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Construction
of a wastewater management pilot plant for the Olive
Mill owned by “P. Kantaris & Co”-Olympos
(Vasilika, Municipality of Polixnitos, Lesvos).
The centrifugal (3-phase) olive mill has a processing capacity of about 2500
tn of olives per harvest period (maximum produce). It is
estimated that approximately 3300 m³ of olive mill
wastewater is produced as a by-product (over a 90
day-period of operation which is the longest harvest
period).
Description
of the Olive mill wastewater Management System selected
for this Olive Mill:
In the case of
this olive mill the technique for olive mill wastewater
separation into phases (fractions), followed by subsoil
disposal was selected. Parallel with the above, a system
for the recycling of the (turbid) water of the vertical
centrifuge (oil separator) will be installed, while the
decanter “ARA” system will be applied. These two
systems will result in a reduction of drinking water
usage. Consequently, less wastewater will be produced.
More
specifically, the basic parts of the management system
are:
1.
Installation and operation of the system for the
recycle of the (turbid) water of the oil separator and
utilization of the “ARA” system in order to reduce
the use of water by the decanter.
2.
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.
3.
Disposal
of the middle liquid fraction into Evaporation-Infiltration
Ponds located 400m from the Phase Separation Tank.
4.
Periodical removal of the settled sludge from the
PST and burial in deep (anaerobic) pits, near the
Subsoil Infiltration Field (SIF).
5.
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.
6.
Removal of the sludge from the anaerobic pits
once a year and its application to agricultural land.
Technical
details of the Management System:
The
total Management System of olive mill wastewater was
initially designed and dimensioned without having the
possibility of reducing the volume of the produced
wastewater. Thus, the volume of the Phase Separation
Tank (PST) initially was estimated (about 530 m³ total
tank volume) according to the production capacity of a
average conventional olive mills on Lesvos. However the
olive mill of “P. Kantaris & Co”
can reduce the volume of the wastewater it produces by:
a) the application of the “ARA” system that its
decanter technology provides (model NX-X20 by Alfa
Laval) and b) the installation and operation of the
system for the recycling of the (turbid) water produced
by the oil separator (this recycling system is also
proposed by Alfa Laval). Thus, at least a 40% reduction
in the total volume of wastewater produced is expected.
Consequently, the Phase Separation Tank (PST) was
eventually dimensioned to be 40% smaller than initially
calculated (318 m³ total volume). The PST 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
middle fraction of the Phase Separation Tank will be
disposed in three ponds of a total effective volume of
637m3. These ponds will be connected via overflow pipe,
allowing phased repletion. Each pond will function as a
separate subsoil infiltration disposal field and
evaporation pond. This integrated system is fed from the
uppermost section of the land, taking advantage of the
natural declination. The slopes of the pond will be
designed especially. Moreover, dry-stone walls will be
built for soil retention. A special type of stone will
be used and especially designed foundations will be laid.
The dry-stone wall will be 1 meter in thickness
and 2 meters high on average, varying according to the
pond’s location and the natural altitude of the
building suite. The wastewater flows naturally from the
Phase Separation Tank to subsoil infiltration /
evaporation disposal field. Pond design was based on the
acceptance that during a peak olive harvest season, the
total volume of the middle fraction ending up in the
ponds would be 1500m3. Other data which were taken into
account for pond design were rainfall, evaporation and
infiltration data of the landsite area. An important
criterion used for the selection of the Subsoil-Infiltration
Ponds was the assumption that their operation would not
contaminate the ground water table or have any other
negative impact on the neighboring crops.
A
basic criteria for the selection of the Evaporation –
Infiltration Ponds was the assumption that it their
operation would have no negative impacts (pollution) on
the ground water table and on the neighbouring
cultivations.
However the following precautions were taken in order to
protect a neighbouring cultivated field, which belongs
to a different landowner:
a)
the allowance of a 5m wide buffer zone towards
the neighbouring field and
b)
at the
site entrance as well as other locations where the level
of the wastewater is perhaps higher than the altitude of
the land, special safety and insulation measures will be
taken by fitting 400μm thick HDPE sheets concluding
at the pond coronation to insure their insulation. These
sheets will be retained in position by depositing
excavation material over the membrane.
The
total (maximum) annual volume of the produced sludge is
estimated to be 400m3. It will be stored in deep pits
whose surface will be covered with various materials (e.g.
leaves). Thus it is estimated that a 130 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 € 138.300. The
operational costs are estimated at approximately €
4.100/ year.
Note:
The “ARA” system is an innovation in horizontal
centrifuges (decanters) which operate in three (3)
phases. It allows a reduction of up to 75% of the water
used for the dilution of the olive paste (in the
decanter) compared to decanters of older technology.
This results in a direct reduction of the wastewater
produced (roughly a 30% reduction in the total volume of
wastewater produced by the olive mill).
In
brief, the oil separator (turbid) water recycling system
consists of a stainless steel tank that receives the
(turbid) water of the oil separator and from a pump
which feeds this water back to the decanter in order to
(partially) substitute the water from the water supply
system which is required for the dilution of the olive
paste. It is a simple
system (to install and operate), which is
expected to reduce the total volume of wastewater
produced by the olive mill by 20% (recycled water from
the rinsing of the oil separator is also included).
Photographs
from the worksite
Photo
1 (Tank)
Photo
2 (Tank)
Photo
3 (Tank)
Photo
4 (Lakes-Infiltration Field)
Photo
5 (Lakes-Infiltration Field)
Photo
6 (Sludge pitch)
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