Analysis of water supply system maintenance costs from the aspect of water quality

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.

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.

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.

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.

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.

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).

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 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.

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.

• 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’.