As I have written before, we have built the first CAMBI thermal hydrolysis and digester system in North America which also happens to be the largest in the world. The benefits and costs of this project are both enormous: at a sticker price of $470 million dollars, the new system will generate 10 net megawatts of clean, renewable power; cut our carbon footprint by one-third; reduce our truck traffic by half; and in the end save our ratepayers money.
Bringing such a huge project to fruition has been a monumental task and I will be posting about the entire effort once it is complete. I wanted to discuss just one small piece of the research work we did as part of the full job.
First, some background. Biosolids is the byproduct left over after all the processes at Blue Plains are finished cleaning the “waste” water we receive every day. The digesters are large tanks that convert our biosolids into methane, a process that is performed by microorganisms called methanogens, which are so exotic they actually are not even bacteria – for any biologists in the audience, they are classified as archaea.
Each of our four digesters is around four million gallons in size, and we have to grow our methanogens ourselves – they’re not the sort of thing you can buy at Home Depot! Unfortunately, methanogens do not grow very fast. Our operators control the process by slowly adding more and more food to the tanks: the more food that gets added, the faster the organisms grow. But adding too much food too quickly can drive down the pH, turning the digester into a tank of acid and killing all the organisms that had been painstakingly grown so far. It’s a testament to the skill of our process team that we are able to get these temperamental digesters running at all!
The original plan was to ramp up our digester tanks at a rate of 3 percent per day – that is, each day we add 3 percent more food than the day before. Working with our design-build contractor, PC Construction/CDM Smith, the research team thought there might be an opportunity to speed up the commissioning process by carefully balancing the acidity in the tanks. And the faster the tanks come online, the sooner we start reaping all the benefits of the digesters and saving money!
In our laboratory, the research team started two parallel processes. The first used the planned process, at 3 percent per day. In the second, the team added some magnesium hydroxide to counteract the expected rise in acidity and used a 5 percent ramp-up rate. The second digester was more stable and produced more gas faster – in other words, the experiment was a success!
The graph shows the theoretical difference between the two rates. At 5 percent, we reach the maximum loading rate for each digester (1.4 million pounds/day) in less than 50 days – at 3 percent, it takes 100 days! And each and every pound of processed biosolids is one less pound that we need to stabilize and ship off the plant, saving ratepayer money on hauling and disposal costs. Potentially, the new method could save as much as $200,000 on each digester – that’s $800,000 for the entire project!
Working with the process group and the research team, our contractor carefully monitored the acidity of the tanks as they were seeded and growing and made the decision to go ahead and add magnesium hydroxide and start ramping up faster – this decision was much easier and less nerve-wracking thanks to the successful experiments. Though there have been some obstacles in practice that prevented us from realizing all the potential savings, our digesters still came online much earlier and with more stability thanks to the amazing collaboration between all three groups.
This incredible benefit is just one demonstration of the value of our research program at Blue Plains. Research doesn’t always pan out, but when it does, the return can be immense, as this example illustrates. One of our advantages at DC Water, thanks to our facilities and resources, is the ability to conduct research of this complexity and scale on an ongoing basis. Boutique laboratory work is difficult, requiring specialized knowledge and expertise, and many utilities around the country simply can’t afford the overhead, despite the benefits such studies can offer. We plan to begin offering to perform similar studies in our laboratory for utilities, at their request. They benefit from the fruits of the research studies; DC Water and the ratepayers benefit from the service fee. It’s another way we can take advantage of our fantastic research team to benefit our ratepayers while improving the state of the art in our industry!