A digester gas system upgrade can be carried out while the plant keeps running, if the work is phased around the digesters, the boilers and the flare so that no gas is vented. A tender for a sewage treatment works in New Zealand set that rule, and the specification shows how to plan it: new pipework, traps and relief valves, an automatic flare that replaces a hand-lit one, and ventilation for the basement.

The challenge: a system from around 1980

The works treats the sewage of a town with screening, grit removal, clarification and trickling filters. Sludge goes to two digesters, then is dewatered on a belt filter press. The gas from the digesters heats the digesters through two boilers, drives the mixing recirculation, and any excess is burned in a flare.

The gas system had been installed around 1980 and had been serviced and partly upgraded. A recent inspection of the pipework, regulating equipment and flare found it needed an upgrade to keep operating and to meet current safety standards.

The approach: what the tender asked for

Design flows

Each digester produces about 30 m³/h on average and 40 m³/h on a peak day. The system had to pass a peak design flow of 100 m³/h. The gas is typical of domestic sewage sludge, with the usual high methane content.

The existing and new systems

Two identical lines took gas from the digesters to the boilers and to one common flare, linked so either could serve both. The flare was lit by hand and had no automatic flame control. Condensate drains had no drip traps.

The new system kept the arrangement but replaced the equipment:

• Pipework in epoxy-coated mild steel, internally and externally, with a constant fall to a new drip trap where it crosses the basement.
• A condensate and sediment trap near each digester connection.
• A low-pressure check valve after the flow meter, and drip traps at low points.
• At the flare, a new isolation valve, regulating valve and flame arrester, plus a drip trap, a low-pressure manometer and a relief valve at a safe distance.
• An automatic flare with automatic ignition, flame control, re-ignition on flame failure and an alarm if the flame fails for a preset time. If a pilot is used, it must stay lit so the main flame can be relit.
• A security fence around the flare with room for servicing.
• A fan and duct to the basement, sized so the space can be made safe to enter within 10 minutes.

The flare flame temperature had to stay at a level that met the odour guarantees given in the tender.

Why the sequence matters

The works had to remain in operation. The flare could be isolated and bypassed through the boilers, and gas could be stored in the digesters, but only for a limited time. The specification therefore capped the whole job at 16 weeks, including two weeks of commissioning, and banned venting gas on purpose. It also required the contractor to plan the work with the plant operator, to follow the operator’s instructions on safety, and to tell the principal before stopping the gas system.

Performance guarantees covered flame temperature, odour removal, and flame failure detection with automatic re-ignition. Commissioning took a minimum of two days, followed by two months of testing and adjustment.

What to consider for your own project

1. Plan the shutdowns. Work out how long the boilers and digesters can cover for an isolated flare.
2. Treat condensate seriously. Missing drip traps are a common weakness in older systems.
3. Replace hand-lit flares. Automatic ignition and flame monitoring improve safety.
4. Ventilate enclosed spaces. A basement with gas pipework needs a fan and a safe-entry procedure.
5. Specify performance, not just equipment. Guarantees on temperature, odour and flame failure set clear acceptance tests.

For a comparable flare see our post on sewage biogas flares. The UK’s rules differ from New Zealand’s, so check with your own regulator.

FAQ

Can a digester gas system be upgraded while the works runs?

Yes, if the flare is bypassed through the boilers and gas is stored in the digesters, but only for a limited time. Plan carefully with the operator.

Why replace a manual flare?

An automatic flare relights after a flame failure and raises an alarm. A hand-lit flare does neither.

What is a drip trap for?

It collects condensate that forms in the gas pipework so that liquid does not block the line or reach the equipment.

How long should commissioning take?

This tender asked for at least two days, followed by two months of testing and adjustment.

Upgrading a digester gas system?

Contact our engineers to discuss the flare and gas train for your works.