**Valorization of Organic Wastes by Composting Process and Soil Amendment**

Hafedh Rigane and Khaled Medhioub

*Unité de Recherche: Etude et gestion des Environnements côtier et urbain, University of Sfax Tunisia* 

#### **1. Introduction**

390 Material Recycling – Trends and Perspectives

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Agricultural wastes disposal is becoming a serious environmental problem. Indeed, these residues may be highly polluting and phytotoxic. The removal of the produced solid and liquid residues is causing serious environmental problems. The direct application of organic wastes, such as olive husks or olive mill wastewaters, to soil has been considered as an inexpensive method of disposal in addition to the recovery of their mineral and organic components. Nevertheless, due to their ligno-cellulosic contents, olive husks are potentially environmentally harmful biomass. Fresh olive husks (not biologically stabilized) may have phytotoxicity due to their monogenic chemical composition (only lignin and cellulose), to phenols coming from olive oil processing, to its high Carbon/Nitrogen (C/N) ratio and to the presence of hormon inhibitors (De Bertoldi et al., 1986). According to several authors (De Jager et al., 2001; Palm et al., 2001), the improvement of soil fertility mainly under low input agricultural systems requires the input of stabilized or mature organic wastes. Many studies (Gallardo-Lara and Nogales, 1987; He et al., 1992; Ouédraogo et al., 2001; Stamatiadis et al., 1999) have shown that application of mature composts at reasonable rates improves plant growth, soil physical properties and increases available soil nutrient levels.

Composting in a controlled biooxidative process that involves a heterogeneous organic solid substrate may resolve this problem. It evolves through a thermophilic stage and the temporary release of phytotoxins, leading to the production of carbon dioxide, water, mineral salts and stabilized organic matter containing humic like substances. These kinds of fertilizers are used to improve soil fertility and plant production either in organic and conventional agriculture.

The main problems that can occur from excessive application of compost are plant toxicity due to salt content (Stamatiadis et al., 1999) and accumulation in plants of trace metals which may pose a health risk when humans or farm animals consume the plant (Petruzzelli, 1996; Cabrera et al., 1989).

Many wastes produced in important quantities are used for composting process such as solid wastes (olive husk, poultry manure) and also liquid wastes mainly the olive mill wastewaters which may be transformed into an organic fertilizer by composting. This is an inexpensive method of disposal leading to important advantages. The composted olive

Valorization of Organic Wastes by Composting Process and Soil Amendment 393

other chemical elements at low concentrations (Vlyssides et *al*., 1996). The direct application of these solid wastes in soil causes problems because of their phytotoxicity (De Bertoldi and *al.*, 1983). The sifting operation uses a densymetric method that allows their separation into 40% pulp and 60% some fragments. The sifted olive husk compost gives more nitrogen and more synthesised and polymerised humic acids than unsifted olive husk compost (Hachicha, 2002; Hachicha and *al*., 2003). Poultry manure contain relatively high concentrations of total nitrogen (more than 2.5% of dry matter), of ash (more than 40% of

During the composting process several parameters were followed such as temperature, pH, electrical conductivity (EC) and ash ratio. Maturity was evaluated by the determination of the C/N ratio, cation exchange capacity (CEC) and humic acid concentrations (Mustin, 1987). Moreover, during the composting process, moisture was kept at 45-50% by adding water and aeration was assured by windrow turning. Temperatures were measured daily basis at different positions in the core of windrow using mercury thermometers and the average of all measurements was recorded. Samples for analysis were collected weekly at

pH 4.92 4.73 8.62 6.43 7.27 E.C. (mmhos/cm) - - - 0.5 0.68 C/N 48.74 37.5 10.44 32 28.5 CEC (meq/100g OM) - - - 73.56 92.93

Total Kjeldhal N (%*d.s.*) 1.06 1.31 2.88 1.5 1.56 Total Organic C (%*d.s.*) 51.67 49.13 30.08 48 44.5 Cellulose (%*d.s.*)" 39.75 28.09 23.17 18.6 13.9 Lignin (%*d.s.*) " 24.72 18.73 17.15 14.3 12.2 polyphenols (%*d.s.*) 0 0 0 0.435 0.18 Fatty substances (%*d.s.*) 4.08 4.29 0.8 3.02 3.67 Total P (%*d.s.*) 0.1 0.13 1.51 0.4 0.42 Total K (%*d.s.*) 0.59 0.71 2.98 0.97 1.06 Mg (%*d.s.*) 0.08 0.10 0.50 0.15 0.23 Ca (%*d.s.*) 0.14 0.17 6.52 1.66 2.13 Na (%*d.s.*) 0.21 0.29 3.58 1.32 1.25

Table 1. Properties of the initial solid wastes and mixtures (Hachicha et al., 2003).

For amendment tests, we applied two types of composts: compost 1 and compost 2. The

Sifted olive husks

Poultry manure

5.22 10.77 41.65 24.5 30.2

Windrow 1

Windrow 2

dry matter) and relatively low C/N ratio (Table 1).

different points along the windrows.

Ash content (% dry solid

**2.1.2 Soil amendment** 

essays were realized in plots and in field.

(*d.s.*))

Component Non sifted

olive husks

husks are organic fertilizers that improved soil fertility (Fiestas Ros de Ursinos, 1986; Tomati and Galli, 1992). The olive husk composts bring essentially potaasium (K), in addition to Nitrogen (N), Phosphorus (P) and Magnesium (Mg) and obviously organic matter (OM). Soil amendment with compost enhances its microbial activity and improves its physical and chemical properties. Thus, farmer's interest for recycling organic wastes in agriculture as fertilizers or amendments is increasing. The composting studies were performed on olive husks mixed (80 %) with 20% poultry manure. The initial mixture was realized in order to reach a C/N ratio of 30. The obtained compost was characterized physically and chemically in order to define its fertilizing capacities and use safety.

The objectives of the present work were to assess the suitability of a waste compost to supply some essential plant nutrients such as N, P, K, iron (Fe), manganese (Mn), zinc (Zn) and copper (Cu); evaluate and compare the effects of manure and compost on soil chemical properties of studied soils in the same zone and to prove the effect of compost on crop productivity.
