What It Takes to Turn STP Biogas Into CBG
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India's CBG build-out under the relaunched GOBARdhan scheme leans heavily on municipal sewage. Sewage treatment plants run their digesters continuously and put out a steady gas flow, which makes them attractive anchor feedstocks for a CBG project. The chemistry is another matter. Of the common CBG feedstocks, sewage digester gas carries the widest contaminant load, and that load is what the gas train has to be built around. It is also the part most projects under-size.
A plant can meet the CBG certificate on a good day and still run below its rated output the rest of the time. The reason usually sits upstream, in a conditioning train sized against a generic inlet assumption rather than the gas the digester actually produces.
What sewage digester gas actually contains
Start with hydrogen sulphide. Municipal sludge is sulphur-rich, so H₂S in sewage digester gas commonly runs from several hundred ppm to a few thousand, and reaches 1 to 2 percent by volume where the feed carries a heavy sulphur load. At those levels it corrodes carbon steel, copper alloys, and downstream instrumentation, and it poisons the catalysts and media used later in the train.
Ammonia rides along with it. Sewage sludge is high in nitrogen from proteins and urea, and ammonia partitions into the gas as the digester runs. It attacks copper and brass, raises NOₓ when the gas is burned, and has to be stripped before upgrading. Every serious upgrading scheme lists ammonia among the components it must separate out, alongside CO₂, H₂S, water, oxygen, VOCs, and siloxanes.
Siloxanes are the contaminant that makes sewage distinct. They enter the sewer from shampoos, detergents, and cosmetics, volatilise in the digester, and travel with the gas. D4 and D5 dominate, together accounting for more than 90 percent of the total siloxane content. Measured concentrations in wastewater digester gas typically fall in the range of 20 to 60 mg/m³, and sewage plants hold that load fairly steadily, unlike landfills where it drifts with age. The damage happens in combustion. Siloxanes convert to silicon dioxide, a glassy microcrystalline silica that deposits on engine surfaces and drives wear. Engine makers cap siloxane intake at roughly 5 to 28 mg/m³ for reciprocating units, and as low as 0.03 to 0.1 mg/m³ for turbines. Raw sewage gas sits well above both.
Then the VOCs. Sewage digester gas carries toluene, benzene, xylenes, halogenated compounds, mercaptans, and traces such as HCl. These foul membranes and adsorbent beds, add odour, and feed the acidic condensate that attacks piping.
What IS 16087:2025 requires
CBG has to meet IS 16087:2025 before it enters a cascade or a pipeline. The standard sets seven limits. Methane must reach at least 95 mole percent. Carbon dioxide is capped at 4 mole percent on its own, and carbon dioxide, nitrogen and oxygen together at 5 mole percent. Oxygen alone stays under 0.5 mole percent. Hydrogen sulphide is held to 3.7 mg/m³, total sulphur including H₂S to 10 mg/m³, and moisture to 5 mg/m³.
For pipeline injection, PNGRB's access-code specifications apply on top of the standard, and the stricter of the two governs.
Two of these numbers set the desulphurisation design point. H₂S at 3.7 mg/m³ and total sulphur at 10 mg/m³. Against a raw inlet carrying sulphur in the thousands of ppm, that is a reduction of three orders of magnitude or more, held continuously rather than on a good day. What the table does not list is siloxanes or ammonia. Those reach the engine and the membrane whether or not they appear on a certificate, so meeting the spec is necessary without being the whole design brief. The conditioning still has to clear the full load the digester puts out.
How a sewage-gas conditioning train is built
A sewage-gas train works in stages. Bulk H₂S removal comes first, sized to absorb wide swings, through a biological trickling filter or a chemical scrubber. Moisture is dropped by chilling. Siloxanes and residual sulphur are polished on activated carbon. Carbon dioxide is then separated by pressure swing adsorption or membrane to lift methane past the 95 percent mark. Sequence and sizing decide whether the polishing media lasts months or weeks, and getting that order wrong is a common way STP-gas projects erode their economics before they reach rated output.
Where CRA fits
Keeping CBG on spec starts at the desulphurisation step. CRA sizes chemical, biological, or dry-bed scrubbing to the feedstock's actual H₂S profile rather than a generic inlet assumption, and has more than 100 chemical scrubbers in service worldwide. The outlet H₂S figure that scrubber holds is the number the membrane upgrading system downstream is rated against, so when one partner designs both steps, that outlet spec is set to protect the membrane's rated service life rather than a figure written into a separate vendor's data sheet. A final activated-carbon polishing stage removes siloxanes and trims residual H₂S to protect engines, turbines, and catalytic units.
That coordination is not new territory here. The same in-house engineering group that has designed flares and thermal oxidisers for three decades now carries a whole-plant performance guarantee across CBG and RNG trains, measured at the plant boundary rather than section by section. For sewage gas, where no single stage clears the whole contaminant load, that single point of accountability is what keeps a plant on spec once it is running.
If you are scoping CBG from an STP, send us your feedstock profile and target output, and we will walk you through what the gas train needs to look like to hit IS 16087:2025 at rated capacity.
References
3. CRA Energy, “How SATAT, GOBARdhan and IS 16087:2025 Are Reshaping Compressed Biogas in India.”
4. “Hydrogen sulphide management in anaerobic digestion,” Bioresource Technology.
5. “Removal of hydrogen sulfide in biogas from WWTP sludge,” IWA Water Practice & Technology.
6. “Influence of sampling on volatile methyl siloxanes in biogas,” Biomass and Bioenergy.
7. “Removal of siloxane from digestion gas of sewage sludge,” Bioresource Technology.
8. “Purification of wastewater digester biogas from siloxanes,” Renewable Energy.
9. “Biogasification of biowaste and sewage sludge, measurement of biogas quality.”
