The engineering typologies of the anaerobic digestion systems that are currently available vary from extremely simple systems that are mainly applied to the live­stock sewage wastes on a business scale to those that are more sophisticated and use high technology and are applied for industrial effluent treatment.

2.1.1.1 Simplified plants Due to its constructive and managerial simplicity, this plant typology finds many possibilities for application in the zoo technical sector. These plants, in fact, only comprise a storage basin (containing the material to be digested), often pre-existing, equipped with an appropriate gasometric cover. The simpler systems are the ‘cold’ systems (psychrophiles) that have variable yields depending on the season and elevated permanence times (around 60 days). The annual bio-gas production for a swine liquid manure is about 25 m3/100 kg live weight. The systems that are equipped with heating, on the contrary, obtained from the bio-gas produced, allow to work in a mesophily regime and to obtain higher, more constant yields during the year, with more reduced retention times (median of 20 days). In this case, the annual production of bio-gas from swine liquid manure will be around 32 m3/100 kg live weight [2, 32].

The function of the gasometric cover is to retain and store the bio-gas that is formed; it can be dome shaped or have a floating shape [2]:

• Simple dome cover: It is not pressurized and is made of flexible canvas material that is anchored on the basin’s perimeter. The gas, being at very low pressure, is extracted and sent to its place of use through a blower.

• Double or triple membrane dome cover: This type of cover comprises two or three superimposed membrane layers which are fixed at the edge of the basin (see Fig. 3). In this case, the draining of the bio-gas is achieved by overpressure valves that are regulated by sensors.

• Floating cover: These are membranes which are equipped with a ballast system that is realized using flexible pipes filled with water to grant the bio-gas storage pressure (see Fig. 3).

The necessity to store a larger quantities of bio-gas than that which can be stored using a normal gasometric cover can be satisfied by using external gasometers (spherical shaped and made of two or three adjustable volume membranes).

cover device

bioeas

hot

water

heating system

Figure 5: Scheme of a simplified bio-gas plant with heating.

Depending on a census taken at the end of 2004, more than 100 bio-gas plants were present in Italy; of these, 70 are simplified and low cost plants that have been realized by superimposing a plastic material cover over an effluent from intensive animal breeding in a storage basin [32].

We will now discuss two typologies of digestors which are more complex than the simplified plants discussed above: mixed reactors and ‘plug flow’ reactors.

2.1.1.2 Mixed reactors The mixed reactor is the more classic digestion typol­ogy. They are silo-shaped and are built in armoured concrete or steel. These reac­tors, working in the thermophile or mesophily regime, are equipped with a heating system that comprises a heat exchanger and they are insulated at the perimeter. Mechanical agitators at a low rotation regime allow mixing of the material to be digested. Depending on the number and the position of the agitators, the reactor can be completely or partly mixed. The gas produced by the anaerobic digestion process is retained by a gasometric dome placed on the top of the reactor and mainly made of a polymeric sheet that is protected by a steel cover or armoured concrete. This typology of the reactor allows treating liquid manures with a dry substance content that is lower than 10%, with medium permanence times, which are between 15 and 35 days depending on the composition of the substrate and the process temperature.

2.1.1.3 ‘Plug flow’ reactors These reactors, equipped with a heating system, agitators and gasometers, allow the horizontal scroll of the liquid manure. They are only used on a small scale, because of technical and economic constraints that limit their volume to a maximum of 300-400 m3. These systems, which are appro­priate to treat liquid manures with a dry substance content of up to 13%, allow to obtain bio-gas yields that are higher than those obtained with mixed reactors, at equal temperatures.

biogas

liquid manure

cl-*

■ ‘ ■ ‘ exit

agitator

Figure 6: Complete mixing reactor.

In addition to the two reactor versions analysed, which are more commonly applied, the market also offers other typologies of digestors that are more sophis­ticated and use high technology, which are particularly suitable for industrial efflu­ent treatment at high organic load: reactors for contact, anaerobic filters and upflow anaerobic sludge blanket (UASB) reactors.