Discharge to Cooling Tower

Sending RO reject to the cooling tower is the second most common method of disposing of the reject. The concerns with this method of handling of RO reject are 1) scaling in the heat exchange equipment, 2) tower chemistry, and 3) logistics. 

Calcium, and to a somewhat lesser extent, silica, can cause scaling problems for the cooling tower, particularly if the RO reject is a major portion of the cooling tower make-up. Scaling of the heat exchanger equipment results in a loss in productivity on the process side of the 

Location of the RO system relative to the cooling towers is a concern. In most facilities, the RO is not located near the major cooling towers. There may be a small, local tower located nearby, but discharging to this tower may be difficult as the RO reject could be a major portion of the make-up flow to the tower. Pipelines must be run from the RO to the major cooling towers to allow discharge of the RO reject to these towers. 

Prior to making the decision to discharge RO reject to the cooling tower, an analysis should be conducted to determine what impact the reject will have on tower operations. There is a need to balance conservation and recovery of RO reject water with the impact on the cooling system. 

The use of RO to treat and recover reject from another RO system is becoming more common, particularly in ZLD applications. A secondary RO is used to treat the reject from the primary RO to reduce the thermal evaproative requirements of the ZLD system. 

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Lastly, the possibility of recycling treated effluents as make up to cooling towers will allow cost-effective tertiary purification. Discharge of salts remains the last obstacle that needs to be overcome to guarantee optimum environmental protection. 

It amounts to an average flow rate of around 500 m -h. After treatment, 50% is discharged and around 50% recycled as make up to cooling towers. 

Noncontact cooling water is normally supplied to several processes from the utilities area. The system is either a loop that utilizes one or more evaporative cooling towers, or a once-through system with direct discharge. 

Presently there are no EPA pretreatment standards for the oil and gas extraction (oilfield) point source category. The EPA pretreatment standards for discharge from existing and new petroleum refining facilities to publicly owned treatment works include 100 mg/L each for oil and grease (O G) and ammonia (as N). For new facilities a total chromium concentration of 1 mg/L for the cooling tower discharge part of the refinery effluent is also required (40 CPR Part 419). 

Chemicals such as sulfuric acid are used to control scaling in the system. Biofoulants such as chlorine and hypochlorite are widely used by the industry. These additives are discharged in the cooling tower blowdown. 

A key advantage of the PO WW ER system is its ability to treat water contaminated by salts and metals. Also, PO WW ER can reduce high volumes of aqueous waste while producing a high-quality water effluent that can be used as boiler or cooling tower makeup water or discharged to surface water. 

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