## 2 Integrated and Decentralised Urban Water Management: A Resilient Approach
### Greywater  
Greywater is domestic wastewater that does not contain faecal matter, i.e., it is the water that constitutes the non-sanitary component of wastewater. It usually comes from bathtubs, showers, wash basins, laundry, and kitchens [48; 49; 50]. Its characteristics vary according to its source, and it requires different levels of treatment depending on its intended use. Greywater systems can therefore be quite diverse in terms of complexity and scale. Domestic wastewater with a low concentration of pollutants, such as that from baths, showers, and washbasins, can be considered for utilisation [48; 49]. However, those coming from sinks and dishwashers are generally excluded due to the high levels of organic matter, fats, oils, and detergents, which require more complex and costly treatment. The British standard BS EN 16941-2 [49] establishes a hierarchy for the supply and demand of greywater, prioritising the preferential use and collection of this water. For example, toilet flushing is the priority use, followed by external use without a sprayer, laundry, and external use with a sprayer. To facilitate treatment, the collection is prioritised according to source, starting with baths and showers, followed by wash basins, clothes washers, and, lastly, kitchen sinks and/or dishwashers [49]. As a calculation method, there are two main approaches to estimating greywater supply and demand: the simplified approach and the detailed approach. The simplified approach (Table 2) applies to residential buildings and is based on a balance of assumptions such as relatively constant daily demand per person for flushing toilets and washing clothes and constant daily greywater production per person [49]. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Occupancy**} & \multirow{2}{*}{**Supply (1)**} & \multicolumn{3}{c}{**Demand**} \\ \cline{3-5}  & & **Toilet (WC)** & **Laundry (2)** & **Other Non-Potable Uses (3)** \\ \hline \(1\) person & 60 & 35 & 15 & 10 \\ \hline \hline \end{tabular} (1) Yield from showers, baths, and/or wash basins. (2) These figures are based on average daily demand. Note that a clothes washer normally uses 30 L to 60 L per cycle. (3) For example, garden watering.  
\end{table} 
Table 2: Average daily supply and demand—greywater: simplified approach [49]. In Singapore, greywater from bathrooms (bathtub, wash basin, and shower) accounts for around 40% of the total volume of usable greywater, while laundry water contributes around 20% [50]. In Portugal, according to ANQIP ETA 0905, 2023, in the absence of specific studies, in new or refurbished buildings that have water-efficient devices (e.g., ANQIP label \"A\" classification or European unified label green), average water consumption can be estimated at around 110 L per day per inhabitant, with greywater production of approximately 70 L per day per inhabitant (Table 3). When analysing the different standards, there is consistency and similarities in both the supply and demand of domestic greywater. For example, in the Portuguese context (Table 3), around 57% of the total volume of greywater produced comes from bathroom water, while 19% comes from clothes washers. These figures can be compared with Singaporean standards, showing a certain difference in bathroom consumption, which can be attributed to local consumption habits. It should be noted that the quantity and origin of greywater can vary considerably depending on the nature and activity of the buildings, as well as the consumption habits of the occupants [48,50]. These factors directly influence greywater production and use patterns in residential and commercial premises. Another analysis to be made is concerning the 60 L of greywater supplied (Table 2), which are compared to the average values of 40 to 58 L of regenerated water specified in Table 3. For the specific uses, it is possible to compare that 35 L are for bathrooms, confirming the Portuguese specification of 35 L for flushing toilets, while 15 L are for laundry, very similar to the 13 L for clothes washers.