NARRAGANSETT BAY COMMISSION:
SEPTAGE RECEIVING STATION UPGRADES
Lincoln, Rhode Island
The Narragansett Bay Commission (NBC) operates a septage receiving station in Lincoln, Rhode Island. The NBC’s septage facility can receive up to 435,000 litres per day of septage and some incidental domestic grease from septage haulers both in Rhode Island and in the neighboring state of Massachusetts. The average daily flow is 300,000 litres.
The septage from the trucks empties into a series of cascading grit removal chambers and the septage liquids are discharged into a 250mm gravity flow line that is commingled with the sanitary sewer connection on the West Side of the Blackstone River under the George Washington Highway. The commingled flow of septage and domestic sanitary waste then goes under the Blackstone River through a siphon and discharges into a drop manhole on the East Side of the Blackstone River.
Historically, there have been large releases of malodorous hydrogen sulfide gas at discharge end of the siphon. These odors have migrated into the surrounding neighborhood and were the source of numerous odor complaints from the homes along the Blackstone River.
Secondly, the cascading grit removal chambers were not effective in the removal of grit and solids. They allowed solid materials to migrate into the 10” gravity line causing frequent plugging. This plugging not only slowed down receiving operations but it also required weekly jetting of the line at premium overtime rates after the receiving of septage trucks ended for the day.
To ensure that hydrogen sulfide is not released in the siphon’s drop manhole, the utility’s engineering staff members visited other septage installations looking for other odor control solutions that worked and that would be applicable for their specific situation in Lincoln, RI, and for effective screenings and grit removal technology. They visited septage receiving stations that use the combined Lakeside Complete Plant screening and degriting system on the front end and the Venturi Aerators technology for septage odor control and conditioning on the back end. Among all of the various options reviewed they determined that the Lakeside/Venturi approach that they saw at the Windham, CT WPCA was the best configuration and solution to addressing the persistent problems with hydrogen sulfide releases at the siphon discharge and effective removal of the solids.
SCREENING & DEGRITTING
The first step is to screen and degrit the septage on a Lakeside Rotomat Complete Plant and then to “condition” the filtrate off the Lakeside unit that is discharged in a 76,000 litre in-ground tank with a Venturi Aerator system. The septage haulers pull their trucks into the septage receiving facility’s truck bay and connect to a discharge hose, which flows directly to the Lakeside Complete Plant. The flow can be either gravity feed into the Lakeside unit or pumped to increase the flow rate. However, when the discharge from the truck exceeds the capacity of the influent tank, it reverts to a gravity flow so as not to flood the pump pit adjacent to the Lakeside unit.
The Lakeside Complete Plant accomplishes the bulk removal of the solids and grit and discharges them into a rolloff container in the adjacent bay of the septage receiving facility. This configuration was selected so that the rolloff could easily be removed for changeout and disposal. Solids and grit have sufficient dryness to pass a Subtitle D “paint filter test,” so the solids and grit are hauled off to a landfill for disposal.
SEPTAGE AERATION: Odor Control & BOD Reduction
Small particles of organic materials that are not removed by the screens will fall through the screens and are in the “filtrate” coming out of the Lakeside Complete Plant. These organic particles have high BOD demand and they directly contribute to the high levels of hydrogen sulfide in the septage. They are discharged into a 76,000 litre “treatment tank” where a Model VA-500 venturi aerator continuously recirculates the septage filtrate adding “dissolved oxygen” in order to oxidize hydrogen sulfide into soluble SO4—a form of sulfur which is not malodorous. In the startup field testing dissolved oxygen (DO) added to the septage filtrate by the venturi aerator was measured at 4.35 mg/L at 22.5°C. This provides sufficient “residual” dissolved oxygen after satisfying the OUR (oxygen uptake rate) requirement of the septage to prevent the reformation of hydrogen sulfide gas before it reaches the discharge side of the siphon. Further, the high level of residual DO aids in BOD reduction in the commingled sanitary sewer line and begins treatment upstream of the wastewater treatment plant at Buckland Point.
The utility had been adding sodium hypochlorite to the septage to aid in odor control releases at the end of the siphon. Using the venturi aerator will reduce the amount of hypochlorite required for treating the septage when it is mixed with the domestic sanitary main.
The venturi aerator is located immediately outside the pump room and is operated by a “modified” Gorman-Rupp T4 [100mm] self-priming sewage pump in a lift suction configuration, pumping at 31 L/s into the Venturi Aerator unit. Being located outside the building the Venturi Aerator aspirates only outside ambient air into the venturi unit. The modification to the T4 self priming pump prevents hair and other materials that may bypass the screen from clogging the impeller of the pump. There are two pumps for redundancy and isolation valves to allow for the process to be switched between pumps and for periodic emptying of the tank and for annual cleaning and inspection.