Wastewater...

Outside of the laboratory there is no such thing as "pure" water. Even rain water, for example, will take on gases, solid particles and other "impurities" as it falls through the air to the earth. As water strikes the ground and flows across or through the surface of the earth, as would be expected, it takes on the characteristics of the materials it has encountered. For example, minerals are dissolved and contribute to the dissolved salts that are normally found in waters. At the same time organic matter from decomposed vegetation or from the soil, will also dissolve or be carried along within the water. Thus, waters will have many natural impurities. Generally these impurities do not detract from utilization of the water, e.g., drinking, washing, etc. Our main concern will be with waters that have been used by man and then discarded. While such waters have in the past been commonly referred to as sewage, over the more recent years they have come to be called "Wastewaters".

The term "wastewater" is a broad, descriptive term. Generally it includes liquids and waterborne solids from domestic, industrial or commercial uses as well as other waters that have been used (or "fouled") in man's activities, whose quality has been degraded, and which are discharged to a sewage system. The term "sewage" has been used for many years and generally refers to waters containing only sanitary wastes. However, "sewage" technically denotes any wastewaters which pass through a sewer.

Two general categories of wastewaters, not entirely separable, are recognized : domestic wastewaters and industrial wastewaters. Domestic wastewaters originate principally from domestic, household activities but will usually include waters discharged from commercial and business buildings and institutions as well as ground water. Surface and storm waters may also be present. Domestic wastewaters are usually of a predictable quality and quantity. Industrial wastewaters, on the other hand, originate from manufacturing processes, are usually of a more variable character, and are often more difficult to treat than domestic wastes. While domestic wastewaters can be dealt with in general terms with respect to character and treatment, industrial wastewaters must be examined on an industry-by-industry basis. Generally we deal primarily with domestic wastewaters.

Collection of Wastes...

A network of pipes, pumps and pump stations collect and transport wastewater to the treatment plant.

The length of time required for the wastes to reach a treatment facility is very important and can affect treatment plant efficiency. A velocity of at least two feet per second (2 fps) should be maintained within the collection system to prevent any settling of solids which tend to clog pipes and cause odors.

Manholes should be located every 300 - 500 feet to allow for inspection and cleaning of the sewer. When low land areas and areas a great distance away from the treatment facility must be sewered, pump and lift stations are normally installed. These pump stations lift the wastewater to a higher elevation where it again can flow by gravity or may be pumped under pressure to the treatment facility.

Even though pump stations convey the wastewater to the treatment plant, they can cause operational problems throughout the treatment units. A pump station located just ahead of the plant can cause problems by periodically sending large volumes of flow to the plant one minute and virtually nothing the next minute.

If the velocity of the wastewater is not maintained at 2 fps or higher and solids are settling, anaerobic decomposition of the wastes may take place, releasing hydrogen sulfide gas. This can cause odor problems, damage concrete in the collection system and headworks of the treatment plant, and make the wastes more difficult to treat (i.e. settling of solids is more difficult).

In most wastewater systems the sewer coming into the treatment plant that carries wastes from households, commercial establishments and industry is called a sanitary sewer. While a storm sewer carries storm water runoff from street catch basins, land, roofs of buildings, etc., a system which conveys both sanitary wastes and storm runoff is called a combined sewer.

Combined sewers can cause operational problems at a treatment plant. Unfortunately, most plants with combined sewers are not designed to handle the increased flow loads during storms and usually cause a decrease in plant efficiency. During high flow periods, detention times are decreased, solids may be washed out of the secondary system and large amounts of grit, sand and silt may be washed into the plant.

Even in areas where sanitary and storm are separated, infiltration of storm and/or ground water through joints, cracks or breaks can cause high flow problems, usually witnessed during a storm, replacement or sealing of leaky sections will have to be accomplished.

Sources and Types of Wastewater...

Domestic wastewaters consist primarily of liquid discharges resulting from sanitary facilities, bathing, laundering and cooking activities as well as from other sources. The principal sources of domestic and commercial wastewaters are shown below :

Domestic and Commercial Wastewater Sources...

Human Wastes : Urine and feces
Household Wastes : Laundry, bathing and kitchen
Storm Flows/Street Washings : Sand, grit, etc. and animal wastes
Groundwater Infiltration : Leaky pipes and manholes
Industrial Wastes : Manufacturing process wastes, equipment cleaning and cooling waters

Human Wastes...

