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WATER & WASTE TREATMENT HOW TO MANAGE SUSPENDED


Simon Mattock from Advanced Water Technologies (AWT), ICS Cool Energy Preferred Water Treatment Specialist Partner, discusses the origin of suspended solids and their impact on chiller systems


contaminates are often poorly reported, and further introduction of organic and inorganic ‘dirt’ (contamination) adds to this collective over time. Even newly commissioned systems, if not meticulously monitored, are often seen with corrosion debris due to ‘flash’ corrosion during filling. The installation process itself can introduce all manner of dirt, dust, filings, oils, grease, swarf, flux residues as well as potential bacterial issues especially if systems have been left untreated and allowed to stagnate between commissioning and operation. Equally over time, more established systems can become fouled. To help assess the levels of ‘dirt’ in the form


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of solids in our closed water system we can test for what is referred to as suspended solids. In general, suspended solids are particles present in water, but for our specific purposes, we will understand them as solid particles that remain suspended in water and have a particle size of 2 microns or larger. You might also hear the phrase ‘dissolved


solids’, which means certain particulates smaller than our 2-micro differentiator, and not what we are discussing in this article. If the presence of suspended solids is


elevated beyond the maximum guidance threshold of >30 mg/litre and excessive suspended solids are not removed, they can have adverse effects on a chilled circulating system, that include, but are not limited, to: • Energy efficiency that directly increases


running costs • Blockages leading to ‘dead spots’ around


the system. • System erosion (generating further


suspended solids and adding to the rate of erosion). • Clogging by sludge debris of heater


exchangers, which puts the exchanger at risk of localised failure.


22 NOVEMBER 2023 | PROCESS & CONTROL


e need to recognise that the contamination of closed systems can never be fully prevented. Legacy


• Increased wear on pumps that can lead to


possible leaks and failure. • Reducing the effectiveness of water


treatment chemicals thus increasing corrosion potential. • Encouraging bacteria proliferation, made


possible by creating ideal growth conditions. Addressing the presence of suspended


solids becomes a key part of any system management strategy. The BISRIA BG 50/2021 standard and guidance states that suspended solids should be controlled at less than 30 mg/litre, and that ‘in the circulating water and a well-controlled system consistently achieve less than 10mg/litre’. As much as ‘low’ suspended solid values are a good measure of system integrity, we need to be mindful that it does not guarantee that active corrosion is not occurring from elsewhere in the system, hence strategic sampling needs to be considered. In consideration of individual reported levels


of solids, any trends upward in solids over time also need to be monitored as this is likely a direct result of internally corroded metals from the system. It is worth mentioning that as much as periodic water testing provides information on changes to water quality, it is limited. This alone does not provide evidence of the actual condition of the physical system. Providing this additional system evidence can be achieved by utilising equipment such as corrosion coupon racks and/or online electronic sensors. It is accepted that low level rates of


corrosion are very difficult to avoid. This is recognised by the guidance that offers predefined millimetre-per-year thresholds against various metals typically found in such systems. In a well-controlled system, however, corrosion rates should be well below these ‘threshold’ values and shouldn’t necessarily be seen as maximum control limits. A simple indicator of increased levels of


suspended solids is often seen as a ‘cloudy’ sample of water, or as a colouration of a collected water sample (see above). This is


why a visual evaluation is included in the ‘analysis’ undertaken during routine water treatment sampling. The insoluble nature of these solids means


that we can readily measure the levels of total suspended solids (TSS) within a system using a solids recovery filtration method (gravimetric). There are other, quicker and less accurate means of TSS measurement, like colourimetric for example. As referenced above, the gravimetric and


preferred (more accurate) means of TSS measurement in water is a method of analysis which is described by the Blue Book. In simple terms, a fixed volume of water is filtered under a vacuum with those solids of a certain size retained on the filter medium. The residue material is dried in an oven, removing all moisture and then weighed. This final weight of remaining solids is reported as the TSS of that water sample. As much as this process will give us an


indication of TSS in a system, for larger and more complex systems the higher the potential for variability of TSS can be. These systems are dynamic, and as such subject to velocity variability, which has a bearing on our results. Certainly, gravity, coupled with a poor flow profile for larger particulates may result in settlement and pockets thus the concentration of solids locally. In recognition of volume size, and increased variability, guidance (BG 50) recommends an increase in sample locations for a more holistic overview. It is important to be able to recognise that


should samples be taken from areas of low velocity (for example terminal units or the pipework feeding them), then these samples may contain ‘settled’ solids and as such would contribute to reports of elevated total suspended solids. Unfortunately, laboratory testing would not be able to differentiate between ‘suspended’ and ‘settled’. In situations where higher overall counts are recorded, localised flushing may be recommended to remove the excessive build- up of such ‘settled’ solids.


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