The size of a landfill gas bioscrubber is set by three choices: the gas flow, the inlet hydrogen sulphide (H₂S) level and the outlet target. In a proposal for a landfill with H₂S around 17,000 mg/Nm³, the same biological scrubber needed four, five or nine vessels depending on those choices. Treating the gas to meet the flare’s sulphur dioxide limit needed far less equipment than treating it to engine-grade levels.
The challenge: very high sulphide, uncertain trend
Landfill gas normally contains some H₂S, but this site recorded up to about 17,000 mg/Nm³ (roughly 14,000 ppm by mass). The gas was being burned in a flare at about 1,000 Nm³/h, although the flare was designed for 2,000 Nm³/h.
Two things made sizing difficult. First, there was no single obvious target. Second, the data suggested H₂S might be falling over time, but there were too few samples to be sure, so the design had to use the maximum measured value.
Two ways to set the target
1. Flare exhaust: sulphur dioxide limit
Burning H₂S creates sulphur dioxide (SO₂). The South African air quality regulations for gas combustion installations set an exhaust limit of 400 mg/Nm³. Working back through the combustion, the fuel gas needed H₂S reduced to about 2,350 mg/Nm³.
2. Gas engine: total sulphur in the feed
If the gas later runs an engine, the manufacturer’s technical instruction limits total sulphur to under 700 mgS per 10 kWh without a catalytic converter. This is a tighter requirement, but because the starting point is so high, the gap between the two targets is smaller than it first appears.
Three scenarios, three sizes
| Scenario | Flow | Outlet H₂S | Vessels |
|---|---|---|---|
| 1. Design capacity, engine-grade | 2,000 Nm³/h | About 350 mg/Nm³ | 9 |
| 2. Current flow, engine-grade | 1,000 Nm³/h | About 350 mg/Nm³ | 5 |
| 3. Current flow, flare compliance only | 1,000 Nm³/h | 2,350 mg/Nm³ | 4 |
Each vessel was 3.5 m in diameter and 12 m high. Halving the flow roughly halves the equipment, and relaxing the target from engine-grade to flare compliance saves about one more vessel. The proposal priced each scenario separately so the owner could see exactly what each requirement cost.
How the bioscrubber works
Gas enters at the base of a sealed vessel filled with a high-surface-area packing. Bacteria that eat H₂S grow on the packing, and a small air blower supplies the oxygen they need. A recirculating liquid keeps the packing wet and the pH controlled, and excess bacteria are drained off on a timer. No chemicals are dosed in normal running, which is the main reason a biological scrubber costs less to operate than caustic or iron-based systems.
Three site-specific points came out of the design:
- Make-up water. Each vessel needs about 1 m³/h. Landfill leachate was the first source to investigate, because it also supplies carbon for the bacteria. Without it, soft water dosed with nutrients is needed, in the order of 24 m³/day and some 7 tonnes of nutrient a year for the whole plant.
- Effluent. The drain liquid has a pH of about 2 to 5. At pH 2 that equals roughly 0.5 g/L of sulphuric acid, which is dilute, but it has to go somewhere permitted, possibly back into the landfill.
- Temperature. Cold spells call for insulation and heating. In this location nights can be cool but days are warm, so heating was left as an option.
Operating limits to plan for
- The scrubber must always see at least 20% of design flow to keep the bacteria active.
- It must sit upstream of any gas drying, because the gas leaves saturated.
- For lower outlet levels, a polishing step such as iron sponge vessels, or more scrubber surface, would be needed.
- Explosion protection applies, so electrical equipment in the gas line is rated for Zone 2, and the vessel has pressure and vacuum relief.
How performance is proven
Commissioning acceptance was a 24-hour run at the specified maximum flow, checked with in-line chemical cells or detector tubes. If maximum flow was not available, the maximum available flow was used.
What to consider for your own project
- Decide the end use first. Flare-only compliance and engine feed give very different targets.
- Size on measured peaks, then check the trend. Take enough samples to know whether H₂S is rising, flat or falling.
- Do not size for flare capacity if the real flow is half. Flow is the biggest cost driver.
- Secure make-up water and an effluent route. They are easy to overlook.
- Compare fabrication locations. Here, building in Thailand and shipping beat local manufacture.
For monitoring and operating standards of enclosed flares, see the Environment Agency’s LFTGN05. For a mill-gas example of the same technology, see our post on bio-scrubber process design for high-sulphide mill gas.
FAQ
What is a biological H₂S scrubber?
A vessel of packing on which sulphide-oxidising bacteria grow, fed with air and a circulating liquid, converting H₂S to sulphur compounds without chemical dosing.
How much H₂S can an engine tolerate?
It depends on the manufacturer. The limit used here was under 700 mgS per 10 kWh without a catalytic converter.
Why not scrub everything to engine standard?
Because it costs more. If the gas is only flared, a looser target meets the SO₂ limit with less equipment.
Does a bioscrubber need chemicals?
Not in normal operation, but it needs make-up water, nutrients if leachate is not available, and a safe effluent route.
Dealing with high-sulphide landfill gas?
Send us your gas flow and H₂S data and we will outline the scrubber size for each target.