Co-composting of solid and liquid olive mill wastes: Management aspects and the horticultural value of the resulting composts I. Aviani a,b , Y. Laor b , Sh. Medina a , A. Krassnovsky a , M. Raviv a, * a Agricultural Research Organization, Institute of Plant Sciences, Newe Ya’ar Research Center, Ramat Yishay 30095, Israel b Agricultural Research Organization, Institute of Soil, Water and Environmental Sciences, Newe Ya’ar Research Center, Ramat Yishay 30095, Israel article info Article history: Received 24 December 2009 Received in revised form 15 March 2010 Accepted 20 March 2010 Keywords: Compost Fusarium oxysporum Olive mill waste Phytotoxicity Suppressiveness abstract Successful co-composting of solid and liquid olive mill wastes (OMW) and obtaining a product of horti- cultural value may increase the viability of this recycling approach. Two composting cycles were per- formed, in which olive mill solid wastes (OMSW) were used to form five mixtures, wetted either with fresh water or with olive mill wastewater (OMWW). Up to 0.3 m 3 of OMWW could be applied to each m 3 of the raw materials without negatively affecting the chemical, physical and horticultural properties of the resulted composts. A growing media composed of perlite amended with 25–33% OMW-composts showed higher suppressiveness against Fusarium oxysporum f. sp. melonis as compared to equivalent per- lite:peat moss mixtures. The yields of tomato plants grown in peat moss amended with 20% (v:v) of OMW-composts were not significantly different than plants grown in unamended peat. The viability of co-composting as a treatment approach for OMWW is discussed in the context of management aspects and the horticultural value of the final product. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction The worldwide production of olive (Olea europaea L.) oil has been constantly growing in the last 15 years. Consequently, olive oil producing countries are currently facing severe environmental contamination problems caused by the lack of economically feasi- ble solutions for the wastes associated with this industry. The most common olive oil extraction process in Middle-East- ern countries is the three-phase continuous centrifugation in which water is added to the crushed olives, forming three phases: olive oil, olive mill solid waste (OMSW) (50% moisture) and olive mill wastewater (OMWW). OMWW generated in the three-phase process is typically acidic (pH 4–5), has extremely high biological (BOD) and chemical (COD) oxygen demand (as high as 100,000 and 220,000 mg l 1 , respectively), high concentrations of fatty com- pounds and a high concentration of polyphenols (Azbar et al., 2004). The presence of phenolic compounds as well as short- and long-chain fatty acids is believed to be responsible for the phyto- toxic (Casa et al., 2003; Isidori et al., 2005; Kistner et al., 2004) and antimicrobial (Fiorentino et al., 2003) nature of these wastes. Due to the high organic load and the toxic nature of OMWW, its direct discharge into domestic wastewater treatment facilities is forbidden in most countries (Rozzi and Malpei, 1996). A few man- agement approaches are being considered for OMWW, including biological (Fountoulakis et al., 2002; Sabbah et al., 2004) and vari- ous physico-chemical treatments (Brunetti et al., 2008; Mantzavi- nos and Kalogerakis, 2005), controlled land spreading (Saadi et al., 2007) and co-composting of the solid and liquid wastes associated with the olive oil industry (Filippi et al., 2002; Tomati et al., 1995). Like most organic materials, OMSW can be aerobically stabilized (composted), in a process during which heat is generated and moisture content decreases. Adequate moisture is essential for effi- cient composting (Richard et al., 2002). In the case of OMSW com- posting, the liquid waste (OMWW) can be considered as an alternative liquid source, such that both OMSW and OMWW can essentially be detoxified during their co-composting process. This approach has the benefit of treating both the solid and the highly problematic liquid waste, while aspiring to achieve a product of high agronomic and horticultural value. From an environmental standpoint, co-composting of solid and liquid olive mill wastes results in a relatively safe product, which may be used as a soil additive or as an ingredient of growing media (Papafotiou et al., 2005). Therefore, co-composting is an advanta- geous solution, as compared to other land application approaches such as controlled spreading of raw OMWW, which may result in long- and/or medium-term risks to ground water and soil fertility (Saadi et al., 2007; Laor and Raviv, 2009). Previous studies reported efficient co-composting of olive mill wastes (OMW) (Filippi et al., 2002; Mari et al., 2003). If adequately processed, the resulting com- posts were shown to have high agronomic value as soil additives (Alburquerque et al., 2007). However, knowledge is still lacking 0960-8524/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2010.03.096 * Corresponding author. Tel.: +972 4 9836505; fax: +972 4 9836936. E-mail address: mraviv@volcani.agri.gov.il (M. Raviv). Bioresource Technology 101 (2010) 6699–6706 Contents lists available at ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech