Biosolids Odor Control via Sodium Chlorite and Calcium Nitrate

Project Scope

In upstate New York, a wastewater treatment facility historically utilized potassium permanganate to control odors at two locations in their biosolids processing operations. The first location was into the waste activated sludge (WAS) fed to a gravity belt thickener. The second location was into a mixed stream of primary sludge and thickened WAS blended in holding tanks and pressed before incinerating. In October 2014, the plant’s incinerator went down. Pressed solids were hauled off-site for disposal in a landfill. The plant began to receive odor complaints from the truck drivers and the landfill personnel, so the treatment rate of permanganate was doubled. This attempt at durational odor control was unsuccessful and the cost was $480,000 per year. The plant had been using a nitrate product for durational odor control in its collections system and learned from a nearby plant about USP Technologies’ successful application of a sodium chlorite based oxidant blend (chlorite) for immediate odor control supplemented with a nitrate solution (nitrate) for durational odor control. USP Technologies (USP) was then brought in to trial a similar solution.

Technology

Chlorite oxidizes hydrogen sulfide and organic odors without producing any harmful by-products such as chlorinated organics. Chlorite is very fast reacting, on the order of seconds, with hydrogen sulfide and organic odor compounds. As a result, it can be added directly to the sludge in the feed lines prior to the presses. The chemical reaction at neutral-acid pH:

2 H₂S + NaClO₂ → 2 S<_>0 + HCl + NaOH + H₂O

and at alkaline pH:

S<_>2- + 2 NaClO₂ → SO<_>42- + 2 NaCl

Neutral-acidic conditions are favorable, as it only requires 3 mg/L sodium chlorite per 1 mg/L sulfide.

Like permanganate, chlorite is not as effective at durational odor control, so nitrate was selected to perform this role. Sulfate reducing bacteria, under anaerobic conditions (as is the case in sludge holding tanks), typically use sulfate as a source for oxygen. When nitrate is present, however, the bacteria will preferentially select nitrate as its source of oxygen, preventing the sulfate from being reduced to sulfide and subsequently preventing the formation of hydrogen sulfide gas. Also, with an alternative oxygen source, other common bacteria can now consume dissolved sulfide ions and oxidize them back to sulfate and/or elemental sulfur, further reducing sulfide available to form hydrogen sulfide gas.

Solution

USP replaced the two potassium permanganate dosing locations with chlorite, and a separate nitrate dosing location was installed at the presses. Three 1,000 gallon tanks were installed for chlorite as well as a 14 gph diaphragm chemical metering pump at each dose point, which was interlocked into the plant’s SCADA system. At the gravity belt thickeners, the H₂S levels were typically low but there are other problematic organic odors which are effectively oxidized with chlorite. At the belt filter press, H₂S gas levels were higher (35-55 ppm) since the primary and waste activated sludges fed to the press typically sit in holding tanks for anywhere from a day to two weeks. The plant has ventilation hoods above each press that bring the gas to a scrubber, but the odors in the room were still strong enough to cause discomfort to the operators in the vicinity.

An H₂S monitor was hung at the press for continuous monitoring of H₂S. Once the chlorite feed system was installed, sulfide in the sludge was able to be effectively oxidized, reducing the H₂S gas at the press from 35-55 ppm down to 0-3 ppm. Figure 1 shows the effect of the chemical program. Additionally, as the data towards the right hand side of the graph depicts a point in time when the chemical feed was lost (a valve was closed).

When not incinerated, the dewatered solids are hauled off-site for disposal at a landfill. The potassium permanganate provided very little, if any, odor control once the solids were loaded into the trucks, leading to many odor complaints. The addition of nitrate at the belt filter press via a 2,500 gal storage tank and dosing system proved

to be effective as it completely eliminated further odor complaints. It is the plant’s plan to stop using their incinerator in March 2016, at which point nitrate will be used continuously.

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Biosolids Odor Control via Sodium Chlorite and Calcium Nitrate