PROCESS WATER REUSE

Karen S. Eble, Jennifer Feathers
Betz Industrial
Trevose, Pa.

Once the contaminants from process water streams have been identified, refiners must match these to appropriate treatment and reuse options to create an optimal water reuse program.

The selection of treatment options depends on the contaminants present and their concentrations.

This second of two articles discusses the available treatment methods and the contaminants for which they are most suitable.

WATER CLEANUP

In many cases, water that has been used once in the refinery is not of appropriate quality for immediate recycle.

This does not mean that the stream must be treated in the waste treatment plant or sour water stripper. It might be more economical to minimize a particular contaminant, or type of contaminant, before recycle.

A minimum of treatment in a small on site contaminant-specific treatment facility is often sufficient to make the stream acceptable for reuse. Table I lists treatment methods appropriate for various contaminants.

Because of the tremendous amount of contaminants in refinery water, several types of treatment may be required before a stream is suitable for reuse. It is critical that the best method for cleanup be chosen.

Many factors play a part in the selection of methods, including:

Determination of which method is best suited for the specific contaminants targeted for removal
Cost of equipment and installation, with regard to available capital
Cost of operating and maintaining equipment, including future cost factors
Piping and pumps required for water transport
Cost of present water source
Cost of waste water discharge treatment
Flow rate of stream to be treated (continuous vs. fluctuating)
Quality of stream to be treated and variability of quality.
To follow are descriptions of the equipment and processes most commonly used to treat water targeted for reuse.
CHEMICAL OXIDATION
Oxidizing agents are used to reduce the potential for microbiological growth and chemical oxygen demand (COD) of a stream before discharge or reuse. Such agents include chlorine, chlorine dioxide, hypochlorite, potassium permanganate, peroxide, ozone, and oxidizing biocides.
Note that some units are unable to tolerate any free chlorine in the influent. Sulfite is most commonly used to neutralize free chlorine.
AIR STRIPPING
Air stripping is the process of transferring undesired contaminants (ammonia, volatile organics, etc.) from water to air. This is accomplished by injection of water into air by means of spray systems, spray towers, packed towers, and diffused or mechanical aeration systems.
Factors that determine the effectiveness of air stripping are contact area, contaminant solubility, temperature, and the diffusivity of the contaminant in air and in water.
PHYSICAL SEPARATION
Physical separation is used to reduce total suspended solids (TSS) and turbidity. There are a few processes that accomplish solids/water separation effectively.
In many applications, the separation can be enhanced and accelerated by the addition of inorganic and organic coagulants (polymers).
AIR FLOTATION
Air flotation units are specifically designed for removal of oil and grease as well as suspended solids. They are generally preceded by a gravity separation unit, such as an American Petroleum Institute (APT) or CPI separator, to remove gross quantities of free oil and settled solids.
With proper operation and chemical treatment, these units remove both free and emulsified oil, grease, and suspended solids at 8595% efficiency.
Separation in a flotation unit is dependent on the density difference between the particle and the fluid (water). To enhance this driving force, low-density buoyant gas bubbles can be added to attach to the contaminants, thereby increasing the density differential.
The gas most frequently used in this process is air. The flotation process-dissolved air or induced air flotation (DAF and IAF) -is defined by how the air bubbles are formed and attached to the solids.
FILTRATION
Filters use anthracite, or a combination of media (commonly called "mixed bed") to remove suspended solids. Most systems are downflow units, which operate by gravity flow.
Upflow units are operated under pressure. They are designed specifically for TSS and turbidity reduction and do not remove dissolved solids.
Most filters continuously produce effluent quality of < 5 nephelometric turbidity units (NTU) with proper operation. Often, coagulants can be added to enhance removal of solids across the filter.
CLARIFICATION
Clarification is used to separate suspended solids from water by coagulation, flocculation, and sedimentation.
Suspended particles are destabilized by charge neutralization, which allows them to come together to form a floc. The floc settles to the bottom of the clarifier, leaving clear water as the effluent.
BIOLOGICAL TREATMENT
Biological treatment is used primarily to remove soluble and insoluble organics and inorganics from waste water after the primary separation process.
The soluble contaminants -biological oxygen demand (BOD), COD, hydrocarbons, ammonia, phenol, phosphates, and some heavy metals -act as biological "food" to the microorganisms. The microorganisms convert this food to a microbial suspension, which can be settled out easily from the waste water.
The most common industrial biological treatment is the use of activated sludge. The activated sludge process brings waste water into contact with microorganisms under aeration, which is required to provide the dissolved oxygen necessary for biological degradation of organics.
