Abstract
Wastewater treatment is an energy intensive process responsible for 3 percent of global Greenhouse Gases emissions which will increase with growing population and more stringent discharge regulations. Water companies have a high demand for energy for pumping, mixing, aeration and transport. This thesis proposes an alternative treatment strategy for wastewater, using renewable energy to produce hydrogen and oxygen gas by electrolysis. Hydrogen could be produced to power transport fleets, reducing plant emissions, while the oxygen could be used to replace the large volumes of pumped air required for aerobic biological treatment. Renewable hydrogen production is produced by splitting water, via electrolysis. For each kg of hydrogen produced via electrolysis, 8 kg of oxygen are co-produced and vented to the atmosphere. As oxygen is a key requirement for aerobic wastewater treatment , there is a strong incentive to co-locate electrolysis plants near or at wastewater treatment plants (WWTP) to reduce the energy required to treat sewage and the related greenhouse gases emissions.A 3.56 litre laboratory scale sequential batch reactor (SBR) using electrolytic oxygen was operated with Food to Microorganism (F/M) ratios from 0.5 to 1 and dissolved oxygen (DO) concentrations ranging from 1 to 8 mg/L. High COD removal (from 80 to 90 %) was achieved consistently. Dissolved oxygen concentration was shown to influence bacteria respiration and oxygen uptake rate (OUR) values increased proportionally at DO concentrations between 1 to 8 mg/L. The activity of certain enzymes, linked to the biodegradation process, such as protease and glucosidase, increased at higher DO concentrations. However, higher enzymatic activity and OUR values did not result in better COD removal. The evolution of bacteria communities’ relative abundances was assessed according to DO concentrations. The most abundant phyla included Proteobacteria, Bacteroidetes, Candidatus Sacharibacteria and Actinobacteria. Significant changes in terms of relative abundance were observed during the operation time. Increasing the DO concentration to 5 mg/L and above reduced the diversity of organisms present in the reactor which was attributed to the absence of an anaerobic zone and period during the settling at higher DO. Other factors, such as the composition of the substrate and its availability played a role in the observed changes in the bacteria community.
DO concentration or F/M ratio had no significant effect on sludge production. Solid yield oscillated between 0.35-0.40 g of total solid/g of COD removed which is comparable to what was reported in the literature for conventional activated sludge (CAS). Sludge settling was characterized by measuring the sludge volume index (SVI) and hindered settling speed. Values for SVI followed a similar pattern as the F/M ratio used and remained in a normal range (80 to 180 mL/g) throughout the 130 days of operation, although it deteriorated at a higher F/M ratio. Similar observations have been made on the hindered settling speed values which decreased at a higher F/M ratio. Increasing the DO concentration did not reverse the effect the higher F/M ratio had on sludge settleability.
To assess the financial attractiveness and the feasibility of the hypothesis of using both electrolytically produced hydrogen and oxygen to reduce GHG emissions in wastewater treatment operation, a techno-economic analysis was performed. The relationship between electrolyser size and WWTP oxygen demand was studied, and it was found that electrolysers ranging from 0.5 MW to 60 MW would be required to supply enough oxygen to a WWTP size range (25-3500 kpe). The use of electrolytic oxygen to replace existing air aeration systems was estimated to reduce WWTP energy expense related to aeration by more than 80 %, while increasing indirectly the electrolyser power efficiency by 8 to 12 % for an electrolyser with an initial Higher Heating Value (HHV) efficiency of 71 %.
Leveraging energy savings from electrolytic oxygen in a WWTP to subsidize hydrogen production would yield a hydrogen price between £7.6 and £8.8 per kilogram, depending on the WWTP size, with calculations based on 2021 conditions. This corresponds to 8% to 13% savings compared to the minimal hydrogen price achieved without using the oxygen. If oxygen was valued at its market price, between £0.4 to £0.7/kg, the hydrogen selling price could be lowered by 59 % down to a value of £3.84/kg, which would be a major step in making electrolytic hydrogen competitive with grey or blue hydrogen alternatives.
The relationship between WWTP size and hydrogen production was studied, revealing that an 8 MW electrolyser co-located on a WWTP serving 400 to 450 kpe could supply the transport fleet of a UK water company, consisting of 1000 to 2000 light vehicles and 100 to 150 larger HGVs. This setup would meet both the fleet’s hydrogen demand, estimated between 1068 and 1221 tonnes per year, and the oxygen demand of the host WWTP. This would result in significant indirect and direct carbon emissions reduction for a total of around 15 kilo tonnes of CO2e/year. Assuming a larger hydrogen demand could be secured with an industrial partner, it would be beneficial to co-locate the electrolyser on larger WWTP to take advantage of the scaling up effect and further promote the hydrogen industry while reducing overall carbon emissions.
In summary, combining electrolytic hydrogen and oxygen production with WWTP operation could provide a promising opportunity to decrease carbon emissions in the water industry while improving the competitiveness of green electrolysis with other hydrogen production technology.
| Date of Award | 2025 |
|---|---|
| Original language | English |
| Sponsors | Horizon Europe & Dŵr Cymru Welsh Water |
| Supervisor | Alan Guwy (Supervisor) & Jaime Massanet-Nicolau (Supervisor) |