PhD report - Executive Summary
1. EXECUTIVE SUMMARY
This review discusses bioreactor considerations to leverage the value to be obtained from waste. Section 2 gives a general introduction to the wider project, which takes lessons from both environmental and bioprocess engineering. This project is concerned with proving the technology to produce valuable products from dilute waste streams, and less concerned about the performance and application of the produced material. Reactor design is the main area that can be optimized for bioproduction, to place greater emphasis on the optimal functioning of the system as a whole.The energy utilisation of reactors needs to be improved, and the reactor needs to be designed with downstream processing in mind.
Section 3 introduces the reader to the concept of wastewater biorefineries. The wastewater biorefinery is established to maximize biomass productivity by ensuring that, not only is the wastewater treated to the necessary standard (yielding the outgoing water product), but that components removed from this wastewater are converted to products of value (economically or socially or both). The concept of a wastewater biorefinery is built around the overarching ability to function in the face of significant infrastructural limitations and, through its operation, improve these limitations.
Wastewater biorefineries move away from the traditional wastewater treatment objectives.Section 4 discusses how the system objectives change, and are affected by the differences in approach between the different stakeholders involved in their design and operation. From a process point of view multiple aspects need to be considered, including
- Efficient product recovery of both the bioproduct and water product;
- Energy efficiency through utilising both biomass retention and product retention;
- The ability to be retrofitted into existing plants;
- The implications of maintaining product productivity in non-sterile, mixed, cultures.
The social implications that arise when a new discipline emerges needs to be considered. These develop from disciplines that differ in their approach to reactor design, the objectives that the reactor is designed for and the prevalent culture and values. Emerging disciplines often co-incide with changing generations, which influence the preferred technologies used and ease of adapting to more advanced forms of analysis. The different backgrounds in especially the younger generation often lead to tensions, where bioprocess engineers often have a stronger scientific background, which leads to a different professional discourse. As the field of wastewater biorefineries develops its own language and discourse, supportive technologies mature and the boundaries between the parent biotechnologies are reduced, development of new design parameters and the development of new design approaches will occur, leading to a new generation of professionals. Section 4.3 highlights some of these tensions in the research environment, where data obtained in the laboratory for bioprocessing purposes often cannot be applied directly to wastewater bioprocesses. Section 4 also highlights emergent phenomena that arises when existing systems are adapted for use that they were not originally intended for.
Section 5 illustrates the suitability of Bacillus as a production organism in a mixed culture, specifically in its capacity to produce polyglutamic acid (PGA). Section 5.1 gives a brief introduction to the widespread use of Bacillus in industry. PGA's role in serving an ecological function is highlighted, which supports its value towards contributing to Bacillus dominance and hence higher potential productivity. The rationale of this project is illustrated by highlighting the uses of PGA in industry, most notably that of bioflocculant and soil conditioner. Section 5.4 argues for the production of PGA in wastewater biorefineries by showcasing previous work that produced PGA from waste resources. The value of using Bacillus in wastewater biorefineries is used as an example to show how working with Nature can give overall benefit to both humans and the environment. In Section 5.5 the positive role that Bacillus plays in wastewater treatment is discussed, alluding to the possibility to improve the natural system while providing opportunity for economic gain. Section 5.6 shows how producing PGA, using Bacillus, from a dilute waste can provide benefits over using conventional bioprocessing.
The first half of this review considers which factors are crucial to develop a bioreactor in a wastewater biorefinery. Section 6 provide the required review of reactor systems available to meet the needs of biomass retention, processing of large volumes and integrated product recovery. While not considered in this project, solid substrate fermentation (SSF) reactors are included for its potential in using biosolids to produce valuable products. Based on the combination of the need for biomass retention and the need for product recovery, the two reactors that were selected for further discussion is the aerobic granular sludge (AGS) process, an example of a particle based biofilm reactor (Section 6.6) and a hybrid rotating biological contactor (RBC) based on the HYbrid Bacillus ACtivated Sludge (HYBACS) process (Section 6.7). Scale up will not be considered in this project, but where possible the relevent calculations and data will be included to develop scale-up effectively and build confidence in reactor types as viable alternatives to the stirred tank reactor (STR). Specifically, laboratory data will be generated with an awareness of that required to inform scale-up.
This review focuses on the reactor configurations to produce PGA-containing product to be used for bioflocculant or soil conditioning applications. Downstream processing (DSP) is often only considered after the reactor is designed and separately to it. This may lead to an unoptimised system. To design the reactor system with DSP in mind to facilitate their combined optimisation, a brief overview and proposed DSP options are given in Section 7.
Closing remarks are given in Section 8, and Section 10 lists the numerous acknowledgements, illustrating that with the more transdisciplinary approach required when optimising the system as a whole, more and more players become important.
The greatest value in wastewater biorefineries lies not in the technology or adequate engineering design, but in the connections between people, adequate communication and a willingness to engage.