Bioremediation and Wastewater Treatment Engineering Notes
Principles of Bioremediation and Engineering Approaches
Definition and Engineering Philosophy: Bioremediation involves using biological organisms to clean up environmental damage. Engineers typically do not simply add external fungi or bacteria to an ecosystem. Instead, they manipulate the existing environment to stimulate the growth and activity of indigenous fungi and bacteria already present at the site to perform remediation faster and in a more controlled manner.
Primary Pollutants: BTECs: The most common targets for bioremediation are hydrocarbons, specifically a group known as the "big four" or BTECs, which include: * Benzyl * Toluene * Ethyl benzene * Xylene
Characteristics of BTECs: These are small, volatile hydrocarbons. They are highly toxic and ubiquitous in petroleum-based materials (e.g., gas stations, oil spills). Despite their toxicity, they are readily biodegradable by various fungi and bacteria.
Other Targets: Beyond hydrocarbons, bioremediation techniques target pollutants such as PFAs, heavy metals (e.g., mercury from factories), and other industrial contaminants remaining at brownfields.
Ex Situ Bioremediation Techniques
General Concept: Ex situ involves excavating contaminated soil or material and moving it to a different location or setup for treatment. This is common when soil is compacted and lacks the air necessary for aerobic biodegradation.
Biopiles: Soil is excavated and piled under a shield to control humidity and temperature. Digging the soil changes its texture, reducing compaction and increasing air pockets and surface area to facilitate aerobic conditions.
Windrows: This method involves mechanical turning of excavated soil. The soil is repeatedly aerated by being turned over with added water. * Environmental Risks: As BTECs degrade, they release greenhouse gases such as . If conditions become anaerobic within the pile, Archaea may produce methane ().
Bioreactors: These are closed tanks or big containers where conditions are strictly controlled. Mechanical mixing and various additives are used to stimulate degradation. * Wastewater Treatment: Technically a large-scale bioreactor. * Biofilters: Used for gaseous products like toluene. For example, air containing toluene is piped over gravel covered in a mix of microbes that adsorb and utilize the toluene as the air passes through.
Land Farming: Similar to windrows but utilizes standard farming equipment. Soil is spread over flat land, repeatedly turned to aerate it, and watered to maximize microbial interaction with pollutants.
In Situ Bioremediation Techniques
General Concept: Treatment of pollutants directly in the ground without excavation. This is preferred when the volume of land is too large to dig up or when excavation would pose a health hazard (e.g., exposing workers to trapped toxins).
Bioventing: Air is injected into the soil to stimulate aerobic microbes. This is often used for contamination layers deep below a clean surface. Pipes are drilled deep into the site to pump oxygen into deeper layers of soil and groundwater.
Bioslurping: A complex machine pumps air into the ground while simultaneously vacuuming air and water out. This process separates hydrocarbons from the water, collecting them in tanks on-site while discharging the purified water back into the ground.
Biosparging: Air is pumped into the groundwater to push volatile compounds into the soil's air pockets, where they can be broken down by soil microbes.
Permeable Reactive Barrier (PRB): An underground filter/barrier that purifies water as it flows through. It is often combined with other methods to protect downstream water supplies while an upstream site is being cleaned.
Phytoremediation and Heavy Metals
Mechanism: Plants (and associated microbes) are used to pull pollutants into their tissues. While plants struggle with BTECs, they are effective for heavy metals like mercury, lead, zinc, and copper.
Phyto-degradation vs. Volatilization: Plants can break down some organic pollutants or pull heavy metals into their tissues. In some cases, they volatilize the pollutants (like mercury), respiring them into the air in highly diluted, safer concentrations.
The Problem of Metals: Unlike hydrocarbons which can be broken down into and water, heavy metals are elements. They cannot be destroyed; they can only be trapped, moved to toxic waste sites, or diluted to harmless levels.
BioSwales: These are long, sloped rows or hills covered in plants. They slow down water runoff and erosion, trapping nutrients like manure/ammonium from agricultural sites before they flush into rivers.
The Populus Tree: The Department of Energy (DOE) frequently studies Populus trees for remediation because they grow fast, have deep root systems, are hardy, and can be used for the paper industry or biofuels. They can grow in full sunlight without a canopy.
Wastewater Treatment Mechanisms
Biological Oxygen Demand (BOD): A critical measure of how much oxygen microbes require to break down organic waste. High BOD indicates high organic content ( for domestic sewage; for industrial waste). The goal is to reach an effluent level of or less.
Process Stages: * Primary Treatment: Physical filtration and sedimentation to remove "big stuff" like toilet paper and solids. The resulting primary sludge is trucked to landfills or incinerated. * Secondary Treatment: Biological process involving aeration tanks and sedimentation tanks to remove dissolved organics. * Disinfection: Final step using UV or chlorine before discharging effluent.
Flocculation and Activated Sludge: Microbes grow in clumps or "flocks" held together by Extracellular Polymerase Substance (EPS). These flocks act like a biofilm matrix that adsorbs dissolved organic matter. This allows the water to be cleaned quickly as the heavy flocks settle out into "sludge."
Microbiology of the Tank: * Chemoheterotrophs and Chemoautotrophs are the dominant guilds. * The entire nitrogen cycle is present, including nitrogen fixation and denitrification. * Filamentous bacteria, Actinomycetes, and Beta-proteobacter Zoogloea are responsible for flock formation. * Anoxic vs. Anaerobic: Some distinguish anoxic (nitrate-based respiration) from anaerobic (no oxygen or nitrate; fermentation and sulfate/iron/ reduction).
Questions & Discussion
Question: Is there a common plant used in phytoremediation?
Answer: The Department of Energy (DOE) often uses Populus. It's used in the paper industry, grows fast, is tolerant to diverse sites, and is a candidate for biofuels. In an example provided, a Populus site that appeared healthy was actually situated on a dying factory site full of cloth dyes, resulting in a strange microbial population.
Question: Is the goal of wastewater treatment to clean water for release into rivers without killing fish?
Answer: Yes, that is the overall goal. It involves removing organic pollutants, nitrogen, phosphorus, and industrial hazardous materials like lead, copper, and acids.
Question: How long does the sedimentation/treatment process take?
Answer: Generally, water stays in the tanks for approximately to hours. A student mentioned visiting a plant where it seemed to take about a day.
Danbury Water Treatment Plant Anecdote: The Danbury sewer system recently underwent an expensive upgrade to improve phosphate reclamation. Reclaiming phosphate is currently about times more expensive than just removing it to clean the water. A student recalled visiting a plant (possibly the John Oliver Memorial site after the 2018 naming controversy) and seeing a screen filter out "nasty" solids, including six or seven Barbie doll heads caught in one day.