Basic unit operations in wastewater treatment

Abstract

Municipal wastewater treatment typically comprises a string of physical, chemical, and biological processes aimed at the removal of the polluting load and the production of a final product that can safely be disposed of in watercourses and=or reused. Of the various processes involved, physical processes, which are also commonly referred to as unit operations, play a major role throughout the various treatment stages. In a conventional wastewater treatment plant (WWTP), which is schematically illustrated in Figure 3.1, one can easily identify several such stages and typical efficiencies in terms of biochemical oxygen demand (BOD) and suspended solids (SS) removal. 1. Preliminary treatment (also referred to as pretreatment): it aims to remove bulky and large solids, thus preconditioning the effluent prior to the remainder of the treatment. This stage involves several unit operations such as screening, comminution, sedimentation for grit removal, skimming, and flow equalization. 2. Primary treatment: unit operations such as sedimentation and flotation are employed to remove the suspended and colloid fractions of the effluent. 3. Secondary treatment: it aims to remove organic material through biological processes. 4. Tertiary or advanced treatment: it aims to remove nutrients such as nitrogen and phosphorous, residual suspended solids, inorganics and toxic and refractory organics that have escaped from previous stages. This can be done through physical (e.g., filtration, screening, air stripping, adsorption, ion exchange), chemical (precipitation, oxidation), or biological processes. However, with the exception of very few countries that have nutrient removal regulations for ecological reasons, typical WWTPs do not go beyond secondary treatment. 5. Disinfection removes water pathogens by chemical (e.g., chlorination or ozonation) or physical processes (e.g., UV irradiation). The treated effluent can then be safely discharged in natural receivers or partially reused (e.g., for irrigation). 6. Sludge treatment includes processes such as thickening, dewatering, drying, and digestion and aims to reduce the volume of the sludge to be handled as well as to stabilize biologically the final product, which is usually then sent to landfills (although it also may be used as fertilizer or fuel). The aim of this chapter is to describe the principles of the main unit operations typically employed in WWTPs such as sedimentation, flotation, screening, skimming, and flow equalization. It should be pointed out that particular emphasis is given on sedimentation, since it is found in nearly every single treatment stage. On the other hand, unit operations associated with tertiary treatment are not discussed here, since they lie outside the scope of this book. © 2007 Springer Science+Business Media, LLC.