Tapping sewage as a source of useful materials

Sewage stinks. It causes pollution. Few people care where it goes. Fewer chemists want to work with it. And yet, with the help of the right technology, sewage could become a reliable, low-cost feedstock for chemicals and other materials.

Commercial products that could be made from municipal liquid waste include phosphorus, nitrogen-rich fertilizers, fuels, biodegradable plastics, thickeners for paints and other products, and oils for making a range of chemical intermediates.

Sounds great, if a little icky, but a few not-so-small hurdles stand in the way. One is how to economically extract useful chemicals from municipal wastewater when more than 99% of it is, well, water.

The presence of pathogens, heavy metals, and toxic compounds such as solvents adds complexity. And social taboos associated with the use of material that was once sewage could prevent investors and regulators from supporting technologies in this field. One thing is for sure: Companies aren’t going to shout about where the raw materials have come from.

These challenges have led to project failures. Nevertheless, the opportunity is substantial. More than 300 km³—yes, cubic kilometers—of municipal wastewater, and more than 600 km³ of industrial wastewater, are generated around the world each year. And more is on the way: Only 8% of domestic and industrial wastewater is treated in developing countries, compared with 70% in high-income countries.

In developed countries, solid sludge resulting from bacterial digestion in wastewater treatment plants is commonly incinerated or applied to farmland, where it can contaminate soil. In the U.S., about half of the 7 million dry metric tons of sewage sludge generated annually is applied to farmland.

But so much more can be done with this stinking stuff. Taking out high-value chemicals reduces solid-waste generation and subsequent waste treatment costs. The practice can also benefit water utilities looking to repurpose sewage into industrial or potable water.

Recovering chemicals from wastewater is at the nexus of water, energy, and food security. “Interest in using wastewater is only going to increase,” says Shrinivas Tukdeo, an analyst who covers water treatment for the market research firm Frost & Sullivan.

The Netherlands, a small, low-lying nation where water management is part of daily life, is leading the push to extract products from wastewater. It has programs to recover energy, phosphorus, and cellulose at more than a dozen wastewater plants.

Cellulose can be readily recovered from wastewater in a simple pretreatment phase using fine sieves (Water Res. 2013, DOI: 10.1016/j.watres.2012.08.023), says Enna Klaversma, a technology expert at Energy & Raw Materials Factory, a company that was formed by Dutch municipal water treatment companies and is known by its Dutch acronym, EFGF.

Klaversma studied environmental science at Wageningen University & Research, which has become something of a Dutch center for wastewater treatment research. Being an expert in sewage sludge and biogas generation isn’t for everyone, but she has been involved in a number of wastewater-to-chemical projects in the Netherlands as a well as a volunteer for water management projects in Indonesia.

After the recovered cellulosic material is pressed to remove water, it can be used as feedstock for biofuels or higher-value industrial chemicals. Removing cellulose can also cut the energy consumed in conventional sludge treatment by up to 40%.

In 2014 in Amsterdam, EFGF helped establish one of Europe’s first facilities to recover phosphorus from sewage. Another plant opened in Amersfoort, the Netherlands, in 2016.

The Amersfoort facility’s one-step process was developed by the University of British Columbia and commercialized by Vancouver-based Ostara Nutrient Recovery Technologies. A few other phosphorus technologies are also in use around the world, including Berlin Waterworks’ AirPrex process.

Sewage sludge with a solid content of about 30% is formed at the Amersfoort facility by passing the water through a centrifuge. The sludge is then pressed, releasing a nutrient-rich liquid. Magnesium chloride is added, causing phosphorus to crystallize into magnesium ammonium phosphate (NH₄MgPO₄), which can be readily separated and used as a fertilizer.

Ostara’s NH₄MgPO₄ crystals, branded as Crystal Green, dissolve slowly over about eight months once applied to farmland. Ostara’s technology is now being used in 15 wastewater treatment plants across North America and Europe.

The Amersfoort plant produces about 900 metric tons per year of NH₄MgPO₄ and generates 1,250 kW of energy in the form of biogas. The plant is fed with wastewater generated by a population of 315,000.

Harvesting phosphorus cuts sewage sludge volume at Amersfoort by more than 50%, according to Eliquo Water & Energy, the engineering contractor for the project. Sludge disposal costs are about $50 per metric ton. “This can reduce wastewater plant operating costs in the Netherlands—where almost all sewage sludge is burned—by hundreds of thousands of dollars per year,” Klaversma says.

But even the enthusiastic Dutch are cautious about selling sewage-derived products on the open market. Under Dutch law, products generated from sewage plants—including NH₄MgPO₄—are still classified as a waste, and the market is restricted to companies licensed to receive waste.

Dutch regulators are concerned about contamination by pathogens as well as contaminants such as drug metabolites. It could be years before the government recognizes materials derived from wastewater treatment as conventional products, Klaversma acknowledges.

