Selecting an algal strain because of its beneficial properties is unlikely to be the most successful method of recovering energy from its cultivation. As recent studies have shown, producing high value biofuel from algae may not be the most effective means of energy recovery. Instead it seems that anaerobic digestion or combustion may be more appropriate. Given this situation, it may be more important to utilise strains of algae that are most suited to individual scenarios (wastewater type, climate etc.). It is also likely in many locations with climatic variation in seasons that the species of algae dominating will change as temperatures and amount of sunlight vary. There are examples of such species change in the literature, Professor Shelef in his study of algal cultivation in raw wastewater in open ponds found that in Spring Micratinium dominated, in Summer Chlorella was most common and in autumn and winter Euglena became dominant [23].

The alternative to allowing various strains to dominate naturally is to select a strain that is capable of tolerating extreme conditions or recycling the favoured algae. Spirulina is a species of algae renowned for good bio­mass control due to high pH requirements [111, 112]. Conditions could be manipulated to promote the growth of species such as Spirulina by adjust­ing pH in wastewater streams. In a study conducted by Olguin et al. [113]

Spirulina was cultivated in piggery wastewater and seawater. In the study, continuous cultivation of Spirulina was achieved with no issues relating to contamination. Calculations would be necessary to understand whether or not promoting specific strain dominance would be worthwhile from an energy recovery perspective. It may be more productive to simply allow a naturally dominant strain to develop requiring fewer inputs. As studied by Park et al. [91], it is also possible to recycle algae improving dominance of selected strains. This may provide a robust method of selectivity and could allow for improved productivity with little input required.

a Wastewater treatment plant; b Secondarily treated wastewater [104]; c Raw dairy manure [105]; dRaw swine manure [106]; e Bioethanol distillery [102], f Distillery stillage [107]; g Grape distillery [108]; h Brewery wastewater [109]; i [110].