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With the increased focus on phosphorus removal in wastewater treatment, utilities are dosing more coagulant which in turn puts a greater strain on ensuring sufficient alkalinity levels. The more metal-based coagulant dosed to combat phosphorus, the less alkalinity is available in the wastewater to control pH which may lead to an ammonium compliance risk.

Ammonia removal in wastewater

In wastewater operations, aerobic metabolisms use dissolved oxygen to convert ammonia and ammonium into energy. These bacteria (nitrosomonas and nitrobacter) prefer these nitrogenous compounds to organic, carbon based compounds. This type of aerobic metabolism, which uses dissolved oxygen to convert ammonia to nitrate (via nitrite), is the nitrification and denitrification process.

Nitrification/Denitrification process in wastewater.

During the nitrification process, alkalinity in the wastewater is lost as acid is produced by the bacteria.

The standard rule of thumb is that for every one mg/l of ammonium oxidised,  7.14mg/l of alkalinity (as CaCO3) is consumed.  A reduced alkalinity reading means the wastewater is more prone to pH shifts which may lead to a slowing of the nitrification process.

In many plants, especially where the incoming wastewater is naturally low in alkalinity, alkaline compounds such as sodium hydroxide or sodium carbonate are added to ensure nitrification process occurs at the desired rate.

Optimum conditions, therefore, consist of a near neutral pH value, preferably slightly above, and importantly, a steady state. The more stable the environment for the microorganisms, the more effectively they consume ammonia and ammonium. Alkalinity is the key to controlled operations and can lead to improved processing performance and higher treatment capacity. Monitoring pH is useful, but monitoring alkalinity will enable you to control pH.

How to monitor alkalinity in wastewater treatment works

Alkalinity is the sum of all the acid consuming bases present in water sample, it is therefore often thought of as the buffering capacity of a sample. It is not the same as pH!

It is reported in units of mg/l CaCO3 and measured by titrating a sample with an acid until it reaches a defined pH end point. Titrating a known volume of the sample with dilute acid of a given strength to pH 4.3 is the standard approach. Titrating until this pH ensures all the species in the sample contributing to alkalinity have been neutralised. This detection method is generally unaffected by solids in the samples unlike a colorimetric approach.

A Tytronics Sentinel Process Titrator has been used at many UK sewage plants to determine the actual Alkalinity value of primary and final settled samples, as well as final effluent samples, with excellent results and reliability.

The analyser uses the pH/mV probe endpoint determination method, with any range of alkalinity being possible to determine. Reagents manufactured by PMA are low cost and automatic calibrations ensure analysis results are accurate.

The Tytronics Sentinel Titrator was designed specifically for wastewater samples; hence it is less prone to blockages, downtime and high maintenance cost than clean water analysers and all without filtration.

The New Accuseries

Interested in the Tytronics?

Check out the new model of the analyser, the Accuseries. The Accuseries has the same setup in terms of optics and fluidics but with an updated interface and dual parameter capability. Want to do phosphate and alkalinity measurement on the same analyser? Get in touch and ask about the Accuseries.

Discover Accuseries
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