Industrial wastewater treatment solutions in refining

More than ever, refining and petrochemical facilities are under pressure to increase efficiency and achieve the goals of sustainability initiatives. Processing heavier, sulfur-laden crude oil increases levels of difficult-to-remove metals and contaminants. As a leader in process technology and contaminant removal solutions, Honeywell UOP receives lots of questions from customers in refinery and petrochemical plants on best practices for water and wastewater management. Below are some of the frequently asked questions we receive. Don’t see your question below? Ask a question here.

A: Typical refinery wastewater treatment include primary treatment such as oil/water separation, and a clarifier to remove solids; secondary treatment to remove organics, metals, and other contaminants; and tertiary treatment for polishing before discharge into the environment. As a secondary treatment, biological wastewater treatment is the most widely used technology for removing organic compounds in refining and petrochemical industries. Biological treatment is generally classified into two categories: 1) suspended growth processes and 2) attached growth processes.

Suspended growth

The most common suspended growth process in refinery wastewater treatment is the activated sludge process. In activated sludge, aerobic microorganisms are suspended in the water column and air is continuously added for both aeration (providing an oxygen source for the microbes) and mixing (maintaining biological solids suspension). The dissolved organic and reduced nitrogenous compounds serve as the carbon “food source” for the microbes, which breakdown (oxidise) the compounds, typically producing carbon dioxide (CO₂).

Attached growth

In attached growth processes, an inert packing material is provided in the bioreactor on which the microbes attach. This material provides a stable surface for the microbes to grow and form a biofilm. In this system the wastewater is passed by the microbes, instead of mixing the microbes throughout the water, as in suspended growth processes.¹

A: In the attached growth process the microbes have a surface to adhere to, allowing them to form a biofilm. Biofilm reactors generally allow for greater solids retention time, which typically results in a high biomass-to-contaminant ratio that promotes efficient organics removal, including biochemical oxygen demand (BOD) and chemical oxygen demand (COD). Fluctuations in wastewater influent contaminant levels are common in refinery and petrochemical plants running a multitude of process units. Attached growth bioreactors can minimize the impact of toxic shocks and organic overload on system performance, while establishing a stable microbial population and handling influent fluctuations better than activated sludge processes.

A: Hard to degrade complex organics such as aromatics (BTX), phenolics and polynuclear aromatics present challenges for many wastewater treatment processes. Complex organics contribute to high levels of chemical oxygen demand (COD). High solids retention time is required to efficiently degrade these compounds, which are difficult for traditional activated sludge processes to treat.

Scrubbing hydrogen sulfide creates spent sulfidic caustic wastewater containing sodium sulfide (Na₂S), which is toxic to microorganisms, along with organic compounds. Elevated levels of sulfide can promote the growth of filamentous bacteria, which can result in severe sludge bulking and loss of biomass from aeration basins. Wet-air oxidation is often used as a pre-treatment for spent sulfidic caustic streams to oxidise sulfur and breakdown complex organic compounds, but it results in high operating and maintenance costs.

Processing heavy sour crude oil can result in higher levels of metals and other contaminants like selenium. The biological oxidation and reduction of many metal species can prove difficult for traditional wastewater treatment processes.

A: Selenium is an essential trace element for many organisms. However, selenium discharges are an environmental concern because of bioaccumulation by algae and subsequent toxic concentrations in aquatic organisms higher in the food chain. Elevated aquatic selenium levels are toxic to fish and can cause waterfowl deformation and mortality.

Many refineries are seeing increased regulatory limits on selenium discharges, typically at the micrograms per liter (µg/L) level. Many refineries today are processing heavier, high-sulfur sour crudes, which also contain higher levels of metals and other contaminants like selenium. Selenium levels vary based on the crude source, but are generally present in crude oil at ~1:1000 ratio to sulfur. Selenium is distributed across various water sources and refinery process units. The highest concentrations are typically found in the sour water stripper.

Various forms of soluble selenium species are found in refinery wastewater, in particular selenocyanate, selenite and selenate. Selenite and selenate are particularly difficult to remove from wastewater. Selenocyanate can be biologically oxidized (aerobically) to selenite/selenate, and then selenite/selenate can be biologically reduced (anoxically) to elemental Se.

A: Honeywell UOP’s XCeed™ bioreactor system is an advanced, biofilm treatment technology ideal for bulk contaminant removal, along with difficult to degrade organics, from industrial wastewater streams. Based on Honeywell’s immobilised cell bioreactor technology, the system has delivered exceptional results in more than 50 installations worldwide. The technology has been directly applied to wastewater applications as diverse as food and beverage, textile and garment industries, mining, specialty chemical plants and refinery and petrochemical plants. It can remove more than 99 percent of organics and up to 95% of inorganic contaminants from waste streams.

A: The system uses multi-chamber packed beds with quasi-plug flow design that provides long solids retention times while minimizing the hydraulic retention time. The increased solids retention time results in a high biomass-to-contaminant ratio that promotes biological oxygen demand (BOD) removal while minimising sludge formation. The system’s design incorporates unique mixed-media support for immobilized bio-catalysts. High concentrations of immobilized bacteria provide natural resistance to, and fast recovery from, process upsets. The high rates of BOD reduction and flexible process configuration also allows for a small system footprint. Unique up-flow design eliminates the need for mechanical mixing, while compact packed bed design minimizes aeration requirements, resulting in both low energy and maintenance costs and minimal operator attention. The XCeed bioreactor is most commonly provided as a pre-fabricated modular unit. For larger installations, it can be built onsite as a concrete or panel tank system, or as a retrofit for existing aeration basins.

For more information on UOP’s XCeed Bioreactor technology, technical papers, customer case studies and more, visit us online.

References:

1. Petroleum Refining Water/Wastewater Use and Management, IPIECA Operations Best Practice Series, 2010.

Read the article online at: https://www.hydrocarbonengineering.com/special-reports/01072016/industrial-wastewater-treatment-solutions-in-refining-3616/