COMPOSTING PLANTS The composting process allows to recycle municipal, commercial, industrial or agricultural organic residuals. It recovers organic matter from the residuals that can be returned to the floor. It should be kept in mind that this is a treatment to reduce residuals, and not, necessarily, a business. To assure the agricultural and commercial quality of the resulting compost, the content of nutritious and organic matters should be controlled, as well as the presence of undesirable substances. If several types of residuals will be together composting, firstly they should be mixed completely. The mixture is needed to balance the nitrogen relationship and coal, to distribute the humidity homogeneously along the pile and also to assure a distribution of the oxygen to make spongy the group. If the material to compost has high contents in nitrogen, the blend is particularly critical. The mixture is carried out with appropriate machines. The mud produced in plants of treatments of municipal and industrial waters are other candidates for composting. The organic matter of mud can oscillate from the 50 to 70% of its total solids, depending on its origin. The elimination of the mud of purifying water plants is a problem of difficult solution. First for their high humidity that should be reduced through a mixture with dry materials before they can be composted. The sewer mud should stay at high temperatures to destroy the pathogen agents. Finally, the presence of heavy metals and other pollutants can limit the utility of the mire composted mud. Many communities and industries already use diverse composting technologies to process the mud. The different composting systems try to optimise each one of the factors that intervene in the composting through diverse technical means. In principle, no system is objectively the best, and the peculiar conditions of each installation should be evaluated to develop a successful program of composting. STATIC PILES (WINDROWS) The technology for the composting in piles is relatively simple, and it is the most economic system and the most used one. The materials gather on the floor or pavement, without compressing them in excess, being very important the form and measure of the pile. The good measures oscillate among 1,2 -2 meters high, for 2-4 width meters, being the variable longitude. The section spreads to be trapezoidal, although in very rainy areas it is semicircular to favour the drainage of the water. The piles are ventilated by natural convection. The hot air ascends from the centre of the pile creates the aspiration of the air of the sides. The form and good size of the pile depends on the particle size, content of humidity, porosity and level of decomposition, all that affects the movement of the air toward the centre of the pile. The size and the form of the piles is designed to allow the circulation of the air along the pile, maintaining the temperatures in the appropriate range. If the piles are too big, the oxygen cannot penetrate in the centre, while if they are too small they won't heat appropriately. The good size varies with the material type and the ambient temperature. Once constituted the pile, the only necessary administration is the turn or blended with an appropriate machine. Their frequency depends on the material type, of the humidity and the speed with which we want to carry out the process, being habitual to carry out a turn each 6 - 10 days. The turns are good to homogenize the mixture and their temperature, in order to eliminate the excessive heat, to control the humidity and to increase the porosity of the pile to improve the ventilation. After each turn, the temperature descends 5 or 10 ºC, ascending again in case that the process has not finished. Automatic controls of temperature, humidity and oxygen are usually carried out to determine the good moment to make the turn. The composting in simple piles is a very versatile process and with scarce complications. It has been used with success for composting manure, pruning remains, mires and R. S. U. The process achieves good results with a wide variety of organic residuals and it works satisfactorily while they stay under aerobic conditions and the adequate content of humidity. The composting operations can continue during the winter, but will get slower as a result of the cold. The project should be made avoiding that the tumbling machines pass above the pile. The sides of the piles can be as vertical as it allows the accumulated material; that usually leads to piles that are twice wider than high. At the moment the composting tends to be carried out in covered ships, without walls, to reuse the water of those leached and of rain to control the humidity of the pile. The duration of the process is of about two or three months, more the period of maturation. VENTILATED STATICS PILES The following level of sophistication of composting is the ventilated static pile, in which the materials are placed on a group of perforated tubes or a porous reserve, connected to a system that aspires or inserts air through the pile. Once the pile is constituted, it is not moved, in general, until the active stage of composting is complete. When the temperature in the material exceeds the adequate, some sensors that control the fan activate it so that it injects the necessary air to cool the pile supplying oxygen. Because there are not mechanisms to mix the material during the composting process, the ventilated static piles are used for homogeneous materials as the mud blended with a dry and porous substratum such as wooden chips or sawdust, they form a thin liquid film in which takes place the decomposition. The heterogeneous materials, just as the R. S. U., tend to require more mixture and removed. This system allows the quick transformation of organic residuals into fertilizers. The controlled ventilation impels the activity of the microorganisms authors of the composting process. The system is also more economic for the little mechanical intervention that is required. The composting capacity varies according to the number of blower's units and its model type, as well as the nature of the organic residuals to treat. The process usually lasts 4 - 8 weeks, and then the product