The management and removal of residuals, sludge, and biosolids has historically been a burden on wastewater utilities, accounting for nearly 50 percent of treatment costs. But this waste may hold the key to additional revenue if reclaimed and sold.
More companies and water-treatment facilities are no longer considering it waste and instead are finding new and innovative ways to use it to produce energy to run their plants, power cities and towns, and create biofuel. Still, others are sending it to their agricultural fields as high-performing fertilizers to enhance crops or as fresh water to irrigate the land.
In the United States alone, billions of gallons of water are treated each day at water resource recovery facilities, according to the Water Research Foundation (WRF). However, once the water is treated, the question is what to do with the solids removed during the treatment process.
The World Bank in 2020 reported that the world’s wastewater—80 percent of which is released into the environment without adequate treatment—is a valuable resource from which clean water, energy, nutrients, and other resources can be recovered.
A World Bank initiative, Wastewater: From Waste to Resource,1 calls for smarter wastewater management, including reuse and resource recovery, and looks at wastewater projects around the world that have paid dividends for people, the environment, and economies in the short and long terms.
According to the initiative, wastewater treatment offers a double value proposition. In addition to environmental and health benefits, wastewater treatment can bring economic benefits through reuse in different sectors, and revenue generated from this process can help cover water utilities’ operational and maintenance costs.
“In this sense, wastewater should not be considered a ‘waste’ anymore, but a resource,” said Diego Juan Rodriguez, author of From Waste to Resource: Shifting Paradigms for Smarter Wastewater Interventions in Latin America and the Caribbean and a senior water resources management specialist at the World Bank.
“This is at the core of a circular economy, an economic system aimed at minimizing waste and making the most of resources. As cities continue to grow, future urban development requires approaches that minimize resource consumption and focus on resource recovery, following principles of the so-called circular economy.”
Jennifer Sara, global director, World Bank water global practice, said, “Once treated, wastewater can be used to replace freshwater for irrigation, industrial processes, or recreational purposes. It can also be used to maintain the environmental flow and by-products from its treatment can generate energy and nutrients.”
WRF described the resulting mixture as “a unique semi-solid blend of organic and inorganic materials, trace elements, chemicals, and even pathogens, so there is no across-the-board solution for handling and processing the combinations of constituents that may be present.”
Rodriguez said that one of the key advantages of adopting “circular economy principles in wastewater management is that resource recovery and reuse could transform sanitation from a costly service to one that is self-sustaining and adds value to the economy. This will help countries bridge the funding gap in sanitation to achieve the sustainable development goals.”
‘Substantially Better’ Fertilizer
According to the Water Research Foundation (WRF) website, biosolids are often rich in nutrients such as nitrogen and phosphorus, which also happen to be ingredients for promoting healthy soil and plant growth. The waste undergoes rigorous treatment to meet stringent regulations before facilities sell the biosolids to be applied to land as fertilizer.
For example, almost 10 years ago, Lystek International introduced a system that allows wastewater treatment plants (WWTPs) to reduce sludge volumes by up to 30 percent, increase biogas production by up to 40 percent, and produce (for potential sale) a pathogen-free, nutrient-rich fertilizer for farmers.
According to Lystek’s president, Rick Mosher, Lystek’s LysteGro product fertilizer is unique due to its liquid form, allowing it to be subsurface-injected for even distribution and a tenfold reduction in odor as compared to historical Class B application.
“It’s not as good as commercial fertilizer—it’s substantially better,” Mosher contended in a post published by data and intelligence platform Statista.
From a plant equipment and operations perspective, the system is basically “a severe pasteurization process” that incorporates heat (at 70 degrees Celsius/158 degrees Fahrenheit), alkali (typically potassium hydroxide), and high shear mixing to achieve hydrolysis, or breakdown, of the organics present in biosolids.
The resulting low-viscosity liquid is not only available as a U.S. Environmental Protection Agency Class A, Exceptional Quality (EQ) and Canadian Food Inspection Agency-registered fertilizer for land application but can also be leveraged for plant optimization.
WRF has taken on more than 100 projects in this area, building a $20 million body of research and relationships with partners such as the U.S. EPA, the Water Environment Federation, the New York City Department of Environmental Protection, the California Association of Sanitation Agencies, the North East Biosolids and Residuals Association, and the Mid-Atlantic Biosolids Association.
Energizing Power Plants and Then Some
The District of Columbia Water and Sewer Authority’s (DC Water) Blue Plains Advanced Wastewater Treatment Plant is the largest plant of its kind in the world, according to a Pepco Energy Services combined heat and power (CHP) case study. On an average day, the facility treats close to 370 million gallons of wastewater and can treat over one billion gallons a day at peak flow.
