Published: 12 April, 2017
Accurate measurement of wastewater flows is needed to help maintain compliance with increasingly stringent legislation aimed at minimising the impact of human activities on the environment. Alan Hunt, Electromagnetic Flow product manager for ABB Measurement & Analytics in the UK & Ireland, explains why electromagnetic flowmeters can offer the best solution for keeping track of wastewater flows.
Industrial companies account for a significant proportion of water and energy consumption in the UK. These companies produce considerable volumes of waste, which need to be accurately monitored in order to assess and limit their impact on the environment.
Under the Environmental Permitting Regulations (EPR), any industrial company discharging 50m3 or more of effluent per day to a watercourse or the sea is obliged to self-monitor its effluent flows. In addition, some companies that discharge to a particularly sensitive aquatic life area may have to self-monitor their effluent flows even if they are discharging below 50m3 per day if this is specified in their permit.
Introduced as part of a move to improve the measurement and control of discharge and waste levels from industrial companies, the self-monitoring obligation requires operators to comply with the Environment Agency’s MCerts certification scheme. Under this scheme, companies should be able to demonstrate to the satisfaction of an MCerts inspector that they are using the Best Available Technique (BAT) to protect the environment.
Where the self-monitoring of effluent flow is concerned, operators are subject to a ±8% uncertainty target for the measurement of total daily volume of effluent discharged, with the calculation including consideration of the characteristics of the flowmeter being used.
In practice this means that, if there are instruments or systems using a particular technology that have passed all the necessary tests and received an MCerts compliance certificate, operators must use them. Where self-monitoring of effluent flows is concerned, it is important to use a technology that not only represents the Best Available Technique but also offers the best balance of operational benefits, not least the ability to continue to deliver accurate and reliable measurement throughout its life.
From an economic perspective, accurate flowmetering is also important where an industrial site, business or other heavy water user is billed for discharging large quantities of wastewater to a public water treatment works.
In these situations, billing needs to be exactly matched to the amounts discharged. In most cases, the point at which wastewater flows are measured is the pumping station, which relays wastewater from industrial sites for treatment at one or more wastewater treatment plants.
One way of measuring these quantities is to estimate the flow by multiplying the volume of water that flows each time the pump is started by the number of pump starts over a set period of time. This gives an approximate total volume figure for a known period which can then be used to estimate the flow. One problem with this approach, however, is that does not take account of any potential losses in the system, which may arise due to factors such as leakage or unexpected variations in flow.
Another, more precise way, is to use a flowmeter to provide an exact and continuous measurement of the volume of water that flows through a wastewater distribution system.
Choosing the right flowmeter
As the data collected on wastewater will be the starting point for ensuring any improvements in measurement accuracy as well as compliance with legislation, it is important to ensure that the flowmeters selected offer the best long-term accuracy, repeatability and reliability for the task.
There are various options available, each offering their own distinct set of advantages and drawbacks depending on the characteristics of the application, including the ability to handle high velocity flows of particulates and suspended solids as well as the overall installation space available.
The ability to handle high velocity flows where solids may be present, for example, rules out many conventional flowmetering technologies which can quickly suffer damage that reduces both their measurement accuracy and their overall working life.
Differential pressure meters such as orifice plates and flow nozzles, for example, can become clogged by the build-up of particles in the flow medium, requiring them either to be removed for cleaning or replaced. Mechanical meters are also unsuitable. With moving parts subject to wear and tear, these meters can quickly suffer reduced accuracy, leading to either under or over-registration of flows. Furthermore, the need for mechanical meters to be periodically tested, recalibrated and repaired means that they have to be removed, requiring users either to replace the meter with a temporary device or cease measurement until the meter is refitted into the line.
Ultrasonic flowmeters also suffer various drawbacks which can make them unsuitable. Transit time meters in particular can struggle to handle flows with high levels of particulate matter, requiring a strainer to be fitted. Both transit time and Doppler meters can also be affected by velocity profile distortions, requiring from 10 to 40 upstream diameters, depending on the severity of the disturbance. The turndown of ultrasonic meters is also limited within an ideal range of 20:1 to 40:1. Ultrasonic meters can also be difficult to install and set up, especially where high accuracy is required. As a further consideration, there may also be the need for data from the meter to be accessed from a central location, particularly on large, dispersed sites or where the meter is situated in a hard to reach location.
Electromagnetic flowmeters are well-suited to the demands of wastewater flow measurement applications. Compared to other flowmeter types, electromagnetic flowmeters offer greatly enhanced accuracy and repeatability throughout their operational life, with uncertainty of ±1% reading or better.
With no moving parts, they do not suffer from problems with wear and tear, minimizing maintenance, and require no upstream strainers to filter sediment. A choice of flow primary linings affords further protection against coating and high sediment flows, with users able to choose from a variety of materials, including ceramic linings for particularly abrasive flows.
The ability of electromagnetic flowmeters to better handle distorted velocity profiles also means that the amount of piping upstream and downstream of the meter is greatly reduced. Modern electromagnetic flowmeters are also capable of being buried, eliminating the need for the construction of costly installation chambers.
These inherent benefits are being augmented by ongoing developments in technology, both at sensor and transmitter level.
An added incentive
An added incentive to opt for electromagnetic flowmeters is the Water Technology List (WTL), operated by jointly by the Department for Environment, Food and Rural Affairs (Defra) and HM Revenue and Customs.
The WTL scheme allows any business that purchases a water meter for monitoring water use to claim a 100% Enhanced Capital Allowance (ECA) on that purchase, allowing them to write off the cost of purchase against their taxable profits in the first year of purchase.
As well as the purchase cost, the scheme also applies to the cost of installation, covering the work involved in fitting and commissioning a water meter.
Available in the product search section of Defra’s web site at www.defra.gov.uk, the list details the various flowmeter suppliers and technologies within the WTL scheme, including ABB’s WaterMaster flowmeters.
The combination of potential operational savings and the financial incentives around the Water Technology List scheme, provide an attractive case for re-examining the possibilities arising from using flowmeters in water and wastewater pumping applications.
For further information please visit: http://new.abb.com/uk