TWO-STEP PROCESS TREATS VARIETY OF WASTE WATER STREAMS

With the advent of more stringent regulations affecting the disposal of contaminated waste water streams, industrial generators must consider treatment processes that separate and concentrate contaminants to ensure producing an effluent that meets discharge criteria.

An innovative new technology, the PO**WW**ER process, treats contaminated waste water by vaporization of most of the water and volatiles, followed by catalytic oxidation to destroy the remaining volatile and nonvolatile contaminants. As a result, essentially pure condensed water is produced, says Fredric Schwartz, general manager, PO**WW**ER process systems, for ARI Technologies Inc., Pallatine, III.

The nonvolatile constituents are concentrated as brine solution, which can be reused, further processed, or disposed of safely.

Schwartz says the technology is best applied to isolated "problem" waste water streams. In the refinery, candidate streams include water from the sour water stripper and alkylation unit.

The process was developed by Chemical Waste Management (CWM) of Oak Brook, III., which subsequently entered into an agreement with ARI Technologies to act as exclusive licensor in the U.S., Canada, and Mexico.

ARI Technologies, who developed the fluidized bed catalytic oxidation system technology used in the process, will be responsible for the marketing, engineering, and construction of PO**WW**ER units. Marketing plans will be directed toward chemical manufacturers, refiners, commercial industrial, and municipal waste water treatment plants, and landfill operators.

The first commercial application of the process is under construction as part of a $125-million hazardous waste treatment plant contract, according to ARI Technologies. The contract is between Enviropace Ltd. - a joint venture affiliated with Waste Management International plc-and the government of Hong Kong.

The unit will have the capacity to treat 50 gpm of waste water containing varying levels of toxic contaminants. It is expected to produce a nearly equivalent volume of pure water for reuse in the plant. The facility is scheduled to begin operation at the end of the year.

PROCESS DESCRIPTION

This treatment alternative combines evaporation and catalytic oxidation, which separates and individually treats volatile and nonvolatile organic compounds, volatile inorganic compounds, salts, and metals. The result is a concentrated inorganic slurry that may have reclamation value or will lend itself to further stabilization to yield an approved landfill product.

The purified water can be recycled back to the front end of the plant for reuse, or if the appropriate National Pollutant Discharge Elimination System (Npdes) permit has been obtained, it can be discharged.

Evaporation is a nonspecific separation process; all volatile material is vaporized to some degree (depending on the relative volatility of each compound and the mixture present).

This nonspecificity leads to the contamination of the distillate and has been the main drawback of evaporation technology within the waste treatment industry. The distillate from the evaporator requires further treatment to remove or destroy the volatile contaminants.

The process technology, according to Schwartz, uses this lack of specificity as an advantage in the complete treatment of waste waters.

EVAPORATOR

The evaporator has three main components: the heater, vapor body, and entrainment separator (Fig. 1).

As feed enters the evaporator system, it is mixed into a recirculating brine. The brine is pumped through a heat exchanger where it is heated. The material then returns to the vapor body, where the mixture boils, and the vapor exits the vessel to the entrainment separator.

Mist particles from the boiling action in the vapor body are removed in the entrainment separator. The vapor then passes into the oxidation system.

The concentrated brine is removed from the system through batch or continuous purging. The resulting brine - only a small fraction of the original waste volume - can be treated by stabilization.

All metals in the brine can be stabilized to below U.S. Environmental Protection Agency (EPA) limits for the reaching potential of metals, as measured by the Toxicity Characteristic Leaching Procedure, or TCLP.

The process utilizes catalytic oxidation of the volatile compounds to prevent contamination of the product water. The volatile materials are oxidized in the presence of oxygen on the surface of the catalyst. This process converts the organic material to CO2 and the inorganic volatile material, such as ammonia, to nitrogen.

The inert gases (CO2, N2, etc.) are then vented from the system as the stream is condensed to product water.

The oxidizer has three main components: the recuperative heat exchanger, the reactor heater, and the catalytic reactor.

The inlet vapor is preheated, along with oxidation air, in a recuperative heat exchanger, with the vapor exiting the reactor. Volatile materials contained in the vapor stream are oxidized at the elevated temperature (700-1,000 F.) in the fluidized catalyst bed.

The catalyst is a proprietary metal oxide catalyst on a specific support. The metal oxide is composed of a nonprecious metal; therefore, it is not as expensive, nor as limited in versatility, as precious metal catalyst.

The catalyst has been designed to withstand problems common to precious metal catalysts, including fouling, activity suppression, and poisoning. Because of the nature of the catalyst, change-out is not required; catalyst makeup alone is sufficient to maintain activity, says Schwartz.

