Esther Posadas

Carbon and nutrient removal from centrates and domestic wastewater using algal–bacterial biofilm bioreactors

The mechanisms of carbon and nutrient removal in an open algal-bacterial biofilm reactor and an open bacterial biofilm reactor were comparatively evaluated during the treatment of centrates and domestic wastewater. Comparable carbon removals (>80%) were recorded in both bioreactors, despite the algal-bacterial biofilm supported twice higher nutrient removals than the bacterial biofilm. The main carbon and nitrogen removal mechanisms in the algal-bacterial photobioreactor were assimilation into algal biomass and stripping, while stripping accounted for most carbon and nitrogen removal in the bacterial biofilm. Phosphorus was removed by assimilation into algal-bacterial biomass while no effective phosphorous removal was observed in the bacterial biofilm. Carbon, nitrogen and phosphorus removals of 91 ± 3%, 70 ± 8% and 85 ± 9%, respectively, were recorded in the algal-bacterial bioreactor at 10d of hydraulic retention time when treating domestic wastewater. However, the high water footprint recorded (0.5-6.7 Lm(-2)d(-1)) could eventually compromise the environmental sustainability of this microalgae-based technology.

Influence of Biogas Flow Rate on Biomass Composition During the Optimization of Biogas Upgrading in Microalgal-Bacterial Processes

The influence of biogas flow rate (0, 0.3, 0.6, and 1.2 m(3) m(-2) h(-1)) on the elemental and macromolecular composition of the algal-bacterial biomass produced from biogas upgrading in a 180 L photobioreactor interconnected to a 2.5 L external bubbled absorption column was investigated using diluted anaerobically digested vinasse as cultivation medium. The influence of the external liquid recirculation/biogas ratio (0.5 < L/G < 67) on the removal of CO2 and H2S, and on the concentrations of O2 and N2 in the upgraded biogas, was also evaluated. A L/G ratio of 10 was considered optimum to support CO2 and H2S removals of 80% and 100%, respectively, at all biogas flow rates tested. Biomass productivity increased at increasing biogas flow rate, with a maximum of 12 ± 1 g m(-2) d(-1) at 1.2 m(3) m(-2) h(-1), while the C, N, and P biomass content remained constant at 49 ± 2%, 9 ± 0%, and 1 ± 0%, respectively, over the 175 days of experimentation. The high carbohydrate contents (60-76%), inversely correlated to biogas flow rates, would allow the production of ≈100 L of ethanol per 1000 m(3) of biogas upgraded under a biorefinery process approach.

Simultaneous biogas upgrading and centrate treatment in an outdoors pilot scale high rate algal pond

The bioconversion of biogas to biomethane coupled to centrate treatment was evaluated in an outdoors pilot scale high rate algal pond interconnected to an external CO2-H2S absorption column (AC) via settled broth recirculation. CO2-removal efficiencies ranged from 50 to 95% depending on the alkalinity of the cultivation broth and environmental conditions, while a complete H2S removal was achieved regardless of the operational conditions. A maximum CH4 concentration of 94% with a limited O2 and N2 stripping was recorded in the upgraded biogas at recycling liquid/biogas ratios in the AC of 1 and 2. Process operation at a constant biomass productivity of 15gm-2d-1 and the minimization of effluent generation supported high carbon and nutrient recoveries in the harvested biomass (C=66±8%, N=54±18%, P≈100% and S=16±3%). Finally, a low diversity in the structure of the microalgae population was promoted by the environmental and operational conditions imposed.

Microalgae-based Wastewater Treatment

Microalgae-based wastewater treatment (WWT) relies on the ability of phototrophic microorganisms to supply oxygen to aerobic organic pollutants degraders and enhance the removal of nutrients and pathogens. Microalgae photosynthesis also boosts biomass productivity, thereby providing new capabilities for the recovery of energy and nutrients and/or climate change mitigation. Microalgae-based WWT engineering requires optimization of light supply and biomass recovery. Various technology platforms are being researched to enable microalgae-based WWT, but full-scale implementation is currently only realistically achievable in shallow, well-mixed raceway ponds (high-rate algal ponds). While full-scale experience is limited, implementation is increasing and supported by vigorous research efforts worldwide.

Evaluation of the dynamics of microalgae population structure and process performance during piggery wastewater treatment in algal-bacterial photobioreactors

The dynamics of microalgae population during piggery wastewater (PWW) treatment in four open photobioreactors operated at 27days of hydraulic retention time, and inoculated with Chlorella sp. (R1), Acutodesmus obliquus (R2), Oscillatoria sp. (R3) and in the absence of inoculum (R4), were evaluated for 6months. In addition, the algal-bacterial biomass concentration, removal of organic matter, nutrients and heavy metals were also assessed. The results revealed a high diversity and rapid variations in the structure of microalgae populations, Chlorella sp. being dominant in R4 throughout most of the operational period. Steady state average biomass concentration ranged from 2445-2610mg/L in R1-R3 to 3265mg/L in R4. No significant differences were recorded in the removal efficiencies (REs) of total organic carbon (86-87%), inorganic carbon (62-71%), total nitrogen (82-85%) and total phosphorous (90-92%). Finally, Zn-REs accounted for 26% in R3, 37% in R2, and 49% in R1 and R4.

