Wastewater treatment
Design engineers and decision makers need to take into account technical and economical criteria of each alternative when choosing a suitable technology.
[7]: 215 Often, the main criteria for selection are: desired effluent quality, expected construction and operating costs, availability of land, energy requirements and sustainability aspects.
In developing countries and in rural areas with low population densities, sewage is often treated by various on-site sanitation systems and not conveyed in sewers.
After treatment, the treated industrial wastewater (or effluent) may be reused or released to a sanitary sewer or to a surface water in the environment.
[11]: 60 Agricultural wastewater treatment is a farm management agenda for controlling pollution from confined animal operations and from surface runoff that may be contaminated by chemicals in fertilizer, pesticides, animal slurry, crop residues or irrigation water.
Agricultural wastewater treatment is required for continuous confined animal operations like milk and egg production.
Where land is available for ponds, settling basins and facultative lagoons may have lower operational costs for seasonal use conditions from breeding or harvest cycles.
[13]: 6–8 Animal slurries are usually treated by containment in anaerobic lagoons before disposal by spray or trickle application to grassland.
By progressively converting dissolved material into solids, usually a biological floc or biofilm, which is then settled out or separated, an effluent stream of increasing purity is produced.
Phase separation may occur at intermediate points in a treatment sequence to remove solids generated during oxidation or polishing.
Disposal options for dried solids vary with the type and concentration of impurities removed from water.
[16] Solids such as stones, grit, and sand may be removed from wastewater by gravity when density differences are sufficient to overcome dispersion by turbulence.
More complex clarifiers also have skimmers to simultaneously remove floating grease such as soap scum and solids such as feathers, wood chips, or condoms.
Secondary treatment converts organic compounds into carbon dioxide, water, and biosolids through oxidation and reduction reactions.
[18]: 363–408 Disinfection by chemical oxidation kills bacteria and microbial pathogens by adding hydroxyl radicals such as ozone, chlorine or hypochlorite to wastewater.
[2]: 1138 Filtration through sand (calcium carbonate) or fabric filters is the most common method used in municipal wastewater treatment.
