Microbiology of the Built Environment: Wastewater, Water Treatment, and Indoor Microbiota

Microbiology of the Built Environment

Introduction

The built environment encompasses all human-made surroundings that provide the setting for human activity, including homes, workplaces, and public spaces. Microorganisms play critical roles in these environments, influencing water quality, waste management, and human health.

Bioremediation and Microbial Processes in the Environment

Bioremediation of Chlorinated Solvents

• Bioremediation is the use of microorganisms to degrade or detoxify environmental contaminants, such as chlorinated solvents found in degreasing agents, dry cleaning fluids, and chemical feedstocks.

• These contaminants can pollute groundwater if not properly disposed of.

• Biostimulation involves adding nutrients (e.g., molasses) to stimulate the growth of indigenous microorganisms capable of degrading pollutants.

• Bioaugmentation is the addition of specific bacteria to initiate or accelerate the bioremediation process.

• Dehalococcoides is a bacterium known for its ability to perform bioremediation of chlorinated solvents for extended periods (up to 4 years).

Example: Molasses is added to contaminated groundwater to stimulate native bacteria, while Dehalococcoides is introduced to enhance the breakdown of chlorinated compounds.

Wastewater and Drinking Water Treatment

Importance of Water Treatment

• Water can be a source of infectious diseases (e.g., cholera) and chemical intoxications.

• Ensuring water purity is essential for public health.

• Microorganisms are used in treatment and purification schemes to identify, remove, and degrade pollutants.

Primary and Secondary Wastewater Treatment

• Wastewater includes domestic sewage and liquid industrial waste.

• Gray water is water from washing, bathing, and cooking.

• Sewage contains human and animal fecal material.

• Wastewater treatment relies on industrial-scale microbial bioconversion to make effluent water suitable for release into surface waters or further purification for drinking.

Goals of Wastewater Treatment

• Reduce organic and inorganic materials to levels that do not support microbial growth or contain toxic materials.

• Efficiency is measured by reduction in biochemical oxygen demand (BOD):

$\text{BOD} = \text{amount of dissolved oxygen consumed by microbes to completely oxidize all organic and inorganic matter in a water sample}$

Stages of Wastewater Treatment

Stage	Main Process	Purpose
Primary Treatment	Physical separation (screening, sedimentation)	Removes solids and particulate matter
Secondary Treatment	Biological (aerobic and anaerobic digestion)	Decomposes organic matter using microbes
Tertiary Treatment	Physicochemical/biological (e.g., nutrient removal)	Removes inorganic nutrients and remaining contaminants

Primary Treatment

• Uses physical methods to separate solid and particulate organic/inorganic materials from wastewater.

Secondary Treatment

• Aerobic secondary treatment uses microbes under aerobic conditions to digest organic materials (e.g., activated sludge and trickling filter systems).

• Anaerobic secondary treatment involves digestive and fermentative reactions by microbes under anoxic conditions, typically in sludge digesters or bioreactors.

• Slime-forming bacteria such as Zoogloea ramigera form flocs that aid in the removal of organic matter.

• Most plants chlorinate effluent after secondary treatment to reduce biological contamination.

Tertiary Treatment

• Further processes secondary effluent to remove additional organic matter, suspended solids, and inorganic nutrients (e.g., phosphate, nitrate, nitrite).

• Includes chemical precipitation and enhanced biological phosphorus removal (EBPR) using phosphorus-accumulating organisms in sequential anaerobic and aerobic bioreactors.

• Most complete method but not widely adopted due to cost.

Sludge Processing and Contaminants of Emerging Concern

• Wastewater treatment is primarily designed for human and industrial waste.

• New pollutants (pharmaceuticals, personal care products, household products, sunscreens) are increasingly found in sewage.

• Advanced treatment systems are needed to degrade these emerging contaminants.

Drinking Water Purification and Stabilization

• Secondary-treated wastewater is not potable; further treatment is required to remove pathogens, chemicals, and turbidity.

• Typical steps in water purification:

  • Sedimentation: removal of large particles

  • Coagulation and flocculation: formation and settling of aggregates

  • Filtration: removal of remaining particulates and microbes

  • Disinfection: typically with chlorine gas or UV radiation

Water Distribution Systems

Challenges in Water Distribution

• Water must travel through extensive municipal and domestic piping systems to reach consumers.

• Problems include taste and odor issues, and the potential for growth or selection of pathogenic and resistant bacteria.

• Elimination of microbial growth requires complete nutrient removal and maintenance of residual chlorine, but neither is fully achievable.

• Opportunistic pathogens (e.g., Legionella pneumophila, Pseudomonas spp., Mycobacterium spp.) and grazing protists can persist in distribution systems, especially in showerheads.

The Microbiology of Homes and Public Spaces

Microbial Ecology of Indoor Environments

• Humans spend most of their time indoors, sharing their environment with diverse microbiota inhabiting air, dust, surfaces, ventilation, and water systems.

• Indoor microbiota can increase exposure to pathogens but may also reduce allergies by promoting immune tolerance.

• The composition of home microbiota is highly predictive of the specific family and changes rapidly with occupancy.

• Flushing toilets can aerosolize large numbers of bacteria, potentially transmitting enteric bacteria.

• Sexually transmitted diseases are unlikely to be transmitted via indoor air or surfaces due to their sensitivity to drying.

Transmission and Resistance of Microbes

Microbial Transmission Pathways

• Microbes are ubiquitous and can be transmitted via hands, fomites (inanimate objects), and vectors (e.g., flies, mosquitoes).

• Selective pressure from overuse or misuse of antibiotics and disinfectants leads to resistant strains.

• Not all microbes are pathogens; understanding transmission routes is essential for infection control and public health.

Summary Table: Key Microbial Processes in the Built Environment

Process	Purpose	Key Microbes/Mechanisms
Bioremediation	Degrade environmental pollutants	Dehalococcoides, biostimulation, bioaugmentation
Primary Wastewater Treatment	Remove solids/particulates	Physical separation
Secondary Wastewater Treatment	Decompose organic matter	Aerobic/anaerobic microbes, Zoogloea ramigera
Tertiary Treatment	Remove nutrients/contaminants	Phosphorus-accumulating organisms, chemical precipitation
Drinking Water Purification	Produce potable water	Sedimentation, filtration, disinfection
Distribution System Management	Prevent pathogen growth	Chlorination, nutrient control

Additional info: Advanced water treatment and distribution systems are continually evolving to address new contaminants and microbial threats, emphasizing the importance of microbiology in public health and environmental management.