Tank manual pdf

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Ice-rette Promo Sheet. Soil strata in which the percolation rates are in excess of 30 minutes per inch should not be included in computing the absorp- tion area.

Although few data have been collected comparing percola- tion test results with deep pit performance, nevertheless the results of such percolation tests, while of limited value, combined with compe- tent engineering judgment based on experience, are the best means of arriving at design data for seepage pits.

Table 4. A pit of 5 foot diameter and 16 foot depth has an area of 94 -f , or square feet. Table 1 or Figure 3 page 9 gives the absorption area requirements per bedroom for the percolation rate obtained. The effective area of the seepage pit is the vertical wall area based on dug diameter of the pervious strata below the inlet. No allowance should be made for im- pervious strata or bottom area. With this in mind, Table 4 may be used for determining the effective side-wall area of circular or cylindri- cal seepage pits.

Sample Calculations. According to Table I, 3 X or square feet of absorption area would be needed. Assume also that the water table does not rise above 27 feet below the ground surface, that seepage pits with effective depth of 20 feet can be provided, and that the house is in a locality where it is common prac- tice to install seepage pits of 5 feet diameter i.

In other words, one 5 foot diameter pit 36 feet deep would be needed, but since the maximum effective depth is 20 feet in this particular location, it will be necessary to increase the diameter of the pit, or increase the number of pits, or increase both of these. Experience has shown that seepage pits should be separated by a distance equal to 3 times the diameter of the largest pit. For pits over 20 feet in depth, the minimum space between pits should be 20 feet See Fig.

The area of the lot on which the house is to be built should be large enough to maintain this distance between the pits while still allowing room for additional pits if the first ones should fail. If this can be done, such an absorption system may be approved; if not, other suitable sewerage facilities should be required. Digging in wet soils should be avoided as much as possible. Cutting teeth on mechanical equipment should be kept sharp. Bucket augered pits should be reamed to a larger diameter than the bucket.

All loose material should be removed from the excavation. Pits should be backfilled with clean gravel to a depth of one foot above the pit bottom or one foot above the reamed ledge to provide a sound foundation for the lining. Preferred lining materials are clay or concrete brick, block, or rings. Rings should have weep holes or notches to provide for seepage. Brick and block should be laid dry with stag- gered joints. Standard brick should be laid flat to form a four inch wall.

The outside diameter of the lining should be at least six inches less than the least excavation diameter. The annular space formed should be filled with clean, coarse gravel to the top of the lining as shown in Figure Either brick dome or flat concrete covers are satisfactory. They should be based on undisturbed earth and extend at least 12 inches beyond the excavation and should not bear on the lining for structural support.

Bricks should be either laid in cement mortar or have a two inch covering of concrete. If flat covers are used, a prefabricated type is preferred, and they should be reinforced to be the equivalent in strength of an approved septic tank cover. A nine inch capped opening in the pit cover is convenient for pit inspection. All concrete surfaces should be located with a protective bitumastic or similar compound to minimize corrosion.

Connecting lines should be of a sound, durable material the same as used for the house to septic tank connection. All connecting lines should be laid on a firm bed of undisturbed soil throughout their length. The grade of a connecting line should be at least two percent.

The pit inlet pipe should extend horizontally at least one foot into the pit with a tee or ell to divert flow downward to prevent washing and eroding of the sidewalls.

If multiple pits are used, or in the event repair pits are added to an existing system, they should be connected in series. Abandoned seepage pits should be filled with earth or rock. Functions of Septic Tanks Untreated liquid household wastes sewage will quickly clog all but the most porous gravel formations. The tank conditions sewage so that it may be more readily percolated into the subsoil of the ground.

Three functions take place within the tank to provide this protection. Removal of Solids. As sewage from a building sewer enters a septic tank, its rate of flow is reduced so that larger solids sink to the bottom or rise to the surface. These solids are retained in the tank, and the clarified effluent is discharged. Biological Treatment. Bacteria present are of a variety called anaerobic which thrive in the absence of free oxygen. This decomposition or treatment of sewage under anae- robic conditions is termed "septic," hence the name of the tank.

Sewage which has been subjected to such treatment causes less clogging than untreated sewage containing the same amount of suspended solids. Sludge and Scum Storage. Sludge, and scum to a lesser degree, will be digested and compacted into a smaller volume. However, no matter how efficient the process is a residual of inert solid material will remain.

