Clean water and renewable energy are essential requirements to build resilience towards the adverse effects of climate change and global warming. Advanced wastewater treatment options may provide a unique opportunity to recover various useful resources such as energy (biogas), fertilizers, minerals, and metals embedded in the wastewater stream. However, considerable challenges remain when it comes to designing and planning sustainable wastewater treatment systems. This thesis focuses on the avenues of energy recovery from wastewater treatment plants (WWTP), by evaluating the potential for biogas recovery from wastewater and sewage sludge treatment in WWTPs. Various available technologies for biogas recovery are examined and evaluated to understand their viability in different applications and relative performance. Further, the methodologies and tools employed to assess such energy recovery systems are evaluated, covering the technical, economic, and environmental performance aspects. A sustainability assessment framework is then developed, using appropriate sustainability indicators to assess performance. The framework is applied to a case study of a WWTP in the emerging city of Tbilisi, Georgia. A spreadsheet tool is also developed to aid the sustainability (technoeconomic and environmental) assessments for the case study. The case study results reveal a significant biogas recovery potential, with annual energy generation potential of 130 GWh from combined heat and power (CHP) recovery, and a potential to avoid 28,200 tCO2eq emissions every year, when biogas is recovered only from the wastewater. The recovery potential increases when biogas is recovered from both wastewater and sewage sludge. Further, the contribution of overall resource (energy and nutrient) recovery in WWTPs to the Sustainable Development Goals is examined. By studying the linkage of various benefits to the different SDGs, the multilateral and cross-cutting nature of benefits from resource recovery is clearly illustrated. The thesis concludes with the discussion of possible future technologies and perspectives that can enhance the sustainability of WWTPs and help transform them into Wastewater Resource Recovery Facilities (WRRFs). / Rent vatten och förnybar energi är väsentliga krav för att bygga motståndskraft mot de negativa effekterna av klimatförändringar och global uppvärmning. Avancerade avloppsreningsalternativ kan ge en unik möjlighet att återvinna olika användbara resurser som energi (biogas), gödselmedel, mineraler och metaller inbäddade i avloppsvatten strömmen. Det finns emellertid stora utmaningar när det gäller att utforma och planera hållbara reningssystem. Denna avhandling fokuserar på möjligheterna till energiåtervinning från avloppsreningsverk (WWTP), genom att utvärdera potentialen för biogasåtervinning från avloppsvatten- och avloppssrening i WWTP. Olika tillgängliga tekniker för återvinning av biogas undersöks och utvärderas för att förstå deras livskraft i olika applikationer och relativa prestanda. Vidare utvärderas de metoder och verktyg som används för att utvärdera sådana system för energiåtervinning som täcker de tekniska, ekonomiska och miljömässiga aspekterna. En ram för hållbarhetsbedömning utvecklas sedan med hjälp av lämpliga hållbarhetsindikatorer för att bedöma prestanda. Ramverket tillämpas på en fallstudie av en WWTP i den framväxande staden Tbilisi, Georgien. Ett kalkylarkverktyg utvecklas också för att underlätta bedömningarna av hållbarhet (teknisk ekonomi och miljö) för fallstudien. Resultaten från fallstudien avslöjar en betydande återvinningspotential för biogas, med en årlig energiproduktions potential på 130 GWh från kombinerad värme och kraft (CHP), och en potential att undvika 28.200 ton CO2-utsläpp varje år, när biogas endast återvinns från avloppsvattnet. Återvinningspotentialen ökar när biogas utvinns från både avloppsvatten och avloppsslam. Vidare undersöks bidraget från den totala återhämtningen av energi (energi och näringsämnen) i WWTP till målen för hållbar utveckling. Genom att studera kopplingen mellan olika fördelar till de olika SDG: erna illustreras den multilaterala och tvärgående karaktären av fördelarna med resursåtervinning. Avhandlingen avslutas med diskussionen om möjliga framtida tekniker och perspektiv som kan förbättra WWTP: s hållbarhet och hjälpa till att omvandla dem till anläggning för återvinning av resurser från avloppsvatten.

