Microbiology of Sewage Treatment

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Sewage, or wastewater, is composed of all the material that flows from household plumbing systems, including washing and bathing water and toilet wastes. Municipal wastewater also includes industrial wastes. In many cities, storm water runoff that flows into street drains enters the sewage system as well.

The most obvious reason that sewage must be treated before discharge is that pathogenic microbes can be transmitted in feces, including those that cause diarrheal diseases and hepatitis. If untreated sewage is released into a river or lake, the receiving water, that is then used as a source of drinking water, disease can easily spread. In a similar manner, if marine waters become contaminated with untreated sewage, consumption of the shellfish grown there can result in disease. Shellfish are filter feeders and they concentrate microbes from the waters in which they live.

A less obvious problem is the impact that the high nutrient content of sewage has on the receiving water. When any nutrient-rich substance is added to an aqueous environment, microorganisms quickly utilize the compounds as energy sources, employing metabolic pathways such as glycolysis and the TCA cycle (see figure 6.9). As a result, microbes that use aerobic respiration consume available O2 in the water, using it as a terminal electron acceptor (see figure 6.19). The amount of dissolved O2 is limited and can easily be depleted during the microbial breakdown of nutrients. Fish and other aquatic animals in the environment die because they require O2 for respiration. Thus, effective treatment of sewage must decrease the level of organic compounds substantially, in addition to eliminating pathogens, toxic materials, and other pollutants. ■ aerobic respiration, pp. 137,148

Reduction of Biochemical Oxygen Demand

An important goal of sewage treatment is to lessen the environmental impact of sewage by reducing the biochemical oxygen demand (BOD), the amount of O2 required for the microbial decomposition of organic matter in a given sample. The BOD is roughly proportional to the amount of degradable organic material present in a sample. To determine the BOD, the O2 level in a well-aerated sample of microbe-containing test water is measured. The sample is then incubated in a sealed container in the dark under standard conditions of time and temperature, usually 5 days at 20°C. The O2 level is then determined again. The difference between the dissolved O2 at the beginning of the test and at the end reflects the BOD of the sample. In many cases the sample must be diluted in order to accurately determine the BOD. High BOD values indicate that large amounts of degrad-able materials were present in the test water, resulting in correspondingly large amounts of O2 being used during its biological degradation. The BOD of raw sewage is approximately 300 to 400 mg/liter, which could easily deplete the dissolved O2 in the receiving water. The dissolved O2 content of natural waters is generally 5 to 10 mg/liter.

The methods used to reduce the BOD of sewage employ controlled environments that accelerate natural processes. Aerobic treatments allow microbes to oxidize organic compounds in the presence of O2, yielding carbon dioxide and various other inorganic compounds. The processes occur rapidly, but the methods are relatively expensive because they must be engineered to maintain sufficient O2 levels. Anaerobic treatments involve a succession of microbial populations. First, microbes ferment the organic compounds. Then, methanogens utilize the small breakdown products including acetate, CO2, and hydrogen, forming CH4 (methane). The methane can be either discarded, conserved for fuel, or oxidized to CO2 by the methane-oxidizing bacteria (see Perspective 31.1, page 788) ■ methanogens, p.271

Municipal Sewage Treatment Methods

Large-scale sewage treatment plants in the United States use a series of two processes, primary and secondary treatment, as mandated by the 1972 Federal Water Pollution Control Act, now known as the Clean Water Act. Once treated, the liquid portion, or effluent, may be discharged into a body of water. Increasingly, that effluent is given additional processing, tertiary treatment, before release. The solid portion, or sludge, is further treated in an anaerobic digester.

Primary Treatment

Primary treatment of sewage is a physical process designed to remove materials that will settle or sediment out, removing approximately 50% of the solids and 25% of the BOD. During this step, raw sewage is first passed through a series of screens to remove large objects such as sticks, rags, and trash (figure 31.1). Skimmers then remove scum and other floating materials. The sewage is allowed to settle in a sedimentation tank, facilitating removal of the solids. Once the settling period is complete, the sludge is removed and the primary treated sewage that remains is sent for secondary treatment.

