Variation of biomass energy yield in wastewater treatment high rate algal ponds

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CHAPTER 3 Beneficial colonial algal species for wastewater treatment and biomass energy production in high rate algal ponds (HRAP

This chapter is based on the following publication:
Mehrabadi, A., Farid, M. M., Craggs, R. 2017. Potential of five different isolated colonial algal species for wastewater treatment and biomass energy production. Algal Research, 21, 1-8.

Chapter 3. Selection of beneficial colonial species
Chapter preface

While several unicellular algal species have been evaluated extensively for their wastewater treatment and biofuel production potential, there has been little focus on the colonial species that typically predominate in high rate algal ponds and which have the benefit of being easily harvested by cost-effective, simple gravity settling. This chapter investigates the wastewater treatment performance of five wastewater colonial algal species that are common in high rate algal ponds: Mucidosphaerium pulchellum, Micractinium pusillum, Coleastrum sp., Desmodesmus sp. and Pediastrum boryanum and their potential value for biofuel production under simulated New Zealand summer and winter conditions. The results showed Mucidosphaerium pulchellum and Micractinium pusillum were the most beneficial colonial species for both treatment and biofuel production.

Introduction

The techno-economic feasibility of biofuel production from algal biomass has been questioned for several years and it has been highlighted that even the use of conventional raceway ponds for biofuel production alone is not currently financially viable [3, 15, 106-109]. One opportunity to lower algal-based biofuel production costs is where the algal biomass is produced as an essentially free by-product of tertiary-level wastewater treatment in High Rate Algal Ponds (HRAP) [6, 76]. Wastewater treatment HRAPs are a component of enhanced wastewater treatment pond systems which have received much attention worldwide as an upgrade option for traditional wastewater treatment ponds, due to their higher nutrient removal rates and the ability to recover resources in the form of algal biomass [103, 110].
To effectively combine low-cost tertiary-level wastewater treatment and low-cost algal production in WWT HRAPs for biofuel, the WWT HRAP must be mainly dominated by algal species that have both high treatment and energy production potential. The main characteristics of beneficial algal species for wastewater treatment are: 1) high nutrient removal capacity at typical wastewater nutrient loads, 2) ability to grow under seasonally variable environmental conditions, and 3) easy harvest by simple gravity settling [11, 18]. The important characteristics of algal species for low-cost biomass energy yield in WWT HRAP without impacting their wastewater treatment function include: 1) high year-round productivity, 2) high energy content (resulting from beneficial biomass chemical composition), and 3) high settleability to achieve the highest settleable algal biomass yields [76].

Chapter 3. Selection of beneficial colonial species

Over recent years, many studies have been conducted to investigate beneficial algal species for biofuel production using wastewater (municipal/industrial/animal manure) as a nutrient source [100, 111-115]. The majority of these studies have found high biofuel production and wastewater treatment potential of the tested algae. However, only a limited number of algal species including Chlorella sp., Scenedesmus sp., Ankistrodesmus sp., and Monoraphidium sp. have been assessed [100, 111-115] and these are typically motile or poorly-settleable unicellular algae that require chemical flocculation / energy consumption for efficient removal. Moreover, most studies have been carried out under simulated moderate conditions without taking into account natural seasonal variation in performance. In addition nutrient and operational stress conditions (such as cultivation under N starvation or high light intensity) have been suggested to improve algal biomass quality for biofuel production while they may negatively affect wastewater treatment performance [100, 111, 114-117].
While poorly-settleable unicellular species have been cultivated on wastewater under controlled conditions for biofuel production, research on algal-based wastewater treatment ponds has shown that WWT HRAPs are often populated by colonial species which have similar wastewater treatment performance to unicellular species but can be cost-effectively harvested using gravity settling [11, 118]. Park et al. [12] conducted an experiment in outdoor pilot-scale WWT HRAPs populated by >80% Pediastrum boryanum (a readily settleable colonial species) and showed that >80% NH4+-N and 50-75% PO43--P were removed year-round. They found that not only high nutrient removal was achievable by Pediastrum boryanum dominance but also high harvest efficiency was achieved, which would improve the economic viability of WWT HRAP for combined wastewater treatment and low-cost energy production. Our previous study has also shown >70% year-round nutrient removal in outdoor pilot-scale WWT HRAPs dominated by colonial species such as Micractinium sp., Mucidosphaerium pulchellum, Coleastrum sp., Desmodesmus sp., and Pediastrum sp. [119]. In a comparative lab-scale experiment, Sutherland et al. [103] found that under continuous mixing the nutrient removal capacity of colonial species (Mucidosphaerium pulchellum and Pediastrum boryanum) and poorly settleable unicellular species (Chlorella sp.) were similar. However, compared with the two other species, Pediastrum boryanum was highly settleable so that >55% of culture biomass settled at 10 min. While a number of colonial microalgal species have been assessed for wastewater treatment potential [11, 12, 85, 93, 103] there has been little focus on algal biomass quality (i.e. biochemical composition and biomass energy content) for biofuel production. It has been found that colonial species can be maintained in

