FIGURE 4-1 Treatment processes commonly used in water reclamation. Note that some or all of the numerous steps represented under advanced processes may be employed, depending on the end-product water quality desired and whether engineered natural processes are also used. All possible combinations are not displayed here.

that does not require primary treatment and secondary sedimentation (LeClech et al., 2006). Instead, raw wastewater can be directly applied to a bioreactor with submerged microfiltration or ultrafiltration membranes. These applications may only employ a fine screen as a preliminary treatment step. MBR processes combine the advantage of complete solids removal, a significant disinfection capability, high-rate and high-efficiency organic and nutrient removal, and a small footprint (Stephenson et al., 2000). In the past 10 years, reductions in the cost of membrane modules, extended life expectancy of the membranes, and advances in process design and operation have resulted in many domestic and industrial applications using MBRs. Its integrated design, which can be scaled down more easily than conventional secondary treatment processes, can facilitate decentralized water reclamation. However, membrane fouling and its consequences regarding plant maintenance and operating costs limit the widespread application of MBRs (LeClech et al., 2006; van Nieuwenhuijzen et al., 2008). Challenges that require research relate to maintaining productivity (or flux, i.e., the amount of water produced per membrane area) and minimizing the effects of membrane fouling. Other MBR research needs include the effluent quality that can be achieved and improvements in oxygen transfer and membrane aeration to lower operational costs of MBRs (van Nieuwenhuijzen et al., 2008).

In the United States, 45 percent of wastewater treatment plant effluent as of 2004 received only primary and secondary treatment (see Table 2-1). EPA (2008b) reported that 49 percent of all wastewater treatment plant effluent received “greater than secondary” treatment. This could include MBR treatment or

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