Apr 13, 2022
Affluent with Effluent
Have you ever used a hot tub thermometer to measure the temperature of sewage effluent? I have.
The year: 2006. The place: the Whistler wastewater treatment plant (WWTP).
My team had been engaged by a private industry stakeholder. The first question I was asked when I met with their project manager was how many boreholes needed to be drilled to support the district energy system that was being proposed.
The project manager had been consulting with geo-exchange construction companies who were primarily engaged in drilling vertical borehole heat exchangers and was led to believe this was the only option for an ambient temperature district energy system. They had been left with a sense of urgency that they needed to hire drill rigs ASAP, or they would be snapped up for other projects.
After some convincing that geo-exchange system technically feasibility and financial viability is site-specific, I was given approval to walk the site with a geological engineer to assess options and develop a report.
With the proximity to the Cheakamus River, I thought that an open-loop system might be favorable compared to the large, expensive vertical GHX that would have been required to meet the loads of the heating dominant building load profile. I also knew that areas of Whistler had granite with high thermal conductivity because I had tested a borehole installed in that granite in 2001. The formation thermal conductivity there was approximately 3.2 W/m-K, meaning perhaps a borehole GHX would be viable.
We knew from experience that other areas of Whistler had horrendous drilling conditions that led to challenging borehole heat exchanger construction.
When we were walking the site, we noticed the effluent outfall from the WWTP across the river from where the new neighbourhood would eventually be. My colleague perked up with a big grin and mentioned that the former Whistler Public Works Manager had been interested in reclaiming energy from the municipal sewage system. We added this opportunity to our options assessment and decided to immediately investigate how much energy we could extract from this plentiful, renewable resource.
A quick trip to the local plumbing store and we were back at the effluent discharge site with a hot tub thermometer, some tape, and a stick we had foraged along the way.
We removed a manhole cover with our pinky fingers and dipped our new, highly scientific instrument into the effluent. We watched the gauge rise, reaching its final destination around the 20°C mark. We then measured the temperature of the river upstream of the outfall, and it was about 2°C.
From experience, we expected that the undisturbed ground temperature in the region would be below 10°C, and with the heating dominant load profile we knew the GHX temperature would only go down from there. We have developed a solution for that predicament now, but I’ll save that for another day.
We knew that the effluent would increase the heat pump COPs significantly compared to the other options that were being considered.
We went to the WWTP and had a chat with the operator. He told us that the WWTP would be retrofitted for the 2010 Winter Olympics and the retrofit would double its capacity from 12,500 m3/day to 25,000 m3/day.
We did our homework, confirmed the opportunity to work a district heating system energy source into the new WWTP design, and recommended that the effluent be the source of energy for the new neighbourhood.
Whistler eventually went ahead with the WWTP effluent-based district energy system with distributed heat pumps in every building. I had the pleasure of assisting with commissioning the system. It was an amazing thing to see our pipe dream turn into a pipe dream.
I always emphasize that the site will tell the story and that an upfront “Phase 1” geo-exchange assessment by a geological engineer specialized in geo-exchange is a good idea. If there are unique options, they will know more quickly and with greater certainty than anyone.