During the processing of oil in the refinery, large volumes of water flow through the processing units in the form of Cooling water, Process water, vapour etc. These streams pick up small quantities of hydrocarbons or Oil from equipment or from spillage. The objective of the treatment is to separate the oil from the waste water before it is allowed to be discharged into the sea or any water body with very minimum negative environment impact.
This treatment work happens at the Effluent Treatment Facility (ETF) of Refineries and it involves the removal of free or emulsified oil contained in waste water streams and a Biological treatment in the aerated lagoon before it is discharged to the sea. The treatment consists of free or emulsified oil removal (Pre treatment), Sulphide removal, BOD/COD reduction through Biological oxidation and suspended solids removal (Post treatment).
The existing facilities for free oil and emulsified oil removal will be reused and extended with Chemical Oxygen Demand (COD) degradation, sulphide oxidation, nitrification and filtration steps. Free oil removal is accomplished in the existing Corrugated Plate Interceptors (CPI). The oil is then sent to the existing wet slop system while the effluent is routed to the Dissolved Air Flotation (DAF) unit for emulsified oil removal.
The skimmed oil from the DAF unit is further handled in the existing oily sludge handling systems. The oily effluent is mixed together with the stripped sour water effluent streams and directed to the pre-aeration equalization tanks. Effluent from the equalization tanks will be mixed with spent caustic. The total effluent is cooled before it enters the pre aeration basins for sulphide oxidation and COD degradation. The effluent in the pre-aeration basins overflows into the bio heaters for nitrogen removal. The effluent leaving the bio-reactors contains biomass which will be recovered in the clarifier and surplus sludge will be de-watered. The treated effluent will be fed through sand filters to remove suspended solids and through a post aeration basin to increase the oxygen content before discharging to the sea. It is required that the treated effluent should contain very minimum ppm of sulphide. The effluent is treated with bacteria to remove sulphides present. The effluent temperature needs to be maintained at a specified temperature to retain bacteria and remove sulphides.
Process Improvement :
This process of maintaining effluent temperature is facilitated by Thermax’s Absorption Chiller. In order to remove sulphides from the effluent to achieve the desired ppm level, bacteria are added to the total effluent. If the temperature of the effluent is higher than specified temperature, bacteria present in the effluent would be killed and thus the process sulphide removal would not be achieved. To accomplish this task, Thermax’s Absorption Chiller is used to maintain the temperature of the effluent at the desired levels and facilitate sulphide removal.