Odor & Corrosion Juggling Act Part II: Lessons Learned with Maintaining a Major Municipal Collection System Odor and Corrosion Control Program.

Manatee County Utilities (MCU) recently completed a strategic enhancement to their decades long commitment to be a good neighbor to those in close proximity to wastewater collection system lift stations and treatment facilities. Historically a variety of liquid and vapor phase treatment approaches were utilized to prevent nuisance odor complaints. In 2018, MCU embarked on implementation of an enhancement to their successful odor control program that targeted reducing the hydrogen sulfide (H2S) related corrosive conditions in much of their collection system. As MCU operates an expansive collection system involving an extensive (over 700) lift station (LS) network with many long retention time force mains, achieving significantly lower H2S in the head spaces within lift stations and gravity sectors downstream of force main discharges was a major challenge. This paper provides further perspective gained with longer term operating experience and full system implementation. The collection system conveys 22 Million Gallons per Day (MGD) of flow across three service areas to a corresponding wastewater reclamation facility for each service area. The North Service Area (NSA), Southeast Service Area (SESA), and Southwest Service Area (SWSA) are comprised of many outlying lift stations (LS) with manifolded force main networks that feed re-pump stations. Some re-lift stations are known as master lift stations (MLS), which receive wastewater that in some cases is repumped four times; such as MLS 1M. Figures 1 and 2 provide perspective on the geographic expanse and force main network complexity. As MCU was experiencing significant corrosion of concrete and metallic components of the gravity system upstream of the MLSs and in the MLS wet wells while successfully preventing public odor complaints with a combination of chemical liquid phase treatment applied at key upstream LSs and biological vapor phase treatment with biofilters an enhancement to H2S treatment goals began in 2018. To significantly reduce corrosive conditions MCU set a target MLS atmospheric H2S control objective of 20 ppm average and 50 ppm peak. In most cases historical levels were five to twenty times these target values, so accomplishing these odor and corrosion (O&C) control targets required incorporating new treatment and operational strategies. By targeting such lower MLS H2S levels it was expected that corrosive conditions in upstream re-pump stations/gravity sectors and related downstream WWRF headworks would also be reduced. Based on a 2017 engineering study report, one key new liquid phase treatment approach implemented was to raise the wastewater pH from the typical 6.8-7.4 range into the 8.0-8.5 range. This was done in order to significantly increase the percentage of dissolved sulfide (DS) in the nonvolatile HS- and S2- forms rather than volatile H2S. Due to extremely long retention times generating very high DS levels (10-40 mg/L) in some force main networks upstream of most MLSs it was anticipated a combination of chemical treatments targeting pH elevation along with DS reduction could achieve MLS H2S control objective at many locations without a major increase in historical H2S treatment expense. Implementation started in the NSA, then moved to the SWSA, and finally the SESA. Continuous H2S monitoring of the MLS wet wells, provided by MCU's vapor phase treatment provider, was critical to evaluating impacts of introducing upstream magnesium (mag) hydroxide chemical treatment to raise the wastewater pH and optimize dose requirement of calcium nitrate at higher pH conditions. Ability to access web-based data on nearly a real time basis facilitated a highly data driven implementation approach on a MLS by MLS basin basis. Figure 3 shows an example of post implementation online H2S data for Heritage Harbour MLS, which is the result of two upstream mag dose locations to raise wastewater pH, two upstream nitrate dose locations to reduce DS, raised MLS wet well level to reduce turbulence, and MLS wet well ventilation to historical onsite biofilter. The technical paper will summarize significant H2S performance improvement achieved and enumerate lessons learned from the implementation periods and subsequent ongoing operation timeframes across sixteen MLS basins. While utilization of pH elevation was generally effective in reducing H2S release from wastewater received by the MLSs, practical limitations experienced due to the complex upstream manifolded force main networks, significant DS levels, and degree of wastewater turbulence will be covered. Ultimately a blend of wastewater pH elevation, DS reduction and turbulence reduction along with MLS biofilter ventilation adjustments unique to each MLS basin has resulted in dramatic reduction in H2S induced corrosive conditions within MLS wet wells, upstream LSs and gravity systems. Adjusting chemical dosing to reflect individual MLS basin sulfide generation dynamics related to seasonal flow/wastewater temperature changes, significant storm events, addition of new LS & MLSs, altering existing LS force mains, and collection system maintenance activity is an ongoing operational goal/challenge for MCU staff, their consultants, and O&C service providers. Tools and processes developed to facilitate ongoing optimization of the O&C program will also be presented.