Double Membrane Biogas Holders for Biogas Plants
Created on 09.02
Double Membrane Biogas Holders for Biogas Plants
A double membrane biogas holder is a low pressure gas buffer rather than a gas vessel. The inner membrane holds biogas at about 2 to 3 kPa and forms the gas boundary, while the outer membrane carries wind, snow and rain load so the gas never sees those loads. The space between the membranes is vented or lightly pressurised to keep both layers stable, and the whole system is held inside a ±3 to 5 kPa band by a pressure relief valve and a flame arrester on the roof manifold. Because the envelope flexes instead of stressing, it matches a bolted glass-fused-to-steel digester much better than a rigid dome would.
Biogas plants have a gas supply problem that looks nothing like a refinery gas problem. Production follows the digestion reaction, which means it follows temperature and loading, while consumption follows the boiler or engine duty curve, which does not. Without a buffer the relief valve opens and you flare gas you paid to produce, or the pressure drops and air enters a vessel that must stay oxygen free. A double membrane biogas holder solves this at a fraction of the cost of a gas holder vessel, and for plants running anaerobic treatment on industrial wastewater it is usually the practical answer. The engineering that makes it work is not the membrane itself, it is the pressure control and the interface with the digester roof.
Two membranes, two load cases
The value of a double membrane is load separation. The inner membrane is the gas boundary: it holds the biogas at a nominal 2 to 3 kPa, so the pressure the digester works at stays in the kilopascal range. The outer membrane sits above it and takes wind uplift, snow and rain, which means the gas space is never exposed to the outside weather and the inner layer is not pumped or beaten by gusts. The interspace between them is either open to a vent stack or gently inflated with a small blower; the vented version is common for digestion tanks because it keeps the interspace at atmosphere and removes any chance of collecting gas in the cavity, while the pressurized version gives a tidier profile on the tank rim for sites with a height restriction.
The pressure band is the specification. Running the system inside ±3 to 5 kPa with a relief valve set near the upper limit and a vacuum breaker on the lower side means the envelope moves a little as gas volume fluctuates, instead of putting stress into the shell or lifting a flange gasket. That is the whole reason a flexible holder and a bolted enamel wall belong together: both are designed for an atmospheric vessel with a low pressure differential.
Where the holder sits in the gas train
The roof manifold is the critical component. Gas leaves the digester through a nozzle that should combine a flame arrester and a pressure relief valve on the same connection, so that a blocked burner or a stopped engine does not become an overpressure event on the tank. From there the gas goes to the desulfurisation step, where ferric chloride dosing or a dry desulfurisation box removes hydrogen sulfide, and then to the boiler, engine or flare. The holder itself is not a gas cleaning device; it is the buffer that keeps pressure steady so the cleaning and combustion equipment sees a predictable flow.
Sizing the buffer starts from the peak hourly gas production minus the minimum burner or engine demand, multiplied by the time you want to cover. If the holder is too small the membrane will bottom out or lift constantly, and if it is too large you have paid for volume you never use. A short side note on chemistry: the enamel lining of the digester tolerates sulfide across its pH range, so the vessel is not the constraining item, but the downstream equipment needs the desulfurisation sized from the peak load of the feed.
Tank integration on a bolted enamel digester
The rim is where the holder, the shell and the process all meet. For a glass-fused-to-steel digester, panels about 1.2 m wide are fired at 820 to 930 °C, carry an enamel layer of 0.25 to 0.45 mm rated above 3,450 N/cm², and are joined with 8.8-grade bolts and EPDM gaskets. The roof flange of the bolted shell is the anchorage for the membrane, so it has to be detailed with the gas outlet nozzle, the relief and vacuum protection, the pressure transmitter port that feeds the SCADA alarm, the interspace vent, and a safe access hatch. The enamel covers the pH range from 1 to 14, which helps when the feed is an industrial stream that can shock the pH.
Operationally the plant wants a simple procedure: watch the pressure trend, keep the relief valve set point verified on the annual shutdown, inspect the membrane surface and the flange anchors at the same shutdown, and keep the interspace vent clear through the winter. Because the holder is flexible, a developing problem shows up as a slow drift in the pressure band well before it becomes an odor or a flare event.
Technical Specification
Parameter | Typical Value / Range | Note |
Inner membrane pressure | ~2–3 kPa nominal | Gas boundary for the biogas |
Operating pressure band | ±3–5 kPa | Held by relief valve and vacuum breaker |
Outer membrane | weather layer | Wind and snow load, ADM 2015 / ASCE 7-10 |
Interspace | vented or lightly pressurized | Keeps both membranes stable |
Protection devices | pressure relief valve, flame arrester | On the roof gas manifold |
Digester operation | mesophilic 35–38 °C | Typical HRT 20–30 days |
Typical gas yield | 0.5–0.6 m³ biogas/kg VS removed | Municipal sludge reference |
Enamel lining | 0.25–0.45 mm, fired 820–930 °C | 1500 V DC spark test per panel |
Vessel class | atmospheric / low pressure | Not a pressure vessel boundary |
Practical notes for a biogas plant order
When you write the specification, fix three things explicitly. First, the pressure band and the relief set point, stated as numbers with the tolerance, so the supplier cannot deliver a cover that requires 8 kPa to work. Second, the roof interface list, including the flame arrester location and whether it is inside or outside the envelope, the placement of the pressure instrument, and the access route for membrane inspection. Third, the maintenance access: a membrane roof is inspectable but not walkable, so the hatch and the rim walkway belong on the drawing.
