I imagine there's a real concentration problem there. Sewage is incredibly diluted if you think about it (divide your estimate of how much organic waste you generate by the amount of water used in your water bill).

If, instead, houses had grey water systems (i.e. the sink in you bathroom is used to fill up your toilet tank), or if black waters had a separate sewer (sure, not feasible in suburbia), the energy budget would change considerably.

Either way, the affluent is full if nutrients even if we can't recover the energy. Nutrients our soil is rapidly being depleted of (thanks to our modern use of sewers!)