Lime Feed

FIG. 7.31.9 Public utility district water reclamation plant at South Lake Tahoe, California. aCarbon dioxide is added to water in the reaction basin after it has passed through the ammonia-stripping column. bThermal disk is a processing unit with a series of hollow disks filled with a heat-transfer medium. The solid, reclaimed lime is cooled by the disks as it passes between them.

FIG. 7.31.9 Public utility district water reclamation plant at South Lake Tahoe, California. aCarbon dioxide is added to water in the reaction basin after it has passed through the ammonia-stripping column. bThermal disk is a processing unit with a series of hollow disks filled with a heat-transfer medium. The solid, reclaimed lime is cooled by the disks as it passes between them.

FIG. 7.31.10 Water reclamation plant at Windhoek, SouthWest Africa.

plant next adds lime to the overflow stream for coagulation and to precipitate phosphates and raise the pH. The high pH converts the ammonium nitrogen into ammonia form. In the next process, a cooling tower strips it out. The plant pretreats lime sludges and puts them through a recalcining furnace that burns CaCO3 into CaO. The furnace exhaust gas provides carbon dioxide for the recar-bonation (neutralization of the high pH) of the water immediately following the ammonia stripping. A two-stage centrifuging station sends about one-fourth of the recycled lime back to the chemical treatment stage for reuse.

Following nitrogen removal, the wastewater effluent passes through separation beds (mixed-media filters) that remove any remaining SS. Finally, the nearly pure water undergoes adsorption in upflow activated-carbon columns to remove soluble organic contaminants, including pesticides and ABS. Each of the eight columns contains 24 tn of carbon, providing about 4.8 million acres of adsorption surface for the removal of pollutants. Before being discharged to the Indian Creek Reservoir, the sparkling clear effluent is chlorinated to insure against bacterial contamination.

The plant regenerates granular carbon in a complete system using a multiple-hearth furnace with a capacity of 6000 lb or 6,000,000 surface acres per day. The plant scrubs and cools all furnace gases from the organic-sludge and lime-sludge furnaces to eliminate any air pollution.

One of the keys to the economy of the Lake Tahoe plant is the regeneration of the activated granular carbon. Larger plants can also find economy in a lime recovery system.

The median BOD concentration of the effluent produced is 0.98 mg/l, and the median COD is 10.83 mg/l. SS consists mostly of carbon fines in the effluent, amounting to 0.53 mg/l in concentration. The lowest phosphorus concentration (0.09 mg/l) was achieved when waste streams were recycled to the lime basin for 6 months. All systems are efficient, and testing continues to generate more knowledge on further improvements to operation.

The carbon system is loaded at a rate of 6.2 gpm per sq ft and has a short contact time of 17 min.

The greatest difficulties after startup have stemmed from the ammonia air stripper, which initially was sized for 50% of the required capacity. The plant now incorporates a second stripping tower. This custom-built unit incorporating thousands of redwood slats has 50 to 60% efficiency at low winter temperatures. This efficiency is due to the dependence of the transfer mechanism on temperature. To achieve 90% ammonia removal in the winter, the plant must increase the air-to-water ratios from 250 cubic feet per minute (cfm)/gpm to about 800 cfm/gpm. The temperature effect becomes negligible at high pH levels.

Calcium carbonate sludges have precipitated in the tower and caused a heavy, sticky residue to develop. The plant must periodically wash this limestone residue to remove the precipitate. The removable slats in the second unit overcome this problem.

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