User:Velella/sandbox/STW processes

User:Velela/sandbox

Process selection
Many technical, environmental and economic factors affect the design of a sewage treatment plant and, with the exception of small rural works, each one will be designed to suit the particular conditions encountered.. Each design therefore depends on an analysis of all these factors to determine the optimum combination of processes to suit the expected range of loadings, both with regards to quality of the input, the required output standards and the maximum and minimum rates of flow expected. A life cycle assessment (LCA) can be used, with criteria or weightings attributed to the various aspects. This makes the final decision subjective to some extent. A range of publications exist to help with technology selection.

In industrialized countries, the most important parameters in process selection are typically efficiency in both output quality and energy use, reliability, and space requirements. In developing countries, they might be different and the focus might be more on construction and operating costs as well as process simplicity.

Choosing the most suitable treatment process is complex because of the range of constraints and requirements. Where geography is appropriate a gravity sewerage system may feed the works which may allow a system driven largely by gravity such as trickling filters and a gravity effluent discharge. In low-lying areas energy costs may be more significant because of the need to pump sewage. Where land is expensive, the use of compact systems such as activated sludge processes may be indicated with thermophilic sludge digestion to reduce sludge volumes and generate biogas. The nature of the influent sewage also affects the choice of processes. A high strength and low-flow influent would require much more oxidative treatment capacity than a high flow, low strength influent, which might in turn require much larger settlement capacity. Highly variable strength and flow pose special difficulties in design and may indicate the need for duplicate process units that can be witched in or out depending on the prevailing influent composition. Such solutions are typically found in towns or holiday developments with large seasonal swings in population.

Industrial processes in the catchment can contribute difficult to treat components which may require extended treatment or more specialized processes such a roughing filters, or polishing lagoon provision.

Where treatment plants are installed close to housing or work-places, odor control and disease vector control becomes a significant constraint, especially in sludge handling. Design requirements may include covered sludge processing units with active air extraction and treatment.

Automation of sewage treatment plants has reduced the size of the work-force need to keep the plant operational for many larger works in the developed world treatment have resisted automation including managing issues such as sludge bulking, excess ragging in the influent, sludge transport off site etc. The need for expert personnel remains a constraint especially in high pay, low unemployment areas.