In short. Landfill gas and sewage digester gas can fuel gas engines, but only after their variability is understood. Landfill gas yield rises and then declines over years and is diluted by air drawn into the wells; sewage gas follows the treatment plant’s sludge load. Both carry siloxanes and other contaminants that damage engines. Size the generating capacity to the clean gas that can be collected dependably, not to the electrical demand, and budget for gas treatment and frequent maintenance.
Two waste gases, two different behaviours
Landfill gas forms as buried municipal waste decomposes anaerobically. It is collected through vertical wells and horizontal trenches connected to a header under suction from a blower. Its methane content is typically 40 to 55 percent, with carbon dioxide making up most of the balance, and nitrogen and oxygen present whenever the suction draws air in through the cap or cover. Sewage gas is produced in the anaerobic digesters of a sewage treatment plant, from primary and secondary sludge. It is usually richer, around 60 to 65 percent methane, and more consistent hour to hour because the digester is a controlled vessel, but it depends on sludge feed and digester temperature. In Telangana, STPs serving large urban catchments and the larger municipal dumpsites are the realistic sources; small STPs without digesters produce no usable gas at all.
Gas quantity changes over time
A landfill’s gas generation follows the age and composition of the waste. Production builds over the first few years after placement, peaks, then declines slowly over decades once filling stops. Indian municipal waste is high in food and wet organic matter, which decomposes quickly, so the peak can come earlier and fall faster than in older international models. Collection efficiency also matters: a poorly capped site may capture only a fraction of what is generated. Seasonal rainfall changes moisture and therefore generation rates, and heavy monsoon rain can flood wells and cut collection for weeks.
Sewage gas tracks the plant’s inflow and the operator’s management of the digesters. Temperature is critical; mesophilic digesters work around 35 degrees Celsius, and gas production falls sharply if heating fails. Digester cleaning, foaming events and changes in sludge thickening cause step changes in yield. For both sources the fuel is dynamic, and the plant must be designed to follow it.
Contaminants and what they do
| Contaminant | More typical of | Engine consequence | Usual control |
|---|---|---|---|
| Siloxanes | Both, often higher in sewage gas from cosmetics and detergents | Abrasive silica deposits on valves, pistons, plugs, turbocharger and catalyst | Activated carbon or regenerable adsorbent with breakthrough monitoring |
| Hydrogen sulphide | Both, very variable | Acidic oil, corrosion of bearings and exhaust | Biological or iron-based scrubbing, digester air or iron dosing |
| Halogenated compounds | Landfill gas | Hydrochloric and hydrofluoric acid formation, rapid corrosion | Carbon adsorption, oil analysis, limits from the engine manufacturer |
| Moisture | Both, gas is saturated | Condensate in headers and fuel train, acid carry-over | Knock-out pots, gas chilling, sloped pipework with drains |
| Oxygen and nitrogen | Landfill gas under suction | Lower heating value, risk of explosive mixture in the header | Wellfield balancing, oxygen analyser with blower interlock |
Engine manufacturers publish limits for each contaminant per unit of methane or per unit of fuel energy, and several build sewage and landfill gas versions of their engines with hardened valve seats, adapted oil specifications and lower compression for this reason. The treatment train is selected to bring the measured gas inside those limits with margin.
Size from the gas, then match the engines
P (kW) = Q × m × 35.8 × η / 3.6
Q is the dependable collected gas flow in Nm³/h, m is the methane fraction, 35.8 MJ/Nm³ is the lower heating value of methane, and η is the engine’s electrical efficiency as a decimal. Worked example (illustrative): an STP’s digesters produce between 180 and 260 Nm³/h at 62 percent methane, with 200 Nm³/h available more than 90 percent of the time. At 200 Nm³/h the fuel energy is 200 × 0.62 × 35.8 = 4,439 MJ/h, and at 37 percent efficiency that gives about 456 kW. Rather than one 600 kW engine that would often run lightly loaded, two engines of around 250 kW let the plant run one set on lean days and both on good days, with surplus gas going to the flare. For landfills, a gas yield forecast over the project life is needed, and modular engines that can be removed or relocated as yield declines protect the investment.
Operating strategy and electrical integration
Most waste gas plants follow gas availability rather than electrical demand. The engine controller trims load to hold the gas header pressure or holder level within a band, which means the electrical output floats and the grid or another source must absorb the difference. That makes grid-parallel operation the normal mode, with synchronising, reverse power, loss of mains and anti-islanding protection, and approval from the DISCOM. At an STP the electricity is usually consumed by the plant’s own aeration blowers and pumps, which are large and steady and suit captive use; export depends on the regulatory framework and should not be assumed. Jacket water heat keeps the digesters at temperature, closing the loop described in CHP and cogeneration. A flare remains essential for start-up, maintenance and surplus gas, and should be sized for the full collected flow. Where the plant cannot run gas engines continuously, converting an existing diesel set to dual fuel can use whatever gas is available without making power depend on it.
Maintenance on a hard fuel
Expect shorter intervals than on natural gas. Spark plugs, oil and filters are changed more often; valve recession and cylinder head deposits are inspected at each major service; and oil analysis for wear metals, silicon, acidity and total base number guides the schedule. The treatment media consumption is a running cost, and carbon beds must be changed on breakthrough rather than on a fixed calendar. Telemetry of methane, oxygen, hydrogen sulphide, header pressure and engine load turns problems into trends rather than failures. The wider guide to biological gas fuels is in biogas generators, and the water sector context in water and wastewater.
Common mistakes
- Sizing engines to the landfill’s peak predicted yield. Within a few years the engines run part loaded or idle as production declines.
- Omitting siloxane removal because the first gas test showed low levels. Silica builds up gradually and a top overhaul arrives at a fraction of the expected interval.
- Running the wellfield blower hard to raise flow. Air is drawn through the cover, oxygen rises, methane falls and the header approaches an explosive mixture.
- Removing the flare once engines are installed. Surplus gas is then vented, which is a safety and environmental breach.
- Assuming the electricity can be exported. Without a sanctioned arrangement the plant can only use what it consumes itself and the revenue model fails.
What to send us
Send any gas flow and composition records, the digester or landfill design data, the plant’s electrical load list and DISCOM connection details, and information on existing flares, blowers and treatment equipment. Photographs of the gas header, flare and proposed engine location help us judge the installation. Use the application engineering enquiry and we will screen the gas against engine requirements and propose a capacity that matches dependable yield.
Questions engineers ask
How much power can a sewage treatment plant generate from its own gas?
As a rough guide, digester gas can supply a significant share of an activated sludge STP’s own electricity demand, often quoted between a third and a half, depending on sludge characteristics, digester performance and the efficiency of the aeration system. A plant-specific estimate requires measured gas flow and composition.
Why are siloxanes such a problem in gas engines?
Siloxanes from personal care and industrial products volatilise into the gas and burn to silicon dioxide. The silica forms hard deposits that wear valves and seats, foul spark plugs, score liners and poison exhaust catalysts. Removal before the engine costs far less than the overhauls they cause.
Can landfill gas run a generator continuously?
Yes, if the collection system is well managed and the engine capacity is matched to the dependable yield. The engine controller should follow header pressure and step load down when gas falls, and the plant should plan for engine removal or relocation as the site’s production declines over the years.
What methane content does a gas engine need?
Many engines designed for landfill and sewage gas run on methane content down to around 40 percent, and some lower with special calibration, at reduced output. The engine manufacturer states the minimum and the rating at each level. Below that range, gas is better flared or used in a boiler.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.