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land remains inexpensive. Furthermore, because photosynthesis is the driving force, algae production has generally been restricted to geographies where sunlight abounds. But recent technological advances in high-rate algal ponds (HRPs) and photobioreactors are rapidly broadening the geographic regions that are fit for production. Additionally, the use of artificial light technology (including LEDs) minimizes the need to be in locations of abundant sunlight. Technological advances not only expand the geographic possibilities for growth but affect the productivity of the algae. Where algal biomass productivity cultured in conventional large-area ponds rarely exceeds 300 mg/L, high-rate ponds typically exceed 3,000 mg/L, with photobioreactors achieving more than 10,000 mg/L (though the capital costs of photobioreactors today exceed that of HRPs by about 10 times). At these high rates of biomass production, nutrients can be reduced to very low levels in discharged effluents, and the biomass can be harvested for energy production via either biofuel combustion or conversion to biogas through anaerobic digestion. From Algae To Energy Biofuel production from algae requires maximizing produc- tivity while simulta- neously maximizing the concentrations of intracellular lipids during productivity. Maximum productivity occurs when neither nutrients nor carbon are limiting. Municipal wastewaters have excess nutrients but lack sufficient car- bon, requiring carbon dioxide supplements to sustain productiv- ity. Maximizing lipid content is achieved by "starving" the algae of nutrients, primarily nitrogen. Thus, when using municipal wastewater to produce algae for biofuel, a balance must be struck between making sufficient nutrients available to produce enough algae, while at the same time limiting these same nutrients sufficiently to enhance enough lipid production to support enough biofuel production to be economically viable. Today's process for producing com- mercial biofuel from the algae involves a series of steps between harvesting and thickening the algae biomass to producing the final product via complex chemical reac- tions, rendering it an expensive proposition. These factors make direct biofuel production for the public utility dif- ficult to justify. On the other hand, algae production can be beneficial to the public utility because the energy "stored" in algal biomass can be recovered via anaerobic digestion, a process that is already present in many wastewater treatment facilities. In full-scale digestion systems, the methane generation potential for algal biomass is about 50 percent that of raw sludges, thus providing an opportunity to supplement biogas production from the raw sludges to enhance energy generation. Additionally, the carbon dioxide in the biogas can be extracted and then reinjected into the HRPs or photobioreactors to balance the stoichiometric inorganic c arbon requirements for maintaining optimal algae productivity. The energy challenges of this century require a new way of thinking about the treatment of wastewaters to meet the growing demand. Producing algae from the nutri- ents present in waste- waters offers two significant benefits. First, algae productiv- ity removes nitrogen and phosphorus from treated effluents to reduce negative envi- ronmental impacts. Second, energy can be recovered from those same algae — via anaerobic diges- tion — and used to reduce the amount of power needed to operate a wastewa- ter treatment facility. Think about it! Nutrients wateronline.com ■ Water Online The Magazine 46 Dr. Umble is the wastewater practice leader for MWH and provides technical analysis and support to design teams for new and rehabilitated municipal wastewater treatment facilities. Umble is a leader in initiatives promoting environmental stewardship, serving as a technical advisor/reviewer for Water Environment Research Foundation, International Water Association, and the WateReuse Foundation collaborative research projects. The energy challenges of this century require a new way of thinking about the treatment of wastewaters to meet the growing demand. 4 4 _ V E R T _ 0 2 1 4 E Z i n e _ M W H _ D G . i n d d 2 44_VERT_0214 EZine_MWH_DG.indd 2 1 / 3 1 / 2 0 1 4 1 : 2 3 : 0 9 P M 1/31/2014 1:23:09 PM