systems, except for many that are used for agricultural irrigation, almost always employ filtration New directions and concerns in wastewater treatment are evident in various specific areas. The changing be treated, emerging health and environmental industrial was stes, and the impact of new regulations, are among the most important. Further, other important concerns include ng infrastructure.(2) new methods of process sis and control (3 treatment plant performance and reliabil ity, (4)wastewater disinfection, (5) combined sewer overflows. (6) impacts of stormwater and sanitary overflows and nonpdint sources of pollution, (7) separate treatment of return flows,(8)odor control and the control of VOC emissions, and (9) retrofitting and Some of the problems that have to be addressed in the United States deal with renewal of the aging wastewater collection infrastructure and upgrading of treatment plants. Issues include replacement of leaking and undersized sewers. control and treatment of overflows from sanitary and combined collection systems, control of nonpoint discharges, and upgrading treatment systems to achieve higher removal levels of specific constituents ollection systems. particularly those in the older cities in the eastern and midwestern United States, are older than the treatment plants. Sewers constructed of brick and vitrified clay with mortar ioints. for example, are still used to carry sanitary wastewater and stormwater. Because of the age of the pipes and ancillary structures, the types of materials and methods of construction, and lack of repair, leakage is common. Leakage is in the form of both infiltration and inflow where water enters the collection system, and exfil tration where water leaves the pipe. In the former case, extraneous water has to be collected and treated, and oftentimes may overflow before treatment, especially during wet weather. In the latter case, exfiltration causes untreated wastewater to enter the groundwater and/or migrate to nearby surface water bodies. It is interesting to note that while the standar r treatment have increase significantlv, comparatively little or no attention has been focused on the discharge of untreated wastewater from sewers through exfiltration. In the future, however, leaking sewers are expected to become a major concern and will require correction Because of the changing characteristics of the wastewater, studies of wastewater treatability are increasing. especially with reference to the treatment of specific constituents. Such studies are especially important where new treatment processes are being considered. Therefore, the engineer must understand the general approach and methodology involved in:(D assessing the treatability of a wastewater(domestic or industrial).(2) conducting laboratory and pilot plant studies, and(3) translating experimental data into Computational fluid dynamics (CFD) computer-based computational methods for solving fundamental equations of fluid dynamics (ie, continuity, momentum, and energy), is now being used to improve and optimize the hydraulic performance of wastewater treatment facilities. Applications of CFD include the design of new systems or the optimization of systems such as vortex separators, mixing tanks. sedimentation tanks. dissolved-air flotation units. and chlorine contact tanks to reduce or eliminate dead zones and short circuiting Improved UV disinfection systems are being designed using CFD. One of the main advantages of CFD is simulating a range of operating conditions to evaluate performance before designs and operating changes are finalized. Another advantage is that dynamic models can be integrated with the process control system to optimize ongoing operation Important factors in process selection and design are treatment plant performance and reliability in meeting permit requirements. Because wastewater treatment effluent quality is variable because of varving organic loads changing environmental conditions. and new industrial discharges it is necessary to design the treatment system to produce effluent concentrations equal to or less than the limits prescribed by the maintained such as in reuse applications. On-line monitoring of critical parameters such as total organic carbon(TOC), transmissivity, turbidity, and dissolved oxygen is necessary for building a database and for improving process control, Chlorine residual monitoring is useful for dosage control, and pH monitoring assists in controlling nitrification systems e defined as the probability that a system can meet established performance criteria consistently over extended periods of time. Two components of reliabilitv. the inherent reliability of the process and mechanical reliability will be discussed. As improved microbiological techniques are developed, it will be possible to optimize the disinfection process The need to conserve energy and resources is fundamental to all aspects of wastewater collection. important parts of treatment plant design and operation as significant savings can be realized by selecting 1-61-6 systems,except for many that are used for agricultural irrigation, almost always employ filtration. New directions and concerns in wastewater treatment are evident in various specific areas . The changing nature of the wastewater to be treated, emerging