The digestate leaving the anaerobic digestion plant must undergo further aerobic treatments in order to obtain a stabilized, mineralized, hygienized and environmentally compatible material.
Depending on the anaerobic digestion technology and the available matrices, the digestate as it is, or its palatable fraction, is mixed with lignocellulosic (structuring) material and, finally, sent for composting.
Composting
All anaerobically degradable substances are also aerobically degradable and the two processes differ in the kinetics of the process, the type of reaction, the families of bacteria involved and the production of excess muds. For this reason, aerobic and anaerobic treatment systems often coexist in the same plant.
The integration of anaerobic digestion with composting represents a technological evolution, with mutual benefits between the process phases, which can be summarized as follows:
- improvement of the energy balance of the plant with energy production
- better emission control capacity at lower costs
- less surface commitment for the same waste treatment
- reduction of CO2 emissions into the atmosphere (zero or positive balance)
- guaranteed reduction of pathogenic organisms thanks to the double thermal passage
- a higher purity level of the compost
Composting is an aerobic process, at the end of which a stabilized, mineralized, sanitized and phyto-compatible material is obtained. Downstream of the anaerobic digestion process, the digestate as it is, or its shovelable fraction, is mixed in appropriate quantities with other biomasses in order to obtain a mixture with particular characteristics of structure, texture, porosity of the material and nutritional balance.
Subsequently, by controlling the conditions of ventilation, temperature and humidity, the ideal conditions are created for the growth of aerobic microorganisms.
Nitro-denitro and depuration
In some cases, the agronomic use of the liquid fraction of the digestate is not allowed. In general, this event occurs when:
- there is not enough land available for the scattering of nitrates present in the digestate
- agronomic use is not possible for law obligations normally related to the type of by-products used
In the first case, the liquid fraction is subjected to a Nitro-denitro treatment for the removal of nitrogen; in the second, the Nitro-denitro treatment is combined with a physical treatment of ultrafiltration and reverse osmosis.
Normally the system components are:
NITRO-DENITER SYSTEM (bioreactor)
The biological treatment consists of a biological reactor with suspended biomass under aerobic/anoxic conditions. The oxygen necessary for the oxidative treatment is supplied by a group of air diffusers, placed on the bottom of the tank.
ULTRAFILTRATION SYSTEM
The mixture formed in the reactor (supernatant + suspended biomass) is pumped towards tubular membranes, installed in an external skid: the solids (suspended biomass) are separated from the membrane, while the water (supernatant) permeates and moves towards the outlet.
REVERSE OSMOSIS
The water leaving the ultrafiltration system is pressurized and sent to the reverse osmosis modules, where it loses almost all the dissolved salts, so that the pure water can continue its course through the semi-permeable membranes.
The physical treatment with reverse osmosis membranes allows the finissage treatment of the permeate from UF, guaranteeing high quality indicators (up to the discharge into surface water).
