Removing Highly Toxic Phenolic Compounds:

Wastewater Treatment Options

April 24th 2020

Key Takeaways

  • Phenol and its derivatives (phenols, or phenolic compounds) are moderately water-soluble pollutants, common to the wastewaters of various industries, including oil & gas, paint manufacturing, phenolic resin production, paper and pulp factories, and pharmaceutical industries. Phenolic compounds are used in low concentrations in disinfectants, and are also present in many alcoholic beverages, pharmaceuticals, and cosmetics.
  • Excessive phenolic compounds are harmful to human health and the environment. Chlorophenols, by-products of chlorinating phenol-containing water, are carcinogens.
  • Existing treatment options include biodegradation, distillation/evaporation, adsorption and extraction, membrane separation, and chemical oxidation. A treatment system needs to be chosen and engineered carefully, with consideration of specific wastewater chemistry, operating conditions, and economics.
  • Recent advances in membrane separation technology can help optimize the treatment of phenolic compounds-contaminated wastewaters, making zero liquid discharge (ZLD) or minimum liquid discharge (MLD) options more cost-effective.
phenol3
Phenol (C6H5OH), the simplest member of the phenols family.

Uses, Toxicity, & Wastewater

Phenolic compounds are profoundly toxic to humans, animals, and aquatic life, and can also form carcinogenic chlorophenols in the presence of chlorine. They are considered priority water pollutants by the EPA and the NPRI in the USA and Canada, respectively. There are strict discharge limits for phenols in many jurisdictions, typically <0.5 mg/L.

 

The phenolic compounds consist of an aromatic hydrocarbon group bonded to a hydroxyl group (–OH). Phenols are often moderately water-soluble and smaller phenol molecules can be volatile. Phenols occur naturally in small, relatively harmless concentrations. They are also synthesized on an industrial scale for use in disinfectants, medicinal products, and as ingredients for many polymers, resins, and rubbers for the plywood, tire, construction, automotive, and appliance industries. Small quantities are present in alcoholic beverages, pharmaceuticals, and cosmetics. They can be found in the wastewaters of these industries and others, such as oil & gas.

discharge limits
Phenols face strict discharge limits.

Treatment Options for Phenols in Wastewaters

There are many technologies available for the treatment of phenols-laden wastewaters—with highly varied advantages and disadvantages. For example, some are destructive, while others allow the recovery of phenols for potential reuse. The economics of the different processes vary significantly depending on water chemistry, scale, treatment requirements, and other operating conditions. Options need to be carefully analysed for a specific application before treatment decisions are made. Many of the available phenol treatment processes are discussed below and summarized in this table. 

Treatment Description Recovery Pros Cons
Bio- Degradation Uses microorganisms to biodegrade phenols Destructive
  • Cost-effective
  • Safe & easy
  • Simple, harmless products
  • Not suitable for high phenols concentration/ TDS/acidity etc.
  • May require other chemical treatment/aeration etc.
  • Large space requirements
  • Membrane Separation Uses high pressure & fine membranes to reject phenols, other compounds, & ions Possible
  • Cost-effective & reliable
  • Small footprint, scalable
  • Produces low volume of concentrated brine
  • Risks such as fouling & scaling need to be managed
  • Cannot meet very low discharge requirements, but can be combined with other steps
  • Distillation/
    Evaporation
    Uses heat & difference in volatility to separate phenols from water Possible
  • Good for high concentrations
  • Good potential for recovery & reuse
  • Costs relatively high
  • Energy intensity
  • Adsorption & Extraction Phenols adsorb to solids such as activated carbon (AC)

    Solvent extraction uses relative solubilities to separate liquids
    Adsorption: typically unfeasible

    Extraction: possible
  • AC/solvents cost-effective for low concentrations.
  • Recovery from solvents possible
  • Polish to extremely low concentrations
  • Not economic for high concentrations due to high consumable costs
  • Requires disposal/recycling of spent AC/solvents
  • Other organics increase cost
  • Chemical Oxidation Uses one of a variety of oxidising methods, e.g. the Fenton reaction Destructive
  • Different strategies available
  • Scalable
  • Costs can be high for some chemistries
  • High consumable or energy costs
  • Products may require further treatment/precipitation/solids management
  • Biological and Enzymatic Degradation

    Biological treatment is a highly established, successful method for the removal of phenols from water. It uses either aerobic or anaerobic microorganisms to biodegrade phenols. It is relatively inexpensive, safe, easy-to-operate, and environmentally friendly. It generally produces simple, harmless products. Membrane bioreactors (MBRs) are a variant of conventional bio-degradation. They use an activated sludge process combined with a membrane filtration step to produce very high-quality water with a small footprint.

     

    Another phenol treatment uses enzymes to catalyze phenol polymerization for precipitation from water. Depending on the circumstances (e.g. availability and cost of appropriate enzymes) it can be even more cost-effective than biological treatments.

