# Suitable net head for a hotel wastewater

## Main Question or Discussion Point

Hi, I am here again. Sorry if this thread is not that related here in this field. I just want to ask, if I have to build a micro hydropower scheme in a wastewater of a hotel, what net head is suitable to use? (Sorry, I can't construct a clear sentence).

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CWatters
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What is your definition of micro hydropower? Google suggests 5kw to 100kw. Some work with a 2m head.

• Ballena Joseph
What is your definition of micro hydropower? Google suggests 5kw to 100kw. Some work with a 2m head.
In my computations, I used 3m net head. But I'm not sure if it is suitable for hotel wastewater. And also, I have this problem in determining the average wastewater discharge of a hotel. Google suggest that the average is 150 cubic meter per day. I don't know if that is enough to produce a much more power to the turbine.

CWatters
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How many floors in the hotel?

Let's imagine you put a storage tank on the 1st floor (3m above the turbine). That could collect waste water from all the floors above it.

If you raised the tank to say the 6th floor you would increase the head to say 18m but you couldn't collect the waste water from the floors below it.

So there might be no gain by raising the tank/head.

Have you done the energy calculation for 3m and 150m^3 a day?

• Ballena Joseph
How many floors in the hotel?

Let's imagine you put a storage tank on the 1st floor (3m above the turbine). That could collect waste water from all the floors above it.

If you raised the tank to say the 6th floor you would increase the head to say 18m but you couldn't collect the waste water from the floors below it.

So there might be no gain by raising the tank/head.

Have you done the energy calculation for 3m and 150m^3 a day?
I already calculated the power. The value of power is 2.33 kW. I also convert the units of volume flow rate, that is 0.001736m^3/s then multiplying it by the average number of room for hotel, which is 75 rooms. Then I have this value of 0.13m^3/s. I am not sure if I need to multiply the 150m^3/per day to the average number of rooms for hotel. But I multiply it because the 150m^3/day is not enough to calculate a higher value of power.

anorlunda
Mentor
Real turbines and generators are less than 100% efficient. But at 100% efficiency, use the following approximate numbers

1 watt = 1 joule/second = 0.1 kg*m/second

So, with 3 meters head, you need 0.033 kg/second or 2880 kg/day, or 2.9 m3/day for 1 watt of power.

For 1 kw of power, you need 2880 m3/day

If one toilet flush uses 1.3 l of water, then to make 1 kw with 3 m of head, you need about 2 million flushes per day

If your turbine generator is 50% efficient, you need twice as much.

[someone please check my calculations. I too make errors.]

• Ballena Joseph
Real turbines and generators are less than 100% efficient. But at 100% efficiency, use the following approximate numbers

1 watt = 1 joule/second = 0.1 kg*m/second

So, with 3 meters head, you need 0.033 kg/second or 2880 kg/day, or 2.9 m3/day for 1 watt of power.

For 1 kw of power, you need 2880 m3/day

If one toilet flush uses 1.3 l of water, then to make 1 kw with 3 m of head, you need about 2 million flushes per day

If your turbine generator is 50% efficient, you need twice as much.

[someone please check my calculations. I too make errors.]
I don't get it. Does the volume flow rate of 150m^3/day is enough to get as much as more power?

CWatters
Homework Helper
Gold Member
In my computations, I used 3m net head. But I'm not sure if it is suitable for hotel wastewater. And also, I have this problem in determining the average wastewater discharge of a hotel. Google suggest that the average is 150 cubic meter per day. I don't know if that is enough to produce a much more power to the turbine.
150 cubic meters a day per room sounds too high..

When I did a search I found typical figures of about 0.5 cubic meters per day per room..

https://www.4hoteliers.com/features/article/1889
This varies, but most studies indicate hotels use between 100 and 200 gallons of fresh water per occupied guestroom per day.
choosing the middle.. 150 US Gallons per day is about 0.57 cubic meters per room per day.
and
https://www.savemoneycutcarbon.com/learn-and-save/buildings/hotels/water-saving-for-hotels/?keyword=+hotel +water +consumption&medium=ppc&network=g&gclid=Cj0KCQjw6MHdBRCtARIsAEigMxG6oIncBeVMUxh6Q-i3fq9RnkucJJW_1Ki0M4YBDYUoQY0Xo3s_820aAgKCEALw_wcB
On average an occupied hotel room uses between 140,000 and 275,000 litres of water each year.
200,000 L/year is about 549L/day or 0.5 cubic meters per room per day.

so if there are 75 rooms the total volume of water is around 75 * 0.5 = 38 cubic meters per day.

If the head is 3m that gives a total energy of

E = mgh = (38 * 1000) * 9.8 * 3 = 1.1MJ

Average power would be = total energy/seconds in a day..

Pav = 1.1*10^6 / 86400 = 13W

I already calculated the power. The value of power is 2.33 kW. I also convert the units of volume flow rate, that is 0.001736m^3/s then multiplying it by the average number of room for hotel, which is 75 rooms. Then I have this value of 0.13m^3/s. I am not sure if I need to multiply the 150m^3/per day to the average number of rooms for hotel. But I multiply it because the 150m^3/day is not enough to calculate a higher value of power.

Gravity is a surprisingly weak force.

• Ballena Joseph
150 cubic meters a day per room sounds too high..

When I did a search I found typical figures of about 0.5 cubic meters per day per room..

https://www.4hoteliers.com/features/article/1889

choosing the middle.. 150 US Gallons per day is about 0.57 cubic meters per room per day.
and
https://www.savemoneycutcarbon.com/learn-and-save/buildings/hotels/water-saving-for-hotels/?keyword=+hotel +water +consumption&medium=ppc&network=g&gclid=Cj0KCQjw6MHdBRCtARIsAEigMxG6oIncBeVMUxh6Q-i3fq9RnkucJJW_1Ki0M4YBDYUoQY0Xo3s_820aAgKCEALw_wcB

200,000 L/year is about 549L/day or 0.5 cubic meters per room per day.

so if there are 75 rooms the total volume of water is around 75 * 0.5 = 38 cubic meters per day.

If the head is 3m that gives a total energy of

E = mgh = (38 * 1000) * 9.8 * 3 = 1.1MJ

Average power would be = total energy/seconds in a day..

Pav = 1.1*10^6 / 86400 = 13W

Gravity is a surprisingly weak force.
Does it mean that the 3 meters of head and 150 cubic meters per day is not enough to produce much more power? Then what should I do now? Should I continue making a micro hydro scheme?

CWatters
Homework Helper
Gold Member
Check the figures that Anorlunda and I posted and decide if the idea is feasible.

Perhaps compare the cost/benefit of a micro hydro system with alternatives such as heat recovery from showers waste water or fitting low energy light bulbs.

• Ballena Joseph
Check the figures that Anorlunda and I posted and decide if the idea is feasible.

Perhaps compare the cost/benefit of a micro hydro system with alternatives such as heat recovery from showers waste water or fitting low energy light bulbs.
For now, I don't know if it is feasible. I can't decide if I have to do about it or not.

CWatters
Homework Helper
Gold Member
How much does electricity cost? If you recovered all of the energy available using microhydro how much would it be worth? Eg what would the saving be per year?

How much would the hydro plant cost?

Does the saving justify the cost?

• Ballena Joseph
How much does electricity cost? If you recovered all of the energy available using microhydro how much would it be worth? Eg what would the saving be per year?

How much would the hydro plant cost?

Does the saving justify the cost?
I can't answer your questions. My mind Isn't thinking for a solution to my problem.

anorlunda
Mentor
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