Driving Net-Zero at DC Water, Washington

CDM Smith, in an equal joint venture with PC Construc­tion, partnered with DC Water on the design-build delivery of the biosolids main process train project, which incor­po­rates Cambi’s thermal hydrolysis process (THP) - the first facility in the United States to use this technology, and the largest Cambi plant in the world.

The Blue Plains advanced wastewater treatment plant (AWWTP) serves more than 2 million residents in densely populated Washington, D.C., USA, and in surrounding communities in Maryland and Virginia. The 62 hectare plant has an average capacity of 5.3 billion litres per day and a peak capacity of more than 14.3 billion litres per day - an expensive, energy-intensive operation.

Incor­po­rat­ing Cambi THP
While DC Water had already developed a successful biosolids reuse programme, upgrading the facility provided consid­er­able additional economic and envi­ron­men­tal benefits. Originally, the utility pursued a design that used anaerobic digestion—a process that would have required 10 large, egg-shaped digesters—but building the digesters proved expensive, and the utility considered alternative approaches.

Obtaining a fixed price for the $210 million project gave DC Water certainty in moving forward with the detailed design and construc­tion. Another advantage of selecting the CDM Smith/PC team was an alternate approach to design and construc­tion of the digester building, which shaved several months from the delivery schedule. “DC Water was impressed by our commitment to produce the design using virtual design and construc­tion technology, and we demon­strated the value of these tools through enhanced visu­al­i­sa­tion of the project at design and operational review meetings,” says David Schwartz, CDM Smith vice president. After thorough evaluation, DC Water settled on a design-build approach to reduce the likelihood of unexpected costs. Using Cambi THP also reduced the size of the digesters needed by 50 %, lowering construc­tion costs and making the project more affordable. Delivering the project through a design-build approach helped add innovation throughout the project. “Design-build can provide the creativity and flexibility to address challenges that arise, and greater certainty of project costs well before bid day,” notes Schwartz.

Building Better Biosolids
The Blue Plains advanced wastewater treatment plant previously used lime stabilisation to treat sludge, producing class B biosolids used for agricultural fertiliser. However, transporting these biosolids to where they can be used was costly - the plant produced 1,200 wet tons of biosolids each day, or about 65 truckloads.

Implementing Cambi THP at Blue Plains advanced wastewater treatment plant resulted in an improved, class A biosolid, which DC Water is now supplying to soil blenders, landscapers, community groups and governments in DC and Maryland. “THP is a digestion preprocessing step that uses heat and pressure to kill pathogens in the biosolids and also breaks down biosolids so they are more readily digested,” says Peter Loomis, CDM Smith project manager. “THP allows higher loading to the digesters, reducing the required retention time and the corresponding digester volume. It also improves the reduction of volatile solids in the digesters, resulting in higher gas production compared to conventional digestion.”

The process reduces the volume of biosolids, produces more biogas to generate more power, and reduces odour issues associated with the transporting and disposing of class B biosolids. These changes are expected to reduce trucking by 50 %, resulting in less traffic congestion and significantly lower carbon emissions.

Biogas Fuels Savings

Through a separate project, DC Water is introducing a combined heat and power (CHP) component, which will provide the plant with renewable energy. Capturing and using the biogas created through anaerobic digestion will generate between 8 and 10 megawatts of power, approximately one third of the energy needed to operate the Blue Plains facility. By reducing demand from the electrical grid, the plant will reduce the impact of hot summer days on the mid-Atlantic coast, which can strain the grid and contribute to brown-outs. Offsetting the plant’s electricity use now will also help to prevent future rate increases, despite the rising price of power.

Combined, reduced biosolids hauling and onsite power production are expected to reduce the facility's greenhouse gas emissions by 40 %. And in addition to reducing waste, generating energy and improving air quality, the project will save residents of the district and surrounding metropolitan areas an estimated $20 million annually. The investment in the new main process train has enabled DC Water to expand capacity if necessary and to operate sustainably in the future. “This project brings a new focus on recovering resources,” says George S. Hawkins, DC Water general manager. “The digesters allow us to extract valuable nutrients, energy and carbon from the wastewater and make use of these resources within our service area."