3.3. Drinking Water Temperature in the Transport and Distribution System
The temperature gradient between soil surrounding the water main and water in the pipe drives temperature change in the DWDS. The temperature of the shallow underground soil (1–2 m depth), where drinking water mains are often installed, shows seasonal variations. The ‘frost depth’ is the depth to which the ground water in the soil is expected to freeze in subzero conditions, and it depends on climatic conditions. Frost depth is considered in many countries to determine the minimum installation depth of drinking water mains to avoid freezing of water in the pipes, or breaking pipes from freezing and thawing of the soil around the pipes. Typical installation depths in central Europe vary between 0.8 m and 1.5 m, whereas in countries such as Finland at higher latitudes, installation depths increase, up to 2.5 m. In other countries, where frost is not an issue, the minimum depth of the trenches is determined in such a way that the pipes are protected from traffic and external loads. In Cali (Colombia) an installation depth between 1.0 m and 1.5 m was reported. In Spain, for instance, the minimum depth will be such that the upper border of the pipeline is at least one meter from the surface; under sidewalks it should be a minimum of 0.60 m. In South Africa, the cover should be no less than 0.9 m [27
], although older South African standards stipulated 0.6 m minimum cover. Pipes in South Africa are typically installed at approximately 1.5 m. Water reticulation design guidelines provided by WaterCare in New Zealand suggest 1.0 m cover in roads and 0.75 m in berms and open country [28
Soil temperature is influenced by the weather (air temperature, solar radiation, etc.), the environment (rural vs. urban), land-cover (bitumen/tar vs. natural vegetation), soil type and conditions (sand vs. clay and moisture content), as shown below. The energy transfer rate from the soil to the inner pipe wall is determined by the conductivity of the pipe material and the thickness of the pipe wall. Subsequently, the energy is transferred from the inner wall to the flowing water. Within a few hours, drinking water reaches the surrounding soil temperature, depending on factors such as the pipe diameter, wall thickness and flow velocity. Based on the equations presented by Blokker and Pieterse-Quirijns [11
] it is possible to calculate the time needed to warm up the water contained in a pipe of a certain diameter, given an initial drinking water temperature and the soil temperature. Figure 2
shows the number of hours needed for drinking water in distribution pipes to heat up from 15 °C to 25 °C and number of minutes in connection pipes to warm up from 20 °C to 25 °C. Plastic and asbestos cement pipes are thermal insulators and this means a relatively long heating time. Cast iron pipes, even with cement lining, show a much shorter time for the water to heat up from 15 to 25 °C for the same diameters, e.g., less than 1 h for a 150 mm cast iron pipe with cement lining [11
The term “urban heat island” describes built up areas that are hotter than surrounding rural areas due to limited evapotranspiration, heat storage in buildings and urban surfaces, and anthropogenic heat sources. Sources of anthropogenic heat include cooling and heating of buildings, manufacturing, transportation, lighting, etc. [30
]. Recently it was proven that the temperature of the shallow underground is also strongly influenced by anthropogenic heat sources such as district heating pipes, electricity cables, underground parking garages, etc. and it can lead to which is known as the ‘subsurface heat island effect’ [32
]. Analysis of German cities has shown that superposition of various heat sources leads to a significant local warming [32
]. Measurements of soil temperatures in The Netherlands have shown that soil temperatures at depth of 1.0 m in a warmer than average summer with a heat wave can reach very local up to 27 °C and can heat up at a rate of 1 °C per day, in so-called ‘hot-spot’ locations. Examples of ‘hot-spot’ locations are industrial areas with large anthropogenic heat sources, with no vegetation and good drainage that prevents infiltration and fully exposed to the sun radiation [21
Blokker et al. [10
] modelled drinking water temperature in the DWDS using EPANET-MSX [35
]. The use of EPANET-MSX facilitates the calculation of temperature at each node in the distribution network. The model was developed assuming a constant soil temperature over 24 h. Figure 3
shows that tap temperatures vary from 10 °C close to the WTP to 25 °C further downstream. Machell and Boxall [19
] reported measured temperatures in the networks and showed that temperature increases with increasing water age along flow routes. Figure 4
shows different pipe routes for a network with two Service Reservoirs (SRs) and demonstrates a range of temperature increases. Although several soil temperature models for rural areas have been proposed, little is known about the soil temperature profile in urban areas. A one-dimensional soil temperature model was developed by Blokker and Pieterse-Quirijns [11
] and extended by Agudelo-Vera et al. [21
] to include anthropogenic heat sources, as seen in Figure 5
3.5. Drinking Water from Source to Tap
Water has a relatively large heat capacity; therefore, considerable amounts of energy are required to heat up water. Additionally, water has a relatively high heat transfer coefficient, so it takes some time for the water to heat up; note that the time required to reach a certain temperature is decreased by convection (i.e., flowing water enhances heat transfer). A heat transfer model can calculate that it takes tens of hours to heat up water in a reservoir or a transport main (pipe diameter 300–800 mm), a few hours in a distribution pipe (diameter 60–150 mm), and a few minutes in a property connection pipe (diameter 15–30 mm). This is shown in Figure 2
]. This simple heat transfer model assumes that the driving force is the temperature at the pipe wall, which is not affected by the temperature of the drinking water. This means that the temperature of the pipe wall can be assumed to be equal to the undisturbed soil temperature at installation depth. The undisturbed soil temperature can easily be determined by a one-dimensional micrometeorology model. However, there is a heat exchange between soil and drinking water.
