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To review methodologies of sewage treatment and sludge handling.
One of the problems with sewage waste is the same as all other pollution problems. People do not want to pay to handle, treat, or otherwise dispose of something they no longer have any use for. Sewage treatment is in many cases expensive and getting more expensive. Sewage disposal is accomplished by:
The simplest land disposal method is the pit privy or outhouse. With the advent of indoor plumbing came the septic tank and subsurface disposal field lines. Large scale, high density development required something more sophisticated that used less land area. The modern municipal sewage treatment plant of today employs mechanical, biological, and chemical treatment processes prior to discharging our treated waste into a river or stream. There are also smaller "package" treatment plants, lagoons, and aerated lagoons, which serve smaller residential, commercial, or industrial developments.
Sewage consists of water plus solids and other impurities which are dissolved or carried in suspension. The solids are usually less than 0.1% by weight of the liquid waste, but are the main reason for sewage treatment.
The construction of sewers permits the conveyance of dangerous and offensive wastes from the vicinity of dwellings, but the problem of their treatment and disposal must then be solved.
Although sewage is more than 99.9 per cent water, the small proportion of solids in suspension and solution is highly important in its effects. The solids of sewage are in solution and suspension and include both organic and inorganic matters. The solids may be suspended, dissolved, settleable or in the form of colloids.
Sewage contains inorganic chemical compounds from the water supply and many complex organic materials derived from feces, urine, and other wastes that reach the sewers. The organic substances in sewage can be divided into compounds containing nitrogen and those free from nitrogen. The principal nitrogenous compounds are urea, proteins, amines and amino acids.
The principal non-nitrogenous compounds are fats, soaps, and the carbohydrates, which include cellulose. Industrial wastes in the sewage may result in the presence of unusual chemicals. A typical sewage may contain the following contaminants (mg/liter):
Contaminants Weak Medium Strong
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Total Solids (Suspended and dissolved) 450 800 1250Suspended Solids 96 200 360
BOD 96 210 400
Ammonia Nitrogen 5 12 21
Organic Nitrogen 8 21 40
Chlorides 21 42 83
Phosphorus 6 10 20
Grease 50 100 150
The total count of bacteria in sewage is large but widely varying, principally with strength. Counts at 20°C may be expected to be somewhat lower.
Sewate treatment is somewhat arbitrarily classified as "primary", "secondary", or"tertiary", or "advanced". Probably a better way is to establish the degree of contaminant removal necessary (dictates degree of treatment) and group the necessary processes into a functional group. However, for discussion, this arbitrary classification will be used. In primary treatment, solids are allowed to settle and are removed from the water. Secondary treatment, a further step in purifying waste water, uses biological processes. As sewage enters a plants for primary treatment, it flows through a screen. The screen removes large floating objects such as rags and sticks that may clog pumps and small pipes. The screens vary from coarse to fine, from these with parallel steel or iron bars with openings of about half an inch or more to screens with much smaller openings.
12.4.1.1 Solid Materials.
Some plants use a device known as a comminutor which combines the functions of a screen and a grinder. These devices catch and then cut or shred the heavy solid material. In the process, pulverized matter remains in the sewage flow to be removed later in a settling tank. After the sewage has been screened, it passes into what is called a grit chamber where sand, grit, cinders and small stones are allowed to settle to the bottom. Disposal of screenings can be accomplished by burial, incineration and digestion in sludge digestion tanks.
12.4.1.2 Grease Removal.
More or less oil and grease will be found in all sewages. They tend to form scum in sedimentation chambers, clog fine screens and are considered as interfering with filtration and reducing the efficiency of activated sludge.
12.4.1.3 Scum.
In tanks treating raw sewage, more or less scum is to be expected. The amount appearing will depend upon the amount of greasy and floating material in the sewage and the efficiency of screening. Where much grease is expected, it may be advisable to remove it in a grease-removal tank before sedimentation. When the scumming is completed and the grit removed, the sewage still contains dissolved organic and inorganic matter along with suspended solids. The latter consists of minute particles of matter that can be removed from the sewage treatment in a sedimentation tank. When the speed of the flow of sewage through one of these tanks is reduced, the suspended solids will gradually sink to the bottom. This mass of solids is called raw sludge.
Secondary treatment removes up to 90 per cent of the organic matter in sewage by making use of the bacteria in it. The two principal types of secondary treatment are trickling filters and the activated-sludge process.
