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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