Hydrogen Peroxide - The Cedar Rapids Experience

Hydrogen Sulfide Control in Bio-Solids Processing

Hydrogen Peroxide – The Cedar Rapids Experience

The City of Cedar Rapids Water Pollution Control Facility (WPCF) treats an average of 42 MGD of mixed industrial and domestic wastewater.  Due to the large percentage of industrial wastewater contribution (e.g. paper mills, corn processors, agricultural products), influent to the Cedar Rapids WPCF is “higher strength” than most municipal sewage, particularly with respect to BOD, sulfate, and nutrients.   These conditions result in efficient production of hydrogen sulfide (H2S) in both solid and liquid treatment processes, and thus create significant safety and operational challenges for the Cedar Rapids staff.

One of the highest priority areas for improved H2S control was the bio-solids gravity belt thickener (GBT) building. During normal operations, H2S levels in the GBT building routinely exceeded the Cedar Rapids WPCF mandated levels for worker exposure (10 ppm).  Significantly, H2S spikes in the 100-200 ppm range have been measured directly above the gravity belt thickener table.  Due to these unsafe levels of H2S, shut down of the GBT’s and ventilation of the building was required before equipment maintenance or other work could be started.  This procedure negatively impacts the efficiency of the bio-solids processing operation.  High H2S levels in the GBT building over time has also caused severe corrosion of electrical components and equipment, and contributed to odors in and around the GBT building.

Due to the safety and operational issues noted above, a program was undertaken to evaluate hydrogen peroxide (H2O2) oxidation of bio-solids sulfide prior to gravity belt thickening.  The objectives of this program were to: 1) Improve worker safety by maintaining GBT building H2S levels in the 0-2 ppm range 2) Reduce bio-solids processing “down time”, 3) Reduce corrosion and odors in the GBT building.

This paper will present results from this ongoing program, which employs injection of hydrogen peroxide into the waste activated sludge line, several minutes ahead of the GBT’s.   Results from initial laboratory testing, through full scale implementation of this program will be reported.

To date, the use of hydrogen peroxide has reduced H2S levels at the GBT’s by an average of  >90% which has maintained H2S in the GBT building to the target level of 0-2 ppm 95% of the time.  These significant reductions in H2S have accomplished the program objectives, e.g.:

• H2S levels in the GBT building are regularly maintained below Cedar Rapids 10 ppm exposure limits, resulting in safer working conditions in the GBT building.
• Routine equipment maintenance and other work in the GBT building can now be done without shutting down the bio-solids processing operation, improving efficiency.
• The environment in the GBT building is less corrosive. It is expected that this will extend the life of electronics and other equipment.
• Odors in and around the GBT building have been reduced

Further optimization of this H2S control program are underway, and include (bio-solids) flow pacing of the hydrogen peroxide metering pumps, and/or automated H2O2 dosing control based on gaseous H2S monitors in the GBT building.

Download Cedar Rapids – IWPCA Abstract (doc)

Download Cedar Rapids – IWPCA Presentation (ppt)