12.4.2.1 Trickling Filters.
The trickling filter, also known as the sprinkling or percolating filter is widely used. A trickling filter is a bed of crushed stone, gravel or slag of relatively 12-4 large size, to which settled sewage is applied by sprinkling on the surface. The applied sewage trickles in a thin film over the surfaces of the filtering medium which have become coated with a zoological film. Fine suspended solids are removed and held by the film and colloidal material is absorbed by it. Since air is present in the filter, a large population of aerobic bacteria will inhabit the film and work upon suspended, colloidal and dissolved organic solids which have become concentrated in and upon it. This brings about a reduction of B.O.D., ammonia and organic nitrogen and particularly in the lower part of the bed formation of nitrates. The film includes zoologic-forms and other bacteria, fungi, protozoa and algae. Protozoa feed upon bacteria and will reduce the number of coliforms in the applied sewage about 50 per cent in a 5-ft.-deep filter.
Pretreatment.
Effective presedimentation of the applied sewage is essential to good performance of the filters; otherwise suspended solids may cause clogging, in addition to applying a heavier load than is desirable. It is considered good practice to provide a sedimentation period that will reduce the B.O.D. of the raw sewage about 30 to 50 percent.
12.4.2.2 Activated Sludge.
One of the most important methods of sewage treatment is that employing activated sludge. As the name indicates the method employs a sludge which by aeration and agitation has become flocculent and accumulated a population of aerobic bacteria. This is added to sewage and the mixture or mixed liquor, is agitated in the presence of oxygen. The mixed liquor is then settled, it will be found that much more will have been accomplished than with aeration along. The sludge, which is light and flocculent serves as a vehicle for aerobic bacteria which oxidize organic matter both in solution and suspension and as it moves rapidly through the sewage, absorbs suspended and colloidal matter from the sewage.
There are many methods and processes for dealing with the sludge disposal problem, which is sometimes referred to as the problem of the ultimate disposal. The most common method for disposing of sludge and other waste concentrates consist of digestion followed by filtration and incineration.
Sludge Digestion. Sludge digestion is practiced for the following reasons:
(a) It breaks down the organic matter of the sludge into simple compounds. If the process is continued long enough, a stable and inoffensive sludge will result.(b) It transfers a portion of the solids into liquids and gases, thereby reducing the sludge volume to be handled. The volatile matters of the sludge will be reduced 55 to 75 per cent.
(c) Compaction and reduction of water content occur to amounts given in the preceding article.
(d) The sludge gives up its remaining water more readily and drying on beds can be accomplished economically.
(e) Digested sludge is used as a fertilizer, and the gases produced during digestion are used for fuel.
(f) Coliforms will be reduced by 99.8 per cent by digestion for 30 days at 95 to 100°F.
Results from the activities of two groups of bacteria which are capable of living in the same environment. One group which is normally present in large numbers in sewage, attacks the complex substance which are capable of living in the same environment. One group which is normally present in large numbers in sewage, attacks the complex substance the fats, carbohydrates and proteins and converts then to simple organic compounds. This group includes many acid-forming bacteria which form fatty acids, as butyric, acetic and proprionic and are called volatile and organic acids.
The second group of organisms of importance in anaerobic sludge digestion include those which form methane and carbon dioxide by using the acid, ammonia and other products of the first group of bacteria. The methane formers reproduce more slowly than do the acid formers and also require a rather narrow pH range, from 6.5 to 8.0. Digested sludge consist of 90 to 95 per cent water and the next step in disposal must be the removal of as much of the water as possible.
Water can be removed from sludge by use of a rotating filter drum and suction. As the drum rotates in the sludge, the water is pulled through the filter and the residues are peeled off for disposal. For more effective dewatering, the sludge can be first treated with a coagulant chemical such as lime or ferric chloride to produce larger solids before the sludge reaches the filter.
Drying beds which are usually made of layers of sand and gravel can be used to remove water from sludge. The sludge is spread over the bed and allowed to dry. After a week or two of drying, the residue will be reduced in volume and consequently, will be easier to dispose. Although digested sludge has considerable value as a fertilizer and soil conditioner, most farmers do not want to bother with sludge, and thus other methods of disposal are necessary. Incineration consists of burning the dried sludge to reduce the residue to a safe non-burnable ash. The ash can be disposed of by filling unused land or by dumping it well out into the ocean. Since most of the pollutants have been removed by the burning, the ash should cause very little change in the quality of the ocean waters.
If properly done, chlorination can kill more than 90% of the bacteria in the effluent of a secondary treatment plant.
Chlorination is also used for other objectives in waste water treatments such as:
Biological treatment is capable of solving the majority of our sewage treatment problems. However, today's pollutants are becoming tougher to treat, and increasing in volume while the capability of our streams to assimilate these wastes remains constant. Advance treatment techniques range from extensions of biological treatment capabilities to physical chemical separation techniques such as absorption distillation and reverse osmosis.
