Mixotrophic algal systems improve the economics of wastewater treatment
Traditional methods of wastewater treatment are expensive and energy demanding. Algae-based systems can be deployed to recycle the energy, nutrients and valuable materials present in wastewater, improving the economics of the process for a range of wastewater generators. Thus, microalgae can be exploited in a dual approach to clean up wastewater from various sources (e.g., municipal, industry and agriculture), while simultaneously producing valuable biomass for biofuels and bioproducts in the process.
Traditionally, wastewater treatment involves a series of energy- and resource-intensive steps that sequentially remove debris, organic matter, nutrients, other contaminants (e.g., heavy metals) and microorganisms from the water before discharge into waterways. Water and wastewater treatment services can account for the majority of municipal electricity use and greenhouse gas emissions.
The secondary treatment process (aeration step) is responsible for more than half of a municipality’s energy usage at the wastewater treatment plant. This step involves the use of electro-mechanical aeration mechanisms to provide the oxygen needed for bacteria to metabolize and remove organic material from the wastewater. This step reduces the biochemical oxygen demand (BOD) of the discharged water by introducing less organic material, thus leading to a healthier waterway. Subsequent tertiary treatments are then needed to remove excess nutrients to meet discharge regulations, which add to the operational costs.
In the 1950s, William J. Oswald at the University of California-Berkeley began his pioneering work to use algae in natural wastewater management systems. He developed the Advanced Integrated Wastewater Pond Systems technology that passes wastewater through a series of high-rate algal ponds for BOD treatment. This approach takes advantage of photosynthetic algae to generate the oxygen needed by the aerobic bacteria to degrade organic matter in a mixotrophic system, and thus it removes the need for expensive aeration processes. Building on the successful work of Oswald, researchers and industry partners affiliated with ATP3 and the Arizona Center for Algae Technology and Innovation (AzCATI) at Arizona State University continue to advance the use of microalgae in innovative approaches to clean up wastewater. These approaches focus on solving environmental issues related to wastewater treatment in energy-positive ways.
Novel approaches to algae-based wastewater treatment systems
Current work ongoing by ATP3 and AzCATI affiliates in the area of algae-based wastewater treatment systems are taking a “circular economy” approach to reduce energy input into the system and harness the value of wastewater contents.
Peter Lammers (ATP3 Chief Scientist) is a Research Professor within AzCATI, and his group is leading the way in developing novel approaches to wastewater treatment in hot and arid environments. According to Lammers, “Our goal is to transform the economics of wastewater treatment with our disruptive, energy-positive approach to meeting rigorous water quality treatment standards. We plan to enable a truly circular economy by capturing and re-using energy, nutrients and valuable inorganic materials trapped in urban waste streams.”
His unique approach is to use algae extremophiles in a mixotrophic system within a closed photobioreactor to treat water for excess nutrients, organics and metals. “Delivery of gaseous O2 for wastewater treatment turns to out have the highest impact on the cost of wastewater treatment, while CO2 delivery cost is a major driver of the cost of renewable fuels from algae. Mixotrophy solves the gas delivery problem by avoiding it,” Lammers explains. The closed system allows the recycling of gases produced during the simultaneous respiration and photosynthetic processes ongoing by algae in the mixed system and reduces evaporative losses. In addition, the mixotrophic approach enables the concurrent removal of carbon, nitrogen and phosphorous. Further, the extremophiles used thrive in acidic water at values similar to soft drinks (pH 2.5) in addition to high temperatures. Such conditions reduce contamination in the system and kill pathogenic organisms. This approach significantly increases the yield of algae biomass, which lowers the land requirement for wastewater treatment five-fold relative to conventional algae-based systems.
The resulting biosolids and algae biomass can then be converted into energy through hydrothermal liquefaction, a process pioneered with Lammers’ long-term collaborator Shuguang Deng (School for Engineering of Matter, Transport and Energy at ASU). Lammers comments, “We plan to partner with local governments in the greater Phoenix area to test our process at regional wastewater treatment plants to assess seasonal variations in process parameters. We are also working to partner with food and beverage processing firms in the Phoenix area to obtain sugar- and protein-rich wastewater.”
Tryg Lundquist (ATP3 California Site Lead) is Associate Professor at the California Polytechnic State University at San Luis Obispo (Cal Poly) and Chief Technical Officer at MicroBio Engineering, Inc. (MBE), a consulting and engineering firm specializing in the design and construction of algae ponds for wastewater reclamation, biofuel production, microalgae feeds and specialty products. Lundquist was a long-time associate of Oswald at UC Berkeley and worked with him on the design and construction of the two largest systems incorporating high-rate raceway ponds at municipal wastewater treatment plants. Cal Poly and MBE are jointly developing the RNEW® process, an innovative technology for treating and reclaiming wastewater, using native microalgae and bacteria grown in shallow, mixed raceway ponds. Lundquist explains, “The innovative algae treatment we are developing and demonstrating in California achieves nitrogen and phosphorus removal from wastewater, in addition to organics destruction. We have determined that CO2 addition to the wastewater allows us to grow sufficient algae to remove both nitrogen and phosphorus to reach the most stringent discharge limits in the US.” The algal biomass generated from the process can then be used as a biofuel feedstock. This process can significantly reduce costs for wastewater treatment plants.
The open pond system is suitable for environments with moderate climates where evaporative losses are less of a concern, but conventional wastewater treatment methods may be needed in the winter months when there is less sunlight. However, the use of this algae-based system for only part of the year is expected to result in capital and operating costs at one-third to one-half less than conventional methods alone. Lundquist notes, “Cal Poly’s work with ATP3 has been focused on quantifying the algae production rates (akin to crop yields) for various promising algal strains under a wide variety of cultivation conditions. In upcoming experiments, wastewater will be used as the liquid growth medium for algae in experiments using ATP3 methodologies.”
Milton Sommerfeld (ATP3 Education and Training PI) is Co-Director of AzCATI, and his team works on the removal of excess nutrients and contaminants from wastewater with the possibility of recycling these materials for reuse. According to Sommerfeld, “Depending on the wastewater type, the nutrients provided in wastewater provide a desirable water and nutrient source for cultivation of algae for biofuels and feeds.” Recently, his team collaborated with other Arizona state universities and local municipalities to demonstrate the growth of algae on various wastewaters to produce algal biomass for end products. In addition, Sommerfeld specializes in algal strain selection for diverse growing conditions, including partnering with public utility companies to investigate the use of algae to remove contaminants from discharge waters. Sommerfeld adds, “AzCATI is capable of providing and evaluating algae strains for treatment of different types of wastewaters and providing advice/education and support for the use of algae in wastewater treatment.”
Future Collaborations with Public-Private Partners
A number of challenges around the deployment of algae in wastewater treatment systems must be addressed to advance the technology even further. Such issues include limited availability of land at wastewater treatment sites, seasonality of algal growth and chemicals present in the wastewater that reduce light availability or inhibit growth. As these issues are solved and the technology continues to mature, there will be numerous opportunities for ATP3 and AzCATI partners to collaborate with the public and private sectors. Contact us to discuss ideas or needs you may have for the design and deployment of algae-based systems in wastewater treatment.