More than 8 million tons (dry weight) of sewage biosolids are generated each year by publicly owned wastewater treatment plants, according to EPA estimates. Across the country, communities are making a green effort to upgrade wastewater facilities not only to save money but, in some cases, to generate revenue as well.
What they’re discovering, though, is that so many variables, technologies, options and regulations must be considered that finding the right solution can be daunting.
To help utility managers wade through this universe of options and enable their projects to move forward with confidence in their success and the support of stakeholders, Brown and Caldwell recently used three distinctly different approaches — Triple Bottom Line monetization, process analysis and intense research — to put a trio of wastewater projects on the right track.
City of St. Petersburg, Fla.
As Florida’s first “Green City,” St. Petersburg wanted to recover the stored energy in biosolids and yard wastes and convert it into energy. Through a public-private partnership, St. Petersburg proposed to build, own and operate a facility that will use city-generated biostocks (such as biosolids, yard and wood waste, grit and screenings) to fuel a biomass gasification and energy facility. By doing so, the city not only would reduce its costs, but also generate renewable energy and eliminate the release of methane gas and the potential of groundwater pollution from landfills or land spreading.
|By pumping all biosolids to a single plant using existing pipelines, the city will save about $30 million over 20 years
To evaluate alternate methods of processing dewatered biosolids from its four water reclamation facilities and excess yard wastes, Brown and Caldwell performed detailed, quantitative analysis of 35 options, settling on nine possible solutions. The key was staying open to creative solutions; avoiding a traditional approach that would have been more costly for the city.
The BC team looked at higher quality biosolids (considering that the state was considering a ban on land application of Class B biosolids), more efficient operations and a reduced carbon footprint.
The solution? Upgrade a single plant (instead of all four) to a temperature-phased digestion process, and treat all biosolids at this plant, which will produce Class A biosolids. By pumping all biosolids to a single plant using existing pipelines, the city will save about $30 million over 20 years.
Capital Regional District, Victoria, B.C.
The challenge here was to design four new wastewater treatment plants and a centralized biosolids treatment facility, while placing values on social and environmental impacts. The BC team weighed three options against 30 economic, environmental and social criteria, then monetized all factors and impacts to create an “apples to apples” understanding of total costs and risks. By creating a common unit of measure for all factors that is universally understood — the dollar — the district would be able to make decisions on benefits and risks more objectively.
|Capital Regional District, Liquid Waste Management Plan Area, Victoria, B.C.
The district evaluated biosolids utilization alternatives as part of a planning effort for the Core Area Wastewater Treatment Program. A two-stage analysis identified a biosolids management program that met its goal of cost-effective, environmentally sustainable, and socially responsible wastewater treatment (the Triple Bottom Line). Biosolids production and utilization could provide multiple benefits, including reducing the district’s carbon footprint, generating revenue and recovering additional resources.
Their analysis determined that the most beneficial options promoted solids stabilization through anaerobic digestion. Resource recovery was maximized through capture of biogas for sale to the local utility, reuse of heat energy for thermal drying, and production of a Class A biosolids product. Dried fuel product was particularly attractive, in part, because the carbon tax in British Columbia provided an economic incentive. Development of a topsoil blend and mine reclamation were recommended to diversify the biosolids program.
Results of this analysis enabled the district to make an informed decision about methods of production and biosolids utilization to maximize benefits. The district’s plan is to produce a dried biosolids product that will be available as fuel for cement kiln or alternative energy production.
This solution will maximize resource recovery, reduce greenhouse gas emissions, integrate with solid waste management, provide end-use reliability, and acceptable life cycle cost. The concept— which regulators, political leaders and the public could understand and support —not only meets all system requirements, but minimizes the chance that minor issues would become major stumbling blocks. And it provides about $29 million in savings.
As part of an ambitious program to improve the way it processes and handles biosolids at the Blue Plains Advanced Wastewater Treatment Plant, DC Water developed a long-range biosolids management program. This program is focused on recycling nutrients and organic material in an environmentally safe and beneficial manner. Although the program of biosolids recycling for its 1,200 wet tons per-day production is highly regarded, DC Water decided that changes were needed in the long term.
DC Water spent more than 10 years examining solutions that would significantly reduce the amount of biosolids, improve product characteristics, and be cost- and energy-efficient — a tall order for this 370-mgd plant, the world’s largest advanced wastewater treatment facility. Brown and Caldwell has been an integral part of that evaluation of options, and, most recently, has led the development of the new biosolids management program.
The evaluation included examining previous research and testing, as well as a review of evolving technologies, which included thermal hydrolysis (as pretreatment for anaerobic digestion) and other anaerobic digestion approaches that could produce Class A biosolids. The BC team visited and tested facilities in Europe and the United Kingdom to confirm system “unknowns,” which included product dewatering performance as well as product odor and regrowth potential after dewatering.
After extensive due diligence on the Cambi Thermal Hydrolysis Process, the team determined that this process (from Norway) could be implemented successfully in the United States as the world’s largest installation of thermal hydrolysis. Using more efficient combustion and energy recovery methods for handling digester methane maximized the economic gains of the DC Water program.
The Cambi process cuts digester capacity needs in half while producing Class A biosolids. The new digestion/energy system, using new combustion turbine technology, will generate 13 MW of green, renewable power to supply more than one-third of the plant’s power needs and reduce greenhouse gas emissions by about 40 percent. This new system essentially pays for itself by reducing biosolids volumes, generating renewable power, and reducing operating and maintenance costs.