Some of the substances found in wastewater that were the focus of advance treatment are:
In addition to sophisticated advanced treatment, improvement in performance of existing secondary plants can be achieved by eliminating overloading. There are two basic ways a plant can be overloaded. One is too much wastewater or too strong (organically) wastewater.
To reduce the problem of too much waste, we can eliminate or reduce infiltration construct surge basins or convert from low rate trickling filters to high rate activated sludge treatment.
To improve the problems associated with organic overloading, surge basins are often helpful. These plants can also be converted to activate sludge processes or improve settling in activated plants by chemical additives.
In summary, the limited capability of waterways to accept any waste regardless of degree of treatment and accessibility of waterways in the main deterrent to continuous expansion of conventional treatment processes.
Ingenuity in applying existing and new methods singularly or in combination may be part to the answer. Inclusion of land disposal as a link in the treatment process may be another answer. Septic tanks have been used as a land disposal method for years. Although modern thinking tends to promote sanitary sewers in lieu of septic tanks, perhaps even this method can be used in combination with others to alleviate our wastewater problem.
Sewage is the liquid conveyed by a sewer. It may consist of any one or a mixture of liquid wastes which will be separately defined. Sanitary sewage, also known as domestic sewage, is that which originates in the sanitary conveniences of a dwelling, business building, factory or institution (industrial making). Storm sewage is liquid flowing in sewers during or following a period of rainfall and resulting there from. Infiltration is the water that has leaked into sewers from the ground.
A sewer is a pipe or conduit, generally closed but normally not flowing full, for carrying sewage. A common sewer is one in which all abutting properties have equal rights of use. A sanitary sewer is one that carries sanitary sewage and excludes so far as possible storm sewage, surface water and ground water. Usually it will also carry whatever industrial wastes are produced in the area that it serves. It is occasionally although improperly, called a separate sewer. A story sewer carries storm sewage, including surface run-off and street wash. A combined sewer is designed to carry domestic sewage, industrial waste and storm sewage. A sewer system composed of combined sewers is known as a combined system, but if storm sewage is carried separately from the domestic and industrial wastes, it is said to a separate system. The term sewerage is applied to the art of collecting treating and disposing of sewage. Sewerage works or sewage works are comprehensive terms encompassing all the structures and procedures required for collecting, treating and disposing of sewage.
A house sewer is a pipe conveying sewage from the plumbing system of a single building to a common sewer or point of immediate disposal. A lateral sewer has no other common sewer discharging into it. A submain sewer is one that receives the discharge of a number of lateral sewers. A main sewer, also known as the trunk sewer receives the discharge of one or more submain sewers. A sewer outfall receives the discharge from the collecting system and conducts it to a treatment plant or point of final disposal. An intercepting sewer is one that cuts transversely a number of other sewers to intercept dry-weather flow, with or without a determined quantity of storm water, if from a combined system. A relief sewer is one that has been built to relieve an existing sewer of inadequate capacity.
Sewage treatment covers any artificial process to which sewage is subjected in order to remove or alter its objectionable constituents so as to render it less dangerous or offensive. This term is to be preferred to sewage purification. Sewage disposal applies to the act of disposing of sewage by any method. It may be done with or without previous treatment of the sewage.
Sewage reclamation. This term implies the use of sewage after treatment to the extent necessary for the expected use.
BOD means Biological or Biochemical Oxygen Demand and is the amount of dissolved oxygen required for the aerobic decomposition of the organic matter present. It is also used as a treatment plant's efficiency indicator.
"Package" plants are fairly small primary and secondary treatment plants built in modules and are used primarily to serve areas where public sewers are not available. They are designed to remove 80-95% of the BOD and suspended solids and practically all of the settleable solids.
Lagoons and aerated lagoons provide nearly the equivalent of secondary treatment but require a large land area. A lagoon is a scientifically constructed pond three to five feet deep, in which sunlight, algae and oxygen interact to reduce BOD. An aerated lagoon is deeper, therefore requires less land, with mechanical aeration added to keep oxygen levels in waste high enough to eliminate odors and to speed up digestion.
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2. How is sewage disposal
accomplished? 3. What is B. O. D.? Why is it
important? 4. Discuss primary and secondary
treatment procedures for sewage. 5. What is activated sludge and
what is its significance in sewage
treatment? 6. What is
sewerage? 7. What are some of the
substances found in wastewater? Are any of these
substances of public health significance?
1. What is
sewage?