Space must be provided in the tank to store this residue during the interval between cleanings; otherwise, sludge and scum will eventually be scoured from the tank and may clog the disposal field. If adequately designed, constructed, maintained, and operated, septic tanks are effective in accomplishing their purpose.

The relative position of a septic tank in a typical subsurface disposal system is illustrated in Figure 11 page The liquid contents of the house sewer A are discharged first into the septic tank B , and finally into the subsurface absorption field C.

The heavier sewage solids settle to the bottom of the tank, forming a blanket of sludge. The lighter solids, including fats and greases, rise to the surface and form a layer of scum.

A considerable portion of the sludge and scum are liquefied through decomposition or digestion. During this process, gas is liberated from the sludge, carrying a portion of the solids to the surface, where they accumulate with the scum.

Ordi- narily, they undergo further digestion in the scum layer, and a portion settles again to the sludge blanket on the bottom. This action is re- tarded if there is much grease in the scum layer. The settling is also retarded because of gasification in the sludge blanket.

Furthermore, there are relatively wider fluctuations of flow in small tanks than in the large units. This effect has been recognized in Table 5 page 29 , which shows the recommended minimum liquid capacities of household septic tanks.

Underground pollution usually moves in the same general direction as the normal movement of the ground water in the locality. Ground water moves in the direc- tion of the slope or gradient of the water table, i. In general, the water table follows the general contour of the ground surface. For this reason, septic tanks should be located downhill from wells or springs. Sewage from disposal systems occasionally contaminate wells having higher surface elevations. Obviously, the elevations of disposal systems are almost always higher than the level of water in such wells as may be located nearby; hence, pollution from a disposal system on a lower surface elevation may still travel downward to the water bearing stra- tum as shown in Figure 12, below.

It is necessary, therefore, to rely Figure Tanks should never be closer than 50 feet from any source of water supply: and greater distances are preferred where possible.

The septic tank should not be located within 5 feet of any building, as structural damage may result during construction or seepage may enter the basement. The tank should not be located in swampy areas, nor in areas subject to flooding.

In general, the tank should be located where the largest possible area will be available for the disposal field. Consideration should also be given to the location from the standpoint of cleaning and maintenance. Where public sewers may be installed at a future date, provision should be made in the household plumbing system for connection to such sewer. Although the sewage undergoes treatment in passing through the tank, this does not mean that infecti- ous agents will be removed; hence, septic tank effluents cannot be considered safe.

The liquid that is discharged from a tank is, in some respects, more objectionable than that which goes in; it is septic and malodorous. This, however, does not detract from the value of the tank.

As previously explained, its primary purpose is to condition the sewage so that it will cause less clogging of the disposal field. Further treatment of the effluent, including the removal of patho- gens, is effected by percolation through the soil. Disease producing bacteria will, in time, die out in the unfavorable environment afforded by soil.

In addition, bacteria are also removed by certain physical forces during filtration. This combination of factors results in the eventual purification of the sewage effluent. Studies have proved that liberal tank capacity is not only important from a functional standpoint, but is also good economy. The liquid capacities recommended in Table 5 allow for the use of all household appliances, including garbage grinders. Table 5. Specifications for Septic Tanks Materials.

Properly cured precast and cast-in-place reinforced con- crete tanks are believed to be acceptable everywhere. Steel tanks meet- ing Commercial Standard of the U. Department of Commerce are generally acceptable. Special attention should be given to job built tanks to insure water tightness.

Heavyweight concrete block should be laid on a solid foundation and mortar joints should be well filled. Some typical septic tanks are illustrated in Figure 13 page Suggested specifications for watertight concrete are given in Appendix E.

Precast tanks should have a minimum wall thickness of 3 inches, and should be adequately reinforced to facilitate handling. When pre- cast slabs are used as covers, they should be watertight, have a thickness of at least 3 inches, adequately reinforced.

All concrete surfaces should be coated with a bitumastic or similar compound to minimize corrosion. Settlement of backfill may be done with the use of water, provided the material is thoroughly wetted from the bottom upwards and the tank is first filled with water to prevent floating.

