## 1 Introduction
### Olive Oil Industry Wastes  
The cultivation of olive trees for the production of olive oil is a well-known and established practice in the Mediterranean region for millennia. The largest area under olive groves worldwide is located in Spain, with over 2.5 million hectares [1], accounting for more than half of the total area dedicated to this crop in Europe, followed by Italy and Greece. In terms of olive oil production, Spain is also the world's leading country, with a yearly production of about 1.35 million tonnes on average in the last ten years [2].  
The olive oil supply chain generates a wide variety of by-products, particularly during the agricultural and oil production phases, which are currently not suitably managedand usually dealt with as waste. The amount and physicochemical properties of these wastes depend on the olive oil extraction method. Figure 1 shows the main products and by-products of the most common and efficient olive oil extraction process in Spain (continuous two-phases). It is estimated that for every 100 kg of olives harvested, approximately 80 kg represent wastes: olive leaves and twigs (\(\sim\)8 kg), olive pits (9-10 kg), olive pomace (70-75 kg) and waste water [3]. The two-phase olive pomace from oil mills is deposited and dried in large ponds for subsequent extraction of crude olive pomace oil, generating exhausted olive pomace as a by-product. Olive oil industry wastes not only include those generated in the oil mill, but also those originated in the olive grove after the pruning activities, which represent another abundant source of wastes in the olive oil value chain. On average, the residual woody biomass from olive pruning in Mediterranean areas ranges between 2.31 t/ha for irrigated olive groves in annual pruning and 4.52 t/ha for rainfed olive groves in biennial pruning [4]. As a result, the yearly production of olive tree pruning in Spain can be estimated at roughly 5-7 million tonnes.  
In the past years, the olive oil sector has experienced important investments in the modernization of oil mills that led to many of the older, traditional mat presses being replaced by more efficient, high volume continuous flow plants for centrifugal separation  
\begin{table}
\begin{tabular}{l r r r r} \hline \hline
**By-Product** & **Olive Tree Pruning** & **Olive Pits** & **Olive Pomace** & **Leaves and Twigs** \\ \hline Location & Olive grove & Olive mill & Olive mill & Olive mill \\ Yearly production rate & 2–3 t/ha & 90–100 kg/t & 650–750 kg/t & \(\sim\)80 kg/t \\  & & of olives & of olives & of olives \\ Ash (wt. \%) & 3–5 & 0.5–2 & 2–5 & 8–10 \\ Moisture (wt. \%) & 15–20 & 30–35 & 65–70 & 5–10 \\ LHV (MJ/kg) & 16–18 & 17–19 & 16–18 & 10–12 \\ Current application & None (burnt or & Heating, sold to & Olive pomace oil & Livestock feed \\  & scattered onsite) & biomass traders & extraction plants & \\ Selling price (€/kg) & Free & 0.15–0.20 (dry) & Disposal fee & Free \\ \hline \hline \end{tabular}
\end{table}
Table 1: Main properties of olive oil industry by-products (two-phase extraction process).  
Figure 1: Products and by-products of the olive oil industry supply chain (two-phase extraction process).  
(two or three phases) of the oil from the rest of the olive fruit. As a result, there are currently over 1800 olive mills operating in Spain [8], of which more than 90% are based on the two-phase extraction process. However, the olive oil sector is nowadays facing several additional challenges toward environmental sustainability concerning waste management and recovery, as well as energy efficiency improvement.