Biological wastewater treatment processes focus primarily on the effective removal of nitrogen and phosphorus compounds, thereby reducing their negative impact on the aquatic environment. Currently, wastewater management is in transition, i.e. linear wastewater treatment is moving to a circular model . Although wastewater treatment plants are achieving the goal of improving environmental quality, they are still not using the full potential of wastewater as a raw material rather than waste.
Wastewater treatment – challenges of modern treatment plants
Annually the world produces about 380 billionm3 wastewater, and their global production, relative to current levels, is expected to increase by 24 percent by 2030 and by 51 percent by 2050. . This volume generates a huge load of pollutants. Not all of it can be effectively removed, and in addition, greenhouse gases are emitted at each stage of treatment (directly or indirectly). Wastewater treatment accounts for about 5 percent of global greenhouse gas emissions, including about 13 percent of nitrous oxide and 5-8 percent of methane. In the next decades, methane emissions from wastewater are projected to increase by up to half, and nitrous oxide by 25 percent, compared to current values .
Wastewater treatment is getting more expensive. Expectations for zero-emission facilities are also growing. Increasingly, wastewater treatment plants are looking for long-term technological solutions that would enable them not only to reduce their operating costs, but also to gradually transform their facilities into raw material and energy factories. Along with the world’s wastewater production, among other things, there is the creation of 17 million tons of nitrogen and 3 million tons of phosphorus, which can be recovered and used in agriculture, partially replacing synthetic fertilizers . In addition, wastewater is an increasingly important source of water and energy, not only for agriculture and industry, but also for housing. Combining the challenges of recovering and recycling raw materials with clean production is being efficiently implemented by environmental biotechnology, within which the trend of “circular biotechnology” is gaining strength.
Environmental biotechnology methods promote cost-effective and sustainable wastewater treatment. Currently available technological, analytical and microbiological knowledge makes it possible to modify processes toward higher pollutant removal efficiency, lower greenhouse gas emissions and recovery of useful raw materials. Although experts estimate that on a technological scale this trend is not likely to enter facilities until around 2050, we are already seeing intensive research development at laboratory and pilot scale (Figure 1). We are also seeing tremendous interest in new technologies from the facilities themselves. Among the currently leading pro-climate or circadian trends we can highlight: reduction of nitrous oxide emissions, modern methods of removing nitrogen compounds, and biological recovery of ammonium in the form of nitric oxide.
Why have nitrous oxide emissions come under special scrutiny?
No stage of transport, wastewater treatment or sludge disposal sees as much potential for reducing greenhouse gas emissions as biological removal of nitrogen compounds. . Bioreactors are recognized as a major source of gases in facilities. This is where up to 90 percent of the release occurs. strongly greenhouse-generating nitrous oxide (whose global warming potential is 310 times higher than for carbon dioxide) (Figure 2). Nitrous oxide in wastewater treatment plants comes primarily from three microbial reactions that require aerobic or anaerobic conditions. They are mediated by bacteria that oxidize ammonia (nitrification process) and heterotrophs involved in denitrification.
As a result of increasing anthropogenic pressures (industry, new pollutants, variable quality of incoming wastewater treatment plants) and climatic pressures (increasingly frequent extreme weather events, increased drought and intense precipitation events), we will see an increase in greenhouse gas emissions. In the case of nitrous oxide, sudden fluctuations in biological process conditions (changes in dissolved oxygen and ammonium concentrations, unstable transformations of anaerobic to aerobic environments) will be particularly conducive to this phenomenon [5, 6]. Although interest in continuous monitoring of nitrous oxide in facilities is growing, and in the near future EU legislation will also require a look at the problem, in most cases the scale of emissions is still out of control.
Technological know-how related to optimizing the aeration, mixing or total denitrification process, as well as providing an external source of carbon, is crucial in nitrous oxide mitigation strategies . Introducing complex technological changes requires collaboration with experts and researchers outside of those operating the facility. Therefore, it is important that national and international funds support the initiation of projects between industry and science.
Do novel nitrogen removal methods give room for recovery?