Although the Dutch are particularly keen on extracting chemicals from waste, officials from across Europe are promoting such technology because of the high cost of waste disposal on the Continent and a desire to shift to a circular economy in which resources are reused.

The European Commission is helping fund a string of efforts to make products from wastewater. Among them is Waste2NeoAlginate, a project aimed at alginatelike exopolymers, high-molecular-weight polymers made of exopolysaccharides, a sugar residue generated by bacteria that populate wastewater treatment plants.

The exopolymers are similar to alginate, a fairly expensive polymer extracted from seaweed and used in applications such as food and paint thickening.

In a world where the human population is projected to grow to 9.7 billion by 2050 and demand for resources will rise, the generation of products and clean water from wastewater will be essential, a United Nations report affirmed earlier this year. Recovering phosphorus—an essential element for growing food that could be in short supply in coming decades—is particularly important, the UN concluded.

In a four-year project that began in August, Waste2NeoAlginate’s partners, including Delft University of Technology, are testing a two-step process of extracting and refining alginatelike exopolymers from sewage sludge.

Modest volumes of exopolysaccharides are normally secreted by the sewage sludge bacteria, but plant operators can tune conditions in treatment tanks to promote the process. A metal cation is then added to the sludge to form an insoluble alginate-like gel. The process is similar to that used to recover alginate from seaweed.

In October, Water Authority Rijn en IJssel, a Waste2NeoAlginate partner, began building the first plant that will use the technology. Located in Zutphen, the Netherlands, the plant is due to open in early 2019 with the capacity to make 400 metric tons per year of alginatelike exopolymers from wastewater generated by nearby dairy factories. The partners anticipate recovering 20 kg of exopolymers from every 100 kg of sewage sludge.

The process is expected to reduce sludge generation by about 30%. “The regional water authorities expect to save costs on wastewater treatment by recovering the alginatelike exopolymer,” says Philip Schyns, senior project adviser for Rijn en IJssel. “They can sell it, which generates money, and they have less sewage sludge to process, which saves money.”

The Zutphen plant will cost about $13 million, paid for by the European Commission and the Dutch government. Return on investment is expected within five to 10 years of operation. Waterschap Vallei en Veluwe also plans a second wastewater-to-alginatelike exopolymers plant in Epe, the Netherlands.

About 35,000 metric tons of alginate is derived annually from seaweed, but relatively high extraction costs mean that it is not used in low-value applications. The alginatelike exopolymer’s lower production costs mean it will open up markets such as ink thickening for textile dyes, according to the water technology firm Royal HaskoningDHV, a partner of Rijn en IJssel.

Waste2Aromatics, another consortium funded partly by the EC, has made good progress since its launch in 2015. Featuring 12 partners, including the chemical maker SABIC, the consortium says it will soon unveil a blueprint for two pilot processes to generate furanic building blocks—intermediates for a range of plastics—from waste.

Both processes were developed primarily by the Dutch technology institute TNO. A steam-based process uses a feedstock of diaper fill or sieved sewage combined with household organic waste. A second, biphasic-reactor-based process uses manure as feedstock. On Nov. 30, waste management companies in the Waste2Aromatics consortium will pick one of them as the basis for a pilot plant in the Netherlands.

But it’s no easy thing to move from simple phosphate and cellulose recovery to polymers or polymer intermediate projects that require substantial capital investment.

For example, last year the Paris-based wastewater treatment firm Veolia halted an effort to make polyhydroxyalkanoate (PHA) from municipal wastewater and shut down a pilot plant in Belgium.

PHA is a biodegradable polymer with properties similar to polylactic acid, a bioplastic commonly used to make disposable cutlery. Bacteria present in wastewater can generate PHA within their cells.

Despite Veolia’s exit, research for making PHA from wastewater appears to be gathering pace. “Waste to PHA was a very interesting field of research in the 1970s when there was an oil crisis. And now with an interest in the bioeconomy rising, it’s coming back in a big way,” says Jeroen Hugenholtz, R&D manager for industrial microbiology at Wageningen University & Research.

The big hurdle to making PHA from wastewater is the cost of isolating a solid from a very dilute stream of water. “It is all about this,” Hugenholtz says.

Scientists are starting to get around the problem, although some of the projects are pretty far-out. For example, Mark A. Blenner, a Clemson University chemical engineer and synthetic biologist, has developed a process for astronauts to recycle their urine into PHA when they are on space missions.

Blenner’s approach is to engineer Yarrowia lipolytica yeast to digest urine. By splicing genes from algae and phytoplankton into the yeast and by adding a source of carbon in the form of CO₂ from astronauts’ breath, Blenner has developed a route to PHA.

The idea is that PHA can be fed into a three-dimensional printer to make whatever tools or components astronauts might require. By modifying certain Yarrowia lipolytica genes, Blenner can also generate omega-3 fatty acids, essential components of the human diet that can’t be made in the body.

While the omega-3 process would be unpalatable for many consumers on terra firma, Blenner’s technology could provide the basis for PHA production in wastewater treatment plants.

