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Evolution In Wastewater Goes With The Flow While options for clean water flow measurement abound, there is room for improvement on the wastewater side — but new technologies are closing the gap. By Andy Godley F low measurement underpins almost every aspect of the water and wastewater industries. In the clean water part of the cycle, flows are monitored and measured at all stages from source to delivery, driven by factors such as environmental protec- tion (abstraction), reduced leakage, and revenue generation. On the wastewater side, however, there is generally less flow measurement in place as, historically, the drivers for wastewater flow metering have not been as strong. Playing Catch-Up There are significant technical difficulties when metering wastewater that make it much more challenging to measure than clean water and may also partly explain why there is less metering of wastewater flows. Firstly, once abstracted, most clean water travels through pipes that are pressurized by pumping or gravity and run full. Wastewater, on the other hand, is often running through partially-filled pipes, sewers, and channels with a free surface flow. Closed-pipe flow is, on the whole, much easier to meter than open-channel flow, where the level of fluid and its velocity can vary independently. Thus, a given volumetric flow in a specific channel may be a shallow, fast-moving flow or a deep, slow-moving flow. Unless a control structure such as a weir or flume is in place, two measurements — velocity and liquid level — are often required to calculate volume. Clean water flows also tend to be contained in round pipes. In wastewater, we find all kinds of interestingly-shaped sewers and chan- nels. When no structure is in place, the shape needs to be characterized with respect to depth to calculate the wetted area (volumetric flow being wetted area multiplied by mean velocity). Then, of course, clean water is, by definition, clean; wastewater isn't. Wastewater can be highly variable in content, carrying heavily-fouling substances such as fats, oils, and greases (FOG), as well as light solids and heavy solids, such as grit and other debris. Pressing For Measurement In the U.K., regulations stemming from the Urban Wastewater Treatment Directive and the Water Framework Directive have changed the emphasis on wastewater flow measurement. It is set to change even further with the new pressures on water companies caused by the Outcome Delivery Incentives (ODIs) put in place as part of the latest industry price review. These include, for example, incentives to reduce the incidence of sewer flooding. The consequences of pollu- tion spills from malfunctioning combined sewer overflows (CSOs) are now more severe, with higher fines being levied by the Environment Agency for such incidents. This is leading, in the words of David Tyler, Environment Strategy Manager at Southern Water, "towards a more resilient and adaptive sewer network, one which is inexorably under- pinned by in-sewer flow monitoring." Finally, there is also the opening of the retail market for water and wastewater services in 2017 in England that will allow non-household customers to buy their wastewater services from any provider, not just their local water company. This should stimulate new ideas for service provision with better understanding of wastewater discharges, under- pinned by flow measurement. The majority of wastewater, whether from residential premises, industrial, or commercial sites, is discharged to the sewer network. A large industrial user will have a trade effluent meter (TEM) monitor- ing its discharges so that charges can be levied based on the Mogden formula, which combines flow, solids, and biochemical oxygen demand (BOD) to assess loading on the treatment process. The opening of the retail market for non-household water services in 2017 is stimulating new interest in this area. Those offering services based on more accu- rate flow measurement, and hence more accurate charges, are likely to develop additional services that can be provided using this data. Applying Solutions This may be an area where insertable sensors, such as the Nivus cor- relation pipe sensor, can be used to good effect. Such sensors can be installed in a live pipe without disruption, but more importantly, can be removed for cleaning, thus providing ongoing accuracy. Some are already in use for monitoring trade discharges to the environment from on-site treatment plants. At sites without trade effluent meters, charges are based on the metered potable water supplied. Where waste flows to a combined sewer that also receives surface runoff, adjustments are made in the charging mechanism. An interesting development for sites without a specific TEM, therefore, is the wastewater meter offered by Dynamic Flow Technologies. This uses microwave technology to measure the actual discharged waste flow. Models have currently been developed for typical drain flows in 4-inch (100 mm) and 6-inch (150 mm) pipes and will allow charg- ing based directly on the quantity of foul discharge, rather than some assumed relationship with the water in and unmeasured adjustments for runoff. This technology is currently on trial with Wessex Water in the U.K. and could lead to new charging mechanisms for sites where clean water usage is relatively low but where there are large surface areas for rainwater runoff (for example, an out-of-town superstore where relatively little clean water is used in toilets and canteen facilities, but there are large roofs and parking areas). In-sewer flow measurement is perhaps one of the most chal- lenging flow applications in the water industry, due to the highly fouling nature of the fluid. However, it is also one of the most nec- essary for the reasons cited above. Non-contact sensors that are less 30 wateronline.com n Water Innovations