Construction of a wastewater management pilot plant for the Olive Mill owned by “Agricultural Cooperation of Anemotia” (Anemotia, Lesvos).

The centrifugal (3-phase) olive mill has a processing capacity of about 2450 tn of olives per harvest period (maximum produce). It is estimated that approximately 3200 m³ of olive mill wastewater is produced as a by-product (over a 70 day-period of operation which is the longest harvest period).

Description of the System for the Management of the olive mill wastewater:

For this olive mill the technique of separation of the olive mill wastewater into phases (fractions), followed by subsoil disposal was chosen.

More specifically, the basic parts of the management system are

1.      The initial separation of the olive mill wastewater into 3 fractions (phases) by natural settling. For this purpose a concrete Phase Separation Tank (PST) will be constructed. This tank is dimensioned to provide a five (5) day hydraulic retention period.

2.      Disposal of the middle (light) liquid fraction over a Subsoil Infiltration Field (SIF) located 500m from the Phase Separation Tank.

3.      Periodical removal of the settled sludge from the PST and burial in deep (anaerobic) pits, near the Subsoil Infiltration Field (SIF).

4.      Periodical removal of the floating grease/oil from the PST and its return back to the olive mill for the separation of industrial olive oil.

5.      Removal of the sludge from the anaerobic pits once a year and its application to agricultural land.

Technical details of the Management System:

The selected management system presented above was approved by the Board of the olive mill “Agricultural Cooperative of Anemotia” following a comparative study between the selected system and an alternative management system also opted for. The latter consisted of an anaerobic lake for stabilization/ storage/ infiltration (without pretreatment – separation into phases with natural settling in a concrete tank). There was only a 10% difference in the investment costs of the two management systems, while both systems were expected to have the same treatment efficiency. It must be noted that the implementation of the selected management system requires a suitable block of land with an area of almost 1000 m². This land will be purchased by the Cooperative Olive Mill (the land purchasing costs are not eligible within the program).

The Phase Separation Tank (PST) total volume resulted in 556 m³. The tank is divided into three compartments communicating with each another via baffles and overflows (baffled reactor type). The bottom of the tank is given a slight inclination (2%). The PST is designed in such a way that the sludge and the floating grease/oil are temporarily stored inside the tank. These fractions will be periodically removed (2-3 times during the harvest period) with the help of pumps.

The Subsoil Infiltration Field (SIF) will have an effective area of 850 m². It will be landscaped in order to be flat (without inclinations). With the SIF initially being dug at a total depth of 1m, starting from the bottom towards the top (natural ground level), it will be filled with rocks of 0,3m - 0,6m diameter reaching a height of 0,70m. Then a 15 cm layer of pebbles will follow. The rest of the SIF, until it reaches the normal level of the ground, will be filled with soil. For the uniform distribution of the middle (light) liquid fraction of the olive mill wastewater over the SIF, a collector and diffusion pipes will be placed under the ground. The transfer of the light fraction of the wastewater to the SIF has been designed to be operated by natural (gravitational) flow. However a pump-station will be installed for the safe transfer of the wastewater to the SIF.

A basic criteria for the selection of the Subsoil Infiltration Field was the assumption that its operation would have no negative impacts (pollution) on the ground water table and on the neighboring cultivations. However the following precautions were taken in order to protect a neighboring cultivated field, which belongs to a different landowner:

a)      the allowance of a 5m wide buffer zone towards the neighboring field and

b)      the installation of a protective membrane between the SIF and the cultivated field, whose ground level is below the ground level of the SIF.

The total (maximum) annual volume of the produced sludge is estimated to be 640 m³. It will be stored in deep pits (with a 3m effective depth, near the SIF) whose surface will be covered with various materials (e.g. leaves). Thus it is estimated that a 213 m² area will be needed for the storage of the sludge. It is also expected to have a sludge volume reduction inside the pits of about 70%-80% until the beginning of the next olive harvest period. The digested sludge will then be collected in order to be spread over non-cultivated lands (for safety reasons) and even over cultivated fields (olive groves, vineyards, etc.) as it is expected to serve as a soil enhancer / organic fertilizer.

The transfer of the produced sludge from the Phase Separation Tank to the pits will be achieved by a tank-car.

The total investment cost is estimated at € 117.200. The operational costs are estimated at approximately € 3.000/ year.

Photographs from the worksite  

Photo 1 (tank)

Photo 2 (tank)

Photo 3 (Infiltration field)

Photo 4 (Infiltration field)



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