Harnessing heat from wastewater

https://www.bbc.co.uk/future/article/20240103-sewage-a-low-cost-low-carbon-way-to-warm-homes

by cannibalXxx • 3 years ago

130 152 3 years ago

bbc.co.uk

That photo of steam coming from the manhole cover in NY is pretty goofy. NY has 100s of miles of super-hot steam pipes underground. You'll see that same steam come up from manhole covers after a good rain in August. I'm pretty sure it's not from wastewater pipes.

notkaiho• 3 years ago

Some of it is, and the picture editor could be excused for sticking with a generic "steam manhole NYC" stock image for illustration imo.

calamari40652• 3 years ago

Most cities in the world don't have steam infrastructure like that and still have steaming manholes in the winter from hot wastewater.

Groxx• 3 years ago

Doesn't even have to be hot, it just has to be hotter than the air above ground, and relatively humid (how much depends on the temperature difference). You can also see rivers steam when it gets very cold, and they're often only a few degrees above freezing at that point.

wolverine8765• 3 years ago

Is all the steam from hot wastewater? With all the things going on underground in cities, I've assumed there must be heat (i.e., from basic thermodynmics), and of course it needs a place to escape.

elric5• 3 years ago

Capturing heat before we flush it down the train is probably preferable to extracting it after, given that delta T will be higher. There are shower drain heat exchangers which can recover some of the heat in the water going down the drain.

bee_rider3• 3 years ago

I wonder what the breakdown is by usage. I imagine the combination of: dish washer, sink, and clothes washer, might make it appealing to collect it all at once.

IggleSniggle3• 3 years ago

Clothes detergents have been able to effectively clean in cold water for decades. The only reason people still wash using these detergents in hot water is ignorance. It's such a huge waste. Please stopping washing clothes with brand name detergents using hot or warm water. It costs more money, does more environmental damage, and doesn't even make your clothes marginally cleaner.

dzhiurgis• 3 years ago

My dishwasher uses 1kwh of power per load, thats hardly worth capturing

Shower tho… esp if you purchase ready made shower capsule that does not require external hot water supply and produces it’s own from recycled water? That might be the one.

jahewson• 3 years ago

Not much time to capture heat in a flowing drain, many small heat exchangers will be less efficient and more expensive than one large one, very hard to fit in a small space without utilising a compressor, plus additional plumbing - the captured heat pipe would have run all the way back to the water heater, losing heat all the way.

elric5• 3 years ago

A quick google suggests 50-70% efficiency on shower heat recovery. Sounds pretty good to me.

hinkley11• 3 years ago

Since we premix the hot and the cold water and then spray it over a human body, 30% recovery rate sounds impressive honestly. I doubt you can go much higher than that even with the counter flow design they undoubtedly already have.

ak217• 3 years ago

I'm amazed at how little district heating is used in northern climates like Canada and the northern US. In dense areas, district heating using waste heat from power plants should really be required by urban planners, or at least it would make a huge carbon footprint difference if we invest in it.

On an individual household scale, I've always wondered why nobody has come up with basic drain heat exchangers to capture most of the heat coming out of shower and washing machine wastewater before it leaves the building. (Edit: I guess they do exist - https://www.homedepot.com/p/Power-Pipe-4-in-x-48-in-Drain-Wa... - but I've never seen one in the wild)

calamari40652• 3 years ago

The problem is adding the infrastructure where it doesn't already exist. District heating is great when building a new city, but we don't do a lot of that anymore. Just imagine the cost of laying new pipes under an entire city. I'm sure someone smarter than me can do the math, but I'd suspect it'd take many decades to recoup the environmental and economic costs of installation.

quickthrowman2• 3 years ago

In dense areas, district heating using waste heat from power plants should really be required by urban planners

Power plants generally aren’t located in dense areas…

I'm amazed at how little district heating is used in northern climates like Canada and the northern US.

District heating and cooling are available in both Mpls and St Paul in the downtown core. This is the coldest large metro area in the United States.

