The Composting Process from a Waste Management Method to a Remediation Procedure

Composting is a controlled technology to enhance the natural aerobic process of organic wastes degradation. The resulting product is a humified material that is principally recyclable for agricultural purpose. The composting process is one of the most important tools for waste management, by the European Community legislation. In recent years composting has been increasingly used as a remediation technology to remove biodegradable contaminants from soil, and to modulate heavy metals bioavailability in phytoremediation strategies. An optimization in the recovery of resources from wastes through composting could enhance soil fertility and promote its use in the remediation biotechnologies of contaminated soils. Keywords—Agriculture, biopile, compost, soil clean-up, waste recycling. I. COMPOST UTILIZATION IN AGRICULTURE PRODUCT-ORIENTED PERSPECTIVE A. Advantages of Compost Utilization HE European directives regarding wastes priorities the various options for waste management. These are, in order of importance, prevention (product, no-waste, reduction of waste production and hazard), preparing for re-use, recycling, other recovery and disposal [1]. This hierarchy is based on the effects that each option has on the environment and sustainability is the guiding principle. Moreover, it is considered the total life cycle of the product introducing the prevention concept. There is a large agreement on the fact that whatever the waste management selected, it must contribute to the recycling of nutrients, conservation of organic matter and reduction of environmental impact. The Landfill Directive [2] establishes to diminish the quantity of biodegradable waste disposed in landfills, such that only 35% of the total amount (by weight) disposed in 1995 will be permitted by 2016. This strategy should include measures of recycling, composting, biogas production or materials/energy recovery [2]. This will be a hard task given that the amount of wastes yearly generated is increasing. However, the conclusions of report from the Commission to the Council and the European Parliament [3] refer that all strategies adopted by Member States show the promotion of composting, recycling of paper and energy recovery, stressing the importance of using source segregation of organic waste to obtain good quality compost [3]. G. Petruzzelli, F. Pedron, M. Grifoni, F. Gorini, I. Rosellini, B. Pezzarossa are with the Institute of Ecosystem Study – National Research Council, 56124 Pisa, Italy (phone: +39-050-3152489; fax: +39-050-3152473; e-mail: [email protected]). Recent estimates indicate that 45% of European soils have low organic matter content, principally in the southern regions of Europe, but also in areas of France, the UK and Germany, and more than 70% of the soils contain less than 4% of organic matter in the topsoil (0-30 cm), a condition that sometimes is defined of pre desertification stage [4]. In this context the amount and quality of the organic matter present in many urban wastes, above all those deriving from some selected uses, justify the choice of a biological treatment, for the production of compost, as a means of exploiting the waste itself. However the most common destination of the about 220 million tons of municipal solid wastes (MSW) generated in the Community every year are incineration and land filling. For instance, in the Europe (EU27), 486 kg per person of MSW has been treated in 2010, of which about 37% is disposed in landfills (on average 185 kg/inhabitant per year), approximately 23% has been sent to incineration (109 kg/inhabitant per year), while about 25% (121 kg/inhabitant per year) and about 15% (71 kg/inhabitant per year) are sent, to recycling and composting, respectively, depending on local conditions, and degree of industrialization [5]. The utilization of compost in agriculture with its contribution of humified organic matter is a powerful means to maintain or restore the quality of soil, contrasting the organic matter depletion. The use of compost greatly improves soil physical conditions, which can promote root growth and penetration into the soil, with an increase of root biomass production [6]. Fertility is enhanced as a result of re-establishing the balance between the withdrawal and restitution of organic matter in the biosphere. Moreover compost also increases soil biodiversity and locks to the soil organic carbon that otherwise would go in the atmosphere. In addition we have to consider the presence in the compost of essential elements for plant nutrition such as nitrogen, phosphorus, potassium and it should be noted that organic nitrogen is much more slowly released from compost than from mineral fertilizers thus reducing nitrogen losses in the environment, potassium is protected by the organic matter from absorption by clay minerals, and phosphorus is protected from co-precipitation with calcium. Many studies indicate that utilization of compost can increase the yield and in part the quality of plants depending on the kind of compost and soil. B. Limitations to the Use of Compost The main constraints to the use of biomasses in agriculture derive from the presence of inert substances such as