Preliminary Treatment

Wastewater delivered to the plant from the collection system first goes through preliminary treatment. Preliminary treatment includes a Grit facility, which consists of 6 tanks, and the usage of mechanically cleaned screens where objects such as rags, sticks, and boards are removed and disposed of in landfills. These objects are removed in order to prevent damage to the downstream equipment and to prevent the clogging of pipes. The flow then travels through the Fine screens, which removes material down to 0.25 inches in diameter. The screenings are disposed of in landfills.

Primary Treatment

Following preliminary treatment, the wastewater flows into the primary settling tanks. During primary treatment, large and denser suspended solids organic particles settle in 6 sedimentation basins. Each of the basins has a design capacity of 17.5 million gallons per day and removes the majority of the suspended organic matter in the form of sludge.

The activated sludge process uses microorganisms to feed on the suspended and dissolved organic material. The primary effluent is conveyed to six parallel covered basins, each having a design capacity of 17.5 million gallons per day. Two cryogenic oxygen plants produce up to 80 tons per day each of 98% pure oxygen, which is fed to the covered basins to aid in the treatment process.

Secondary Treatment

Flow from the aeration basins is conveyed to four secondary clarifiers. Each of the four 210-foot diameter tanks has a design capacity of 17.5 million gallons per day and the four 155-foot diameter tanks have a design capacity of 9.4 million gallons per day. The settled material is collected at the bottom of the clarifiers and returned as “seed” organisms to maintain the activated sludge process in the reactors. The remainder is pumped for thickening to the gravity sludge thickening tanks. The clarified effluent flows out of the tanks to the chlorine contact chambers.

Tertiary Treatment

Disinfection coupled with follow-up dechlorination is the last treatment step before discharge to the Patapsco River. Chlorine is used at the treatment plant as a disinfectant to destroy disease-causing bacteria and other organisms. Chlorination and disinfection of the secondary effluent are accomplished in four contact chambers, with each chamber having three compartments, providing a design retention time of 30 minutes. A new dechlorination facility began operation in 1991 and uses sulfur dioxide to remove chlorine before discharge to the Patapsco River.

Solids

The thickening of sludge at the Patapsco plant is accomplished by gravitation. Solids from primary and secondary treatment are pumped into the two 65 feet diameter sludge-thickening tanks. These two fiberglass domed tanks take primary sludge (1% solids) and thicken it to 6% solids and secondary sludge (1.5% solids) to 2.5% solids. The thickened solids are pumped into the sludge storage tanks.

The primary and secondary sludges are mixed in the blended sludge storage tanks. The 2% to 6% blended sludge is pumped to the Heat Drying Facility, which first dewaters the sludge with four high solids centrifuges. The cake is then transported to the dryers where it is converted to pellets that are then sold for use as fertilizer, used for land application, or disposed of in landfills. The centrate and spent process water is pumped back to the Patapsco Facility for treatment. The Heat Drying Facility has a nominal capacity of 54.8 dry tons per day and a maximum capacity of 109.6 dry tons per day.