St. Paul: https://www.districtenergy.com/

Minneapolis: https://cordiaenergy.com/our-networks/minneapolis/

On an individual household scale, I've always wondered why nobody has come up with basic drain heat exchangers to capture most of the heat coming out of shower and washing machine wastewater before it leaves the building. (Edit: I guess they do exist - https://www.homedepot.com/p/Power-Pipe-4-in-x-48-in-Drain-Wa... - but I've never seen one in the wild)

$851 just for the heat exchanger, not including piping and labor. It will take a lot of showers and laundry to pay that back. I’d probably add one to a new house build but would pass on retrofitting one into an existing home unless I was doing a complete plumbing remodel. Similar to daylight harvesting LED lighting, it’s possible you’ll never see a payback on the equipment and labor.

danjrslp• 3 years ago

I was going to mention the Minneapolis system, I work across the street from this plant.

kube-system• 3 years ago

I was just about to post that same link. I suspect the reason you don't see them much is because installing one will often require more plumbing and more labor. Residential installations in the US are particularly sensitive to up-front cost, and tradespeople in the US tend to be conservative in choosing 'new' technology.

MattGaiser• 3 years ago

And also exacerbate an already challenging shortage of trades.

FredPret2• 3 years ago

This would work as a sort of energy storage - you could overheat your water reservoir during cheap power times and then transfer the extra heat to the entire city the rest of the day.

yetihehe2• 3 years ago

You would need a lake sized insulated reservoir for that to work. Too much water is used already by those systems, they don't typically want to add another storage. Typically they are gigawatt scale systems burning something because just using electricity is too much strain on grid, unless you have a power plant nearby (like cogeneration, which uses waste heat from electricity generation from fossil fuels).

bombcar• 3 years ago

I remember reading on a plumbing blog (or YouTube or something) about someone who installed one of these - high end house, massive shower, owner who liked to take hour long hot showers or something.

In that case, it paid for itself pretty quickly.

For "normal use cases" the cost savings over time just isn't there, when you compare the added complexity. You're better off saving heat with on-demand heaters or HVAC recirculators, etc.

Places where it could make sense already use them (I believe car washes recycle water and heat in some locations).

hooverd• 3 years ago

That someone just needs a sauna.

euroderf2• 3 years ago

So how does one keep all the showered-off gunk from clogging up the works ?

blacksmith_tb2• 3 years ago

The builders of my house (who were angling for, and got, PassivHaus+ certification for it), built a heat exchange for the drains on the second floor that warms the intake to the hot water heater. It works, but it's pretty limited, in practice it does pass some of the heat from a bath or shower or dishwasher run back, so that's something. Of course the timing matters, water drains pretty quickly, even from a bath, the there's no coordination to guarantee the water heater runs while that happens (other than the good chance that if we just pulled hot water out it will be running to replace it, but sometimes that isn't immediate).

thinkcontext• 3 years ago

The DoE quoted a figure of 30% savings on showers, which is pretty impressive. All houses should be built so this is standard. It's so much more expensive to retrofit it later especially if the layout doesn't have the shower over the water heater.

hinkley11• 3 years ago

The water heater is always refilling. Otherwise you’d have no hot water pressure. It’s just that the new water goes to the opposite end of the tank from where the output is attached.

How twin heating element water heaters work:

https://youtube.com/watch?v=Bm7L-2J52GU

tvb12• 3 years ago

$851?!

Well, at second glance, it's definitely larger than it appeared to be in the picture. Home depot says 35 pounds, so ~$100 worth of copper.

elric5• 3 years ago

I'm amazed at how little district heating is used in northern climates like Canada and the northern US.

District heating is very expensive to build and difficult to retrofit. Doesn't seem very surprising to me. Any time a piece of road has to be dug up, costs skyrocket and people are inconvenienced for what seems to be an eternity. As far as I can tell, this is universally true among western nations.

mschuster913• 3 years ago

Any time a piece of road has to be dug up, costs skyrocket and people are inconvenienced for what seems to be an eternity.

Former trench digger here. The cause are usually two problems... first, the "underground line maps" ("Leitungskataster" in German) are usually not up to date because not everyone bothers to update them. Quite the "fun" if you suddenly hit a cable at 50cm when the map says you should be clear up to 2m!

The second problem is that Western societies almost always choose the cheapest bidder, which means no work on weekends or, heaven forbid, 24/7 as that would be too expensive. That means everything is slow as molasses.

miketery• 3 years ago

It's a cost thing. Prices for energy have been so cheap that investing in recapture capex didn't make sense. With energy prices going up, it does make sense.

A funny anecdote (or sad?), after gas prices spiked in much of Europe due to the war many manufacturing facilities were looking to cut energy use, they found a lot of waste. Many places were able to reduce their gas usage by as much as ~33% if I recall correctly.

