Nitrates

Nitrates

Bacteria in wastewater collection systems will use dissolved oxygen, nitrate, and sulfate as oxygen sources for respiration, in that order of preference. Dissolved oxygen is usually present in fresh wastewater, but is rapidly depleted by biological activity. There is typically very little nitrate present in the wastewater, while sulfate is typically abundant. Since little or no nitrate is available, the bacteria begin utilizing sulfate when the dissolved oxygen is depleted. The byproduct of the sulfate uptake process is dissolved sulfide. The dissolved sulfide combines with hydrogen ions to form hydrogen sulfide, causing odor and corrosion problems.

Nitrate solutions (calcium nitrate and sodium nitrate) are typically fed at pumping stations to control sulfide formation in force mains by providing an alternate source of oxygen. Feeding nitrate solutions to control sulfide formation and release is the most common form of chemical treatment in collection systems but there are many others including: magnesium hydroxide, iron salts such as ferrous chloride, hydrogen peroxide, enzymes, liquid oxygen and many other proprietary blends.

Typical Chemical Storage Tank and Control Panel

Most nitrate solutions are considered non-hazardous and are relatively easy and safe to store, handle, and inject. The chemicals are typically injected into pump station wetwells through the use of chemical metering pumps.

Applicable Treatment Processes

Wetwells, force mains and downstream gravity sewers.

Typical Design Criteria

Dosage: Varies based on numerous factors including flow rate, detention time, wastewater source and upstream conditions but rules of thumb are:

  • To oxidize existing sulfides = 0.7 gallons of calcium nitrate per pound of dissolved sulfide
  • To prevent formation of sulfides = 2.1 gallons of calcium nitrate per pound of dissolved sulfide

Since less nitrate is required to oxidize existing sulfides than to prevent the formation, nitrates are often added about 2 hours upstream of the control point. In other words, if the H2S needs to be controlled at the force main discharge manhole, then the nitrates would be most efficient if fed into the force main 2 hours upstream of the discharge manhole.

Major Design Considerations

Chemical Choice: Whether to use a nitrate solution or one of the many other chemical treatment alternatives will depend on many factors. In general, nitrate solutions have proven to be a cost effective alternative in a wide range of applications but seem to be most effective in forcemains with shorter detention times.

Injection Point: Nitrate solutions are typically dripped into wetwells or just upstream of the wetwell to encourage mixing. Nitrates are typically used to control sulfides at forcemain air release valves, discharge manholes and further downstream. It is usually not the best option when trying to control odors at the pumping station itself.

Fats/Oils/Grease (FOG): One of the common complaints about feeding nitrate solutions to wetwells is its tendency to increase the amount of FOG in the wetwell and downstream, especially when it is overfed. Increased FOG pumping at some wetwells has been observed at some stations when using nitrate solutions which is both time consuming and expensive for operators. This should be taken into consideration when evaluating chemical addition options. Some suppliers now offer nitrate solutions with additives that prevent or reduce the amount of FOG formed.

Secondary Containment: Most calcium nitrate solutions are considered hazardous, therefore, secondary containment is typically required.

Automatic Feed Control: Evoqua is the largest supplier of calcium nitrate.  They have developed software that can pace the feed of calcium nitrate based on flow rates, wastewater temperature, downstream H2S concentration and other operating parameters.  These automatic controllers can reduce chemical feed rates significantly.