First, I don't measure ammonia, but do measure lots of other stuff & have a UV/Vis spectrophotometer & do have to break ammonia complex for WWT to precipitate metals.
I’m not familiar with the chemscan brand.
What is their procedure for UV/colorimetric analysis?
I looked at Hach's site and it seems you must add some reagent to release bound ammonia plus an indicator in order to measure. Don't know how this could be truly 'on-line' measurement. Also, looked in Vogel's Textbook of Quantitative Inorganic Analysis. Mentions Nessler's reagent which contains mercury. But measurements are in vis, not UV range, so maybe chemscan has an improved method.
My opinion is that you should try to use an ISE (ion selective electrode), Orion shows a measurement range 0.01-17000 ppm. If bound ammonia cannot be measure directly by ISE, you can first add whatever reagent is used to liberate the bound ammonia for UV.
Since writing the above, I found that there was a 90-day valuation of 9 online analyzers conducted in Houston.
Only the abstract is available for free, but I note that of the instrument types, there was 1 UV absorbance (Chemscan), 1 colorimeter, & 5 ISE. I would say from this that if the conditions below are similar to yours, ISE is the most proven of the methods, and further, you have a choice of ISE electrode suppliers (I think that they have all standardized on BNC connectors). It's also possible that you can contact the vendors listed below, and if they did well on the test, you might be able to obtain a portion of the report.
"This report includes evaluations of nine online ammonia analyzers representing three different technologies (ion-selective electrodes–ISE, colorimetric, and UV absorbance) that were tested under field conditions, over a period of 3 months (90 days) at the City of Houston's Beltway Wastewater Treatment Plant in Houston, Texas.
This report contains evaluations of:·
ASA, Inc., ChemScan, Model UV 2150 NH3-N; ·
Bran + Luebbe, Model 90 S ISE; ·
Danfoss Analytical, Model Evita Insitu Colorimetric; ·
FPM Analytics, Model IonChem 2005 Ammonia ISE; ·
Hach Company, Model APA 6000 NH3-N ISE; ·
Waltron Ltd., Model l Al-8232 Ammonia Monitor ISE, ·
WTW Measurement Sys., Model TresCon BA43111e02 NH4-N ISE; and ·
Zellweger Analytics, Model Z polymetron 8810 Ammonia ISE. This test was sponsored by the Instrumentation Testing Association (ITA) and the City of Houston, Department of Public Works and Engineering, Wastewater Quality Control Laboratory and Beltway Wastewater Treatment Facility.Testing was conducted at the City of Houston's Beltway wastewater treatment facility operating with an average flow of 5 mgd (18,925 m3/d) in an activated sludge basin mixed liquor suspended solids environment having the following average characteristics·
flow of 250,000 gal/hr (946,350 L/h) ·
BOD5 between 500 and 1500 mg/L ·
Total Suspended Solids of 3220 mg/L ·
Total Dissolved Solids of 340 mg/L ·
Dissolved Oxygen of 2.1 mg/L ·
pH of 6.7 ·
Ammonia (NH3-N) of 2.1 mg/L ·
Nitrate (NO3-N) of 3.2 mg/L and ·
Seasonal Temperatures between 50 and 75oF (10 to 24oC)."
On-Line analysis typically means automatic analysis with sample lines directly connected to the process. The output from the analyzer is continuous, and is updated at some regular interval. Methods of analysis for ammonia may require sample conditioning and/or reagent addition, as the form of ammonia is in equlibrium as NH3 or NH4, depending on sample pH.
An ISE is not the most reliable option for this application, as evidenced by the Houston test where both in-situ (immersed) and on-line (flow through) ISE systems performed poorly and required substantial maintenance for calibration and cleaning. Any analyzer must deal with the issues of fouling and drift, which are especially challenging in wastewater. ChemScan had the best overall accuracy (measured as a standard deviation compared to a control sample analyzed in the laboratory) at Houston. ChemScan also had the lowest maintenance. Their method is full spectrum uv analysis of a sample reacted with hydroxide and bleach (chloramine formation method). This has been shown to be a very reliable method in treatment plants throughout the US and Canada. A filtered sample is required for aeration basin analysis, but ChemScan has several options for systems, some of which are capable of monitoring multiple sample points and detecting multiple parameters using a central analyzer. Some ChemScan systems detect ammonia, nitrate, nitrite and phosphate at multiple sample points. All ChemScan systems have automatic zeroing and cleaning to deal with fouling and drift issues.
