Before adding capacity to a biological hydrogen sulphide scrubber, check why the existing one is underperforming. More tanks will not fix a scrubber that has lost its biology through low maintenance, a very low pH or cold weather. This post follows the clarification questions raised by an operator who wanted to add four scrubber tanks at an existing landfill gas site.

The challenge: high H₂S at the inlet, poor removal at the outlet

The site already had biological scrubber (BSR) tanks and a flare. Inlet H₂S had risen to about 6,000 to 7,000 ppm, and the outlet was above 1,000 ppm. The original modelling, prepared for an emissions assessment, had predicted an outlet below 200 mg/Nm³ (roughly 145 ppm) from two scrubbers, at an inlet of 1% H₂S and 40 m³/h.

The operator acknowledged that there had been a significant gap in operation and maintenance. In the follow-up call they asked a pointed question: are we missing something? That is the right question to start with.

The approach: working through the clarifications

The operator’s review produced a list of practical questions. Here is what they covered, and the answers recorded.

Layout and flare relocation

A mark-up showed the flare being moved to make room. The contractor pointed out that running all the new cables along the existing cable tray and around the whole plant could be a problem. Cables were outside the scrubber supplier’s scope, so routing needs to be agreed early. Because the flare had its own hazard assessment, the supplier agreed to provide supporting documents for the engineer who would update it.

Pumps and pipework

• Each tank needs its own recirculation pump, so adding scrubbers means adding pumps.
• The recommended pipe was Schedule 80 engineering-grade PVC.
• A recent blow-out on a recirculation line raised the question of whether existing pipework should be replaced. The installed pipe grades were not known, which is a reminder to survey before design.
• The supplier would make the large pipe supports, with the cable tray supplied by others.

Drawings, commissioning and warranty

• The supplier provides the P&ID, layout and general arrangement drawings, with a draft layout by the end of week one and final drawings in week three.
• An installation guide for local plumbers and electricians follows in week six.
• Commissioning is carried out by a commissioning engineer, with an initial engineer followed by a senior one. The controls could be reprogrammed remotely over an installed remote-access link, which avoids doubling up on site work.
• The standard warranty is 12 months, but only where there is an agreed maintenance arrangement with the supplier or an approved contractor.

Why the problem may not be capacity

Two findings stand out.

The water had reached pH 1

A biological scrubber produces acid as it works. Its pH is normally controlled by draining a proportion of the liquid and making up with fresh water. If that does not happen at the right rate, acidity builds up and the bacteria suffer. Here the recirculating water had reached about pH 1, and the acidic water was being discharged to the leachate tank. The agreed response was a pH dosing system and a long-term buffering solution, together with a closer look at the site practice. The operator also noted that the frequency of dumping fresh water could not be changed from the control screen.

Winter temperatures

The bacteria work best at about 35 to 40°C. The discussion covered:

• Fitting thermocouples to the scrubber tanks.
• Logging the tank temperature profile for a year and comparing it with ambient air temperature.
• Trying to avoid large temperature swings.
• Adding sample ports to the tanks, so that the liquid can be checked.

What to consider for your own project

1. Diagnose before you buy. Check pH, temperature, nutrient supply and flow against the design before concluding that the plant is too small.
2. Keep the maintenance regime going. A biological scrubber is a living process. A long gap in operation can undo the performance.
3. Survey existing pipework. Do not assume that old pipe and valves will tolerate the new duty.
4. Agree who supplies what. Cables, trays, supports and drawings should have clear owners.
5. Get monitoring in. Temperature sensors and sample ports make problems visible.

For the design of the original two-stage treatment, see our post on a biological scrubber with activated carbon polishing. Exposure limits for H₂S are set out in the HSE’s EH40 guidance.

FAQ

Why does the pH of a biological scrubber fall?

The bacteria turn hydrogen sulphide into acid. Without regular draining and fresh make-up water, the pH keeps falling.

Can cold weather affect H₂S removal?

Yes. The bacteria work best at around 35 to 40°C, so site winters need to be assessed, and the tanks monitored.

Do I need a pump for every tank?

In this design, yes. Each scrubber tank has its own recirculation pump.

Is a warranty available if I maintain the plant myself?

The standard 12-month warranty depends on a service agreement with the supplier or an approved contractor.

Is your H₂S scrubber underperforming?

Send us your inlet and outlet readings and the scrubber’s operating history, and we will help diagnose the cause.