Algal remediation of CO2 and nutrient discharges: A review

Abstract

The recent literature pertaining to the application of algal photobioreactors (PBRs) to both carbon dioxide mitigation and nutrient abatement is reviewed and the reported data analysed. The review appraises the influence of key system parameters on performance with reference to (a) the absorption and biological fixation of CO2 from gaseous effluent streams, and (b) the removal of nutrients from wastewaters. Key parameters appraised individually with reference to CO2 removal comprise algal speciation, light intensity, mass transfer, gas and hydraulic residence time, pollutant (CO2 and nutrient) loading, biochemical and chemical stoichiometry (including pH), and temperature. Nutrient removal has been assessed with reference to hydraulic residence time and reactor configuration, along with C:nutrient ratios and other factors affecting carbon fixation, and outcomes compared with those reported for classical biological nutrient removal (BNR). Outcomes of the review indicate there has been a disproportionate increase in algal PBR research outputs over the past 5-8 years, with a significant number of studies based on small, bench-scale systems. The quantitative impacts of light intensity and loading on CO2 uptake are highly dependent on the algal species, and also affected by solution chemical conditions such as temperature and pH. Calculations based on available data for biomass growth rates indicate that a reactor CO2 residence time of around 4 h is required for significant CO2 removal. Nutrient removal data indicate residence times of 2-5 days are required for significant nutrient removal, compared with <12 h for a BNR plant. Moreover, the shallow depth of the simplest PBR configuration (the high rate algal pond, HRAP) means that its footprint is at least two orders of magnitude greater than a classical BNR plant. It is concluded that the combined carbon capture/nutrient removal process relies on optimisation of a number of process parameters acting synergistically, principally microalgal strain, C:N:P load and balance, CO2 and liquid residence time, light intensity and quality, temperature, and reactor configuration. This imposes a significant challenge to the overall process control which has yet to be fully addressed.

pCO2 and CO2 fluxes of the metropolitan river network in relation to the urbanization of Chongqing, China

“…In spring, factors including the appropriate temperatures, slow flow velocity due to scarce rainfall, and nutrients enrichment due to point and nonpoint sources led to algal blooms in many tributaries of the Yangtze River, especially after the construction of the Three Gorges Reservoir [ Cao et al ., ; Da et al ., ], decreasing the water p CO 2 . The capture, utilization, and storage of CO 2 by algae in water can draw down the p CO 2 in eutrophic rivers [ Judd et al ., ]. Our investigation showed that the river water p CO 2 was probably influenced at the same time by the dilution effect of persistent rainfall and biogenic CO 2 uptake; however, which mechanism was dominant depends on the rainfall intensity and temperature.…”

Optimization of cultivation conditions for combined nutrient removal and CO2fixation in a batch photobioreactor

“…CO 2 fixation as a function of I appears to follow no recognisable pattern across either different species or different investigations for the same species . Against this, for individual studies subject to the same standardized conditions fixation and biomass productivity both increase with I as expected to some maximum governed by light saturation .…”

“…Microalgae, configured as photobioreactors (PBRs), has attracted considerable attention as the basis of combined biological method for removal of nutrients (nitrogen N and phosphorus P) from wastewaters and CO 2 fixation from flue gases . The microalgae Chlorella vulgaris has been extensively studied for CO 2 mitigation under a range of operating conditions, including gas CO 2 concentration, light intensity and temperature .…”

“…PBRs offer a more sustainable alternative to the established biological nutrient removal (BNR) process, which demands energy for aeration and the pumping of sludge between various tanks in the treatment scheme, as well as supplementary chemical dosing with coagulants to obtain the required P removal . However, while offering a potentially sustainable solution for combined nutrient and CO 2 abatement, the process footprint can be up to two orders of magnitude greater than that of the BNR process . Optimization of the process, so as to enhance the CO 2 fixation (and so biomass growth) and nutrient assimilation, is thus critical.…”

“…A maximum uptake rate of 4–5 mg L −1 d −1 N and 0.4–0.6 mg L −1 d −1 P has been reported for both a classical stirred tank PBR and a membrane PBR operating at a 2–5 d HRT . Nutrient removal for continuous processes of 2–3 d HRT are generally below 85% for both N and P, compared with 75–88% N and 80–99% P for the BNR process . This reduced robustness compared with BNR arises primarily from the combined impact of the lower X max (generally <1.5 g L −1 , cf.…”