Adequate access must be provided to each compartment of the tank for inspection and cleaning. Both the inlet and outlet devices should be accessible. Access should be provided to each compartment by means of either a removable cover or a 20 inch manhole in least dimension. Where the top of the tank is located more than 18 inches below the finished grade, manholes and inspection holes should extend to approximately 8 inches below the finished grade see Figure 14, page 32 , or can be extended to finished grade if a seal is provided to keep odors from escaping.

In most instances, the extension can be made using clay or concrete pipe, but proper attention must be given to the accident hazard involved when manholes are extended close to the ground surface. Typical single and double compartment tanks are illustrated in Figures 15 and 17, pages 33 and This free drop prevents backwater and stranding of solid material in the house sewer leading to the tank.

A vented inlet tee or baffle should be provided to divert the incom- ing sewage downward. It should penetrate at least 6 inches below the liquid level, but in no case should the penetration be greater than that allowed for the outlet device. A number of arrangements com- monly used for inlet and outlet devices are shown in Figure 16 page A vertical section of a properly operating tank would show it divided into three distinct layers; scum at the top, a middle zone free of solids called "clear space" , and a bottom layer of sludge.

The outlet device retains scum in the tank, but at the same time, it limits the amount of sludge that can be accommodated without scouring, which results in sludge discharging in the effluent from the tank. Observations of sludge accumulations in the field, as 30 VII Figure Figure For horizontal, cylindrical tanks, this should be reduced to 35 percent. The outlet device should extend above the liquid line to approxi- mately one inch from the top of the tank.

The space between the top of the tank and the baffle allows gas to pass off through the tank into the house vent. Tank Proportions. Also, for tanks of a given capacity and depth, the shape of a septic tank is unimportant. However, it is recommended that the smallest plan dimension be at least 2 feet. Liquid depth may range between 30 and 60 inches. Although some variation is to be expected, on the average about 30 percent of the total scum will accumulate above the liquid line.

In addition to the provision for scum storage, one inch is usually provided at the top of the tank to permit free passage of gas back to the inlet and house vent pipe. For tanks having straight, vertical sides, the distances between the top of the tank and the liquid line should be equal to approximately 20 percent of the liquid depth.

In horizontal, cylindrical tanks, area equal to approximately 15 percent of the total circle should be pro- vided above the liquid level. This condition is met if the liquid depth distance from outlet invert to bottom of tank is equal to 79 percent of the diameter of the tank.

Use of Compartments. These can be either separate units linked together, or sections enclosed in one continuous shell as in Figure 17, page 35, with water- tight partitions separating the individual compartments. A single compartment tank will give acceptable performance. The available research data indicate, however, that a two compartment tank, with the first compartment equal to one-half to two-thirds of the total volume, provides better suspended solids removal which may be especially valuable for protection of the soil absorption system.

Tanks with three or more equal compartments give at least as good per- formance as single compartment tanks of the same total capacity. An access manhole should be provided to each compartment. Vent- ing between compartments should be provided to allow free passage of gas. Inlet and outlet fittings in the compartmented tank should be proportioned as for a single tank.

See Figure 16, page The same allowance should be made for storage above the liquid line as in a single tank. If either the sludge or scum ap- proaches too closely to the bottom of the outlet device, particles will be scoured into the disposal field and will clog the system. Eventually, when this happens, liquid may break through to the ground surface. When a dis- posal field is clogged in this manner, it is not only necessary to clean the tank, but it also may be necessary to construct a new disposal field.

There are wide differences in the rate that sludge and scum will accumulate from one tank to the next. For example, in one case out of 20, the tank will reach the danger point, and should be cleaned, in less than 3 years. Tanks should be inspected at least once a year and cleaned when necessary. Although it is difficult for most homeowners, actual inspection of sludge and scum accumulations is the only way to determine definitely when a given tank needs to be pumped.

When a tank is inspected, the depth of sludge and scum should be measured in the vicinity of the outlet baffle. The tank should be cleaned if either: a The bottom of the scum mat is within approximately 3 inches of the bottom of the outlet device; or b sludge comes within the limits specified in Table 6 see Figure 18, page Table 6.

Scum can be measured with a stick to which a weighted flap has been hinged, or with any device that can be used to feel out the bottom of the scum mat. The stick is forced through the mat, the hinged flap falls into a horizontal position, and the stick is raised until resistance from the bottom of the scum is felt. With the same tool, the distance to the bottom of the outlet device can be found see Figure 18, page A long stick wrapped with rough, white toweling and lowered to the bottom of the tank will show the depth of sludge and the liquid depth of the tank.

