Lesson 1:
Wastewater Treatment

 

 

Objective

In this lesson we will learn the following:

  • What is involved in treating wastewater.
  • What microbes are beneficial and harmful in the wastewater treatment process.

 

Reading Assignment

Along with the online lecture, read Chapter 1 in Wastewater Microbiology.

 

Lecture

Introduction

The purpose of this lesson is to provide a fundamental background on the relationship between microbes, wastewater, and wastewater treatment. It will explain why we are concerned about microbes in wastewater, what role do they play in wastewater treatment, and what happens when "clean" water is released into the environment.

The main focus of wastewater treatment plants is to reduce the BOD (biochemical oxygen demand) and COD (chemical oxygen demand) in the effluent discharged to natural waters, meeting state and federal discharge criteria. Wastewater treatment plants are designed to function as "microbiology farms", where bacteria and other microorganisms are fed oxygen and organic waste.

Wastewater is teaming with microbes. Many of which are necessary for the degradation and stabilization of organic matter and are beneficial. On the other hand, wastewater may also contain pathogenic or potentially pathogenic microorganisms, which pose a threat to public health. Waterborne and water-related diseases caused by pathogenic microbes are among the most serious threats to public health today. Waterborne diseases whose pathogens are spread by the fecal-oral route (with water as the intermediate medium) can be caused by bacteria, viruses, and parasites (including protozoa, worms and rotifers).

Diarrhea is one of the most common features of waterborne disease. Fecal pollution is one of the primary contributors to diarrhea. Examples of bacteria commonly associated with diarrheal disease are Shigella dysenteriae and Salmonella typhi. Two protozoans commonly associated with diarrheal disease are Giardia lamblia and members of the genus Cryptosporidium.

 

 

Indicators and Detection

Water quality has inspired development of tests designed to measure its suitability for drinking, bathing, and release back to the environment. Water that looks clear and pure may be contaminated with pathogenic microorganisms. Even water that appears "pure" must be tested to ensure that it contains no microorganisms that might cause disease. On the other hand, there are so many potential pathogens that it is impractical to test for them all. Because of this, tests have been developed for indicator organisms. These are organisms that are present in feces (or sewage), survive as long as pathogenic organisms, and are easy to test for at relatively low cost.

Indicator organisms indicate that fecal pollution has occurred and microbial pathogens might be present. Total and fecal coliforms, and the enterocci -fecal streptocci are the indicator organisms currently used in the public health arena.

 

 

Biological Wastewater Treatment

Principal Goals

It was mentioned earlier that many of the microbes present in wastewater are beneficial. In fact, many wastewater treatment technologies are dependent on these beneficial microorganisms for remediation of wastewater so that it won't detrimentally impact the environment. One of the primary goals of biological treatment is the removal of organic material from wastewater so that excessive oxygen consumption won't become a problem when it is released to the environment.

Another goal of biological treatment is nitrification/denitrification. Nitrification is an aerobic process in which bacteria oxidize reduced forms of nitrogen. Denitrification is an anaerobic process by which oxidized forms of nitrogen are reduced to gaseous forms, which can then escape into the atmosphere. This is important because the release of nitrogen to the aquatic environment can also cause eutrophication.

 

Overview of nitrification and denitrification
at the wastewater treatment plant.

 

Another goal of biological treatment is elimination of pathogenic microorganisms either through predation or out-competition. The oxidation/stabilization of organic sludge is also of importance in biological treatment of wastewater.

 

 

Biochemical Oxygen Demand and Eutrophication

Organic material in wastewater originates from microorganisms, plants, animals, and synthetic organic compounds. Organic materials enter wastewater in human wastes, paper products, detergents, cosmetics, and foods. They are typically a combination of carbon, hydrogen, oxygen and nitrogen and may contain other elements.

The oxidation of organic materials in the environment can have profound effects on the maintenance of aquatic life and the aesthetic quality of waters. Biochemical oxidation reactions involve the conversion of organic material using oxygen and nutrients into carbon dioxide, water and new cells. The equation that expresses this is:

Organic material + O2 + nutrients CO2 + H2O + new cells + nutrients + energy

 

It can be seen from this equation that organisms use oxygen to breakdown carbon-based materials for assimilation into new cell mass and energy. A common measure of this oxygen use is biochemical oxygen demand (BOD). BOD is the amount of oxygen used in the metabolism of biodegradable organics. If water with a large amount of BOD is discharged into the environment, it can deplete the natural oxygen resources. Heterotrophic bacteria utilize deposited organics and oxygen at rates that exceed the oxygen-transfer rates across the water surface. This can cause anaerobic conditions, which leads to noxious odors. It can also be detrimental to aquatic life by reducing dissolved oxygen concentrations to levels that cause fish to suffocate. The end result is an overall degradation of water quality.