Comparative evaluation of piggery wastewater treatment in algal-bacterial photobioreactors under indoor and outdoor conditions

This work evaluated the performance of four open algal-bacterial photobioreactors operated at ≈26days of hydraulic retention time during the treatment of 10 (×10) and 20 (×20) times diluted piggery wastewater (PWW) under indoor (I) and outdoor (O) conditions for four months. The removal efficiencies (REs) of organic matter, nutrients and zinc from PWW, along with the dynamics of biomass concentration and structure of algal-bacterial population were assessed. The highest TOC-RE, TP-RE and Zn-RE (94±1%, 100% and 83±2%, respectively) were achieved indoors in ×10 PWW, while the highest TN-RE (72±8%) was recorded outdoors in ×10 PWW. Chlorella vulgaris was the dominant species regardless of the ambient conditions and PWW dilution. Finally, DGGE-sequencing of the bacterial community revealed the occurrence of four phyla, Proteobacteria being the dominant phylum with 15 out of the 23 most intense bands.

Seasonal variation of biogas upgrading coupled with digestate treatment in an outdoors pilot scale algal-bacterial photobioreactor

The yearly variations of the quality of the upgraded biogas and the efficiency of digestate treatment were evaluated in an outdoors pilot scale high rate algal pond (HRAP) interconnected to an external absorption column (AC) via a conical settler. CO2 concentrations in the upgraded biogas ranged from 0.7% in August to 11.9% in December, while a complete H2S removal was achieved regardless of the operational month. CH4 concentrations ranged from 85.2% in December to 97.9% in June, with a limited O2 and N2 stripping in the upgraded biogas mediated by the low recycling liquid/biogas ratio in the AC. Biomass productivity ranged from 0.0 g m-2 d-1 in winter to 22.5 g m-2 d-1 in summer. Finally, microalgae diversity was severely reduced throughout the year likely due to the increasing salinity in the cultivation broth of the HRAP induced by process operation in the absence of effluent.

Microalgae cultivation in wastewater

Abstract Research on biological processes for wastewater treatment (WWT) over the past decade has focused on the development of compact treatment processes at the expense of a high energy demand or a loss of valuable nutrients. The recent worldwide interest in the cultivation of microalgae for energy production, along with the need for more sustainable WWT processes, has turned microalgae-based WWT into a promising alternative to bacterial-based processes from an economic and environmental viewpoint. The O 2 produced photosynthetically by microalgae is used for the oxidation of organic matter and NH 4 + , while the autotrophic and heterotrophic metabolism of algal-bacteria consortia can support an enhanced nutrient recovery. This chapter presents a critical state-of-the-art review of the basis and applications of microalgae in the context of WWT in this 21st century.

Biogas Purification and Upgrading Technologies

The fact that most countries do not promote the use of biogas as energy vector via tax incentives entails the need for an optimization of biogas upgrading technologies in order to support a cost-competitive utilization of this renewable energy source. Nowadays, the contaminants present in biogas such as CO2, H2S, H2O, N2, O2, siloxanes, and halocarbons are removed through the implementation of costly and environmentally unfriendly upgrading processes. Conventional biogas upgrading is based on physical/chemical technologies leading to CH4 purities of 88–98% and removal efficiencies of higher than 99% for H2S, halocarbons, and siloxanes. Unfortunately, their high energy and chemical demands limit the environmental and economic sustainability of these conventional biogas upgrading technologies. In this sense, biological processes have emerged in the past decade as an economic and environmentally friendly alternative to conventional biogas upgrading technologies. Thus, biotechnologies such as microalgae-based CO2 fixation, H2-assisted litoautotrophic CO2 bioconversion to CH4, enzymatic CO2 dissolution or fermentative CO2 reduction have been consistently shown to result in CO2 removals of 80–100% with CH4 purities of 88–100%, while allowing the valorization of CO2 into bioproducts of commercial interest (therefore preventing its release to the atmosphere). Similarly, H2S removals > 99% are consistently achieved in aerobic and anoxic biotrickling filters, algal-bacterial photobioreactors, and digesters under microaerobic conditions. In addition, recent investigations have shown the potential biodegradability of siloxanes and halocarbons under both aerobic and anaerobic conditions. This chapter constitutes a state of the art comparison of physical/chemical and biological technologies for the removal of CO2, H2S, halocarbons, and siloxanes from biogas.

Influence of the seasonal variation of environmental conditions on biogas upgrading in an outdoors pilot scale high rate algal pond

The influence of the daily and seasonal variations of environmental conditions on the quality of the upgraded biogas was evaluated in an outdoors pilot scale high rate algal pond (HRAP) interconnected to an external absorption column (AC) via a conical settler. The high alkalinity in the cultivation broth resulted in a constant biomethane composition during the day regardless of the monitored month, while the high algal-bacterial activity during spring and summer boosted a superior biomethane quality. CO2 concentrations in the upgraded biogas ranged from 0.1% in May to 11.6% in December, while a complete H2S removal was always achieved regardless of the month. A limited N2 and O2 stripping from the scrubbing cultivation broth was recorded in the upgraded biogas at a recycling liquid/biogas ratio in the AC of 1. Finally, CH4 concentration in the upgraded biogas ranged from 85.6% in December to 99.6% in August.