**3. Micronutrients and trace elements from MSW**

 As it has been shown in the previous sections, the use of MSW can be very positive due to the addition of plant essential elements and the availability of them in the soils and due to the improvement of some physical properties [50]. However, trace elements should be identified and considered as environmental risk.

In the EU, as in the rest of the world, several treatments are used for MSW, mainly landfill disposal. However, landfill and composting are not the only treatments for urban wastes. Incineration has been increased (with or without recovering energy), and it is an important treatment used with MSW in the EU.

*The Use of Composted Municipal Solid Waste under the Concept of Circular Economy… DOI: http://dx.doi.org/10.5772/intechopen.83386* 

It is important to consider that countries with limited natural resources should have an interest in resource reuse [57] and the addition of MSW to soil has positive benefits. For these reasons, the composting process for municipal solid waste should be implemented as far as possible due to the great organic fraction of MSW.

Composts have been frequently used as nitrogen and organic carbon amendments to improve soil quality and to support plant growth, with the additional benefit of reducing waste disposal costs [49]. Nevertheless, the environmental risks from the use of MSW begin within the previous treatments before its addition to soil. The composting process is recommendable before its use, although health risk assessment of odor emissions (i.e., sulfides and aromatics) from waste composting is important [58].

Regarding with the major potential environmental impacts related to landfill, the main problem identified in the municipal wastes consisted of untreated leachates [59]. The leachate pollution of groundwater and surface waters can be categorized into four groups (dissolved organic matter, inorganic macrocomponents, heavy metals, and xenobiotic organic compounds) [60]. Kjeldsen et al. [60] defined these groups for MSW landfill leachates as follows:


The inorganic macrocomponents can be complemented considering other nitrogen forms derived from the oxidation of ammonium that are easily leachate as nitrate (NO3 <sup>−</sup>) and nitrite (NO2 <sup>−</sup>) due to the composition of MSW (organic matter is the major fraction) and its biodegradation. The overuse of nitrogen fertilizer can cause the leaching of NO3 <sup>−</sup> to the surrounding water source and the emissions of N2O and NO to the atmosphere [61].

The use of MSW as soil amendment, after a good composting process, can produce several environmental risks, which can be summarized as:


Moreover, the presence of fragments and objects in MSW is a major concern related to the use of MSW as soil amendment because of the addition to the topsoil of undesirable objects. Farmers and other potential users (i.e., gardeners) do not

want to use amendments and fertilizers that look unpleasant and contain materials that cannot be easily integrated into the soil.

Most of these problems (presence of solid fragments of major size, XOCs, etc.) can be solved in the treatments previously carried out on urban waste treatment plants, including sieving processes before and after composting. The problem of MSW is more serious in developing countries [62] without the application of an adequate treatment to the municipal wastes before applying them to soils.

The excess of nutrients and other elements like trace pollutants is a more difficult problem to solve in the treatments carried out in the municipal waste plants before the application of MSW. The urban waste is usually composted before soil addition facilitating the stabilization of the organic compounds although its application has environmental risks due to soluble organic carbon forms [63], nutrients, and the increment of pollutants, especially by leaching them to waters. Yusof et al. [63] found direct influences of leachate from MSW in the form of inorganic nitrogen and heavy metals in waters.

In general, after an adequate treatment of MSW, we pay our attention in the pollution of the soil–plant system and water, due to the excess of micronutrient or pollutants available from this waste.

The heavy metal pollution of surface soil horizons is characteristic for the sites of solid waste storage and their impact zones irrespectively of climatic conditions, ways of waste management, and stages of the life cycle [45]. At the same time, heavy metals accumulate in ruderal herbaceous plants [45]. However, soil moisture, irrigation, and climate conditions (rain) can affect the mobility and displacement of pollutants to surface water and groundwater and favor their presence in the root environment. In this case, it is possible to incorporate the pollutants into the food chain by plant uptake.

So, there would be a serious risk associated with the availability and mobility of trace elements, including the excess of micronutrients. In general, the addition of MSW increases the presence of trace elements in the soil [62]. Long-term application of municipal solid waste compost may result in accumulation of toxic metals in amended soil, as it has been demonstrated [64].

In general, an increment in leaching and changes in plant composition have been observed and can lead to environmental problems related to water contamination and the accumulation in the food chain of trace elements. Rezapour et al. [62] observed that soils were significantly enriched by the available and total fractions of the metals in the sequences of Zn > Pb > Ni > Cd > Cu and Cd > Zn > Ni > Pb > Cu, respectively. Nevertheless, only the Cd content exceeded the standard levels. However, many works found Cd-soluble concentrations in leachate below the detectable rates and an increment of the soluble fractions of Zn and Ni [65]. Cu and Zn availability is increased with MSW [66], as well as it has been reported for landfill sites [67].

Trace elements are accumulated in different parts of the plants. For instance, Cd, Cr, and Pb were accumulated in roots and stems in mulberry trees [64] and the Cu and Zn concentration in grains of wheat [68]. Adamcová et al. [69] found the highest degree of accumulation for Cd under the use of MSW. Cd, as well as Cr, Ni and Zn are accumulated mostly in the leaves, whereas Co, Cu, Fe, Hg, Mn, and Pb are accumulated mostly in the roots in the case of tansy (*Tanacetum vulgare* L.).

 The application of MSW and derived materials from them, as the compost-like output (CLO) is most of the times used based on the nitrogen content as it is an important parameter for soil fertilization. However, metal pollution should be considered as heavy metal concentrations could exceed water quality limits at the higher application rates. This was found when applying amounts over 3000 kg N/ha [70]. However, the type of soil and the irrigation are important factors that can control the pollution to waters.

#### *The Use of Composted Municipal Solid Waste under the Concept of Circular Economy… DOI: http://dx.doi.org/10.5772/intechopen.83386*

Another negatives effects were described. Leachates also pose pressures on biochemical and chemical oxygen demand (BOD and COD), TOC, ammonium and sulfur compositions, and heavy metals in soil and groundwater [71].

Salinity of soils and water can be increased by using MSW and biosolids [44, 69]. Hamidpour et al. [68] detected after a 3-year experiment the increment of soil salinity due to the use of MSW. This means that soluble salts (inorganic ions) are presented in the soil solution derived from MSW. However, in saline soils, MSW compost, with high organic matter content and low concentrations of inorganic and organic pollutants, allows an improvement of physical, chemical, and biochemical characteristics and constitutes low-cost soil recovery [72].

Biological activity of soils can be affected by the addition of MSW, both in positive and negative ways. Farrell et al. [73] showed the increment of microbial activity in contaminated soils with Cu, Pb, and Zn. Composts can successfully immobilize heavy metals and promote ecosystem diversity/function; surface incorporation had little remedial effect below the surface layer over the course of our short-term trial. On the other hand, the presence of XOCs can alter the soil biota.

It is obvious that there are environmental risks associated to the use of MSW due to the possible pollution of water, the plant uptake of pollutants with an impact in the food chain, and the presence of undesirable fragments. Nevertheless, the use of MSW compost, considering these risks and the type of soil where it is applied, can be controlled or minimized the risks.
