The Water and Wastewater Utility Department of the city of Austin, Texas, manages the operation of a regional sludge processing facility where they were met with the challenge of solving an odor problem.
Between October of 1992 and August of 1994, the facility received a total of 23 odor complaints from the surrounding neighbors. Some had complained numerous times. Most of the complaints occurred between the hours of 8 a.m. and 12 p.m. – coming from the residents of apartment complexes, the staff and students at two elementary schools, restaurant patrons, and a local air force base’s personnel.
In an effort to maintain effective control of any odor emissions from the plant site, the city had retained several consultants over the previous years to monitor the site and conduct evaluations of the existing odor control measures.
Sampling was performed during September of 1992, December of 1992, and April of 1994. Though some of the results were inconsistent with previous studies, there was a definite trend that indicated certain processes were emitting appreciable levels of H2S and other sulfur compounds.
In the fall of 1994, the city hired CH2M Hill consultants to perform an intensive audit of the plant and its operations, and to make recommendations on improved odor control. As part of their study, they conducted an audit of the current odor emission sources. They later presented the impact of each individual source, showing its percent contribution to the total odor inventory at the plant site.
The major sources of odor (more than 90 percent of the total odors) and their percent contribution to the total odor inventory were as follows –
Equalization tank -- 34.0
Govalle flares -- 15.6
SAR flares -- 15.6
Thickened sludge storage -- 12.6
Lagoon 1W -- 5.0
Govalle digesters -- 5.0
SAR digesters -- 4.7
The major source of odor emissions at the plant was coming from an existing odor scrubber system. This system consisted of a first stage packed column, using caustic as the chemical reagent, followed by three carbon units in parallel as a second stage. The scrubber system was scrubbing the malodorous air from only the sludge equalization tank and was incapable of handling the total odor load. The sludge equalization tank vent gas had an average hydrogen sulfide concentration of 100 ppm, with peaks to 250 ppm.
However, the consultant explained, relying only on caustic (sodium hydroxide) to react with the H2S, the process merely moved around the sulfide. The reaction is reversible at lower pH and the sulfide can be re-released in some other part of the plant when the liquid blowdown stream from the caustic scrubber was returned to the plant’s main system.
In presenting recommendations to enhance the odor control at the plant, the consultant reviewed several alternatives with the city and recommended that a new two-stage caustic/sodium hypochlorite packed tower system be installed to handle the combined malodorous air from the sludge equalization tank, sludge screening, and the sludge storage tank. The consultant also recommended that a biofilter be considered as a final polishing stage downstream of the packed towers.
The city opted to explore other odor control options. Working with the consultant, they purchased a fixed bed, iron oxide based odor control system. The Sulfatreat® Company of Chesterfield, Missouri, supplied the media. The vessels and other mechanical equipment were competitively bid and supplied by the general contractor. Based on experience gained from more than 1,000 previous applications, Sulfatreat based its recommendation on its proven commercial technology for H2S scavenging used globally by major oil and gas producers.
The Sulfatreat technology provided a low cost, highly effective (over 99.5 percent removal) solution to the major odor generation sources at the facility. The process provides the wastewater plant or sludge processing facility owner/operator an easy-to-use, reliable, and cost-effective tool for odor control. Additionally, the Sulfatreat media is a low pressure-drop product that provides long on-stream operation with minimal maintenance or operational attention.
The H2S reacts with the iron chemistry to produce environmentally safe reaction products consisting of predominantly iron pyrite and some elemental sulfur. The spent media is nonhazardous and can be disposed of in a landfill or applied as a soil amendment product (registered as such in the State of California).
The facility also purchased Sulfatreat media to condition the anaerobic digester gas at the site upstream of the boilers and power generation equipment.
The Sulfatreat odor control system, with the initial charge of media, has exceeded initial run time expectations even though the inlet H2S concentration has routinely been in the 200 ppm range. To date, there have been no operational or mechanical problems with the Sulfatreat system. More importantly … there are no odor complaints!
For further information, contact SulfaTreat in Chesterfield, Missouri – phone 800-726-SOUR.