All autotrophic algal species require a source of nutrients. The most im­portant nutrients, (i. e., those that are needed in greatest concentrations) are nitrogen and phosphorous, but many other nutrients and trace metals are also necessary for optimal growth [16]. There are many media recipes designed to provide complete nutrition for most species of algae. Nutri­ent rich effluents however are often capable of providing almost all of the nutrients required by certain algal species and [17, 18] consequent culti­vation in effluent provides two significant benefits. Firstly, direct uptake of these nutrients and metals, produces cleaner water. Secondly, the algae generate oxygen which aids aerobic bacterial growth leading to additional metal and nutrient assimilation. In the 1950s experiments were carried out by Oswald and his colleagues investigating the symbiotic relationship be­tween algae and bacteria for wastewater treatment in oxidation ditches [3, 19-22]. The experiments which were undertaken used the algae Euglena sp. due to their natural presence in ditches under examination. Oswald and his colleagues discovered that the bacteria and algae in the oxidation ditch develop a symbiotic relationship producing a more stable and less hazard­ous effluent [21]. The economic potential of the algal cells for livestock feed was identified, and the merits of using a faster growing species of algae in effluent specifically for the purpose of livestock feed were dis­cussed [19]. Particular attention in the late 1950s was given to the design of wastewater treatment ponds and relationships between oxygen produc­tion, biological oxygen demand (BOD) removal and light use efficiency over specific periods of time as a function to a variety of species, depths of pond, treatment time, loading rates to identify optimal operational condi­tions [3]. The importance of algae in a heavily loaded oxidation pond to provide the necessary oxygen for sludge oxidation was highlighted, and this early research remains of particular interest as it identified optimal conditions for effluent treatment and demonstrated that oxidation ponds using the symbiotic relationship can achieve considerable BOD removal (>85%) [3].

Further important work was carried out on the use of algae in waste­water treatment in the 1970s and 1980s. Of particular interest was the research lead by Professor Shelef, with a focus on the growth of domi­nant species of algae in open ponds using raw sewage as the main source of nutrients [23]. His research indicated that Micractinium and Chlorella dominated in most cases, with retention times of around three to six days. Using an influent with concentrations of total suspended solids (TSS) ca. 340 mg/L and BOD ca. 310 mg/L, a considerable reduction to levels ca. 60 mg/L TSS and below 20mg/L BOD was reported. Additionally phos­phate was reduced to very low levels and around 10-40 mg/L ammonia remained. This resulted in low levels of organic contamination which al­lowed the use of the treated wastewater for irrigation, and the residual nutrients provided a source of fertiliser as an added value.

In the 1980’s the concept of growing algae was further developed how­ever the focus turned to utilising the biomass for fuel production due to the energy crisis in the United States at the time. Oswald continued his re­search and was joined by Dr. Benemann and together they investigated the potential for the cultivation of algae on a large scale for fuel production. During this time most research moved away from wastewater treatment and more towards high productivity of biomass and high fuel yields with work conducted on the use of flue gas as a source of carbon dioxide to in­crease productivity and provide a method of carbon mitigation. Due to the continued search however for a more economically and environmentally viable system, once again research has turned to the potential benefits of growth in wastewater. Several different wastewater types have been inves­tigated, most common are domestic sewage [24-27], agricultural wastewa­ter (swine and cattle) [26, 28-31] and several industrial wastewaters, e. g., carpet manufacturing [32] and distillation [33]. Previous research suggests that cultivation in the tertiary treatment stages of wastewater treatment may provide the ideal conditions for reasonable algal growth due to high residual nutrient loading and prior removal of organic contaminants [25]. In such a scenario algae can provide an effective means of nutrient polish­ing. Various strains of algae have been shown to effectively remove nitro­gen and phosphorous forms in a number of synthetic and actual wastewa­ters (Table 1).

The table displayed in Table 1 suggests that there is considerable po­tential for nutrient removal in wastewaters with high nutrient loading us­ing a variety of species of common algae and mixtures of locally dominant algae. Much of the research reported has however been conducted at a lab scale in photo-bioreactors which provide controlled conditions (e. g., tem­perature, light and species control) and therefore will improve the produc­tivity of the algae and thus the uptake rate of nutrients. There are however exceptions where polycultures of dominant algae have been grown both using an outdoor turf scrubber [34] and high rate algal ponds [35] as the culture methods. In the cases where biomass has been cultivated on turf scrubbers or in ponds the system has developed using species which come to dominate as species selection is not possible. Despite the lack of con­trollability of species dominance and environmental factors, these studies still demonstrate good N and P removal alongside reasonable productivity rates of biomass.

With many industries looking for cost effective means of nutrient re­moval from wastewaters, the cultivation of algal biomass can provide a suitable method which may return a positive balance of energy as opposed to conventional processes which generally require an energetic input.