In the world, according to the information from the world Bank group released on March 19, 2020, only 20% of the wastewater generated is treated, with the rest being discharged directly to surface water, leaving many water sources contaminated.
LAOTSS® treatment technology offers a cost-effective solution that seeks to be environmentally friendly, with its operation mimicking some of the natural phenomena found in self-depurative water bodies, i.e. a river or lake in which the processes of pollutant degradation occur through bacterial action, that in the presence of dissolved oxygen within the water develop an aerobic microbiota, without generating odors. Such processes allow water bodies to maintain conditions appropriate for aquatic life. However, the purification of a river has limitations in terms of pollutant loads and speed at which these pollutants degrade. Therefore, a LAOTSS® system controls the conditions that affect biological mechanisms for optimization, thus reducing the sanitation time from 20 days that would have taken a natural system to less than 24 hours.
The treatment system consists of a tank equipped with aerators with a suspended bacterial culture called activated sludge. The purification mechanism that takes place is very simple: bacteria consume the organic matter present in the water by incorporating it into their metabolism as a carbon source that allows them to obtain energy to develop its functions. Bacteria form agglomerations that are denser than water allowing their separation by sedimentation, once the purification process has been carried out in a biological reactor (tank with suspended bacteria) the water is passed into a sedimentation tank in which water flows slowly allowing bacteria flocs to deposit at the bottom, obtaining a clear effluent from the top of the tank and a concentrate of sludge-like bacteria from the bottom, this sludge is returned to the biological reactor to maintain a high concentration of bacteria some of which will, eventually, be purged form the system.
In Figure 1 we have a schematic representation of the activated sludge process.
The biological process requires that the water in the influent is free of large waste (e.g. bags, cans, etc.) as well as sand. To achieve this, pretreatment facilities are placed before the biological reactor. These systems commonly consist of fine and coarse grids, in which the large solids are retained. In the case of sand, the units designed to remove it from the water are called sand traps, which basically consist of a channel through which the water is circulated at a very low speed, so that the sands suspended in the water are sedimented at the bottom of the channel from which they are constantly removed.
After the pretreatment stage, the water must be sent to the biological reactor, this is commonly done by pumps, for which a lift station is necessary where the pretreated water will stay for few minutes until it is sent to the treatment plant by one or two pumps1.
The purged sludge from the sedimentation is called residual sludge and should be treated to decrease its organic content and remove as much water as possible in order to facilitate its transportation and/or final use, for this purpose the sludge is treated in aerobic or anaerobic digesters and dehydrated commonly in drying beds, although there are other devices such as press filters.
A complete flow diagram of the processing process is presented in Figure 2.
