Nitrite Violation

Q: Recently, the city performed its semi-annual wastewater sampling at our facility. To our and their surprise, they found that we’re in violation of their sewer use ordinance for nitrites; they had never analyzed for nitrites before, and nitrites aren’t listed in our discharge permit.

We reviewed our Material Safety Data Sheets and did find several that contained sodium nitrite. Before we go and try to investigate alternatives to these materials, we have two questions: Can the city issue a notice of violation for something not in our permit? And is there an easy way to remove nitrites from wastewater? T. J.

 

A: Regarding your first question, I haven’t read your discharge permit, and I’m not an attorney. I cannot give you a definitive answer. Read your permit thoroughly. Chances are it contains general language that states your are required to comply with the city’s sewer use ordinance; if so, it appears that the city could issue a notice of violation regarding nitrites without it being specifically listed in your discharge permit.

Treating nitrites is fairly simple using one of two chemicals found in many plants: sodium hypochlorite (NaOCl, bleach) or hydrogen peroxide (H2O2). Basically, these chemicals oxidize the nitrite (NO2) to nitrate (NO3) through the following reactions: 

NO₂ + H₂ O₂ >>> NO₃ + H₂ O

or

NO₂ + NaOCl >>> NO₃ + NaCl

 

The concentration of nitrites is usually given in terms of “as nitrogen,” that is, the result measures the concentration of nitrogen as part of the nitrite molecule.

Theoretically, to treat one lb of nitrites expressed as nitrogen, it takes about?5.3 lb of sodium hypochlorite or about?2.3 lb of hydrogen peroxide. If 10 mg/L of nitrites as nitrogen (usually expressed “as N”) is present in 1000 gal of wastewater, theoretically, it would take?approximately 8?fluid oz of 35% hydrogen peroxide or about?36 fluid oz of 14% bleach.

Because these chemicals can also oxidize some of the organics in your wastewater, we recommend that you perform some bench-scale tests to determine actual dosage. Because this reaction is fairly rapid and we desire not to have these chemicals interfere with your wastewater treatment system’s precipitation, coagulation and clarification, we recommend that these chemicals be metered into the wastewater after the clarifier. Care should be taken not to overdose these chemicals, because they could oxidize trivalent chromium to hexavalent chromium.

You didn’t state whether or not you also have a nitrate limit. One mg/L of nitrite is converted to one mg/L of nitrate. By getting rid of one problem, are you creating another. We are not familiar with any “easy” chemical process for the removal of nitrates from wastewater. Typically, they are removed from wastewaters using biological activity where the dissolved oxygen in the wastewater is lowered to near zero so that specialized bacteria strip the oxygen atoms from the nitrate molecule and release nitrogen gas.

It is very likely that your wastewater does not have sufficient organic matter or food source for these bacteria to use, and the cost to install a biological treatment system on the back end of your current physical-chemical treatment system would be prohibitive. Nitrates can also be removed by ion exchange, but you either must replace anionic media or regenerate on-site; in either case, you will have expensive disposal.

Lastly, if you find that you can oxidize nitrite to nitrate and be in compliance, keep in mind its potential impact on SARA 313 toxic release reporting. Because you are “manufacturing” nitrate compounds by the conversion of nitrites to nitrates, if you “manufacture” more than 25,000 lb/year of nitrate compounds, the release of nitrate compounds will need to be reported by July 1 of each year.

Assuming sodium nitrate is the “nitrate compound,” 1 lb of nitrites as nitrogen?is converted into?5.1 lb of nitrate compounds. You can estimate your annual nitrite as nitrogen pounds by this equation:

(Nitrite concentration as N) × (million gal/yr)
× 8.34 = (pounds per year)

 

Therefore, to exceed the 25,000-lb threshold, you would need to treat more than 4,900 lb/year of nitrites as nitrogen.
This is the first time we have heard of a metal finishing facility having a compliance issue for nitrites. Good luck.