Evaluation of High Rate Algae Ponds for treatment of anaerobically digested wastewater: Effect of CO2 addition and modification of dilution rate.

High Rate Algae Ponds (HRAP) are the simplest way to grow microalgae biomass and an interesting alternative for wastewater treatment. In this work the performance of these systems was evaluated using anaerobically digested wastewater as culture medium. Two variables were studied in long-term mode: the carbon dioxide supply and the modification of the dilution rates. The results showed that CO2 supply increases the productivity but less than expected considering the potential biomass generation calculated based on the ratios of carbon to nitrogen of microalgae and wastewater. The assimilation into biomass only accounted for 57% of the inlet nitrogen under the best conditions because nitrification and volatilization reduced the availability of this element. The operation under short hydraulic retention times presented a more interesting performance with higher biomass productivities. The biomass produced was efficiently harvested with in a Dissolved Air Flotation (DAF) unit.

Assessing carbon and nitrogen removal in a novel anoxic-aerobic cyanobacterial-bacterial photobioreactor configuration with enhanced biomass sedimentation.

The carbon and nitrogen removal potential of an innovative anoxic-aerobic photobioreactor configuration operated with both internal and external recyclings was evaluated under different cyanobacterial-bacterial sludge residence times (9-31 days) during the treatment of wastewaters with low C/N ratios. Under optimal operating conditions, the two-stage photobioreactor was capable of providing organic carbon and nitrogen removals over 95% and 90%, respectively. The continuous biomass recycling from the settler resulted in the enrichment and predominance of rapidly-settling cyanobacterial-bacterial flocs and effluent suspended solid concentrations lower than 35 mg VSS L(-1). These flocs exhibited sedimentation rates of 0.28-0.42 m h(-1) but sludge volumetric indexes of 333-430 ml/g. The decoupling between the hydraulic retention time and sludge retention time mediated by the external recycling also avoided the washout of nitrifying bacteria and supported process operation at biomass concentrations of 1000-1500 mg VSS L(-1). The addition of additional NaHCO3 to the process overcame the CO2 limitation resulting from the intense competition for inorganic carbon between cyanobacteria and nitrifying bacteria in the photobioreactor, which supported the successful implementation of a nitrification-denitrification process. Unexpectedly, this nitrification-denitrification process occurred both simultaneously in the photobioreactor alone (as a result of the negligible dissolved oxygen concentrations) and sequentially in the two-stage anoxic-aerobic configuration with internal NO3(-)/NO2(-) recycling.

Evaluation of carbon dioxide mass transfer in raceway reactors for microalgae culture using flue gases.

Mass transfer of CO2 from flue gas was quantified in a 100m(2) raceway. The carbonation sump was operated with and without a baffle at different liquid/gas ratios, with the latter having the greatest influence on CO2 recovery from the flue gas. A rate of mass transfer sufficient to meet the demands of an actively growing algal culture was best achieved by maintaining pH at ∼8. Full optimisation of the process required both pH control and selection of the best liquid/gas flow ratio. A carbon transfer rate of 10gCmin(-1) supporting an algal productivity of 17gm(-2)day(-1) was achieved with only 4% direct loss of CO2 in the sump. 66% of the carbon was incorporated into biomass, while 6% was lost by outgassing and the remainder as dissolved carbon in the liquid phase. Use of a sump baffle required additional power without significantly improving carbon mass transfer.

Oxygen transfer and evolution in microalgal culture in open raceways.

The mass transfer characteristics of all sections of a 100 m(2) raceway were evaluated. The efficiency of different diffusers was determined dynamically and the most effective was used for steady state system characterisation at water depth 0.2 m and velocity 0.22 m s(-1). Mass transfer coefficients at a gas flow rate of 6 m(3) h(-1) were 164.50, 63.66, 0.87 and 0.94 h(-1) for the paddlewheel, sump, straight and curved channel sections, with associated oxygen transfer rates of 106, 172, 27 and 39 g h(-1). Oxygen supersaturation during algal cultivation led to a reduction in biomass productivity, which was more severe with pure CO2 than flue gas. Simulations showed the energy required to increase mass transfer and reduce oxygen concentrations was more than compensated for by increased biomass and potential energy yields. Oxygen removal is likely to be a critical criterion, and maintenance of mass transfer by sparging may be necessary even when CO2 is not required.