During oxidation, acid gases (e.g., HCl) may be formed from the conversion of some of the volatile compounds. These gases are removed by the scrubber. The scrubber may be wet or dry, depending on design criteria.

The product vapor is condensed after the scrubber or vented to atmosphere. Vent purges of noncondensible gases are required on heat exchangers.

The condensed water meets stringent discharge standards (chronic bioassay). It can be reused as cooling tower makeup, boiler makeup, or process feed water. It can also be discharged to a publicly owned treatment works (POTW) or, as previously mentioned, via Npdes-permitted discharge.

APPLICABILITY TESTING

Potential treatment streams are tested in the laboratory to demonstrate the ability of the process to concentrate inorganics while yielding a clean water phase.

The next step in the evaluation process involves a detailed pilot plant study to further expand the analytical data to be used in engineering a commercial-scale unit.

PILOT PLANT RESULTS

This technology has been demonstrated in a 0.5 gpm pilot plant at CWM's Lake Charles, La., facility. The pilot plant data show that the process is effective for treating waste waters containing inorganic salts, metals, volatile organics, and nonvolatile organics.

Schwartz says the system easily qualifies as Best Demonstrated Available Technology (BDAT). The product water also meets strict discharge standards.

Some of the key results from tests of typical waste water streams are:

• High concentration ratios were achieved, Waste waters were concentrated from 0.5-2.0 wt % total dissolved solids (TDS) to a slurry containing 60-70 wt % total solids.

• High heat transfer coefficients were maintained. Relatively high steady-state heat transfer coefficients were maintained throughout the tests.

• High catalytic oxidation efficiencies were attained. Total organic carbon (TOC) oxidation efficiencies as high as 99.9%, and specific compound oxidation efficiencies of 99.99% were achieved. Feed TOC concentrations of 500-3,000 ppm were reduced to

• Product water quality was high. Priority pollutants (as defined by EPA under the Resource Conservation and Recovery Act) were oxidized to below 30 ppb, and in most cases, to below detectable limits. Acute toxicity tests were successful at 100% product water.

• Brine was successfully stabilized. All metals in the resulting brine were stabilized to below TCLP characteristic limits using conventional stabilization technology. No priority pollutants were identified in the TCLP leaching solution.

Table 1 presents pilot plant test data for two waste water streams.

TREATMENT COSTS

The costs associated with treating complex waste water are relatively high compared to conventional waste water treatment technologies such as metal precipitation or biological treatment. But combining treatment processes for specific streams (e.g., metals precipitation plus organic oxidation plus carbon polishing) makes treatment costs prohibitive without a comprehensive treatment system such as the vaporization/catalytic oxidation process.

For treating hazardous wastes, this new technology is competitive with conventional disposal charges. Typical commercial rates range from a low of 2-3/gal for a large system, to 50/gal for a VOC-contaminated stream. (Of course, treatment costs for any specific site are a function of many different parameters.)

The technology can also provide a reduction in downtime while increasing reliability, says Schwartz. In addition, the system will not be obsolete before the end of its payback period, which allows for an extended depreciation schedule.

ADVANTAGES

What makes this system unique, according to Schwartz, is its ability to treat water contaminated with a mixture of volatile and nonvolatile contaminants containing salts and metals. Conventional technologies can treat only individual categories of contaminants, relying on the addition of other equipment to meet regulatory specifications.

And while traditional treatment systems can require acreage, this system has a relatively small footprint. Its modular design makes it equally efficient for process or remedial operations.

The process is applicable to all streams amenable to an evaporation process. And with the organic oxidation capabilities, the technology is ideal for waste waters that contain only organics or a mixture of organics and inorganics.

Such applications include volume reduction applications where organic levels are substantially high for simple stripping operations. Likewise, waste waters that contain both high levels of inorganics and low levels of organics are ideal for treatment by this method.

And there are no chemical additives to the process; therefore, the amount of solid waste is minimized.

The process technology lends itself well to the incorporation of a more energy-efficient mechanical vapor recompression evaporation process. Other evaporator types may be more favorable; for example, falling film, depending on the brine chemistry.

Other types of catalysts may also be utilized for more efficient operation.

The key to the process is that it provides effective volume reduction while producing a high-quality effluent. High volume reductions, high heat transfer coefficients, and high oxidation efficiencies have been attained on the pilot-plant scale.

EPA EVALUATION

The U.S. EPA has included the process technology in its Superfund Innovative Technology Evaluaton (SITE) program. The demonstration took place Sept. 1424 at the Lake Charles pilot facility.

Copyright 1992 Oil & Gas Journal. All Rights Reserved.