As part of a recently constructed Anaerobic Digester Plant at Villa Farm, Keisby Estate, Lincolnshire - designed and built for a 155Kw electrical output - the semi-submerged digester tank is constructed of a cast in-situ reinforced concrete base with the tank walls and roof formed from pre-cast reinforced concrete sections.
The tank - measuring 35m long by 6.5m wide and 4m high - is used to mix a feedstock combination of maize & manure under mesophilic (oxygen-free) conditions. Once fed into the tank, the organic matter is heated to around 38oC and during the retention time is broken down by bacteria creating methane-rich biogas.
In order to protect the exposed internal concrete surfaces in the void space above the feedstock (known as the 'Gas Zone') from the corrosive, acidic gases generated, a rubberised, polymer modified emulsified asphalt-based material had been applied to protect the walls and soffits but which subsequently failed even before the tank and plant had been commissioned. Furthermore, the exterior of the concrete roof had inadequate protection and was allowing rainwater ingress through the cold-joints between the pre-cast reinforced concrete planks and into the tank below.
The production of acidic gases (eg H2S - Hydrogen Sulphide) as part of the AD process means that when dissolved in water, Hydrosulfuric Acid is created which is very corrosive when In contact with bare concrete and steel resulting in surface damage and corrosion. In order to prevent damage being caused by this acid erosion, it is necessary to protect the non-submerged surfaces of the digester tank - the 'Gas Zone' - with a highly chemically resistant and elastomeric, crack-bridging coating that provides 100% surface area protection.
The existing rubberised coating that had failed even before the AD plant had been commissioned and put in to service, had to be completely removed by a hydro-demolition process (hydrablasting). Having fully exposed the concrete substrate, a surface levelling mortar - Irathane's CR94 - had to be applied to the soffit & walls to provide a suitable substrate free of blowholes & surface defects prior to the application of the chemically resistant and crack-bridging coating, Aqualine 300.
The vertical joints in the concrete walls and roof of the tank were sealed with a one-part, moisture-curing & permanently elastic sealant. The overall surface area of concrete in the gas zone was then primed with Irabond BC50 followed by three coats of Aqualine 300 to achieve an overall dry-film thickness of approximately 1.5mm. Each layer of coating is applied to walls & soffits in a contrasting colour (grey / black / grey) to ensure consistent film thickness of the coating across the whole area.
The top surface of the digester tank roof was also waterproofed and protected with the application of Makers Ultradeck, a cold, liquid-applied & fully mesh-reinforced rapid-curing protective roofing system.