Anaerobic Digestion
 Illustration 1: Deep bunker and wet pre-treatment in the digestion plant Palma de Mallorca; Capacity 32.500 t/y (extensible to 90.000 t/y in two further steps |
The digestion process of Ros Roca is a wet technology and applicable for different types of organic waste. According to the quality of the organic waste different types of bunkers for receiving the waste are applicable. For reception of humid organic waste a closed deep bunker is very suitable. The bunker is discharged automatically with srews and a pump and can be cleaned afterwards. Also flat bunkers or deep bunkers with cranes can be used. The first step in the process is a wet pre-treatment. First of all the organic waste is mixed very well with process water in a turbomixer and a suspension with a solid concentration of up to 15 % is produced. It is possible to separate out of the turbomixer impurities like glass, stones, bones by means of a grit system. |
The suspension having passed the screen flows through an aerated sand trap where small inert particles like sand, glass, stones are separated. The result of a very efficient wet pre-treatment is a suspension strongly enriched with biodegradable material and almost free of impurities. The suspension then passes a crushing unit to ensure that only particles with a size of less than 12 mm are charged via a suspension buffer tank to the sanitation process.
The sanitation process (> 70 ° C, > 1 h) takes place in front of the digestion. The design of the sanitation process makes possible to lead back suspension which was not correct sanitized to the suspension buffer tank and the suspension can pass the sanitation process again.
The sanitation process works under batch conditions in mixed tanks. Thereby short circuit flow is safely precluded. The retention time as well as the sanitation temperature are controlled and monitored continuously. The exhaust heat from the co-generation unit is usually used for the sanitation process.
After successful sanitation the suspension is charged continuously to the digester for anaerobic treatment. A digester without any moving aggregates inside is used in the process to ensure a high availability. For mixing the digester compressed biogas is used. The produced biogas is normally used in co-generation units for electricity production. The digestion process itself is characterized by a low electricity consumption and a high surplus of electricity which is very important for the economy of the plant.
Illustration 2 and 3: Digesters (2 * 3.800 m³ ) and Co-generation unit (electric power 2* 500 kW) in the digestion plant in Avila, Spain; Capacity 36.000 t/y
The digested suspension is dewatered with centrifuges. The solid digestate is nearly free of impurities and needs no further treatment steps. It can be used directly in agriculture or can easily be upgraded to other products e.g. as potting soil. The solid digestion residue from the process is of high quality directly after the digestion and certified from the Bundesgütegemeinschaft Kompost e.V. (BGK).
A first digestion plant following completely the animal-by-products regulation 1774/2002 is under construction in Västerås, Sweden. The Växtkraft-plant will treat biowaste, silage and industrial organic waste and has a nominal capacity of 23.000 tons per and is under construction. The following illustration shows the process flow diagram of this plant.
Illustration 4: Procees flow diagram, Växtkraft digestion plant, Västerås, Sweden, 23.000 t/y
The plant in Västerås is equipped with two receiving areas for solid as well as for liquid organic waste. After passing the wet pre-treatment and wet crushing to a particle size of less than 12 mm the waste passes the sanitation process. The material is then treated in a biogas mixed digester and dewatered with centrifuges. The liquid as well as the solid digestate are used as fertilizer in agriculture and the high quality of the digestate allows its use also for ecological farming. As precondition for using the material in ecological farming the process is operated without addition of any chemicals. The biogas is upgraded to natural gas quality and used as vehicle fuel for city busses and refuse collectors.
The process realized in Västerås is part of the EU-Agropti-gas Project.
