Thesis Format

Integrated Article

Overcoming Technological Challenges for the Commercialization of the Circulating Fluidized Bed Bioreactor for Municipal Wastewater Treatment

Author

Michael J. Nelson, The University of Western OntarioFollow

Degree

Doctor of Philosophy

Program

Chemical and Biochemical Engineering

Collaborative Specialization

Environment and Sustainability

Supervisor

Zhu, Jesse

2nd Supervisor

Nakhla, George

Co-Supervisor

3rd Supervisor

Yuanyuan, Shao

Affiliation

Tianjin University, China

Co-Supervisor

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.

Summary for Lay Audience

The circulating fluidized bed bioreactor is a process in which a biofilm of bacteria and other microbes is grown on particles to form bioparticles. The bioparticles are then “fluidized” or mixed by air flow or liquid circulation in the system.

One promising application of this technology is biological wastewater treatment. Typical wastewater treatment processes use suspended, or free-swimming, bacteria to consume organic and nutrient pollutants in wastewater. However, these processes require a great deal of space, energy, and money to build and operate. They also produce a lot of excess waste sludge that must be treated before final disposal.

Previous studies on the circulating fluidized bioreactor have shown that it is capable of treating wastewater at significantly higher rates than conventional processes using smaller reactor sizes; ideal for reducing the land use footprint. Furthermore, it also produces far less waste sludge compared to conventional processes. However, the circulating fluidized bed bioreactor does have a high energy footprint due to its air and water circulation needs. Whether this energy footprint is enough to offset its potential cost saving advantages is the purpose of this research work.

Costing estimates of the CFBBR and competing technologies were generated using a software called CapdetWorks. These estimates provided a detailed look at the cost of individual components of the treatment plants using the different technologies and allowed for the identification of major cost contributors. This provided two major findings, one) where the CFBBR currently stands in terms of cost competitiveness and 2) which areas of the CFBBR process should receive more attention by researchers in order to further improve its cost competitiveness.

One the major cost contributors was the fluidization energy, specifically liquid pumping. To explore reducing this cost, two studies on inverse fluidization (using floating particles instead of settling ones) were done. One study explored methods for predicting liquid pumping requirements based on the particle properties, evaluating several literature sources to determine which was the most accurate. The other study evaluated expanded minerals particles for use in inverse fluidized bed bioreactors and determine their treatment capability for municipal wastewater.

Recommended Citation

Nelson, Michael J., "Overcoming Technological Challenges for the Commercialization of the Circulating Fluidized Bed Bioreactor for Municipal Wastewater Treatment" (2021). Electronic Thesis and Dissertation Repository. 8338.
https://ir.lib.uwo.ca/etd/8338