A huge part of contaminants in natural water resources and also in different wastewaters is associated with solids, and thus important removals of polluting species are expected to be achieved by simple and advanced solid/water (S/W) or solid/wastewater (S/WW) separation in adequate operating conditions. Often the effluents from different industries or different natural polluted water resources contain 40-70% of total COD associated with suspended solids (in colloidal and settable forms), and also other contaminants such as metals, bacteria and viruses (size range of 0.01-10 micro m), organic micropollutants (PCBs, PAH) are associated with particulate matter due to their adsorption, agglomeration and incorporation onto solid matrices. Incorporation of S/W or S/WW separation processes prior to advanced chemical or biological treatment such as microscreening, primary sedimentation, direct mechanical filtration, or chemical coagulation-flocculation associated with solids separation, flotation, activated carbon adsorption or membrane processes has improved the performance of treatment steps and increased treatment capacity in advanced chemical or biological units. Consequently, the presentation of the most used S/W and S/WW separation equipments, processes and operations in different treatment systems, overall efficiency and cost is of great importance in this work. All S/W or S/WW separations may be used in any step of a sewage treatment system, but the type of raw sewage limits each S/W or S/WW separation associated with the adopted cleaning regime. Moreover, the water and wastewater services are able to offer efficient products and equipments for the complete water and wastewater treatment sector and worldwide market: (i) very fine screens; (ii) clarifiers, filtration installations (sand-based or multilayers-based filtration); (iii) membrane bioreactors to separate fibrous materials and reduce the COD/BOD ratio of treated discharges in receiving watercourse; (iv) grit separation of digestible organic materials and other mineral materials; (v) sludge treatment (screening-thickening-dewatering-drying from one source). For further improvements, precipitation and coagulation agents are also added, all having a solids’ agglomeration effect. Thus, dissolved and very fine particles from wastewater are converted into ease separable agglomerates to enable filtreable solids to be reduced by up to 95%, COD/BOD by 65% and phosphorus by 60%. In addition, the case study of five different S/WW separation processes applied in a textile wastewater treatment plant (a private cotton manufacturing company) is evaluated in terms of solids and organics removal efficiencies and also risk control of residuals in the treated textile effluent and natural aquatic receptor.