     

    However, both biological and enzymatic treatments are not suitable for all water chemistries, for example, high concentrations of phenols, high salinity, and high acidity. They may also require interventions such as further chemical additives and aeration. This means that they are not suitable or cost-effective for all applications.

    img_wastewater_aeration
    Biological treatment aeration tanks

    Membranes

    Membrane methods use reverse osmosis membranes to reject phenols, as well as other compounds and ions. They are typically cost-effective and reliable, with low power consumption, a small footprint, and scalability. Although they have good rejection performance for many phenols, their performance can be pH dependent. Furthermore, a polishing step may help to further lower the phenol concentration to reach discharge limits, especially for smaller phenol-family molecules. They are particularly suited to treating high concentrations that are unsuitable for biological treatment.

     

    Some risks must be considered such as membrane fouling and scaling. However, comprehensive membrane treatment processes with appropriate pre-treatment can manage the risk effectively, making reverse osmosis (RO) and nanofiltration (NF) very cost-effective and convenient for the treatment of phenols. 

    XtremeRO Brine  Concentrator
    XtremeRO reverse osmosis system

    Distillation/Evaporation

    There is a variety of evaporation/distillation options possible for the separation of phenols from wastewater. They use the relative volatility of some phenols to purify water. Distillation methods generally have high energy costs and are typically viable for high phenol concentrations only. Due to the spread of volatility across different members of the phenol-family, such methods may not be suitable for all phenols.

     

    Pervaporation (PV) applies a vapor pressure difference to pull phenol vapor through a membrane, exploiting the difference between the affinities of phenol and water to the membrane. With relatively low energy consumption and simple operation, it is effective for separating low volatility organics, including some phenols.

    SaltMaker-Evaporator-Crystallizer-at-Saltworks-Technologies-HQ
    SaltMaker MultiEffect evaporator

    Adsorption and Extraction

    Adsorption is commonly used to remove phenols using solid consumables such as activated carbon (AC).  The economics depend on the cost of the adsorbent and the cost of its disposal or recycling. Once spent, the management of phenol-saturated solids is critical and includes options for reuse or disposal. Although usable for the treatment of any concentration of phenols, adsorption is most cost-effective for removing low concentrations, making it an excellent final step polish. The presence of other organics in the water will increase the cost since these may also occupy adsorption sites in the AC. 

     

    Solvent extraction is a similar, liquid-liquid process. It is standardized and non-destructive  for phenols compounds, useable over a wide range of concentrations. However, it is only cost-effective in some circumstances, typically on a small scale.

    activated carbon
    Activated carbon

    Oxidation and Fenton’s Reaction

    Oxidation is a destructive method for the treatment of phenols and other organics. It can use a variety of conventional oxidising agents (ozone, chlorine, permanganate etc.), catalysts, and conditions, including irradiation. The choice of oxidation strategy depends heavily on the economics, wastewater chemistry, and other conditions. Costs are typically low to moderate, and oxidation can be used in varied operating conditions. Reaction products may require further treatment or be precipitated (meaning that consideration may need to be given to appropriate solids management).

     

    The Fenton reaction (and variants) use H2O2 and Fe2+ to produce hydroxyl radicals, which oxidize organics such as phenols. As a phenol treatment method, it is usually cost-effective, and suitable for high concentrations. However, it is pH dependent and may not suit all water chemistries. As with many other chemical treatments, sludge is produced that requires management.

    Contact Saltworks for Phenols Solutions

    Saltworks’ process experts support our clients by providing options analysis—we help you to decide on the right treatment for your phenols-laden wastewater. Providing you with the right solution is what matters most, so our recommendations may include solutions from third-party vendors rather than our own products (or combinations of both). We have extensive experience developing specifications for third party vendors for our clients. Given our position in the water industry, we know who builds the best sub-processes and process elements.

     

    Saltworks offers two ‘in-house’ solutions for phenols that are adaptable for new or existing treatment chains:

    • Option 1 uses reverse osmosis/nanofiltration, combined with our ultra high pressure XtremeRO system to remove most of the phenolic compounds. We optionally add polishing to remove the rest cost-effectively. Option 1 is better suited for low TDS wastewater.
    • Option 2 uses our SaltMaker evaporator crystallizers to reduce the volume of high TDS wastewater to minimal or zero liquid discharge. Post-treatment systems can polish the phenol-containing condensate. Option 2 is better suited for high TDS wastewater.

    Once a treatment technology is chosen, we can bring our testing, automation, and piloting expertise to the project. Get in touch below to see how Saltworks can help you to meet your phenols challenges.

    Saltworks Engineers Working at an Ammonia Splitter Pilot Plant 1
    Saltworks engineers operating a pilot plant

    Phenols Related Resources

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