The stick should be lowered behind the outlet device to avoid scum particles. After several minutes, if the stick is carefully removed, the sludge line can be distinguished by sludge particles cling- ing to the toweling.

In most communities where septic tanks are used, there are firms which conduct a business of cleaning septic tanks. The local health department can make suggestions on how to obtain this service. Tanks should not be washed or disinfected after pumping. A small residual of sludge should be left in the tank for ceding purposes.

The material removed may be buried in uninhabited places or, with permission of the proper authority, emptied into a sanitary sewer system. It should never be emptied into storm drains or ducharged directly into any stream or watercourse.

Methods of dis- posal should be approved by the health authorities. When a large septic tank is being cleaned, care should be taken to enter the tank until it has been thoroughly ventilated and ;ases have been removed to prevent explosion hazards or asphyxiation the workers. Figure T8. Grease Interceptors.

The discharge from a garbage grinder should never be passed through them. The septic tank capacities recommended in this manual are sufficient to receive the grease normally discharged from a home. In general, the addi- tion of chemicals to a septic tank is not recommended. Some proprie- tary products which are claimed to "clean" septic tanks contain sodium hydroxide or potassium hydroxide as the active agent.

Such compounds may result in sludge bulking and a large increase in alkalinity, and may interfere with digestion. The resulting effluent may severely dam- age soil structure and cause accelerated clogging, even though some tem- porary relief may be experienced immediately after application of the product. Frequently, however, the harmful effects of ordinary household chemicals are overemphasized.

Small amounts of chlorine bleaches, added ahead of the tank, may be used for odor control and will have no adverse effects. Small quantities of lye or caustics normally used in the home, added to plumbing fixtures is not objectionable as far as operation of the tank is concerned.

If the septic tanks are as large as herein recommended, dilution of the lye or caustics in the tank will be enough to overcome any harmful effects that might otherwise occur.

Some 1, products, many containing enzymes, have been placed on the market for use in septic tanks, and extravagant claims have been made for some of them. As far as is known, however, none has been proved of advantage in properly controlled tests.

Soaps, detergents, bleaches, drain cleaners, or other material, as nor- mally used in the household, will have no appreciable adverse effect on the system. However, as both the soil and essential organisms might be susceptible to large doses of chemicals and disinfectants, moderation should be the rule.

Advice of responsible officials should be sought before chemicals arising from a hobby or home industry are discharged into the systems. For household installations, it is usually more economical to provide a single disposal system than two or more with the same total capacity. Normal household waste, including that from the laundry, bath, and kitchen, should pass into a single system. Roof drains, foundation drains and drainage from other sources pro- ducing large intermittent or constant volumes of clear water should not be piped into the septic tank or absorption area.

Drainage from garage floors or other sources of oily waste should also be excluded from the tank. Toilet paper substitutes should not be flushed into a septic tank. Paper towels, newspaper, wrapping paper, rags, and sticks may not decompose in the tank, and are likely to lead to clogging of the plumb- ing and disposal system. Waste brines from household water softener units have no adverse effect on the action of the septic tank, but may cause a slight shorten- ing of the life of a disposal field installed in a structured clay type soil.

Adequate venting is obtained through the building plumbing if the tank and the plumbing are designed and installed properly. A separate vent on a septic tank is not necessary. A chart showing the location of the septic tank and disposal system should be placed at a suitable location in dwellings served by such a system. Whether furnished by the builder, septic tank installer, or the local health department, the charts should contain brief instructions as to the inspection and maintenance required.

The charts should assist in acquainting homeowners of the necessary maintenance which septic tanks require, thus forestalling failures by assuring satisfactory opera- tion. The extension of the manholes or inspection holes of the septic tank to within 8 inches of the ground surface will simplify maintenance and cleaning. Abandoned septic tanks should be filled with earth or rock. The septic tank should be filled with water and allowed to stand overnight to check for leaks. If leaks occur, they should be repaired.

The soil absorption system should be promptly inspected before it is covered to be sure that the disposal system is installed properly. Prompt inspection before backfilling should be required by local regulations, even where approval of plans for the subsurface sewage disposal system has been required before issuance of a building permit.

Backfill material should be free of large stones and other deleterious material and should be overfilled a few inches to allow for settling. In general, the usefulness of the septic tank system decreases as the size of the establishment served increases.

