Hello, and welcome to the third module about treatment. In the last module we looked at the different anaerobic treatment technologies. Now we will look at the aerobic technologies, which can be used as stand-alone systems or to polish the effluent of anaerobic units. Aerobic systems are biological systems which rely on the action of bacteria needing oxygen to live, like us. Thus, in all of them oxygen provision is a key aspect. Without sufficient oxygen, the environment becomes anaerobic, which leads to processes such as described in the last module. Some aerobic technologies try to imitate nature, such as the "waste stabilization ponds", or the "constructed wetlands". Some of these are more engineered, such as the "trickling filter" or the "activated sludge". Let's start with the waste deposition ponds, or lagoons. It is a system that consists in a succession of ponds with different functions. First one anaerobic pond, then one or two facultative ponds, and finally, one or two maturation ponds. Let's look in more detail at the differences between these three types of ponds. The anaerobic pond is the deepest, with a depth of two to five meters, and also the smallest. It is highly loaded because it receives the rawest water. The anaerobic pond has a hydraulic retention time from one to three days, and its main function is sedimentation and anaerobic stabilization of sludge, thus, settling. The facultative pond or facultative ponds if they are several, are shallower. Usually, less than 1.5 meters, but large. The idea here is to maximize the oxygen supply either through algae, wind, or artificial aeration. The hydraulic retention time there is longer: from ten to twenty days. Here, the main function is the aerobic degradation of suspended and dissolved matter, thus, degradation. Finally, the maturation ponds are even shallower, usually less than one meter, but large. The hydraulic retention time is about ten days. Here the main function is the final sedimentation of suspended solids, the bacterial mass, and pathogens, thus polishing. Waste stabilization ponds can treat high strength wastewater to a high quality effluent. They are generally reliable and good functioning and inexpensive, compared to either centralized options. However, they require a lot of space and may generate bad odors, especially the anaerobic pond, if poorly designed. It requires expert design and supervision, especially to avoid short-circuiting, in which case, the effluent goes from the inlet to the outlet without mixing properly, that is, not staying through the intended retention time. Besides, if they work at best in warm climates, waste stabilization ponds are not always appropriate for colder climates. This picture shows large waste stabilization ponds for the city of Cuenca, in Ecuador. A way to increase the performance of waste stabilization ponds is to provide artificial aeration, which is done in Cuenca in the first pond, upstream of this picture. It is also a good way to upgrade the ponds when then reach design capacity, especially extension, is not always easy. In aerated ponds, mechanical aerators provide oxygen and keep the aerobic organisms suspended to achieve a high rate of organic degradation. Increased mixing and aeration means that the ponds can be deeper and tolerate much higher organic loads than the maturation pond. The depth may be from two to five meters and a retention time from three to twenty days. In some cases, it can even replace the anaerobic pond, thus avoiding the bad odors that anaerobic ponds may generate. The downside of it is that aeration implies a high energy consumption, and thus costs, and requires a constant source of electricity. Interruption of electricity service may cause the pond to turn anaerobic, thus changing completely the inner biological processes. We will now have a look at another family of treatment technologies: the "constructed wetlands". "Constructed wetlands" aim to replicate the naturally occurring processes of a natural wetland, marsh or swamp. We distinguish three types: the "free water surface constructed wetland", the "horizontal subsurface flow constructed wetland", and the "vertical flow constructed wetland". Let's start with the "free water surface constructed wetland". In that case, the water slowly flows through the wetland, particles settle, and pathogens are eliminated through the combined action of sun, settling, adsorption, and predation from higher organisms. The organisms and plants utilize the nutrients. This style of constructed wetland is commonly used as a polishing step after secondary or tertiary treatment processes. It is only appropriate for low strength wastewater. This is a really important fact. The depth of the water is ten to 45 cm above ground level. Wastewater needs to be well distributed at the inlet. Good operation and maintenance is needed for it not to turn into a mosquito breeding area, and to avoid short-circuiting. Let's