Your Path to Doing More with Less: Resource Recovery
- Wendell Khunjar - Hazen and Sawyer
For many years now, the water industry has been shifting from a paradigm of wastewater treatment and disposal to one of water resource recovery. Newer technologies enable recovery of resources that can be used to generate products with economic value in the facility and/or secondary markets, while also helping utilities meet stringent discharge limits.
Recovery of Nutrients: Nansemond Treatment Plant
The Nansemond Treatment Plant in Suffolk, Va., is a 30-mgd facility operated by the Hampton Roads Sanitation District (HRSD) that has implemented nutrient recovery. The recovery process at the plant consists of patented fluidized bed reactors that recover ammonia and phosphorus from the centrate stream as struvite. The nutrient-rich centrate stream is mixed with appropriate stoichiometric doses of magnesium chloride and caustic to precipitate struvite pellets, which are then harvested from the reactors, dried, and bagged for sale.
Since May 2010, more than 1,000 lbs of struvite have been recovered on a daily basis from the NTP. The recovered struvite is used as a slow-release fertilizer (Crystal Green®) in the agricultural market. In addition to re-covering nutrients, implementation of struvite recovery at the NTP has reduced the soluble phosphorus and nitrogen content in the centrate by up to 85% and 25% respectively. This has minimized the need for ferric addition throughout the NTP and reduced the phosphorus content of the biosolids by 10 to 15%. Combined benefits of nutrient recovery at the NTP can translate to annual savings of up to $450,000 per year.
Recovery of Digester Gas: F. Wayne Hill Resources Center
The F. Wayne Hill Water Resources Center (FWHWRC) is a 60-mgd facility owned and operated by the Gwinnett County Department of Water Resources (GCDWR).
Consistent with its values to be wise stewards, GCDWR embarked on a project to maintain full treatment capacity (60-mgd) at the FWHWRC while increasing digester gas production so that it could leverage the capacity of a 2.1-megawatt biogas engine generator. The engine generator is equipped with a natural gas/biogas blending system to enable continuous operation of the engine generator even during periods of low digester biogas production, maximizing both energy production and return on investment, while minimizing the plant’s carbon footprint. The project also included a gas conditioning and handling system, as well as a waste heat recovery system that produces hot water to heat the anaerobic digesters.
Since installation of the engine generator, GCDWR has also implemented a co-digestion program, whereby high strength organic wastes from the region are received and processed at the FWHWRC prior to injection into anaerobic digesters to boost gas production. Leveraging the benefits of co-digeston with the engine generator and conversion to a real-time power rate structure has allowed FWHWRC to lower its average power cost, resulting in a savings of over $1 million (USD) per year.
Recovery of Water: Sustainable Water Initiative For Tomorrow (SWIFT) Research Center
Hampton Roads Sanitation District (HRSD) arrived at an innovative single solution to solve several challenges – using reclaimed water to recharge the Potomac Aquifer. The SWIFT Research Center, currently under construction in Suffolk, VA, is an advanced treatment demonstration facility that incorporates an 8-step process to facilitate water reclamation for recharging the Potomac Aquifer.
Recharging the Potomac Aquifer with reclaimed water will replenish eastern Virginia’s dwindling groundwater supply, increasing the region’s water supply stability; reduce the rate of land subsidence, mitigating some of the impact of sea level rise on the coastal communities area; and support Chesapeake Bay restoration by diverting water from disposal to beneficial reuse.
The demonstration facility is expected to begin operations in April 2018 and will be able to recover one million gallons of water per day. It will also be used as a research and learning facility for training HRSD staff and providing public education. The information collected during different loading rates at the research center will enable HRSD to improve the efficiency of both design and operation of future full-scale facilities, which are expected to encompass seven of the utility’s nine treatment plants and over 100 million gallons of water per day of managed aquifer recharge.
Recovery of the Future: Beyond Nutrients, Energy, and Water
Throughout the past century, our efforts in the water industry have shifted from mastering the basics of biological wastewater treatment to focusing on reclaiming water for reuse and recovering energy from biomass. During the transition into the 21st century, we developed and implemented full-scale nutrient recovery applications. We now sit at the crossroads where recovery of cellulose, high value carbon, bioelectrochemical products, rare earth elements and plasmids represent the next generation of resource recovery.
• Cellulose is an end product of microsieving of raw influent wastewater. Recovered cellulosic material can be processed into paper or used as a feedstock for energy and/or high value carbon recovery.
• High value carbon products include volatile fatty acids (VFAs), polyhydroxyalkanoates (PHAs), and various alcohols. VFAs are an end product of fermentation and represent a building block for creating polymers used within the manufacturing industry. PHAs are a biopolymer produced by microorganisms that can be used as a bioplastic precursor. Alcohols like methanol and/or ethanol are end products of fermentation and can be used in industrial and transportation applications.
• Bioelectrochemical products like hydrogen peroxide and caustic can be used within industrial applications for scaling and pH control.
• Rare earth elements are redox-stable metals with unique electrochemical properties that have extensive use in electronics, energy systems, and transportation technologies.
• Plasmids are mobile genetic elements that can be coded for novel enzymes which can be exploited by the biotechnology industry.
The next generation of resource recovery is promising, and new directions in resource recovery will undoubtedly occur in the next few decades. Translating these concepts into practice will require concerted effort by all water industry stakeholders to understand how these emerging technologies, recovery products, and markets can be best leveraged to achieve multiple benefits at utilities.
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Horizons showcases significant water, wastewater, reuse, and stormwater projects and innovations that help our clients to achieve their goals, and can help you achieve yours. Articles are written by top engineers and process group leaders, demonstrating and explaining the beneficial application of a variety of technologies and tools.