Challenges in Using Waste Water as Growth Media

Although algal biomass production using waste water as a growth media seems attractive, we face several challenges thereof. One problem with such system is the availability of production sites (Slade and Bauen 2013) because we can only inte­grate biomass production with city waste water treatment plants. Overall, it is clear that reducing the inputs (energy and fertilizer) of the process makes algae an ideal feedstock in such integrated systems as compared to any other biofuel feedstocks, such as canola, corn, and switch grasses (Clarens et al. 2010). The requirement of fresh water and nutrients may decrease up to 100 % by using waste water as source nutrients (Li et al. 2010; Udom et al. 2013).

Presence of potentially toxic compounds in municipal waste water is another problem, especially when industrial waste water is being mixed. Heavy metals are believed to inhibit the important enzymes involved photosynthetic pathways of microalgae (Kumar et al. 2010). A noteworthy reduction in specific growth rate and biomass productivity of B. braunii was observed when cultivated in secondary efflu­ents of a municipal sewage treatment plant and it was shown that it was due to the presence of phenolic compounds and heavy metals in the waste water (Orpez et al.

2009) . Very fortunately some microalgae (Scenedesmus and Pseudochlorococcum.) have displayed tolerance to higher concentrations (80-100 mg mL-1) of heave met­als such as Pb2+. On the other hand, mercuric ions (Hg2+) inhibited chlorophyll bio­synthesis even at lower concentrations (5-10 mg mL-1) and a complete destructed algal cells at concentration >20 mg mL-1 (Shanab et al. 2012). It is believed that it is related to the amount of heavy metal ions bound to the cell surface, and to the amount of up-taken heavy metal ions (Franklin et al. 2001). However, the growth inhibition may due to extracellular ions concentration, for instance in the case of zinc (Wilde et al. 2006). In the algal stabilization ponds, the heavy metal ions, Zn2+ and Pb2+ were removed up 72 % and 73 % respectively, by Chlorella sp. (Kumar and Goyal 2010 ) . These studies have shown that there are important benefits to be derived from integrating algal production systems with nutrient-rich waste water streams (Dalrymple et al. 2013).

Although, people argue that using waste water for algal biomass production may pose contamination risks yet Life Cycle Analysis (LCA) studies have confirmed that it is a very productive approach and ensure the viability and sustainability of the complete biofuels production process in monetary terms (Kumar and Goyal 2010). An LCA study was carried out to evaluate the energy balance and environmental impacts from biomass to biodiesel production and combustion. It was shown that substantial (>50 %) cost reductions may be achieved if CO2, nutrients, and water can be provided at lower cost, i. e., sourced from waste water (Lardon et al. 2009).

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