Bioprospecting microalgae for growth on undiluted anaerobic digestate of piggery effluent

Microalgae cultures do not generally compete with agricultural food crops except for their reliance on fertilisers (Borowitzka and Moheimani 2013). Comparatively, some agricultural wastes such as anaerobic digestion of piggery effluent contain very high concentrations of nitrogen and phosphorous (Buchanan et al. 2013). The use of microalgae culture for the treatment of anaerobic digestate of piggery effluent offers attractive advantages over current wastewater treatment systems used by piggeries. This effluent is very high in ammonium, which at high pH is toxic to most organisms (Buchanan et al. 2013). If microalgae can recover nutrients from anaerobic digestion of piggery effluent in the form of biomass, this could potentially be used as a source of feed or bioenergy. If the undiluted anaerobic digestion of piggery effluent is treated by selective microalgae, this can also improve water recycling and economic returns (Buchanan et al. 2013). This study utilised bioprospecting strategies (indoor and outdoor) incorporating the selection and culture of microalgae that were capable of growing on undiluted, untreated anaerobic digestate of piggery effluent.

Detailed bioprospecting was conducted to isolate suitable microalgal species capable of growth on anaerobic digestion of piggery effluent (Ayre 2013). As a result, Chlorella, Scenedesmus and a pennate diatom were isolated using a synthetic medium with up to 500 mg NH₃-N/L.

The next step involved the culture of isolated species in outdoor paddle-wheel-driven raceway ponds over a course of 20 weeks with ammonia concentrations of up to 1,600 mg NH₃-N/L. Maintaining a steady culture density in the raceway ponds over the course of cultivation demonstrated the potential for on-going long-term nutrient removal using microalgae and translation to large-scale applications.

The highest ammonium removal rate achieved was equal to 83.3 mg NH₃- N/L/d. Under the batch mode, the phosphorus (P) and carbon (C) removal rates were 5.2 mg P/L/d and 562 mg IC/L/d, respectively. The average biomass productivity of 25.6 mg dry matter or ash-free dry weight/L/d was achieved. It was also found that CO₂ addition could significantly (P < 0.05) enhance microalgae growth (repeated measure one-way ANOVA). This proof-of-concept study illustrated the potential for culturing microalgae in untreated and undiluted anaerobic digestion piggery effluent having high ammonium content.