Wastewater heat recovery

[Emilly_Rose]

Hi all,

As part of the construction of an apartment building, we are evaluating the possibility of recovering heat from wastewater for heating the DHW. The mixed wastewater is collected in a pit where there is a heat exchanger connected to a water-water heat pump. There is therefore a heat stock formed in the pit.

I would like to be able to evaluate the splicing options of various streams. That is, I would like to be able to estimate the changes in the amount of energy available as a function of the flow rates and temperatures of the different connected wastewater streams. Example: what is the energy gain if I connect the waste water from the showers (given volume and given temperature per day) or if I do not connect them.

Does anyone have any idea how to proceed to make a consistent approximation?

Thanks

 

[1503-44]

The energy content you can try to capture in each stream is directly proportional to its temperature and flow rate. Select a base temperature in order to reference the potential gain. Let's assume that is 20°C. Then you can evaluate the heat content of each stream in regard to 20°C.

(I have not been able to download your diagram. That could be a problem at my end.)

If you have 1 stream of 5 liters/minute at 30°C, it's heat content relative to 20°C is (30-20)C° x 5 liters/minute = 50 kilocalories/minute.
Say stream 2 is 1 liter/min at 33°C. So that's (33-20) x 1 = 13 kilocalories per minute.

Stream one contains about 4X more available heat.

Of course what of that amount you will be able to extract depends on your exchanger operating temperature and efficiency.

A black swan to a turkey is a white swan to the butcher ... and to Boeing.

 

[LittleInch]

For a domestic setting I'm pretty sure you will find that the amount of heat recovery is negligible and the practical issues large.

Flow is very unstable and tends to occur in bursts. The water is "grey", full of dirt,suds and not very warm. Your HX will clog up and get fouled.

So whilst a noble effort, I would spend the money on better insulation or heat exchangers for the warm or cold air exiting the building.

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[IRstuff]

The OP's image

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[MedicineEng]

I think LittleInch hit the nail in the head.
The practicality and maintainability of a system like this vs the potential energy recovery doesn't seem to make a lot of technical/economical sense.
If you want to proceed with this as a conceptual exercise, I would stick with shower water as it is the least dirty stream of all wastewater produce in a domestic setting and as such, the one that might give you better results.

Good luck.
Engineering progress is made of ideas that at first glance look odd/impractical and then someone comes with a solution that nobody thought about before.

 

[LittleInch]

The image looks rather concept like to me. The pump power required may exceed the heat energy recovered....

Normally you need to create the hot water before you use it, e.g. early in the morning. How does that work?



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Emilly_Rose]

Hi there Thank you very much for your answers.
Sorry for the long response time from me.

I was asking the question from a theoretical point of view to be able to adapt to different situations with the same type of heat exchangers.
In fact, for this type of installation to be interesting, you need large daily volumes of wastewater, which is the case in our two current projects (building with 1000 residents, shops and restaurants) .

To explain the principle more clearly, it is really all the wastewater that is collected in the pit (showers, kitchens, wc...)which ensures all-day supply for a large-scale building. Solid matters (excrement, paper,..) are retained by the grilles and the dirty water comes into contact with the exchanger. The cooled water then goes into the sewers by gravity (the pump is not used to pump the water). The pump is used once a day for 3 minutes to extract solid waste and take it to the drains. The energy used by it is very low compared to the energy harvested. Once the heat has been extracted by the exchanger, it is taken to a heat pump which is used to heat the domestic hot water to over 60°C. We know that the system works for our water volumes and have previous examples with great results.

An example of a problem we encounter: In the building project, we are also evaluating the possibility of installing (passive) heat recovery units directly in the shower channel for preheating the hot water in the showers. In this case, the water leaving the shower drain will be 36°C without that heat recovery and 23°C with heat recovery. We would like to know how detrimental this is to the general heat collection system in the pit as previously described.


Simpler example: in the case of a municipal swimming pool project, it was decided to harvest heat from greywater (every day, part of the pool water must legally be renewed). We would like to know how interesting it would be to also connect the (warmer) shower water to this heat recovery pit. This has a significant cost and it would be interesting to have a simple tool to just figure out if, neglecting the losses, it has a big impact or not.

In my idea, I imagined calculating a mixing temperature of the wastewater in the pit (average over the day) with the different flow rates and incoming temperatures and according to the options chosen, then calculating the COP of the heat pump according to this temperature. Finally, calculate the electricity needed to bring to the heat pump according to the COP and therefore see how much energy is saved with a higher temperature. This is not so negligible given the volumes involved, but at the same time this very theoretical analysis that does not take into account the mixing, losses, or the differences in water quality seems to me to be completely false. I imagine you need simulation software to come up with something more conclusive. I wanted to know if anyone had ever worked on a problem like this

 

[1503-44]

https://www.youtube.com/embed/AnOoepocsSE

https://www.viessmann.co.uk/heating-advice/how-does-a-heat-pump-work

A black swan to a turkey is a white swan to the butcher ... and to Boeing.

 

[LittleInch]

e is data you need to know such as incoming temperature from the heat pump.

Generally anything below 7C between incoming and outgoing temperature is too low to make a HX work effectively. Heat exchanged is proportional to delta T so the lower the delta T the lower the heat transfer.

But if you know water flows and volume inside the HX, then it is easy to work out heat capacity ( 4.2 KJ / kg or 4.2MJ/m3)

I would look at an overall efficiency of 50-60% max.

The amount of fouling of the HX over time though will be significant or you need very smooth HXs and nothing for things to clog.

I'm surprised foul waste (toilets) went into the mix as they are not normally heated water??

"We know that the system works for our water volumes and have previous examples with great results." Care to share any more information?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.