Nitrous oxide emissions in microbial transformations are inextricably linked to the configuration of technological processes. At the moment, we have access to a number of modifications of the classic activated sludge sequence, which are based on anaerobic oxidation of ammonia (anammox) and shortening of the transformation processes of nitrogen compounds (so-called partial/shortened nitrification). Thus, these technologies can effectively treat ammonia-rich wastewater while reducing energy consumption and sludge production . Removal of nitrogen compounds from municipal wastewater reduces the risk of eutrophication of waters and their potential toxicity to the organisms living in them.
Climate change is also a factor in increasing the risk of blooms. Biological nitrogen removal processes originally focused on oxidation-reduction reactions and did not consider the possibility of resource recovery. In the conventional nitrification-denitrification process, ammonium nitrogen in municipal wastewater is oxidized to nitrate in the presence of oxygen, which generates a high energy demand associated with aeration (2.6-6.2 kWh/kg of nitrogen removed) . In addition, denitrification, that is, the reduction of the resulting nitrates to gaseous nitrogen, needs a carbon source. Currently, it must be supplied from outside for the process to run efficiently. In the context of converting nitrogen compounds into gaseous nitrogen, we are faced with both high energy and substrate procurement costs.
In addition, ammonium fertilizers are produced by the Haber-Bosch reaction, a highly energy-intensive process. Here there is room to combine knowledge of the processes of transformation of nitrogen compounds in treatment plants with the opportunities offered by the recovery of ammonia from wastewater. Currently, urban wastewater is estimated to contain about 40 mg of ammonium nitrogen per liter. Assuming that 380 billioncubic meters of wastewater is generated annually worldwide, this gives the potential to produce about 20 million tons of ammonia per year. Wastewater treatment can be a pro-ecological way to meet the needs of agriculture .
Nitrous oxide is increasingly seen not only as a highly greenhouse gas, but also as a reactive form of nitrogen that has some chemical energy that can be harnessed. Attempts have already been made to recover nitrous oxide from wastewater and use it as fuel. One example is the so-called. The CANDO process, which includes partial nitrification, partial anaerobic reduction of nitrite (nitrate (III)) to ammonium oxide, followed by conversion of nitrous oxide to nitrogen gas with energy recovery by catalytic decomposition to nitrogen and oxygen . Eventually, nitrous oxide is co-fired with methane in biogas, supplying 30 percent more. more energy compared to a standard biogas blend.
About 60-80 percent. nitrite formed in the CANDO process can be converted into useful nitrous oxide . In the process as the so-called. an external carbon source, polyhydroxybutyrate (PHB) derived from microbial communities that have the ability to accumulate this compound (isolated strains from soil, for example) and then use it to reduce nitrous oxide was used. In addition, the process showed high efficiency in removing nitrogen compounds, as much as 98 percent. For more than 200 test cycles. Reduced oxygen demand, lower sludge production and the aforementioned energy recovery in the form of biogas were achieved .
There is now a growing body of evidence that bioelectrochemical systems, such as microbial fuel cells (MFCs), microbial electrochemical cells (MECs) and microbial desalination cells (MDCs), can provide an alternative approach to wastewater treatment along with ammonia and energy recovery [10, 11]. In bioelectrochemistry, bacteria at the anode can convert chemicals stored in organic matter into energy while facilitating the transfer of ammonium nitrogen across the cation exchange membrane.
The concentrated ammonia stream obtained at the cathode can be further collected by various methods, such as ammonia stripping or precipitation. To date, microbial electrochemical cells have been used to treat wastewater with high concentrations of ammonia (e.g., urine, sludge dewatering leachate) . The recovery efficiency of such processes ranged from 30 to as much as 80 percent . Note, however, that for standard municipal wastewater, where ammonium nitrogen concentrations typically do not exceed 40 mg/L, these systems will not be applicable.
