Wastewater treatment and disposal methods must normally meet environmental regulations. These regulations usually consider technology limitations, volume of discharge and concentration of pollutants, method of disposal, nature of the receiving stream, etc. Some wastewater streams either with or without treatment may be recycled for use in systems requiring water of lesser quality. Final treatment usually requires removal of oil and suspended solids, reduction of biochemical and chemical oxygen demand (BOD and COD), and removal of toxic contaminants.
Surface disposal of plant wastewater (except for oncethrough cooling water system effluent which can generally be discharged directly if chlorine residuals are acceptable) using properly-lined evaporation ponds is sometimes allowed and can be used where evaporation rates are favorable. Underground disposal of wastewaters is permitted in some instances. In such cases, a thorough geological investigation is often required to ensure against contamination of protected aquifers. Removal of suspended solids and additional treatment to ensure compatibility among the various wastewater sources and the receiving aquifer must frequently be implemented in order to eliminate plugging of the underground reservoir.
Treatment of wastewater prior to disposal to surface streams is almost always required. Reuse or recycle of wastewater streams within the plant can be considered and is often required by regulatory agencies. This often reduces overall plant wastewater treatment requirements significantly.
Boiler blowdown streams are alkaline (pH ~ 10), low in volume, and generally high in total dissolved solids content (although blowdown from high pressure boilers is normally low in dissolved solids). After cooling, this wastewater is normally compatible with other wastewater streams but may require neutralization prior to discharging. Because it is usually oil free, boiler blowdown may bypass any conventional oil-water separation step. High pressure boiler blowdown can be reused as cooling tower makeup or cooled, deionized, and reused as boiler makeup.
Cooling tower blowdown is often the major source of plant wastewater. Discharge limitations may apply to concentrations of total dissolved solids, specific ions such as sulfate, suspended solids, corrosion inhibitors such as hexavalent chromium or phosphates, biological control agents including chlorine, and various miscellaneous chemicals. The quantity of cooling tower blowdown may frequently be reduced, thereby reducing treatment requirements, by observing higher circulating water control limits, using more effective scale inhibitors, or treating cooling tower makeup water or a sidestream from the system. If the dissolved solids level exceeds established discharge limits, it is usually necessary to blow down at a higher rate in order to lower the concentration, although in some situations pretreatment of the makeup water will effect the necessary reduction. Another alternative would be to
concentrate the blowdown by mechanical evaporation to facilitate disposal. High sulfate concentrations may result from using sulfuric acid for neutralization of alkalinity. Where sulfate concentrations are specifically limited, use of another acid, such as hydrochloric, precipitation softening of makeup water, or use of a scale inhibitor to allow operation at higher alkalinity levels are possible approaches to resolving this problem. Suspended solids can, of course, be reduced by filtration. Sidestream filtration, which would also improve cooling system operation might be utilized. A non-chromate corrosion inhibitor can be substituted for a chromate-based material, or the hexavalent chromium can be removed by chemical reduction, precipitation, clarification, and filtration or by ion exchange. Phosphate compounds can be handled by chemical or biological methods. If high intermittent concentrations of biological control agents are unavoidable, diversion of the blowdown to a holding pond for blending with better quality blowdown may be necessary.
Surface disposal of plant wastewater (except for oncethrough cooling water system effluent which can generally be discharged directly if chlorine residuals are acceptable) using properly-lined evaporation ponds is sometimes allowed and can be used where evaporation rates are favorable. Underground disposal of wastewaters is permitted in some instances. In such cases, a thorough geological investigation is often required to ensure against contamination of protected aquifers. Removal of suspended solids and additional treatment to ensure compatibility among the various wastewater sources and the receiving aquifer must frequently be implemented in order to eliminate plugging of the underground reservoir.
Treatment of wastewater prior to disposal to surface streams is almost always required. Reuse or recycle of wastewater streams within the plant can be considered and is often required by regulatory agencies. This often reduces overall plant wastewater treatment requirements significantly.
Boiler blowdown streams are alkaline (pH ~ 10), low in volume, and generally high in total dissolved solids content (although blowdown from high pressure boilers is normally low in dissolved solids). After cooling, this wastewater is normally compatible with other wastewater streams but may require neutralization prior to discharging. Because it is usually oil free, boiler blowdown may bypass any conventional oil-water separation step. High pressure boiler blowdown can be reused as cooling tower makeup or cooled, deionized, and reused as boiler makeup.
