Why does concrete deteriorate in biogas plants?
Sulphuric acid can form on moist surfaces in the gas zone and attack the cement paste that binds the concrete together. This process is known as biogenic sulphuric acid corrosion.
From H₂S to acid
Hydrogen sulphide (H₂S) produced during anaerobic digestion enters the gas space. On moist surfaces where oxygen is available, sulphur-oxidising microorganisms can convert sulphur compounds into sulphuric acid.
Attack on the cement matrix
The acid attacks calcium compounds in the cement paste, weakening the concrete and causing material loss. Sulphate reactions can also produce expansion and cracking. Chemical damage may extend beyond the visibly deteriorated surface.
Influencing conditions
Condensation, temperature, oxygen availability and gas composition influence the location and severity of the attack. The upper walls, tank rim and areas exposed to condensation are often particularly affected.
Can the damage extend below the visible concrete surface?
An apparently firm surface can conceal chemically affected concrete beneath it. Its appearance alone does not establish the depth of damage.
Near-surface zone
During rehabilitation work, chemically affected zones extending approximately 1–3 cm below the surface have been observed in concrete that still appeared firm. This field observation gives an indication of the possible depth; the actual condition must be established for each tank.
Why it matters
A bonded coating needs sound concrete across the surface to be protected. Mechanical anchors must pass beyond the damaged zone and transfer loads into sufficiently strong concrete. The depth of attack therefore affects preparation, repair and anchorage details.
Why is substrate preparation so important for liquid-applied coatings?
A coating needs a reliable bond to sound, clean and correctly prepared concrete. A chemically resistant material can still fail if it loses adhesion to the wall.
Typical preparation
Preparation may include removing weakened or chemically affected concrete, cleaning, abrasive blasting and drying. The required surface condition, strength and moisture level depend on the coating system.
Operational effect
Concrete moisture, condensation and temperature affect application, adhesion and curing. Surface preparation and the waiting periods between coats or before restart must be included in the shutdown schedule.
Epoxy and polyurethane formulations can offer excellent chemical resistance. Their performance in a tank also depends on the bond to the concrete and the quality of the connections.
Which methods can protect an existing concrete tank?
Available methods include mineral systems, resin coatings, rubber, fibre-reinforced linings and thermoplastic sheets. Selection combines chemical resistance with the condition of the concrete, movement, connection details and installation requirements.
Mineral systems
Mineral systems can be suitable where their formulation matches the chemical exposure and the substrate. Concrete repair and chemical protection serve different purposes; the selected system must provide the required function under the expected operating conditions.
Epoxy, polyurethane and polyurea
Epoxy formulations can provide high chemical resistance and low permeability. Some polyurethane and polyurea formulations offer flexibility and crack bridging. These properties vary with the product, and all bonded systems require suitable preparation and application conditions.
Rubber and fibre-reinforced linings
Rubber linings can combine flexibility with resistance to particular chemical or mechanical loads. Fibre-reinforced resin systems can form continuous protective layers around complex shapes. Material selection, attachment, joints and penetrations determine how each system performs.
Thermoplastic sheet linings
Thermoplastic plates and sheets are welded together to form a protective barrier. With mechanical fixing, the lining is anchored to the concrete without relying on a full-area adhesive bond. Its design must accommodate expansion and maintain protection at the fixings and penetrations.
How does a mechanically fixed thermoplastic lining work?
The concrete remains the load-bearing structure, while a separate thermoplastic layer protects it from the tank contents and atmosphere. Necessary structural repairs form part of the rehabilitation work.
Load transfer
Fixings transfer the lining loads into the concrete at defined locations. Where the surface zone is chemically affected, the anchors must pass through it and achieve the required capacity in the remaining concrete.
System details
In TP·VAS, PP-H plates form the protective surface. PP-H fixing strips and rings, together with an elastic contact and sealing layer, help limit liquid movement behind it. Flexible FPP cover strips are welded to the plates to close the connections and completely cover the metallic fasteners on the side exposed to the tank contents.
Read the TP·VAS technical system description
How do temperature and structural movement affect tank linings?
A fully bonded coating follows movement of the concrete. Mechanical fixing allows relative movement between the lining and the wall. The connection details must provide for the movements expected in service.
Temperature gradients
Direct sunlight, moving shade, daytime heat and night cooling create temperature differences across the tank wall. The resulting expansion and contraction place demands on the protective system and its connections.
Own expansion of the lining
PP-H also expands and contracts as its temperature changes. The plate layout, fixing arrangement and flexible connecting strips must accommodate this movement as well as movement of the concrete.
What installation conditions affect downtime?
Cleaning, concrete repairs, preparation, installation, curing and final checks determine the shutdown period. The choice of protection method changes the work required at each stage.
Weather and handling
PP-H plates can be positioned and mechanically fixed to damp concrete. Their attachment does not require the entire wall to be dried for adhesion. At Haus Düsse, cleaning with a conventional pressure washer was sufficient before installation. Welding areas must be protected from rain and snow, and kept clean and dry; local sealants require suitable application and curing conditions.
Handling and access
Individual PP-H plates can generally be carried into the tank by hand, avoiding the need to lift large membrane rolls. Access dimensions and working height determine the equipment needed. Strong wind makes large plates difficult to handle and must be considered when planning the work.
Restart
Reduced surface preparation and drying can shorten installation time. The schedule must also allow for connection details, local sealant curing, inspection and the conditions required for restart. These stages are planned with the operator for the actual tank.
What determines the service life of a lining?
Durability depends on the complete protective system: material, anchorage, movement, welded joints and sealed connections. A defect at a penetration can expose the concrete even when the lining material itself remains chemically resistant.
Metal fasteners
In TP·VAS, welded FPP cover strips completely cover the metallic fixing components on the side exposed to the tank contents and atmosphere. The protective barrier continues over the fixing zones. Corrosion-resistant stainless steel provides an additional safeguard beneath this covering.
Possible pathways and maintenance
If aggressive liquid enters through a local defect, it can travel behind the lining. Fixing strips and rings with their contact and sealing layer help limit this movement. Selected areas can be opened to inspect the concrete and subsequently welded closed. Local repairs are possible where access and the surrounding material allow a reliable welded connection.
Service life therefore depends on the actual exposure, temperature, workmanship and maintenance. Records of the installed details support later inspections and repairs.
How does POLY·VAS prepare a rehabilitation project?
POLY·VAS reviews the existing conditions, compares suitable methods and develops the technical details and material requirements for the work.
Technical preparation
The assessment covers the tank contents, temperatures, concrete condition, geometry and existing connections. It identifies any further measurements or investigations needed and defines the areas to be repaired or lined, the fixing and connection details, and the materials required.
Execution
Installation can be coordinated with independent companies selected for the technical requirements and local conditions of the project. Access, the work sequence and the available shutdown period are considered during preparation.
First information
Please send the tank dimensions, construction type, contents, operating temperatures, known damage, drawings, photographs and location. Include the intended shutdown period and your contact details so that we can discuss the next steps.
Contact: info@poly-vas.com