Chapter 3. Selection of beneficial colonial species

WWT HRAP by recycling a small (<20%) portion of the harvested biomass back to the pond [11, 12, 85]. This could provide an opportunity to maintain the most beneficial species for both wastewater treatment and production of algal biomass for biofuel in WWT HRAP. Therefore, the aim of this study was to investigate the performance of typical WWT HRAP colonial algal species for efficient wastewater treatment (in terms of nutrient removal) as well as biomass energy yield (in terms of growth rate, biochemical composition and energy content) for further use as biofuel feedstock.

1.1 Introduction
1.2 Low-cost algal biomass production in WWT HRAPs
1.3 The effect of microalgae chemical composition on biomass energy content
1.4 Parameters affecting biomass energy production in WWT HRAP
1.5 Algal biofuel options
1.6 Which conversion route is more beneficial?
1.7 Summary and Research needs
1.8 Conclusions
2. Variation of biomass energy yield in wastewater treatment high rate algal ponds
2.1 Introduction
2.2 Materials and methods
2.3 Results and discussion
2.4 Conclusions
3. Beneficial colonial algal species for wastewater treatment and biomass energy production in high rate algal ponds (HRAP)
3.1 Introduction
3.2 Materials and methods
3.3 Results and discussion
3.4 Which species is more beneficial for combined wastewater treatment and biomass energy
production?
3.5 Conclusions
4. Effect of CO2 addition on biomass energy yield in wastewater treatment high rate algal
mesocosms (WWT HRAM)
4.1 Introduction
4.2 Materials and methods
4.3 Results and discussion
4.4 Conclusions
5. Biodiesel production potential of wastewater treatment high rate algal pond biomass 
5.1 Introduction
5.2 Materials and methods
5.3 Result and discussion
5.4 Potential of biodiesel production from WWT HRAP biomass
5.5 Quality of the WWT HRAP biomass-based biodiesel
5.6 Is WWT HRAP biomass a promising feedstock for biodiesel production?
5.7 Conclusions
6. Pyrolysis of wastewater treatment high rate algal pond (WWT HRAP) biomass
6.1 Introduction
6.2 Materials and methods
6.3 Results and discussion
6.4 Is WWT HRAP biomass a promising feedstock for pyrolytic bio-oil production?
6.5 Conclusions
7 Wastewater treatment high rate algal pond biomass for bio-crude oil production
7.1 Introduction
7.2 Materials and methods
7.3 Results and discussion
7.4 Is WWT HRAP biomass a promising feedstock for bio-crude production?
7.5 Conclusions
8 Conclusions
9 Future work
9.1 Biomass energy production in WWT HRAP
9.2 WWT HRAP biomass conversion
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Wastewater treatment high rate algal pond (WWT HRAP) biomass for low-cost liquid biofuel production

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