New dewatering technologies are emerging that could markedly improve the economics of making PHA and other products at wastewater treatment plants.

Centrifuges and membrane filtration systems currently separate solids from water, but centrifuges use substantial amounts of energy, while filters and membranes can clog. U.K. start-up uFraction8 says it has developed a microfluidic sieve made from plastic or steel that consumes a quarter of the energy of centrifuges (Sci. Rep. 2016, DOI: 10.1038/srep36386). And uFraction8 claims its curved channels don’t clog.

The start-up’s sieves have channels of specific geometry so that at a given flow speed, only certain-sized particles will go through each aperture, says Managing Director Brian Miller. A series of channels separates and concentrates particles into size fractions. Pathogens can be removed using the microfluidic sieve, according to Miller. “We know we can do a single micrometer target, and we suspect we can go down to in the region of 350 nm,” he says.

The company is working with technology partners to capture phosphate crystals in municipal wastewater. It’s also set to participate in a consortium that’s designing a system to recover crystallized dye particles from dye wastewater generated by a plant in India.

Meanwhile, the efficiency of membrane filtration is also improving. Operating costs are falling, making filtration membranes the long-term bet for processes that convert wastewater into products, claims Jens Lipnizki, head of technical marketing for membranes at Lanxess.

Membranes can be used in a cascade system to separate target particles by size. As with uFraction8’s technology, they can also filter out pathogens.

Membrane fouling is currently a problem in wastewater filtration. But Lanxess says it has developed a prototype membrane element featuring a compound designed to disrupt communication between bacteria, discouraging them from forming biofilms. “We’re not adding biocides. We don’t want to kill the bacteria, just make it uncomfortable for them,” Lipnizki says.

While companies such as Lanxess seek to ease the processing of municipal wastewater, other waste streams are gaining attention because of their heavy loading of nutrients.

This is the case in Scotland, which generates just 115,000 metric tons per year of sewage sludge and significantly more whiskey waste. Roger Kilburn, chief executive officer of Scotland’s Industrial Biotechnology Innovation Centre, sees an opportunity to take the 2 million metric tons of waste—known as pot ale—generated by Scotland’s whiskey distilleries and combine it with CO₂ from whiskey fermentation to generate syngas and, from there, high-value chemicals.

A hybrid approach is to combine wastewater with other waste streams. This is the strategy being pursued by California-based start-up T2 Energy. It’s harnessing algae to convert municipal wastewater, glycerin, and CO₂ from power plants into oleic acid—a building block chemical—or fuels.

By weight, half of T2 Energy’s algae cells are biomass, and half are lipids and triglycerides, says CEO Mark Randall. “Oleic acid—a triglyceride—costs in the $400–$500 range per metric ton. We can produce it for less than $100,” Randall says.

T2 Energy is in talks with potential investors to raise money for a demonstration facility. Should it go ahead, the company hopes to bring in the chemical maker AkzoNobel—a major consumer of oleic acid—to assist with the project’s management.

“Many of the inputs are free or low cost. We are ready to go to commercial scale,” Randall says.

While T2 Energy is looking to enjoy the economies of scale afforded by municipal wastewater systems and power plants, some companies want to play with more modest volumes of waste, such as that from a village or a small brewery.

The Santa Clara Valley, Calif.-based start-up NuLeaf Tech is taking this route. The firm’s Nutree device, which it calls a vertical wetland, is intended to process just 2,000 L of microbrewery wastewater into liquid fertilizer each day. The technology can be scaled up by adding modules.

Nutree is a 3-m³, treelike structure that uses microbial fuel cells to pump wastewater to its crown. Plants and microbes commonly found in wetlands digest the waste and convert it into a liquid fertilizer while releasing electrons to power the fuel cells.

The major attraction of such a system over traditional wastewater treatment plants is cost: NuLeaf estimates that it will be able to produce Nutrees for about $10,000 each. At this price the Nutree may prove attractive in developing countries, says CEO and cofounder Rachel Major. The firm is currently testing prototypes with a handful of small breweries in the U.S.

Further down the line, the effluent could even be cleaned up, used as drinking water, or reused to make beer if local water availability is an issue, Major says.

Frost & Sullivan’s Tukdeo likes the potential of artificial wetlands. Using such an approach, researchers could recover and potentially recycle hydrocarbons, fatty acids, nutrients for making fertilizers, and even dyes. This strategy would keep capital and operational costs low—advantages in developing countries, Tukdeo says.

Treelike structures that convert human waste into valuable chemicals and electricity sound like fantasies from some utopian future. The reality is that the technical and economic hurdles for such inventions are myriad. Certainly not all the technology in development will pan out.

But rising water scarcity, and the need to recover clean water from wastewater, is encouraging the adoption of such technology. By 2050, when an additional 2 billion people will be consumers, global water demand will have increased by 55%, the UN predicts.

Already, chemists and waste technologists are establishing processes with commercial potential. The merger of the wastewater and chemical industries is under way.