A biological scrubber followed by an activated carbon polishing stage can bring hydrogen sulphide in biogas from about 1,000 ppm down to below 10 ppm. The scrubber removes the bulk cheaply, and the carbon catches what is left. In a design for a cold-climate biogas plant treating up to 750 Nm³/h, this two-stage arrangement was chosen to meet an outlet target of under 7 ppm.

The challenge: very low H₂S from a variable source

Hydrogen sulphide forms from proteins and sulphates during anaerobic digestion. Concentrations range from a few ppm, where iron salts are present, up to about 4% by volume in some wastes. It has to come out for two reasons: to meet environmental standards, and to prevent corrosion of engines and boilers.

The client asked for treatment of 685 Nm³/h of biogas, with H₂S at 1,000 ppm in and under 7 ppm out. The design was set at 750 Nm³/h to leave headroom.

The approach: two stages with different jobs

Many methods can remove H₂S: chemical wet scrubbers using caustic soda, dry iron-based media, iron salt dosing to the digester, chelated iron solutions, bacterial processes and activated carbon. Most of these have high investment or running costs, including chemicals and waste disposal. The design combined two with complementary strengths.

Stage 1: the biological scrubber

A sealed GRP vessel holds a packed bed. Hydrogen sulphide-consuming bacteria grow on the packing and feed on the H₂S as the biogas rises through the bed. Air is added by a blower to supply oxygen. Key features:

• The bacteria work best at 35 to 40°C. In cold climates the vessel is insulated, and heating coils or engine cooling water can keep it warm.
• The pH is controlled by draining some liquid and topping up with fresh make-up water. The make-up should contain a carbon source, such as lagoon effluent, and not too many suspended solids.
• An auto-drain removes excess bacteria on a timer.
• No routine chemical dosing is needed, and the spent liquid can usually go to a normal wastewater drain.

Two operating rules apply. The scrubber needs at least 20% of its design flow at all times to keep the bacteria active. And because gas leaves the scrubber saturated, it should be placed before any gas drying equipment.

Stage 2: granular activated carbon (GAC)

Activated carbon adsorbs H₂S onto a huge internal surface, up to about 25% of its own mass. The proposal gives a simple working example: if the yearly H₂S load is 2,000 kg, about 8,000 kg of carbon is needed.

The carbon is split between two vessels in a lead-and-follow arrangement. Each holds six months’ worth of carbon, so each is changed once a year, and the changes are staggered. That way the follow vessel always protects against breakthrough from the lead.

Gas velocity inside the carbon vessels

Gas must pass through the bed slowly and evenly, with no dead zones. The recommended velocity is 0.25 to 0.45 m/s. To achieve this without a tall, thin vessel, two internal rings divide each vessel into three flow sections. A blower raises the pressure to about 300 mbar to overcome the resistance of the bed.

Why the combination works

• The biological stage does the heavy lifting at low running cost.
• The carbon stage reduces the load to a trace. Because the incoming gas is already low in H₂S, the carbon lasts a long time between changes.
• Together they give a more reliable low outlet than either could alone.

Proving the result

The guarantee was an outlet of 10 ppm or less from an inlet of 1,000 ppm or less at up to 750 Nm³/h. At commissioning, the scrubber was to hold the specified removal for 24 continuous hours, measured by an in-line chemical cell or detector tubes. If the full flow was unavailable, the maximum available flow would be used.

Safety

Biogas and air can form explosive mixtures. All electrical equipment in the gas or air line is protected to zone 2 (ExN) or is intrinsically safe, and pressure and vacuum relief valves protect the GRP vessels. Workplace exposure to hydrogen sulphide is also limited in the UK. See the HSE’s EH40 workplace exposure limits.

What to consider for your own project

1. Know your H₂S range. It changes with the feedstock and affects how fast carbon is used.
2. Consider the climate. Cold sites need insulation, and a heat source for the scrubber.
3. Plan the make-up water. It needs a carbon source and low solids.
4. Allow for carbon replacement. Schedule and budget a change every year, per vessel.
5. Keep the flow steady enough. A scrubber that runs below 20% of design flow loses its biology.

Our post on expanding an existing biological scrubber looks at what goes wrong when scrubbers are neglected.

FAQ

How does a biological H₂S scrubber work?

Bacteria on a packed bed use the hydrogen sulphide as food and, with a supply of air, convert it to harmless compounds.

Why add activated carbon after it?

To reach a very low outlet concentration and to give a safety margin if the biological stage is disturbed.

Does a biological scrubber use chemicals?

Not on a routine basis. It needs nutrients, which can come from lagoon effluent, and water for make-up.

How often is the carbon replaced?

In this design, each of two vessels is replaced about once a year, on a staggered schedule.

Need to clean up biogas?

Send us your gas flow and H₂S range and we will propose a treatment train.