Detective work solves chlorine mystery

The installation of new water quality monitoring sondes at a desalination plant threw up some puzzling results, as Tony Halker, chief executive of instrumentation specialist, Intellitect, explains

Head of an Intellisonde, which can measure a range of parameters

Following the installation of Intellisonde in-pipe drinking water quality monitoring sondes at a desalination plant in the Middle East, distribution engineers discovered an occasional but significant disparity between the sondes’ data and results from grab samples tested by a diethyl-p-phenylene diamine (DPD) photometer tablet method. Most of the time, there was close agreement between the two methods, but periodically, the photometer results varied wildly, whilst the in-pipe monitor showed relatively stable values.

Initially, distribution managers suspected that the online monitors were at fault and that they were failing to detect occasional spikes in free chlorine. However, as a multiparameter monitor, the Intellisonde was able to provide more clues which ultimately lead to the successful identification of the problem.

Real-time monitoring.
The desalination infrastructure is fairly new and the utility takes water from a processor and a trunk distributor. The Intellisondes at these sites monitored free chlorine, temperature, pH, conductivity, turbidity and flow, at 15 minute sampling intervals.

The solid-state free chlorine sensor in the Intellisonde employs electrochemical technology to provide accurate data without the need for reagents or other consumables. In order to check the performance of the Intellisondes, grab samples were taken and free chlorine tests were conducted with a portable photometer which measured the colour intensity produced when DPD tablets were crushed and mixed with a sample.

Comparative study
In an initial study, regular samples were taken manually over a 24 hour period and tested with the DPD method. These results were compared with the Intellisondes’ logged free chlorine data (see Figure 1).

An unexpected scatter of DPD measurements was observed during night and on multiple occasions. By providing continuous data for other parameters - in this case, temperature, pH, conductivity, turbidity and flow - the Intellisondes were able to highlight other factors that could be affecting the DPD measurements.

The data in Figure 2 shows that at the time when the DPD measurements were most scattered, flow dropped and turbidity increased, so it was suspected that the particles contributing to this turbidity could be interfering with the DPD measurement. The process of sampling caused flow disturbance in the pipe, which appeared to result in a variable level of particles in each sample.

Low flows at night were resulting in the settlement of particles which were disturbed randomly by the sampling process and it was considered likely that these particles were affecting the DPD results.

In order to check this assumption, local sand was added to bottled drinking water and tested by the DPD method. This water had previously been tested and shown not to contain free chlorine, but following the addition of local sand the DPD test gave a positive result for free chlorine. This confirmed that sand was the cause of the false DPD data.

Subsequently, samples of water taken from the Intellisonde locations were allowed to settle and then showed no chlorine in a DPD test. However, when the samples were shaken, DPD tests erroneously reported the presence of chlorine.

The reagents in DPD tablets are known to react with a wide range of substances, including iron and manganese compounds, and it was discovered that local sand grains have a coating of iron oxide which gives them a pink colour. It is highly likely therefore, that this iron oxide coating was reacting with the DPD reagent and causing the false free chlorine measurements.

The sensitivity of the DPD method to iron and manganese compounds is well established and is a consideration in any clean or drinking water analysis application.

Intellitect Water’s Jo Cooper said, “This project highlights the benefits of multiparameter monitoring.”