is put in a pile during 1 - 2 months so that it gets matured. It can be used in combination with other composting technologies. With an appropriate pre-treatment of the organic residuals, the excess of humidity and the anaerobia conditions of fermentation can decrease. CLOSED SYSTEMS The processes in tunnels, containers or in drum are modular processes that allow to enlarge the treatment capacity, adding the necessary treatment units. The recipient can be anything, from a silo to a concrete pit. As it is a closed systems, it is possible to treat the scents taken place by an eventual anaerobic decomposition. Commonly, forced ventilation is used. The operation is similar to a ventilated static pile. The systems of silos use the graveness to move the material through it, and the internal lack of mixture limit the silos to homogeneous materials. Other composting systems in containers can include intern mixture systems that physically move the materials through the container, combining the advantages of the systems of turned piles and statics ventilated piles. Also, it incorporates a ventilation system for the contribution of necessary oxygen to the microorganisms. It is necessary to stand out the low energy consumption, mainly in the case of loads processes, and the low number of personal necessary for the operation. The evolution of the composting systems to closed systems has represented a very important advance in this type of treatments, so much from the process point of view as for the quality of the final product, favouring the use of the composting like modern technology of treatment of the organic matter of the R. S. U. The process variables, such as content of humidity, composition of nutritious, temperature, pH, quantity of gas, time of retention, etc., can be controlled, directed and optimised. This leads to a quicker and more complete degradation with a minimum contamination of the surroundings. In the last 10 years, the development of these treatment techniques of organic matter has been extremely intense, mainly, in the case of the closed systems. COMPOSTING IN DRUM The composting process takes place in a drum of slow rotation. These drums can work in continuous or for loads and they are commercialised in different sizes and ways. They are built in steel and most of they incorporate thermal isolation, mainly in Central European and Nordic countries. The organic residual, once weight and registered, is discharged in the reception area. From here it is deposited by a shovel directly to the feeder of the composting drums. The feeding of the residual and their distribution inside the drum are carried out in a completely automatic way. The process of decomposition takes place inside the composting drum. Thanks to the intermittent rotation of the composting unit, the material is disentangled, homogenized in a selective way with a good result. The emissions of scent, which reach a maximum at the beginning of the decomposition, are extracted by the system of ventilation of the drum and directed to a bio-filter for their elimination. The liquid of the residuals, liberated during the transformation of the organic substances, it is re-fed to the organic residual by the intermittent rotation of the system, staying inside the same one. At the end of the cycle, the material has a good homogenisation grade, it is disentangled, it doesn't have any unpleasant scent, it is innocuous in what refers to the human hygiene, and it has a good content of humidity for the elimination of pollutants and for the secondary composting. It can be a process of pre composting or a pre treatment to facilitate the separation of the pollutants of the R. S. U. COMPOSTING IN TUNNEL Here, the process takes place in a closed tunnel, generally manufactured in concrete, with ventilation controlled by drive or aspiration, for the contribution of O2, indispensable for the microorganisms. The difference with the previous process is that here the residual is static and the process is complete. COMPOSTING IN CONTAINER It is a technique similar to the previous one. The differenceis that, in this system, the composting is carried out in steel containers, generally of smaller size that the concrete tunnels. Often it is a process in continuous, with load of the material to compost in the superior part and the discharge is done for the inferior part. COMPOSTING IN WAREHOUSE The composting process takes place in a closed warehouse. The ventilation is carried out by a badge in the base and with the help of different types of revolving units (tumbling). The modern plants are completely automated and equipped with tumbling, which are moved by cranes and can reach the total composting of the area of the warehouse. The described processes can be defined in static or dynamic; in the first ones the residual is ventilated without rotation (composting in tunnel or in container), while, in the seconds, the residual is ventilated and turned as it happens in the other ones. MATURATION AND USES OF THE COMPOST The maturation can be considered as the final complement of the process of fermentation, diminishing the metabolic activity, with which the oxygen demand ceases. It allows to reach the wanted biological balance in the breast of the mass of organic matter. Once the maturation process has finished, the compost can be stored until the moment of sale or application to the land. The compost is sold to bulk and in containers of 2, 5, 20 and 60 litres The main use of the compost continues being that of amendment or fertilizer in agricultural processes. It is also prominent the compost use like substrata for the cultivation in gavel. TREATMENT OF THE SCENT The control of scent is one of the primary interests in the big composting facilities, especially if they are located near residential areas. The good administration of the process and the careful chore can reduce the scents, but in many cases some method of the treatment of the scent will still be required. There are several options for the treatment of the scent, including the chemist, the thermal destruction and the bio filtration. In many cases, the bio filtration is the most economic option and the most effective, and so, its use is generalized in the composting industry.