Using hydrothermal carbonization for sustainable treatment and reuse of human excreta
Graphical abstract
Introduction
Poor sanitation and energy scarcity are big challenges in the developing world, contributing to health, environmental, economic and social problems. Around 2.3 billion people worldwide, mainly from developing countries, do not have access to safe sanitation facilities. Of these, 892 million still practice open defecation (WHO and UNICEF, 2017). Inadequate water, sanitation, and hygiene problems cause the death of around 842,000 people annually, 280,000 of which succumb to diarrheal death due to poor sanitation (WHO and UNICEF, 2017). Similarly, energy scarcity affects the poorest people. Around 2 billion people use solid biomass, especially wood which is collected and converted into wood-charcoal, to provide their energy needs, such as cooking and heating; these practices have environmental impacts, including air pollution, greenhouse gas emissions, deforestation, and soil erosion (Bailis et al., 2003; Ward et al., 2014).
Human excreta (often termed as black water) are considered hazardous due their potential to transmit diseases. They are also rich in organic matter and nutrients such as nitrogen, phosphorus and potassium, which can lead to environmental problems if not disposed properly. It is estimated that each person generates between 120 and 530 g of wet human feces and 1–1.4 L of urine per day (Rose et al., 2015). These nutrients can potentially be reused and recovered after appropriate treatment (Hu et al., 2016). Hydrothermal carbonization (HTC) could provide the necessary treatment to recover nutrients and sterilize human excreta while addressing sanitation and energy problems.
HTC is a thermochemical process that typically ranges between a few minutes and several hours, in which wet biomass is heated to temperatures ranging from 180 to 250 °C and self-generated pressure maintains water in a subcritical state. During the process, mainly hydrolysis, decarboxylation and dehydration reactions occur, resulting in mass loss, mainly of oxygen and hydrogen molecules. As a result, a carbon-rich solid phase with high calorific value, referred to as hydrochar, a nutrient-rich aqueous phase and some excess gas are formed (Funke and Ziegler, 2010). HTC has the potential to become an attractive treatment alternative because (i) it enables relatively short processing times; (ii) the reaction sterilizes the products; (iii) significant degradation of micropollutants, such as endocrine-disrupting agents and pharmaceuticals, is expected (vom Eyser et al., 2015), and (iv) it is considered an energy-efficient technology (Ramke et al., 2009; Reiβmann et al., 2018). Moreover, it could also be considered as a sustainable treatment with a closed-loop cycle approach that recovers energy and allows the reuse of nutrients.
Only a few studies have investigated human waste as HTC feedstock, and most of those focused on sewage sludge after different levels of treatment: Danso-Boateng et al. (2013, 2015b, 2015c) and Afolabi et al. (2015, 2017) investigated primary sewage sludge, while Escala et al. (2013) and Smith et al. (2016) investigated secondary sewage sludge and stabilized sludge after anaerobic digestion. Interestingly, in the few studies investigating HTC of human excreta (Afolabi et al., 2015, 2017), the excreta were diluted from their “typical” 20–25% solids content (Rose et al., 2015) to about 5% solids content by adding water, and reaction temperatures were up to 200 °C. Danso-Boateng et al. (2013, 2015b) used synthetic feces with solids contents of 5%, 15% and 25% subjected to 140–200 °C, and focused on the kinetics of hydrochar production and its properties.
The main objectives of this study were to explore the properties and major chemical processes occurring during HTC of raw human excreta with typical solids content (∼25% solids) in a temperature range of 180, 210 and 240 °C. Specifically: (i) aqueous and solid phases were characterized for their physicochemical properties, (ii) mass balances of carbon and nitrogen were conducted, (iii) potential use of the aqueous phase as fertilizer was evaluated, (iv) an energy accounting was calculated to explore the relevance of this practice as a sustainable environmental solution, and (v) a pilot scale reactor was operated to validate the observations from laboratory experiments. This study is the first to investigate HTC of actual human excreta with its natural moisture content. Together with the pilot scale experiment, this research is able to represent the process as closely as possible to real-world application to date.
Section snippets
Materials and methods
Raw human excreta were collected from seven people. The participants defecated and urinated into pre-weighed autoclave bags attached to a dry field toilet unit (toilet paper was not introduced) and plastic bags were sealed. At the end of each day, the collected excreta bags were autoclaved to prevent possible infection and contamination by pathogens. Full bags were weighed, dried for 24 h at 105 °C and weighed again. The dry material was then pulverized in a mechanical grinder. The homogenized
Results and discussion
In the following sub-sections, we present and discuss the result obtained in the HTC experiments. We start with the results from the hydrochar characterization, followed by the aqueous-phase characterization. After that, we focus on mass balance of carbon and nitrogen. Lastly, the energy accounting of the HTC process is considered.
Conclusions
Raw human excreta were studied under different HTC reaction severities in laboratory and pilot scale reactors with no distinct effect to the reactor size. Interestingly, raw human excreta did not follow the conventional classification of biomass in terms of its H:C and O:C elemental ratios. Moreover, following HTC, the resulting hydrochar did not resemble elemental ratio classification of coal but was closer to oil shale. The calorific value ranged from 24.7 to 27.6 MJ/kg, and had enough energy
Acknowledgements
This study was funded by the Rosenzweig-Coopersmith Foundation (USA) and the Israeli Water Authority. The authors would like to thank, Drs. Frits Caspers, Beatrice Bressan and Sergio Bertolucci from the European Organization for Nuclear Research (CERN, Switzerland) and Dr. Yaakov Garb (Ben Gurion University) for assisting with the pilot scale reactor design and donation. From Ben Gurion University, we also thank Dr. Itamar Giladi for helping with the statistical analysis, Drs. Sofiya Kolusheva,
References (50)
- et al.
Characterization of solid fuel chars recovered from microwave hydrothermal carbonization of human biowaste
Energy
(2017) - et al.
Life cycle analysis of hydrothermal carbonization of olive mill waste: comparison with current management approaches
J. Clean. Prod.
(2017) - et al.
Assessing the environmental impact of energy production from hydrochar generated via hydrothermal carbonization of food wastes
Waste Manag.
(2015) - et al.
A unified correlation for estimating HHV of solid, liquid and gaseous fuels
Fuel
(2002) - et al.
Immobilization of phosphorus in cow manure during hydrothermal carbonization
J. Environ. Manag.
(2015) - et al.
Process energetics for the hydrothermal carbonisation of human faecal wastes
Energy Convers. Manag.
(2015) - et al.
Kinetics of faecal biomass hydrothermal carbonisation for hydrochar production
Appl. Energy
(2013) - et al.
Hydrothermal carbonisation of sewage sludge: effect of process conditions on product characteristics and methane production
Bioresour. Technol.
(2015) - et al.
Influence of pH on hydrothermal treatment of swine manure: impact on extraction of nitrogen and phosphorus in process water
Bioresour. Technol.
(2016) - et al.
A comparison of product yields and inorganic content in process streams following thermal hydrolysis and hydrothermal processing of microalgae, manure and digestate
Bioresour. Technol.
(2016)