Fixed-bed technology
How does the fixed-bed technology work?
In wastewater treatment plants with fixed-bed technology the polluted water is treated in three steps:
Mechanical preliminary clarification
In the preliminary treatment the sewage is clarified from settled and floating materials. By the slow water flow the suspended solids – with bigger density than water’s – settle at the bottom of the containers. Substances with smaller density swim upon and are held back by submersed barriers. The primary treatment is minimum two-staged (2 chambers) to achieve an optimal preliminary clarification.
Biological clarification
The pre-clarified sewage flows to the fixed-bed tanks in gradual free flow. Here the wastewater gets biologically purified by microorganisms growing on the submersed fixed-bed. The fixed-bed consists of plastic units with a high specific surface. The oxygen needed for the degradation process is brought into the water by membrane aerators. These aerators are located under the submersed fixed-bed. For this process a compressor pumps atmospheric air into the aerator. The air is pressed through the small pores of the membrane and is distributed in the wastewater as fine bubbles, the necessary oxygen dissolves in the water. Bringing the wastewater under the fixed-bed in combination with the microstructure of the fixed-bed material provides an optimal mixture of wastewater and injected air. The microorganisms required for the degradation of the water substances are already present in the wastewater. For starting up the plant it is not necessary to apply starting cultures or any other additives.
Mechanical secondary clarification
After the biological stage, in the secondary clarifier the treated wastewater is separated from the excess bacteria which have come off from the fixed-bed. The excess biomass settles at the bottom of the tank and is pumped periodically into the primary clarifier. Here this biomass is stored with the sludge coming from the primary treatment until transportation.
Advantages of the fixed bed technology
Small treatment plants have to adapt to very variable loading and inflow. It demands high adaptability of the system in order to guarantee adequate purification quality.
In periods of less loading microorganisms which are settled on the fixed-bed capsulate themselves. That is how they can survive longer starvation phases without any damage. The only requirement is to have a minimum of oxygen in the water. If there is no accruing wastewater for longer period the control system of the plant can be switched for holiday mode. This way the aeration of the fixed-bed is reduced to 10 % of the normal electricity consumption. When getting back to normal operating conditions the original purification capacity is achieved in a few days by the fast reactivation of bacteria.
In contrast to activated sludge process or sequencing batch reactor (SBR) technologies, fixed-bed plants work effectively without expensive separators and long settling time. The reason for that: the microorganisms come off from the surface of the fixed bed in flakes with high specific weight. They settle afterwards easily in the secondary clarification. The result is optimal settling performance of the secondary treatment and little amount of solid content in the effluent of the plant.
A further advantage of the fixed-bed technology is that the wastewater can flow through the whole treatment plant in gradual free flow. Neither pumps which are susceptible to failures nor expensive programming and process equipments for observing treatment and settling phases are necessary. For this reason overflow of untreated wastewater into the effluent can not happen.
