Waste water produced by the various processes of the refinery includes:
- Waste water from the refinery’s main production units, as well as from the utility units.
- Tank drainage.
- Ballast and oily residues from tanker vessels cleaning operations.
- Wastewater from the Truck Loading Terminal, and from the port facilities.
- Rain water from the process areas, as well as the dikes and collection basins of the refinery tank farms.
Waste water flows can be characterised according to their physical, chemical and biological properties, i.e.:
- Physical properties: the nature of suspended or dissolved solids in the liquid waste, the temperature, the colour and the smell.
- Chemical properties: the nature of chemical contaminants classified as:
- organic (e.g. hydrocarbons, phenols)
- inorganic (e.g. phosphate, pH, chlorides, sulphur compounds), or
- gaseous (e.g. hydrogen sulphide, ammonia)
- Biological properties: microorganisms.
Waste water, depending on its origin and the nature of the contaminants, can be pre-treated in intermediate units, or treated directly in the refinery’s Effluent Treatment Plant.
The main intermediate waste water pre-treatment units processing waste water – prior to its treatment in the Effluent Treatment Plant – are the following:
1. Sour Water Stripper unit
Waste water containing acid gases, from all the units of the refinery, are collected via a separate network, and fed through to the Sour Water Stripper, where the volatile pollutants are removed from the waste water by evaporation. Waste gases are then sent to the Sulphur Recovery units while the clean water is recycled and used in the desalination of crude oil, or is sent to the Effluent Treatment Plant.
2. Neutralisation unit
Alkaline chemical effluents are collected via a separate network, and fed into the Neutralisation unit, where they are neutralised. The chemically neutral waste water is then sent to the Effluent Treatment Plant for further treatment.
3. Neutralisation unit treating effluents from the Alkylation unit
The liquid effluents from the alkylation process, which may contain acid, are treated with lime – Ca(OH)2 solution – in special tanks before being fed into the Effluent Treatment Plant.
Effluent treatment plant
The effluent from the intermediate treatment processes, together with the effluent originating directly from the other refinery units, are sent for treatment in the Effluent Treatment Plant via a separate Oily Water Sewer (OWS) system.
Liquid effluent drained from storage tanks is intermittently fed via enclosed drains to the refinery sewer system for treatment in the Effluent Treatment Plant. During the drainage process the quality of effluents is visually inspected.
Deballasting of tanker vessels is achieved via the ballast pipelines into ballast tanks, where the oily phase is separated from the aqueous phase by virtue of their different specific gravities. After a specific settlement time, the aqueous phase is sent to the Effluent Treatment Plant, via closed pipelines, while the oily phase is fed to the crude oil tanks for re-distillation.
Waste water is treated in the following stages:
Three oil separators (API-3, API-4, API-5), and two tanks (T-2000 and T-2001), are used for the treatment of rain and ballast water. The operation is straightforward and is therefore not explained in detail or shown in the schematic diagram.
In separators API-1 and API-2, the oil is removed by mechanical disc-type separators (Discoil), while the sludge at the bottom is removed using chain-scrapers and sludge pumps. The recovered oil is fed into the refinery’s oil recycling system (slops), while the sludge is sent to the dehydration unit.
The waste leaving API-2, is sent to a Dissolved Air Flotation unit (DAF-1). Chemicals are added prior to flotation to achieve flocculation of hydrocarbons, while the separated oil that floats on the surface is collected and fed into the refinery’s oil recovery system (slops). The treated water is then pumped to the Sand Filters for further hydrocarbon content reduction.
Following the Sand Filters, waste water undergoes a Biological treatment process that utilises four BIOFOR type Biofilters. Before the Biofiltration stage, waste water is mixed, in a balancing tank, with a recirculation stream from the Biofilters exit. This allows for the pollutant content to be controlled (reduced), and for the efficiency and life expectancy of the Biological Treatment unit to be maximised. In the Biofilters, the organic load of waste water is biologically degraded by microorganisms that develop in the media bed of the Biofilter. Oxygen is introduced into the system by the addition of compressed air.
Following Biofiltration, the treated effluents are discharged to the sea. A certain amount of treated effluent is stored in a tank, to be used for washout and cleaning of the Biofilters (with compressed air).
