A Brazilian initiative is developing the circular economy model for sewage treatment, using anaerobic technology in an approach that will help developing countries achieve universal access. By Thiago Bressani-Ribeiro, Cesar Mota and Carlos Chernicharo
Billions of people still do not have access to improved sanitation. The current biggest deficit in sewage treatment infrastructure is in rural areas and small- to medium-sized urban centres. At the same time, certain large metropolitan areas – including major cities in Latin America, Africa and Asia – also do not have comprehensive sewer networks, or currently treat only a very small percentage of collected sewage. Furthermore, sewage treatment plants in most low- and middle-income countries, where design, construction and/or operation problems are widespread, face increased treatment costs, poor treatment performance, and non-compliance with environmental legislation.
Given the current enormous deficit in sewage treatment in these countries, challenges abound in two areas: implementing new sewage treatment plants (STPs) where there are sewer networks; and properly and sustainably operating existing and future STPs.
The closing of resource cycles in sewage treatment offers great opportunities that could help achieve universal access to sanitation. Integrating STPs into the local and regional economy could drive social and political interest in sanitation, by increasing acceptance and demand for resources recovered from sewage. Treated effluent could be an important source of revenue for sanitation projects, especially in water-scarce regions, where treated effluents could be used in agriculture, industry or to recharge water tables. Nutrients present in treated effluent and sludge can be valuable sources of fertiliser, especially in agriculture-intensive regions.
“STPs could play a pivotal role in universal access to sanitation”
However, the road to realising the full potential of closed-cycle sewage treatment is full of hurdles, including lack of trained personnel to design, build/install and operate certain unit processes, and societal barriers to accepting, valuing, using and demanding resources recovered from sewage. Furthermore, regulations aimed at protecting public health and the environment can sometimes have the opposite effect, by imposing impossible-to-achieve standards that only drive up treatment costs to unaffordable levels. This can result in widespread malpractice, including clandestine discharge of untreated sewage or its by-products (such as sludge) to water bodies.
The anaerobic opportunity
Most people who lack access to improved sanitation live in developing countries with hot climates. Under such conditions, sewage treatment plants based on anaerobic technologies could potentially play a pivotal role in the paradigm shift necessary to achieve universal access to sanitation. They can be used at the heart of treatment plants in circular economy, closed-cycle approaches.
The Brazilian context illustrates this. The current amount of sewage generated in the country (approximately 350m3/s) could potentially generate nearly 42GWh/month of electricity by using upflow anaerobic sludge blanket (UASB) reactors to treat the main sewage flow, coupled with biogas recovery facilities (for example, combined heat and power (CHP) engines). That would be enough to supply around 250,000 households. This is merely an illustrative number, given the vast Brazilian territory and spread of population. The main point is that revenues associated with biogas recovery can foster the implementation of new sewage treatment plants.
Furthermore, in closed-cycle STPs, part of the biogas can be used within the plant as a source of thermal energy for sludge sanitisation. The remaining biogas volume (typically around 75% by our recent estimates) can be used to replace cooking gas (liquefied petroleum gas (LPG)) in households in the vicinity of the STP, representing an important social benefit. This would not only result in carbon-neutral treatment, but also in a significant additional reduction in indirect greenhouse gas emissions, because of the use of biogas – instead of fossil fuel – for cooking.
Also, anaerobic treatment preserves the nutrients present in the liquid phase. It is estimated that the nitrogen load from sewage would be enough to fertilise an area of around 480km2. This represents only a small fraction of Brazilian cropland, which means that sewage alone could never completely replace mineral fertilisers. Nevertheless, this currently unexplored resource potential could boost the implementation of new, decentralised STPs.
In water-scarce regions such as northeastern Brazil, the use of UASB reactors, followed by polishing ponds – which can produce disinfected effluent – could provide sludge and treated effluent for restricted irrigation of crops not intended for direct human consumption, such as sugar cane.
A Brazilian case study
It is because of this potential that, in 2017, a group of seven Brazilian universities partnered with local and international water companies, and other sanitation service providers, to create the National Institute of Science and Technology on Sustainable Sewage Treatment Plants (INCT Sustainable STPs). The Brazilian national research council (CNPq), the Brazilian coordination for the improvement of higher education personnel (CAPES), and the Minas Gerais state agency for research and development (FAPEMIG) provided the initial funding.
The Institute has adopted a multilayered approach based on strategic pillars. Its aim is to bring about a paradigm shift in sewage treatment in Brazil, a country where less than 50% of sewage is treated. Although the impacts are intended for Brazil, efforts are aimed at the wider Latin America region. The approach is focused on closing cycles to make sewage treatment more sustainable and increase access to sanitation.
One area of action is the training and qualifying of technicians, plant operators, design engineers and water company professionals, to conceptualise, design and evaluate various options of operational units that would allow the closing of cycles in sewage treatment. For example, a series of six Technical Notes – aimed at solving current drawbacks related to UASB reactors treating sewage – was published in Portuguese and Spanish in a special issue of an important Brazilian technical journal. In addition, a new book published by IWA Publishing – Anaerobic Reactors for Sewage Treatment: Design, Construction and Operation, edited by Chernicharo, C A L and Bressani-Ribeiro, T – aims to comprehensively address the main aspects of interest in design, construction, and operation of UASB reactors for sewage treatment.
