A horizontal bioscrubber removes hydrogen sulphide from biogas using bacteria growing on a packed bed, with no routine chemical dosing. In a proposal for a palm oil mill, a single horizontal vessel was specified to cut H₂S from up to 3,000 ppm to 200 ppm at 800 Nm³/h. A sister enquiry for a larger flow of 1,600 Nm³/h used the same design.
The challenge: sulphide that corrodes engines
Biogas always carries some H₂S, formed from proteins and sulphates during digestion. Concentrations range from a few ppm, where iron salts are present, up to about 4% by volume in difficult wastes. It has to be removed to meet environmental standards and to stop corrosion in gas engines and boilers.
Options include caustic wet scrubbers, iron-based dry beds, iron salts dosed to the digester, chelated iron solutions and biological scrubbing. Most have high capital or running costs from chemicals and waste disposal. The biological route is lower cost on both counts.
How the horizontal unit works
- A sealed, modular GRP vessel holds a fixed bed of packing, which supports bacteria that consume H₂S. The bacteria are active at low pH.
- Biogas passes from one end of the vessel to the other through wet packing.
- Air is added to supply oxygen, through a modulating valve when the unit is under suction or a frequency-controlled blower when under pressure.
- pH is held by draining a controlled amount of circulating liquid and topping up with make-up water. The make-up should contain carbon and nutrients, for example lagoon effluent.
- The ideal temperature is 35 to 40°C. In warm climates the heating load is small. In cold ones the vessel may need insulation or heating from an engine jacket or boiler.
The vessel is modular so that all components fit in a standard ISO container for shipping. The 800 Nm³/h unit was about 14 m long, 4.5 m high and 3 m wide.
Design basis
| Parameter | Value | Why it matters |
|---|---|---|
| Maximum flow | 800 Nm³/h | Sets vessel volume and contact time |
| Inlet H₂S | up to 3,000 ppm | Sets the bacterial load |
| Outlet H₂S | 200 ppm | Guaranteed at 800 ppm inlet or below |
| Rate of change | 1% per minute for flow and H₂S | Bacteria cope with gradual swings only |
| Minimum flow | 20% of design | Below this, biological activity falls |
| Inlet pressure | -20 to +50 mbar | Suction or pressure operation |
The minimum flow rule is easy to miss. A bioscrubber that is oversized or starved of gas will lose its bacterial population. The guarantee was tested by a continuous 24-hour run at maximum available flow, measured with an in-line cell or Draeger tubes.
Safety and controls
Because biogas and air can form explosive mixtures, electrical equipment in the gas line is Zone 2 rated or intrinsically safe, and pressure and vacuum relief valves protect the GRP vessel. Instruments include oxygen and H₂S cells, a pressure switch to detect flow interruption and level switches. An Allen Bradley PLC runs the system.
Gas analysers for oxygen and H₂S sat in their own panel with dewatering and dust filters. The bioscrubber must sit before any gas drying equipment, because the gas leaving it is saturated.
When a polishing stage is needed
For very low outlet levels, below about 100 mg/Nm³, a polishing stage such as duty and follow activated carbon vessels is suggested. Adding bioscrubber area instead is possible, but the area needed rises steeply as the target falls.
What to consider for your own project
- Know your H₂S range, not just the average.
- Check the minimum turndown. 20% of design flow here.
- Locate it before drying equipment.
- Secure a make-up source of nutrients and soft water.
- Plan the utilities: three-phase power, hot water for heating and a drain for spent liquid.
See also our articles on the packed-bed bio-scrubber and on desulphurisation and dehumidifying for gas engines. For UK context, see government guidance on anaerobic digestion.
FAQ
Does a bioscrubber need chemicals?
Not for routine operation. It needs nutrient-bearing make-up water and, at times, soft water.
Why horizontal rather than vertical?
Horizontal vessels are modular, suit low-height sites and ship in an ISO container. A site visit for one project confirmed a horizontal unit was preferred.
What happens to the spent liquid?
It is described as safe to send to a regular wastewater drain.
How is performance proven?
By a 24-hour test at the maximum available flow.
Need to remove H₂S from biogas?
Send us your gas flow and H₂S range and we will size a unit.