Secondary Treatment

Secondary treatment is chiefly a biological process designed to convert most of the suspended solids in sewage to inorganic compounds and cell mass that can be removed, eliminating as much as 95% of the BOD. Microbial growth is actively encouraged during secondary treatment, allowing aerobic organisms to oxidize the biologically degradable organic material to CO2 and H2O. Note that because secondary treatment relies on the metabolic activities of microorganisms, the processes could be devastated if sufficient toxic industrial wastes or hazardous household materials were dumped into sewage systems, killing the microbial population. Methods used for secondary treatment of sewage include:

■ Activated sludge process. This common system employs mixed aerobic microbes that are adapted to utilize the nutrients available in sewage and grow as suspended biofilms, or flocs. Although the organisms are often naturally present in sewage, large numbers are inoculated into the wastes by introducing a small portion of leftover sludge from the previous load of treated wastes (see figure 31.1b). An abundance of O2 is supplied by mixing the sewage in an aerator. As the microbes proliferate, the organic matter is converted into both biomass and waste products such as CO2. Following

Nester-Anderson-Roberts: I V. Applied Microbiology I 31. Environmental I I © The McGraw-Hill

Microbiology, A Human Microbiology: Treatment of Companies, 2003 Perspective, Fourth Edition Water, Wastes and

Polluted Habitats

31.1 Microbiology of Sewage Treatment

Wastes

Screening Removal Sedimentation Thickening of scum tank

Supernatant to (a) Primary secondary treatment tank treatment

Primary sludge

Digester Sludge removal

Secondary sludge

Aerated Sedimentation Thickening activated sludge tank

(b) Secondary Sludge inoculum treatment

Phosphates removed

Ammonia removal in stripping tower

Mixing Flocculation

Settling

Recarbonation basin

Lime sludge

Lime thickening

Furnace

Recovered lime

Decant tank

Water

Separation bed

Removal of more phosphates

Carbon regenerated

(c) Tertiary treatment

Activated carbon

Recovered water

Figure 31.1 Municipal Sewage Treatment (a) Primary (physical) treatment; (b) secondary (biological) treatment; and (c) tertiary (largely chemical) treatment.

the aeration, the sewage is again sent to a sedimentation tank. There, most of the flocs settle and the resulting sludge is removed; a portion of this sludge is introduced to a new load of wastewater to act as an inoculum. A complication of the activated sludge process occurs when filamentous bacteria such as Thiothrix species overgrow in the sewage during treatment, creating a buoyant mass that does not settle. This problem, bulking, interferes with the separation of the solid sludge from the liquid effluent. ■ biofilm, p. 104 ■ Thiothrix, p. 279

■ Trickling filter system. This method is frequently used for smaller sewage treatment plants. A rotating arm sprays sewage over a bed of coarse gravel and rocks, which become coated with a biofilm of microbes that aerobically degrade the sewage (figure 31.2). The film, about 2 mm thick, consists of a heterogeneous mix of bacteria, fungi, algae, protozoa, and nematodes. The rate of sewage flow over the rocks can be adjusted so that waste materials are maximally degraded.

Nester-Anderson-Roberts: I V. Applied Microbiology I 31. Environmental I I © The McGraw-Hill

Microbiology, A Human Microbiology: Treatment of Companies, 2003 Perspective, Fourth Edition Water, Wastes and

Polluted Habitats

Chapter 31 Environmental Microbiology: Treatment of Water,Wastes, and Polluted Habitats

PERSPECTIVE 31.1 Now They're Cooking with Gas

Farmers in rural China have historically been very frugal when it came to using any waste materials.In rural areas, however, fuel for cooking and heating is often in short supply; as a consequence, up to 80% of the hay that should go for animal feed must be used for fuel. To alleviate this problem, many of the farmers build methane-producing tanks on their farms (figure 1). Near the farmhouse is an underground cement tank that is connected to the latrine and pigpen. Human and animal wastes along with water and some other organic materials such as straw are added to the tanks. As the natural process of fermentation occurs, methane gas

(CH4) is produced.This gas rises to the top of the tank and is connected to the house with a hose. Enough gas is produced to provide lights and cooking fuel for the farm family. By producing its own gas, a family also saves money because it does not need to buy coal for cooking.

Organic material -

anaerobic digestion (several steps)

methane

Figure 1 Production and Use of Methane on a Small Scale ch3cooh + h2 + CO2

ch3cooh ch4 + CO2

■ Lagoons. The sewage is channeled into shallow ponds, or lagoons, where it remains for several days to a month or more, depending on the design of the lagoon. Algae and cyanobacteria that grow at the surface provide O2, enabling aerobic organisms in the ponds to degrade the sewage.

■ Artificial wetlands. These employ the same principles as lagoons, but their more advanced designs not only offer a means to treat sewage, but also provide a habitat for birds and other wildlife (figure 31.3). For example, the sewage treatment processes in Arcata, California, use a series of marshes that now attract a variety of shorebirds and serve as a wildlife sanctuary.

