38 Water/Wastewater Flow Monitors Protect Slow-Sand Filters
The costs that can result from unacceptable drinking water quality are potentially huge, so engineers at a major UK water treatment works have installed a pair of YSI SonTek doppler flow meters at the outlets to two slow sand filters and report a major improvement in purification control which has substantially reduced operational risk.
Water demand commonly fluctuates throughout the day, and in response to weather conditions, by as much as 30% and this can place considerable pressure on slow sand filters, so it is important that they are managed carefully.
Slow sand filters are commonly employed in the purification of surface water. They employ a biological filter in addition to the physical filter provided by the sand itself and as such require careful management in order to ensure that the biological layer, known as Schmutzdecke, is maintained in optimal condition. Failure to do so can result in the presence of bacteria and algae in the produced water.
If a filter is found to produce water of unacceptable quality, there can be serious
repercussions. Firstly, the filter will be taken out of commission for at least 8 weeks. Secondly, this may result in fines by the Drinking Water Inspectorate and thirdly, a drop in drinking water production may mean that it becomes impossible to supply 100% of the distribution network, which in turn will result in compensation payments and the costs associated with the provision of an alternative supply.
Problems associated with slow sand filters usually relate to flow rates – low flow can result in reduced levels of dissolved oxygen that would affect the Schmutzdecke and high flow rates can result in excessive turbidity. The target is to establish a flow rate that is equivalent to a drop in water level in the filter of at least 2 inches per hour.
Conscious of the need to be able to measure flow continuously, engineers at the treatment plant reviewed the flow measurement options and were dismayed to discover that their options were extremely limited.
The Problem
Firstly, the feed channel from the filter had a square cross-section, secondly the water supply pipe contained a natural venturi and (as if that wasn't enough!) the pipe was only 2m long, so the 10 pipe diameters in length required by most traditional flow meters was certainly not available. It would therefore be necessary to excavate and construct a suitable flow path before a traditional flow meter could be installed. The cost of such work was estimated to be £27,000 per filter and the engineers were not comfortable undertaking such work in close proximity to the potable water supply.
The Solution Nick Martin from YSI Hydrodata recommended the installation of a small Doppler
flow meter, the Argonaut-SW. As a result, two Argonauts were installed within the outlets of the two slow sand filters.
No capital works were necessary for the installation. The flow meters were simply attached to blocks made of the same material as the channel in which they were located. A professional diver was employed to reach down and place the meters in the pipes, but other than this no further costs were incurred.
The Argonauts scan and store data continuously; however, 2 minute average data is transferred to the water company's SCADA system (via a 4-20mA output) which in turn controls the flow rate through the filter. This has ensured over 2 years of trouble free operation for the sand filters.
Following the initial installation, a 'drop test' was conducted in which a known volume of water was passed through the beds and compared with the Argonaut, which demonstrated accuracy of better than 99.9%.
How it works
The Argonaut-SW has three acoustic beams. One of these beams points straight up, and the other two point up/down stream at a 45-degree angle. The upward- looking beam measures water level. The two slanted beams measure the water velocity in two dimensions via the acoustic Doppler method and can measure both forward and reverse flow.
Summarising the benefits of Doppler flow measurement, even in very clean water, Nick Martin says, "If this water company had chosen to install traditional flow meters an extra capital cost of over £54,000 would have been incurred and the filters would have been out of action for at least a week. Instead, we were able to install the Argonauts in less than an hour with minimal cost and no interruption to supply.
Reader Reply Card no 128
New Auto-Clean Optical Dissolved Oxygen Monitor Lowers Carbon Emissions in Wastewater Treatment Plants
Specialist electrochemical sensor manufacturer Analytical Technology Inc (ATi) (USA) announces that its recently launched optical Q45D Dissolved Oxygen Monitor with Auto-Clean System reduces energy consumption in wastewater treatment plants. The innovative monitor is designed to provide reliable oxygen measurements in aeration processes and reduces maintenance requirements via the self-clean facility. Using a clean monitor such as the Q45D in wastewater treatment processes produces highly accurate results and lowers carbon emissions by reducing the amount of energy consumed.
In order to minimise carbon emissions, wastewater treatment plants require dissolved oxygen monitors capable of producing consistent and accurate readings. Unreliable monitors providing false readings can lead to a 12-15%
increase in electricity costs per parts per million (ppm) error. To ensure monitor reliability, it is important to keep the sensor clean, as poorly maintained monitors can over-aerate, producing inaccurate measurements and wasting energy. However, maintaining manual and flow-based control systems can prove expensive and these systems require regular cleaning to ensure optimum levels of accuracy. Using an energy-efficient system such as Analytical Technology’s Q45D Dissolved Oxygen monitor can result in energy savings of up to 25%.
The Q45D system is also the only optical dissolved oxygen monitor to incorporate a fully integrated automatic cleaning system. This innovative self-clean system uses high pressure blasts of air to remove any material that builds up on the membrane, preventing inaccurate readings and reducing maintenance. The Auto-Clean system can be programmed at user-defined intervals from once per hour to once per day. When the sensor is kept clean it reads at accuracy levels of better than 0.1ppm meaning energy costs are lower as energy is not wasted on producing false or repeated readings.
Recognising that customers have diverse measuring requirements, Analytical Technology’s Q45D Dissolved Oxygen monitor is available with a choice of two sensors: traditional galvanic or optical. The galvanic sensor uses a robust 5 mil Teflon membrane which acts as a barrier to allow molecular oxygen to diffuse into the reaction cell where it is reduced at the working electrode, producing a small current that is proportional to the oxygen concentrate. The optical system uses a florescence quenching technique to measure dissolved oxygen.
Dr Michael Strahand, General Manager (Europe) at Analytical Technology comments “A reliable and accurate dissolved oxygen monitor is key to reducing both energy waste and costs in wastewater treatment plants. The Q45D monitor ensures accurate results, better process control and with the self-cleaning feature the monitor is near maintenance-free. The user-friendly settings and option of two sensors enable the monitor to produce precise readings in the most demanding situations, making it extremely versatile in assisting plants to reduce their carbon footprint.”
Reader Reply Card no 129
Groundwater Monitoring - Low-flow Purging and Sampling
Geotech (UK) Ground water monitoring. This new extensive white paper of some 3,300 words with cases studies and illustrations examines best practice and the ‘state-of-the-art’ in Low-flow Purging and Sampling. It finds: “Significant timesaving, improved sample process integrity, quality assurance and minimal quantities of potentially contaminated purge water - are attractive as the focus on groundwater quality sharpens and economics demand best cost-benefit based methods.”
As each country looks more closely at its water
resources and maintaining water quality, considerations on water sampling and techniques become more important. For any source, there is an obvious need for sampling process integrity to be able to sample accurately, reliably and repeatably, in compliance with rules and guidelines thus achieving quality assurance - and as economically as possible.
Reader Reply Card no 130
IET
Annual Buyers Guide 2009
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