The basic process consists in the transformation (Oxidation) of the soluble and particular biodegradable constituents in final oxidized products which include carbon dioxide, nitrate/nitrogen and water. The responsible for this conversion is a mixed colony of aerobic microorganisms, which produce extracellular biopolymers to form the sludge flakes; these flakes are settleable and can be separated by gravity from the clarified effluent.
This process can be carried out through different types of aerobic systems with suspended biomass and adherent biomass. In both cases, it is necessary to ensure a sufficiently long contact time between heterotrophic organisms and sewage. Also, you must ensure an appropriate supply of nutrients (N and P) and oxygen. The daily amount of microorganisms produced is in excess respect the purifying needs, so it must be removed from the biological reactor and sent to appropriate treatments, so that the process can maintain high efficiency and operational stability.
Systems with suspended biomass: The proper supply of oxygen is ensured by an aeration system, consisting in fine bubbles disk diffusers, in a station of centrifugal blowers and in an oxygen meter, which continuously monitors the concentration of O2 in water and regulate the speed of the blowers. This system, though being the most efficient under the point of view of the oxygen transfer is however a voice that influences the management costs of an activated sludge plant.
Within the various tested applications, the solution with the lowest energy costs involves the use of channel tanks with flow makers, which ensure a good distribution of the organic load and the mass mixing. In this way, it is possible to double the concentration of MLSS compared to traditional methods, reducing the time of treatment, or it is possible to work with low values of F/M ratio, to increase the process yield, to reduce the amount of activated sludge and to increase the stabilization of sludge directly in the oxidation tank.
Systems with adherent biomass (biofilm): the microorganisms grow on the surface of a support, which constitutes the inert filling material. Depending on the used support, it is possible to subdivide these systems in:
- Systems with fixed supports (trickling beds or bioreactors)
- Systems with mobile supports (systems with biological disks), where the support rotates half-immersed in the wastewater. The microorganisms, adherent on the surface of the discs, treat the effluent during their immersion phase, while they are enriched with atmospheric oxygen during their emersion phase.
In particular, the biofilters deserve a description, because the biofilm of microorganisms develops and grows on the plastic bodies that fill the reactor. The water to be treated enters from the bottom of the basin, is oxygenated by means of blowers and, thanks to the introduced air, is pushed upward inside of vertical pipes that pass through the bed of filler. The aerated water is then distributed to the surface and treated by microorganisms, adhering to the filling bodies; to increase the effect of treatment, the water is re-circulated several times through the bed and then exit in the upper spillway.
Periodically a backwash with air must be performed, in order to separate the metabolic biomass (sludge) from the plastic elements. The sludge is collected on the bottom of the reactor and sent to a small tank that functions as a clarifier/thickener. With this system, a plant can work with a very low organic load, for which the excess of sludge produced is much reduced. The reduction of the contact time is permitted by the particular composition of the biomass. In this case, there is simultaneously the presence of bacteria, algae and fungi that bind in an integrated food chain, reducing the time of removal of organic charge.
The main advantages of biofilters are:
- Possibility to untie the retention time of the biomass from the hydraulic retention time;
- Possibility to increase the biomass concentration, even up to 40 g/l;
- Reduction of volumes and surfaces of the reactors, also thanks to the modular construction;
- Process’s performance are in line with the demands of the limits imposed by local authorities;
- Operative management easier with few maintenance requirements;
- High diversity of the population of bacteria, which leads to better recovery from toxic shock, a quicker adaptation and better applicative versatility in consequence of changes of the organic load.
- Better properties of sludge thickening, so it is not required the process of secondary sedimentation and sludge recirculation.
- This technology can be used either as a pretreatment for high organic loads, and as a preparation for the recycling of treated water.
This technology is an integral part of the treatment process of a Biological plant.