BAF Filtration Mode And Backwashing Control Based On Headloss Analysis

Because of shortage of land in urban area with the rapid development of economy and urbanization, the development of some public facilities, such as sewage treatment plants, are limited. Biological Aerated Filters (BAFs) play important role in developing new wastewater treatment plants and upgrading of existing plants because of several advantages of small footprint, lower operation cost and investment of capital construction, promising treatment performance, convenient for biofilm inoculation and quick start-up ability. However, there are still some defects in the developing of this new technology, for example, the treatment mechanism of BAF is unclear and sometime the operation stability is not satisfied because of backwashing frequently. With the increasing application of BAF in wastewater treatment and water reclaim project, it is urgent to reveal the characteristics of the biofilter clearly for improving the operation stability and treatment performance of BAF.In order to develop the treatment mechanism and optimize the operation controlling of BAF, for improving treatment efficiency and operational stability, the development of headloss was analyzed in this paper, based on the theoretical analysis, mathematical simulation and experimental fitting. A novel biological-physical filtration model of BAF was established based on Kozeny-Carman filtration equation and Eckenfelder equation, and the procedure, operational condition and control countermeasure of BAF backwashing were also discussed.An average porosity ratio equation was established to describe the development of headloss in a clean filter by the method of porosity decreasing, which is described as follows:Based on tradition Kozeny-Carman filtration equation, a physical filtration equation in clean filter was established by subsection analysis of infinitesimal element, which is described as follows: This equation is considered to characteristic the development of headloss along filter bed. Therefore, this equation can overcome the limitation of tradition filtration equation which could only calculate the total headloss. The results of experiment fitting showed that this equation could be used to describe headloss development in clean filter accurately.The headloss development in BAF is the result of synergistic effect of physical interception and biological oxidation. The biological-physical filtration mode must be established on basis of physical filtration and biofilm accumulation, i.e. the novel filtration equation should represent the biological-physical process in BAF. So that the Eckenfelder equation is introduced to porosity model to describe the biofilm accumulation, and then the headloss development model is coupled with physical holding up and biological oxidation. Some simplified parameters, such as water percentage and density of accumulation, were adopted in the process for establishing a hybrid biological-physical filtration equation. The BAF biological-physical filtration equation is obtained by theoretical analysis and mathematical simulation, which is described as follows:Experimental fitting results shown that this novel BAF biological-physical filtration equation could provide a theoretical basis for predicting headloss development. And then, the control and optimization of backwashing condition were carried out in this paper, in order to improve BAF operational stability and treatment efficiency.By comprehensive consideration of the experimental results and technical economical analysis, the optimized backwashing procedure was proposed as follows: Firstly, backwashing air-flow intensity is 12 L/(m2·s)to wash 2~4min. Secondly, air-flow intensity is 12 L/(m2·s) and water-flow intensity is 6 L/(m2·s) to wash 4~6 min. Finally water-flow intensity is 8 L/(m2·s) to wash 7~9 min. The experimental results showed a perfect headloss recovery and a stable effluent quality after backwashing. Although the turbidity and COD increased slightly after backwashing, but that could recover to normal level in 2 h. The results showed that the proposed backwashing procedure can recover the flux of bio-filtration effectively and not destroy the treatment performance. Furthermore, an alternative backwashing procedure can be employed in BAF wastewater treatment plant in which BAFs was usually arranged as multiunit mode, thus the effluent quality can be improved through the mix and dilution of initial effluent after backwashing.Through an analysis of the development of headloss in a bioreactor, a novel biological-physical filtration equation of BAF was established in this paper. This research will provide a new idea and primary basis for more investigation on the development of headloss and the deep analysis of treatment mechanism of BAF. And that the results of investigation of backwashing optimization and control countermeasures will also be useful for the improving of treatment efficiency and operational stability of BAF.