Cooling tower blowdown is often the major source of plant wastewater. Discharge limitations may apply to concentrations of total dissolved solids, specific ions such as sulfate, suspended solids, corrosion inhibitors such as hexavalent chromium or phosphates, biological control agents including chlorine, and various miscellaneous chemicals. The quantity of cooling tower blowdown may frequently be reduced, thereby reducing treatment requirements, by observing higher circulating water control limits, using more effective scale inhibitors, or treating cooling tower makeup water or a sidestream from the system. If the dissolved solids level exceeds established discharge limits, it is usually necessary to blow down at a higher rate in order to lower the concentration, although in some situations pretreatment of the makeup water will effect the necessary reduction. Another alternative would be to
concentrate the blowdown by mechanical evaporation to facilitate disposal. High sulfate concentrations may result from using sulfuric acid for neutralization of alkalinity. Where sulfate concentrations are specifically limited, use of another acid, such as hydrochloric, precipitation softening of makeup water, or use of a scale inhibitor to allow operation at higher alkalinity levels are possible approaches to resolving this problem. Suspended solids can, of course, be reduced by filtration. Sidestream filtration, which would also improve cooling system operation might be utilized. A non-chromate corrosion inhibitor can be substituted for a chromate-based material, or the hexavalent chromium can be removed by chemical reduction, precipitation, clarification, and filtration or by ion exchange. Phosphate compounds can be handled by chemical or biological methods. If high intermittent concentrations of biological control agents are unavoidable, diversion of the blowdown to a holding pond for blending with better quality blowdown may be necessary.
Process water is usually contaminated with hydrocarbons and may be sour (contain dissolved hydrogen sulfide). Normally all wastewater contaminated with heavy hydrocarbons is handled with an API separator or corrugated plate interceptor where the oil is skimmed from the water. Wastewater contaminated with light hydrocarbons is often handled with a flash tank where the hydrocarbon vapors are sent to flare. Sour water is normally handled with a sour water stripper where the hydrogen sulfide is vented to incinerator or flare or sent to a sulfur recovery unit. Stripped sour water may be reusable as cooling tower or boiler makeup. Additional treatment may be needed before reuse is possible. In some cases, conventional treatment techniques are not adequate for process wastewater treatment. Such supplementary techniques as air flotation, biological treatment, activated carbon adsorption, ion exchange, ultraviolet irradiation, ozonation, chlorination, hydrogen peroxide oxidation, and precipitation and clarification may be necessary.
Fig is an example of a complete wastewater treating system. Produced water is normally contaminated with hydrocarbons, may be sour, and may range from very low to extremely high total dissolved solids. Treatment can be similar to that of process water. Produced water is often reusable in reservoir formation flooding operations. Ion exchange process waste streams can have high concentrations of sodium chloride, acid or base, and are high in total dissolved solids. Adjustment of pH may be necessary before discharge with the other plant wastewater streams. In some cases, concentration by evaporation is needed to facilitate disposal.
Reverse osmosis wastes are generally not as high in total dissolved solids and can frequently be reused as cooling tower makeup. Wastewater disposal requirements can often be significantly reduced by selecting the proper makeup water treatment system. Chemical cleaning wastewaters can be acidic or alkaline, and contain high concentrations of dissolved metals. Adjustment of the pH, chemical precipitation of the metals, and removal of the precipitates by clarification and filtration is often necessary.
Plant runoff waters may have to be retained for flow equalization and oil and suspended solids removal prior to discharge. Adjustment of pH may also be necessary. If contaminated from process leaks, treatment similar to that described above for process water may be required, depending upon the degree and type of contamination. Removal of heavy metals, such as dissolved iron, is sometimes required. This is often accomplished by raising the pH to precipitate the metals as oxides or hydroxides, and then clarifying and filtering. Other approaches may be needed, depending upon the type of contamination and required degree of removal. Plant sewage is generally treated by extended aeration, a modification of the activated sludge process. Package treatment plants are available for this purpose.