The plant which has been in operation since 1995 is designed for the treatment of Domestic wastewater (105.500 equivalent inhabitants), Industrial wastewater (5.000 equivalent inhabitants) and sewage from septic tanks (7.000 equivalent inhabitants), giving a total treatment capacity of 117.500 equivalent inhabitants and a flow rate of 26.000 m3/d. The average flow rate for the summer of 2005 was 19.000 m3/d.
Sewage is treated with the method of Activated-Sludge and sludge with the method of Anaerobic Digestion utilizing the biogas which is produced to generate electricity. The plant is designed to give a 96% BOD5 removal and a 95% SS removal.
The plant comprises from the following units:
INFLUENT WASTEWATER PUMPING STATION
The wastewater after passing through two coarse screens is pumped via 4 Archimedian screw pumps (one spare) to a level of 5,60m higher and from there and on it continues by gravity – flow to the outfall. Each screw pump can deliver 830 m3/h.
SEPTAGE TANK & PUMPING STATION
The septage tank has two chambers with a total volume of 420 m3.
Trucks unload septage from houses not connected to the city’s sewage network and from septic tanks from the whole Prefecture. Prior to the septage tank a pre-treatment unit is installed which removes coarse solids and sand.
FINE SCREENING
Fine screening is carried out with three automatic screens (one spare) with 6mm passage. The activation of the CLEANING rotating drum is achieved with differential air pressure sensors. The screenings are washed, compressed and then transported via a belt conveyor to a SOLIDS container.
GRIT – OIL & GREASE REMOVAL TANK
There are two grit and grease removal tanks. The total volume of each tank is 118m3 of which 75m3 are for grit removal and 43m3 are for grease removal.
Wastewater is aerated via 11 ceramic diffusers. With a design settling time of about 7min, about 90% of the grit with diameter varying between 0.16 to 0.20mm is removed. The floating solids and grease are removed by a skimmer and are led to a basin. Grit is led to an automatic unit where it is dehydrated.
PRIMARY SEDIMENTATION
There are two circular tanks 22m in diameter each. Wastewater enters from the centre of the tank giving a radial flow pattern towards the effluent weir. Sludge removal scrappers are supported on a beam spanning the tank thus sweeping the sludge to the centre of the tank. The efficiency of Primary sedimentation is 33% removal of BOD5 and 60% of suspended solids.
SELECTOR TANK
The tank has a volume of 485 m3 and Primary Sedimentation effluent is mixed with recirculation from Secondary Sedimentation.The tank serves as an anoxic zone in order to inhibit filamentous growth in the aeration tank which would retard the settling of sludge in the Secondary Sedimentation tank and cause foaming problems.
AERATION TANKS
Following the Selector tank wastewater is evenly distributed to four aeration tanks, each having the following dimensions. Length: 60m, width: 8.4 m and effective Depth: 4.5 m. Each tank has an anoxic zone followed by an aerobic zone. Wastewater is aerated through fine porous membrane diffusers which are situated on the bottom of each tank. In addition to carbonaceous organic matter removal the aeration tanks provide partial nitrification – denitrification. The total volume of the tanks is 9072 m3.
SECONDARY SEDIMENTATION TANKS
There are three tanks with a diameter of 33m and a volume of 2.130 m3 each. The flow pattern is the same as in the primary sedimentation tanks and for the effluent, the two (older tanks) have weirs and the newer tank has submerged radial pipes. From these settling tanks activated sludge is recirculated to the selector tank and excessive activated sludge is pumped to the centrifugal thickening unit.
CHLORINATION TANK – SEA OUTFAL
Disinfection of wastewater is achieved by Sodium Hypochlorite solution 15% in active chorine. The tank dimensions are 24m in length, 10m in width and 2.5 m in depth with a volume of 600m3. Contact time is about 30 min giving a bacterial remaining content of less than 176/100 ml colonies. (total coliforms). At the end of the chlorination tank, dechlorination is carried out by a solution of Na2SO3.
The outfall pipe is made of HDPE and has a diameter of 710 mm and a length of 110m reaching a depth of 19m below the sea level.
CENTRIFUGAL THICKENING
Secondary sludge is thickened in two Centrifuges (Decanters) each processing 25 m3/h and 175 kg/h. Thickened sludge is pumped to the pre-thickeners, alternatively sludge can be pumped to the digesters or to the post – thickeners.
PRETHICKENING
Primary and Activated sludge enter the two tanks from the centre. Each tank has a diameter of 8.5m and a volume of 200m3. Sludge moves in a radial flow pattern and settles with the aid of a spanning bridge with vertical pickets (in a comb form). Thickened sludge is withdrawn from the bottom of the tank and is fed to the digesters or alternatively to the post-thickeners.
ANAEROBIC DIGESTERS
Thickened sludge is fed, after been heated by means of two heat exchange units to a temperature of 35οC, with two progressive cavity pumps (Μohno) each having a maximum capacity of 22m3/h and a discharge head of 40m, to two anaerobic digesters whose total volume is 3100m3. Anaerobic digestion operates in mesophilic zone, achieving the stabilization of sludge in the absence of oxygen. The produced biogas contains about 70% methane and is stored in a storage tank with a floating cover after sulfur removal. The tank has a diameter of 12.80m and a volume of 650m3. Biogas is used as a fuel in water boilers which in turn heats the sludge, in an internal combustion engine generating electricity for the plant (operating at 100 KW) and in case of excess it’s burnt by a torch.
