BP management had its doubts. John Eastman’s proposal was to eliminate NH3 from wastewater at the firm’s chemical plant in Lima, Ohio, using a two-stage bioreactor. It started with aerobic nitrosomonas bacteria converting NH3 to nitrites in filter substrates, and then used anaerobic denitrifying bacteria to convert the nitrites to N2. In the second stage, these bacteria were to be grown in large ponds. The pilot plant worked flawlessly and we developed a front-end loading (FEL-3) scope and budget for the process. (Adequate front-end loading is crucial for any project; see “Don’t Flub Front-End Loading.”)
Think of what this means! Instead of an elaborate staged process requiring maintenance staff, bugs are your unit operation. There’s a word to describe this: nifty.
Let’s consider the pros and cons of biological wastewater treatment for heavy metal removal and elimination of hydrocarbons. First, most all the heavy metal winds up as a solid; this, in itself, is highly useful because the metal is removed from the wastewater. Aerobic treatment excels at converting compounds of carbon, oxygen, nitrogen and phosphorus into N2, CO2 , phosphates and H2O. Anaerobic digestion of materials produces methane, enabling food-processing and similar wastes to become a source of fuel gas.
The waste composition and type of reactor determine the most-appropriate organism. Aerobic reactors use algae and bacteria. Anaerobic reactors rely on sulfur-reducing bacteria (SRB) and fungi. Anaerobic reactors minimize sludge and loss of chemicals to the atmosphere but tend to require processing in batch because of slow reaction times.
Aerobic fluidized bed bioreactors frequently offer the best economics — but require skilled operators. Often, anaerobic and aerobic reactors are paired in series, as in the NH3 cleanup process at Lima. Some research has shown that an anaerobic treatment stabilized and provided nutrients to the bugs in an aerobic process that followed. This research tends to be project-specific — so it’s sensible to conduct a pilot study before full implementation.
Studies in Poland, Italy and elsewhere concluded that heavy metals are effectively, but slowly, removed from wastewater in an anaerobic environment using SRB or fungus. The resultant heavy-metals-laden sludge, of course, then requires careful disposal. A fast aerobic reaction involving algae has successfully treated wastewater containing copper and cadmium — removing 98% of the Cu and 100% of the Cd.
Waste removal efficiencies generally depend upon the relative proportion of different bugs as well as feed stability. Promoting the growth of the best species may require trace nutrients — e.g., FeIII is added to enhance anaerobic disposal of toluene. The heavy metal or organic present also impacts efficiency. Parameters controlling digestion include: pH, which usually is low — SRB raise the pH to neutral; temperature, which generally should stay between 60° and 100°F — the water can’t be frozen or boiling; initial concentration of the metal or organic — low concentration discourages bug growth while extremely high concentration kills bugs; and nutrients and their transportation. Consistency is crucial to growing and maintaining the bugs you want.
With hydrocarbons, biological reactions are highly efficient in destroying oxygen and nitrogen components. However, chlorinated compounds, like dioxins, merely are converted to materials such as vinyl chlorides or dichloroacetate that may be nearly as bad or perhaps even worse than the original waste. This is strong argument for anaerobic processing.
One refinery study showed the best results with a carbon/nitrogen/phosphorus mole ratio of 100:5:1. The actual ratio may be less important than the ratio’s stability.
Heavy metals like mercury that are mixed in organics often inhibit biological growth of the best organisms. An additional problem is that the heavy metal is converted to an organo-metal that animals and plants can readily absorb. (“Consider the Consequences of Chemistry,” looks at one unfortunate example — people suffering health effects caused by wallpaper dyed with Prussian Green.)
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