New processes and technologies have been added to provide advanced wastewater treatment at the facility, which now uses both primary and secondary treatment, as well as denitrification, multimedia filtration, and chlorination/dichlorination during the treatment process.
This CHP on-site power production reduces the facility’s greenhouse gas emissions by about 40 percent and provides steam to the complete thermal hydrolysis process. DC Water’s thermal hydrolysis facility system generates high-quality sludge that is used as soil amendments (200,000 tons per year). A portion of the sludge is processed in an anaerobic digestion system, which generates 10 megawatts of electricity that is used elsewhere at the treatment plant.
The World Bank’s report casts a light on wastewater management experiences in Latin America and the Caribbean, which are already reaping benefits.2
For example, by using treated wastewater instead of groundwater, the San Luis Potosi power plant in Mexico cut costs by 33 percent, leading to US$18 million in savings over six years for the power utility. For the water utility, the additional revenue from selling treated wastewater helped cover operations and maintenance costs.
Furthermore, a wastewater treatment plant in Cusco, Peru, saves US$230,000 a year in transporting biosolids (nutrient-rich organic materials resulting from the treatment of domestic sewage in a wastewater treatment facility) and landfill fees due to an agreement with the local compost producer. The compost produced with the plant’s biosolids is then used as part of the water management project to preserve Piuray Lake.
The Brazil-based CAESB water and wastewater utility’s use of biosolids for corn production led to higher-than-average grain yields and was 21 percent more efficient than mineral fertilizers.
The operator of the La Farfana wastewater treatment plant in Santiago, Chile, after investing US$2.7 million to retrofit the plant, was able to sell biogas, accounting for an annual net profit of US$1 million for the business.
Its Biogas Produces Electricity, Heat
In Copenhagen, they are burning the sludge from wastewater treatment, turning it into renewable energy, and using some of that energy to heat the entire city of Copenhagen with radiant heat, according to the Technical University of Denmark (DTU).
“Sludge is an organic material that is removed from wastewater, and it primarily consists of feces, food scraps, and paper,” a university report explained. “By feeding the sludge into oxygen-free digesters, bacteria can convert it into biogas, which then produces electricity and heat,” DTU said. “This can make the treatment plants self-sufficient, while excess electricity and heat can be sold back to the power grid.”
Anders Damgaard, a professor at the university, stated in the report, “Since you already have to treat the water, you might as well extract energy from the process and thus the facilities go from having a very energy-intensive impact to a net saving. Several of them are also edging towards becoming climate neutral.”
Ensuring Safe Sludge
Sludge converted to fertilizer in the United States is not without its controversies.
Per-and polyfluoroalkyl substances (PFAS)-tainted substances have ruined farmers’ livelihoods, poisoned water supplies, contaminated food, and put the public’s health at risk, according to a recent report from the Guardian.4
Testing for PFAS, also known as forever chemicals, was occurring only in Michigan and Maine as of 2022 and the results were not environmentally favorable, with contamination found in all tested samples, according to regulators. Public health advocates fear regulators are ignoring the dangers to appease the waste management industry.
“We’re in an absolute mess, and the government knows we’re in a mess, but it seems like they don’t know what to do,” Julie Lay, an Alabama agricultural worker who has organized residents to try to stop sludge from being spread in the state, told the Guardian. “It’s terrible.”
Methane Gas a Valuable Byproduct
Within the waste stream, organics provide a source of energy found in extracted sludge and biosolids, and by diverting the organics-laden sludge into anaerobic digesters, methane gas is produced in the digestion process and can be used to power turbines that generate electrical energy.3
The energy contained in the sludge fed solely from the incoming waste stream to the plant can supplement the plant’s energy needs by as much as 75 percent. By using codigestion, it is possible to cross the threshold of gas production to generate a surplus of energy.
The biogas produced in the digestion process can also be collected and processed to provide fuel to power vehicles. Biogas that has been upgraded by removing hydrogen sulfide, carbon dioxide, and moisture is known as biomethane. The most common examples are WWTPs that are powering their own fleet of vehicles. This opens the door to supplying nearby commercial industrial vehicle fleets.
Huber Technology reported that biodiesel production from wastewater solids is being experimented with using a hydrothermal process.
“Hydrothermal processing mimics the process by which fossil fuels are formed under Earth’s surface” according to James Oyler of Genifuel, in a published report. “It has the ability to produce resources such as oil and gas in a fraction of the time without using heavy infrastructure or leaving a large carbon footprint.”
Paul Bergeron is a freelance writer based in Herndon, Va.