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Each city must have a sanitary sewer system and there are thousands of households in a city. So we will have a lot of methane generated from these sanitary sewer systems which can be utilised.

So is there any way to capture the methane gas from the sewer system of a city so that we can have free energy? Is there a company that has the technology for doing that?

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While some sewer gas is present most of the time in the pipes, the time it takes crap to get from the house to the sewage processing plant is short relative to the amount of gas given off. It would make more sense to limit your operations to the sewage treatment plant.

Secondly. Two major components of sewer gas are methane, and hydrogen sulfide. H2S is present, has some energy, and is very poisonous. This is the 'rotten egg' smell. Human noses detect it at 1 ppm. They go numb to it a a few ppm later. At 15 ppm, it's toxic, although it takes a while. At 1000 ppm, you take 1 breath and fall unconscious. That's 0.1%

There's not a lot of energy in H2S. If you burn it you get a mix of sulfur acids.

So let's look at the methane. Overall you are looking at the partial decomposition of food scraps and crap. The roman Legion figured on about 2 lbs of grain per man per day in the field. Probably 1.5 pounds after the rats got their share. You start with 1.5 pounds at 450 g/lb = 675 grams. x 4 calories per gram (average for cereals) = 2500 calories per day. 80% of that is extracted in the gut. 500 calories get exported with your crap. Suppose that 1/2 the energy is converted into methane. That's 250 calories per person per day.

City of 1 million people. 250 million calories = about 300,000 kWh = 300 MWh or about $9000 a day. On the other hand that's 1/3 of a kWh per person per day.

By comparison that same 1 million people are already using 5 to 80 kWh/day. So the methane is small potatoes. And that's not counting industry and office space.

I suspect that the money can be better spent on mass transist.

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  • your answer misses several facts about wastewater treatment that are relevant, see my answer. I like the way of thinking from 2lbs grain to the energy content of sewage, though. I'll maybe check the numbers.
    – mart
    Dec 23 '14 at 9:45
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Actually, it's quite a good idea to utilize the gas potential of sewage. As mentioned in Sherwood's answer, this will happen at the treatment plant, not in the sewer.

Edit to add

I wrote the part below without real wastewater experience. Now that I have some: A common way is the following: seperate primary sludge and fats in a settler, these go directly into anaerobic digestion. The aerobictreatment of the remaining wastewater with the activated sludge process produces waste sludge, this also goes into the digester. Expect about 18 l/day PE sewage gas at ~60% methane. This can provide a significant part of a sewage plants power consumption, but not more.

End Edit

The ways to do this are not trivial, but doable. Look at the wikipedia page for anaerobic digestion for an overview. The biggest challenge in AD of sewage is that sewage is highly diluted, with lots of water. So you need different concepts than with a anaerobic digester handling manure, or foodwastes or energy crops. But, as I said, it's doable.

While the energy generated maybe small potatoes compared to the energy demand of the population supplying the sewage, it can be significant for the treatment plant. There's also another benefit, less obvious: In traditional wastewater treatment, a large step is the aerobic degradation of the organic contents, by bacteria. Aerobic bacteria can gain far more energy from the matter than anaerobic species, so they grow far faster. The sludge from aerobic digesters is mostly made up of the 'excess' bacteria. Sludge treatment and disposal is a major cost in treatment plants. Anaerobic digesters produce far less sludge. As an example, upflow anaerobic sludge blanket reactors are said to ...

With UASB, the whole process of settlement and digestion occurs in one or more large tank(s). Only the post UASB liquids, which have a much reduced BOD concentration, needs to be aerated.

This leads to a halving of the aeration energy and doubling of the power generated from digestion

If you are interested in the technical challenges, I advise you to look at the most excellent and free book Anaerobic Digestion (and maybe skim the other free IWA titles)

I can't answer why AD is not the standard treatment for wastewater. My guess is that many treatment plants in the developed world where built before the AD technolgoy was mature, and it's a huge headache to change a plant to a completly different process. Apparently, UASB seem to be far more common in Brazil.

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  • Why would anerobes produce less sludge? If the aerobic sludge is mostly bacteria/organic matter then there almost certainly something that will eat it and convert the carbon to either methane or CO2. But your answer is a good one. I give you a point. Dec 23 '14 at 18:11
  • aerobic digestion produces more energy for the bacteria, because the path (say) C6H12O6 + 6O2-> 6CO2 + 6H2O releases more energy than C6H12O2 -> 3CH4 + 3CO2 (because the enthalpy of CH4 is higher). So the bacteria can grow more and you get the sludge. Of course, you can take the sludge and digest it anaerobically (and this is done in many, many WWTPs) afterwards (still leaving you with an enormous vollume of sludge afterwards), but you get less energy than you would if you'd AD the raw sewage. On the other hand, the sewage sludge is easier to handle for most AD systems because it is thicker
    – mart
    Dec 23 '14 at 20:43

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