With over one hundred years of commercial cultivation, sugar cane is one of the most valuable agricultural botanical resources in the World. This position is not only based on production of sugar from sugar cane but also it is, to a great extent, as a result of the increasing importance of sugar cane by-products and side industries. Furthermore, with the advancement of science; awareness of inharmonious growth of materials and energy consumption, and the desire to minimize the negative impacts of industrial pollutants and materials, the scope for using sugar cane is still developing rapidly. Bagasse, molasses and filtered mud are the most important by-products in the process of production of sugar from sugar cane. Among these by-products, bagasse is both a biomass resource for producing energy and is one of the most important agricultural wastes, which can be used in different side industries. Therefore, it was chosen for study in this research as it offers considerable potential as a source of energy. Bagasse is often used as a primary fuel source for sugar mills; when burned in quantity, it produces sufficient heat energy to supply all the needs of a typical sugar mill, with energy to spare. To this end, today a secondary use for this waste product is in combined heat and power plants where its use as a fuel source provides both heat and power. With a suitable energy production technology, bagasse can be used as a fuel in CHP for high efficiency energy generation. Today, with regard to the low efficiency of traditional methods, the high cost of disposal of waste materials and environmental pollution, the use of modern methods such as anaerobic digestion for the production of biogas has increased. The collected biogas from the process of anaerobic digestion provides a renewable energy source similar to natural gas, but with less methane and lower heating value, that is suitable for use in CHP plants. In this research, a comparison with different bagasse energy production technologies leads to the selection of anaerobic digestion as the most suitable for use in Iran. Then a typical biogas CHP is assumed, and the biogas system is designed. Finally, the potential for the development of biogas CHP plants with bagasse in Iran is addressed through a study of the economic and environmental aspects.

The development of anaerobic digestion processes has gained recognition for its potential for producing volatile fatty acids alongside bio-alcohols. In addition, the fermentation process and the products produced from biological fermentation have the possibility to overcome the abundance of organic waste in our society, which is potentially immensely rich in untapped valuable potential products. The current project aims at producing medium chained volatile fatty acids and alcohols from short-chain volatile fatty acids using hydrogen as an electron donor. For efficient conversion, inhibition of methanogenic microorganisms was performed by thermally pre-treating the microorganisms at 90 °C for 15 min. The highest observed concentrations of volatile fatty acid were 6.42 ± 0.09 g/L and the concentration of ethanol was 0.33±0.03 g/L. These concentrations were obtained with the addition of 4 g/L of liquid substrate and gas co-substrate (H2:CO2). Moreover, the predominant product from the present experiment was valeric acid and it reached its highest concentration of 1.41 g/L after 37 days. Interestingly, the addition of H2:CO2 co-substrate showed that this fermentation can be used for carbon capture and utilisation alongside hydrogen consumption in a ratio of 1:1 to increase the value of short-chain fatty acids. Furthermore, this can contribute to decreased CO2 emissions and reduced use of fossil resources for alcohol production which is in line with the global environmental goals.

