Unfortunately, around the world, huge amounts of wastewater 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 this categories, some technologies are based on biological processes, aerobic, anaerobic or both. And some others on physical-chemical ones. In this module, we will have a look at the different treatment stages and then focus on pre and post-treatment, which can be implemented in any system. The learning goals of this series of module 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-solids 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 wastewater 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 phosphorus, or micropollutants from the effluent. Thus, re-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 to a few pre-treatment technologies. Pre-treatment is a preliminary removal of wastewater or starch 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 reduce 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 grease interceptors, to remove the fats, oil, and grease. Finally, settling is used, for example, in the sand trap or grit chambers to remove the heaviest 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 desired end use of the effluent or a national standard for discharge in water bodies, a post-treatment step may be required to remove pathogens, residual suspended solids, or dissolved constituents. This can be mainly achieved through tertiary filtration and disinfection techniques. Filtration process can be classified in two categories: First, the depths 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 receiving, for example, through membranes. As for disinfection, it targets the elimination of pathogenic microorganisms. Chlorine is the most widespread technique. Chlorine is mixed with effluent at the outlet of the treatment plant. As one in this figure, with the chlorine diffuser and the 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 by-products, harmful for the environment. It was 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 the household level, at the street level, or at the inlet of a treatment plant. Post-treatment is not always necessary and the pragmatic approach is recommended. The Efland quality should match the intended end use practice or the quality of the receiving water body.
