The adsorption/bio-oxidation process is a two-stage modification of the activated sludge process used for wastewater treatment. It consists of a high-loaded A-stage and low-loaded B-stage. The process is operated without a primary clarifier, with the A-stage being an open dynamic biological system. Both stages have separate settling tanks and sludge recycling lines, thus maintaining unique microbial communities in both reactors.
History
Adsorption/bio-oxidation process was invented in the mid-1970s by the professor of the RWTH Aachen University Botho Böhnke. It was based on the finding, made by the German engineer Karl Imhoff in the 50th. Imhoff stated that the treatment efficiency of 60-80 percent could be achieved in highly loaded activated sludge basins.
In 1977 Böhnke published his first article on adsorption/bio-oxidation process. The same year the patent was issued. Extensive research of the following years, conducted by prof. Böhnke together with Bernd and Andreas Diering, ended up in 1985 with the establishment of the company Dr.-Ing. Bernd Diering GmbH. The same year AB-process was for the first time applied in a full-scale at the Krefeld, Germany sewage treatment plant (800 000 P.E.). In 1990, 19 full scale installations existed in Western Germany alone. Further application of the process in Europe was hindered by the tightening of the effluent discharge requirements with respect to nitrogen and phosphorus. The process came into notice in 2000th again due to the increased interest in energy recovery from wastewater.
Principle of operation
The A-stage, or adsorption stage is the most innovative component of the process. It is not preceded by primary treatment. Influent organic matter is removed in the A-stage mainly by flocculation and sorption to sludge due to the high loading rates (2-10 g BOD • g VSS−1 • d−1) and low sludge age (typically 4-10 h). Hydrolysis of complex organic molecules occurs improving biodegradability of the influent of the B-stage. High loading rates and low sludge age favours development of dynamic biocoenosis with a large fraction of microorganisms present in the exponential growth phase. Diverse sludge biocoenosis increase variety of organic compounds that can be degraded in the A-stage and makes the process more stable towards the shock loads. Altogether, up to 80% of the influent organic matter can be removed in the A-stage. The required reactor volume and oxygen supply are lower if compared to the removal in the conventional activated sludge process.
The B-stage, or bio-oxidation stage, is a typical low-loaded activated sludge process, where biodegradation of the remaining organic material occurs. The B-stage can be designed for nitrogen and/or phosphorus removal by alternating aerobic, anoxic and anaerobic zones in the reactor.
Typical operational conditions of the adsorption/bio-oxidation process
Advantages of the process
Lower aeration requirements decrease energy consumption and aeration costs for 20 percent if compared with conventional single stage activated sludge plant. The volumes of aeration tanks are 40% lower if compared with conventional single stage activated sludge plant. Increased sludge production in the A-stage results in increased biogas production in the digester (for plants with anaerobic digestion of excess sludge). Stability towards the shock loads (pH, chemical oxygen demand (COD), toxic substances) explained by the wide-ranging biochemical potential, high mutation capacity and adaptability of sludge in the A-stage. A-stage can receive higher organic loads than conventional activated sludge systems. Effluent concentrations are more stable because of the two-stage process configuration employed. Heavy metals are mainly removed with the A-stage sludge. Therefore, B-stage sludge has lower concentrations of heavy metals than sludge from conventional activated sludge process and may comply with the agricultural standards.
Drawbacks of the process
Incomplete denitrification is often observed in the B-stage if the influent C/N ratio is low. Direct by-pass of a part of A-stage influent with high organic matter content to the B-stage is used to increase the C/N ratio. High sludge production in the A-stage is a drawback for WWTPs that are not equipped with anaerobic digestion of sludge because it increases sludge treatment costs. Sludge from A-stage has poor settling properties. High retention times cause an increased need for additional reactors to maintain throughput increasing equipment costs