How To Utilize Nutrients To Your Advantage

Using nitrogen and phosphorus to create algal biomass may be the energy solution for tomorrow.

Few argue that one of the greatest challenges facing our world in the 21st century is to meet the energy demands necessary to support the economic growth resulting from the expanding global population. Though fossil fuels have been the mainstay in providing energy for increasing demands, the limitations of these sources call for more sustainable solutions to be advanced. Algae production has the possibility to rise to the occasion as one of these sources, and wastewater treatment plants may be able to become factories for its production.

Energy experts estimate that global demand for energy will outpace its supply by more than 40 percent by 2030. Though fossil fuels will continue to dominate the supply landscape, algal biofuels potentially could replace more than 10 percent of fossil fuel sources in this time frame, which would stand to be a significant contribution. Moving in the algae direction is attractive — its energy conversion efficiency exceeds that of terrestrial biofuel sources (e.g. corn ethanol) by more than 10 times.

Turning Nutrients Into Algae
Domestic wastewaters contain an abundance of nutrients, specifically nitrogen and phosphorus — key ingredients in algal growth — which have negative environmental consequences if not substantially removed from treated discharges. However, integrating controlled algal production into the treatment process addresses two key environmental objectives: nutrients are removed from the treated effluent, and the algal biomass produced can be harvested and converted into usable energy. It all sounds positive, but could it be too good to be true?

Historically, the deterrent to widespread algae production from wastewater is the fact that large acreages are required to produce significant quantities, making it practical only in rural areas where 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 productivity while simultaneously 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 carbon, requiring carbon dioxide supplements to sustain productivity. 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 commercial biofuel from the algae involves a series of steps between harvesting and thickening the algae biomass to producing the final product via complex chemical reactions, rendering it an expensive proposition. These factors make direct biofuel production for the public utility difficult 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 carbon 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 nutrients present in wastewaters offers two significant benefits. First, algae productivity removes nitrogen and phosphorus from treated effluents to reduce negative environmental impacts. Second, energy can be recovered from those same algae — via anaerobic digestion — and used to reduce the amount of power needed to operate a wastewater treatment facility. Think about it!

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.