In upstate New York, a wastewater treatment facility historically utilized potassium permanganate to control odors at two locations in its biosolids processing operations. The first location was in the waste-activated sludge (WAS) fed to a gravity belt thickener. The second location was in a mixed stream of primary sludge and thickened WAS blended in holding tanks and pressed before incinerating.
In October 2014, the plant’s incinerator broke down. Pressed solids were hauled off site for disposal in a landfill. The plant began to receive odor complaints from the truck drivers and landfill personnel, so the treatment rate of permanganate was doubled.
This attempt at durational odor control was unsuccessful, at a cost of $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 a successful application of USP-OC31, a sodium chlorite oxidant blend for immediate odor control supplemented with a nitrate solution for durational odor control. USP Technologies then was brought in to conduct a trial of a similar solution.
Reducing Hydrogen Sulfide
USP-OC31 oxidizes hydrogen sulfide (H2S) and organic odors without producing harmful byproducts such as chlorinated organics. The solution reacts quickly, 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.
Neutral pH: 2 H2S + NaClO2 ——> 2 S0 + NaCl + 2 H2O
Alkaline pH: S2- + 2 NaClO2 ——> SO42- + 2 NaCl
Neutral-acidic conditions are favorable, as they only require 4.3 mg/L USP-OC31 per 1 mg/L sulfide.
Like permanganate, USP-OC31 is not as effective at durational odor control, so in this case nitrate is used 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 their source of oxygen, preventing the sulfate from being reduced to sulfide and subsequently preventing the formation of H2S gas.
Also, with an alternative oxygen source, other common bacteria now can consume dissolved sulfide ions and oxidize them back to sulfate and/or elemental sulfur, further reducing sulfide available to form H2S gas.
Successful Odor Control
USP provided all equipment as part of the trial program and replaced the two potassium permanganate dosing locations with USP-OC31. A separate nitrate dosing location was installed at the presses. Three 1,000-gal dosing systems were installed, as well as a 14-gal-per-hour diaphragm chemical metering pump, and remote telemetry/monitoring at each dose point, which was interlocked into the plant’s SCADA system. At the gravity belt thickeners, the H2S levels typically were low, but there are other problematic organic odors that are effectively oxidized with the system. At the belt filter press, H2S gas levels were higher (35 to 55 ppm) because the primary slude and WAS fed to the press typically sit in holding tanks for one 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. Once the feed system was installed, sulfide in the sludge was able to be effectively oxidized, reducing the H2S gas at the press from 35 to 55 ppm to 0 to 3 ppm.
When not incinerated, the dewatered solids are hauled off site for disposal at a landfill. The potassium permanganate provided 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.
In early 2016, the plant stopped using its incinerator and now is using nitrate continuously.
Switching odor control treatment programs has provided many benefits to the wastewater treatment facility. The plant has saved $1,000 to $4,000 a month using the new system.
Furthermore, the plant was receiving powdered permanganate in pails and mixing it into solution multiple times per day. By switching to USP-OC31, operators no longer have to make up solutions, saving one to two man-hours per day and reducing operators’ risk of chemical exposure. Additionally, operators no longer need to carry, store and dispose of the contaminated pails.