However, as drinking water pipes distribute water of varying temperatures (5–25 °C throughout the year due to seasonal variation), the soil temperature around the drinking water pipe is also affected by the drinking water temperature. As the pipes are installed for a long period of time, it can be expected that the soil temperature around the pipes is not always equal to the undisturbed soil temperature. Thus, the soil temperature around the drinking water pipe is also affected by the drinking water temperature. The interactions between and within the soil temperature and water temperature are complex. The effect of soil temperature on short and long wave radiation, surface convection, and heat transfer through the soil need to be considered in combination with the effect of drinking water temperature, which is difficult to model. The weather-related variables have a seasonal temporal resolution, whereas the drinking water temperature could change in a few hours depending on the flow rate of the water through the pipe.
Given the above and considering the typical residence times of water in the various parts of the network between source and tap, drinking water temperature at different locations between the source and a tap is estimated as follows:
Drinking water temperature at source or treatment plant (Table 1
): Temperature is often measured here, and hence known. Ground water temperature at the source will be relatively stable (e.g., 12–13 °C in The Netherlands and U.K./Bristol) year-round, and surface water source temperature can vary substantially between 2 and 27 °C.
Drinking water temperature in the transport main: Typically, almost equal to source/treatment plant temperature (difference of ± 1 °C). Firstly, these mains have a large diameter and are usually short enough for the residence time to be much smaller than the heating time given in Figure 2
. Secondly, these large mains substantially influence the surrounding soil temperature, which means there is a limited net heat exchange between the soil and water in the pipe. Furthermore, these mains are typically installed deeper than distribution mains, hence the soil temperature is less affected by the weather.
Drinking water in SRs/tanks: The large volume to surface area of most SRs compared to pipes leads to slower heating/cooling effects during the residence within these critical structures. However, they often have very long residence times. Figure 4
shows the relative impact of flow routes, including a second large SR to retard heating effects during the summer in the UK. It should be noted that this was for an underground tank in a hilly area. Underground tanks are affected by ground temperature as with the buried pipes. Where topology is flatter, such tanks are typically elevated above the ground. In above-ground reservoirs, heating and cooling effects can be very significant due to bigger and more rapid variations in air temperature than in soil temperature. Temperature in the reservoirs can be also affected by material. However, there is not enough data to quantify the level of difference.
Drinking water temperature in the distribution mains: typically quickly approaches the undisturbed soil temperatures at installation depth (typically 1.0 m). These mains have a limited diameter, where the residence time is greater than the heating time from Figure 2
. As these mains influence the surrounding soil temperature to a limited extent, the actual heating time may be longer than that shown in Figure 2
, but the residence times have the same order of magnitude, so there is significant heat exchange. These mains are typically installed at a depth of 1 m, where the soil temperature is subjected to seasonal change.
Drinking water temperature in the connection water supply pipes: typically almost equal to the temperature at the end of the distribution main (so soil temperature at depth of 1 m). Firstly, these small diameter mains have short lengths, where the residence time (during flow, the situation of stagnant water is kept out of the analysis) is much smaller than the heating time. These small mains hardly influence the surrounding soil temperature, and if they do, the equilibrium would be towards the temperature of the distribution mains. These pipes are typically installed at a shallower depth than distribution mains, so the soil temperature is more influenced by the weather.
Drinking water temperature in the premises plumbing pipes: Typically almost equal to the temperature at the end of the connection and thus of the distribution main (i.e., soil temperature at depth of 1 m). These small diameter mains have short lengths, hence their water residence time (again during flow) is much smaller than the heating time. These mains are not located in the soil, but in airshafts, and the air temperature is not affected by the drinking water temperature of these small-diameter pipes.
Drinking water temperature at the tap: Typically (during flow after flushing) equal to temperature at the end of the distribution mains (i.e., soil temperature at the depth of 1 m) when customers are directly connected to the network. For situations where storage occurs between the distribution network and the customer’s tap, other temperatures apply depending on the type of storage (roof or underground), local climate and storage times. Stagnant water will reach the surrounding temperature.
Consequently, it is clear that the soil temperature at the installation depth of the distribution mains is important to know. This temperature is determined by, on one hand, short and long wave radiation (including from above ground anthropogenic sources), surface convection, and heat transfer through the soil and, on the other hand, by the underground (anthropogenic) heat sources. As the anthropogenic sources can have a local effect, it is not easy to predict drinking water temperatures in the entire network. Tap temperatures are not typically measured (Table 1
), and in the soil/ground water, only on a project basis. Therefore, the soil temperatures at installation depth are mostly unknown.
