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Water-Energy (W-E) Nexus of wastewater treatment plants (WWTPs) is one of the crucial research areas of the present decade. WWTPs consume about 3-5% of the global energy and contribute to indirect greenhouse gas (GHG) emissions into the environment. Although responsible for significant energy consumption and aerial emissions, WWTPs have the potential for energy recovery (sector specific renewable energy) from wastewater (WW) and sludge in addition to the energy generation from renewables like solar and wind (non-sector specific renewable energy). Considering these, a review was undertaken on W-E nexus at WWTPs from broad range of research articles collected from different scientific resources such as Scopus, Elsevier etc, and technical and government reports published in last 10 years. The main aim of this review is to identify the energy saving and low carbon measures and tools (modelling) to assess the carbon reduction potential of the WWTPs. From the literature review, it is identified that no models/tools provide the flexibility to assess (a) the energy demand of the WWTP and (b) feasibility of the renewable energy system integration to the WWTP on a single platform. Considering this, a methodological framework was developed specifically for the WWTPs called Water-Energy Nexus Tool (WENT) in a Microsoft Excel environment. This framework provides the flexibility to assess the energy demand of the WWTPs and possibility of renewable energy generation at the plant. The developed framework was applied to WWTPs in Spain (Molina and Alicantarilla WWTPs in Murcia), Italy (Ponte Metauro WWTP in Fano) and Northern Ireland (North coast WWTP) to assess their carbon emission reduction potential. Based on the results obtained from this assessment and carbon emission reduction measures reported in the literature recommendation of low carbon solutions and pathways are proposed for the WWTP managers to reduce the carbon emissions associated with the energy consumption at the WWTPs. The low carbon solution involves (i) most viable operational strategies like repair and replacement of the faulty equipments, online or automatic control of the processes, (ii) assessment of the techno-economic feasibility of the sector specific renewable energy technologies like anaerobic digester, heat pumps, and hydropower turbine, and (iii) techno-economic feasibility of the non-sector specific renewable energy technologies like solar PV and wind turbines.

The production of hydrogen from wastewater is regarded as a viable method for the generation of sustainable energy. H2 synthesis by microbial electrolysis cells (MECs) shows significant advantages over conventional fossil fuel-based hydrogen production methods, however, the anode material and structure are critical to the overall efficiency of the MEC. When the influent water enters the MEC, the microorganisms present in the wastewater, mainly electrochemically active bacteria, degrade organic matter, proliferate and establish a biofilm on the anode surface. These bacteria transfer ions/electrons to the anode with protons transported to the cathode where subsequent reduction reactions produce H2. Given the input of energy from the biofilm, only a small external bias is needed to drive hydrogen production. For these reasons, the anode must be conductive, biocompatible, and economically viable to drive the scaling of this technology to commercialization. In terms of reactor design, a membrane-less single chamber cube design has been considered suitable for practical use due to its improved electrochemical performance and low capital cost. Considering these issues, the goal of this dissertation was to identify cost-effective and recycled materials for MEC anodes, to compare their performance and cost, and through effective system design to optimise the anode's performance in terms of H2 production and chemical oxygen demand (COD) reduction. Both microbial fuel cells (MFCs) and MECs were developed using graphite flexible powder (GFG), recycled water filter block/powder (RWFB/P), and recycled carbon fibre (RCCF) and carbon felt (CF) based anodes. Performance was compared in terms of electricity production (MFC), H2 generation (MEC) and COD reduction rate. GFG based system outperformed all other systems producing gas with a hydrogen composition of 61.96 ± 5.18%, at a volume of 0.280 ± 0.009 L/L/d H2. Based on the lab data and using a Pugh based design approach, a concept design for a novel MEC was proposed to serve as a starting point to inspire further progress towards of commercialisation of MECs using cost-efficient and recycled a node materials.