POST-THICKENERS
Stabilized sludge is fed from the digesters to two circular tanks with 11m diameter each which work in the same manner as the pre-thickener. Sludge is thickened and then pumped to the dewatering unit. Each tank has a volume of 300 m3.
DEWATERING
Sludge from the post-thickeners is fed to two belt filter presses with a belt width of 2m and maximum load 848 kgss/h of coagulated sludge. The concentration of dewarted sludge is about 20% dry solids.
ADDITIONAL BUILDINGS
The plant includes an Administration building with a fully equipped laboratory for water and wastewater analysis. It also includes a control building at the entrance of the plant, a Repair shop, an Industrial water pump station, various pumping stations and other smaller buildings.
ODOR CONTROL UNIT
The odor control unit has a capacity of 45.000m3/h and it achieves a 99% reduction of the odors that are produced in the plant. Odor control is carried out by a three stage wet scrubber with the aid of chemicals. Ammonia and hydrogen sulfide are removed from the air by using a solution of sulphuric acid, caustic soda and hydrogen peroxide. The odor unit treats air from the inlet works, septage handling sludge tanks and the dewatering unit.
PLANT AUTOMATION
The supervision and control system (SCADA) with additional instruments monitors in real time, supports and improves the operation of the plant. Most of the equipment is operated by SCADA which also keeps historical values of various parameters.
PROTECTION FROM THE SEA
Different projects have been carried out for the protection of the plant from the sea waves
PLANTING OF THE PLANT
The plant area and the road through which the plant is accessed have been planted with trees, bushes and flowers. Additional planting is to be carried out in the future.
FINANCING OF THE PROJECT – ASSETS
Until 31/12/2002 it has been executed and financed:
The first phase of the works 3.643.379 ECU, (1.241.481.489 drachmas) which was financed 70% from the European Community (MOP of Crete), 14,7% from the Greek Government and 15,3% from D.E.Y.A.Ch.
Part of the second phase of the works 1.455.612 ECU (495.999.954 drachmas), which was financed 85% from EC (Temporary Cohesion Funds), 5,25% from the Greek Government and 9,75% from D.E.Y.A.Ch.
The second phase works 7.108.613 ECU (2.422.259.627 drachmas), which was financed 80% from the EC (Final Cohesion Funds), 7% from the Greek Government and 13% from D.E.Y.A.Ch.
Additional projects, studies, assets with a cost of 551.868 EC (188.049.021 drachmas) which were financed from D.E.Y.A.Ch.
The total cost was 12.759.472 ECU (4.347.790.091 drachmas)
D.E.Y.A.Ch.’s funds were covered by Loans from the Deposits and Loan Funds of Greece
The above don’t include the cost of supervision of the construction works and the interests of the loans.
OPERATION OF THE PLANT
Sewage Treatment has been in operation since June 1995 and the sludge treatment (digesters) since August 1998. The odor unit has been in operation since March 2001 and the extension of the plant is in operation since January 2001.
OPERATING DATA OF THE PLANT
The average daily capacity of summer 2005 reached 19000m3/d and the average daily septage handling was 1.000m3/d.
The quality of the plant fully complies with the environmental limits, resulting an average removal of: BOD5 98,5%, solids 95,5% and ammonia 99%.
After disinfection the microbiological quality is equivalent to the the quality of surface water (<100 coliform /100 ml of water).
D.E.Y.A.Ch. collaborates with the GREEK CENTRE FOR MARINE RESEARCH (ELKETHE) in order to monitor the plants effect on the marine environment. The results are exceptionally good for the operation of the plant.
In 2005 the plant produced about 1100 tons of sludge (in dry solids).
The total cost for the operation of the plant for 2002 was 0,3266 €/m3 of treated wastewater.
Legent
- ENTRANCE GUARDROOM
- ATTENDANTS HOUSE
- ADMINISTRATION BUILDING CHEMICAL & BIOLOGICAL LABS
- INFLUENT PUMPING – STATION (SCREW PUMPS)
- FINE SCREENS ROOM
- DEBRIS CONTAINER & SAND SILO
- SUBSTATION
- AIR COMPRESSOR BUILDING
- SEPTAGE TANK & PUMPING-STATION
- GRIT , OIL & GREASE REMOVAL TANK
- FLOW DISTRIBUTION FOR PRIMARY SEDIMENTATION
- PRIMARY SEDIMENTATION TANK
- PRIMARY SEDIMENTATION PUMPING-STATION
- MAIN FLOW DISTRIBUTION TANK FOR AERATION TANKS
- FLOW DISTRIBUTION TANK FOR AERATION TANKS
- AERATION TANK
- RE-CIRCULATION PUMPING- STATION OF MIXED LIQUOR
- FLOW DISTRIBUTION FOR SECONDARY SEDIMENTATION
- SECONDARY SEDIMENTATION TANK
- RE-CIRCULATION PUMPING-STATION OF EXCESS SLUDGE
- FLOW MEASUREMENT (VENTURI)
- CHLORINATION TANK
- FLOW DISTRIBUTION & OUTFALL LOADING PIT
- CHLORINATION BUILDING
- INDUSTRIAL WATER BUILDING
- PRE – THICKENING TANK
- PRE – THICKENING PUMPING – STATION
- DIGESTOR
- DIGESTION – ENERGY BUILDING
- GAS HOLDER
- DESULPHURIZATION INSTALLATION
- TORCH
- POST-THICKENING
- POST-THICKENING PUMPING-STATION
- DEWATERING BUILDING
- VEHICLE, MACHINERY GARAGE
- CENTRIFUGAL THICKENING BUILDING
- ODOR CONTROL BUILDING