Wastewater often contains large amounts of the nutrients, particularly nitrogen and phosphorous, which are essential for growth of all organisms and are typically limiting in the environment. Nitrogen is a complex element existing in both organic and inorganic forms. The forms of most interest from a water quality perspective are organic nitrogen, ammonia, nitrite, and nitrate. Phosphorous is found in synthetic detergents and is used for corrosion control in water supplies.

The introduction of large concentrations of these nutrients from untreated or improperly treated wastewater can lead to eutrophication. Eutrophication is the process by which bodies of water become rich in mineral and organic nutrients causing plant life, especially algae, to proliferate, then die and decompose thereby reducing the dissolved oxygen content and often killing off other organisms.

 

 

Fundamentals of Biological Treatment

The basic mechanisms of biological treatment are the same for all treatment processes. Microorganisms, principally bacteria, metabolize organic material and inorganic ions present in wastewater during growth. Which brings us to the fundamental differences between catabolic and anabolic processes. Catabolic processes are those biochemical processes involved in the breakdown of organic products for the production of energy or for use in anabolism. Catabolic processes are dissimilar because the reactants can be though of as redox reactions because they involve the transfer of electrons resulting in the generation of energy to be used in cell metabolism. In contrast, anabolic processes are the biochemical processes involved in the synthesis of cell constituents from simpler molecules. These processes usually require energy and are assimilatory. That is the processes result in the incorporation of the reacting molecules or compounds into new cell mass.

 

 

Biofilms

The growth of bacteria in pure culture has been the mainstay of microbiology, specifically the mainstay of microbiological technique. Solid media techniques have allowed the isolation of individual species from complex natural populations. In natural environments and in pathogenic relationships, bacteria are different than the same organisms grown in vitro. In natural systems, mixed bacterial populations grow as biofilms.

There are three steps necessary for the formation of biofilms. First, there must be a macromolecular conditioning of the surface to be colonized. This is a purely chemical process that occurs on the order of microseconds. If you put any clean surface into the environment, low molecular weight compounds possessing their own unique hydrophilic (readily absorbing or dissolving in water) and hydrophobic (repelling, tending not to combine with, or unable to dissolve in water) character will bind to that surface. Step two, microbial binding, is a two-step process. First there is reversible binding (colonization) by bacteria. Next if the cell senses the proper conditions, irreversible binding takes place, often triggering capsule formation. Finally, there is further permanent attachment of cells and cell division leading to microcolony formation and biofilm generation.

One important distinction of biofilms is that they can provide a variety of microenvironments and are chemically heterogeneous throughout. They establish their own gradients of nutrients, oxygen saturation, and pH relative to the bulk environment. Because the capsule is hydrated, biofilms are greater than 95% water and thus they will trap inorganic and organic material that is soluble or particulate in nature. The solid/liquid interface between the biofilm and the environment is important as well to current/flow rates. There is a critical role of transport and transfer processes which are generally rate controlling in biofilm systems. For example, high flow rates in oligotrophic ( Lacking in plant nutrients and having a large amount of dissolved oxygen throughout. Used of a pond or lake) environments will be well nourished due to high transfer rates across the interface.

In natural systems, biofilms are responsible for the removal of dissolved and particulate contaminants and are important in the cycling of chemical elements. These concepts are equally important in wastewater treatment systems. Also, in the natural environment, enhanced growth may result from nutrient trapping.

 

 

Microbiology of On-Site Systems

The septic tank works by a combination of sedimentation and anaerobic digestion. Anaerobic bacteria are responsible for the digestion. Anaerobic bacteria are non-pathogenic and are present in large numbers in the human intestine. A new supply of these bacteria are regularly added to the septic tank with each flush of human fecal material. Anaerobic digestion represents an incomplete digestion. Methane, hydrogen sulfide, and sulfur dioxide gases are produced, as well as a sludge of high molecular weight hydrocarbons. This sludge will readily decompose further when exposed to oxygen and aerobic bacteria. This further decomposition will take place in the municipal sewage treatment plant or landfill if either of these places is used to dispose of sludge pumped periodically from septic tanks.

 

Wastewater Treatment

Introduction

Wastewater treatment refers to the process of removing pollutants from water previously employed for industrial, agricultural, or municipal uses. The techniques used to remove the pollutants present in wastewater can be broken into biological, chemical, physical and energetic. These different techniques are applied through the many stages of wastewater treatment.

