The Biological Treatment Step
In the biological treatment step, the dissolved and fine dispersed organic loads are converted into clean water and sediment solids through bacteria in an aerobe purification process. The sediment solid is often referred to as excess sludge.
In the aerobe biological wastewater treatment there is a differentiation between the activated sludge technology and the fixed film technology.
In the activated sludge systems the biomass is suspended in the wastewater and supplied with oxygen by aerators. The wastewater can either pass continuously through the treatment tanks or can be treated in a batch system (SBR-systems).
In fixed film systems, the biomass grows on supporting plates. The bacteria consume the required oxygen from the surrounding atmosphere. In trickling filters, the growing surface consists of an open grating structure, which allows the biofilm to be surrounded at the same time by wastewater and atmosphere. In immersion disc plants, the biofilm situated on the growing surface is intermittently exposed to the atmosphere and the wastewater to be treated.
For special applications, combinations of the above mentioned technologies are used.
The aerobe biological treatment process leads first of all to the reduction of organic carbon compounds, described by the values of BOD5 and COD. Furthermore, the harmful and volatile ammonia nitrogen is converted into immobile nitrate nitrogen. This process is called nitrification. For both processes, treatment efficiencies of up to 99% can be reached. If the treated wastewater is re-circulated into an upstream reaction tank, denitrification can also be achieved. In this process, the carbon compounds of the reaction tanks detract the oxygen fixed in the nitrate so that the nitrogen can escape in elemental form.
The advantage of the aerobe biological technologies is the almost odorless process of the wastewater treatment.
For high loaded industrial wastewaters, it is best to then use biological anaerobe technologies using a digestive processes.
System S&P® uses the rotating immersion disc technology for the biological wastewater treatment. This technology represents a combination of trickling filter and biological contactors. The rotating immersion disks provide the most energetically efficient technological process, as only the rotor with the biomass turns so as to keep mechanical abrasion minimal. For the dimensioning of the biological rotating immersion disk step, System S&P® follows the ATV-guideline A 281. For other climate zones, this guideline is appropriately modified thanks to the System S&P® experience of more than 20 years. Plants in which the required oxygen is brought into the wastewater via compressed air or surface aerators have an energy demand of 50% more than plants based on biodisk technology.
The high sludge age of the biomass achieved through the fixed film process used by System S&P® proves that the excess sludge production is up to 50% less.