Cost is usually better than a rigid steel or concrete dome for the same gas volume, and the installation is lighter because there is no heavy rigid roof to crane and no field welding on the shell. On the other hand, a flexible holder has a finite membrane life and needs planned replacement, whereas a rigid dome is a longer asset but with a much heavier structural scope. For plants where the digester is a bolted tank, the flexible option is usually the one that lines up with the pressure class of the shell.
Project Case
Project | Location | Product | Capacity | Scope |
Anaerobic treatment train, soy processing wastewater | Hebei, China | GFS digesters with gas tight roof cover | project specific | EPC anaerobic section supply + supervision |
Brewery wastewater storage | Sichuan, China | GFS tank | 14,655 m³ | supply, wastewater storage |
Industrial effluent tank programme, 27 units | Xinjiang, China | GFS tank | 30,469 m³ | supply + supervision |
Center Enamel Engineering Capability
Center Enamel (Shijiazhuang Zhengzhong Technology Co., Ltd) has designed and fabricated bolted storage tanks since 2008. As the first glass-fused-to-steel (GFS) tank manufacturer in China, the company holds close to 200 enamel-related patents, produces roughly 300,000 enamel-coated steel plates a year, has completed more than 30,000 installed projects and supplies its tanks to over 100 countries. The new 150,000 m² production base was added to raise output capacity, and single tanks are supplied up to 60,000 m³. Manufacturing runs under ISO 9001 and ISO 45001, with product certification including NSF/ANSI 61, WRAS, FDA, LFGB, CE (EN 1090), ISO 28765, FM, BSCI and EUROCODE, and design referenced to AWWA D103-09, AWWA C550 and NFPA where the application requires it.
For biogas plants this includes the double membrane holder design, the roof manifold with relief valve and flame arrester integration, and the bolted enamel digester shell that the holder is anchored to.
Frequently Asked Questions
Q1: What is the difference between a gas holder and a double membrane cover?
A1: A conventional holder is a vessel or a rigid structure that stores gas at a higher pressure. A double membrane holder is flexible and stores gas at 2 to 3 kPa nominal inside a ±3 to 5 kPa band, so it suits an atmospheric digester shell.
Q2: Can a biogas holder be added to an existing tank?
A2: Often yes, if the rim can take the membrane anchor and the nozzle layout can be revised. Send the existing drawings and the permitted gas pressure and the retrofit feasibility review comes back quickly.
Q3: How do I size the holder volume?
A3: Take the peak hourly gas production, subtract the minimum burner or engine demand, and multiply by the buffering time you need. The result is the gas space the membrane envelope has to cover.
Q4: Is a flame arrester required?
A4: Yes as a rule, on the gas outlet manifold of a digester delivering biogas to burners or engines, together with the pressure relief valve and the vacuum breaker. Ask the supplier to show the manifold arrangement.
Q5: Does the membrane resist hydrogen sulfide?
A5: The envelope is outside the gas boundary for the outer layer and the inner membrane is selected for biogas duty, while the digester enamel lining tolerates sulfide across its pH range. Desulfurisation is still required for the downstream equipment.
Q6: What is the inspection and replacement interval?
A6: Visual inspection at each shutdown, flange anchor and surface check annually, and membrane replacement at the end of its service life, which the supplier should state as a number in the proposal.
Q7: Does the cover need a special foundation?
A7: The holder itself is light, but the digester shell it sits on does not: the ring beam or concrete foundation with controlled settlement stays the critical item, as it is for any bolted tank.
A double membrane biogas holder is the cheapest reliable way to keep a digester's gas pressure in a controlled band and stop the plant from flaring or drawing air. Separating weather load from gas load is the design idea, and the ±3 to 5 kPa band with a relief valve, a vacuum breaker and a flame arrester is what makes it safe on an atmospheric shell. Combined with a fused enamel digester wall fired at 820 to 930 °C and spark tested at 1500 V, the assembly handles sulfide and pH upsets without turning the tank into a pressure boundary. If you are planning a biogas plant or retrofitting a digester, send the gas production profile, the burner demand, the digester diameter and the site wind and snow data, and the holder configuration and tank sizing will come back with the document package.
Talk to an Engineer
Send your biogas plant data: feed type and flow, digester volume and operating temperature, expected biogas volume per hour and per day, gas end use and its minimum stable demand, hydrogen sulfide level, existing tank drawings if any, and site wind, snow and seismic conditions. We will return the holder configuration, the pressure band and relief settings, the roof manifold layout, the digester diameter and height, and the full tender document package. Engineering review first, and nothing is sold until you are comfortable with the design.
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