Unfortunately, around the world, huge amounts of waste water and sludge end up untreated into rivers, lakes, and into the sea. What does it take to treat them? I will now introduce you to the centralized and semi-centralized treatment technologies. The treatment process can be divided into several stages. The pre-treatment, the primary treatment, the secondary treatment, the tertiary treatment, or post-treatment. Within these categories, some technologies are based on biological processes: aerobic, anaerobic or both. And some others on physical, chemical ones. In this module, we'll have a look at the different treatment stages and then focus on pre- and post-treatment, which can be implemented in any system. we will review the anaerobic technologies, The learning goals of this series of modules on treatment are: get to know the different treatment technologies, understand the different treatment stages and processes, and understand the different functions of the treatment technologies. Let's start. First, let's review the different treatment stages. The two main treatment stages are called primary and secondary. "Primary treatment" consists in the liquid/solid separation. For example, through sedimentation. As for the "Secondary treatment", it consists of the removal of organic matter and suspended solids. The "pre-treatment" is the preliminary removal of waste water or sludge constituents, such as oil, grease and various solids, like sand and trash. Pre-treatment is a must. On the other hand, the "tertiary treatment" and "post-treatment" consist of removing elements like remaining pathogens, nutrients such as nitrogen and phosphorous, or micropollutants from the effluent. Tertiary treatment embraces a large spectrum of technologies, and most of them are not implemented in low- and middle-income countries. Too expensive and too complex. Let's have a look at a few pre-treatment technologies. Pre-treatment is a preliminary removal of waste water or sludge constituents, either before conveyance, or before treatment systems. It is meant to prevent the accumulation of solids in further stages and avoid blockages. It can also help produce abrasion of mechanical parts and extend the life of sanitation infrastructure. Pre-treatment technologies use physical removal mechanisms such as screening, flotation, settling, and filtration. "Screening" is used, for example, in the screens to prevent trash and coarse solids to end up in the sewerage system or the treatment plant. "Flotation" is used, for example, in grease traps and grids interceptors to remove the fats, oil and grease. Finally, "settling" is used, for example, in the sand trap, or grit chambers, to remove the heavier solids. As you can see in this figure, grease traps and grit chambers can be combined in one technology. On the other end of the treatment process, there are different possibilities to polish the effluent: the so called post- treatment technologies. Depending on the design and use of the effluent, or on national standards for discharging water bodies, a post-treatment step may be required to remove pathogens, residual suspended solids, or its soil constituents. This can be mainly achieved through tertiary filtration and disinfection techniques. Filtration process can be classified in two categories: first, the depth, or "packed-bed filtration", as shown in this figure, where the effluent passes through a filter medium, such as sand or activated carbon. On the other hand, “surface filtration“ removes undesired elements through sieving; for example, through membranes. As for "disinfection", its target is the elimination of pathogenic micro-organisms. Chlorine is the most widespread technique. Chlorine is mixed with effluent at the outset of the treatment plant, as shown in this figure, with a chlorine diffuser and a chlorine mixer. You can also do disinfection through UV radiation or ozonation, the two being more energy intensive. However, you have to be cautious with disinfection. For example, chlorine together with organic matter can form toxic byproducts, harmful for the environment. It should thus only be applied if strictly necessary. You don't want to create more problems than you actually solve. To sum up, we saw that treatment is a succession of different steps, the main ones being the primary and secondary treatment. Pre-treatment is a must, as a kind of preventive measure, either at the household level, at the street level, or at the inlet of a treatment plant. Post-treatment is not always necessary, and a pragmatic approach is recommended. The effluent quality should match the intended end use practice, or the quality of the receiving water body.