The Biofilters’ washout water is stored in a separate storage basin, and is treated in the Dissolved Air Flotation unit DAF-2. The main purpose of this air flotation is the removal of the suspended solids in sludge form. The treated effluent water from DAF-2 joins the exit stream from DAF-1 to the Sand Filters, being recirculated in the treatment process.
Sludge produced at various stages of the waste water treatment process (mainly from DAF-2, API-1/2, and a small amount separated in DAF-1) is stored in a separate tank, from which is pumped in a Centrifuge for dehydration.
Sanitary waste treatment plant
The sanitary waste water of the refinery is fed into a biological filter for treatment before disposal. The treatment is carried out using the activated sludge method.
Sanitary liquid waste (which does not include oil contaminants), originating from the facilities on the refinery site, is sent for treatment at the Sanitary Waste Treatment Plant by natural flow, or by using tank trucks in areas where pumping is not technically feasible.
The Sanitary Waste Treatment Plant comprises a combination of the following five subsystems:
I. Primary treatment
The aim of primary treatment is the removal of solids from liquid effluent which is achieved by:
Screening: i.e. filtering out of sizable solids.
Sedimentation: removal of organic and inorganic suspended solids.
Solids removal is carried out to reduce the pollution load on the subsequent treatment processes. Primary sedimentation takes place in a settling tank, where the solids are allowed to settle under the force of gravity.
Flow balance: two submerged pumps are installed in the balancing tank, which under controlled flow rate, drive the waste to the Sanitary Waste Biological Treatment unit.
II. Secondary (biological) treatment
Biological treatment is achieved using an activated sludge process comprising:
- An aeration tank by means of which:
- Oxygen is supplied, which is required by the microorganisms that break down organic materials in the waste. The oxygen is provided by generating bubbles in the waste; the bubbles are created by air compressors, which push air through bubble generating devices.
- Mixing is carried out, to avoid the microorganisms settling to the bottom of the tank, and to maintain uniform concentration of microorganisms.
- A secondary sedimentation tank: following the aeration process, liquid waste then enters the secondary sedimentation tank, where the biomass settles down, while the clear effluents are sent to the de-nitrification tank. The settled sludge is recycled, being pumped back into the aeration tanks, at a rate that ensures that the activated sludge content of the system remains at suitable levels.
III. Tertiary treatment
Clear effluent from the aeration process is sent to the de-nitrification tank where, under anoxic conditions, de-nitrification occurs resulting in nitrogen removal.
Disinfection is intended to destroy pathogenic microorganisms present in waste water, so as to prevent the transfer of these microorganisms, via the effluent, to the final recipient (sea). The process of disinfection takes place in the chlorination tank, where sodium hypochlorite is introduced into the system via a dosing pump to achieve a free chlorine content of 0.5-1 mg/l.
V. Sludge silo
The sludge silo stores excess sludge from the sedimentation tank.
After treatment, sanitary effluent passes through a 394 metre-long, under-sea pipeline (together with the treated industrial wastewater, and cooling water), and is discharged into the sea at a depth of more than 30 metres below sea level.
Sea water is used as a cooling agent for the refinery’s heat exchangers; sea water is also fed into the desalination units for producing process water.
The industrial facilities of MOTOR OIL have a procedure – approved by the competent ministry – for the collection and processing of waste generated by ships sailing in its port facilities.
Systematic research into the quality of coastal waters in the vicinity of the MOTOR OIL refinery is being conducted by Professor S. Varnavas of the Laboratory of Applied Geochemistry at the Geology Department of the University of Patras. The objective of this research is the monitoring of water quality in these coastal waters. The latest report contains the following conclusions:
“Variations in temperature and conductivity in the area of survey were found to be within normal values for the corresponding period of measurement for a coastal Mediterranean environment.
Oxygen content in the water during all periods of measurement, including the summer period, indicates good oxygenation levels.
The pH values are at the normal levels of a marine environment.
Comparison of the values of the parameters, as well as of their geographic variations, shows that the conditions remain stable, at normal levels for a coastal Mediterranean marine environment.
The water content per litre of suspended solids, and the concentrations of the elements studied are at low levels. They are within the limits of the concentrations of suspended solids found in other Greek coastal areas.
In conclusion, the low levels of suspended solids found in the coastal waters, and the low concentration of metals found in them, confirm that normal conditions exist in the coastal waters in the vicinity of the refinery”.