Liquid Treatment Processes
Wastewater to the BackRiver Wastewater Treatment Plant currently receives four levels of treatment: Preliminary, Primary, Secondary, and Tertiary.
The preliminary treatment process includes six fine screens (one quarter inch openings) and four grit removal basins. The fine screens remove larger solid objects such as cigarette butts, rags, sticks, rubber, and plastic items, which may clog or damage downstream pipes, pumps, or collection mechanisms. After fine screening, the flow enters the grit removal basins where the flow velocity is reduced to 1 foot per second to allow sand and other grit particles to settle out. Collected screenings and grit are removed and transported for disposal to the Quarantine Road Landfill.
Following preliminary treatment, the wastewater is distributed among eleven primary sedimentation tanks (nine 170-ft. diameters and two 200-ft. diameters) where large and denser suspended organic particles settle. Approximately 65 percent of the suspended organic material normally settles in these tanks and is removed continuously by rotating scraper mechanisms as primary sludge. Primary sludge removed from the tanks averages approximately one-percent solids content. While the sludge goes to solids processing, the clarified effluent proceeds to secondary treatment.
Waste Pickle Liquor, which contains iron and is obtained as a by-product from Severstal steel mill at Sparrows Point, is added to the primary effluent prior to secondary treatment to chemically precipitate phosphorous with iron thus reducing the amount of this nutrient discharged to the Bay.
Effluent from the primary process, which still contains dissolved and fine suspended solids, flows to the activated sludge process (secondary treatment). At the BRWWTP, flow from primary treatment enters two separate fine bubble activated sludge facilities where a culture of microorganisms is maintained to absorb and metabolize organic pollutants. Air is continuously added to the aeration basins to meet the oxygen demand of the bacteria and other organisms that comprise activated sludge. After treatment, the activated sludge is then separated from the wastewater by sedimentation in secondary clarifiers similar to those used in primary treatment.
After sedimentation, approximately 35 to 40 percent of the aeration tank influent flow is then recycled back to the aeration basins in a form commonly referred to as Return Activated Sludge (R.A.S.). This maintains the required biomass concentration, while the remaining sludge commonly referred to as Waste Activated Sludge (WAS) is pumped to solids handling processes. Collectively, the facilities generally utilize approximately 200 million cubic feet of generated air per day.
All Activated Sludge Facilities are retrofitted to operate as a Biological Nutrient Removal (BNR) Facility. The retrofit allows various single stage nitrification/denitrification modes to be compared so that the most efficient design can be selected for future plant designs.
To further reduce the phosphorus loading to Back River, ferric chloride and polymer are added separately to the aeration tanks prior to secondary clarification. As an option, polymer can also be added to the effluent after secondary clarification. Together, these chemicals cause a precipitate to form that can be removed by sedimentation or during filtration.
Advanced Treatment (Tertiary)
After secondary treatment, flow enters the Sand Filtration Facility (Tertiary Treatment). This facility consists of forty-eight, 16 ft. x 116 ft. sand beds with a total filter surface area of slightly over two acres. Each filter bed has an 11 inch sand depth with a sand particle size between 0.55 and .65 mm.
Similar to a swimming pool sand filter, the Back River filtration process entraps the solids particles on the filter bed to remove most of the remaining suspended solids from the wastewater. The suspended solids are then automatically backwashed from the bed and recycled back to the influent of the activated sludge process.
Chlorine solution is added at the influent of the filtration process for the purpose of minimizing algae growth on the filter beds, and as the primary point of chlorination for disinfection of the plant effluent. Chlorine can also be added downstream of the filtration process for final trim dosing.
Disinfection by chlorination (bleach), dechlorination by bisulfite, and post-aeration in a step-dam cascade system were the original three step process design objectives at the Chlorination/Dechlorination Facility. However, with the March 1993 start-up of the filtration process, the location for chlorinating final effluent was eventually modified and chlorine is now added primarily at the Filtration Facility. As a result of this process modification, the overall contact time has been greatly increased. After filtration, the effluent passes through four, three-pass chlorine contact tanks located at the Chlorination/Dechlorination Facility. Each chlorine contact tank measures 43 ft. by 165 ft. by 15 ft. deep giving a total volume of over 3 million gallons and provides up to an additional 44 minutes of chlorine contact / disinfection time.
The overall contact time currently achieves a disinfection level as measured by E-Coli concentration, that meets a permit requirement of less than a Most Probable Number (MPN) of 126 per 100 ml. After chlorination, bisulfite solution is added to neutralize the chlorine residual, thus minimizing the potential toxic affects that chlorine, and compounds that form as a result of its addition, could have on aquatic life within the Bay.
Finally, the effluent spills down a step-dam cascade system where it is re-aerated to a dissolved oxygen concentration in excess of five milligrams per liter. This treated, neutralized, and oxygenated effluent then passes through a 1,200 ft. long outfall pier where, because of its high dissolved oxygen content, it attracts large populations of minnows and other aquatic and bird life.
Inherently, odors are generated at all wastewater treatment plants; the Back River Plant is no exception. However, the BRWWTP continues to make significant improvements in controlling odors emanating from the plant to the surrounding community. The odor control measures employed to date have accounted for a steady and marked reduction in odor complaints.
The current method of odor control is achieved through capturing and collecting odorous gases and treating them through a series of individual chemical scrubbing systems located at different unit process areas. There are seven of these chemical scrubbing systems and they are located at the plant’s Fine Screen Building, Rapid Sludge Loading Facility, Thickener Tanks, Gravity Belt Thickeners, and three servicing the Primary Settling Tanks.
In addition to the chemical scrubbing systems, a hydrogen peroxide (H2O2) addition system is used to control odors from the incoming wastewater to the plant. Two 10,000 gallons above ground H2O2 tanks deliver chemical to the liquid stream at a flow rate up to 7.5 gallons per minute. Hydrogen peroxide, having a strong oxidizing characteristic, is utilized primarily to oxidize hydrogen sulfide (rotten egg smelling compound) to a more stable form of sulfur.