Among others, the following components are welcome: • oil and grease separators; • biological reactor, for example activated sludge, aerated fixed-bed, sequenced batch reactor; • secondary clarifier (guaranteeing very low suspended solid concentrations in the effluent) for example laminar settling. A compact system, with relatively simple installation procedure, will lead to greater energy and material efficiency. For example, facilitating the mixing and supply of raw materials such as oxygen leads to greater energy savings and consumables. In addition to the savings in energy and materials, the less land use lead to consider this system as economically more advantageous and with higher performances when compared to other existing products already in the market. This kind of solutions is an appropriate wastewater treatment solution for small residential buildings, holiday homes and other facilities with a low number of inhabitants. Also for larger applications such as, specific industrial facilities, housing areas, isolated campsites or research stations, compact wastewater treatment plants can be applied for the purpose of sewage treatment. Besides being compact, those solutions should also be flexible in order to adapt to different industrial effluents, or to specific sizing parameter, for example in seasonally operated facilities, such as hotels or agro-industries showing fluctuations in organic load and flow rate and toxicity peaks. Treatment plants must be especially capable of adapting to changing conditions and eventually have stand-by mode during off-season.