Ammonia Measurement in Raw Wastewater

A wastewater treatment plant

Introduction

The action of the micro-organisms on organic and complex inorganic materials, leads to the production of ammonia. When ammonia is present in water at high enough levels, it will have detrimental effects on animal life. For this reasons ammonia in a wastewater plant might be monitored and removed at different points in the process:

  • Inflow of the WWTP - Raw wastewater intake (Using our MS3500)
  • In the aeration lanes
  • At the outflow of the clarifier
  • At the works discharge

The problem and the opportunity

Ammonia levels at the intake can change a lot from day to day and from hour to hour. No instrument to date is able to measure ammonia at the raw wastewater inlet (soon after the screens) without an extensive maintenance regime or filtration system. If there was an instrument able to measure ammonia levels soon after the screens operators could:

  • Improve the overall outgoing water quality
  • Save on electricity costs
  • Reduce chemical usage
  • Optimise their process
  • Identify serial offenders

Measurement at the Intake: Today

Wastewater Intake

Today’s paradigm for measuring ammonia at the intake is:

  1. Grab a sample once every week/month
  2. Send the sample to the lab
  3. Get the result
  4. Assume ammonia levels will be higher
  5. Oversize the nitrification process
  6. Waste a lot of energy/chemicals

Ammonia Measurement – The Challenge

  • Optical systems will suffer from fouling and biological growth
  • If a system uses filtration it will need filters to be replaced on a daily basis
  • If a system uses auto-clean with compressed air/water it will need a source of clean air/water
  • High levels of turbidity will affect some measurements methods
  • NO system has been specifically designed for this application

Ammonia Measurement – The Solution

The 3500 Ammonia Monitor is the only instrument in the market that has been specifically designed to monitor the levels of ammonia at the raw wastewater intake. The principle of operation is the measurement of headspace gases from a sample tank containing the waste water to be measured.

The MS3500 works by passing water through a sample tank as shown below. Sodium Hydroxide (NaOH) is added to increase the pH of the waste water, converting NH4+ to Ammonia. Through the use of Henry’s Law the concentration of gases in the headspace is proportional to the concentration of the substance in the water. The Ammonia and other volatiles in the waste water will pass into the headspace above the waste water where it will be trapped. This will continue until equilibrium is reached.

A sample of the headspace gases are then passed across sensors in the MS3500 sensor head which respond to the Ammonia in the headspace but reject other contaminants. This response is then analysed by the instrument and a concentration value is generated based upon the relationship between the concentration present in the headspace and that in the water.

Case Study - Ammonia Monitoring at the intake in a WWTP in London

This is from a site in London that serves a population equivalent of more than 1 million people. The test lasted three months and again 24-hours sample sets were obtained. A very good correlation between MS3500 and the lab results was demonstrated. After months of tests the customer is now confident of the solution and of the correlation with his own lab. Three systems will be used to monitor the two inlets of raw wastewater into the WWTP and help the plant to:

  • Improve process control
  • Bring down energy costs
  • Accurately determine the ammonia profile