I can't find the original source, but here's [1] one that claims 23% lower consumption across Germany in 2022 adjusted for temperature.

1 - https://www.nature.com/articles/s41560-023-01260-5

philwelch3• 3 years ago

Reducing consumption as a response to increased prices doesn’t mean the difference in consumption was originally “wasted”; it only means it was not worth the higher cost.

joeyo• 3 years ago

David MacKay discusses this in Sustainable Energy Without the Hot Air. He concludes that, while it's on first glance appealing to want to use waste heat from power generation, heat pumps are strictly superior to combined heat and power except for in a few specialized circumstances (e.g. industrial uses that require high temperatures). The book is fifteen years old now, so I suspect the math even more strongly favors heat pumps than when it was published.

See the section, "Heat pumps, compared with combined heat and power": https://www.withouthotair.com/c21/page_147.shtml

renhanxue3• 3 years ago

But district heating is a distribution mechanism, not a heat generation mechanism. Cogeneration plants are one way to power it, but you can also use waste heat from industrial processes, very large scale heat pumps, or burning household waste. All of these are in common use in places where district heating is common. The public district heating utility in Stockholm claims to have the world's largest heat pump installation, which has the capacity to extract 225 MW of heat from treated sewage, in the process generating both heat that's sent into the district heating network, cooling that's sent into the district cooling network, and finally a small amount of electric power by releasing the treated water into a lake via a turbine. This facility opened in 1986. The utility also has various other facilities, both cogeneration plants that burn biofuel (mostly byproducts from the lumber industry) or household waste, as well as other heat pumps that extract energy from seawater.

District heating/district cooling has three advantages: you can change the heat source centrally without having to refit every single dwelling, there are economies of scale in the heat generation, and you can take advantage of heat or cooling that's just in the wrong place and transport it to where it's needed.

hedora• 3 years ago

Yeah; if just used for hot water, this doesn’t make much sense compared to a hybrid heat pump water heater. Here in California, with a well insulated/partially passively heated/cooled house, our hot water is well under 10% of our electricity bill (ignoring our EV).

In places where it’s really hot or cold, it would be an even lower percentage. The money that it would cost to trench and maintain the hot water distribution lines for a house would probably be better spent on heat pumps, architectural features or upgraded insulation (Even just $2000 of extra insulation does an amazing amount of good vs. bare minimum code insulation, and I doubt you can get district hot water installed for less than that.)

chris222• 3 years ago

Heat pumps move heat so the heat from the waste water is still useful to them.

lucumo• 3 years ago

On an individual household scale, I've always wondered why nobody has come up with basic drain heat exchangers to capture most of the heat coming out of shower and washing machine wastewater before it leaves the building.

I have had one of those installed in my home recently. A QB1-16 from Q-Blue https://www.q-blue.nl/en/products/q-blue-showersave/ Supposedly it saves about 50% of the loss heat. Since I dislike hasty showers, I figured that would save me plenty.

I'm amazed at how little district heating is used in northern climates

In my country we can pick our own electricity and gas provider, but for district heating you're tied to the provider for your area. Consequently, people with district heating get fleeced. When buying a house, I gave preference to the ones without district heating.

wenc• 3 years ago

I'm amazed at how little district heating is used in northern climates like Canada and the northern US.

It is used on most if not all Canadian university campuses. Canadian university buildings are also far less sprawling because they're all connected to a steam generation plant.

Toronto has district heating.

https://www.toronto.ca/services-payments/water-environment/e...

In the reverse direction, Lake Ontario is part of a cooling system as well.

https://en.wikipedia.org/wiki/Deep_Lake_Water_Cooling_System

chefandy• 3 years ago

Similarly, I know Harvard and MIT, and also parts of Boston have centralized steam heat systems. Harvard uses theirs extensively still, at least in the old buildings, but I'm not sure about MIT. Boston's is fed by an old commercial steam plant on the outskirts of Chinatown. I think it covers a pretty limited area though. I never lived anywhere with it.

jstsch• 3 years ago

When I renovated my house, I installed such a heat exchanging pipe. It reduces hot water usage (=fossil gas) by about 40% and with my usage payed itself back in about two years. It's not a drop-in replacement, since you should use it only for the shower drain.

megak1d2• 3 years ago

So the shower drains through it and the 'wrap around' pipe is the input to your hot water tank?

ed_balls• 3 years ago

The nice thing about district heating is the extra space you get and no fire/Co hazard.

btbuildem3• 3 years ago

I guess if everyone did that (residential heat exchangers on your house drain), there could be a real risk of the sewage cooling and congealing in the downstream pipes, causing a blockage.

seabrookmx• 3 years ago

Atleast in BC, our power plants are hydroelectric dams far from the urban center so there's no waste heat to collect.

