PhD report - closing remarks
A PhD report that could just be my proudest achievement to date has just been submitted. This is the closing remarks section. The Executive Summary was posted in a previous post. If you want the full report, please let me know.
8. CLOSING REMARKS
The concept of industrial metabolism requires the maximisation of resource productivity by ensuring that all resources exploited provide the maximum possible products and services. This approach is a key premise to achieving sustainable systems for our society and minimising the environmental burden associated with anthropogenic acitivity. In addressing this, it is valuable to view wastes, including wastewater, as a potential raw material resource. This approach can be used through the wastewater biorefinery concept which is centred on two major outcomes: the treatment of wastewater to the desired water quality and the simultaneous production of products of value, as either commodity or energy products. Unit operations in such a wastewater biorefinery include reactors that can reduce nutrient loads in wastewater in a variety of environments, while producing a range of valuable bioproducts that can potentially sustain jobs, meet commodity needs and fund capacity building through their revenue. This review addresses the requirements of such reactor systems in which microbial bioprocesses are used for the production of commodity polymers while reducing nutrient loads. Because of the non-sterile nature of the wastewater environment, it is best to select micro-organisms for product formation that also fulfill a role in the microbial ecology and culture conditions and product which contribute a selective advantage to the microbial population. Hence, wastewater biorefineries are not suitable for all bioproducts. The overall project considers this application in terms of the model system in which microbial communities enriched for Bacillus species are used for the production of poly-glutamic acid.
Poly-glutamic acid (PGA) is hypothesized to fulfil an ecological role to give Bacillus (or other species producing PGA) a competitive advantage that can be exploited for bioproduction through microbial community engineering in wastewater biorefineries. The reactor design is crucial to achieve this microbial community engineering. In order to achieve the production of relatively pure, N-rich PGA from wastewater, the following need to be addressed:
- Establishment of Bacillus as the dominant organism in the mixed microbial community used for PGA production;
- Determining a reactor configuration suited to retention of biomass while processing large volumes of dilute wastewater;
- Selection of a downstream process train that takes cognisance of the dilute product environment, and intercalates with the reactor system in the optimal manner;
- Selection of a reactor configuration and associated downstream processing unit operations that are appropriate for the production of a low value product in dilute solution.
Two reactor designs have been highlighted that are proposed to fulfill this function as well as contribute to effective product recovery: the Aerobic Granular Sludge (AGS) process, and the hRBC process.
The AGS process is defined by the characteristics of the sludge particles. The granules are defined as "aggregates of microbial origin, which do not coagulate under reduced hydrodynamic shear ('sludge bulking') and which settle significantly faster than activated sludge flocs" (15 sec vs 20 min) (de Kreuk et al 2007a).
The hRBC process is a modified Rotating Biological Contactor (RBC) system. RBC's are non-submerged, attached growth bioreactors, similar to trickling filters, with circular media mounted (approx 3.6m in diameter for standard units) on a horizontal shaft, partially submerged (typically 40%) in the wastewater, and rotated at a speed of one to six revolutions per minute (Grady et al 2011). The circular support media is modified to form a high surface area mesh disc to support an active biofilm.
These reactors were selected for detailed discussion because the management of biofilm detachment - and hence product recovery - are controllable. The support material of the hRBC is relatively planar, while the granular system does not have a carrier, with settling properties that make product recovery possible. In addition, the biofilm zonation in both reactor types (allowing for heterotrophy, nitrification, phosphate removal and denitrification zones intrinsic in the biofilm) is advantageous.
The AGS process is operated as a Sequencing Batch Reactor (SBR). It is a time-oriented process that can be designed and operated to simulate virtually all conventional continuous-flow activated sludge systems, from contact stabilization to extended aeration, making it a useful approach in a laboratory environment. This aspect, combined with the rapid sedimentation velocity possible with aerobic granules allows product formation and downstream processing to form part of the reactor design at discrete steps (Johnson, 2010).
The hRBC process can accommodate a wider range of substrate loads and is a simpler process to operate than the AGS process. The combination of the meshed internal structure and planar disk macro structure gives product recovery options that compliment the AGS process, and the overall operation of the hRBC is thought to promote Bacillus dominance.
PGA is generally accepted to be a product excreted into the bulk solution and not even weakly associated with the biomass. Several biofilm-based approaches allow cultivation at relatively low flow rates and may be used to minimise the separation of the weakly associated polymer from the biomass. In terms of considering PGA recovery, three potential scenarios have to be considered:
- PGA is excreted into the bulk solution and is not associated with the biomass at all
- PGA is excreted, but weakly associated with the biomass. This association is easily disrupted
- PGA is excreted, but strongly associated with the biomass. Significant physical or chemical means need to be employed to disrupt this association.
The AGS and hRBC processes will have different utility towards PGA product recovery depending on which of these scenarios come into play. Scenario 1 and 2 are more suited to the hRBC, while scenario 3 favours the AGS process.
This work carries significance in arguing for a different approach to waste, and serves to introduce the discussion on how to use significant infrastructural limitations to the advantage of wastewater biorefineries. Challenges in optimising the system rather than individual unit processes are highlighted, which include the different hegemonies present in the interface between environmental and bioprocess engineering disciplines. This review particularly considers the aspects to consider when reactors are designed as unit processes in a wastewater biorefinery, and contributes to a better understanding of reactor selection in areas where systems are required to meet the needs of biomass retention, processing of large volumes and integrated product recovery.
This work has broader applicability by complementing current research to understand and better control erosion and abrasion while minimising sloughing (considered in this project as product recovery). The remaining challenge in the engineering of these wastewater biorefinery systems, as with the wider body of work on biofilm processes is directed towards the control of biofilm structure and morphology, which depends on, and at the same time affects, the reactor hydrodynamics and mass transfer characteristics.