Before discharge into the receiving water, the effluent is disinfected with chlorine, ozone, or UV light to decrease the numbers of microorganisms and viruses. The disinfected wastewater may then be dechlorinated to avoid releasing excessive amounts of the toxic chemical into the environment. Even without disinfection, few pathogenic bacteria survive secondary treatment

Nester-Anderson-Roberts: I V. Applied Microbiology I 31. Environmental I I © The McGraw-Hill

Microbiology, A Human Microbiology: Treatment of Companies, 2003 Perspective, Fourth Edition Water, Wastes and

Polluted Habitats

31.1 Microbiology of Sewage Treatment

PERSPECTIVE 31.2 Sludge for Trees

Sludge is a by-product of sewage treatment that can be difficult to dispose of. If trucked to landfills, it takes up valuable space. If discharged into waterways or incinerated, it can become a major source of pollution.

Scientists from the Forestry Department at the University of Washington in Seattle are experimenting with at least one strategy for dealing with the problem of sludge disposal.They have taken the sludge from the city's sewage treatment plant to their experimental forest and used it as fertilizer for Douglas-fir trees.They have found that the sludge boosts the growth of these trees by over 400%.

There are two major problems with using sludge as fertilizer.The first problem is that heavy metals and other similar pollutants remain in the sludge following digestion of the sewage.The second is that pathogenic microorganisms and viruses also may be left in the residues. Because of these two problems, sludge cannot be used as fertilizer on food crops or in other situations in which humans might be contaminated.The sludge can be used, however, to grow trees that will be processed for pulp or lumber.The sites where sludge is used must be carefully monitored to ensure that heavy metals, organic pollutants, large amounts of nitrogen, and pathogenic microorganisms do not present public health problems in the runoff or in groundwater leaching.Whether large-scale use of sludge for forest production is an economical or practical solution to the disposal of sludge is still open to debate.

because they are rapidly overgrown by other organisms that are adapted to grow best at the temperature and conditions provided. Viruses and protozoan cysts and oocysts might remain, however, because the treatment process does not necessarily destroy them. ■ chlorine p. 118 ■ ozone, p. 120 ■ UV light, p. 123

Tertiary Treatment

Tertiary treatment encompasses any purification process beyond secondary treatment; it may involve physical, chemical, or biological processes, or any combination of these (see figure 31.1c). The treatment is generally designed to remove nitrates and phosphates, compounds that foster the growth of algae and cyanobacteria in the receiving waters. The concentrations of these nutrients, which are very low in unpolluted waters, normally limit the growth of photosynthetic organisms. Consequently, addition of nitrates and phosphates allows the overgrowth of photosyn-thetic organisms, eventually leading to an odiferous scum called a nuisance bloom (see figure 11.8). In addition, the photosynthetic organisms provide a source of organic carbon for other microorganisms, increasing the BOD and, consequently, threatening other forms of aquatic life.

Tertiary treatment processes are expensive and have not been common in the past. However, as water supplies are becoming scarce, and in order to comply with discharge standards designed to protect surface and ground waters, an increasing number of communities are finding them necessary. The treated water can be used to irrigate parks and playfields.

Nitrates are often removed by exploiting the activities of denitrifying bacteria. These organisms use oxidized nitrogen compounds such as nitrate (NO3:) as terminal electron acceptors during anaerobic

Figure 31.2 Trickling Filter Sewage wastes are channeled into the revolving arm, and they trickle through holes in the bottom of the arm onto a gravel and rock bed.The rocks are coated with microorganisms growing within a biofilm that convert much of the organic materials into inorganic matter as the sewage waste trickles through the bed.The effluent has a greatly reduced load of degradable organic materials when it reenters the environment.

respiration, ultimately reducing them to form nitrogen gas (N2) (see figure 6.20). The nitrogen gas is inert, non-toxic, and easily removed. ■ denitrification, p. 776

Phosphates are generally removed using chemicals that combine with phosphates, causing them to precipitate. Microbial processes can also be used, however. For example, bacteria that take up phosphate in a medium and accumulate the excess in intracellular storage granules can be used to extract the compound. The bacteria, along with their polyphosphate granules (volutin), can easily be removed. ■ storage granules, p. 67

Anaerobic Digestion

Within the anaerobic digester, anaerobic microorganisms act on the solids removed during the sedimentation steps of primary and secondary treatment. Various populations act sequentially, ultimately converting much of the organic material to methane:

■ Organic compounds —> organic acids, CO2, H2

Many sewage treatment plants are equipped to use the methane that is generated, thereby avoiding the cost of other sources of energy to run their equipment.

