Introduction

The way we selectively manage water flows (water supply, sewerage and rainwater runoff) in a city, directly affects the form and operation.

Contrary to common perception, a function does not imply a single form.

The form of the water infrastructure (and the function that goes with it) shaped the profile of our cities: for example, the overthrow of the human geography of Paris of 1850-1900 by the possibility of raising water to the upper floors!

Sustainable design of water resources

Water supply

Freshwater usage

Freshwater is important for the day-to-day operation of households. However, freshwater is used in various human activities. The sectors with the most significant water withdrawals are agriculture and industry. Based on available statistics, it can be observed that the overall demand for freshwater increases in time. As can be seen in Fig. 1.4, the rate of change in the demand has increased abruptly after the mid-50s.

Fig. 1.4. Global freshwater use over time.

Freshwater demand

In order to answer the above question on whether it is possible to sustain the demand or not, it is first required to better understand the demand. To do so, we need to analyse the statistics on actual consumption and identify the elastic and inelastic demands in it. Table 1.2. shows a breakdown of overall consumption.

Table 1.2. Analysis of overall demand into different types of water consumption and losses.

Source: Waterworld

As can be seen above, the overall consumption can be divided into (a) authorised consumption) and (b) water losses. It is already clear that water losses comprise a number of different factors that lead to consumption from the system, which should typically not take place. Any unauthorised consumption whether it is intentional (someone is stealing from the system) or unintentional (leak) could be limited and, so increase the overall available freshwater resources.

However, when it comes to management, of freshwater, additional parameters need to be considered. While it is undisputable that access to freshwater is imperative for all, the supplier of freshwater needs to make a profit (private suppliers), or at least cover its costs including life-cycle costs of equipment and facilities (when managed by the state). Hence, planning for the future, it is important to distinguish between revenue water and non-revenue water.

Also, from Table 1.2 it becomes clear that the overall consumption is not the same as the overall demand. The larger the water losses, the bigger the variation between the two.

Management of water losses

If there are so many possible scenarios for water losses, how could these be limited efficiently?

There are different strategies applicable on different types of water losses, such as:

1. zoning and definition of distinct metered areas (DMAs),
2. localisation of leaks by field tests and measurement of the night flow,
3. reduction of the time needed to repair the source of leak,
4. pressure management, and
5. identification of leaks inside properties.

​ 

Control of leakage

Figure 1.6 shows the average daily flow in a DMA. When a leak occurs, there will be an abrupt increase in the total flow. Similarly, this should return to normal levels once the leak is repaired.

Fig. 1.6. New leak detection using real-time data / Graph © Fluksaqua

So, how could we control the leakage without isolating the whole area? What would happen if we decided to reduce the pressure? Figure 1.7 shows a breakdown of the variation of the overall consumption within a day (24hrs). It can be noticed that the background leakage is related to the pressure in the system and, so, for reduced pressure, the background leakage could be limited.

Fig. 1.7. Pressure management graph. - No source may require suitable replacement from author

Pressure reduction in the pipes

Pressure reduction is achieved using relief valves. Figure 1.8 shows the configuration of a typical manually operated valve. The valve keeps the outlet pressure constant even if the inlet pressure is bigger. The diaphragm moves due to the pressure at the outlet. High pressure (at the inlet) raises it and this means that the (red) screw goes up and therefore less water passes and the pressure drops. The pressure at the exit (target pressure) is adjusted based on the demand downstream.

Pressure reduction in properties

To define leaks inside properties, separate data needs to be collected from the individual meters. This can be achieved with the installation of Smart Meters, which collect real-time data and send it to the supplier. Figure 1.9 shows real time usage data from a house throughout the day (24hrs). It can be noticed that, even in late night hours, there is some minimum usage which would typically not be expected. This is an indication of a possible leak and additional data need to be collected to confirm that. There are cases where there will be consumption during late night hours, such as use of a washing machine to benefit from low rates on electricity. However, this scenario would not be observed continuously (e.g. for a few days). If the minimum usage is relatively unchanged this is a stronger indication of possible leak, so the consumer should be contacted and an inspection would need to be scheduled and conducted as soon as possible.

