Hi,

I believe that the most common interference for nitrate measurement is nitrite, However you would not usually expect this to be present at more than 0.1mg/l_N unless you have issues. What is the wastewater you are treating, and what is the mixed liquor suspended solids TSS/VSS.?

Depening on which method other suspects are ferric iron, free chlorine (or other oxidising agents), or chrome 6. I would try to measure samples pre-chlorination, or use a reducing agent to remove it (SMBS is good).

We use two different methods to measure nitrate in our MBRs.

1) The filtrate of both the domestic wastewater and industrial (pulp and paper) wastewater is analysed daily on an ion chromatograph. The nitrate (and other ions) in the supernatant of the mixed liquor of the industrial MBR's are measured twice a week. An IC proved to be the only way of reliably measuring many ions in this wastewater due to high colour and dissolved organic load.

2) Twice a week we also do nitrate on all four zones in both of the domestic reactors to monitor the nitrification / denitrification. For this we use a very cheap UV method on a Hach DR5000. It is Hach Method 10049. The range is 0.1-10mg/l NO3_N.

Sample Prep consists of centrifugation of the solids (2400rpm) and filtration (0.45um).

You then add 200ul of 1N HCl for every 10ml of sample (prepare blank and standard in the same way).

Using a 1cm quartz cell, you zero and read samples.

I have copied the following from the DR5000 manual.

Summary of Method

The UV nitrate direct screening method offers rapid determination of nitrate. Because both nitrate and organic constituents absorb at 220 nm and nitrate does not absorb at 275 nm, the second reading at 275 nm is used to correct for the absorbance attributed to organic matter. Although this method is useful for monitoring nitrate, it is not recommended for samples containing high concentrations of organics. Adding hydrochloric acid prevents interference from hydroxide or carbonate concentrations up to 1000 mg/L CaCO3.

This method works about the same as others (trialled all the Merck and Hach kits) providing the domestic wastewater is free of organics. None of them were excellent when used in real life matricies. Therefore we went with the UV method due to its almost non-existant cost. However it does not work if there is significant organics (and therefore it is not used for the industrial waster). Other than tha It is a good cheap and easy method.

Hope this hellps

Emily

Thanks Emily, BUT:

the water samples is almost free of organic matter, BOD below detectable level, we use Ion Chromatograph [IC] method, which is also an accurate analytical methods which gives you results on SO4, PO4, as well.

Domestic WW is being treated by the MBR. TSS in the oxi/anoxi zones 8-10 g/L. The VSS/TSS us about 85% in both. No NO2 is available, May you list other data or info re interference materials re this IC method ?

Sorry for the lengthy reply re spectophotometric measurements. For IC the case is different

The IC setup we use are dual Metrohm 881 compact IC Pros. The anions one has with dual suppression and conductivity detection. The column is a Metrosep A Supp 5 150/4 and a RP2 precolumn. We use the AccuIon Multi Component Anion Mix 4 (Fl, Cl, Br, NO3, PO4, SO4). You can get a separate NO2 standard which we used during the commissioning period, but determined that it was not worth the effort. The anions eluent is Na2CO3 at 340mg/l and NaHCO3 at 84mg/l , and the regenerant is sulfuric acid 0.1M. Sample loop of 20ul.

This and the cations IC are linked to an 858 Professional Sample Processor and undergou 0.22um inline filtration prior to analysis.

The elution order is :

Flouride, Acetate, Formate, Chlorite, Chloride, Nitrate, Chlorate, Bromide, Nitrate, Phosphate, Sulfite, Sulfate, Thiosulfate.

None of these ions seem to elute near nitrate however we are having some difficulty with the Fl, Acetate, Formate,Chlorate region. Therefore we hope to move to A Supp 5 250/4 column to assist with this.

What system, eluent and column are you using? If there is some coelution you can try variation of the eluent strenght to increase the run time, or getting a longer column.

What ever nitrite present is already changed to nitrate. Reported value will be of nitrate. Spectro photometric method may give good result. However preparation of standard solutions of different range is important. Calibration of instrument is must.