Abstract

There is no current obligation in pipeline dimensioning practice to consider free residual chlorine (FRC) consumption. The only requirements are the hydraulic parameters defined by the ‘codes of practice’ and legal regulations. The objective of this paper is to give insight into the potential additional (hidden) costs that can arise when maintaining water supply systems, i.e., parts that, from the water quality aspect, can often be considered over-dimensioned. An algorithm is used to analyse FRC consumption that is implemented in EPANET 2.0 software. EPANET 2.0 is proven to be able to yield reliable descriptions of FRC consumption in parts of water supply systems. Here it is simplified for use in relation to a single pipeline as an example.

INTRODUCTION

Most Western and Central European countries have high connection rates to public water supply systems. In Croatia it is presently about 87% (Croatian Institute of Public Health 2017). It is noted that approximately 1.6% of the population is connected to local systems, while the remaining 11.4% obtain water by other means and are not connected to any system. As most large settlements (towns) have high levels of connection, further increases in the connection and coverage rates are most likely to occur in rural areas. In accordance with EU Directive 98/83/EC (1998), every settlement in Croatia with more than 50 inhabitants or supplied with 10 m3 of water a day must be covered by a public water supply system.

When smaller settlements remote from existing infrastructure are connected, the dominant technical solutions are those with long pipelines, and are dimensioned according to the ‘codes of practice’ and legal regulations. In Croatia, water supply pipelines have a double function – to supply water for human consumption and quantities sufficient for firefighting. Because of this, many engineers dimension the pipelines for the sum of peak sanitary consumption plus the quantity required to meet firefighting needs.

Water quality is monitored using chemical and (micro-)biological indicators. A standard operating condition for water supply systems is that all indicators must be within the maximum permissible concentrations (MPC). This is achieved by the treatment processes, with mandatory disinfection before release into the system, to eliminate or reduce the water's microorganism content. There is no trend among the chemical indicators toward the MPCs in the water supply network and that would not be expected. This is not the case with microbiological indicators, however.

When monitoring water quality in supply systems with respect to human consumption, it is standard practice to monitor free residual chlorine (FRC). The use of mathematical models to predict the FRC levels for individual parts of water supply systems is becoming more widespread nowadays, as the knowledge and capabilities of computer tools increase.

Others have shown that it is possible to establish a firm link between pipeline condition (age, corrosion, etc.), medium (water) and hydraulic operating conditions in systems, as well as projecting the FRC level in parts of the system, including those whose construction is still only being planned.

It must be stressed now that, when analysing FRC consumption in a system, the critical periods are those with minimum water demand. In this paper, in order for the problem to be presented clearly, the calculation is presented as if it was based on the average year annual water demand.

If a small, distant settlement with only one category of consumer is analysed, the average annual water demand is defined using Equation (1):
formula
(1)
where q is the average flow (l/day), qspec the specific water demand by one person (l/c/d), and N the estimated number of people for whom the water supply system is being built.

When dimensioning a system's hydraulic conditions, the peak consumption values are defined by introducing safety factors to isolate the system's peak consumption hour. Alternatively, the calculation can be based on simultaneity of consumption and consumption units, such as the methods suggested by Brix or H. Charlent (Margeta 2010). In addition, the Ordinance on firefighting hydrant network (Republic of Croatia Ministry of the Interior 2006) is in force in Croatia, and specifies an additional supply of 10 l/s over and above the calculated peak domestic consumption (Figure 1), and a pressure of 2.5 bar at the least favourable point in the (sub)-system.

Figure 1

Example of calculated consumption for a settlement of 45 inhabitants with 125 l/c/d specific demand, following effective legislation.

Figure 1

Example of calculated consumption for a settlement of 45 inhabitants with 125 l/c/d specific demand, following effective legislation.

In some countries (e.g. Germany, Netherlands), legislation regulating firefighting demands is already recognized as conflicting with the maintenance of water quality for human consumption. Compliance with such legislation often results in the construction of infrastructure that requires additional maintenance to retain satisfactory quality parameter levels with which domestic water is monitored – e.g., the FRC content, which changes (decreases) over time.

METHODOLOGY

Concentration changes for substances in water supply systems are calculated using reaction equations, i.e., kinetic models (Vasconcelos et al. 1997). To select a suitable model, it is important to know the reaction level, i.e., the manner and rate of change in concentration of the substance. Reactions can be analysed within a clear pipeline profile (the bulk fluid), as well as along the pipe wall.