Improved energy generation methods are required to guarantee sustainable energy sources for the future. Gasification technology allows for waste and low-quality biomass materials to be converted into a producer gas, which can in turn be utilised in a CHP system for sustainable heat and electricity generation. To improve the downdraft gasification process, blends of biomass materials were investigated for their impact on process parameters such as producer gas lower heating value, conversion efficiency, tar production and producer gas application processes. This work was carried out through a combination of experimental and simulation analysis. Novel work analysing agricultural residues such as poultry litter and anaerobic digestate, along with the bioenergy crop miscanthus showed improvement in gas lower heating value when using blended feedstocks. From previous feedstock blending literature, the blending ratios of interest were identified as 80/20, 60/40 and 40/60 by weight percentage, with wood pellets the blending feedstock introduced. Small-scale experimental analysis was carried out on the Fluidyne downdraft gasification system in Ulster University, using the results collected to validate the computational simulations generated with ECLIPSE simulation software. Experimental results from the blended feedstocks displayed improvements up to 12.9% in producer gas lower heating value, from the 24% increase in carbon content of the feedstock after the introduction of wood pellets. Increases in the combustible portion of the producer gas led to a 6.1% improvement in thermal efficiencies along with 2.5% increase in electrical efficiency of the simulated CHP process. A cradle to grave life cycle assessment carried out using SimaPro software to evaluate the environmental impact that blending feedstocks for downdraft gasification and the application process had showed improvements in environmental impacts when compared to traditional energy generation methods. Further ECLIPSE software was used to carry out the techno economic analysis of the process and using financial indices to evaluate the impact of blends, decreases in net present value, break even selling price and payback period were found due to the purchasing cost of the wood pellet feedstock.

The EU renewable energy directive (2009/28/EC) has set a range of national targets to collectively raise the average renewable share across Europe to 20% by 2020. Northern Ireland and Ireland have set their own ambitions on 40% electricity consumption from renewables by 2020 and 10% and 12% renewable heat by 2020 respectively for Northern Ireland and Ireland. Air source heat pumps have been identified as a major contributor towards reaching renewable heat targets and decarbonisation of heat. In order to maintain a working grid however system constraints result in limitations on wind generation often resulting in curtailment overnight in low demand hours with high wind. Full electrification of heat also gives rise to the necessity to increase generation and reinforce the electricity grid. A more cost-effective solution would be a more sophisticated method which would balance energy production and demand through energy storage. Using high temperature heat pumps domestic retrofit application is possible and when combined with energy storage, demand side management (DSM) control strategies, and user incentives it could be possible to provide a deployable means of balancing non-dispatchable wind generation with heating demand. The research presents operational performance of a retrofit high temperature cascade airsource heat pump (ASHP) and thermal store which uses publicly available real-time and forecast electricity grid data for Northern Ireland using a low cost network connected Raspberry Pi computer to automate dynamic charging and discharging of a thermal store based on the state of the grid demand in real-time. The ASHP can reach a flow temperature of 80°C at 11kW nominal output, thus avoiding replacement of radiators in the occupied test house. The ASHP impact on peak grid demand is minimised by shifting thermal energy stored at low grid demand. Operational data shows the system is capable of providing 8.7% of the daily demand from storage but the overall system COP drops to 1.91 compared to a COP of 2.27 when heating the house directly. The impact is an increase of on average 1.2p/kWh thermal delivered to the house compared to direct only heating. However, the CO2e intensity of the combined system was 30 gCO2e/kWh thermal less intensive than a gas boiler tested in the same house. The ASHP was also run as a hybrid coupled with the test house existing gas boiler in parallel mode (one heat source active at any one time only). In this situation the gas boiler effectively replaced the thermal store to provide DSM avoiding the HP impacting on peak grid demand. In addition, the hybrid was run using a real-time electricity market price signal as an vii alternative to a real-time grid demand signal. A small test was also run with the HP in series with the gas boiler. Due to the lack of a combined control, this mode resulted in reduced operating efficiency, however this could be removed with simple control optimisation. The Raspberry Pi enabled low cost robust fully automated smart grid capability for relatively little cost. The occupants of the test house were unaware of the source of the heat and maintained the existing heat controls. Occupant thermal comfort was never compromised, and occupants were never encouraged or discouraged to use their heating system any differently to the gas boiler installation. The research shows that carbon intensity of domestic heating can be lowered when using an ASHP. Coupled with well-designed thermal storage the impact of electrified heating on the grid network can also be managed with simple and cheap technology without compromising on user thermal comfort. This will come with an efficiency compromise due to storage losses however if storage coincides with otherwise curtailed wind energy overall grid efficiency will increase.