Activated Sludge Process Optimisation – DO control via Ammonium measurement: ISE vs. WET Chemistry.


Investment Reaps Rewards

A recent trial of the Xylem WTW VARiON Ammonia ISE against a competitor’s wet chemistry analyser on the inlet to a major ASP on a flagship works in the UK revealed just how far the ISE technology has come through the intensive investment in R&D that Xylem has made. 6 weeks of data is presented across the plots below.
Two ISE sensors were installed: one with additional air cleaning and one “straight out of the box”. These sensors were calibrated to the liquor matrix during the initial commissioning and then remained untouched for the remainder of the trial.

Figure 1. Week 1: WTW Trial data VARiON ISE vs Competitor Wet Chemistry Analyser. Highlighted regions show missed diurnal peaks and troughs in analyser data (red) and unactionable analyser data (orange). For more graph analysis, please download the PDF at the bottom of the article.

Data Analysis

As can be seen from the above data, it is the ISE sensors which have proven to be the most reliable, the difference between the two ISEs by the 6 week mark highlighting the importance of the air cleaning to the installation location. The wet chemistry analyser required two emergency service visits to correct major issues in week 2 and week 4 of the above data, and repeatedly produced unactionable data when operating in the intervening times, and during the first two weeks of the trial struggled to measure the diurnal load variation.

This data challenges the historical preference for wet chemistry analysers to assess the Ammoniacal loading of an ASP. Due the cost (both capital, installation and operational) of these instruments, a single measurement point at the inlet to the ASP has been the only commercially viable solution, with the assumption that the feed to each of the lanes in the plant will have the same hydraulic characteristics and therefore the loading will be identical. This is rarely the case and the differences between lanes can become more severe as the size of plant increases. These differences limit both the treatment efficacy achieved through control via a single measurement point and limit the efficiency savings which can be made through the availability of a larger volume of reliable and actionable data.

The improvements in ISE technology as evidenced in the data presented offer a number of key benefits. Whole life cost of ownership saving in comparison to the wet chemistry analysers is sufficient to enable the instrumentation of individual lanes for targeted feed-forward ammonia based dissolved oxygen control to become a viable commercial reality.

Utilising a combined Ammonium/Nitrate ISE sensor further enhances the offering and provides the opportunity for feed-forward led dissolved oxygen control to meet consent with a feed-back trim to save energy to be implemented with a rapid payback time for the customer. The reliability and stability of the ISE sensor coupled to stable, actionable data increases the payback by reducing plant energy usage and improves control efficiency without the requirement for costly service contracts, emergency service visits and extended periods of downtime (as seen with the analyser) where the plant must be run from recorded profiles or in hand, adversely impacting site OPEX.

Enhancing Potential Efficiency

The WTW IQ SensorNet system further enhances the potential efficiency through the networking of zones, lanes and even entire ASPs into a single instrumentation system. This is not limited to WTW sensors only but can also take in signals from flow and level sensors enabling visualization and provision of data from a single point on each lane or ASP. The reduction in cabling, both power and signal, offers further savings in both material and installation time. This is of particular benefit as plants move from analogue to digital communications systems.

New Methodology

Alongside the market leading performance of the ISE sensors, Xylem have developed the revolutionary Alyza system for wet chemistry measurement of PO4 and NH3. This system dramatically reduces reagent usage and maintenance requirement while enabling the low range measurements required for the new tightening discharge consents.
Combine both instruments with IQ SensorNet to fully optimise the control of your plant.

Policing The Process

ISE sensors are best applied in secondary treatment processes. Wet chemistry has its place in the final effluent acting as a guardian to monitor discharge against consent levels.

Here the Alyza system comes into it's own.

  • Alyza’s revolutionary liquid handling reduces reagent consumption to 1ml per test meaning that reagent replacement is only required once a year.
  • Uptime is greatly increased by the simplicity of component replacement and extended time between service
  • Extremely low limits of detection enable the Alyza to comfortably measure the ranges required by the new tightened discharge consents - the Alyza can be connected directly into the IQSensorNet system, separate controllers and pumping systems are not required

For more information regarding any of the products mentioned or to arrange a call from one of our experienced team members, contact us today.

  Download the application note here