The production of municipal solid waste has continued to grow in recent years. In Italy, municipal solid waste production reaches about 29 million tons per year. The organic fraction of municipal solid waste (OFMSW), which accounts for 30-40% of the total waste, usually undergoes biological treatments such as anaerobic digestion or composting, or is incinerated or landfilled. Biological treatments are considered not economically viable due to the long processing time (20-30 days), while incineration and landfilling are considered as low cost but polluting processes. In contrast, Hydrothermal Carbonization (HTC) is a cost-effective process to treat organic waste especially for the shorter processing time (0.5-8 h) and the possibility to treat directly wet heterogeneous materials. This thesis aims to investigate the potential use of HTC to upgrade OFMSW and other biomasses to biofuels or valuable byproducts. Chapter 1 gives an overview on the state of the art of HTC technology applied to organic wastes, focusing both on mechanisms and on the characteristics of reaction products. Chapter 2 investigates the potential use of HTC to upgrade the organic fraction of municipal solid waste (OFMSW) into biofuel and byproducts. The impact of process conditions (process time, temperature and solid load) on the formation, chemical and energy properties of hydrochar was deeply investigated. To analyze the behavior of hydrochar as a solid biofuel, the combustion (oxidation) of hydrochar and the co-firing (co-oxidation) of hydrochars and coals was also investigated. The results show that, especially at HTC harsher conditions, hydrochar is a “coal-like” material, that can be used as a valuable solid biofuel. The results evidenced that hydrochar is composed of primary char and secondary char. Primary char (non-extractable with organic solvents) has characteristics similar to coal and could be recovered for combustion, while secondary char is an amorphous and more volatile solid, easily extracted with organic solvents, which found possible application as a source of biochemicals and liquid bio-fuel. Chapter 3 investigates the use of HTC as a promising pre-treatment to enhance the biomethane potential during anaerobic digestion of OFMSW. Anaerobic digestion experiments were carried out using the HTC process liquid and the entire HTC reaction mixtures. Results proved that, when compared to the raw OFMSW, the use of HTC liquid and HTC mixture into AD lead to an increase of biomethane production of up to 37% and 363% by volume, respectively. Chapter 4 reports an HTC kinetics study and a kinetic model, which accounted for reactions leading to the production of primary and secondary char, as well as the liquid and gas phases. The model was optimized using experimental data performed on a lignocellulosic feedstock (olive trimmings) and validated on two other types of biomasses (grape marc and Opuntia Ficus Indica) and was used as a reliable tool to predict the carbon distribution among HTC products. In this chapter an in-depth analysis was also carried out to understand the evolution of feedstock characteristics during the heat-up transient phase before reaching the HTC set-point temperature. The results show that during heat up, the feedstock carbonized to a considerable extent at 220-250 °C. Tests clearly show evidence of the transition between thermal hydrolysis and HTC. Chapter 5 presents a study conducted to evaluate the economic feasibility and the detailed energy and cost analyses of a hypothetical HTC plant transforming wet biomass into pelletized dry hydrochar. To achieve these goals, a model was developed on the basis of experimental results obtained previously on two other organic materials (grape marc and off-specification compost). The results show that, when operating the HTC plant with grape marc at the optimal HTC conditions (T=220 °C, t=1 h, dry biomass to water ratio=0.19), the production cost of hydrochar were determined to be 157 €/ton, competitive with the price of wood pellets (150-200 €/ton). This makes HTC a promising process for a large development at the industrial scale.

Recently, there has been an increased interest in the production of volatile fatty acids from pulp and paper sludge using an immersed membrane bioreactor during anaerobic digestion. The production of biogas through anaerobic digestion has been a hot topic in recent years, but it is no longer economically viable due to competition from fossil fuels. As a result, the production of volatile fatty acids has been investigated in this study using pulp and paper sludge as substrate. To investigate the effect of methane inhibition on enhanced volatile fatty acid production, various parameters, and pre-treatments such as pH, O2 presence, thermal heat shock, and chemical BES-addition were used. Heat shock pre-treatment produced the most volatile fatty acids (2.4 g/L) while producing the least methane (50 mL/g VS). The immersed membrane bioreactor was successfully used to produce volatile fatty acids for 54 days. / En av dagens problem är den ökade populationen vilket har lett till ökade mängder av avfall från mat och slam. Innan låg ett stort fokus på att använda dessa rester i en anaerobisk rötning för att producera biogas. Dock har detta visat sig att inte vara ekonomiskt hållbart just på grund av att inte kunna konkurrera med fossila bränslen, därför har man i stället fokuserat på att ta fram de intermediära produkterna från rötning processen, dvs flyktiga fettsyror. Detta projekt har fokuserat på att producera flyktiga fettsyror från pappersmassa och papperslam som är potentiellt lättare nedbrytbart, än andra lignocellulosa rika substrat. När man vill maximera flyktiga fettsyraproduktionen, då är det viktigt att samtidigt förhindra produktion av biogas. Effekten av flera olika parametrar, såsom närvaro av syre, pH, för behandling med värmechockeller BES-tillsats, för att inhibera produktion av metan har därför undersökts. Förbehandlingen med värmechock ledde till runt 2,4g/L fettsyraproduktion, tillsammans med den minsta mängden av metan, runt 50 mL/gVS. Membranreaktorn kunde användes framgångsrikt för kontinuerlig produktion av flyktiga fettsyror under 54 dagar.

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