Cover story In brief Sewage treatment plants are chemical factories in waiting. By combining engineering expertise with chemistry and biology, plant operators can convert the solid sludge they generate into an array of useful chemical products. Some wastewater treatment plants already make phosphate and cellulose. But this is no cake walk. There are major hurdles still to jump, and project failure is very much a reality.
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C&EN | CEN.ACS.ORG | NOVEMBER 27, 2017
Wastewater
alchemy With sometimes offbeat technology, innovators seek to extract useful chemicals from waste ALEX SCOTT, C&EN LONDON
C R E D I T: S H UT T E RSTO CK
S
ewage 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. 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. 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 NOVEMBER 27, 2017 | CEN.ACS.ORG | C&EN
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Down the drain
cautious about selling sewKlaversma, a technology expert age-derived products on the at Energy & Raw Materials FacHuge volumes present challenges and opportunities. open market. Under Dutch law, tory, a company that was formed products generated from sewage by Dutch municipal water treat3 plants—including NH4MgPO4— ment companies and is known by its Dutch acronym, EFGF. domestic wastewater are still classified as a waste, and amount of household Klaversma studied environgenerated annually the market is restricted to comwastewater that is water worldwide mental science at Wageningen panies licensed to receive waste. University & Research, which has Dutch regulators are conbecome something of a Dutch cerned about contamination by 3 center for wastewater treatment pathogens as well as contamiresearch. Being an expert in sewnants such as drug metabolites. industrial wastewater share of the world’s waste age sludge and biogas generation It could be years before the generated annually water that is released isn’t for everyone, but she has government recognizes materials worldwide untreated to the environment been involved in a number of derived from wastewater treatwastewater-to-chemical projects ment as conventional products, in the Netherlands as a well as a Klaversma acknowledges. volunteer for water management Although the Dutch are parsewage sludge generated annually in the U.S. projects in Indonesia. ticularly keen on extracting After the recovered cellulosic chemicals from waste, officials material is pressed to remove from across Europe are promotwater, it can be used as feedstock ing such technology because of demand for expected proportion of for biofuels or higher-value inthe high cost of waste disposal phosphorus that increase in freshwater left over dustrial chemicals. Removing on the Continent and a desire could be met by global demand from U.S. industrial cellulose can also cut the enerto shift to a circular economy in recovery from for water by processes that gy consumed in conventional which resources are reused. wastewater 2050 industry reuses sludge treatment by up to 40%. The European Commission is Sources: UN, U.S. EPA In 2014 in Amsterdam, EFGF helping fund a string of efforts to helped establish one of Europe’s make products from wastewater. first facilities to recover phosAmong them is Waste2Neophorus from sewage. Another plant opened technologies are also in use around the Alginate, a project aimed at alginatelike world, including Berlin Waterworks’ Airin Amersfoort, the Netherlands, in 2016. exopolymers, high-molecular-weight polyPrex process. The Amersfoort facility’s one-step promers made of exopolysaccharides, a sugar Sewage sludge with a solid content of cess was developed by the University of residue generated by bacteria that popuabout 30% is formed at the Amersfoort British Columbia and commercialized by late wastewater treatment plants. facility by passing the water through a Vancouver-based Ostara Nutrient RecovThe exopolymers are similar to alginate, centrifuge. The sludge is then pressed, reery Technologies. A few other phosphorus a fairly expensive polymer extracted from leasing a nutrient-rich liquid. Magnesium seaweed and used in applications such as chloride is added, causing phosphorus to food and paint thickening. crystallize into magnesium ammonium In a four-year project that began in Nutrients make up less than 1% of phosphate (NH4MgPO4), which can be August, Waste2NeoAlginate’s partners, municipal wastewater. including Delft University of Technology, readily separated and used as a fertilizer. are testing a two-step process of extractOstara’s NH4MgPO4 crystals, branded MAJOR CONSTITUENTS OF AMOUNT, DOMESTIC WASTEWATER mg/L ing and refining alginatelike exopolymers as Crystal Green, dissolve slowly over from sewage sludge. about eight months once applied to farmWater >990,000 Modest volumes of exopolysaccharides land. Ostara’s technology is now being Dissolved and suspended solids, 700 are normally secreted by the sewage sludge used in 15 wastewater treatment plants of which: bacteria, but plant operators can tune across North America and Europe. a) Carbonaceous organic matter 280 conditions in treatment tanks to promote The Amersfoort plant produces about b) Protein compounds 175–350 the process. A metal cation is then added 900 metric tons per year of NH4MgPO4 to the sludge to form an insoluble algiand generates 1,250 kW of energy in the c) Carbohydrates 28 nate-like gel. The process is similar to that form of biogas. The plant is fed with wasteChloride 100 water generated by a population of 315,000. used to recover alginate from seaweed. Alkalinity (as CaCO3) 100 In October, Water Authority Rijn en Harvesting phosphorus cuts sewage Oils and grease 100 sludge volume at Amersfoort by more than IJssel, a Waste2NeoAlginate partner, began building the first plant that will use the 50%, according to Eliquo Water & Energy, Potassium (as K) 50 technology. Located in Zutphen, the Neththe engineering contractor for the project. Phosphorus (as P) 30 erlands, the plant is due to open in early Sludge disposal costs are about $50 per Nitrogen (as N) 10 2019 with the capacity to make 400 metric metric ton. “This can reduce wastewater tons per year of alginatelike exopolymers plant operating costs in the NetherHeavy metals 300 km
99%
>600 km
>80%
7 million dry metric tons
22% 55%