A desired concentration of bio-life, commonly called "mixed liquor," is maintained in an aerated basin. The secondary clarifier is used to settle the microorganisms after they have absorbed the contaminants from the waste water. Some of the concentrated mixed liquor is returned to the basin, and some is wasted to maintain the desired concentration.
Optimization of the activated sludge plant requires integrating mechanical, chemical, and operational control for the most effective treatment program.
SOLIDS REMOVAL
Solids removal processes are used to remove a variety of contaminants from water. To date, most applications are used as a tertiary step in waste water treatment, because they are designed to treat low-turbidity waters with relatively small amounts of suspended contaminants.
These processes would be applicable, however, to many refinery process water streams.
CARBON FILTERS
Activated carbon filters are used:
- To reduce the dissolved organic content-total organic carbon (TOC), COD, and color-of the water
- To remove heavy metals, chlorine, and chloramines
- To reduce TSS and turbidity.
The effectiveness of a carbon filter at removing the desired constituents is called its "adsorptive capacity." A carbon filter has a definite point of exhaustion, at which it must be regenerated or the carbon replaced.
LIME SOFTENING
The lime softening process removes calcium hardness, magnesium hardness, phosphates, silica, and alkalinity from water. It also removes metals such as copper, iron, lead, and zinc.
When caustic and/or soda ash are used in addition to lime, calcium noncarbonate hardness is also removed. (Conductivity is not reduced in lime softening.)
Softening is the conversion of the aforementioned contaminants to insoluble contaminants, which are then removed by mechanical separation. Frequently, carry-over occurs in these units in the form of turbidity and precipitation products of the softening reaction. Polymers are often used to control carry-over.
A hot or warm process softener reduces hardness and silica more effectively than a cold lime softener because it operates at higher temperatures, and because calcium carbonate, magnesium hydroxide, and magnesium silicate are less soluble at higher temperatures.
DEMINERALIZATION
A demineralization system uses the process of ion exchange to create high-purity water. Cation and anion resin beds are used to exchange H+ and OH- ions, respectively, for removal of contaminants in the inlet stream.
These units are closed vessels, which frequently have a degasifier for CO2 removal between the cation and anion units. They are also exhausted after a period of time, causing the cation unit to leak small amounts of sodium, which exits the anion unit as sodium hydroxide. When this occurs, the unit requires regeneration.
REVERSE OSMOSIS
Reverse osmosis is a separation technique that employs a semipermeable membrane cell. A pressure differential is created to drive fresh, clean water to one side of the cell while concentrating contaminants on the inlet, or rejection, side of the cell.
The inlet stream requires very low-turbidity water - generally < 1 NTU-so the unit is usually preceded by a cartridge filter. The membrane cannot tolerate free chlorine or oil and grease and does not remove any dissolved gases.
In this process, fresh water is literally "squeezed" out of the feedwater as a result of the pressure differential. A typical single-cell reverse osmosis system has a 75% volumetric recovery rate; the remaining 25% is the concentrated reject stream.
Typically, reverse osmosis removes 92-95% of the dissolved salts from the water.
EDR
Electrodialysis reversal (EDR) is a method for extracting and concentrating ions in solution by the use of an electric field. The field allows ion passage through anion-selective and cation-selective semipermeable membranes.
The applied direct-current (dc) electrical field inhibits the ions from passing through the membranes, which allow neither diffusion nor convection.
These units are good for treating high total dissolved solids (TDS) water (up to 10,000 ppm TDS). A 1,000 ppm TDS feedwater produces water of 3-5 ppm TDS. Weakly ionized contaminants such as silica, however, are not removed.
The membranes are stacked sheets, each sheet having 100-1,000 unit cells. Feedwater is introduced into alternate membrane compartments. The applied dc voltage draws anions to the anode and cations to the cathode, producing alternate concentrated and purified solutions as the final effluent streams.
A single-stage unit gives 50% concentrate and 50% purified water. Frequently these units are used in a series of 2-6 stages to increase the quantity of purified water and decrease the concentrated stream.
The dc current flow is reversed several times per hour with resultant cleaning and descaling. This prevents dissolved salts and metals from accumulating at the cathode and anode without the use of large quantities of acid or completing agents.
PROCESS WATER REUSE
Because process water streams are many and varied, and individually comprise a small percentage of overall refinery water use and production, they tend to be lumped together in the overall refinery water balance and overlooked when considering water reuse options.
A complete refinery reuse plan requires knowledge of the individual process water streams and their contaminants. Possible treatment processes have been suggested for these contaminants so that their levels can be reduced to an acceptable range.
Copyright 1992 Oil & Gas Journal. All Rights Reserved.