A biological scrubber can bring hydrogen sulphide in biogas from about 3,000 ppm down to below 100 ppm without a regular chemical feed. In a proposal for a 3,000 Nm³/h landfill in North Africa, three fibre-reinforced plastic (FRP) columns, each handling a third of the flow, were offered with a guarantee proved by a 24-hour performance test at commissioning.
The challenge: H₂S damages engines and breaks limits
Biogas always contains some hydrogen sulphide, formed from proteins and sulphates during digestion. Concentrations range from a few ppm, where iron salts are present, to about 4% by volume in extreme cases. It is removed to meet environmental standards and to prevent corrosion in gas engines and boilers.
The proposal listed the usual options: wet scrubbers using sodium hydroxide, dry iron-based adsorbents, iron salt dosing into the digester, chelated iron solutions, dry-bed adsorption and bio-catalytic removal. Most have high investment or operating costs from chemicals and waste disposal. The biological scrubber was chosen for lower cost on both counts while remaining highly efficient.
The approach: bacteria on a packed bed
How it works
Biogas enters at the base of a sealed vessel filled with random packing, which supports a film of sulphur-oxidising bacteria. The gas travels upwards and H₂S reacts biochemically. A frequency-controlled blower adds a small amount of atmospheric air, because the bacteria need oxygen. A spray system keeps the packing wet, and the circulating liquid is kept in range by draining some and making up with fresh water. The make-up should contain a carbon source, such as effluent from a wastewater lagoon, and must be free of excess suspended solids.
Design conditions
| Parameter | Value | What it means |
|---|---|---|
| Design flow | 3,000 Nm³/h across 3 vessels | Each vessel takes about a third |
| Inlet H₂S | Up to 3,000 ppm | The guarantee basis |
| Outlet H₂S | 100 ppm or less | Removal of at least about 97% |
| Vessel size | About 3.5 m diameter by 12.7 m high | Tall columns give contact time |
| Operating temperature | 30 to 40°C | Warm water from engine cooling can supply the heat |
| Minimum flow | 20% of design | Below this the biology is not sustained |
| Nutrient | NPK, in the order of 3.5 tonnes a year | Feeds the bacteria |
| Make-up water | About 9.5 m³/day | For pH and liquid balance |
Equipment and safety
- Insulated FRP reactor, buffer vessel, recirculation pump selected for acid duty, spray system and an automatic time-controlled drain for excess bacteria.
- An automated in-situ cleaning system that flood-washes the packing once a day to once a week without stopping the gas flow, removing sulphur deposits before they harden.
- Oxygen and H₂S analysers on the outlet, pressure switches and level switches.
- Because biogas and air mixtures can be explosive, electrical equipment in the gas and air lines is Zone 2 (Ex n) or intrinsically safe, with pressure and vacuum relief on the vessels.
Why it works, and its limits
The proposal guaranteed the outlet figure on the stated inlet data, with acceptance by 24 hours of continuous operation at the maximum available flow, measured with proprietary monitoring equipment such as a chemical cell or detector tube set.
Limits matter. The scrubber must sit before any gas drying equipment, because the gas leaving it is saturated. It needs a heat source in cooler climates, a carbon-bearing make-up water, and the 20% minimum flow. A discharge route for the drained liquid is also needed; the proposal described the spent liquid as safe for a normal wastewater drain.
For UK context on gas quality for engines and flares, see the Environment Agency’s landfill gas management guidance.
What to consider for your own project
- Measure the H₂S range, including peaks, not just the average.
- Check the heat source for the 30 to 40°C operating range.
- Plan the make-up water and the drain.
- Keep flow above the minimum or provide a bypass strategy.
- Place it before the dryer.
See also our post on packaged landfill gas flares with a blower.
FAQ
Does a biological scrubber need chemicals?
Not regularly. It needs a nutrient and a carbon-bearing make-up water, but not a continuous caustic or iron dosing.
What temperature does it run at?
Ideally 30 to 40°C. In cold climates this can be maintained from an engine cooling circuit.
How is performance proved?
By a continuous 24-hour test at commissioning, measuring inlet and outlet H₂S.
Why a minimum flow?
The bacteria need a continuing supply of H₂S and oxygen to stay active.
Removing H₂S from biogas?
Send us your gas flow and H₂S range and we will advise on a suitable scrubber.