The material presented in Part I is intended to provide insight into the possible land treatment planning objectives, the status of land treatment research and implementation, the renovative processes that occur in the various components of these systems, and the potential for optimizing the configuration of these components. The structure and application of nine models, which include methods to optimize the regional planning, design and operation of slow-rate land treatment systems, are briefly discussed. General comments follow on the overall status of research in land treatment modeling and design and directions for future work. (Author).