Microbiology and Immunology

Wastewater treatment WORLD OF MICROBIOLOGY AND IMMUNOLOGY

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antibodies to the bacterium are not prevalent in the early

stages of the disease. Thus, a patient who had contracted

syphilis—but who is in the earliest stages of infection—could

produce a negative Wasserman test. This can compromise

patient health and treatment, as syphilis becomes more serious

as the disease progresses with time.

See alsoBacteria and bacterial infection; Laboratory tech-

niques in immunology

WWastewater treatmentASTEWATER TREATMENT

Wastewater includes the sewage-bearing water that is flushed

down toilets as well as the water used to wash dishes and for

bathing. Processing plants use water to wash raw material and

in other stages of the wastewater treatment production

process. The treatment of water that exits households, pro-

cessing plants and other institutions is a standard, even man-

dated, practice in many countries around the world. The

purpose of the treatment if to remove compounds and microor-

ganismsthat could pollute the water to which the wastewater

is discharged. Particularly with respect to microorganisms, the

sewage entering a treatment plant contains extremely high

numbers of bacteria, viruses, and protozoathat can cause dis-

ease if present in drinking water. Wastewater treatment lowers

the numbers of such disease-causing microbes to levels that

are deemed to be acceptable from a health standpoint. As well,

organic matter, solids, and other pollutants are removed.

Wastewater treatment is typically a multi-stage process.

Typically, the first step is known as the preliminary treatment.

This step removes or grinds up large material that would oth-

erwise clog up the tanks and equipment further on in the treat-

ment process. Large matter can be retained by screens or

ground up by passage through a grinder. Examples of items

that are removed at this stage are rags, sand, plastic objects,

and sticks.

The next step is known as primary treatment. The

wastewater is held for a period of time in a tank. Solids in the

water settle out while grease, which does not mix with water,

floats to the surface. Skimmers can pass along the top and bot-

tom of the holding tank to remove the solids and the grease.

The clarified water passes to the next treatment stage, which is

known as secondary treatment.

During secondary treatment, the action of microorgan-

isms comes into play. There are three versions of secondary

treatment. One version, which was developed in the mid-nine-

teenth century, is called the fixed film system. The fixed film

in such a system is a film of microorganisms that has devel-

oped on a support such as rocks, sand, or plastic. If the film is

in the form of a sheet, the wastewater can be overlaid on the

fixed film. The domestic septic system represents such a type

of fixed film. Alternatively, the sheets can be positioned on a

rotating arm, which can slowly sweep the microbial films

through the tank of wastewater. The microorganisms are able

to extract organic and inorganic material from the wastewater

to use as nutrients for growth and reproduction. As the micro-

bial film thickens and matures, the metabolic activity of the

film increases. In this way, much of the organic and inorganic

load in the wastewater can be removed.

Another version of secondary treatment is called the

suspended film. Instead of being fixed on a support, microor-

ganisms are suspended in the wastewater. As the microbes

acquire nutrients and grow, they form aggregates that settle

out. The settled material is referred to as sludge. The sludge

can be scrapped up and removed. As well, some of the sludge

is added back to the wastewater. This is analogous to inocu-

lating growth media with microorganisms. The microbes in

the sludge now have a source of nutrients to support more

growth, which further depletes the wastewater of the organic

waste. This cycle can be repeated a number of times on the

same volume of water.

Sludge can be digested and the methane that has been

formed by bacterial fermentationcan be collected. Burning of

the methane can be used to produce electricity. The sludge can

also be dried and processed for use as compost.

A third version of secondary treatment utilizes a spe-

cially constructed lagoon. Wastewater is added to a lagoon and

the sewage is naturally degraded over the course of a few

months. The algae and bacteria in the lagoon consume nutri-

ents such as phosphorus and nitrogen. Bacterial activity pro-

duces carbon dioxide. Algae can utilize this gas, and the

resulting algal activity produces oxygen that fuels bacterial

activity. A cycle of microbiological activity is established.

Bacteria and other microorganisms are removed from

the wastewater during the last treatment step. Basically, the

final treatment involves the addition of disinfectants, such as

chlorine compounds or ozone, to the water, passage of the

water past ultraviolet lamps, or passage of the water under

pressure through membranes whose very small pore size

impedes the passage of the microbes. In the case of ultraviolet

irradiation, the wavelength of the lamplight is lethally disrup-

tive to the genetic material of the microorganisms. In the case

of disinfectants, neutralization of the high concentration of the

chemical might be necessary prior to discharge of the treated

water to a river, stream, lake, or other body of water. For

example, chlorinated water can be treated with sulfur dioxide.

Chlorinationremains the standard method for the final

treatment of wastewater. However, the use of the other sys-

tems is becoming more popular. Ozone treatment is popular in

Europe, and membrane-based or ultraviolet treatments are

increasingly used as a supplement to chlorination.

Within the past several decades, the use of sequential

treatments that rely on the presence of living material such as

plants to treat wastewater by filtration or metabolic use of the

pollutants has become more popular. These systems have been

popularly dubbed “living machines.” Restoration of waste-

water to near drinking water qualityis possible.

Wastewater treatment is usually subject to local and

national standards of operational performance and quality in

order to ensure that the treated water is of sufficient quality so

as to pose no threat to aquatic life or settlements downstream

that draw the water for drinking.

See alsoBiodegradable substances; Biofilm formation and

dynamic behavior; Disinfection and disinfectants; Disposal of

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