Algal biofuels from wastewater treatment high rate algal ponds

Abstract

This paper examines the potential of algae biofuel production in conjunction with wastewater treatment. Current technology for algal wastewater treatment uses facultative ponds, however, these ponds have low productivity (∼10 tonnes/ha.y), are not amenable to cultivating single algal species, require chemical flocculation or other expensive processes for algal harvest, and do not provide consistent nutrient removal. Shallow, paddlewheel-mixed high rate algal ponds (HRAPs) have much higher productivities (∼30 tonnes/ha.y) and promote bioflocculation settling which may provide low-cost algal harvest. Moreover, HRAP algae are carbon-limited and daytime addition of CO(2) has, under suitable climatic conditions, the potential to double production (to ∼60 tonnes/ha.y), improve bioflocculation algal harvest, and enhance wastewater nutrient removal. Algae biofuels (e.g. biogas, ethanol, biodiesel and crude bio-oil), could be produced from the algae harvested from wastewater HRAPs, The wastewater treatment function would cover the capital and operation costs of algal production, with biofuel and recovered nutrient fertilizer being by-products. Greenhouse gas abatement results from both the production of the biofuels and the savings in energy consumption compared to electromechanical treatment processes. However, to achieve these benefits, further research is required, particularly the large-scale demonstration of wastewater treatment HRAP algal production and harvest.

The Environmental Biorefinery: Using Microalgae to Remediate Wastewater, a Win-Win Paradigm

“…CO 2 can be added in a sump of about 1.5 m depth. Compared to stabilization ponds, HRAPs reduce the surface needed by a factor of 5 [66] and the biomass productivity while achieving a three-fold improvement in yield from 10 ton/year/ha [67]. Despite needing 50 times more space land than activated sludge systems (the most common wastewater treatment technology), HRAPs' costs are significantly reduced compared to activated sludge systems: by a factor of two for the capital costs and by a factor of five for the operational costs [67].…”

High rate algal pond systems for low-energy wastewater treatment, nutrient recovery and energy production

“…HRAP tend to select for algal strains that thrive under the diurnally varying conditions for sunlight, temperature, pH and dissolved O 2 (Weissman et al 1988). The annual biomass productivity of wastewater treatment HRAPs without CO 2 addition at moderate latitudes and Mediterranean climates is typically 30 t ha −1 y (ash free dry wt), which is 2-3 times higher than the annual productivity of conventional facultative ponds (10-15 t ha −1 y; Benemann et al 1980b;Craggs et al 1998Craggs et al , 2003Craggs et al , 2011. CO 2 addition HRAP performance, particularly nutrient removal and algal production, is often depressed by severe daytime carbon limitation, indicated by high pond water pH levels (typically above 10), due to the photosynthetic uptake of CO 2 and bicarbonate (Oswald 1988a;Garcia et al 2000;Craggs 2005;Kong et al 2010;Park & Craggs 2010).…”

“…Wastewater treatment ponds provide opportunities for low-energy wastewater treatment and energy recovery from wastewater solids, as well as biofuel production from the harvested algal biomass (Oswald & Golueke 1960;Benemann & Oswald 1996;Craggs et al 2011). Worldwide many thousands of communities, industries and farms use two-stage pond systems for wastewater treatment (e.g., in New Zealand, NZMWD 1974;NZDEC 2006).…”

Lucrative future of microalgal biofuels in Pakistan: a review

“…Lipid accumulation becomes higher in the limited conditions [32]. Depending on algal species, availability and types of nutrients, N:P ratio in algal biomass ranges from 4:1 to 40:1 [33].…”