Human wastes consist of body discharges, mostly feces and urine, which become part of the wastewater through toilet flushing. These wastes have obvious public health implications and are of importance since they may contain organisms which produce diseases in man. The safe and effective treatment of sanitary wastes constitutes a major objective in wastewater disposal.

Household Wastes...

Household wastes are derived from home laundry operations, bathing, kitchen wastes, from washing and cooking foods and dishwashing. Most of these wastewaters will contain synthetic detergents. Kitchen wastes will consist of food residues as well as greases. Household wastes may also contribute significantly to the wastewater burden when home garbage grinding units are used.

Storm Flows...

Storm flows from rains, storms, and street washings will contain grit, sand, leaves, and other debris from the drained surfaces. In some communities these flows are collected separately for disposal and do not become a part of the community wastewater. Storm flow volume varies with the intensity of rainfall, topography, pavements and roof areas. Combined wastewaters area mixture of domestic or sanitary wastewater and storm waters when both are collected in the same sewers.

Ground Water Infiltration...

Sewers, the collecting devices for wastewaters, are buried in the ground and in some instances may be below ground water levels, particularly when such levels are high because of excessive seasonal rainfall. Because the joints between sections of the pipe forming the sewers are not all tight, ground water at times seeps or enters the sewers by infiltration. Since collecting sewers are usually not under pressure and flow is by gravity only, such infiltration is not only possible but at times amounts to appreciable volumes. The volume of ground water infiltration is sometimes difficult to determine accurately. It is influenced by soil composition, the type of sewer construction, ground water conditions, and rainfall and other weather conditions.

Industrial Wastewaters...

Industrial wastewaters are the discharges of industrial plants and manufacturing processes. Industrial wastewaters can represent, collectively, an important part of community wastewaters and must be considered for successful wastewater treatment plant operation. In some locations industrial wastewater discharges are collected together with other community wastewaters and the mixed wastes are treated together. In other instances, the industry may provide some pretreatment or partial treatment of its wastewaters prior to discharge to the municipal sewers. In still other situations, the volume and character of the industrial waste is such that separate collection and disposal is necessary.

Industrial wastewaters vary widely in composition, strength, flow and volume, depending on the specific industry or manufacturing establishment in the community. The specific composition and volume of the industrial waste will, of course, depend on the use to which the water has been put. Typical industries which produce significant volumes of wastewaters include paper and fiber plants, steel mills, refining and petrochemical operations, chemical and fertilizer plants, meat packers and poultry processors, vegetable and fruit packing operations and many more. Industrial discharges may consist of very strong organic wastewaters with a high oxygen demand, or contain undesirable chemicals which can damage sewers and other structures. They may contain compounds which resist biological degradation or toxic components which interfere with satisfactory operation of the wastewater treatment plant. A less obvious source which must be considered an industrial waste, is thermal discharge since it lowers dissolved oxygen values. Many industries use large quantities of cooling water, with the electric power industry being the largest user. However, the primary metal and chemical industry also use substantial quantities of cooling waters.

Appearance of Wastewater...

Domestic wastewater is a turbid or cloudy appearing liquid containing solid material in suspension. When fresh, it is gray in color, and has a musty but not unpleasant odor. Domestic wastewaters will have present, in varying amounts, all kinds of floating matter such as fecal solids, bit of food, oil, garbage, paper, rags, wood, plastics and other materials disposed of in the daily life of a community. Under certain conditions, as a result of biochemical changes caused by bacteria, the color of the wastewater will gradually change from gray to black. As this happens, foul and unpleasant odors develop and black solids appear on the surface or throughout the liquid. A wastewater that has undergone such a change is called septic.

Composition of Wastewater...

Wastewaters consist of water in which solids exist as settleable particles, dispersed as colloids, which are materials that do not settle readily, or solids in a dissolved state. The wastewater mixture will contain large numbers of microscopic organisms, mostly bacteria, that are capable of consuming the organic component (fats, proteins and carbohydrates) of the mixture and bringing about rapid changes in the wastewater. Since the sources of wastewater as well as the inputs are highly variable and since there is also an active microbial component, the composition of all wastewaters is constantly changing. Prior to entering a wastewater treatment plant, a wastewater is sometimes called raw wastewater or raw sewage.

