## 4 Discussion
The various methods utilize different waste streams that would otherwise contribute to costly waste management and environmental issues like toxic leaching and emission of air pollutants from landfills and waste incineration. * In terms of GWP, waste valorization methods that are integrated with CCS can produce negative carbon emissions with the utilization of waste that otherwise would have been incinerated. Otherwise, when only considering carbon emissions, these technologies are not necessarily more sustainable than the conventional methods, with the exception of APR, with a promising GWP of 4.11 kg CO\({}_{2}\)/kg H\({}_{2}\)[18]. Therefore, incorporating CCS technology into waste valorization technologies will enhance their environmental benefits. * Waste plastics gasification (wPG), food waste fermentation (FWF), and biomass gasification with and without CCS are somewhat more expensive than the conventional methods (around USD 2-4/kg H\({}_{2}\)), but not so expensive as to be uncompetitive, particularly when potential for technological improvements and economies of scale are taken into consideration. The energy requirements of the processes range significantly, from 20 to 240 MJ/kg H\({}_{2}\), but as noted earlier, these values vary widely with plant location, energy sources, and many other externalities. Therefore, it is perhaps more meaningful to examine energy efficiency, which is fairly average for wPG and biomass gasification (40-50%) and slightly higher for FWF. Improvements in technology will improve energy efficiency and subsequently further decrease production costs. * Examining the four methods of glycerol conversion (data in Table 1) reveals that APR is more attractive than both ATR and SCWR, not only due to its lower carbon emissions but also due to its lower energy consumption [17]. Therefore, APR outperforms its glycerol-based counterparts in sustainability and energy consumption, but with a TRL of 4-5, APR is not a mature technology yet and is therefore currently the most expensive, followed by steam reforming (SR), then SCWR, and finally ATR [17]. Also, the energy efficiencies of the four technologies range from 45 to 74, with APR performing the least efficiently. Considering all factors, APR is the most worthwhile technology of the four glycerol-based hydrogen production processes to improve upon. * The compiled literature data about water electrolysis reveal that it is a very promising technology for green H\({}_{2}\) production, producing close-to-zero carbon emissions. The performance of electrolysis varies depending on the specific technology and electricity source. First examining costs in Table 2, it can be seen that hydropower and nuclear power currently produce the least expensive H\({}_{2}\), at around USD 2.7/kg H\({}_{2}\). The grid mix, wind power, and solar PV power are slightly more expensive, with wind generally being less expensive than solar PV. In terms of the types of electrolyzer, SOE and AEME are more cost-effective than PEME and AKE. For wind and solar PV power, the most optimal costs for SOE and AEME are USD 3.36-6.72/kg H\({}_{2}\), and for PEME and AKE, the most optimal costs are USD 4-10.13/kg H\({}_{2}\). The energy requirements for the various technologies and power sources are comparable to one another, ranging from 165 to 280 MJ/kg H\({}_{2}\). However, energy efficiency does differ, with wind-powered electrolysis being slightly more efficient than solar PV, and with SOE being the most efficient type of electrolyzer. * Examining the TRLs of water electrolysis technologies in Table 8, it can be seen that PEME and AKE are the most technologically mature, but their material supply chain -availabilities are not very high in terms of availability. PEME specifically has a lower material supply chain availability score because it requires a rare metal (platinum) catalyst.