Overcoming Technological Challenges for the Commercialization of the Circulating Fluidized Bed Bioreactor for Municipal Wastewater Treatment
Abstract
The fluidized bed bioreactor as an attached growth wastewater treatment process has demonstrated advantages over suspended growth processes for municipal wastewater treatment applications. However, previous studies have also demonstrated potentially serious disadvantages in terms of energy consumption and maximum reactor size of high flow applications.
In this work, a cost analysis using the CapdetWorks, supplemented by calibrated model data taken from GPS-X was performed to determine the cost effectiveness of the circulating fluidized bed bioreactor (CFBBR). This study demonstrated that the CFBBR is most cost competitive at low flow below 5 MGD. A 10%-20% reduction in net present values on a 30-year basis was estimated for the circulating fluidized bed bioreactor at flows of 5 MGD and lower, and similar costs (
The cost analysis identified small scale wastewater markets as the best for the CFBBR. One of the largest of such markets is rural China, with over half a billion people living in rural villages in China (and only the capacity to treat 3% of their sewage generated). A study on a pilot-scale twin fluidized bed bioreactor system was conducted in Guangzhou, China, treating a septic tank effluent from a residential building. The TFBBR, demonstrated COD and nitrogen removal rates of 92% and 82%, respectively. It further demonstrated a low biosolids production, corresponding to a cost for biosolids management roughly 50% that of a typical suspended growth treatment process. A cost comparison of estimates for COD addition (to facilitate denitrification) and biosolids treatment for the TFBBR and a conventional attached growth process showed that the TFBBR would be less expensive than conventional processes
To explore the energy and cost saving potential of inverse fluidization for the circulating fluidized bed bioreactor, several expanded mineral materials were tested as carriers for inverse three-phase fluidized bed bioreactors. After overcoming operational challenges, expanded clay as a carrier demonstrated good COD and ammonia removal efficiencies (93% and 98%, respectively) at loadings of 2.2 kgCOD/m3/d and 0.2 kgN/m3/d, similar to previous studies on the inverse fluidized bed bioreactors. However, the observed high suspended biomass concentration indicated that clay could not operate strictly as an attached-growth process, but instead more as a hybrid process of attached and suspended growth.
Accurate methods for estimating liquid velocity would be a key tool for the design of fluidized bed bioreactors, enabling precise delineation of energy demands. Different methods for estimating bed voidage by particles properties and liquid velocity were explored. For low density and low Archimedes number particles, the Khan and Richardson correlation for estimating the n-index of the Richardson-Zaki equation was shown accurate within an average error of ± 3%. Furthermore, using Karamanev’s correlation for the drag coefficient coupled with Newton’s equation for the terminal velocity of free settling particles was accurate within ± 10% error.