The solid components of wastewaters actually represents a very small part of most discharges, usually less than 0.1 percent by weight. However, it is this small component of the wastewater that presents the major challenges in wastewater treatment, operation and disposal. Essentially, the water component, the other 99.9 percent can be viewed as providing the volume and the vehicle for transporting the solid and microbial component of the wastewater.

Although the solid component of wastewaters was noted above as consisting of less than 0.1 percent by weight of the wastewater, the common method used to express the components of water is not percentage. The amount of materials commonly found in or added to wastewater are more easily expressed as a concentration in milligrams per liter. This is sometimes still called parts per million. For practical purposes, these terms may be considered equal. For purposes of conversion, one milligram per liter is equivalent to 8.34 pounds per million gallons.

Considered chemically, wastewater is a very complex mixture of components that would be difficult to completely define. In broad terms, it consists of an organic and an inorganic component. Although a variety of chemical tests are used to characterize wastewaters, not all of the chemical components will be discussed, only the most important. Probably the most often measured characteristics of wastewater are suspended solids and BOD. Because solids are an important category in wastewaters, their composition is explained in some detail.

Municipal Wastewater...

Biochemical Oxygen Demand ( BOD )...

Biochemical Oxygen Demand, BOD, as it is commonly abbreviated, is one of the most important and useful parameters (measured characteristics) indicating the organic strength of a wastewater. BOD measurement permits an estimate of the waste strength in terms of the amount of dissolved oxygen required to break down the wastewater. The specifics of the analysis are discussed in detail in Standard Methods for the Examination of Water and Wastewater. The BOD test is one of the most basic tests used in the wastewater field. It is essentially a measure of the biological and the chemical component of the waste in terms of the dissolved oxygen needed by the natural aerobic biological systems in the wastewater to break down the waste under defined conditions. Generally the BOD test is carried out by determining the dissolved oxygen on the wastewater or a diluted mixture at the beginning of the test period, incubating the wastewater mixture at 20°C, and determining the dissolved oxygen at the end of 5 days. The difference in dissolved oxygen between the initial measurement and the fifth day measurement represents the biochemical oxygen demand.

While BOD describes the biological oxidation capacity of a wastewater, it is not a measure of the total potential oxidation of the organic compounds present in the wastewater. A number of chemical tests are used to measure this parameter, either in terms of the oxygen required for virtually complete oxidation, or in terms of the element carbon. Probably the most common test for estimating industrial wastewater strength is the Chemical Oxygen Demand (COD) Test. This test essentially measures the chemical oxidation of the wastewater by a strong oxidizing agent in an acid solution. The value for the COD test is always greater than the BOD test and is not always a good indication of BOD values for the same waste.

A test which measures carbon and which is being used to a greater extent in measuring wastewater strength is the TOC (Total Organic Carbon) test where the carbon is oxidized by catalytic combustion to carbon dioxide and the carbon dioxide is measured.

Dissolved Oxygen ( DO )...

The dissolved oxygen concentrations in a wastewater before and after treatment are very important. While dissolved oxygen concentrations are necessary to carry out the BOD determination, as described above, dissolved oxygen levels are also quite important in determining how satisfactory a biological wastewater treatment plant is operating. For example, for satisfactory biological wastewater decomposition (i.e. treatment) some dissolved oxygen must be present. If it is not, the system will be inefficient and is said to be anaerobic. Septic conditions follow, accompanied by a variety of nuisance conditions such as odor and color changes.

Normally, oxygen is not a very soluble gas and dissolved oxygen concentrations in wastewaters are very low. For example, dissolved oxygen concentrations of a few milligrams per liter (or parts per million) are commonplace in water. The solubility of oxygen is such that dissolved oxygen levels in clean water are affected by temperature and salt concentrations expressed as chlorides.

When microorganisms and an available food supply are present, dissolved oxygen will be consumed. Since many of the components present in a raw wastewater can serve as a nutrient for microorganisms, most domestic wastewaters will undergo some decomposition and usually any available dissolved oxygen supplies are consumed during travel through the sewer system. Generally, raw wastewater will have little if any dissolved oxygen present while wastewater in the aeration tanks, final settling tanks, or in the final effluent will probably have at least measurable dissolved oxygen concentrations.

Nitrogen Compounds...

