Investigating why recycling gravity harvested algae increases harvestability and productivity in high rate algal ponds

It has previously been shown that recycling gravity harvested algae promotes Pediastrum boryanum dominance and improves harvestability and biomass production in pilot-scale High Rate Algal Ponds (HRAPs) treating domestic wastewater. In order to confirm the reproducibility of these findings and investigate the mechanisms responsible, this study utilized twelve 20 L outdoor HRAP mesocosms operated with and without algal recycling. It then compared the recycling of separated solid and liquid components of the harvested biomass against un-separated biomass. The work confirmed that algal recycling promoted P. boryanum dominance, improved 1 h-settleability by >20% and increased biomass productivity by >25% compared with controls that had no recycling. With regard to the improved harvestability, of particular interest was that recycling the liquid fraction alone caused a similar improvement in settleability as recycling the solid fraction. This may be due to the presence of extracellular polymeric substances in the liquid fraction. While there are many possible mechanisms that could account for the increased productivity with algal recycling, all but two were systematically eliminated: (i) the mean cell residence time was extended thereby increasing the algal concentration and more fully utilizing the incident sunlight and, (ii) the relative proportions of algal growth stages (which have different specific growth rates) was changed, resulting in a net increase in the overall growth rate of the culture.     

Modification, calibration and verification of the IWA River Water Quality Model to simulate a pilot-scale high rate algal pond

We implemented the IWA River Water Quality Model No. 1 (Reichert et al., 2001. River Water Quality Model No. 1, IWA Scientific & Technical Report No. 12) to simulate water-quality characteristics in two pilot-scale High Rate Algal Ponds. Simulation results were compared with two years' of data from the ponds. The first year's data from one pond were used for model calibration; the remaining data were used for validation. As originally formulated and parameterized, the model consistently yielded summer-time algal biomass concentrations which were too low - with consequent failures in its reproduction of dissolved oxygen, pH and nutrient dynamics. We experimented with various structural/parametric changes to improve the model's performance. The most effective strategy was to greatly increase the respiratory losses suffered by the heterotrophic osmotrophs (thereby giving the algae access to a larger fraction of the incoming dissolved organic carbon and nitrogen). This suggests that CO₂-bubbling alone cannot entirely preclude resource-limitation of algal production. We doubt that our parameterization of heterotrophic osmotrophs is correct and infer that the algae derive a large fraction of their nutrition by direct osmotrophic uptake of dissolved organic matter. This inference is supported by the literature concerning the physiology of the dominant algal species in our ponds.     

Decrease of phosphate concentration in a high rate pond by precipitation of calcium phosphate: Theoretical and experimental results

The mechanism involved in the decrease of phosphate in a high rate pond (HRP) is investigated. The calcium concentration in the water (1.25–3.75 mM) and the high pH obtained in the reactors (8–10), result in the precipitation of calcium phosphate minerals. Although calcium hydroxyapatite [Ca₅(PO₄)₃OH, pK₅ = 57] is the thermodynamical stable state, the phosphate concentration is determined by the solubility of the amorphous tricalcium phosphate [Ca₃(PO₄)₂, pK₅ = 25.2]. This is confirmed by the calculation of the theoretical predicted solubility as well as various experiments. The relationship between pH and phosphate concentration may be used to optimize the efficiency of this type of reactor to produce a minimum concentration of phosphate in the effluent.