Kinetic n-order reactions within the free profile are described by Equation (2):
formula
(2)
where R is the instantaneous rate of reaction (mg/l/s) dependent on the substance's concentration; Kb (bulk coefficient) is the coefficient of reaction within the bulk fluid; C is the concentration of the substance, and n is the reaction order.
The rate of reaction at or near the pipe wall is a function of the concentration in the bulk fluid and derived using Equation (3):
formula
(3)
where Kw is a wall reaction rate coefficient of zero or first order, and A/V is the surface area per unit volume in the pipe.
Calibrating mathematical models on different water supply systems gives different coefficient values and ranges for chlorine reaction with the pipe wall, as well as the coefficient of chlorine consumption in the bulk fluid. The solution for the kinetic reaction for a pipeline with a known profile (Cooper 2009) is shown in Equation (4) and takes both reactions into account:
formula
(4)
where C is the required concentration (g/m3), C0 the initial concentration (g/m3), D the pipeline profile (m), and t (days) the time passed. Equation (4) can also be written with the time (t) as the unknown – Equation (5):
formula
(5)

If the minimum required value (e.g., the limit of chemical-analytical detection rate determined by the sampling equipment) is defined as C, analysis will identify the time when the FRC level falls below the minimum value chosen.

Assuming that the initial concentration, at the head of the pipe concerned, has not changed in relation to the increased flow through it, the average water demand – by all consumer categories and including losses – can be calculated (Equation (6)):
formula
(6)
where q2 (m3/d) is the minimum demand with the minimum FRC level to the final consumers at time t.
The volume required for discharge through pipeline flushing is calculated using Equation (7) (Figure 2):
formula
(7)
where ti is the arbitrarily selected time on which the balance will be based – usually, this will be one year (365 days).
Figure 2

Schematic defining the quantity of water required for flushing to maintain the minimum FRC concentration for final users.

Figure 2

Schematic defining the quantity of water required for flushing to maintain the minimum FRC concentration for final users.

The calculations below are based on Equations (1) to (7), and the data on the coefficients of chlorine reaction with the pipe wall and of chlorine consumption in the bulk fluid. These were collected while calibrating six mathematical models of FRC consumption in water supply systems (Table 1).

Table 1

Chlorine bulk and wall reaction coefficients by individual water supply systems

Water supply systemBulk reaction coefficient (d−1)Wall reaction coefficient (m/d)Network length (km)
0.0924 0.01 to 0.75 367.6 
0.8592 0.24 to 1.5 207.8 
0.15408 0.01 to 0.05 132.0 
0.024 0.008 to 0.1 801.5 
2.4 0.2 to 0.6 874.3 
0.24624 0.014 to 0.035 677.3 
Water supply systemBulk reaction coefficient (d−1)Wall reaction coefficient (m/d)Network length (km)
0.0924 0.01 to 0.75 367.6 
0.8592 0.24 to 1.5 207.8 
0.15408 0.01 to 0.05 132.0 
0.024 0.008 to 0.1 801.5 
2.4 0.2 to 0.6 874.3 
0.24624 0.014 to 0.035 677.3 

RESULTS

If the formulae are applied to a large number of cases, the results for particular parameters can be presented. Sample calculations were made for HDPE pipes with nominal bores DN63 to DN315, rated operating pressure PN10 (SDR17), and lengths ranging from 100 to 10,000 m, using the bulk and wall reaction coefficients shown in Table 1, for the needs of a settlement with 45 inhabitants with 125 l/c/d specific demand. It was assumed that the pipes were of constant cross-section throughout their length.

Figure 3(a) (left) shows a decrease in FRC concentration for the DN160 pipeline along a 4,000-meter section depending on different wall reaction coefficients (the bulk coefficient is constant for all curves). Figure 3(b) (right) shows changes in FRC concentration from identical initial conditions but when only the wall reaction coefficient is constant; the bulk reaction coefficient changes.

Figure 3

(a) Decreasing chlorine concentration with respect to pipeline length, and differing wall reaction coefficients. (b) Decreasing chlorine concentration with respect to pipeline length, and differing bulk reaction coefficients.

Figure 3

(a) Decreasing chlorine concentration with respect to pipeline length, and differing wall reaction coefficients. (b) Decreasing chlorine concentration with respect to pipeline length, and differing bulk reaction coefficients.

Figure 4(a) (left) shows FRC consumption for an initial bulk reaction coefficient of 0.24624 (d−1) and wall reaction coefficient 0.014 (m/d) in relation to pipeline length. The initial chlorine concentration was 0.2 mg/l. A little over 2,000 m along the DN110 pipeline – i.e., after more than 50 hours (Figure 4(b)) and with the load defined in this way – the FRC concentration drops to 0.02 mg/l, which is often considered the minimum detectable concentration, when chemical analysis (FRC detection) of samples from the water supply network is being done.

