DESIGN AND OPERATIONAL ASPECTS OF ASP-BIOFILM REACTOR

Abstract

Aerobic hybrid systems exhibiting both suspended and attached growth have been widely in use. These systems, which are termed as hybrid systems have advantages of the both growths. There are various types of hybrid system in use based on the carrier materials. Linpor process and PAC-AS are examples of the hybrid reactors with movable carrier, which use small pieces of sponge and powdered activated carbon for biofilm cultivation respectively. ASP with fixed biomass or aerated biofilm with sludge recycle are typical examples of the hybrid reactors with fixed carrier. In the present day, scenario many of the old treatment plants are not able to meet the effluent standards or the treatment demand, hence either upgrading or expansion is required. Toincrease the capacity for the existing ASP large landarea is required, which may not be available or is very costly. Large area requirement for the extension of the existing activated sludge plant warrants the adoption of hybrid system, as the biofilm can be easily cultivated in the activated sludge reactor. Combination of both growths in the hybrid reactor achieves high efficiency for BOD removal and nitrification even at lower HRT. Further it has lower sludge production, high F/M ratio, good settleability of sludge and small loading on settling tank. The hybrid system, in which the biofilm is cultivated on a fixed carrier, has been studied in the present work. Despite the widespread use of the system, sufficient models are not available for design. This is probably due to the difficulty to include substrate flux into the biofilm in the suspended growth models. In the present work an attempt has been made to develop design tools in the form of design curves (nomographs), mathematical model, and empirical model for faster design. The uses of the above design tools have been exemplified in the respective chapter. The proposed models have been verified by experimental results. As substrate flux is a powerful parameter for biofilm and ASP-biofilm analysis, Saez and Rittmann model as well as Suidan and Wang model has been discussed. These models do not have explicit solution. Hence anew model having an explicit solution has been developed. Moreover, acomparison was made for the above two models of biofilm. It was found that, Saez and Rittmann model is more accurate with respect to the fundamental principals and assumptions of the biofilm in one-dimensional space. In addition to the above work, the operational conditions of ASP-biofilm system have been identified mathematically and verified using an experimental work. As aresult, the best operating conditions of the system such as the sludge age, sludge yield, and nitrification have been identified. The operational conditions about the sludge age revealed that, there exists acritical value of sludge age beyond which the system will not run as a perfect hybrid reactor. Also the optimum and critical conditions for minimum excess sludge and nitrification have been identified. Optimum nitrification and sludge yield are found at low COD level in the reactor. Contribution of the present work shall be useful for the researchers and practitioners. in