As discussed, the only potentially sustainable cultivation method currently available is the use of open ponds. This is because of their lower energy requirements compared to PBRs. Open ponds require a far greater area of land for the mass of biomass produced and area requirements need to be considered for individual cases. The necessary area will be dependent upon the volume of wastewater that requires treatment, pond depth, nutri­ent loading, discharge limits and hydraulic retention times (HRT).

If the focus of algal cultivation is for wastewater treatment the treat­ment efficiency will have a great impact upon the pond area required. Treatment of water with algae depends upon the productivity of the algae, the higher the productivity the greater the nutrients assimilated. Hydrau­lic retention times of around 10 days are most common [55, 96] and as the HRT increases, the area required increases proportionally. It may be important to minimise the area requirements by reducing cultivation time but nevertheless if the wastewater discharged from the system is above the required limits then the time is likely to be too short. The HRT of each sys­tem will depend upon the influent nutrient loading and limits of discharge and therefore must be calculated accordingly.