The CSTR biogas reactor is designed for anaerobic digestion of liquid and mixable organic substrates. Within the sealed tank, bacteria break down organic matter to produce biogas. It is suitable for agricultural and industrial biogas plants. Feedstock composition, dry matter content and daily input determine the working volume, mixing system and operating temperature. The design combines a digestion tank with a flexible gasholder above it.

The shell is assembled on site from factory-made steel panels. A glass-fused-to-steel coating protects the steel from the process medium, combining a smooth surface with the strength of a steel structure. Bolted joints are sealed with a sealant selected for the substrate, biogas and operating temperature. Edges, holes and the gas zone require particular attention where condensation may occur. Panel thicknesses and stiffeners depend on diameter, height and design loads.

The modular construction simplifies transport and assembly: panels are shipped in compact packs and the wall is built in successive rings. The main panels require no on-site welding. Diameter depends on the number of panels per ring; height depends on the number of rows and the assembly module. This allows volume to match the available space and process layout. Replacement of individual parts or structural changes requires an assessment of the shell and a revised design calculation.

The flexible gasholder buffers biogas production and supply to consumers. In a double-membrane design, the inner membrane moves as the gas volume changes, while the outer membrane provides weather protection. A support-air blower maintains the outer shape. Membrane fixings must suit the upper shell ring; a support structure may be required. Gasholder capacity is selected separately from the liquid working volume.

Mixing distributes fresh substrate, heat and bacteria throughout the reactor and reduces floating crusts and sediment. Agitator type, position, power and operating cycle depend on viscosity, dry matter and fibrous material. Low-speed agitators can provide gentle mixing. The speed must maintain substrate movement without excessive local stress; low speed alone cannot replace correct impeller selection and positioning.

Heating compensates for heat loss and warms incoming feedstock. Depending on the project, heat is supplied by internal pipes or an external substrate circulation loop. Wall insulation with protective cladding helps maintain the operating temperature and reduces weather effects. The temperature regime follows the digestion process. Pipework, supports and agitators are arranged together to provide circulation and maintenance access.

Process nozzles and flanges serve feeding, discharge, recirculation, gas extraction and instruments. Manholes allow access after the reactor has been shut down and prepared for maintenance. Viewing windows show the substrate surface; ladders and platforms provide access to service points. The number, size and position of openings are agreed before panel manufacture so that equipment and pipework can be connected without site modifications.

Level, temperature and pressure monitoring helps maintain stable operation. Pressure and vacuum relief valves belong to the gasholder package, not the reactor. The shell, anchors and foundation are designed for substrate weight, wind, snow, seismic and equipment loads. Joints, coating and tightness are checked after assembly. The scope includes the shell, accessories, heating and equipment supports. The membrane gasholder, agitators and automation are specified separately. Insulation is included for the 2.000–4.000 m³ package.

Sizing takes account of retention time, freeboard and possible foaming. The size table gives the geometric volume of the cylindrical shell. The usable digestion volume and gas storage volume beneath the membrane are calculated separately to compare reactor options consistently.