Domestic wastewaters will contain a number of nitrogen containing compounds. Nitrogen is a significant element in wastewater treatment since it is a necessary nutrient for satisfactory bacterial growth during biological treatment. One group of nitrogen containing compounds, proteins, and their breakdown products, amino acids, have already been noted as an organic food component of wastewaters.

Another organic nitrogen compound that is commonly found in domestic wastewaters, since it is excreted by humans, is urea. Urea will breakdown readily in the presence of most microorganisms to ammonia in domestic wastewaters. Ammonia, an inorganic nitrogen compound, has several sources - humans as an excretory product and as a decomposition product from urea or from protein breakdown.

Two forms of inorganic nitrogen not usually found in raw domestic wastewaters, but which are significant are nitrate and its unstable precursor form, nitrite. Nitrate is actually formed from the oxidation of ammonia in the presence of dissolved oxygen. This is called nitrification and occurs in the course of biological wastewater treatment or in the receiving stream. Because this ammonia oxidation consumes oxygen in the receiving stream and since microorganisms as well as aquatic plants can use nitrates or nitrites as a nutrient and thus be stimulated to excessive growth, the reduction of nitrogen levels in wastewater discharges is sometimes required.

Phosphorous Compounds...

Since early in the 1970's, the presence of phosphorous in domestic wastewaters has received increasing attention. Phosphorous, like nitrogen, is required for the nutrition of bacteria in a wastewater treatment plant. In addition, phosphorous is one of the prime nutrients for plant growth in lakes, ponds, and streams, and as such, can cause, under certain conditions, excessive enrichment or plant growth. These considerations have led to a close examination of phosphorous, its presence in wastewater discharges, and its effect on receiving waters. Phosphorous is most commonly found in the form of inorganic phosphates and originates from human urinary excretions, agricultural run-off and from household detergents. Recent legislation in New York and other states banning or limiting phosphate containing detergents has limited the significance of phosphorous in domestic wastewater.

Inorganic Compounds...

While many inorganic compounds are present in domestic wastewaters, they are not often measured. This includes substances such as sodium, bicarbonates, chlorides, sulfates, calcium, and potassium, as well as others. Among those that have a potential utility through measurement are chlorides and sulfates. Chlorides originate from urine as well as from the original waster supply. Since chlorides are relatively inert and are not affected by passage through the wastewater treatment plant, their concentrations will essentially reach more or less predictable levels. Thus, any deviation from these levels may indicate an unusual input to the treatment plant such as from industrial waste streams or from infiltration of the sewer systems.

Sulfate levels in domestic wastewater treatment are relatively unimportant and are not normally measured. Sulfates are readily broken down under anaerobic conditions to hydrogen sulfide gas which causes safety, odor, and color problems.

The presence of hydrogen sulfide can be dangerous to personnel since this gas is toxic. Generally, when it is formed, being heavier than air, it will collect in manholes, pits or any stagnant areas. When this gas is detected the area should be thoroughly ventilated prior to entering.

While at higher concentrations, hydrogen sulfide affects and deadens the mucous membranes in the nose and no odor can be detected even though the gas is present. Hydrogen sulfide is directly responsible for the rotten-egg odor from septic wastewaters. It also acts as the indirect source of other odors and chemical and physical changes in wastewaters. Since it is a very soluble and reactive gas, only a small amount is sufficient to cause an odor or to cause it to enter into reactions with other compounds. A number of organic compounds react with hydrogen sulfide to form unpleasant, odorous end products. Hydrogen sulfide also reacts readily with inorganic compounds, for example, with any iron or manganese in the water to form dark sulfide residues.

Other Compounds...

Other significant compounds found in domestic wastewater will include various oils, fats, and greases. Oils and greases can be the source of problems in the treatment plant where they may accumulate in the form of "grease-balls" or floating mats. These mats can be a general nuisance to operations, or may even cause explosions or fires.

Domestic wastewaters will also contain small and varying concentrations of dissolved gases. Among the most significant of these are dissolved oxygen and hydrogen sulfide. In addition, domestic wastewaters may contain other gases, such as carbon dioxide resulting from the decomposition of organic matter or nitrogen dissolved from the atmosphere. These gases, although small in amount have important roles in the decomposition and treatment of wastewaters.