Fig. 1.9. Hourly hydraulic return for a household within a day (24hrs). / No source and poor quality require replacement from writer

(a) No source and poor quality require replacement from writer

(b) No source and poor quality require replacement from writer

(c) No source and poor quality require replacement from writer

(d) No source and poor quality require replacement from writer

Fig. 1.10. Usage recordings on a (a) daily, (b) hourly, (c) one-minute, (d) quarter of an hour scale.

Figure 1.10 shows indicative household usage recordings on a daily, hourly, minute, quarter of an hour scale. Characteristic spikes correspond to different events. For example, on a one-minute scale, one can notice the spikes in usage due to use of taps, toilet flushing, or shower. Such assumptions cannot be made safely using a less detailed scale (e.g. an hourly one). However, they can be used to identify abnormal increases to the hourly or daily usage which would be indicative of potential leaks.

Fig. 1.11. Plot of the average minimum supply between 1 and 4am for a household for a year. / No source require replacement from writer

Fig. 1.12. Plot of the average minimum supply between 1 and 4am for a household for a year. / No source require replacement from writer

Economic level of leakage

Assuming that we do have the technology to identify leakages in real time and repair them as soon as possible, which would be the optimum leakage level? Would it be optimum to minimise it?

When it comes to total management of water resources, economical factors need to be considered. Hence, the more efficient the leakage control is (faster detection, smaller repair time, better quality of work to prevent future leakage), the higher its cost. So, to answer the above question, one needs to consider the cost of the water lost and the cost of leakage control. If they can be predicted using a mathematical function, then this becomes an optimization problem where the minimum of this function is sought. Figure 1.13 shows the cost versus water losses plot of the aforementioned. The minimum of this function is what we call the Economic Level of Leakage (ELL).

Fig. 1.13. Illutrative representation of the determination of the economic leakage level on a cost vs. water losses graph.

Credit: INTERNATIONAL WATER ASSOCIATION via Waterworld

Management of water usage

Figure 1.14 shows the water consumption and its breakdown as outlined in Table 1.2. The management of water losses was discussed in the previous section. Here, we will take a look at the authorised consumption.

Fig. 1.14. Analysis of overall demand into different types of water consumption and losses / Image: UHI

Figure 1.15 shows a breakdown of the water consumption in a typical household, as given by MWH Global and the American Water Works Association. It can be notices that 31% is used for outdoor activities and 26% of it on the toilet. However, is it a requirement for this water to be clean tap water? Would it be possible to use water from other sources instead, and so reduce the overall demand for water per household? Also, would it be possible to reduce the water usage for other activities, such as showering, or dishwashers?

Fig. 1.15. Breakdown of the water demand for a typical household / Credit: Ensia

In literature, there are various methods proposed in order to reduce the overall water consumption in households, such as:

• Use of grey water
• Collection of rainwater
• Use of flow rate reduction devices on showers and taps
• Use of equipment that require less water to perform the same operation
• Change of the users’ behaviour with regards to the usage of existing equipment.

Grey water

What is grey water? Grey water is water that comes from the use of clean water for a household activity. Sources of grey water in households could be:

• Basins
• Showers
• Washing machines
• Dishwashers

Grey water could be used instead of clean water in:

• Toilets,
• Irrigation,
• Washing machines (not always)

Figure 1.16 shows grey water collected from different sources.

Fig. 1.16. Grey water collected from shower, hand basin, washing machine, kitchen basin, dishwasher (from left to right) / Image source?

Grey water can be collected and appropriately treated so that it is recycled within the household. To do so, except for the treatment equipment, a collection tank and a storage tank would be needed (see Fig. 1.17).

Fig. 1.17. Illustration of the collection, treatment and reuse of grey water in a household / Image: UHI