With population growth and increased energy intensity, increased public concern and regulatory guidance is obliging refining and petrochemical manufacturers to exercise far greater care of wastewater. At the same time, more widespread use of heavier sour crude oil necessitates removal of greater concentrations of metals and contaminants from the water that is used in processing.
The water used in refineries typically is treated for separation of solids from oil and water, while at a secondary level organics, metals, and other contaminants are removed. Finally, at a tertiary stage, adsorbents may be used to polish the water before discharge into the environment. Biological wastewater treatment is the most popular technology for removing organic compounds in the petroleum industry.
The activated sludge process – which recycles microbes – is the most common process for treating refinery wastewater. Aerobic microorganisms are suspended in a water column and mixed with air to continuously provide an oxygen source for the microbes and keep the biological solids suspended. The microbes consume the carbon in the dissolved organic and reduced nitrogenous compounds as a ‘food source’ for the microbes, which organically oxidise the compounds and produce carbon dioxide.
Another process is the attached growth process, where an inert packing material in the bioreactor provides a home for the microbes. The packing material provides a stable and extensive surface area where the microbes can flourish and form a protective biofilm that can help the colony withstand chemical insults.
Wastewater flows over or through packing material and where the microbes can consume their food sources. In these systems, packing material may be static, or floating and mixed, within the reactor. These biofilm reactors permit longer retention time, increasing the biomass-to-contaminant ratio and making the organics removal more efficient in terms of biochemical oxygen demand and chemical oxygen demand.
A typical problem with refinery and petrochemical plants, which can comprise several different types of process units, contaminant levels in wastewater can fluctuate widely. Attached growth processes are far better than activated sludge processes at resisting or surviving these toxic shocks, and better maintain consistent performance and a stable microbial population.
Refinery and petrochemical operations produce complex organics including BTX aromatics, phenolics and polynuclear aromatics. These typically are difficult to degrade because they produce high levels of chemical oxygen demand. And, they require high solids retention time to degrade these compounds – something beyond the capability of traditional activated sludge processes.
Another source of contaminants in these plants is hydrogen sulfide scrubbing, which produces sulfidic caustic wastewater that contains sodium sulfide (Na2S) as well as organic compounds. Sodium sulfide (Na2S) is toxic to the microorganisms.
In fact, high levels of sulfide can cause filamentous bacteria to grow, creating severe sludge bulking and clogging while reducing biomass from the aeration basins. A common pretreatment is wet-air oxidation, which oxidises sulfur and breaks down complex organics, in spent sulfidic caustic streams, but which is costly and requires frequent maintenance.
Fracking, or hydraulic fracturing, has proven to be highly effective at coaxing liquid and gaseous hydrocarbons from tight shale deposits. As its efficacy has improved through technological improvement, fracking is credited with massive increases in the productivity of natural gas and oil wells, particularly in North America.
Fracking also requires large volumes of water. In areas constrained by availability of fresh surface water and potable groundwater, fracking operations have begun to rely more on deep groundwater brines. But these brines contain high levels of hydrogen sulfide, and removing them by chemical oxidation or by stripping or adsorption can be expensive due to the high cost of the chemicals required and the environmental limitations of disposing of the sludge.
Here again, these problems can be overcome by an effective high-retention time attached growth water purification process.
The increasingly prevalent processing of heavy sour crude oil brings higher levels of metals and other contaminants such as selenium in the feedstock. Biological oxidation and reduction of many metal species is beyond the capability of traditional wastewater treatment processes.
In fact, selenium is an essential trace element for many organisms, but discharges of selenium can cause bioaccumulation by algae, resulting in toxic concentrations in aquatic organisms higher up in the food chain. High levels of aquatic selenium are toxic to fish and can cause deformation and mortality of waterfowl.
As a result, many refineries are facing stricter regulatory limits on selenium discharges. Selenium levels vary based on the crude used but are generally present at a ratio to sulfur of about one part to 1000. Selenium is found in water throughout refinery process units, but is most often highest in the sour water stripper.
Various forms of soluble selenium species are found in refinery wastewater, in particular selenocyanate, selenite and selenate – the latter two of which are especially difficult to remove from wastewater. Selenocyanate can be biologically oxidised to selenite or selenate, which than can be biologically reduced to elemental Se.
New bioreactor systems
For all of these applications, an advanced bioreactor system, such as Honeywell UOP’s XCeed™, is an effective solution. A biofilm treatment technology, it works well for bulk contaminant removal and difficult organics from industrial wastewater streams. As an immobilised cell bioreactor technology, the XCeed system has proven effective in more than 50 installations worldwide, where it removes more than 99% of organics and as much as 95% of inorganic contaminants from waste streams.
The technology also is flexible enough for applications as varied food and beverage manufacturing, textile and garment production, and even mining and specialty chemical manufacturing – not to mention refinery and petrochemical operations.
The system uses packed beds in a series of chambers, with mixed-media support for immobilised bio-catalysts and a quasi-plug flow design that ensures long-duration solids retention with minimal hydraulic retention time. The high solids retention time results in a high biomass-to-contaminant ratio, promoting biological oxygen demand removal while producing minimal amounts of sludge.
High concentrations of immobilised bacteria provide natural resistance to – and fast recovery from – process upsets. The unique up-flow design eliminates the need for mechanical mixing, while the compact packed bed design minimises aeration requirements, resulting in both low energy and maintenance costs and minimal operator attention. Moreover, the reduced biological chemical demand flexible process configuration also allows for a small system footprint.
The XCeed bioreactor is most commonly delivered as a pre-fabricated modular unit to ensure high quality and quick installation. Larger installations are built onsite using a concrete or panel tank system, or as a retrofit for existing aeration basins.
As a wider range of feedstocks come into use in the refining and petrochemical industries worldwide, and with increasingly stringent environmental regulations in the offing, effective treatment of wastewater will become a more urgent – and even limiting – factor for profitable and sustainable operation.
Adapted from a press release by David Bizley
Read the article online at: https://www.oilfieldtechnology.com/special-reports/08082016/industrial-wastewater-treatment-solutions-in-refining/