Mass Flow Meter to Enhance Water Treatment

Anglian Water Services cleans water to the highest standard, delivers it to millions of homes, and carefully manages it to ensure it never runs out in an area of the UK. They started a project to optimize and further control dosing of phosphates in the public water system.

The functionality of orthophosphoric acid in the public water system

Public water systems commonly add phosphates to the drinking water as a corrosion inhibitor to prevent the leaching of lead and copper from pipes and fixtures. Inorganic phosphates (e.g. phosphoric acid, zinc phosphate, and sodium phosphate) are added to the water to create orthophosphate, which forms a protective coating of insoluble mineral scale on the inside of service lines and household plumbing. The coating serves as a liner that keeps corrosion elements in water from dissolving some of the metal in the drinking water. As a result, lead and copper levels in the water will remain low and within the norms to protect the public health..

What was the original process?

In the original process a down-steam analyser was in-place to measure the concentration of orthophosphoric acid in the main flow. The measurement results were checked against the required concentration and used to adjust the pump speed and therefore the level of orthophosphoric acid in the main flow. With this process Anglian Water Services can secure copper and lead concentration levels in the water acceptable to protect the public health. Nevertheless the process had room for improvement, which will be discussed in this blog.

The reactive feed-back loop mechanism for dosing phosphates was not a preferred working method. We could not react quickly enough to the changing main flow to reduce or increase the dose proportionally. We had to ensure that we dosed to a level meeting the regal requirements assuming the station was processing maximum flow. Secondary costs were added to the system by needing double redundancy on the analyser to ensure there is no break in the measurement of orthophosphoric acid levels.

Project objectives

1. Reducing phosphate levels

2. Reducing the cost of meeting legal environmental standards for the business.

3. Remove the downstream analyzer and redundant space in the process of record.

Two sensor technologies were evaluated to enhance the process ; Differential Pressure and Coriolis technology. The Differential Pressure instrument was the most cost effective and allowed us to meter the Orthophosphoric acid flow as a volume, it would take an advantage an analogue signal input and adjust the dose proportionally to the main flow.

The Coriolis Mass Flow Meter utilizes direct Mass Flow Measurement, which is preferable over volume flow for this application and is more accurate and repeatable, but is more expensive. It would also take an analogue signal input and adjust the dose proportionally to the main flow.

Making a decision appeared to be based around return on investment. Essentially the time taken to generate sufficient savings. However, during the demonstration of the Coriolis Mass Flow Meter we learned something new that would change the direction of our final design. The Coriolis Mass Flow Meter gave the density of the fluid being metered as an output.

Why was this important?

Phosphoric acid it sold in diluted concentrations , usually 80% in solution. What we have found is that there is a variation in the actual concentration at the point of use.

At this point we already knew that either the Differential pressure or Coriolis technology could support us to enhance the process of record. Now we had the chance to go to the next level and take a previously unavailable but very important parameter and use it to really refine the dose ratio.

The extra density parameter available with the Coriolis Mass Flow Meter made the decision for us. Dosing would now be controlled proportionally to the main flow and the density/quality of the phosphoric acid being used.

The enhanced water treatment process

What are the projected benefits using Mass Flow Meters:

As we look to go live on the first five installations of this technology, we are projecting the following:

1. Stable concentration of orthophosphoric acid in the public water system.
2. Maintaining the public health commitments of the Water Industry.
3. Decreasing the addition of phosphoric acid into the environment by significant levels.
4. Two-fold cost reductions: by eliminating the down-stream analyzers and the consumption of phosphoric acid.

At Anglian Water Services they live with a Love Every Drop approach. The Love Every Drop approach is a vision for how they believe a modern utilities company should be run. That vision means creating a country with a resilient environment that enables sustainable growth and can cope with the pressures of climate change. Creating infrastructure that is affordable and reliable, meeting the needs of customers, communities and the environment. We want our people and our communities to be resilient too. Phosphoric acid is connected with the concept of planetary boundaries according to Rockström et al. 2009. Anglian Water Services was able to reduce the consumption of phosphoric acid in their processes without sacrificing the quality of the water. This fits with the way they run their business.