Lack of competent sanitary engi- neering advice in the development of such systems generally will lead to failures, excessive costs, and a multitude of troubles. The soundest advice available to anyone contemplating such a system is the early retention of competent sanitary engineering consultation, whose first determination should be the suitability of this method of sewage dis- posal for the proposed establishment. Any institutional septic tank system must incorporate appurtenances and supplemental features of design to meet the requirements of the establishment and varying site conditions.

These can be generally suc- cessful when appropriate experience, study, and planning are employed in the choice and development of such a system. This manual cannot present all of the results of experience gained in the design and opera- tion of such systems, but describes the most generally successful proce- dures and practices as a guide to engineers designing them.

While many effluents from institutional septic tanks are disposed of by soil absorption methods, others are discharged to available water- courses after suitable treatment. When soil absorption systems are con- templated, it is essential, as described in Part I, to determine the char- acteristics and suitability of the soil as the first step toward design.

As a matter of fact, the wise builder of an establishment will explore this feature on a contemplated site before the site is purchased. In using water meter readings for estimating the quantity of sewage to be contributed, some allowance should also be made for maximum conditions that may not be readily apparent from the readings.

For example, water consumption by an ordinary family of four in an apart- ment building may average 48 gallons of water per person per day over a period of 5 months, but actually range from perhaps 30 gallons per person on certain days to something in excess of 80 gallons per person on days when water consumption is heaviest, as on washdays.

Besides these peak loads, some allowance should be made for the sewage con- tributed by occasional guests. Therefore, when computing sewage flows from average meter readings, a minimum factor of safety gf about 25 percent should be allowed to cover the range of variations. Accordingly, the design of a disposal system for the apartment house referred to, where the average usage is 48 gallons per person per day, should be based upon a computed maximum usage of at least 60 gallons per person per day.

Conversely, unusually high meter readings may be caused by lawn sprinkling or by leakage of water that does not enter the disposal sys- tem. Due allowances should be made for abnormalities of this kind. Where measurements of water consumption are not possible, as where water meter records are not available, or where disposal facilities are being planned for a new building, it is necessary to use other methods of estimating the amount of sewage to be discharged.

One way is to base the estimated flow on the number of bedrooms, as in Part I. Another way is to compute the flow on the basis of the number and kinds of plumbing fixtures.

If the building is used as a restaurant, the number of patrons or the number of meals served may be the best cri- terion. The competent designer will base his estimates upon a combina- tion of the various influencing factors. He will consider each case on its own merits, especially when disposal facilities are being designed for a large institution where the cost of construction will amount to a con- siderable sum.

If definite information and accurate water measurements are not available, the quantity of sewage may be estimated from experi- ences at establishments similar to that for which the new sewage dis- posal facilities are intended. Table 7 page 43 may be helpful in such cases. The quantities listed in the table are merely the best averages avail- able at this time, and they should be modified in localities or estab- lishments where experience indicates a need for so doing.

The decision as to the number of disposal systems may be influenced by conditions of terrain, topography, and locations of the buildings con- tributing to the wastes. At large camps, for example, and at some re- sorts, kitchens and central dining facilities may be located at appre- ciable distances from the barracks or cottages and cabins. Under such circumstances, the kitchens may be provided with separate disposal systems, including facilities for the removal of grease ahead of the septic tank.

Table 7. When this is done, the total per capita flow must be broken down into its component parts, and some allowance should be made for the amount of sewage tributary to the different disposal systems. Table 8 below illustrates how this may be done where there are no definite data as to the exact distribution of flow. Table ft. Kitchen wastes Toilet wastes Showers, washbasins, etc..

Laundry wastes 30 10 15 15 J0 40 7 15 18 10 50 10 20 20 75 10 25 25 15 15 30 35 20 1 No wastes from these uses. For certain types of new establishments, the designing engineer may be unable to obtain from his clients accurate estimates as to the num- ber of patrons to be served by the disposal facilities.

This is particularly true in the case of restaurants and at recreational places, such as picnic areas, country clubs, and the like. In such cases, computations and esti- mates may best be made from the number of plumbing fixtures in- stalled. Table 9 indicates average values for quantities of sanitary wastes per fixture at country clubs with modern plumbing. Table 9. Lavatories Gallons per day per fixture Type of fixture Toilets Urinals Sinks Gallons per day per fixture 50 Estimates of sewage quantities from golf clubs should be checked and calculations based on the weekend population.