DanielHB• 3 years ago

In Sweden most of the population is covered by district heating. I think it is mostly a matter of population density, in Sweden most people live in 3-6 floor residential buildings clustered together close to public transport hubs. In Canada and US you have urban sprawl and a lot of rural population, so running district heating pipes to every single house is just not worth it.

To be fair people who live in detached houses in Sweden often don't have district heating either, but the % of the population living in detached houses is tiny.

raincom• 3 years ago

How to deal with bad odors from sewage heat?

crest• 3 years ago

Heat exchangers.

quickthrowman2• 3 years ago

Heat exchangers, the sewage and working fluid never mix with each other.

https://en.m.wikipedia.org/wiki/Heat_exchanger

netsharc• 3 years ago

Heat exchanger, the same technology that doesn't spray you with coolant when you turn on the air conditioning in your car...

pacbard• 3 years ago

The idea is to coil the sewage pipe with the cold water pipe before it goes into the water heater. That way the cold water will pick up some of the excess heat from the sewage pipe from the shower and it will cost less to heat overall.

shagie2• 3 years ago

https://www.energy.gov/energysaver/drain-water-heat-recovery

Drain-water heat recovery technology works well with all types of water heaters, especially with demand and solar water heaters. Drain-water heat exchangers can recover heat from the hot water used in showers, bathtubs, sinks, dishwashers, and clothes washers. They generally have the ability to store recovered heat for later use. You'll need a unit with storage capacity for use with a dishwasher or clothes washer. Without storage capacity, you'll only have useful energy during the simultaneous flow of cold water and heated drain water, like showering.
...
Purchase prices for drain-water heat recovery systems range from $300 to $500. You'll need a qualified plumbing and heating contractor to install the system. Installation will usually be less expensive in new home construction. Paybacks range from 2.5 to 7 years, depending on how often the system is used, and the temperature of the incoming water, which is dependent on ground temperatures.

zbrozek• 3 years ago

How do you actually implement this? Waste pipes are huge compared to supply pipes, and rarely take even remotely similar paths. In my home, which I'm repairing right now, I have one water heater but three different waste egresses from the house.

shagie2• 3 years ago

https://www.kitchenandbathclassics.com/eco-friendly/advanced...

A DWHR unit consists of copper pipe tightly wound around a vertical section of copper drainpipe. As water flows down the drainpipe, it clings to the inside surface of the drainpipe. The heat from the drain water is transferred through the copper drainpipe to fresh cold water flowing in the outer copper coil.
The warmed water is then sent either to the hot water tank or other permitted end-use. In either case, the amount of energy needed to provide hot water is reduced. DWHR systems provide greater potential for energy savings as the number of simultaneous flows increases.

The Home Depot version: https://www.homedepot.com/p/Power-Pipe-3-in-x-48-in-Drain-Wa...

You wouldn't necessarily need this on every waste water egress. In my house, I've got one shower egress route that it would make sense on. The kitchen (I don't have a dish washer) doesn't run hot water for long enough for it to be reasonable - nor the 2nd floor bathroom with just a toilet and sink.

(edit)

There are spots where (in my house for example), this could be more useful. For example, that 2nd floor bathroom I've looked at a tankless water heater for the sink (so that I don't have to pull hot water from far away and then could simplify to only need to run cold water). Capturing the hot water there and returning the heat there there may make some sense... though for just a sink that would be a long ROI time.

RicoElectrico• 3 years ago

I thought about the exact same thing, inspired by my admittedly too long showers. Although I pondered whether too cold sewage could lead to more blockages.

mdf• 3 years ago

There's a nice heat pump in Helsinki, Finland, as well, producing district heating from waste water.[1]

[1] https://www.helen.fi/en/news/2023/waste-heat-plays-a-signifi...

CrazyStat2• 3 years ago

Dynomight suggested [1] recovering the heat from hot showers by showering with the drain stopped and letting the water cool to room temperature before draining it.

