Solids Treatment Process
Equally as important as the treatment of wastewater is the handling of its by-product, sludge. At the Back River Plant, solids receive three stages of handling prior to disposal: Thickening, Stabilization, and Dewatering.
At the BRWWTP, primary sludge is thickened by gravity while waste activated sludge is thickened by gravity belt thickeners and / or the air floatation process. Up to eight gravity thickeners, each 65 feet in diameter are used to thicken primary sludge from a dilute liquid of one percent solids content to a thickened sludge concentration between four and seven percent solids.
Similarly, solids from secondary treatment, which are lighter and more readily floatable than primary solids, are processed through five gravity belt thickeners and up to four air flotation thickeners. During the sludge thickening process, waste activated sludge, at a dilute liquid of 0.8%, is thickened to a sludge solids concentration between 3-7% solids.
At the BRWWTP, sludge stabilization is currently achieved by the use of six, 1.3 million gallon conventional and two, 3 million gallon, egg-shaped anaerobic sludge digesters. The egg-shaped digesters were started in December of 1992. They are slightly over eighty feet in diameter and one hundred and fifty feet high. They were selected because of their life-cycle cost, operating efficiencies (never require cleaning) and large volume/small footprint aspect.
Anaerobic digestion is a biological treatment where anaerobic bacteria decompose and stabilize the organics in the thickened sludge. Consequently, this sludge stabilization process reduces the volume of the sludge by destruction of volatile compounds while producing combustible gas as a by-product. Digester gas typically consists of about 65% methane and 35% carbon dioxide and is used for heating the digestion process, which operates between 96 and 98 degrees Fahrenheit. The gas is also used to heat various plant buildings and facilities. Digestion generally destroys about 45% of the volatile solids while producing an average 1.5 million cubic feet of gas per day. Reduction of sludge volume greatly reduces our costs for disposing of plant sludge,
After digestion, the sludge is conditioned with polymeric coagulants and dewatered in solid-bowl centrifuges. These centrifuges use a large solid walled bowl with horizontal axis-rotation. Centrifugal forces developed by the rotation causes the solids, which are denser to adhere to the bowl wall and are continuously scraped off by a helical screw conveyor.
The solid bowl centrifuges concentrate the sludge from thick liquid slurry of 3-5% solids content to a cake of 20 to 25% solids content. Currently, averages of 450 wet tons of dewatered digested sludge are produced per day.
The Back River Plant produces an excellent quality municipal sludge, characterized as low in metals and relatively high in nutrients. Because of these two characteristics, Back River’s digested sludge is currently recycled using three methods. They include: direct agricultural land application, composting of digested sludge cake for marketing as a soil amendment and fertilizer, heat drying of digested sludge to produce a dry pelletized product which is also marketed as a fertilizer. Collectively these sludge utilization options respectively represent 5, 45, and 50 percent of the plant's sludge management program. While agricultural land application is managed on a yearly contractual basis, composting and heat drying are both managed under 20-year privatized service contracts.
The Back River Environmental Laboratory consists of these sections: General, Toxics, and Quality Control. The general lab performs analyses for various parameters such as nutrients, BOD, COD, FOG, TOC, VSS, SS, TP, DO, and pH. Similarly, the toxics lab consists of metals and organics sections. While the metals section tests for a wide spectrum of metals, the organics section tests for acid phenols, base neutrals, pesticides, and volatiles. The Quality Control Section monitors the performance level of the laboratory. It provides check standards to all laboratory sections to verify the precision and accuracy of the various analytical systems.
The Plant’s environmental laboratory currently receives numerous samples each day that are analyzed to determine plant-wide performance. The results of certain of these analyses are used in completing the plant’s daily, weekly, and monthly technical reports. Results are also used for the plant’s monthly discharge monitoring report submitted to the Maryland Department of the Environment and EPA.