Role of typical pipes in disinfection chemistry within drinking water distribution system

Deep insight into the forces driving chloramine decay in different pipe materials is the key to taking sound action to cope with pipe water quality deterioration. By using the newly developed RTCDM (refined Total Chloramine Decay Model) and pipe section reactor, the role of four typical pipes in disinfection chemistry was qualitatively and quantitatively compared, and the mechanism of pipe wall mediated chloramine decay was further described. As for the four typical pipes studied, the characteristics of deteriorating water quality, especially for accelerating total chloramine decay was in the order of cast iron pipe > steel pipe > cement lined ductile iron pipe > polypropylene-random pipe. Cast iron pipes, cement-lined ductile iron pipes, and steel pipes of long service age are characterized by one or two driving forces leading to TCR decay. Aged cast iron pipes could take up chloramine by Fe(0) and microbes (especially nitrifiers) spreading over the inner wall. Aged steel pipe is characterized by aggressive electrochemical corrosion and weak nitrification. Lime and gypsum leaching is the main cause, and nitrification/denitrification may also occur in aged cement-lined ductile iron pipe. Polypropylene-random pipes have a minimum effect on disinfection chemistry. This knowledge is of value in speculating the reasons leading to TCR loss in the full scale distribution system.

Distribution systems are the necessary infrastructure to deliver treated water to the terminal user. Except for frictional head loss leading to hydraulic pressure decrease, pipes especially aged iron pipes, would also lead to water quality deterioration during long distance delivery, as complicated biochemical reactions occur within the pipe system. As water age increases within the ‘biochemical reactor’, the shift in water quality would be more significant compared with the treated water. Hence, stagnant water points or end points of a distribution system are usually characterized by deteriorated water, e.g. red water, bacteria breeding, high turbidity, etc., which have always been the hotspot focused on by water utility and other authorities.

Another driving force that may accelerate disinfectant decay is electrochemical corrosion that occurs in unlined cast iron, galvanized iron, and steel pipes. Chlorine/chloramine are all strong oxidants with standard electrode potentials significantly higher than Fe(0), which makes chlorine/chloramine-mediated corrosion commonly occur in the distribution system. In addition to disinfectant, the exact corrosion process is dependent on flow characteristics and water quality, e.g., Cl−, DO, ⁠, pH, alkalinity. Among them, increase in Cl−, DO, could lead to higher corrosion rates (Pisigan and Singley 1985). By conducting a two year pilot study, Imran et al. (2005) found that alkalinity, Cl−, ⁠, sodium, and DO of the source water or blend of source waters had a significant effect on release of corrosion by-products in the form of red water, temperature and hydraulic retention time were the significant physical and operational parameters. Recently, Li et al. (2014) proved that α-FeOOH was the main corrosion product for both chlorine- and chloramine-disinfection systems, whereas much denser crystallized particles were formed in drinking water distribution systems (DWDSs) with chloramine. This characteristic may prevent further corrosion within the downstream iron pipes for the chloramine-disinfection system. However, it is necessary to keep in mind that, accompanied by electrochemical corrosion, a series of shifts in water quality (increase in pH, alkalinity, Fe residual) happens as well, which may lead to water quality deterioration. Analysis and summary of these data could help us to make an adverse inference on the reason for the disinfectant fast decay and water quality degeneration.

Many mechanism studies have been conducted at the laboratory scale under delicately controlled conditions. In a full-scale distribution system, a delivery path consists of many sections of pipes with different diameters and materials. As a result, the disinfectant decay is caused by a variety of factors, including microorganism-mediated consumption and electrochemical corrosion. Researchers have been focusing on the effects of certain pipe materials on disinfectant decay speed by constructing suitable physical models. Digiano & Zhang (2005) developed the pipe section reactor (PSR) to quantify ‘wall demand’ chlorine decay rate, and found that higher velocity, lower pH, and lower DO could promote chlorine decay for old cast iron pipes. By using the same reactor, Westbrook & Digiano (2009) found the chloramine decay rate was much slower for cement-lined ductile iron pipe than it was for tuberculated cast iron pipes. Another field study reported that unlined cast iron pipes had the largest chlorine decay rate relative to unlined ductile iron pipes and PVC pipes (Huang & McBean 2008). Lately, Zhang et al. (2017) developed a model full-scale water distribution system to examine the effects of pipe materials on chlorine decay, and found that stainless steel pipes had the largest decay rate, followed by cement-lined ductile iron pipes, and polyethylene (PE) pipes. Even the relationship between water quality indices and disinfectant decay rate were deeply explored, but this failed to establish the relationship between the exact driving forces (e.g., biofilm consumption, corrosion) and the decay process, which makes the diagnosis on fast disinfectant decay ineffective. So it was suggested to explore the exact biochemical/physico-chemical forces embedded in different pipes by analyzing water quality variations simultaneously.

Other than experimental study, model calculation is another effective method, and has always been a way for improvement since the 1950s. Data-based empirical models and mechanistic models are the two major types of models capable of giving total chlorine residuals at key locations in the distribution system with certain travel time. The empirical model is developed based on historical data, and could reflect the differences in disinfectant decay coefficient under certain conditions, e.g. the difference caused by the pipe material (Westbrook & Digiano 2009). However, when environmental conditions change even in an invisible manner, the prediction accuracy may be not high enough to be used in the distribution system. The mechanistic/semi-empirical model includes the previous research findings on reaction processes of interest, which could gradually ‘squeeze out’ prediction uncertainties in the course of improvement of the mechanistic model. Hence if the key reaction processes could be described in the mechanistic/semi-empirical model, the model calculation could assist administrators to select proper disinfection strategies by presenting quantifiable modelling results.

Characterizing the disinfection chemistry for different pipes is essential to obtain a deep insight into the full-scale distribution system. To achieve this, we developed new type PSR (Figure S2, supplementary material), which was used to study the influence of five pipes on drinking water quality. Secondly, to quantify pipe wall consumption, the wall decay rate was calculated by using the refined Total Chloramine Decay Model (TCDM). Finally, the characteristic driving forces were inferred for the four typical pipes by analyzing the variations in water quality indices. Based on which, the diagnostic method was put forward to find the reasons for pipeline water quality deterioration.