DC Water’s Biosolids Management Plan was originally adopted in 1999. This plan calledfor long-term Biosolids processing, treatment and disposal by land application. The plan was developed with input from the community, environmental groups, and other stakeholders. However, because of the cost of the project DC Water to deferred the project for a number of years until pushed by regulatory mandate. The decision by the DC Water to defer the project was based on an independently conducted economic analysis and an internal cost-benefit evaluation. All capital expenditure are paid for by DC Water (and Virginia and Maryland) rate payers so costs and benefits have to be considered when planning capital projects will increase customer water and sewer rates.
Sitting on the southernmost tip of Washington DC, across the river from Alexandria is the Blue Plains Advanced Wastewater Treatment Plant. While there are larger sewer treatment plants, that remove the solids and bacteria, the modern day Blue Plains also has Tertiary Treatment to remove nitrogen and phosphorus making Blue Plains the largest advance treatment plant in the United States at 150 acres and with a rated annual average day capacity if 370 million gallons per day (mgd) and a peak wet weather capacity of 1,076 mgd and serving 2 million people. The system needs such a large storm rated capacity to accommodate the old central city section which accounts for one third the area of the District and still has the original combined sewer system that overflows with predictable regularity during large storms
Being an advanced waste water treatment plant is not as modern as it sounds. At Blue Plains and other sewer treatment plants primary treatment uses screens to remove large solids from wastewater, and performs some rudimentary treatment to remove crude solids of human waste and skim off grease, oil and fat. Wastewater sits in settling tanks, which are designed to hold the wastewater for several hours. During that time, most of the heavy solids fall to the bottom of the tank, where they become a thick slurry known as primary sludge. The material that floats is skimmed from the surface of the tanks. Secondary (or biological) treatment involves feeding oxygen to bio-organisms that break down any organic matter still in the wastewater.
Tertiary treatment further treats the effluent water to remove nitrogen, phosphorus, fine suspended particles and microbes, and to kill or slowdown disease-causing organisms and viruses. It is the tertiary treatment that makes Blue Plains an Advanced Wastewater Treatment Plant. At this time, Blue Plains cannot remove enough nitrogen from the waste stream to meet the EPA mandated limit for nitrogen. In addition, when it rains the performance of the clarification units deteriorates because of the large flow of water from the combined sewer system and infiltration overwhelms the system causing turbulence and not enough time to settle the solids and scum. The deterioration in performance cascades down the treatment train and results in a reduced treatment for the sewage that lasts not only during the rain, but can last for several weeks. Blue Plans is currently engaged in a $7.8 billion 20 year improvement program called the Clean Rivers Project that will meet the reduced total nitrogen released requirements of their operating permits and increase the control of the system during rain storms in addition sludge treatment will be improved and sewer piping improved in many areas.
The sludge is separated from the wastewater during the primary treatment is further screened and allowed to gravity thicken in a tank. Then the sludge is mixed with the solids collected from the secondary and denitrification units. The combined solids are pumped to tanks where they are heated to destroy pathogens and further reduce the volume of solids. With treatment sludge is transformed (at least in name) to Biosolids.
On Wednesday at DC Water unveiled the recently completed and operational sludge treatment system. The plant now has Cambi thermal hydrolysis trains, four digesters, new dewatering equipment and a combined heat and power plant at a cost of $470 million. The new digestor system uses thermal hydrolysis (heating to over 160 degrees under high pressure) followed by anaerobic digestors. The system produces methane gas which is captured and used to run turbines to produce power that will meet over one third of DC Water's electric demand at Blue Plains (about $10 million in electric costs) and the digestion process destroys nearly one half of the Biosolids and producing Class A Biosolids reducing the chemical treatment costs and the transportation costs to get rid of the Biosolids. This will save DC Water an additional $13 million a year. Even with all these savings the project has a payback of over 20 years, so this was not about savings, but rather better sewage treatment in an urban environment. Class A Biosolids are safer and easier to use in agriculture.
At the unveiling on Wednesday DC Water indicated that they intend to sell a substantial portion of the residual Biosolids product as a soil amendment in Home Depot now that the Biosolids produced by the plant will be Class A . To ensure that Biosolids applied to the land as fertilizer do not threaten public health, the EPA created the 40 CFR Part 503 Rule in 1989 that is still in effect today. It categorizes Biosolids as Class A or B, depending on the level of fecal coliform and salmonella bacteria in the material and restricts the use based on classification. The Biosolids are tested for fecal coliform and salmonella and composite sampling is done for metals and hydrocarbons; the presence of other emerging contaminants in the Biosolids is not tracked. The land application of Class B Biosolids has been a growing area of concern. Research at the University of Virginia found that organic chemicals persist in the Class B Biosolids and can be introduced into the food chain. The new Biosolids treatment system will not only reduce the stench at Blue Plains, but will also reduce the overall amount of Biosolids and improve their safety by producing only Class A Biosolids. DC Water under the able leadership of George S. Hawkins is moving forward. Mr. Hawkins has launched an ambitious agenda at DC Water that complements the very broad Clean Rivers program to improve aging infrastructure and comply with ever more stringent regulatory requirements from the U.S. EPA.