Handling process byproducts has always been a concern for ethylene producers. When it comes to wastewater treatment, spent caustic has always been the most challenging waste. In water-stressed locations, treating toxic components is not the only factor in achieving wastewater treatment and water recycling goals affordably.
Part One of this article addresses the cost-saving implications of combining Zimpro® wet air oxidation (WAO) (Figure 1) with a biological and powdered-activated-carbon treatment (PACT) plus membrane bioreactor (MBR) technology in a compact, comprehensive system to support water-recycling goals. Water Recycling Efficiency In Ethylene Spent Caustic Treatment, Part II: Process provides a more in-depth look at the technology of the process and addresses the compliance implications of using it in water-starved regions such as the Middle East.
Figure 1. Wet air oxidation breaks down the sulfide-laden caustic stream into inert salts and biodegradable short-chain compounds, enabling WAO + Advanced Biological Polishing effluent to be discharged directly into a seawater environment.
A Tall Challenge
The biggest challenge to recovering water from ethylene production waste water treatment is the fact that it contains a lot of salt. This is typically the result of the need to polish the high TDS, oxidized spent caustic from wet air oxidation or incineration.
Using a divide-and-conquer approach to treat the salt-laden oxidized spent caustic stream separately can be economically attractive, rather than diluting it in a larger integrated wastewater stream where it can complicate both biological treatment processes and reuse efforts. Segregated treatment is particularly desirable because oxidized spent caustic typically represents less than a few percent of the total flow to the biological wastewater treatment plant (WWTP) yet would represent most of the total salt to the WWTP.
Among the alternatives for segregated ethylene spent caustic treatment — including offsite disposal, incineration, and advanced oxidation — Zimpro® WAO + Advanced Biological Polishing offer attractive cost and compliance alternatives. They help users maximize onsite water recycling and minimize concerns about effluent compliance.
Figure 2. Using WAO + Advanced Biological Polishing allows for complete treatment of ethylene spent caustic, allowing for water reuse of the remainder of the petrochemical facility’s wastewater without reverse osmosis.
A Trio Of Proven Technologies
The combined process uses WAO to destroy sulfides completely in ethylene spent caustic while converting the remaining organics to lower molecular weight compounds that are easily biodegradable before the high TDS stream is polished using a combination of activated carbon and biological treatment followed by membrane clarification.
A Space-Saving Cost-Saving Solution
Packaging three proven technologies into one compact integrated process yields a complete solution that saves space over other approaches and costs less to operate over the long term. It also avoids the downside costs of using evaporation ponds that require a considerable footprint and eventually need to be cleaned out, with the salts being sent to a landfill for disposal.
Figure 3. The bottom-line operating costs of WAO + Advanced Biological Polishing compare favorably to a variety of other approaches that demand expensive fuel or chemicals. (NOTES: (1) Includes electricity, steam, boiler feedwater, cooling water, acid neutralization, nutrient, carbon, membrane cleaning chemicals, and instrument air, (2) Includes electricity, steam, boiler feedwater, cooling water, acid, caustic neutralization, peroxide, and instrument air, (3) Fuel only, (4) NaOCl only, (5) H2O2 only. Maintenance costs are not included.)
Economic Sense For Global Applications
Based on both reduced operating cost and high effluent quality, WAO + Advanced Biological Polishing offer promise in a variety of ethylene production environments around the world where water scarcity and regulatory drivers for water reuse are mounting— including the Middle East, China, India, and Latin America. In the Middle East, for example, the volume of reclaimed water made practical by this recycling-friendly approach reduces the need to desalinate additional seawater to replenish freshwater aquifers.
Equally important, the high-reliability, low-downtime nature of the process addresses key concerns about the unique demands in a petrochemical facility. Learn more about specific process performance aspects of WAO + Advanced Biological Polishing in Water Recycling Efficiency In Ethylene Spent Caustic Treatment, Part II: Process.