Wastewater Treatment and Water Purification

Microorganisms and Water Quality

Microorganisms from sewage can cause human disease, but can also negatively affect important ecosystems on which humans rely.

Learning Objectives

Explain the relationship between microorganisms and water quality

Key Takeaways

Key Points

  • Scientists typically measure water quality by testing for the presence of “indicator species ” of bacteria, harmless microorganisms that are found in the human gut alongside pathogenic species.
  • Typical indicator species include coliform bacteria (related to the pathogenic E. coli) and Pseudomonas aeruginosa. When levels of these bacteria are high, scientists begin testing for disease causing bacteria.
  • Ecosystems may also suffer from contaminated water. In aquatic ecosystems, sewage bacteria may cause “dead zones” when they use up the oxygen in the water while decomposing nutrients. Coral reefs may also become infected with sewage bacteria and die.

Key Terms

  • hypoxic: Of, pertaining to, or suffering from hypoxia (lack of oxygen)

Water Quality and Human Health

Waterborne diseases are a infections transmitted by contaminated drinking water. Although there are many pathogens which can be transmitted through water, bacteria and protozoa are some of the most common organisms that cause disease. Monitoring for waterborne disease can be difficult because humans often shed very low numbers of pathogenic bacteria when they are infected. To test whether disease causing bacteria might be present, researchers measure the presence of indicator species, such as coliform bacteria (which are the group to which the pathogenic E. coli belongs) or Pseudomonas aeruginosa. Although most coliform bacteria do not cause disease, they are commonly found in the human gut and in sewage, and their presence implies that human waste has reached the water supply. Researchers usually test water quality by sampling water and measuring the concentration of all bacteria in a sample. If the number of bacteria exceeds the limits set by water quality standards, the next step is to test for the presence of specific pathogens. Scientists can use genetic probes, or specific culture techniques to check whether harmful pathogens are present. Standards for testing may differ depending on the water source: drinking water is held to very high standards, while water quality in lakes and rivers may be held to more lax standards because recreational swimmers typically ingest very little water.


E. coli Bacteria: Researchers monitor water quality by testing for coliform bacteria, relatives of E. coli

Water Quality and The Environment

Microrganisms can also have important impacts on the environment. All healthy ecosystems have their own communities of bacteria that decompose biological matter. However, contamination by sewage and human waste can disrupt the natural balance of bacteria and affect aquatic ecosystems. An influx of human pathogens can cause problems for ecosystems in several ways. First, sewage bacteria can cause hypoxic “dead zones” in aquatic ecosystems. The foreign bacteria rapidly reproduce and consume debris and nutrients in the sewage, but use up all the oxygen in the water in doing so. The de-oxygenated water is harmful to fish and other aquatic life. Coral reefs are also affected by sewage contaminated water. Coral can become infected by human gut bacteria, and this can cause “coral bleaching disease” where coral lose their normal bacterial and algae communities and die. Water quality is not just important for human health, it is important for the human communities that depend on aquatic and marine ecosystems.

Wastewater and Sewage Treatment

Wastewater is treated in 3 phases: primary (solid removal), secondary (bacterial decomposition), and tertiary (extra filtration).

Learning Objectives

List the steps of wastewater/sewage treatment

Key Takeaways

Key Points

  • Primary treatment is the first phase of sewage treatment: wastewater is placed in a holding tank and solids settle to the bottom where they are collected and lighter substances like fats and oils are scraped off the top.
  • Secondary treatment is where waste is broken down by aerobic bacteria incorporated into the wastewater treatment system.
  • Tertiary treatment is designed to filter out nutrients and waste particles that might damage sensitive ecosystems; wastewater is passed through additional filtering lagoons or tanks to remove extra nutrients.

Key Terms

  • Effluent: Sewage water that has been partially treated and is released into a natural body of water; a flow of any liquid waste.
  • zooplankton: Small protozoa, crustaceans (such as krill), and the eggs and larvae from larger animals.
  • aerobic: Living or occurring only in the presence of oxygen.

Sewage is generated by residential and industrial establishments. It includes household waste liquid from toilets, baths, showers, kitchens, sinks, and so forth that is disposed of via sewers. In many areas, sewage also includes liquid waste from industry and commerce. The separation and draining of household waste into greywater and blackwater is becoming more common in the developed world. Greywater is water generated from domestic activities such as laundry, dishwashing, and bathing, and can be reused more readily. Blackwater comes from toilets and contains human waste.

Sewage treatment is done in three stages: primary, secondary and tertiary treatment.


Diagram of Sewage Treatment Process: Sewage passes through primary, secondary, and tertiary treatment.

Primary Treatment

In primary treatment, sewage is stored in a basin where solids (sludge) can settle to the bottom and oil and lighter substances can rise to the top. These layers are then removed and then the remaining liquid can be sent to secondary treatment. Sewage sludge is treated in a separate process called sludge digestion.

