A comparative evaluation of microalgae for the degradation of piggery wastewater under photosynthetic oxygenation

Two green microalgae (Scenedesmus obliquus and Chlorella sorokiniana), one cyanobacterium (Spirulina platensis), one euglenophyt (Euglena viridis) and two microalgae consortia were evaluated for their ability to support carbon, nitrogen and phosphorous removal in symbiosis with activated sludge bacteria during the biodegradation of four and eight times diluted piggery wastewater in batch tests. C. sorokiniana and E. viridis were capable of supporting the biodegradation of four and eight times diluted wastewater. On the other hand, while S. obliquus and the consortia isolated from a swine manure stabilization pond were only able to grow in eight times diluted wastewater, S. platensis and the consortium isolated from a high rate algal pond treating swine manure were totally inhibited regardless of the dilution applied. TOC removal efficiencies (RE) ranging from 42% to 55% and NH4+-RE from 21% to 39% were recorded in the tests exhibiting photosynthetic oxygenation. The similar oxygen production rates exhibited by the tested microalgae under autotrophic conditions (from 116 to 133 mg O2 L−1 d−1) suggested that factors other than the photosynthetic oxygenation potential governed piggery wastewater biodegradation. Microalgal tolerance towards NH3 was hypothesized as the key selection criterion. Further studies in a continuous algal–bacterial photobioreactor inoculated with C. sorokiniana, S. obliquus and S. platensis showed that C. sorokiniana, the species showing the highest NH3-tolerance, rapidly outcompeted the rest of the microalgae during the biodegradation of eight times diluted wastewater, achieving TOC and NH4+-RE comparable to those recorded in the batch biodegradation tests.