Turning Waste to Energy in Washington DC
Tom Lombardo posted on October 26, 2015 |
Thermal hydrolysis makes the wastewater treatment process more efficient. DC’s water treatment plant...

Every day, Washington DC produces nearly a billion gallons of raw sewage. (Sometimes the political jokes write themselves; feel free to insert your own punchline.) Norwegian technology is helping engineers at DC Water accomplish a task that seems to elude the nation’s voters: efficiently turning excrement into something productive, while saving money at the same time.


For years, wastewater treatment facilities have been extracting methane from the waste digesters and burning it as a fuel to produce electricity and heat, as shown below:


The waste heat is fed back to the digesters, helping the beneficial bacteria do their job, and the electricity is used on site, and in some cases sold to the grid, both of which reduce the facility’s energy costs. The remaining solid waste is dried and sold as fertilizer. The Water Environment Federation has a nice interactive illustration of the process. Check it out here.) There’s nothing new about that system. So what’s the big deal about it now? Three words...


Thermal Hydrolysis Process

The Cambi Thermal Hydrolysis Process (THP) was developed and patented in the early 1990s, and has been used in Europe since 1995. DC Water’s Blue Plains plant is the first wastewater treatment facility in North America to employ the technique. Better late than never, I guess.


THP inserts a three step process before the digester:


  1. Solids are preheated to 96oC (205oF) and sent to a reactor.

  2. The reactor exposes the solids to a pressure of 87 psi and heats the material to 165oC (329oF) for 22 minutes. This softens and sterilizes the mixture.

  3. The sterilized waste goes to a flash tank, where a quick return to atmospheric pressure turns the sludge into food that beneficial microbes can easily digest.


Here’s an animation:




Video courtesy of Cambi



THP speeds up the digestion process, allowing material to flow through the system more quickly. As a result, the same sized facility can handle two to three times as much waste as a facility that does not employ THP. Another advantage of THP is that the resulting solid material (the fertilizer) has a lower water content, so there’s less volume to store and transport.


Money

DC Water is the largest consumer of electricity in Washington DC. Although it won’t put electricity onto the grid, the Blue Plains plant will generate 10 MW  - about one third of its own electricity - reducing its electric bill by $10M each year*. Since the fertilizer is sterilized before it goes into the digester, the facility will save another $2M on chemical treatments. Finally, the reduction in volume will decrease trucking expenses by $11M per year. All told, the facility will save the DC residents $23M per year. That gives the plant’s $470M pricetag a 20 year payback period. That’s simple payback, so it doesn’t take into account the possibility of investing money in something else. On the other hand, electricity and trucking costs are likely to increase, and the decrease in the impact on the electric grid, coupled with the reduced wear and tear on the roads, offers some other “spin-off” financial gains.


And of course, there’s the environmental benefit of reducing the plant’s carbon footprint by 33%, a factor whose importance is recognized by at least some of the power brokers in Washington. Maybe DC’s wastewater treatment plant will start a trend in that city: less waste, more productivity.


* 10MW x 24 hours/day = 240 MWh/day x 365 days/yr = 87600 MWh/yr = 87600000 kWh/yr

87600000 kWh/yr x  $0.11/kWh = $10M/yr



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