*6.7. Integrated Algal Biodiesel*

Historically, algal biodiesel has been suggested to be financially feasible only with concomitant wastewater treatment or animal feed production, valuable secondary products, or supplemental energy products. More recent analysis shows that recovery of algal biodiesel from wastewaters can be beneficial after 2 to 4 years with a fair breakeven [40,100]. There has been a lot of speculative interest in algal biodiesel recently, which had been fueled largely by the increased price of diesel before the reversal in 2008. Many new companies have been set up to use algae to develop biofuels and to obtain certified emission reductions (CERs) by reducing CO2 (IGV GmbH, Nuthetal, Germany, undated) [35]. One example is Aquaflow Bionomic (Nelson, New Zealand), which reported harvesting crude oil to refine it into para ffinic kerosene for use as jet fuel, from wild algae grown on oxidation ponds used in the domestic and agro-industrial waste stream treatment trains. Within a 60 ha facility, this plant treats 5 billion liters of water per year [101]. More innovations will show whether these ventures are financially viable and will be introduced not for demonstration purposes, but for growth.

#### *6.8. Advanced Integrated Wastewater Pond Systems*

These consist of an anaerobic digester and algal pond with high concentrations. For nine years, a facility in Grahamstown, South Africa, has been tested for e ffectiveness in wastewater treatment. Nutrient and organic removal levels were reached comparable with traditional wastewater treatment works and negligible *E. coli* counts [24,50]. Anaerobic digestion biogas provides energy, and the algae may be used as fertilizer or fuel (e.g., biodiesel). Despite these and many other international examples of wastewater energy projects, in many countries there is no overall view of the potential or a plan for harnessing this renewable energy source. Anaerobic digesters are used by many urban wastewater treatment plants as part of the wastewater treatment process [76,102]. Although some use heat internally to control digester temperatures and to heat building space, however, the majority vent or flare the gas. This shows the weak integration of energy usage and the potential for reducing greenhouse gas emissions have not been understood [103]. Wastewater treatment plants must be integrated to use biogas to dry and pellet the wastewater sludge, thus reducing the on-site disposal costs and environmental burdens while providing a potential source of energy. The pellets have an energy content of ~16.6 MJ/kg and were used as additional fuel in their kilns by a local cement factory [99].

#### **7. Benefits of Using Wastewater for Energy**

Examples of where energy is extracted from wastewater to produce a range of energy products at varying scales (from small rural to large industrial operations) exist worldwide. A limited range of examples to demonstrate the energy potential from wastewater technologies is presented.
