In-pipe monitor shows up flaws in DPD testing

Comparison of in-pipe water quality data in a desalination plant using a Intellisondes™ multi-parameter monitor, instead of subjecting grab samples to a photometer tablet (DPD) test, successfully identified what caused spikes in free chlorine.

For most of the time, there was close agreement between the two methods, but periodically, the photometer results varied wildly, whilst the in-pipe monitor from Intellitect Water Ltd of the UK showed relatively stable values.

Initially, distribution managers suspected that the online monitors were failing to detect occasional spikes in free chlorine. However, the Intellisonde was able to provide more clues leading to successful identification of the problem.

Intellisondes™ provide real-time monitoring data for water within pipeline distribution networks. They feature up to 12 sensors inside a tiny sonde head that fits in a water pipe through a 3.8 or 5 cm valve. Measurement options include Free Chlorine, Mono-chloramine, Dissolved Oxygen, Conductivity, pH, ORP/Redox, Flow, Pressure, Temperature, Turbidity and Colour. An ISE channel is also available for Fluoride, Ammonium and Nitrate.

The solid-state free-chlorine sensor in the Intellisonde employs electrochemical technology to provide accurate data without the need for reagents or other consumables.

Infrastructure in the desalination plant was fairly new, and the water company took 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.

In an initial study, regular samples were taken manually over a 24-hour period and tested with the DPD method. An unexpected scatter of DPD measurements was observed at night on multiple occasions.

Continuous Intellisondes data for other parameters, (in this case, temperature, pH, conductivity, turbidity and flow) highlighted other factors that could be affecting the DPD measurements. 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.

A test with sand and bottled water 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 species, 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.