Current ways of managing Spent coffee grounds (SCGs) are uneconomical and have significant environmental impacts.Alternative approaches, such as hydrothermal carbonisation (HTC), which can utilise their rich organic matter for energy recovery purposes are essential. Here we present, a response surface methodology (RSM) for understanding the combined interactive effects due to the main HTC operating conditions, i.e. reaction temperature and residence time, as well as optimising them to produce hydrochar from SCGs of maximal yield and calorific value. The fuel properties and combustion behaviours of hydrochar were further evaluated to assess its suitability to replace coals for energy applications. Depending on the operating conditions, the atomic carbon content and calorific values of the hydrochar were significantly improved: by 11.2-30.7% and 15.8-44.7% respectively. The highest hydrochar calorific value recorded, 33.5MJ.kg -1 , resembled that of anthracite and dry steam coal, generally used in the UK. At optimal conditions of ~216°C and 1hr -guided by the RSM -a maximal hydrochar yield of ~64% and a calorific value of 31.6MJ.kg -1 are feasible. Using this as a benchmark, the 500,000 tonnes of SCGs generated annually in the UK has the potential of replacing 4.4% of the coal used for electricity generation in the country.

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This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT Highlights-Microwave hydrothermal carbonization can convert human biowaste to solid fuel chars -Physicochemical, structural, energetic and combustion properties of chars are enhanced -Higher heating value (HHV) of chars recovered increased by up to 41.5%-HHV of chars -up to 25.6MJ/kg -is greater than that of low-ranking coals/fuels -Processing human biowaste into solid fuel is promising for energy applications M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT KeywordsFaecal biomass, microwave, hydrothermal carbonization, chars, sanitation, renewable energy IntroductionTwo of the key issues facing more than 2 billion people in developing countries are poor sanitation and energy scarcity. Having missed the Millennium Development Goals' targets by wide margins, achieving access to adequate and equitable sanitation, ending open defecation/untreated faecal wastewater and increasing the share of affordable, renewable clean energy by 2030 now constitute the key targets of the recently adopted Sustainable Development Goals 6 and 7 (1). Annually, an estimated one billion tons of faecal wastewater is generated (2) and as much as 90% of this is discharged untreated (3, 4). Open defecation is still practised by almost one billion people, and about 2.4 billion people still lack access to improved sanitation (5). The consequences of poor sanitation are pervasive. Open defecation fields (which serve as breeding sites for insects, vectors/disease pathogens), odour nuisance, exposure to human faecal biowaste (HBW) during manual pit emptying and the indiscriminate disposal of this waste intosurface water, open drains, near slums and other fragile settlements constitute serious public health and environmental risks. Faecal contamination of drinking water resources is primarily responsible for the high infant mortality rates due to waterborne diseases such as diarrhoea, which kills 700,000 children per year (6). Aside the environmental and health impacts, experts estimate that lack of access to sanitation cost the global economy US$223 billion in 2015 -based on an economic valuation of the costs associated with premature death and loss in productivity, the healthcare costs of the sick and time forgone due to lack of access to improved sanitation (7).In common with poor sanitation, energy scarcity affects the least well off; an estimated 90% of people in developing economies lack access to reliable energy supplies (8). More than two billion people rely on firewood, charcoal and other related forest biomass to meet...

organic-rich liquor. Also discussed are the influence of HTC heating methods and temperature on HBW 10 processing conversion into solid fuel char, i.e. yield and post-HTC management, i.e. dewaterability rates, particle 11 size distribution and the carbon and energy properties of solid fuel char. While HTC temperatures influenced all 12 parameters investigated, especially yield and properties of end products recovered, heating source effects were 13 noticeable on dewatering rates, char particle sizes and HBW processing/end product recovery rate and, by 14 extension, energy consumed. The microwave process was found to be more efficient for dewatering processed 15 HBW and for char recovery, consuming half the energy used by the conventional HTC method despite the 16 similarity in yields, carbon and energy properties of the recovered char. However, both processes reliably 17 overcame the heterogeneity of HBW, converting them into non-foul end products, which were easily dewatered at 18 <3 seconds/g total solids (c.f. 50.3 seconds/g total solids for a raw sample) to recover energy-densified chars of 19 ≈17MJ/kg calorific value and up to 1.4g/l of ammonia concentration in recovered liquor. 20