Another area of activity is to conduct basic and applied research to foster the development of integrated and sustainable wastewater treatment plants, with the recovery and valorisation of its by-products. Currently, the INCT has an interconnected research programme comprising: management of sewers and user awareness; energy generation and management of gaseous by-products; valorisation and management of solid by-products; and removal of micropollutants, and nutrient removal/recovery. It also looks at decision-making tools and legal/institutional aspects.
Another priority is to engage with regulatory agencies at the state and national levels to bring forward adequate legislation that allows for the safe use of sewage treatment by-products. For example, the INCT is supporting local government agencies and civil society in developing a new legislation framework for water reuse and sludge to agriculture.
In addition, INCT works to transfer knowledge to the general public, the business sector and the government. Low-cost solutions and products for the sanitation sector are under development (for example, an automated sampler and modular three-phase separators for UASB reactors), as well as adapting consolidated technologies in developed countries for the different local realities of developing economies.
The INCT is also engaging with schools through theatre plays and workshops to promote interactive dialogue about so-called flushables into the sewerage system. To raise awareness and disseminate the importance of sustainable sanitation solutions, a free online course (“The route of sewage: from generation to sustainable treatment”) has been developed to train secondary and high-school teachers.
Important outputs from those activities are due to be shared during the 17th IWA Small Water and Wastewater Systems and 9th IWA Resource Oriented Sanitation joint conference to be held in Belo Horizonte, southeast Brazil, on 13-17 September 2020 (see swwsros2020.com). This joint conference is being organised by the INCT in partnership with IWA.
Research efforts
Research work has included making assessments to illustrate environmental gains associated with the use of by-products (biogas, sludge and water) in small (5000 population equivalent) and large (500,000 population equivalent) anaerobic-based plants. In typical, conventional small sewage treatment plants, gas may be flared, sludge disposed of to landfill, and treated effluent released to a receiving water body. In closed-cycle STPs, the biogas is used as a source of thermal energy to sanitise the sludge, with a significant energy surplus (as mentioned before). This surplus could be used to replace cooking gas, serving about 200 families – or approximately 800 people – living in the vicinity. This suggests the potential for complete neutralisation of the carbon footprint of the sustainable STP and an important positive social impact.
Furthermore, sanitisation allows the sludge to be used in agriculture, avoiding its disposal in landfills and, consequently, eliminating fugitive greenhouse gas emissions associated with this approach. Moreover, shorter transport distances – assuming the existence of a storage area near the closed-cycle STP, where the sludge would be made available to farmers – allows a further substantial reduction (about 80%) of greenhouse gas emissions.
The studies also simulated use of sludge and treated effluent in agriculture. Here, the focus was on use of by-products in crops such as sugar cane, not intended for direct human consumption, based on use of post-treatment in polishing ponds, and taking account of the low generation of sludge and effluent compared with large plants. The assessment showed the potential nitrogen load is greater than 7.0 tons per year (effluent and sludge), which would be enough for annual crops fertilisation of around 90ha – an area equivalent to approximately 120 football fields.
Similarly, research work has included assessment to compare conventional and closed-cycle options for large sewage treatment plants. Fugitive greenhouse gas emissions associated with the conventional STP were also shown to be fully eliminated in the proposed design for the sustainable plant, as the sludge generated in the latter is used for agricultural purposes after drying and sanitisation in a thermal dryer fed on biogas. The shorter transport distance to a sludge warehouse, compared with the distance to a landfill for a conventional plant, allows a reduction of approximately 90% of greenhouse gas emissions associated with fossil fuel burning. In this way, the overall carbon footprint of the sustainable STP is significantly reduced (about 98%) compared with the conventional one, approaching the neutralisation of greenhouse gas emissions.
In view of the high amount of sludge and treated effluent produced in large sustainable STPs compared with small plants, and so the greater demand for final disposal area, these by-products were considered for fertilising grazing land. There is potential for a nitrogen supply of more than 1700 tons per year (effluent and sludge), which would be enough for the annual fertilisation of about 34,200ha of grazing land.
Conclusion
Closed-cycle STPs are those that are designed and operated according to the concept of a circular economy and sustainable development. Raw sewage is seen as an input of a manufacturing process in which, besides the production of treated effluent and/or water for reuse, two main by-products are generated: sludge and biogas. The work being carried out under INCT Sustainable STPs highlights some of the possibilities for recovery of by-products, considering aspects of technical feasibility and local socioeconomic realities.
It provides a foundation for the change needed in the role of an STP, moving from a simple sewage conditioner to a resource supplier and revenue generator. In this role, an STP would become a crucial element for local planning, appropriate to particular realities, allowing the population to perceive its benefits in a more direct way. In many cases, the economic development in the surroundings of the STP would be pushed forward, tariff abatement would be possible, and the implementation of new STPs could be self-funded. In developing countries, this paradigm shift in sanitation is essential for increasing sewage collection and treatment.
Professor Carlos Augusto de Lemos Chernicharo, an IWA Fellow, and Professor Cesar Rossas Mota Filho are at the Department of Sanitary and Environmental Engineering, Federal University of Minas Gerais. Dr Mota is also deputy coordinator at the National Institute of Science and Technology for Sustainable Wastewater Treatment.
Thiago Bressani-Ribeiro is a PhD student at Federal University of Minas Gerais and Ghent University.
