saturation levels: exceeding causes corrosion in distribution piping.

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