Considerations on the operation of high rate algal ponds for wastewater treatment and microalgal biomass production

Author: Paul Young

Young, Paul, 2020 Considerations on the operation of high rate algal ponds for wastewater treatment and microalgal biomass production, Flinders University, College of Science and Engineering

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High rate algal ponds (HRAP) are novel natural wastewater treatment systems which have been shown to have many advantages over other similar systems. In recent years they have also received considerable interest as the most promising system for coupling microalgae cultivation for biofuel feed and wastewater treatment. However, despite their advantages and potential, their application has been sluggish. The overall aim of this thesis was to investigate key factors limiting HRAPs application as wastewater treatment systems and microalgae bioreactors using large-scale, operational systems.

One major factor limiting HRAPs application as wastewater treatment systems is their absence as a treatment option in any official regulatory guidelines. Chapter 3 recounts the only independent validation of a HRAP system for inclusion as a wastewater treatment system option in official regulatory guidelines – the South Australian Community Wastewater Management Scheme (CWMS). Validation involved assessing the HRAP system’s ability to inactivate three indicator organisms under winter conditions. The system met the national guideline validation objectives, resulting in two HRAP system designs being included in the CWMS. It is hoped this result will lead to the wider application of HRAPs, with a system based on one of these designs having already been constructed in Peterborough, Australia.

With the inclusion of a HRAP system as a wastewater treatment option in the CWMS likely resulting in their wider application, a well-designed pathogen inactivation model would help guide the design and operation of new systems. Unfortunately, there has been little research in this area with the only such model published 16 years ago. Chapter 4 describes the development and validation of a mechanistic pathogen inactivation model for HRAPs. The model attributes pathogen inactivation in HRAPs to solar radiation and uses laboratory measured inactivation values. This design is unique for pathogen inactivation models and allows for greater utility and flexibility when compared to traditional models based on system measurements. The model was successfully validated for two indicator organisms using a large-scale, operational HRAP. These results support the model design and encourage its further development. The model also provided valuable insight into HRAPs operation that will guide the design of future systems.

After investigating the factors limiting HRAPs application as wastewater treatment systems, it was decided to investigate the key factors limiting their other main application as combination wastewater treatment systems and microalgae bioreactors. Biomass productivities below economically viable levels is one of the most significant limitations to this application. This is presumed to be caused by insufficient carbon in the wastewater, with the addition of CO2 the most cited solution. Chapter 5 outlines a case study on the effect continuous CO2 enrichment of wastewater has on HRAP wastewater treatment and biomass productivity. A HRAP was retrofitted into a major wastewater treatment plant and received secondary treated wastewater enriched with CO2 by industrial biogas scrubbers. An identical HRAP receiving identical wastewater that had foregone enrichment was used as a control. CO2 enrichment had no significant effect on biomass productivity and had a slightly negative effect on wastewater treatment – suggesting the microalgae were not carbon limited. This study is believed to be the closest representation in the literature to how such a design would perform in the real-world and the only study at such a scale to employ an adequate control.

Another major limitation of using HRAPs treating wastewater to cultivate microalgae for biofuel production is the lack of a cost-effective harvesting method. Autoflocculation, via magnesium hydroxide precipitation, is considered a potential method; however, it has not yet been demonstrated in wastewater treating HRAPs at a large-scale. Chapter 6 details the assessment of autoflocculation, via magnesium hydroxide precipitation, as a harvesting method for microalgae in HRAPs treating wastewater. Autoflocculation was induced in a large-scale, operational HRAP containing 33 m3 of HRAP treated wastewater populated by a heterogenic mix of wild strain microalgae. A high level of flocculation efficiency, solids removal and nutrient removal was observed, suggesting this is a viable method for harvesting microalgae and treating wastewater in HRAPs. However, limitations with the method were identified. This is believed to be the largest assessment of microalgae autoflocculation in HRAP treated wastewater and the only assessment conducted in-pond.

Overall, this thesis presents a unique collection of work on large-scale, operational HRAPs that not only supports the application of HRAPs as wastewater treatment but also provides essential information regarding their real-world application. It is also clear from this work that further research is required before HRAPs treating wastewater can be considered viable microalgae bioreactors for biofuel production, with previously overlooked complications to their real-world application elucidated. Nevertheless, this should not be considered an impediment to their application as wastewater treatment systems.

Keywords: wastewater, microalgae, biomass, high rate algal ponds

Subject: Water Management thesis

Thesis type: Doctor of Philosophy
Completed: 2020
School: College of Science and Engineering
Supervisor: Howard Fallowfield