| WATER QUALITY control and fill-water monitoring are identified within literature as of high importance to ensure the system is adequately protected from oxygen ingress and inhibitor depletion (where applicable). Introducing high volumes of fill water into the system can also indicate if leaks are occurring, providing insight as to whether the system is watertight. To maintain control over water quality, the amount of fill water entering a system should be measured and routinely checked, regardless of the methodology de-oxygenated or chemically dosed used to maintain water quality. Categorising all water-quality parameters into lead and lag indicators adds weighting to the parameters that identify when system water trends towards corrosive conditions before corrosion occurs. Therefore, monitoring lead water quality parameters continuously increases the effectiveness of the online monitoring equipment, enabling proactive maintenance works to take place before (significant levels of) corrosion takes place. Because of the increasing uptake of de-oxygenated systems within the UK, this study was conducted over conventional dosed and de-oxygenated systems. De-oxygenated systems provide a more clear-cut set of monitoring parameters because of the lack of complexity within the water chemistry. De-oxygenating a district heating system would shift the relevant guidance documents from BSRIA BG 29 and BSRIA BG 50 to the German water standard VDI 2035. Monitoring options Regardless of the system type, the monitoring parameters with the highest relative importance are consistent and in line with literature findings. The following combinations of monitoring options have been selected for life-cycle analysis based on the key parameters highlighted above across both systems. Option 1 monitors all the parameters with a relative importance deemed greater than a value of 3, shown in Table 1 within a single monitoring unit. Option 2 provides analysis on a hybrid monitoring regime with standalone probes for all key monitoring parameters with a relative importance greater than 4. The results from the financial assessment show there is good economic feasibility for online real-time water quality monitoring. The increased capital investment upfront is quickly recovered by the reduced requirement for sampling to BSRIA BG 29 standards during the commissioning phase before system handover. From an install and commissioning perspective, installation of online monitoring would be economically viable in cases where larger system sizes and longer commissioning phase durations are present, as the cost variation from sampling is improved. The overall net present value observed at system handover and year 20 are significantly impacted by an increase in commissioning phase duration of a project, regardless of system size. Although overall costs increase with extended handover periods, the financial benefits of having online monitoring installed are reduced compared to BSRIA sampling. This is because of the costly nature of laboratory sampling every fortnight. 5 Relative capital and maintenance cost for real-time monitoring MONITORING 4.5 4 3.5 3 2.5 2 1.5 1 0.5 0 0 0.5 1 1.5 2 2.5 3 3.5 Relative importance 4 4.5 5 Dissolved oxygen Conductivity pH Corrosion rate Biological activity Inhibitor concentration Pressurisation unit water metering Pressure fluctuations Sulphate monitoring Chloride monitoring Metal content Figure 2: Relative importance and cost of lead (large markers) and lag indicators (small markers) for de-oxygenated systems Option BSRIA sampling Monitoring type Commissioning phase: BSRIA BG 29 fortnightly sampling between flush completion and practical completions Post-handover operation: Quarterly sampling with results sent for laboratory analysis. Remedial maintenance conducted, based on sample results Option 1 Dissolved oxygen, conductivity, inhibitor concentration, pH, pressure fluctuations, corrosion rate, water metering Option 2 pH, conductivity, dissolved oxygen, pressure fluctuations, with water meter installed Table 2: Combinations of monitoring options selected for life-cycle analysis There is good financial benefit for both online monitoring options over a 52-week period, regardless of system size, and it could be considered financially beneficial to install temporary monitoring during the development commissioning phase. Post-handover maintenance of the system, with the additional maintenance costs incurred by the monitoring equipment, increases overall water quality parts per million costs by around 1,300-3,000. However, the improved visibility and control on water quality enables cost reductions on reactive maintenance works, heat interface unit servicing time costs, and major equipment replacement costs. It has been found that online monitoring equipment is economically viable for medium and large systems. The relative costs recovered by improved operation of small systems does not outweigh the increased equipment maintenance costs. So, installation of online monitoring equipment on small hydraulic systems is not recommended. CJ To read the full research paper, visit the FairHeat website bit.ly/CJOct22PB PETER HORNE is a consulting engineer at FairHeat References: 1 J Greaves, Water quality assessment in UK district heating systems, CIBSE, Sheffield, 2019 2 R Thorarinsdottir, L V Nielsen, S Richter and T Hemmingsen, Monitoring corrosion in district heating systems, The Icelandic Building Research Institute, 2004. 76 October 2022 www.cibsejournal.com CIBSE Oct 22 pp75-76 Water quality monitoring.indd 76 26/09/2022 15:25