But Trees Can Grow on Money: Blue Plains WWTP Tour
In my interest to learn about phosphorus recovery from wastewater, I recently toured the Blue Plains Advanced Wastewater Treatment Plant located in Washington DC. Since it serves the nation’s leaders, the plant operators see themselves as being leaders in implementing the paradigm shift from wastewater treatment to resource recovery. The plant collects 370 million gallons each day from the District of Colombia and parts of Virginia and Maryland. The water is treated to meet stringent carbon and nutrient requirements before discharging into the Potomac River just downstream of the Kennedy Center for the Arts and the Thomas Jefferson Memorial. They are most proud of their newly commissioned biosolids management train.
The goals of the system upgrade was to reduce the volume of solids for hauling, increase the marketability of the remaining biosolids, and recover energy. To that end they installed four anaerobic digesters that each hold 3.8 million gallons. The system includes a thermal hydrolysis pretreatment system that handles 450 dry tons per day of feed sludge to improve solids degradation and methane production in those digesters. Thermal hydrolysis heats and pressurizes the solids to provide pathogen reduction and achieves class A biosolids. Then when the pressure is released into the anaerobic digesters, the remaining cellular material “pops”, similar to what happens when you open a can of soda. The energy from the increased methane production from digesting these broken cells is captured by a 10 megawatt steam turbine generator that offsets the energy demand for the thermal hydrolysis.
My eye of course was tracking the phosphorus through the plant. A majority of the influent P would end up in the anaerobic digester supernatant in a concentrated inorganic form. This flow would be the single best point to implement a direct P recovery process. Unfortunately, this opportunity is missed, and the supernatant is simply sent back to the head of the treatment plant. It is perhaps telling to the P sustainability community that the world’s largest advanced wastewater treatment plant, which prides itself on resource recovery, chose to focus on energy and not on nutrients. That indicates that current P recovery technologies simply do not yet yield the necessary economic or regulatory benefits for widespread adoption.
On a positive note, P is indirectly recovered from the second best place: the resulting biosolids. These solids are trucked to the neighboring states for agricultural land application. The class A designation means they have very low pathogens and odor and can be widely used as a fertilizer, where the organic and inorganic P can become incorporated into the soil and taken up by crop plants. They are actively working to develop an even higher value product from these remaining solids though, and the current frontrunner is composting it to produce a marketable soil amendment. In what I consider infinitely ironic, the solids are low in carbon content and must be blended with an organic substrate. They are experimenting with mixing different types of bark, sawdust, and even shredded currency from the Bureau of Printing and Engraving. So while the Blue Plains Advanced Wastewater Treatment Plant doesn’t yet recover P (since money doesn’t grow on trees), it is possible that together with P your trees can grow on money.