H₂S Treatment Approaches for AD Biogas

Adding Iron Salts to the digester

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.

Use of Activated Carbon as a filter

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.

Biological Scrubbers

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 + H₂O

Failure 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.

1. As noted above, control of temperature, oxygen and nutrient levels are critical to effective performance. To maintain these variables within target levels, requires the addition of costly instrumentation and control systems and higher levels of operator attention.
2. Importantly the biological system requires time from startup to reach maximum efficiency. The correct balance of system parameters noted above, take time to reach the desired target range, and once achieved, the biological culture takes time to grow to the size necessary to manage the design level of H₂S concentrations. This means that when the system is shut down for maintenance (removing the sulfur deposit build up from the packed columns, there is inherent additional time delay in bringing the system back to operating performance levels.
3. Close control and supervision are required to operate these systems because of the inherent intolerance to upset conditions in the incoming gas stream. Biological processes do not respond well to fluctuations in concentrations and flow rate or other conditions such as temperature or nutrient levels. Even moderate changes in these process variables can push a biological system out of compliance with the target outlet condition. As the amount of H₂S present in the outlet gas from an anaerobic digester can vary significantly as described above, a biological scrubber system can be highly challenged to maintain a tight range of outlet H₂S concentration. This challenge is often addressed by conservatively “over-sizing” the system or through the addition of solid media “polishers” at the bio-scrubber outlet.