Will drain water heat recovery devices wreak havoc on city sewer systems?
My province of British Columbia is moving forward with aggressive energy efficiency standards.
It is fully expected that in order to meet the higher levels of standards, homes will increasingly be installing Drain Water Heat Recovery devices.
My question:
I had looked at the prospect of these devices a number of years ago and had a very interesting (but informal and short) conversation with a sewage engineer. He basically told me that these types of devices should never be deployed widely because city sewage systems would come to a halt. Our sewage systems rely on the hot waste water to prevent everything from turning into a cold sludge and clogging up the city sewer pipes. They want the sewage to be warm so that it can make its way to the treatment plant. We didn’t get into this much detail but I suspect that the ongoing efforts by cities to reduce storm-water leakage into the sewer system may make this even more important. Although storm water is cold, it helps to dilute the consistency of the waste stream so that it can flow better. I suspect that the warmth of the sewage also helps with the treatment process.
I think the general idea is that if there is a desire to extract heat from sewage, it should be done at the very end of the process at the sewage treatment plant, not at each individual house.
This was a fairly informal conversation and I haven’t heard anything about this since.
Has anyone else heard anything about the impact on the sewer system of removing “waste” heat from sewer water at individual houses?
If we get to the point where every house is recovering a meaningful part of the waste heat in water before dumping it into the city sewage system, might we see a need for additional pump stations or even the need to pump hot water into the city sewers; fully negating the positive impact?
There is a good document about the entire code here and it includes this excerpt on these devices:
https://www.bchousing.org/publications/BC-Energy-Step-Code-Builder-Guide.pdf
“Drain Water Heat Recovery
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Replies
Until quite recently hot water use was a lot lower than it is today. When I first started designing houses it was accepted that they would have a 40 gal tank. Now you often see a 70 or two 40s (I'm talking about the general housing stock, not the hot water sippers here on GBA). So probably there is at least the same amount of hot water flushing sewers as there has ever been. Remember too that the devices only work on simultaneous loads (showers), so you still have everything else not being captured.
If it really did become a problem, the solution would be to install small grease-traps like they do in restaurants. If you have a grease-trap, hot water in the drains makes the sewers worse by stopping the solids from congealing in the trap.
It seems the real risk to sewers is very different - as you mentioned, cities are increasingly diverting storm water from entering the sewage system. This results in a higher concentration of solids in the sewage as there is less stormwater to dilute them. Concurrently, residential fixtures are getting more efficient as WaterSense standards reduce gpm flow in residential fixtures, further reducing the dilution of sewage.
Also, as water becomes increasingly expensive, economics drive homeowners to conserve beyond the WaterSense standards. Fixtures to support this are available. Niagara has a 0.8/gpf toilet, and 0.5 gpm sink aerators are available. These further reduce the dilution and may affect flow in sewers.
It seems that whatever heat is extracted from hot water by site based recovery systems may be quickly replaced by the heat generated as a result of the breakdown and decomposition of the solids in the sewers. By the time it gets to the plant, the difference is probably minimal.
A more local issue may be that hotter water helps flush solids and grease through smaller site based pipes into larger collectors. If the temperature at the site is reduced, those pipes may be more susceptible to clogging.
I have a different perspective on this issue. I would think that the total contribution of heat from houses wouldn’t be terribly huge compared to the massive surface area of the sewage pipes. Basically I doubt the heat that the heat exchangers would recover would make a significant difference to the sewers. Sewage is going to make some of its own heat too I would suspect, similar to how septic systems essentially heat themselves the same way compost piles do.
Even after the waste recovery heat exchanger, the discharge water to the sewer is significantly above freezing. I can’t claim to be an expert in sewer design and operation, but sewers have been around for literally millennia. Romans started building them. Even in the more recent past, there was less hot water use in general so less total heat contribution to the sewer system.
Perhaps the extra heat helps to cover up some other issue with the sewer systems that needs to be fixed. It would be interesting to hear from others who work in the utility field dealing with large sewage systems to get their thoughts on this.
Bill
Definitely a good idea to ask about scalability.
Sewer lines should be below the frost line - but this isn't always true. I expect that such a problem would be addressed by adding insulation (as is used for exposed sewer lines) - not heat.