## 2 Materials and Methods
### MFCS GHG Impact Calculation Method  
The steps of the onsite non-potable water use cycle were developed by the author for each water type proposed for reuse. Each m\({}^{3}\) extracted, diverted, conveyed, treated, and distributed has an energy expenditure (kWh/m\({}^{3}\)) and a GHG effect expenditure (tCO\({}_{2\text{e}}\)/m\({}^{3}\)) [69]. The energy intensity for Scopes 2 and 3 will be converted to GHG using the baseline emission factor from [70]. The results will be a GHG metric for onsite non-potable water use by water type as per Equation (11) [35].  
\[\text{Water Energy GHG intensity}\ (\text{kgCO}_{2\text{e}})=\frac{\text{Total GHG}\left(\text{kgCO}_{2 \text{e}}\right)}{\text{Total water consumption}\ (\text{m}^{3})}\times 1000 \tag{11}\]  
First, calculation-based methodologies [42] will be used to quantify Scope 2 emissions (onsite pumps and treatment systems power consumption used to move and treat both desalinated water and non-potable water sources) based on activity data and the EAD [70] and IEA [31] emission factors,which are directly given in CO\({}_{\rm 2e}\) equivalent units. This means that the global warming potential (GWP) is equal to 1 CO\({}_{\rm 2e}\)[57; 58; 71] as Equation (12).  
\[\text{GHG emissions}=\sum\
olimits_{gns}\ (\text{{emissions}}_{gas}\times GWP _{gas}) \tag{12}\]  
Second, Scope 3 emissions will be calculated based on the Water-Energy GHG Guidance [35] and WRI/WBCDSD [50] protocol. Electricity consumption associated with the production of desalinated water falls under Scope 3 because the fuel and energy related activities to produce this water type are not included in Scopes 1 and 2 [35; 50]. Whereas, non-potable water supply such as RORW, food waste effluent wastewater, A/C condensate water, and fire sprinkler pump test water fall within Scope 3 as energy:water decarbonization, which is currently not defined by the GHG protocol standards [50; 56] nor the GRP standards [35; 42].  
Calculation Process for Scope Two and Scope Three  
For Scope 2, the formula for calculating the total GHG emissions of the non-potable water system for electricity is calculated by using Equation (13) [57; 58; 42], where the GHG emission factor (EF) from electricity generation in the UAE is 0.64337 kgCO\({}_{\rm 2e}\)/kWh) [31]. This greenhouse gas emission factor (GHGEF) excludes transmission and distribution losses between the power station and the end-user (ibid.)  
\[\text{Electricity generation }(\text{kWh})\ \times\ \text{GHG}_{\rm EF}\ (\text{IEA}, 2016)\ (\text{tCO}_{\rm 2e}\text{/kWh}), \tag{13}\]  
The electricity consumption in kWh is the power demand of the water system for all scenarios (MFCS, Baseline Case (S2), Proposed Research Project (PRP S3) because power will be needed to distribute the water to the point of consumption regardless of the water type used. The only exception is the ultraviolet (UV) disinfection treatment system which was excluded for scenario two (Baseline Case S2). The kWh/month will be the result of Equation (5).  
For Scope 3, the GHG emissions for desalinated water is calculated by using Equation (14) [70] where the GHGEF for water consumption in AD is 0.0151 tCO\({}_{\rm 2e}\)/m\({}^{3}\)[69]. The GHG water:energy decarbonization is calculated by using Equation (15) [50] with the same GHGEF [70].  
\[\text{Desalinated Water Consumption }(\text{m}^{3})\times\text{GHG}_{\rm EF}\ ( \text{EAD},2012)\ (\text{tCO}_{\rm 2e}\text{/m}^{3}), \tag{14}\]  
\[\text{Non-Potable Water Consumption }(\text{m}^{3})\times\text{GHG}_{\rm EF}\ ( \text{EAD},2012)\ (\text{tCO}_{\rm 2e}\text{/m}^{3}), \tag{15}\]