Domestic wastewaters may also contain volatile liquids. These are, in general, organic liquids which boil at less than 100 degrees C, as for example, gasoline or many solvents.

Solids in Wastewater...

Since solids are classified in a variety of ways, they should be discussed with regard to the various categorizations that are used as well as with respect to their chemical make-up. There will, of course, be some overlap in the classification method.

Organic Solids...

In domestic wastewater, solids are about 50 percent organic. This fraction is generally of animal or vegetable life, dead animal matter, plant tissue or organisms, but may also include synthetic (artificial) organic compounds. These are substances which contain carbon, hydrogen and oxygen, some of which may be combined with nitrogen, sulfur or phosphorous. The principal organic compounds present in domestic wastewater are proteins, carbohydrates and fats together with the products of their decomposition. These compounds are subject to decay or decomposition through the activity of bacteria and other living organisms and are combustible, that is, they can be ignited or burned. Since the organic fraction can be driven off at high temperatures, they are sometimes called volatile solids.

Inorganic Solids...

Inorganic solids are substances that are inert and not subject to decay. Exceptions to this characteristic are certain mineral compounds or salts - such as sulfates - which under certain conditions can be broken down. Inorganic solids are frequently called mineral substances and include sand, gravel and silt as well as the mineral salts in the water supply which produce the hardness and mineral content of the water. In general, they are non-combustible.

The amount of these solids, both organic and inorganic gives to a wastewater the characteristic frequently termed as its "strength". Actually, the amount or concentration of the organic solids and their capacity to undergo decay or decomposition is the most important part of this strength. The greater the concentration of organic or volatile solids, the stronger the wastewater. A "strong" wastewater can be defined as one containing a large amount of solids, particularly organic solids. A "weak" wastewater is one containing only a small amount of organic solids.

Solids can also be grouped depending on their physical state as suspended solids, colloidal solids and dissolved solids, each of which can include both organic and inorganic solids.

Suspended Solids...

Suspended solids are those which are visible and in suspension in the water. They are the solids which can be removed from the wastewater by physical or mechanical means, such as sedimentation or filtration. More precisely, they are the solids which are retained on the filter mat or glass fiber pad in a Gooch Crucible. Suspended solids will include the larger floating particles and consist of sand, grit, clay, fecal solids, paper, pieces of wood, particles of food and garbage, and similar materials. Suspended solids are approximately 70 percent organic solids and 30 percent inorganic solids, the latter being principally sand and grit. The suspended solids portion consist of settleable solids and colloidal solids.

Settleable Solids...

Settleable solids are that portion of the suspended solids which are of sufficient size and weight to settle in a given period of time, usually one hour. There are those which will settle in an Imhoff Cone in one hour. The results are reported as milliliters of settled solids per liter of wastewater. Settleable solids are approximately 75 percent organic and 25 percent inorganic.

Colloidal Suspended Solids...

Colloidal suspended solids are solids that are not truly dissolved and yet do not settle readily. These are somewhat loosely defined as the differences between the total suspended solids and the settleable solids. There is, at present, no simple or standard laboratory test to specifically determine colloidal matter. Most colloids will not settle out even after long quiescent periods of settling. They constitute that portion of the total suspended solids (about 40 percent) which are not readily removed by physical or mechanical treatment facilities but may be filtered out in a Gooch Crucible. Colloidal solids are about 65 percent organic, 35 percent inorganic, subject to rapid decay, and are an important factor in the treatment and disposal of wastewater.

Dissolved Solids...

The term "dissolved solids" as commonly used in discussing wastewater is not technically correct. Dissolved solids are smaller in size than suspended and colloidal solids. As used, the term means all of the solids which pass through the filter pad of a Gooch Crucible. Of the total dissolved solids, about 90 percent are in true solution and about 10 percent colloidal. Dissolved solids, as a whole, are about 40 percent organic and 60 percent inorganic.

Total Solids...

Total solids, as the term implies, includes all of the solid constituents of a wastewater. Total solids are the total of the organic and inorganic solids or the total of the suspended and dissolved solids. In an average domestic wastewater, total solids are about half organic and half inorganic, and about two-thirds in solution (dissolved) and one-third in suspension. The organic solids, which are subject to decay, constitute the main problem in wastewater treatment.

Solids Determinations...