plastics, G. Petruzzelli, F. Pedron, M. Grifoni, F. Gorini, I. Rosellini, B. Pezzarossa The Composting Process from a Waste Management Method to a Remediation Procedure T World Academy of Science, Engineering and Technology International Journal of Environmental, Chemical, Ecological, Geological and Geophysical Engineering Vol:8, No:6, 2014 432 International Scholarly and Scientific Research & Innovation 8(6) 2014 scholar.waset.org/1999.6/9998547 In te rn at io na l S ci en ce I nd ex , E nv ir on m en ta l a nd E co lo gi ca l E ng in ee ri ng V ol :8 , N o: 6, 2 01 4 w as et .o rg /P ub lic at io n/ 99 98 54 7 glasses and textiles or from toxic substances such as heavy metals and synthetic organic substances, whereas a process carried out efficiently can eliminate the lack of stabilization of the organic matter. Particular attention has been reserved from many years to heavy metals, given that organic micro-pollutants are subjected to biodegradation phenomena in the composting process and therefore end up in the finished products only in extremely low concentrations. In addition, the soil, by itself, is able to biodegrade almost all the organic composts in the wastes, whereas the same cannot be said of the heavy metals, which have the tendency to persist and accumulate. Heavy metals can be present in compost in various chemical forms [7], such as carbonate salts, sulphites, linked to organic substances, or in an adsorbed or exchangeable form. The predominance of one chemical species with respect to others depends on the type of metal and on the composting process. Cationic species have a different mobility from that of complexed species being often the chelating agents humiclike substances with a considerable variety of molecular weights deriving from the cellular matter. The nature of the metal considerably influences its distribution between the solid and aqueous phases in composts and can in some way also influence the initial availability for plant nutrition after application of the biomasses to the soil. Although we must take account of the important diversity of the chemical forms of heavy metals in compost, it is above all in the soil that we have the fundamental processes that move these elements into the food chain. It is essential to recognize that any detrimental effect on soil quality only occur if the metals are present in “bio-available” forms. Also the most recent advances in the procedures for conducting ecological risk assessment (ERA) of heavy metals, stress the need to incorporate bioavailability in the ERA procedures [8]. Because plant uptake of heavy metals occurs only via soil solution, it is necessary to assess the amount of heavy metals in soluble and/or solubilisable forms, which are also the chemical species able to enter nearly all the environmental processes. The main factors that influence solubility, and therefore environmental mobility, are pH, cationic exchange capacity (C.E.C.), organic matter content and the water and thermal regime of the soil. The activity of a metal ion in the soil solution depends directly or indirectly on the pH. Most of the ions able to precipitate the metals are weak acids, which become soluble following protonation and shifting of the metal in its solid phase. Besides, an increase in acidity reduces the number of specific adsorption sites available for heavy metals. The cationic exchange capacity regulates the mobility of metal ions. As this is a measurement of the negative charge on the constituents of the soil, the C.E.C. is an index of the soil's capacity to adsorb and hold metal cations. Both the organic matter and the clay minerals contribute to the C.E.C.: that deriving from the clays is generally little influenced by the pH, unlike the C.E.C. deriving from the organic matter. The humic substances can interact with the metals forming complexes of varying stability. The complexing ability of the humic substances essentially depends on the content of functional groups containing oxygen and on the amino and imino groups. The complexes of the heavy metals with the organic matter of the soil can have different solubility and therefore a different environmental mobility. The effect of the chemical and physical properties of the soil distributes heavy metals into different pools of different availability, from simple or complexed ions in the soil solution, to ions in the crystalline lattices of the primary minerals. The aim of soil chemistry is to use the appropriate tools (chemical, biological and physical) to evaluate bio-available fractions of metals, while models based essentially on assessment of the following parameters could predict the behaviour in the long term of these elements: a) original content of heavy metals in the soil b) amount of metals applied each year with compost c) the plant species that determines the amount of metals removed with the harvest d) the amount lost due to leaching In the models used to describe the long term behavior of metals the variables should be considered as derivatives of time. Defining, for example, the amount of metals added to the soil as / , the amount that accumulates in the arable layer / , the amount removed by the plants / and that lost through leaching / , for a given soil layer the following relation applies [9]: