advanced processes, including activated carbon, chemical oxidation (ozone, advanced oxidation processes [AOPs]), nanofiltration (NF), and reverse osmosis (RO). Dissolved solids are retained during softening, electrodialysis, NF, and RO. Various processes can be combined to produce the desired effluent water quality depending on the reuse requirements, source water quality, waste disposal considerations, treatment cost, and energy needs.
Nutrient removal is often required in reuse applications where streamflow augmentation or groundwater recharge is practiced to prevent eutrophication or nitrate contamination of shallow groundwater. Nutrient removal can be either an integral part of the secondary biological treatment system or an add-on process to an existing conventional treatment scheme.
All of the biological processes for nitrogen removal include an aerobic zone in which biological nitrification occurs. An anoxic zone and proper retention time is then provided to allow biological denitrification (conversion to nitrogen gas) to reduce the concentrations of nitrate to less than 8 mg N/L as illustrated in Table 3-2 (Tchobanoglous et al., 2002). Gas stripping for removal of ammonia or breakpoint chlorination as the primary means for nitrogen removal is not commonly employed in wastewater reclamation applications in the United States.
To accomplish biological phosphorus removal via phosphorus-storing bacteria, a sequence of an anaerobic zone followed by an aerobic zone is required (for more detailed information see Tchobanoglous et al., 2002). Phosphorus removal can also be achieved by chemical precipitation by adding metal salts (e.g., Ca(II), Al(III), Fe(III)) with a subsequent filtration following the activated sludge system. Although chemical precipitation for phosphorus removal is practiced in many water reclamation facilities, biological phosphorus removal requires no chemical input. Biological phosphorus removal, however, requires a dedicated anaerobic zone and modifications to the activated sludge process, which usually is more costly during a plant retrofit than an upgrade to chemical precipitation. A biological phosphorus removal process is also more challenging to control and maintain because it depends upon a more consistent feedwater quality and steady operational conditions. Biological and chemical phosphorus removal can result in effluent concentrations of less than 0.5 mg P/L (see Table 3-2).
Suspended Solids Removal
Filtration is a key unit operation in water reclamation, providing a separation of suspended and colloidal particles, including microorganisms, from water. The three main purposes of filtration are to (1) allow a more effective disinfection; (2) provide pretreatment for subsequent advanced treatment steps, such as carbon adsorption, membrane filtration, or chemical oxidation; and (3) remove chemically precipitated phosphorus (Asano et al., 2007). Filtration operations most commonly used in water reclamation are depth, surface, and membrane filtration.
Depth filtration is the most common method used for the filtration of wastewater effluents in water reclamation. In addition to providing supplemental removal of suspended solids including any sorbed contaminants, depth filtration is especially important as a conditioning step for effective disinfection. At larger reuse facilities (>1,000 m3/d or >4 MGD), mono- and dual-media filters are most commonly used for wastewater filtration with gravity or pressure as the driving force. Both mono- and dual-media filters using sand and anthracite have typical filtration rates between 2,900 and 8,600 gal/ft2 per day (4,900–14,600 L/m2 per hour) while achieving effluent turbidities between 0.3 and 4 nephelometric turbidity units (NTU). Because large plants with many filters usually do not practice wasting of the initial filtrate after backwash (filter-to-waste), effluent qualities with elevated initial turbidity are commonly observed, and as a consequence, the overall effluent quality can be less consistent in granular media filtration plants compared with reclaimed water provided by a membrane filtration plant.
As an alternative to depth filtration, surface filtration can be used as pretreatment for membrane filtration or UV disinfection. In surface filtration, particulate matter is removed by mechanical sieving by passing water through thin filter material that is composed of cloth fabrics of different weaves, woven metal fabrics, and a variety of synthetic materials with openings between 10 to 30 m or larger. Surface filters can be