look now at the "horizontal subsurface flow constructed wetland". Its performance regarding reduction of BOD, suspended solids, and pathogens is higher than in the freewater surface constructed wetlands, and it does not have the mosquito problem as the water flows only under the surface, as its name indicates. As in the previous technology, the good distribution of the influent at the inlet is very important. The gravel bed is between 50 cm and one meter deep. The water level is maintained at 5 to 15 cm below the surface. As wastewater flows horizontally through the basin, the filter material filters out particles. It acts both as a filter and as a fixed surface upon which bacteria can attach, forming large biofilms. The plant roots play an important role in maintaining permeability in the filter. It is very important to have a good primary treatment before the wetland. Otherwise, the risk of clogging will be very high. Expert follow-up is necessary to monitor the plants, especially the startup periods. As a final remark, we have to mention that the wetlands need a lot of space, generally a surface of about five to ten square meters per person equivalent. This picture illustrates the last type of constructed wetland, the "vertical flow constructed wetland", which is, of course, difficult to differentiate from outside. This is the most sophisticated and performance type of wetland By injecting the wastewater from above the whole surface, the distribution is greatly improved. The water flows down and is collected by a drainage system. The important difference between vertical and horizontal wetlands is not simply the direction of the flow path, but rather, the aerobic conditions. The wastewater is applied intermittently, four to ten times a day. Thus, the filter goes through stages of being saturated and unsaturated, and accordingly, different phases of aerobic and anaerobic conditions. With this regime, there is less plugging risk than in horizontal subsurface flow wetland. Other than that, the treatment processes are the same as in the horizontal flow wetland. Because of the mechanical dosing system, this technology is most appropriate where trained maintenance staff, constant power supply, and spare parts are available. Constructed wetlands can reach a good performance with processes close to nature. However, one has to be aware of the risk of clogging and the management of the plants, especially the startup and maintenance, and beware high ammonia levels which may prevent the plants from growing properly. In all cases, a good primary treatment is crucial. Before finishing this module, we will look at two higher end technologies which we may call <i>Conventional</i>: the "trickling filter", and the "activated sludge". These systems are really performant in developed countries, but imply high capital costs, operation and maintenance by skilled personnel, and a constant source of energy. The "trickling filter" is a fixed bed biological reactor that operates mostly under aerobic conditions. After primary treatment, wastewater is continuously trickled or sprayed through the filter, for example, through a rotating sprinkler. Organics are degraded by the biofilm covering the filter material. The filter is usually from one to 2.5 meter high, but some filters may reach a height up to 12 meters. Adequate airflow is important to ensure sufficient treatment performance and prevent odors. The air can circulate vertically through the filter. The advantage of a trickling filter over the technologies presented before is the small land requirement; however, flies and odors are often problematic. The "activated sludge system" is the best known and most widespread in Western countries under different forms, such as sequencing batch reactor, oxidation ditches, moving beds, or membrane bioreactors. It makes use of highly concentrated micro-organisms to degrade organics and remove nutrients, leading to a high quality effluent. To maintain aerobic conditions and to keep the activated sludge suspended, a continuous and well-timed supply of oxygen is required. Aeration and mixing can be provided by pumping air or oxygen into the tank or by surface aerators. Agglomerations of sludge particles called <i>flux form in the aerated tank and are removed in a settler called in that case "<i>secondary clarifier"</i>. We are now at the end of this module on aerobic treatment technologies. We saw very different technologies that can all perform well, but have different requirements. To sum up, we can say that the main decision factors are: the availability of space, financial resources, skills, and electricity. We now reviewed all the technologies for wastewater treatments present in the <i>Compendium</i>. However, as you observed, all these technologies produce sludge. Besides, onsite systems such as <i>pit septic tanks also produce tons of sludge. How to treat sludge? That's what we'll see in the next module. See you then.
