CHAPTER V. SEWERAGE AND DRAINAQK (Part 1)

CHAPTER V. SEWERAGE AND DRAINAQK Public sewers .generally receive, and should be constructed to carry off as rapidly as possible, rain water from roofs and roads, waste winter from houses, liquid and solid excreta, and refuse matters from factories. Such sewers must be large enough to carry off flood waters in exceptionally severe storms, or be provided with storm water overflows. In some cases two systems of sewers are constructed — one to carry the rain and surface waters of roofs and roads, the other to carry only the waste and soil from dwellings. This is known as the separate system, and has in some instances enabled local authorities to utilise open surface channels and large existing sewers for conveying the rain and surface waters to streams and rivers, while providing, at a minimum of expense, a very perfect system of sewers of small area for the conveyance of house drainage in a concentrated form to the outfall, perhaps on a sewage farm. It has been shown to be a mistake to suppose that rain water from roofs and roads, thus separated from house drainage, is sufficiently pure to be admitted safely into streams and rivers; analysis shows that the road water of populous towns, thus separated, contains all the elements of danger in almost equal intensity as when mixed indis- criminately in the whole volume of town sewage. The question is one which cannot }ye settled definitely for universal adoption; but local authorities are supposed to consider and be guided by the particular circumstances of 126 SEWERAGE AND DRAINAGE. 127 each district — in Bome the combined system of sewerage with large sewers of the best form, and in others the separate system of sewerage, will be found to present the greatest advantages for adoption. We may here remember that old town sewers were con- structed and used before the introduction of water-closets, for the purpose of carrying off rain waters and the slop and sink waters from houses, and that in many cases these sewers are quite unfit for water-closet drainc^e, although they are employed recklessly for the purpose. Sewers should be laid or built in right lines from point to point; manhole chambers, giving easy access to the sewers, should be built at every point where a change of direction or of gradient occurs. Branch sewers should always enter the main sewer at a manhole chamber, and house drains should enter the sewer, not at right angles, but with a splay or curve in the direction of the current of the sewer. Frequent ventilating openings should also be made. The fall or inclination for public sewers should be such as to give the sewage a velocity of flow of three feet per second to ensure a proper scour. Frequently it is impossible to obtain the necessary fall to produce this result, and then the engineer has to make the best of his opportunities and secure intermittent scouring by means of automatic flushing. The usual fall for public sewers is found to range between 1 in 250 and 1 in 750, though in some towns the sewer has perforce , been laid nearly level, having a fall of only 1 in 5,000, as in Southport, Lancashire. The ventilation of public sewers is of paramount import- ance. Many plans have been devised and tested, but the general practical conclusion arrived at by the most ex- perienced engineers is this: that every main sewer should be provided with large effectual openings for ventilation 128 DOMESTIC SANITARY DRAINAGE AND PLUMBING. every hundred yards at least, or eighteen to each mile of sewer, as a minimum provision for ventilation, and that this allowance may be doubled with advantage to public health, no corresponding disadvantage arising but that of the first cost of the ventilating chambers and gratings. Fifty ventilating oj)enings per mile is not, in my opinion, an excessive provision on town sewers to allow for a certain proportion Ixjing out of use, especially if the gradient be unsatisfactory. When offensive smells are observed to issue from such ventilating gratings the remedy is not found by closing them, but rather by opening additional ventilators, and by taking proper steps to flush and cleanse the sewers, and to provide that the private house drains shall Ije properly constructed, so as to discharge their contents freshly and directly into the main sewer before putrefactive action has been set up in them. • The adoption of charcoal as a deodoriser for such ventila- tion gratings over public sewers has been found unsatisfactory. It has been shown that for every square inch of surface outlet ventilator fifty square inches of charcoal, arranged on open trays, should be provided. This charcoal must be kept dry to be of any practical use, it must be frequently changed, and it becomes clogged with dust. This experience has in- duced engineers and local authorities to disapprove of charcoal for this purpose in public sewering. The importance of the ventilation of sewers and drains arises from various causes, which have been admirably classified and explained by Mr. Baldwin Latham. Heat, he states, is one of the most powerful forces at work in unventilated sewers or drains. The confined air is subjected to repeated expansions and contractions as hot and cold water passes through, so that ordinary water-traps SEWERAGB AND DRAINAGE. 