The sequence of treatment units is comprised of seven major sections:
Conventional treatment is used to enhance mainstream water quality by a reduction in the particulate and soluble fractions of organic and inorganic compounds. The processes involved consist of screening, grit removal, primary treatment, high purity oxygen activated sludge secondary treatment, and advanced biological phosphorus removal.
Advanced Biological Phosphorus Removal (Phostrip™)
Phostrip™ is an integral part of the high purity oxygen activated sludge process. Its function is to remove mainstream phosphorus and concentrate this element into a rich sidestream (stripper overflow). This sidestream is normally the only water to be chemically treated. Typically, the bulk of the secondary effluent bypasses the chemical treatment section.
The phosphorus-rich sidestream is subjected to high pH lime treatment. The lime reacts with soluble phosphorus forming a relatively insoluble complex of calcium hydroxylapatite. Subsequent steps in chemical treatment consist of two stages of recarbonation, using carbon dioxide, separated by clarification. The first recarbonation process involves calcium polishing while the second addresses pH control. The chemically treated water is then blended with the bypassed secondary effluent upstream of the ballast ponds.
Biological Nitrogen Removal (BNR)
The cornerstone of the most recent major expansion project is the BNR system, which includes submerged biologically aerated nitrification and denitrification filters. Because of its ability to biologically remove most of the nitrogen species in the water, the BNR process is essential to achieving compliance with the rigorous discharge requirements and addressing concerns of downstream users of the Truckee River.
Advanced Wastewater Treatment
These processes are designed to reduce suspended solids, decrease refractory organics, further lower the nitrogen level, and destruct/inactivate pathogens. These goals are achieved by dual media filtration, activated carbon adsorption, ion exchange, and disinfection, respectively. The activated carbon adsorption and the ion exchange process are only used intermittently, as water quality dictates.
Land Disposal of Effluent
The last step in the treatment process is to apply the final effluent to the soil via a subsurface percolation system. The effluent is further polished as it percolates through the ground.
As a byproduct of treating the wastewater, inorganic and organic sludges are generated. These sludges are thickened prior to processing further. Organic sludges are digested for the purpose of stabilizing the material prior to ultimate dewatering and disposal. Through proper blending and conditioning, certain sludges that are produced may be put to beneficial reuse.