I remember thinking it was interesting at the time, but I have not tried it (my shower has a low lip so it couldn’t hold very much water).

[1] https://dynomight.net/hot-water/

fbdab1032• 3 years ago

Being a cheapskate, I did this when I lived in Chicago winters. My concern was sometimes I would forget to drain the water. I was creating a big thermal mass which my heater would have to work to maintain at equilibrium.

This also has the benefit of potentially raising the indoor humidity, which is otherwise going to be quite dry.

mcbishop2• 3 years ago

But in some areas / seasons, the added humidity is a negative.

krisoft• 3 years ago

My concern was sometimes I would forget to drain the water. I was creating a big thermal mass which my heater would have to work to maintain at equilibrium.

Is that a bad thing? I honestly ask. I'm twisting and turning the idea in my head and can argue in both direction.

I guess it can be a bad thing if you live by letting the interior go to cold frequently and then try to heat it back to room temp. For example if you would turn off the heating for the day, and then try to heat it back up from near freezing to room temp. That would be bad in itself, and the the extra mass would make it worse.

But if it is just maintained near the room temperature point then it is not that clear to me if it is bad. Would be happy to hear either way.

onetimeuse923042• 3 years ago

Don't forget, the heat is also keeping the pipes unclogged. If you remove the heat a lot of the fats turn into hard mass that is extremely difficult to get rid of.

This is what happens when sewage is given time to cool: https://www.youtube.com/watch?v=3i_axpk0a7Q

Reminds me of the story of what happened when traffic lights got "upgraded" to LED to save on energy. First winter all traffic lights got completely covered in snow and traffic ground to a halt. The "waste" heat as was not such a complete waste after all, it was continuously melting snow off the lights.

dvfjsdhgfv• 3 years ago

It can get much worse than that:

https://eng.obozrevatel.com/section-news/news-karelia-is-fre...

RadiozRadioz• 3 years ago

Examples of the danger of "but sometimes!" https://www.youtube.com/watch?v=GiYO1TObNz8

iknowstuff• 3 years ago

Incandescent lights still waste heat 90% of the time when there’s no snow. Adding some resistive heating to run when needed on LED lights is much preferable.

chongli• 3 years ago

Yeah really. It’s not a story about waste heat at all, it’s a story about revision 1.0 not being tested in all weather conditions.

throwbadubadu• 3 years ago

This is what happens when sewage is given time to cool: https://www.youtube.com/watch?v=3i_axpk0a7Q

First thought when scrolling quickly to the fatberg: How can those guys operate there without any "smell protection", could never do.

Next scene: See the guy repeatedly choking and puking.

Thanks, now need to get rid of pictures in head (:

btbuildem3• 3 years ago

It's wild to me how they're strolling through the sewer tunnel, with the voiceover explaining how it was put under ground to shelter people from all the disease carried in it... and they're wading through it, no respirator or any other protection of the sort.

grecy• 3 years ago

Not just fat either. The cold snap has me dealing with frozen pipes in our house right now...

schneems4• 3 years ago

Tucked under a Vancouver bridge, an energy centre sits on top of the existing sewage pumping station so heat can be captured before sewage reaches the treatment plant

Seems like if they’re doing this right before a treatment plant that they’ll be able to localize the problem and hopefully handle it effectively. Though it would be more assuring if they also talked about some of possible problems.

hinkley11• 3 years ago

They’re also likely picking up heat of fermentation, rather than my shower or dishwasher water.

abhibeckert• 3 years ago

I feel like there must be better options than resistive heating to melt the snow.

For example instead of a separate visor for each light, use a single visor that covers all three lights. And maybe angle the whole thing forwards slightly so that there isn't even a vertical surface for snow to rest on - gravity would simply cause it to fall off.

thsksbd• 3 years ago

Ya you really cant cool much unless you're willing to scrape. Unless they turn into a slurry? I dunno

kiba• 3 years ago

So, just add heating element when you need it? It's probably more efficient to use LED lights than to have incandescent light bulb for the few days of the year when you need it.

If you need to keep the heat with the wastewater, don't harvest it unless there's excess heat to be harvested.

fragmede3• 3 years ago

The solution was to have the yellow light be an incandescent bulb.

foofie• 3 years ago

So, just add heating element when you need it?

