In biogas technology hygienisation is the process by which fermentable substances are subjected to a temperature of 70 °C for at least an hour. This process can partially take place both before the start of the fermentation process and also at the end, but then 100%. This measure is intended to render hazardous substances pathogen-free. In addition to this effect, the fermentation of the treated substances is also improved and undesirable weeds seeds are destroyed, which can prevent their spread on to your own or neighbouring fields during the fermentation residue application.
Hygienisation is becoming increasingly important due to more stringent standards of hygiene in biogas engineering. This is a topic that has concerned TEWE Elektronic for over ten years, during which time they have developed and tested various processes. One that is particularly safe is the batch process, in which the mixture is heated to over 70° C in a tank under constant aitation and maintained at that temperature for over one hour. Another that is used is the run-through process, in which an independent heat exchanger heats the material and then collects it in a tank.
The heat energy is usually taken from an existing cogeneration unit that provides hot water at a temperature of approximately 90 °C. In all cases this is transferred to the substrate through heat exchange. Because the end of the process results in a substrate at a minimum temperature of 70° C, it is sometimes necessary to recool to 40 °C. To a limited degree this can be achieved through heat exchange, but it takes place primarily by means of cooling water or air coolers.
The substrate is delivered by being pumped out of a storage tank. Various safety devices and interlocks in the feed and discharge pipes precludes any interference or manipulation. On no account may substrate be supplied or discharged during the process.
The control systems that we have developed in house not only regulate the process but also provide a complete registration of all important parameters needed for verification during inspections by the veterinary or licensing authorities.
The parts of the hygienisation tanks designed and produced by us that come in contact with the substrate are made form high-quality stainless steel V4A, 1.4571). The tank is insulated on all sides with 80 mm mineral wool and clad externally in stainless steel. Each tank is equipped with three temperature measuring tubes and is usually supplied with a conical bottom for better discharge. A slowly rotating agitator stirs the contents during the heating process. The V4A stainless steel heating coil is inserted directly into the medium and therefore has an optimal heat transfer. This arrangement minimises the electrical power required for stirring. The weighed tank has a compensator on the discharge in place of a rigid connection to the pipework.
Our hygienisation tanks are usually mounted on electronic weighing sensors. This enables control of the filling and discharging processes without having to mount the sensors in the hot substrate. It also make it possible to generate a complete weighing record of the quantities that have been processed.
The temperature, agitator operation and other parameters during the process are recorded on a six-colour paper chart recorder, so that it is always possible to track back with the date and time over the long term.
Pasteurisation can be used in the case of very problematic substrates. This technology uses a pressure-resistant vessel into which the substrate is pumped. The required heat can be taken from the cogeneration plant at the hence necessary exhaust gas heat exchanger. The substrate is heated to 133° C, remaining in this state for at least 20 minutes. After that, the boiler pressure is released and the substrate can then be supplied for further use.
Plants for this purpose are subject to inspection by the TÜV and may only be operated by trained personnel. TEWE Elektronic can supply and install these systems in accordance with detailed preliminary designs.