Decentralised sanitation systems based on source separation and urine recycling can contribute much to the existing wastewater regime. Prithvi Simha and Bjorn Vinnerås show how innovative technologies such as urine drying can improve the circularity of sanitation systems.
The norms and standards controlling the quality of the effluent released by municipal wastewater treatment plants have gradually become more stringent over the years. The current focus is on removing nitrogen and phosphorus from wastewater, as these nutrients can affect aquatic life, cause algal blooms, and degrade downstream recreational waters. While phosphorus removal and recycling are well-established industrial practices, biological removal of nitrogen is more challenging because the processes involved (nitrification, denitrification) depend on the biological oxygen demand:nitrogen ratio of the incoming water.
Human urine is by far the largest contributor to the nitrogen load in wastewater treatment plants, and is generally viewed as a pollutant and nuisance. However, researchers working with decentralised wastewater management take a different view, pointing out that the volumetric load of urine to treatment plants is small – just 1% of total volume. This means that urine can viably be collected locally and separated from other wastewater streams, which can continue to be transported to existing plants and treated using existing technologies. Research has shown that reducing urine inflow can reduce sewer corrosion, energy demand for nitrification, space required for denitrification, and emissions of greenhouse gases, such as nitrous oxide, thereby reducing the carbon footprint of treatment plants.
Recycling urine as liquid fertiliser – scale matters
Urine-separating toilets have been around for years. The latest toilets can separate urine using a concealed outlet and just surface tension, replacing the conventional two-bowl design. Using human urine as a liquid fertiliser is not uncommon and, in fact, is traditional practice in agriculture in many parts of the world. However, urine recycling currently works well only at small scale, with urine collected from suburbs, rural areas, and remote locations such as national parks. For instance, Sweden has around 135,000 urine source-separating toilets, installed mostly in holiday homes, while around 80,000 urine-separating toilets have been installed in rural and peri-urban households in eThekwini municipality, Durban, South Africa, the only large-scale implementation of urine source separation. Many have questioned whether urine recycling can actually be mainstreamed to serve dense urban areas and what is needed to make this happen.
The benefits of drying urine
When recycling the plant-essential nutrients present in urine, it is currently necessary to collect and transport water, which makes up 95% of urine, with each individual excreting about 500 litres of urine per year. Considering how little space is available in most urban households and bathrooms, the long transport distances between cities and rural cropland, and the high application rates of urine (15,000 kg per hectare) compared with synthetic fertilisers (>500 kg per hectare), one can readily conclude that large-scale recycling of urine is unfeasible. The solution could be to collect and apply the nutrients in urine without the water.
At the Swedish University of Agricultural Sciences, we have been studying urine chemistry and biochemistry since the early 2000s. Human urine is chemically quite complex and can typically contain hundreds of metabolic breakdown products. Its composition also changes depending on how and where it is collected. In ‘fresh urine’, collected as soon as it is excreted, nitrogen is mostly in the form of urea. However, waste pipes in our homes are colonised by bacteria that produce an enzyme called urease, which rapidly degrades urea to ammonia, thereby producing ‘hydrolysed urine’. Biological nitrogen removal at treatment plants would be very different if urine were not exposed to urease in our pipes, as uncatalysed degradation of urea to ammonia is very slow.
When drying urine, urease must first be inhibited to preserve urea and keep the urine fresh, because, otherwise, almost all nitrogen can be lost to ventilation as ammonia. In recent research, we found that adding, sparingly, soluble metal hydroxides such as magnesium hydroxide and calcium hydroxide to fresh urine can shift its pH to values above 10, inhibiting urease. Drying this alkalised urine results in complete recovery of nutrients, including all nitrogen, and a powder containing 20% nitrogen, 2% phosphorus and 5% potassium. Because water is removed, less than 30 kg of solids per person need to be collected every year, and the powder can be easily stored and applied safely as crop fertiliser, as pathogens are rapidly inactivated during treatment.
Engineering the urine drying system
In practice, a urine drying system can be configured in two ways. First, urine can be alkalised at the toilet, so that it can be transported without degradation of urea and dried elsewhere (see ‘Urine drying at VA SYD’, below). Second, urine can be alkalised and dried at the toilet, so that only dried urine is transported (see box ‘Closing the nutrient loop in Gotland’, on previous page). Both approaches have pros and cons. Drying at the toilet removes the need to install additional pipes for transporting urine, especially in multi-storey buildings, and any existing toilet can be upgraded to include source separation. The challenges are to design a dryer that can be placed in unused spaces in bathrooms and to keep the energy demand for evaporation low. If alkalised urine is collected centrally – for instance, from multiple public toilets or in the basement of a multi-storey building – then larger volumes can be treated, several different types of drying technologies can be used, and better process control can be achieved. Through demonstration projects at real-life settings in Sweden, we have shown that both systems can be implemented successfully.