And now you have twice the systems and twice the failure modes.

stephen_g• 3 years ago

It says in the article that it sits on an existing pumping station, so that's probably the ideal place to avoid this problem, since those pumps would often be designed to grind/mascerate the incoming sewage anyway.

elmolino892• 3 years ago

Since with the heat pumps we need to remove extra heat in the summer heating the sewage may help to disolve some of the fat blobs.

But it is a risque idea, since we are likely to open Pandora's box of higher methane production, pathogens and smell.

kkfx• 3 years ago

Honestly? It's simply not worth the effort. Yes, we can put a grey water reservoir who receive water from washing machines, dishwasher, showers, sinks and so on, with a simple three way thermostatic valve who divert just hot water to the reservoir and than a heat pump suck heat for it to push in a room, but the quantity of heat we can recover is simply too little except maybe for some industrial activity that produce hot water as a result of some process (like an industrial laundry).

It's simply too costly creating such system than the energy we can recover.

szundi• 3 years ago

Haha my girlfrient thought I am crazy when I asked her not to let it down for 30 minutes to extract the heat, LOL

WalterBright• 3 years ago

In the winter, I'll let heated water cool to room temperature before draining it, releasing all its extra heat into the house.

nick222226• 3 years ago

And humidifying in the process

hinkley11• 3 years ago

Late season canning I have a giant vat of hot water on the stove. I let it cool if nobody needs the space.

In the fall I pour it on weeds. Heat is a great herbicide.

wolverine8765• 3 years ago

In some places, India for example, hot water is sometimes supplied by a smallish reservoir at the showerhead. The reservoir's normal state is full of tap-temperature water. For a hot shower, you turn on the reservoir's electric heater; it warms up (pretty quickly - think of electric kettles, and I don't want to shower in 100 deg C water) and you turn on the shower.

I've always wanted something like it at home; it seems much more efficient than a central hot water heater that's always warming the water. (I've also read about at-the-spigot devices that heat water like a coffee maker, with water running past something hot.)

I wonder how the heat capture of wastewater could work with it. It becomes a sort of closed loop hot water device, with heat exchanged with showering humans, the capture device, and the electrical device to top it off.

soperj• 3 years ago

Would work the same, since you're still supplying hot water to the sewers.

wolverine8765• 3 years ago

I'm saying the sewer-bound water's heat could be captured and used for the reservoir.

hinkley11• 3 years ago

I don’t know what current wisdom is, but when these were new they (eg This Old House) would suggest putting one at the master bath and maybe the common bath, then run one for the rest of the house.

Effectively daisy chaining two of them for piping hot showers, but still warm enough for the washer and dishwasher.

Pretty sure the deal was that none of them could really pull a couple gallons a minute at full temperature at the time.

wolverine8765• 3 years ago

Pretty sure the deal was that none of them could really pull a couple gallons a minute

Why not just run them in parallel? If you need more flow, just hook up two or three or N in parallel, feeding the same pipe?

papertokyo• 3 years ago

I live in a building connected to the False Creek system. Some details that may be interesting:

- Most of the apartment blocks currently served are located in a former industrial area that was redeveloped for the 2010 Winter Olympics and gradually built up since then.

- All new buildings in the utility's service area are required to be connected to the system for space heating and domestic hot water. The city charges a connection fee and the building must be designed with the requisite plumbing to integrate with the system, but some space is saved by not needing a large central heating plant.

- The metering and billing is done with https://www.wysemeter.com/ (at least in my building).

- The hot water is indeed very hot, and reliably so. I wouldn't be able to tell the heat source is sewage if the owner didn't mention it.

- In Winter, it's easy to identify which buildings which are connected to the utility, since they don't have have large steam clouds coming out the top.

City council info page: https://vancouver.ca/home-property-development/southeast-fal...

lemper• 3 years ago

bro, that's really cool (or hot)! why didn't smart people harness this back then?

elmolino892• 3 years ago

Clean water flowing in the pipes to a house will often be warmer than outside air during winter and have a higher heat capacity than air used in heat pumps.

No idea if using another heat pump and lowering the temperature from say 10C (ignoring any water heating in the house) to 6C of the gray water flowing out of the house makes sense.

timthorn• 3 years ago

There's an installation near Cambridge that uses the heat to warm industrial greenhouses. It has a methane powered generator which runs the heat pumps and releases its exhaust CO2 into the greenhouse to promote growth.

https://www.cibsejournal.com/case-studies/growing-interest-u...

Harnessing heat from wastewater

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