The solid components of domestic wastewater can be classified in a number of ways. For example, wastewater solids can be categorized on the basis of several operational procedures used in the wastewater treatment laboratory. Total solids may be determined by driving off the water fraction, and suspended solids may be determined by filtering out the solid fraction on a porous pad and drying. Settleable solids may be determined by permitting a sample to settle in a special Imhoff Cone apparatus. The categories used most often in the wastewater treatment field are suspended solids and total solids. The colloidal fraction of domestic wastewaters comprises about 20 percent of the solid component of an "average" wastewater. As stated, this component is characterized by being non-settleable, that is, usually long periods of time would be required for them to settle by gravity alone. Colloidal components of wastewater do not fit neatly into a component that can be determined in the laboratory and colloids are found in both the suspended and dissolved solid fractions. Any estimate of wastewater composition can give only an average composition. The amounts of solids indicated cannot be applied equally to all wastewaters at all times.

Biological Components of Wastewater...

Domestic wastewater also contains countless numbers of living organisms, most of them too small to be visible except when viewed under a microscope, which is why they are called "microorganisms". Typically, a domestic wastewater prior to entering the treatment plant will contain from 100,000 to 1,000,000 microorganisms per milliliter. These microbes have their origin from two general sources: sanitary wastes and the soil. Both wastewaters and soils contain large numbers of microorganisms - especially bacteria. Generally the microorganisms can be regarded as a natural living part of the organic matter found in wastewaters and their presence is most important because they serve a primary function in the degradation of wastes in biological wastewater treatment. In a sense the successful operation of a biological wastewater treatment plant is dependent upon a knowledge of the activities of the microorganisms - especially the bacteria. Efficient treatment then depends on understanding the requirements for optimal growth as well as recognizing unfavorable conditions.

While the majority of the microorganisms found in wastewaters are not harmful to man - that is non-pathogenic (do not cause disease), some microorganisms are pathogenic (disease causing) and always are of great concern in wastewater treatment. Among the diseases that are associated with wastewaters are typhoid fever, dysentery, cholera, and hepatitis.

The microorganisms found in wastewaters are commonly classified by their appearance (morphology). While all microorganisms found in wastewater treatment plants have some role in the decomposition of wastes, probably the three most significant microbial groups in biological treatment are the bacteria, fungi, and protozoa. Bacteria have the primary role of decomposing wastewater compounds, forming settleable solids, and at times are the source of operational problems. The general group called fungi are significant since many operational problems are caused by members of this group. Protozoa are microorganisms that play a key role as predators and help control the bacterial populations.

Bacteria are living organisms, microscopic in size. They consist of a single cell organism and are capable of growth in suspended masses as in the activated sludge process or attached as in trickling filters. There are many different kinds of bacteria, too numerous to elaborate. The group best known to those in the wastewater field are the fecal coliforms - a group of bacteria commonly associated with human excretions. Bacteria have the ability to reproduce rapidly when in intimate contact with their nutrient material (e.g., wastes) and feed readily by taking in food directly through their cell wall. Bacteria occur in three basic shapes: rods (or bacilli), spheres ( or cocci) and spirals. While all of these forms are found in wastewaters, quite often they are found individually enmeshed or associated in masses, slimes or "flocs" as in the activated sludge process. While bacteria have a principal role in biological treatment, under some conditions certain bacterial forms (e.g filamentous bacteria) can cause serious operational problems, especially in settling.

Parasitic Bacteria...

Parasitic bacteria are those which normally live off of another living organism, known as the host, since they require a food supply already prepared for their consumption, and generally do not develop outside the body of the host. The parasitic bacteria are of importance in wastewater. They originate in the intestinal tract of human beings and animals and reach the sewage by means of body discharges. Included among the parasitic bacteria are certain specific types which, during their growth within the body of the host, produce toxic or poisonous compounds that cause disease in the host. These bacteria are called pathogenic bacteria. They may be present in sewage receiving the body discharges of persons ill with such diseases as typhoid fever, dysentery, cholera, or other intestinal infections.

Saprophytic Bacteria...

The saprophytic bacteria are those which feed on dead organic matter, thus decomposing organic solids to obtain their needed nourishment, and producing in turn waste substances which consist of both organic and inorganic solids. By this activity they are of utmost importance in sewage treatment methods designed to facilitate or hasten natural decomposition of the organic solids in sewage. Such processes of decomposition will not progress without their activity. In the absence of bacterial life - sterility - decomposition will not take place. There are many species of saprophytic bacteria, each of which plays a specific role in the breakdown of the organic solids of sewage. Each species tends to die away following completion of its part in the process of decomposition.

