Chemical & Engineering News | Tapping sewage as a source of useful materials
November 27, 2017 – Read full article
With sometimes offbeat technology, innovators seek to extract certain chemicals from municipal waste
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.