 

Primary treatment usually includes the removal of large solids from the wastewater via physical settling or filtration. The first step in primary treatment is screening.

Secondary treatment typically removes the smaller solids and particles remaining in the wastewater through fine filtration aided by the use of membranes or through the use of microbes, which utilize organics as an energy source. Energetic techniques may also be employed in tandem with biological techniques in the secondary phase to break up the size of particles thus increasing their surface area and rate of consumption by the microbes present. A common first step in the secondary treatment process is to send the waste to an aeration tank.

Tertiary treatment involves the disinfection of the wastewater through chemical or energetic means. Increasing the number of steps in a wastewater treatment process may insure higher quality of effluent; however employing additional technologies may incur increased costs of construction, operation, and maintenance.

 

 

Primary Treatment

Screening is the first technique employed in primary treatment, which is the first step in the wastewater treatment process.

This step removes all sorts of refuse that has arrived with the wastewater such as plastic, branches, rags, and metals. The screening process is used primarily to present the clogging and interference of the following wastewater treatment processes.

Screens are considered coarse if their opening are larger than 6mm, fine if their openings are between 1.5 and 6mm, and very fine if their openings are between 0.2 and 1.5mm.

 

This type of screen, called a bar screen, removes debris from wastewater.

 

 

Screens are cleaned manually if the object caught is larger and mechanically if finer particles are caught. The angle of the screen may also be varied to affect the efficiency of filtration.

 

In order to remove coarse solids, numerous types of detritus tanks, grinders, and cyclonic inertial separation are utilized, including a comminutor and a grit chamber. The type of grit removal separation depends upon the characteristics of the grit itself.

A comminutor, also known as the grinding pump, houses a rotating cutting screen. This cutting screen shreds any large chunks of organic matter in the wastewater into smaller pieces. This makes it easier for the microorganisms to use the organic matter as food and prevents the large chunks from harming the internal workings of the treatment plant.

 

A grit chamber allows pieces of rock, metal, bone, and even egg shells, which are denser than organic materials, to settle out of the waste stream. Removal of grit prevents damage to machinery through abrasion or clogging.

 

 

The last step in primary treatment is sedimentation, which occurs in the primary clarifier.

 

Sedimentation simply entails the physical settling of matter, due to its density, buoyancy, and the force of gravity. Certain chemicals known as coagulants and flocculants are often used to expedite this process by encouraging aggregation of particles. Through sedimentation, the larger solids are removed in order to facilitate the efficiency of the following procedures and also to reduce the biological oxygen demand of the water.

The biochemical oxygen demand (BOD) refers to the amount of oxygen required by the microbes within the wastewater to digest the matter that they are using for food. By removing these solids early on, the efficiency of the microbial digestion at later stages in increased.

 

 

Secondary Treatment

Once the wastewater leaves the primary treatment steps, it enters secondary treatment. The first step in the secondary treatment process is the aeration tank.

Bacteria are single celled organisms, which have basic requirements for existence and reproduce rapidly. Many occupy unique niches and consume only certain types of food. Many types of bacteria have been utilized in wastewater processing. If certain bacterium is supplied with an environment in which the proper pH, temperature, micro and macronutrients, and oxygen levels are present, it can quickly and effectively break pollutants present in wastewater down into less harmful components.

The types of bacteria utilized in wastewater processing can be categorized based upon their necessary or intolerance of oxygen to survive. Those bacteria that require oxygen to convert food into energy are called aerobic, those that will perish in the presence of oxygen are anaerobic, and finally facultative anaerobes may thrive in either the presence or absence of oxygen. Typically aerobes, which can degrade pollutants 10-100 times faster than anaerobes, are utilized most frequently. Increases in temperature and pollutant food source have shown to increase the rate of degradation, but if all elements necessary for conversion of food to energy are not in balance, the microbial degradation will be thwarted.

 

The wastewater is then passed through a secondary clarifier, which performs sedimentation again, which is described earlier and occurs in primary treatment as well.

 

 

The disinfection of wastewater through the sue of chemicals such as chlorine typically acts as the final step in wastewater treatment. Disinfection seeks to remove harmful organics and pathogens causing cholera, polio, typhoid, hepatitis, and a number of other bacterial, viral, and parasitic diseases from the water.

Due to security concerns, some wastewater treatment facilities are using sodium hypochlorite to eliminate the need for chlorine. Sodium hypochlorite is more expensive than liquid chlorine, but is also safer. Although chlorine is considered the tried and true solution to reducing pathogens in contaminated water, the method of disinfection, such as UV disinfection, must fit the type of pathogen the wastewater harbors, to be truly effective.