Challenges of mainstreaming
One concern about using urine-based fertilisers in agriculture is the potential risk from micropollutants co-excreted with urine, particularly pharmaceutical residues. However, research indicates a greater likelihood of degradation of these micropollutants in soil and far higher risks associated with the current food and water system, where micropollutants are diluted rather than removed. We believe that micropollutants can be eliminated completely if source-separated urine is treated using the same advanced oxidation techniques currently used at centralised treatment plants. This would also reduce the spread of antimicrobial drugs in the environment which, combined with inactivation of drug-resistant pathogens during the drying treatment, would help significantly in the fight against antimicrobial resistance.
“As a society, we must ask how we want to manage our resources”
Unlike houses connected to the water and wastewater grid, those with on-site sanitation are far too often left to service the system themselves. While we regard urine drying as a disruptive innovation, we recognise the value of integrating the technology with current industrial systems. This integration could take the form of a full sanitation service chain that connects households with fertiliser producers and farmers through solid waste managers, as demonstrated on the island of Gotland. Only then can dried urine be collected and converted to fertiliser pellets, which can be packed, stored and transported using existing supply chain logistics and applied by farmers using existing farm machinery. Going forward, design engineering of the dryer, such as that in our Malmo installation, will be key to convincing actors such as builders, city planners, waste managers and the municipality. Research on the social acceptability of new sanitation systems suggests that most stakeholders are keen to explore the possibility of introducing urine source separation in their cities.
Even if conventional state-of-the-art nitrogen removal processes such as anaerobic ammonium oxidation (anammox) become mainstream, at best only 95% of the nitrogen can be removed from wastewater. Through alkaline drying, all the nitrogen in urine (about 80% of the nitrogen load in mixed wastewater) can be captured without any nitrous oxide emissions during treatment and recycled, which can help create a sanitation system with zero nutrient leakage. As a society, we must ask how we want to manage our resources and whether we want to improve the circularity of nutrient flows in our water and food systems. Many forward-thinking organisations, like VA SYD, certainly seem to think so. Actors who are willing to perform early testing and implementation will be vital to take decentralised sanitation technologies such as urine drying out of the research niche, helping to reconfigure our wastewater regime in line with a bio-based circular economy.
Get regular updates on progress in urine drying and recycling via our blog – Kretsloppsteknik (blogg.slu.se/kretsloppsteknik)
To find out more about commercialisation of the technology, contact Jenna Senecal (www.slu.se/en/ew-cv/jenna-senecal) or visit Sanitation 360 AB (sanitation360.se)
Urine Diversion: One Step Towards Sustainable Sanitation (EcoSanRes publication series). E Kvarnström et al, 2006.
Large-scale peri-urban and rural sanitation with UDDTs, eThekwini Municipality (Durban), South Africa – Case study of sustainable sanitation projects. Roma, E. et al, 2011.
Drying fresh human urine in magnesium-doped alkaline substrates: Capture of free ammonia, inhibition of enzymatic urea hydrolysis & minimisation of chemical urea hydrolysis. A Vasiljev et al, 2022. https://doi.org/10.1016/j.cej.2021.131026
Closing the nutrient loop in Gotland
Gotland, an island off the coast of Sweden, is a popular tourist destination, but suffers from severe water shortages, coastal eutrophication in the surrounding Baltic Sea, and a wastewater treatment plant running at capacity. The ongoing N2 BREW project on Gotland – which includes actors from the food and fertiliser industries, municipality, toilet rental companies, and academia – is demonstrating how the urine nutrient loop can be closed. Fresh urine from dry urinals is being collected, alkalised on site, dried centrally, and applied on farms to fertilise malting barley, which is used to produce beer. Already this year, 300 litres of urine have been dried and used to grow 25 kg of barley and the goal is to produce 10 tonnes of barley during summer 2023.
Urine drying at VA SYD
As part of the Horizon 2020 project REWAISE, we replaced a conventional toilet at the offices of VA SYD, which manages water and wastewater treatment in Sweden’s Skåne region, with a urine-separating toilet. The toilet is integrated with a urine dryer, which we co-developed with the design firm EOOS NEXT, and makes use of just the space below the wall-mounted toilet. The dryer has the capacity to treat 10 litres of urine per day and needs to be serviced once a month. It is equipped with a sensing platform that controls urine inflow and overflow and drying conditions, and alerts maintenance staff when it is time to empty the drying container and collect the dried urine. VA SYD’s motive for testing urine source separation is to reduce the nitrogen load to its Sjölunda treatment plant, which serves the City of Malmö – Sweden’s fastest growing city. Two more toilets drying urine on-site are scheduled to be installed, in Brunnshög, Lund, and Sege Park, Malmö.
Prithvi Simha is Post-Doctoral Researcher at the Swedish University of Agricultural Sciences and Chief Technology Officer of Sanitation360 AB
Bjorn Vinnerås is Professor and Department Chair at the Department of Energy and Technology, Swedish University of Agricultural Sciences.