3.6. Consequences of Higher Temperatures and Legislation
The World Health Organization (WHO) guidelines recommend a maximum temperature limit of 25 °C at the tap [39
]: “Cool water is generally more palatable than warm water, and temperature will impact on the acceptability of a number of other inorganic constituents and chemical contaminants that may affect the taste. High water temperature enhances the growth of microorganisms and may increase taste, odour, colour and corrosion problems”. In a recent review the WHO reports that in a survey of 104 countries, 18 countries have a regulatory/guideline value of temperature [40
]. This review also states that “None of the values for temperature were mandatory, being guiding levels or operational goals. None of the countries and territories’ documents indicated what would happen if temperatures rose above the suggested value. In addition to those with numerical values, seven countries and territories had descriptive levels such as: 2.5 °C above normal; “not objectionable”; “air temperature plus 3 °C”; “acceptable”; and “ambient””. No additional information about the countries or the type of standard is given. In the survey conducted for this paper, a number of legal standards were identified, as summarised in Table 2
Factors such as nutrient concentration, temperature and pH determine microbial community structure and potential for regrowth within DWDSs. Consequently, changes in temperature in DWDSs can influence microbial community composition, promoting the presence of pathogens and the potential for microbial regrowth, particularly of biofilms in the pipe environment [29
]. A temperature increase of drinking water can influence the microbial ecology of DWDSs, affecting parameters such as potential growth (e.g., colony count at 22 °C, bacteria of the coli
group and Legionella
) and the presence of undesirable microorganisms because of their possible role in disease [29
]. There is a difference in the effect of temperature on microorganisms depending on location, either as free-living planktonic organisms in the bulk-water, or as a community within a biofilm attached to the pipe wall. The effect of temperature may also depend on water quality (e.g., disinfectant residual, organic loading) and hydraulics. For example, some microorganisms have their optimal growth at 20 °C, others at 25 °C, and yet others at 30 °C. Thus, the temperature will affect the composition of the biofilm. However, publications about microorganisms in water supplies in many cases do not provide accurate data on water temperature [47
]. It has been shown in a chlorinated DWDS in the UK that a rise of temperature from the average 16 °C in the warmer months to a temperature of 24 °C promoted changes and loss in the complexity of microbial biofilm communities [46
The main concern regarding the impact of temperature increases in DWDS is the potential for the proliferation of pathogens such as Legionella
spp. Legionellosis is a collection of infections that emerged in the second half of the 20th century, and that are caused by Legionella pneumophila
and related species of bacteria belonging to the genus Legionella
. Water is the major natural reservoir for Legionellae, and these bacteria are found worldwide in many different natural and artificial aquatic environments, such as cooling towers, water systems in hotels, domestic water heating systems [48
], ships and factories, respiratory therapy equipment, fountains, misting devices, and spa pools [49
]. Whether or not disinfectant is used, controlling Legionella
spp. in a drinking water installation can be problematic [50
]. Temperature control is a known measure to prevent the proliferation of Legionella
. The WHO states that to prevent Legionella
infection, the recommended temperature for storage and distribution of cold water is below 25 °C, and ideally below 20 °C. Table 2
shows that this recommendation has not been adopted everywhere. Table 2
also shows that temperature standards of building owners are not always matched with temperature standards for drinking water utilities. Laboratory studies of mutant Legionella
strains show that the bacteria may grow below 20 °C under certain conditions [51
will survive for long periods at low temperatures and then proliferate when the temperature increases, if other conditions allow.
When temperatures remain below 25 °C, it is expected that growth of Legionella pneumophila
will not occur or will be limited, whereas at temperatures above 30 °C, it is likely that growth of Legionella pneumophila
will occur at significant levels, providing the biofilm concentration in the drinking water distribution system is high enough. Another prerequisite for the significant growth of Legionella pneumophila
, is that the temperature has to be higher than 30 °C for a prolonged period, reported as more than seven days [29
The results of the survey conducted herein showed that seasonal increase of temperatures can cause unpleasant taste on the palate, which may be related to pipe material (e.g., black alkathane pipework, or lead plumbing pipes). Drinking water companies are generally aware that potential issues can include the occurrence of infections (such as Salmonella
, Mycobacterium), chlorine decay and formation of byproducts. As expressed in one survey response “… it is known that increased water temperature leads to increased biofilm activity in distribution network”. Research in The Netherlands on the influence of temperature on discolouration risk, concludes that it is likely that higher temperatures in the DWDS can augment discolouration risk [52
]. In a tropical DWDS in the city of Cali (Colombia), the formation of disinfection byproducts was clearly influenced by pH, temperature, chlorine dosage, and water age. The interactions observed between these parameters and Trihalomethanes (THMs), were also shaping the microbial characteristics of these systems [24
]. Other studies regarding the effects of temperature in the DWDS are reported in Table 3