To achieve the highest productivity, it is necessary to add a source of nu­trients to produce an effective medium. The fertiliser requirements can be calculated using the stoichiometric requirements of the algae. In their LCA, Clarens et al. [87] used to triangular distributions to calculate mini­mum, maximum and most likely dosing rates for nitrogen and phospho­rous. These dosing rates were found to be just under two times the stoi­chiometric requirement providing a surplus but with the excess allowing other reactions to remove the surplus. The fertilisers used were assumed to have been sourced from urea and superphosphate. Stephenson et al. [96] estimated a nitrogen requirements of 59 kg per ton of biodiesel produced which contrasts significantly with the 6 kg per ton estimated by Lardon et al. [77]. Collet et al. [86] in their LCA study assumed a nitrogen dosing of 221 kg per day which equates to 5.74 kg per ton of biomass. The study assumed the biomass if processed to biogas so to compare the studies it is possible to calculate the nitrogen requirement for the energy produced. In which case the requirements in the study by Stephenson et al. [96] is 1.56 kg N/GJ, for the study by Lardon et al. [77] it is 0.16 kg N/GJ and for the study by Collet et al. [86] the value is

0. 65 kg/GJ (assuming the energy content of biodiesel and biogas is 37.8 MJ/kg and 0.036 MJ/L respectively). Clearly each study makes different assumption and thus the fertiliser requirement estimates vary, it’s most likely that higher dosing is required to provide an abundance of nutrients thus avoiding nutrient limitation.

The major drawback to the use of fertilisers is their energy input, cost and environmental impact. Lardon et al. [77] found fertilisers to be one of the major contributors to energy consumption and to the negative en­ergy balance of the whole process system. When the low nitrogen scenario was considered, the energy consumption was far lower. Clarens et al. [87] came to similar conclusions as nutrient-derived energy consumption ac­counted for the greatest energy use in algae production. Similar observa­tions were made by Shirvani et al. [98]. Not only do fertilisers negatively affect the energy balance, but they also provide a significant source of environmental impacts to the system. Fertiliser production requires a high energy input from both electricity and fossil fuels, both of which are high emitters of greenhouse gases. Some studies have ignored the impact of fertilisers however results in the study by Clarens et al. [87] suggested that using alternative sources of nutrients (wastewater) could in fact uptake CO2 and return a positive energy balance in the best case (using source — separated urine as a nutrient source).