Municipal measurement re-invented

| 19/10/2012 | 0 Comments

SCIAM Worldwide claims that its in-line density measurement system for the City of Jackson Wastewater Treatment plant, which was specified by CDM Smith, will result in a complete and continuous measurement of wastewater slurries as well as annual savings of over $1.3 million.

A recent survey of municipal waste water treatment applications shows a significant proportion of operational cost attributed to the inaccuracy of traditional mass flow and density measurement. One person typically generates 3.27 dry tonnes of sewage per year at a cost of $45 per dry tonne. Accordingly, studies conducted at the City of Jackson, Mississippi waste water treatment plant reveal a total cost of approximately $25.8 million per year.

Using traditional density sensing, such as auto sampling, ultrasonic or microwave techniques, the cost of ambiguity directly related to sensing inaccuracy is typically ± $1.75 million. By replacing their auto-samplers with the more accurate DM3 density meter at the insistence of CDM Smith, the City of Jackson has cost reductions of $1.3 million annually.

SCIAM calibrates the DM3 Density meter in absolute mass per defined unit volume terms. The flow tube volume is the defined and is relatively large such that it is representative of the slurry it measures. All mass measurements are provided by a SCIAM density transducer, which is traceable to USA NIST and other international standards, it is accurate to < ± 0.5% of full scale. The measurements are made continuously. Mass compensation may be programmed due to slight variation in gravitational force at any place on Earth.

Municipal Density Measurement: Technical Comparison

Historically, waste water treatment plants have utilised auto samplers to estimate density by interrogating the slurry periodically during waste treatment processes. As a progressive industry, innovative measurement methods are consistently introduced. The latest development is that of the DM3 density meter which continuously measures slurry passing through a pipe. Often mistaken for a more complicated system, this methodology has never been directly analysed in comparison to auto sampling methods.

By nature, auto sampling is not continuous. The density will likely change between samples and is not monitored during those periods. Auto sampling typically measures a minute sample of slurry which generally does not represent that which actually flowed in the pipe at the time of sampling. There are no test weights used and measurements are not traceable to USA NIST nor international standards. Auto sampling is performed by a technician, which is time consuming, expensive and subject to human error.

The key differentiator with the DM3 method is continuous measurement. Slurry within the flow tube is continuously measured over the full volume and interrogated at 110 times per second. The flow tube volume is the defined and relatively large such that it is representative of the slurry it measures. Knowing the carrier liquid sg and the measured solids sg of the uniformly applied load, the percentage dry solids can be calculated and compared to the actual dry solids determined. All mass measurements are provided by a density transducer, which is traceable to USA NIST and other international standards, and is accurate to < ± 0.5% of full scale. Mass compensation may be programmed due to slight variation in gravitational force at any place on Earth.

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