health and environmental concerns, the problem of industrial wastes, and the impact of new regulations, are among the most important. Further, other important concerns include: (1) aging infrastructure, (2) new methods of process analysis and control, (3) treatment plant performance and reliability, (4) wastewater disinfection, (5) combined sewer overflows, (6) impacts of stormwater and sanitary overflows and nonpdint sources of pollution, (7) separate treatment of return flows, (8) odor control and the control of VOC emissions, and (9) retrofitting and upgrading wastewater treatment plants. Some of the problems that have to be addressed in the United States deal with renewal of the aging wastewater collection infrastructure and upgrading of treatment plants. Issues include repair and replacement of leaking and undersized sewers, control and treatment of overflows from sanitary and combined collection systems, control of nonpoint discharges, and upgrading treatment systems to achieve higher removal levels of specific constituents. Portions of the collection systems, particularly those in the older cities in the eastern and midwestern United States, are older than the treatment plants. Sewers constructed of brick and vitrified clay with mortar joints, for example, are still used to carry sanitary wastewater and stormwater. Because of the age of the pipes and ancillary structures, the types of materials and methods of construction, and lack of repair, leakage is common. Leakage is in the form of both infiltration and inflow where water enters the collection system, and exfiltration where water leaves the pipe. In the former case, extraneous water has to be collected and treated, and oftentimes may overflow before treatment, especially during wet weather. In the latter case, exfiltration causes untreated wastewater to enter the groundwater and/or migrate to nearby surface water bodies. It is interesting to note that while the standards for treatment have increased significantly, comparatively little or no attention has been focused on the discharge of untreated wastewater from sewers through exfiltration. In the future, however, leaking sewers are expected to become a major concern and will require correction. Because of the changing characteristics of the wastewater, studies of wastewater treatability are increasing, especially with reference to the treatment of specific constituents. Such studies are especially important where new treatment processes are being considered. Therefore, the engineer must understand the general approach and methodology involved in: (1) assessing the treatability of a wastewater (domestic or industrial), (2) conducting laboratory and pilot plant studies, and (3) translating experimental data into design parameters. Computational fluid dynamics (CFD), computer-based computational methods for solving the fundamental equations of fluid dynamics (i.e., continuity, momentum, and energy), is now being used to improve and optimize the hydraulic performance of wastewater treatment facilities. Applications of CFD include the design of new systems or the optimization of systems such as vortex separators, mixing tanks, sedimentation tanks, dissolved-air flotation units, and chlorine contact tanks to reduce or eliminate dead zones and short circuiting. Improved UV disinfection systems are being designed using CFD. One of the main advantages of CFD is simulating a range of operating conditions to evaluate performance before designs and operating changes are finalized. Another advantage is that dynamic models can be integrated with the process control system to optimize ongoing operation. Important factors in process selection and design are treatment plant performance and reliability in meeting permit requirements. Because wastewater treatment effluent quality is variable because of varying organic loads changing environmental conditions, and new industrial discharges, it is necessary to design the treatment system to produce effluent concentrations equal to or less than the limits prescribed by the discharge permit. Reliability is especially important where critical water quality parameters have to be maintained such as in reuse applications. On-line monitoring of critical parameters such as total organic carbon (TOC), transmissivity, turbidity, and dissolved oxygen is necessary for building a database and for improving process control. Chlorine residual monitoring is useful for dosage control,and pH monitoring assists in controlling nitrification systems. Treatment plant reliability can be defined as the probability that a system can meet established performance criteria consistently over extended periods of time. Two components of reliability, the inherent reliability of the process and mechanical reliability, will be discussed. As improved microbiological techniques are developed, it will be possible to optimize the disinfection process. The need to conserve energy and resources is fundamental to all aspects of wastewater collection, treatment, and reuse. Operation and maintenance costs are extremely important to operating agencies because these costs are funded totally with local moneys. Detailed energy analyses and audits are important parts of treatment plant design and operation as significant savings can be realized by selecting