Arm rotates slowly as water sprays onto baseball-sized rocks covered with a variety of microorganisms

Figure 31.2 Trickling Filter Sewage wastes are channeled into the revolving arm, and they trickle through holes in the bottom of the arm onto a gravel and rock bed.The rocks are coated with microorganisms growing within a biofilm that convert much of the organic materials into inorganic matter as the sewage waste trickles through the bed.The effluent has a greatly reduced load of degradable organic materials when it reenters the environment.

Arm rotates slowly as water sprays onto baseball-sized rocks covered with a variety of microorganisms

Water collection-goes either to advanced treatment or to a river to reenter environment

Water from primary treatment

Water collection-goes either to advanced treatment or to a river to reenter environment

Water from primary treatment

31. Environmental Microbiology: Treatment of Water, Wastes and Polluted Habitats

790 Chapter 31 Environmental Microbiology: Treatment of Water,Wastes, and Polluted Habitats

790 Chapter 31 Environmental Microbiology: Treatment of Water,Wastes, and Polluted Habitats

Methods Used For Testing Water Nester

Anaerobic digestion of sludge

Sewage is aerated to stimulate aerobic organisms that break down waste.

In this pond, algae feed on and transform harmful nutrients.

Meadow grasses trap remaining pollutants.

Anaerobic digestion of sludge

Sewage is aerated to stimulate aerobic organisms that break down waste.

Water flows into the marsh where pollutants are further metabolized by bacteria and other microorganisms attached to the plant roots.

In this pond, algae feed on and transform harmful nutrients.

Meadow grasses trap remaining pollutants.

Water flowing here Is now cleared of pollutants and carried into a lake, stream, or river.

Figure 31.3 Artificial Wetland

After anaerobic digestion, water is removed from the remaining sludge, generating a nutrient-rich product called stabilized sludge. This may be incinerated or disposed of in landfills, but can also be used to improve soils and promote plant growth. For example, the sludge generated by the city of Milwaukee, Wisconsin, is used to produce Milorganite®, a fertilizer used on lawns, gardens, golf courses, and playfields. An increasing number of sewage treatment facilities are finding similar ways to recycle the treated sludge they generate. Concerns exist, however, about heavy metals, viruses, and other pollutants that may be concentrated in the product.

Individual Sewage Treatment Systems

Rural dwellings customarily rely on septic tanks for sewage disposal. In theory, the septic tank makes sense; in practice, however, it often does not work correctly. Sewage is collected in a large tank in which much of the solid material settles and is degraded by anaerobic microorganisms (figure 31.4). The fluid overflow from the tank has a high BOD and must be passed through a drainage field of sand and gravel designed to allow oxidation of the organic material in the same manner described for the trickling filter. The process, however, depends on adequate aeration and sufficient action by aerobic organisms in the drainage filed. Unfortunately, certain conditions such as a clay soil under a drainage field may prevent adequate drainage, allowing anaerobic conditions to develop. Toxic materials may inhibit microbial activity in the drainage field. Drainage from a septic tank may contain pathogens; therefore, the tank must never be allowed to drain where it can contaminate water supplies.

Nester-Anderson-Roberts: I V. Applied Microbiology I 31. Environmental I I © The McGraw-Hill

Microbiology, A Human Microbiology: Treatment of Companies, 2003 Perspective, Fourth Edition Water, Wastes and

Polluted Habitats

31.2 Drinking Water Treatment and Testing 791

Venting

Venting

Waste water from house _

Scum

Anaerobic digestion

Venting

Anaerobic digestion

Gravel -

Leaching field i^cir

Perforated pipes ,

Perforated pipes , ooooooooooo ooooooooooo

Figure 31.4 Septic Tank Sewage wastes enter the tank from the house.Within the tank, solid materials settle and undergo anaerobic degradation. Materials that do not settle exit through the outlet pipe, which permits seepage into the drainage field. Conditions in the drainage field must be aerobic so that the materials remaining can be degraded by the activities of aerobic microorganisms. If the drainage area is not properly designed, contaminated materials readily enter adjacent waters.

Sludge

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  • veera
    What is sewage microbiology?
    3 years ago
  • alice jamieson
    Can a microbiologist work in a sewage plant?
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    What is microbiology if sewage treatment?
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