Secondary Treatment

Secondary treatment removes dissolved and suspended biological matter, often using microorganisms in a controlled environment. Most secondary treatment systems use aerobic bacteria, which consume the organic components of the sewage (sugar, fat, and so on). Some systems use fixed film systems, where the bacteria grow on filters, and the water passes through them. Suspended growth systems use “activated” sludge, where decomposing bacteria are mixed directly into the sewage. Because oxygen is critical to bacterial growth, the sewage is often mixed with air to facilitate decomposition.

Tertiary Treatment

Tertiary treatment (sometimes called “effluent polishing”) is used to further clean water when it is being discharged into a sensitive ecosystem. Several methods can be used to further disinfect sewage beyond primary and secondary treatment. Sand filtration, where water is passed through a sand filter, can be used to remove particulate matter. Wastewater may still have high levels of nutrients such as nitrogen and phosphorus. These can disrupt the nutrient balance of aquatic ecosystems and cause algae blooms and excessive weed growth. Phosphorus can be removed biologically in a process called enhanced biological phosphorus removal. In this process, specific bacteria, called polyphosphate accumulate organisms that store phosphate in their tissue. When the biomass accumulated in these bacteria is separated from the treated water, these biosolids have a high fertilizer value. Nitrogen can also be removed using nitrifying bacteria. Lagooning is another method for removing nutrients and waste from sewage. Water is stored in a lagoon and native plants, bacteria, algae, and small zooplankton filter nutrients and small particles from the water.

Sludge Digestion

Sewage sludge scraped off the bottom of the settling tank during primary treatment is treated separately from wastewater. Sludge can be disposed of in several ways. First, it can be digested using bacteria; bacterial digestion can sometimes produce methane biogas, which can be used to generate electricity. Sludge can also be incinerated, or condensed, heated to disinfect it, and reused as fertilizer.

Purification of Drinking Water

Water is purified with filters to remove larger protozoans, and by chemical or UV disinfection to kill bacteria and other small pathogens.

Learning Objectives

Illustrate the steps of drinking water purification

Key Takeaways

Key Points

  • Water is first passed through a system of filters and a coagulating agent is added to remove particulate matter.
  • Water is then passed through a membrane filter to remove large pathogens such as cryptosporidum and giardia.
  • To finalize the purification process, chemical disinfection (usually with chlorine or ozone ) or UV light is applied to the water to kill bacteria, viruses, and the hardy cysts produced by cryptosporidium and giardia.

Key Terms

  • ozone: A triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope (O2), breaking down with a half life of about half an hour in the lower atmosphere, to normal dioxygen. Ozone is formed from dioxygen by the action of ultraviolet light and also atmospheric electrical discharges, and is present in low concentrations throughout the Earth’s atmosphere. In total, ozone makes up only 0.6 parts per million of the atmosphere.
  • coagulation: The precipitation of suspended particles as they increase in size (by any of several physical or chemical processes).
  • protozoa: Protozoa are a diverse group of unicellular eukaryotic organisms, many of which are motile. Originally, protozoa had been defined as unicellular protists with animal-like behavior, e.g., movement. Protozoa were regarded as the partner group of protists to protophyta, which have plant-like behaviour, e.g., photosynthesis.

Drinking Water Purification

In order to purify drinking water from a source (such as a lake, river, reservoir or groundwater), the water must go through several steps to remove large particles and different types of pathogens.


Drinking Water Purification: Water is purified for drinking through a system of filters and by chemical disinfectant.

  1. Pumping and Containment: Water is pumped from the source into holding tanks.
  2. Screening: Water is passed through a screen filter to remove large debris.
  3. Storage: Water is stored in reservoirs, tanks, and water towers in preparation for purification. Sometimes water is “pre-cholrinated” in this system to prevent bacterial growth while it is in storage.
  4. Coagulation and Sedimentation: Although there are many processes by which large particles are removed from drinking water, most water purification systems implement some kind of coagulation system. A chemical that causes particle aggregation is added to the water, and clumps of particles form and settle to the bottom of the reservoir. This is called sedimentation.
  5. Membrane Filtration: Membrane filters are able to remove all particles larger than 0.2 um. Larger pathogens such as giardia lamblia and cryptosporidium are trapped in these filters, but the cysts they produce are small enough to pass through.
  6. Disinfection: Before water is considered potable, it must be disinfected to remove any pathogens that passed through the membrane filter.

Methods of Disinfection

  • Chlorination is the most common form of disinfection. Chlorine is a strong oxidant, and rapidly kills many microorganisms, especially bacteria. Because chlorine is a toxic gas, it can also be dangerous to sanitation workers. Chlorine based compounds like choloramine are often used. Although chlorine is very effective against bacteria, it is not as effective against the cysts formed by protozoans (like giardia lamblia and cryptosporidium). Chlorine can sometimes leave residual byproducts in water.
  • Ozone is an unstable molecule that readily gives up one atom of oxygen providing a powerful oxidizing agent. This agent is toxic to most waterborne organisms. Ozone is widely used in Europe, and is an effective method to kill cysts formed by protozoans. It also works well against almost all other pathogens.
  • Ultraviolet Light is very effective at inactivating protozoan cysts, and will also kill bacteria and viruses. However, it is not as effective in cloudy water. It is sometimes used in concert with chlorination.