Chemical Engineering

Biological Wastewater Treatment

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The use of microorganisms to remove contaminants from wastewater is effective and widespread. To choose the right system from the many options offered, understand the various techniques available and evaluate them based on your requirements.

Biological treatment — the use of bacteria and other microorganisms to remove contaminants by assimilating them — has long been a mainstay of wastewater treatment in the chemical process industries (CPI). Because they are effective and widely used, many biological-treatment options are available today. They are, however, not all created equal, and the decision to install a biological-treatment system requires ample thought.

When considering biological wastewater treatment for a particular application, it is important to understand the sources of the wastewater generated, typical wastewater composition, discharge requirements, events and practices within a facility that can affect the quantity and quality of the wastewater, and pretreatment ramifications.

Consideration of these factors will allow you to maximize the benefits your plant gains from effective biological treatment. Those benefits can include:

  • Low capital and operating costs compared to those of chemical-oxidation processes
  • True destruction of organics, versus mere phase separation, such as with air stripping or carbon adsorption
  • Oxidation of a wide variety of organic compounds
  • Removal of reduced inorganic compounds, such as sulfides and ammonia, and total nitrogen removal possible through denitrification
  • Operational flexibility to handle a wide range of flows and wastewater characteristics
  • Reduction of aquatic toxicity

The basics

All biological-treatment processes take advantage of bacteria’s remarkable ability to use diverse wastewater constituents to provide the energy for microbial metabolism and the building blocks for cell synthesis. This metabolic activity can remove contaminants that are as varied as the raw materials, byproducts and products generated by the CPI. For a discussion of factors used to determine biodegradability of specific water compounds, see Reference [1].

Knowing the composition of the water to be handled is essential for planning a treatment process. In petroleum refineries, for example, excessive amounts of spent caustic can quickly overwhelm a wastewater-treatment system due to the normally high chemical oxygen demand (COD) of the spent caustic. Another issue can be a significant increase in ammonia and sulfide loads that result from upsets in the operation of sour-water strippers. These loads can, in turn, upset a biologicaltreatment system if it is not designed to handle ammonia and sulfide.

In addition to understanding the source and composition of the wastewater, one must also recognize when pretreatment steps are needed to provide adequate protection for a biological-treatment system. In most petroleum and petrochemical facilities, for example, raw wastewater normally contains free oil, which can have serious, detrimental effects. Oil can coat and kill bacteria, causing the microorganisms to float out of the system, and can interfere with oxygen-transfer efficiency.

Another source for con- cern at refineries is a potential upset in desalter operations that can lead to significant oil/water emulsions in the wastewater and thereby negatively impact the biological-treatment system. To prevent these types of problems in petroleum-industry systems, process steps prior to biological treatment are normally included. Pretreatment for this industry typically includes the use of oil/water separators, an equalization tank to moderate spikes in wastewater composition, and off-spec wastewater storage. Figure 2 shows a typical wastewater-treatment system for a petroleum facility.

Even properly pretreated wastewater can still contain a wide variety of compounds which may or may not be biodegradable. There can also be significant concentrations of sulfides, ammonia, amines, mercaptans and other compounds that require modifications to the treatment process in order to meet discharge objectives.

Vendors can be helpful in setting up pilot-plant or bench-scale tests to assist in determining if biotreatment is a viable option for a particular wastewater composition. The types of compounds present, the concentration of each and the ultimate discharge requirements are key to selecting a proper biological-treatment system. This is true whether the wastewater is being discharged directly to the environment or to a publicly owned treatment works (POTW), or if it is to be reused within the facility.

Once the factors discussed above have been resolved, selection from the many available options can begin. Biological-treatment methods vary widely, ranging from fixedfilm technologies like rotating and submerged biological contactors to technologies like sequencing batch reactors and continuous-flow activated- sludge systems.

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