Harnessing heat from wastewater
Using wastewater and drain heat as an energy source is emerging as a low‑carbon way to warm buildings, from city‑scale district heating networks to in‑home shower drain heat exchangers. Commenters weigh the technical feasibility, costs, and retrofitting challenges, noting that such systems work best at scale and in dense areas with existing infrastructure, while household devices can pay off only in specific high‑use scenarios. They also highlight trade‑offs, such as the role of residual heat in preventing sewer blockages, and compare wastewater recovery with alternatives like heat pumps and improved insulation.
Image and basic physics
- Several comments note the article’s NYC “steaming manhole” photo is misleading there because city steam mains, not sewage, often cause visible steam.
- Others point out that in most cities, any underground water warmer than very cold air (even just a few °C above freezing) can visibly “steam.”
Household drain heat recovery
- Many are interested in simple drain heat exchangers, especially for showers.
- Reported efficiencies of 30–70% for shower systems; one user cites a government source with 2.5–7 year payback, another says their unit saves ~40–50% of shower hot-water energy.
- Technical notes:
- Typical design: vertical copper drain pipe with fresh cold water coiled around it; no direct contact between sewage and supply.
- Works best with continuous, simultaneous in/out flow (showers), less so for batch uses like dishwashers/washers unless storage is added.
- Cost is a major barrier: examples around $800+ hardware plus labor; many would only add them in new construction or heavy remodels.
District heating & wastewater heat at scale
- Some are surprised district heating isn’t more common in cold regions; others note it exists in specific downtowns and campuses.
- Large-scale systems (e.g., Stockholm, Vancouver, Helsinki) use heat pumps on sewage or treated wastewater, industrial waste heat, and other sources.
- Advantages cited: fuel/source flexibility, economies of scale, ability to move heat from where it’s unwanted (data centers, incinerators) to where it’s needed.
Economic & infrastructure challenges
- Retrofitting district heating into existing cities is seen as extremely expensive due to trenching, outdated underground maps, slow roadwork, and disruption.
- Posters debate whether district heating or individual heat pumps are superior; one cites work arguing heat pumps are usually better than combined heat and power, with district heat mainly a distribution issue.
Side effects and risks
- Concerns that over‑cooling sewage could worsen fatbergs and clogs; some argue existing pumping and maceration near treatment plants mitigates this.
- Comparison to LED traffic lights: removing “waste” heat caused snow accumulation issues; lesson is that “waste” heat sometimes has functions.
Behavioral hacks & small-scale ideas
- Some let hot bath or shower water cool indoors before draining to reclaim heat and humidity; others worry about excess humidity or added thermal mass but consensus is that impact on heating bills is probably small/unclear.
- Ideas include showering over a stopped drain, grey‑water reservoirs, and using wastewater heat for greenhouse heating; skepticism remains that household‑scale systems often don’t justify their complexity and cost.
Laundry and hot-water use
- Debate over cold vs hot water washing:
- One side claims modern detergents make hot water unnecessary and wasteful.
- Others report hot water clearly reduces odors and is needed for hygiene (mites, fecal contamination).
- Overall, domestic hot water from showers is seen as the prime candidate for heat recovery; dishwashers and washing machines are secondary due to usage patterns.