Allowances of 10 gallons per person for showers and 7 gallons per person for toilet and kitchen wastes, both for the average weekend population, have been found reasonable. Table 10 shows one method used in estimating the amount of sewage discharged hourly during the hours when public parks are open.

Simi- lar figures may be used for fairgrounds, carnivals, ball parks, etc. Table Table 11 and the curves in Figure 19 do not allow for wastes from garbage grinders and automatic washing machines. Discharge from these appliances to an institutional septic tank system calls for extra capacity of 20 and 40 percent, respectively, over the calculated absorp- tion area values.

If both of these appliances are installed, the absorption area should be increased by about 60 percent over the calculated value. Part I allowed for these types of waste in giving specifications for small household systems, but they may never occur in institutional effluents, such as those from factories and offices. Other institutions, such as a country club or drive-in theatre with eating facilities, may contribute wastes from garbage grinders, but not from automatic washing ma- chines.

As previously emphasized, all institutions should have their systems designed by an engineer who is competent to place proper evaluations on the kind of wastes to be contributed. Use of Table 11 is demonstrated in the following examples, which illustrates the design for the two types of soil absorption systems. For each example assume: 5, gallons of sewage per day to be disposed of; percolation rate, 1 inch in 5 minutes.

Example I. Example II. Effective sidewall area equals total area needed. Obviously, more than one pit is required. Design for 3 pits, 10 ft. Use 3 pits, 10 ft. Use 4 pits, 10 ft. Use 5 pits, 10 ft. It usually extends from a point 3 feet outside the building, where it is connected to the building drain. The building sewer should be constructed of cast iron, vitrified clay, concrete, bituminized fiber, asbestos cement, or other durable material.

AH joints in the sewer line should be watertight and rootproof. The grade of the building sewer should be at least 2 percent 2 foot fall per feet, or 14 inch per foot except for the 10 feet immediately pre- ceding the septic tank, where it should not exceed 2 percent. Buildings should be planned so that a proper slope of the building sewer can be obtained. Where the terrain is extremely flat, however, it may be ad- visable to allow a slope of only 6 inches per feet, or 0.

The size of the pipeline to a septic tank is generally dependent upon its ability to allow objects to pass, and upon its capacity to conduct high flows for short periods, rather than upon its capacity in relation to the average flow. No building sewer serving water closets should be less than 4 inches in size.

The relationship of minimum sizes to the slopes and to the number of fixture units is indicated in Table That portion of the sewer line within 50 feet of any well or suction line from a well, or within 10 feet of any drinking water supply line under pressure or within 5 feet of any basement foundation should be durable, corrosion resistant, root proof, and so installed as to remain water tight.

Cast iron or other high strength pipe should be required wherever the line crosses under drive- ways with less than 3 feet of earth cover. While no general statement can be made to cover all cases, Table 2 page 10 should be followed in locating components of the sewage disposal system. For example, a small lavatory is rated as 1 fixture unit; a kitchen sink, 2; a bathtub, 2; a tank operated water closet, 4; and a valve operated water closet, 6.

In general, the collection system should be designed for a capacity expected to be adequate for at least one decade, and preferably two. It is usually ad- visable to design the sewers with capacities, when running full, of not less than 10 times the average estimated flow of sewage. Institutional sewers carrying raw or untreated sewage should be at least 6 inches in diameter and have slopes of at least ys inch to the foot 1.

In very small installations, 4 inch sewers at 14 inch per foot 2 percent may be used to carry raw sewage, and 4 inch sewers with slopes of y8 inch per foot are acceptable for carrying settled sew- age. All sewers should be designed and constructed with hydraulic slopes sufficient to give mean velocities, when flowing full, of not less than 2. Use of other practical "n" values will be per- mitted by the plan reviewing agency for longer pipe sections if deemed justifiable on the basis of research or field data presented.

In general, the following minimum grades should be provided: Percent 8 inch sewers 0. The use of asbestos cement pipe or enamel lined or cement lined cast iron pipe is suggested under such conditions.

Sewer lines must be laid on straight alignment and uniform slope between manholes. Manholes should be placed on sewers at all points of change of slope or alignment, at the upper ends of all sewer lines, and otherwise at intervals not greater than feet.

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