Microbes are key when it comes to wastewater treatment and their role will only grow in the coming years. Modern molecular biology techniques are giving us detailed insights into the mechanisms and relationships in microbial communities. It is important to use modern techniques to modify and improve existing processes in parallel with the development of innovative technologies of the future. Wastewater biotechnology has undergone a number of changes in the past decade that have enabled it to implement the principles of the circular economy. In the climate crisis, further development of low-carbon and zero-waste technologies is an absolute necessity if we want to meet future risks and increase the adaptive capacity of the wastewater sector.
Dr. Ing. Edyta Łaskawiec at the 3rd 3W Congress
The author of the above article, an expert in biotechnology in water and wastewater management, will share her knowledge at the upcoming 3rd 3W Congress. This is a unique opportunity to personally listen to a lecture by Dr. Eng. Edyta Łaskawiec, but also other distinguished experts who deal with technologies in the areas of water, hydrogen and coal. We would like to invite you to this unique event, which will be held already on November 27-28 in Warsaw.
Dr.-Ing. Edyta Łaskawiec – water and wastewater technologist, assistant professor in the Department of Environmental Biotechnology at the Silesian University of Technology. He participates in the implementation of international projects on environmental biotechnology:
- Shortcut nitrification in activated sludge process treating domestic wastewater – key technology for low-carbon and clean wastewater treatment (SNIT) – https://snit.pwr.edu.pl/;
- Integrated system for Simultaneous Recovery of Energy, Organics and Nutrients and generation of valuable products from municipal wastewater (SIREN) – https://siren.put.poznan.pl/;
- Anaerobic biorefinery for resource recovery from waste feedstock (WASTEVALUE) – https://wastevalue.put.poznan.pl/.
In the article, I used, among others. From the works:
 Marshall Donna et al, Are you ready for the sustainable, biocircular economy? Business Horizons 2023, 66(6), 805-816 https://doi.org/10.1016/j.bushor.2023.05.002
 Zhang Xiaoyuan, Liu Yu, Resource recovery from municipal wastewater: A critical paradigm shift in the post era of activated sludge, Bioresource Technology 2022, 363, 127932 https://doi.org/10.1016/j.biortech.2022.127932
 Phuong Tram VO et al, A mini-review on the impacts of climate change on wastewater reclamation and reuse, Science of The Total Environment 2014, 494-495, 9-17 https://doi.org/10.1016/j.scitotenv.2014.06.090
 Song Cuihong et al, Methane Emissions from Municipal Wastewater Collection and Treatment Systems, Environmental Science & Technology 2023, 57, 6, 2248-2261 https://doi.org/10.1021/acs.est.2c04388
 Maktabifard Mojtaba et al, Net-zero carbon condition in wastewater treatment plants: A systematic review of mitigation strategies and challenges, Renewable and Sustainable Energy Reviews 2023, 185, 113638 https://doi.org/10.1016/j.rser.2023.113638
 Kehrein Philipp et al, A critical review of resource recovery from municipal wastewater treatment plants – market supply potentials, technologies and bottlenecks, Environmental Science: Water Research & Technology 2020, 6, 877-910 https://doi.org/10.1039/C9EW00905A
 Duan Huan et al, Insights into nitrous oxide mitigation strategies in wastewater treatment and challenges for wider implementation, Environmental Science & Technology 2021, 55, 7208-7224 https://doi.org/10.1021/acs.est.1c00840
 Winkler Mari KH, Straka Levi, New directions in biological nitrogen removal and recovery from wastewater, Current Opinion in Biotechnology 2019, 57, 50-55 https://doi.org/10.1016/j.copbio.2018.12.007
 Scherson Yaniv D. et al, Nitrogen removal with energy recovery through N2Odecomposition, Energy & Environmental Science, 2013,6, 241-248 https://doi.org/10.1039/C2EE22487A
 Ghimire Umesh et al, Transitioning Wastewater Treatment Plants toward Circular Economy and Energy Sustainability, ACS Omega 2021, 6(18), 11794-11803 https://doi.org/10.1021/acsomega.0c05827
 Zhang Xiaoyuan, Liu Yu, Circular economy-driven ammonium recovery from municipal wastewater: state of the art, challenges and solutions forward, Bioresource Technology 2021, 334, 125231 https://doi.org/10.1016/j.biortech.2021.125231