Ferrous or ferric chloride can be added to the digester to react with and neutralize the H2S prior to exhausting the biogas. On a price per pound of H2S removed basis, this is an expensive approach, (typically $/# of H2S removed), and because of the variations in digester biogas formation discussed above, presents the same challenges of controlling FEXX dosing to maintain consistent outlet levels and “over dosing” to compensate is expensive.
Activated carbon filters are often to remove H2S from the biogas stream. This approach does address the issue of fluctuating H2S concentrations as an activated carbon structure, until it has reached capacity, will remove nearly all H2S present in the gas stream, regardless of concentration or changes in concentration.
However, this is again an expensive approach for even moderate concentration levels (typically $/# of H2S removed). Activated carbon suppliers suggest limiting application to less than 200ppmv. At MV Technologies we occasionally apply activated carbon filters in a “polishing” role downstream of other dry scrubber systems where we face extremely low concentration limits.
When limited quantities of oxygen are added to a biogas stream, specific aerobic bacteria such as Acidothiobacillus will oxidize H2S into elemental Sulphur if those bacteria are provided a stable temperature environment, oxygen, water and a supply of additional nutrients.
The typical biological scrubber design consists of columns of packed media that serve as home sites for the bacteria and provide a high contract ratio between the bacteria and the biogas stream. The biogas and water flow in opposite directions. The primary reactions are:
H2S + 2 O2 ➞ H2SO4
H2S + ½ O2 ➞ S + H2OFailure to control oxygen and temperature to the established set points will result in the formation of excess sulfur in the reactor, impede efficiency and require unscheduled reactor cleanout.
Makeup water requirements are significant and effluent streams must be accounted for.
Biological scrubbing systems, while generally moderate to high in initial capital cost offer the potential advantage of lower operating costs as nutrients and makeup water are less expensive than chemicals, or filtration or reactive media.
However, they suffer from the challenges inherent to management of any biological system.