All of the bacteria, parasitic and saprophytic, require in addition to food, oxygen for respiration. Certain types of them can use only oxygen dissolved in water, termed dissolved oxygen and sometimes called free or molecular oxygen. These organisms are known as aerobic bacteria and the process of degradation of organic solids which they carry out is termed aerobic decomposition, oxidation or decay. This type of decomposition proceeds in the presence of dissolved oxygen without the production of foul odors or unsightly conditions. Other types of bacteria cannot exist in the presence of dissolved oxygen but must obtain the required supply of this element from the oxygen content of organic and some inorganic solids which is made available by their decomposition. Such microorganisms are termed anaerobic bacteria and the process of degradation of solids which they bring about is called anaerobic decomposition or putrefaction, that is, decomposition in the absence of dissolved oxygen, which results in the production of foul odors and unsightly conditions.

To complicate the reactions involved in the decay of organic matter, certain aerobic types can adjust themselves to live and function in the absence of dissolved oxygen and are termed facultative aerobic bacteria. Conversely, some varieties of anaerobic bacteria can become accustomed to live and grow in the presence of dissolved oxygen and are thus termed facultative anaerobic bacteria.

Such adaptability of the saprophytic bacteria to various sources of oxygen of great importance in the decomposition of organic solids in wastewater and thus in the various treatment procedures.

In addition to food and oxygen, bacteria require moisture to remain alive. This is adequately provided in wastewater by its water component.

In order to function at maximum efficiency bacteria require a favorable temperature. They are very susceptible to changes in temperature in that their rate of growth and reproduction, which is directly proportional to the amount of work done, is definitely and sharply affected by such variations. The larger proportion of the saprophytic types thrive best at temperatures from 20°C to 40°C, or 68°F to 104°F. These are known as mesophilic types. Variations from this temperature range limit the activity of mesophilic bacteria, practically eliminating it at extremely low temperatures and at high temperatures. Mesophilic sludge digestion proceeds most rapidly at 35°C or 95°F. Other bacteria live best at high temperatures, in the range of 55°C to 60*C, or 130°F to 140°F. These are known as thermophilic types. Thermophilic bacteria function in sewage treatment principally in high temperature digestion of sludge solids. A very few types of bacteria find their optimum conditions at low temperatures, 0°C to 5°C, or 32°F to 40°F. These are known as psychrophylic bacteria. Temperatures are of major importance in the operation of wastewater treatment processes.

When all of these environmental conditions of food supply, oxygen, moisture, and temperature are properly maintained at their optimum amounts for the full functioning of the bacteria, decomposition of the wastewater solids proceeds in a natural orderly manner.

Among the other microbiological components that are found in domestic wastewaters in smaller numbers are viruses. Viruses are very small and can be seen only by use of a sophisticated tool called an electron microscope. Viruses are significant since they all must derive their energy and reproduce from living tissue and are thus parasitic.

Among the viruses which are found in domestic wastewaters which cause diseases in man are hepatitis, polio, as well as a variety of intestinal viruses such as ECHO, coxsackie and adenovirus. A common virus found in domestic wastewater that does not infect man but does attack bacteria is a phage or a bacteriophage. Viruses pathogenic to man are usually present in small numbers in relation to coliform bacteria, for example, it is estimated that for approximately every million coliform bacteria there is one infective virus present. Viruses are of special concern in wastewater treatment since many are not destroyed by conventional chlorination procedures.

In addition to the groups of microscopic organisms described above, many larger more complex organisms are found in the biological component of wastewater and play a part in the decomposition of organic matter. These are termed macroscopic, that is visible to the naked eye. They include varieties of worms and insects in various stages of development. Some are active in wastewater treatment plant facilities and others are prevalent in highly polluted streams.

Certain forms of all of these organisms, microscopic and macroscopic, are essential to the orderly decomposition of organic matter in nature, and hence are equally important for the proper functioning of the usual methods of wastewater treatment. As noted, it is the biological organisms that actually carry on the processes of biological waste treatment. A prime responsibility of the operator is to provide the environmental condition best suited for their growth.