Through disinfection a significant portion of the pathogens are inactivated, however, it is difficult to identify individual pathogens within wastewater, and therefore indicator pathogens are used. In wastewater, fecal coliform acts as the indicator pathogen, but there has been discussion of using E. coli or total coliform, the indicator for potable water, to check wastewater.

 

 

 

Coagulation and Flocculation

Coagulants and flocculants are chemicals used to precipitate insoluble substances. The purpose of coagulation and flocculation is to cause small pollutant particles such as metals to aggregate and form large enough floc so that they can be separated from the wastewater through sedimentation.

There are three main types of coagulants that are used to overcome the repulsive forces of particles, thus causing them to aggregate. Electrolytes, organic polymers, and synthetic polyelectrolites are added to wastewater and then flocculation tanks mix the water to promote flocs and subsequent physical separation.

Rate of flocculation is dependent upon many factors including concentration of particles, particle contact, and range of particle sizes. Coagulation targets dissolved ions such as metal and radionuclides. Some difficulties with this technology include the frequent need to adjust pH levels, the creation of toxic sludge that must be eventually mitigated, and the difficulty that results in trying to address the chemical nature of multiple compounds. This technology has been used consistently in the electronics and electroplating industry as well as for applications in groundwater treatment.

 

 

Membrane Filtration

The three main types of membrane-based filtration technologies include reverse osmosis, nanofiltration, and ultrafiltration. Although categorized as different technologies, the three types of membrane filtration have a great deal in common. All three act as membranes created by coating a thin layer of a very porous polymer, or plastic, onto a backing material. The end result is the finest form of filtration presently known, with reverse osmosis being the smallest, nanofiltration being a slight step larger and ultrafiltration being a bit larger again.

 

The pore sizes are typically measured in angstroms (one billionth of a meter) and thus are extremely tiny. These membrane technologies offer a host of advantages over traditional filtration. Due to the fine pore space and indiscrimination of influents of these membrane filtration systems, a very high quality effluent emerges. Additionally, membrane technologies take up only a fraction of the space needed for other tertiary treatment systems. The disadvantage of having extremely fine pores means that clogging is a frequent and costly problem with membrane filtration technologies.

 

 

Constructed Wetlands

Scientists have long recognized the abilities of wetlands to purify water. Through the correct sequencing of base media, plant species, and microbe species, constructed wetlands can successfully reduce nitrogen content, filter out solids, and reduce the presence of heavy metals.

The type and amount of pollutant removed depends upon the species and oxygen affinity of the organisms present in the wetland. Wetlands utilize physical and chemical processes to clean wastewater and typically serve as the secondary and tertiary steps.

Although constructed wetlands tend to take up a great deal of space, they require less investment of time and money than traditional waste treatment procedures. Ultimately, constructed wetlands area cost-effective and environmentally-benign method of wastewater processing.

 

 

Review

In this lesson we learned the relationship between microbes, wastewater, and wastewater treatment. One goal of biological treatment is nitrification/denitrification. Nitrification is an aerobic process in which bacteria oxidize reduced forms of nitrogen. Denitrification is an anaerobic process by which oxidized forms of nitrogen are reduced to gaseous forms, which can then escape into the atmosphere. If water with a large amount of BOD is discharged into the environment, it can deplete the natural oxygen resources. Eutrophication is the process by which bodies of water become rich in mineral and organic nutrients causing plant life, especially algae, to proliferate, then die and decompose thereby reducing the dissolved oxygen content and often killing off other organisms. Wastewater treatment in the plants involve primary, secondary and tertiary treatment. Primary treatment deals with removing large solids while secondary treatment removes the smaller solids and tertiary treatment involves the disinfection of the wastewater through chemical means.

 

 

Sources

The Fundamental Microbiology of Sewage

"Wastewater Treatment Technology Tutorial"  2006. Earthspace

 

Assignments

Complete the interactive assignment on Wastewater Treatment .

This assignment will give you practice with the topics covered in Lesson 1.  You should print the assignment and become familiar with the exercises before doing them online. You may do the Assignment online to get credit or print it out and send it to the instructor.  It will require the Flash player to view, which should already be installed on your machine.

In addition, the first Project paper is due this week.  Once you have completed the project either mail, fax, or email it to your instructor.

 

 

Labs

There are no labs associated with this lesson.


 

Quiz

Answer the questions in the Lesson 1 quiz .  When you have gotten all the answers correct, print the page and either mail or fax it to the instructor. You may also take the quiz online and directly submit it into the database for a grade.