Figure 4

(a) Changing chlorine concentration with respect to pipeline length, with changing pipeline profile for constant values of bulk and wall reaction coefficients. (b) Changing chlorine concentration with respect to time, with changing pipeline profile for constant values of bulk and wall reaction coefficients.

Figure 4

(a) Changing chlorine concentration with respect to pipeline length, with changing pipeline profile for constant values of bulk and wall reaction coefficients. (b) Changing chlorine concentration with respect to time, with changing pipeline profile for constant values of bulk and wall reaction coefficients.

Using identical initial conditions to those used in Figure 4, Figure 5(a) (left) shows the additional quantity required for daily pipeline flushing (m3/day), and takes the price of water distribution costs (0.15 €/m3) into account. Annual costs are shown in Figure 5(b) and Table 2.

Figure 5

(a) Quantity required for daily pipeline flushing. (b) Annual maintenance cost of daily pipeline flushing.

Figure 5

(a) Quantity required for daily pipeline flushing. (b) Annual maintenance cost of daily pipeline flushing.

Table 2

Costs (€/year) of pipeline flushing to meet the minimum FRC, depending on pipeline length and profile, for constant bulk and wall reaction coefficient values

Length (m)1,0002,0003,0004,0005,0006,0007,0008,0009,00010,000
ProfilePrice of water used for flushing (€/year)
Dv 63 52 124 196 268 340 413 
Dv 75 48 137 226 315 404 494 583 
Dv 90 27 139 250 362 474 585 697 809 
Dv 110 125 269 414 558 702 847 991 1,135 
Dv 125 34 205 376 547 719 890 1,061 1,232 1,403 
Dv 140 90 289 488 687 886 1,086 1,285 1,484 1,683 
Dv 160 170 409 648 886 1,125 1,364 1,603 1,842 2,081 
Dv 180 255 537 818 1,100 1,381 1,663 1,944 2,226 2,507 
Dv 200 19 346 673 1,000 1,327 1,654 1,981 2,308 2,635 2,962 
Dv 225 80 468 856 1,244 1,632 2,020 2,407 2,795 3,183 3,571 
Dv 250 146 600 1,054 1,508 1,961 2,415 2,869 3,323 3,777 4,231 
Dv 280 230 768 1,307 1,845 2,383 2,921 3,459 3,998 4,536 5,074 
Dv 315 337 981 1,626 2,271 2,915 3,560 4,205 4,849 5,494 6,139 
Length (m)1,0002,0003,0004,0005,0006,0007,0008,0009,00010,000
ProfilePrice of water used for flushing (€/year)
Dv 63 52 124 196 268 340 413 
Dv 75 48 137 226 315 404 494 583 
Dv 90 27 139 250 362 474 585 697 809 
Dv 110 125 269 414 558 702 847 991 1,135 
Dv 125 34 205 376 547 719 890 1,061 1,232 1,403 
Dv 140 90 289 488 687 886 1,086 1,285 1,484 1,683 
Dv 160 170 409 648 886 1,125 1,364 1,603 1,842 2,081 
Dv 180 255 537 818 1,100 1,381 1,663 1,944 2,226 2,507 
Dv 200 19 346 673 1,000 1,327 1,654 1,981 2,308 2,635 2,962 
Dv 225 80 468 856 1,244 1,632 2,020 2,407 2,795 3,183 3,571 
Dv 250 146 600 1,054 1,508 1,961 2,415 2,869 3,323 3,777 4,231 
Dv 280 230 768 1,307 1,845 2,383 2,921 3,459 3,998 4,536 5,074 
Dv 315 337 981 1,626 2,271 2,915 3,560 4,205 4,849 5,494 6,139 

CONCLUSIONS AND RECOMMENDATIONS

  • In view of recent major demographic problems in Croatia, some remote settlements in rural regions can be expected to face growing problems in maintaining satisfactory water quality. In other words, over time, due to decreasing user numbers, increasing water volumes will have to be used for pipeline flushing to achieve minimum FRC concentrations.

  • In new systems, pipeline dimensioning in accordance with current regulations could contribute both to decreasing water quality and increasing maintenance costs (even if no account is taken of possible additional costs arising from potential increases in staff's scope of work).

  • While the analysis might lead to the conclusion that losses have a positive impact on water quality, this is not a recommendation to stop trying to reduce them.

  • The primary recommendation is liberalisation of the ordinance on fire-fighting hydrant networks and the preparation of firefighting plans in cooperation with water supply utilities, to define points where ‘there will always be water to fill the tanker trucks’.

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J. P.
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