129 are totally inadequate to resist the pressure thus brought to bear upon them. Hot water discharged from pantry and scullery troughs, baths, boilers, and factory wastes, into unventilated sewers and drains, compresses the volume of air, and consequently increases the pressure at all points of the system. If 1,000 cubic inches of air at 32° be heated to 60°, the volume expands to 1,057 cubic inches; if it be heated to 100°, the volume expands to 1,138 cubic inches; and in seeking to expand thus in any unventilated sewer or drain, the water-traps must give way and admit the sewer and drain air to the houses. Every 20' increase of temperature increases the volume of air about one-twentieth, if it be free to expand; but, if confined, it increases the pressure in a proportion beyond the power of resistance of ordinary water-traps. We thus learn the need of ample ventilation to check pressure. The ebb and flow of sewage also compresses and dilates the air. The pressure of the air is inversely as the space it occupies ; therefore every gallon of water entering a sewer must increase the pressure of the air, unless free ventilation is provided for the escape of a corresponding gallon of air. In sudden storms of rain, and where sewers are backed up in seaboard towns by rising tide, this effect and its dangers are greatest. Strong winds blowing into the open, unprotected mouths of sewers at outfall, or even over open ventilators, cause considerable pressure, and necessitate relieving ventilation at opposite ends of sewers and all along its course, and practically suggest the wisdom of considering the position of outfalls to avoid prevailing winds as much as possible. The too rapid gradient or fall of a sewer is also a danger, as it may act like a chimney shaft, and cause a K 130 DOMESTIC SANITARY DRAINAGE AND PLUMBING, back pressure of sewer air. The "grand fall," so often triumphantly paraded by the owners of a house on a hill, may prove a fatal fall if the drain ends in a cesspool or defective sewer without ample ventilation and other safe- guards. A gentleman who often publicly advocates the necessity for sanitary reform in domestic sanitary arrangements, and resides in a locality where there is no proper system of main drainage, but where many of the house drains have the " grand fall " referred to, and discharge into cesspools over- flowing— it may be near wells, etc., in adjoining premises — on a lower level, found his drain choked and requested a plumber to clear it. The drain was found full of grease and could not be cleared without breaking open the pipes, which he was informed were not laid on concrete or with cement joints ; the sink in the scullery was fitted with mid- feather trap not cemented in, and there was no grease trap on drain. On the matter being reported, the gentleman stated the drains were inspected some years previously, when a grease trap was recommended, but had not been fixed since. He considered the cesspool system an ideal one, but in the locality referred to it cost £5 to have the cesspool cleaned, and therefore the residents as a rule scarcely ever had it done! The subject of sewage disposal and public sewer con- struction belongs rather to the domain of sewerage on a larger scale than plumbers are usually called on to design ; nevertheless, it is part of their business to know something of the outfalls at their disposal. The construction of main sewers is so often placed in the hands of careless contractors, to say nothing of possible errors of design, that such main sewers are often found to SEWERAGE AND DRAINAGE. 131 be sewers of deposit, or elongated cesspools, in which dangerous gases are evolved from the decomposing excreta and other sewage matters; and plumbers may take it for granted that in the sewers which they are compelled to use as drain outfalls, dangerous air exists, and they should protect their work accordingly by every means placed within their reach. When engaged to fit up the general plumbing and sanitary appliances in a house the plumber should also be competent to claim and carry out the laying of drains to the outfall, and be accountable for all such important work at the house side of the sewer. The soil-pipes, waste-pipes, and sanitary appliances may be perfect of their kind, yet if the house drains are badly arranged or badly laid all the skill and care of the plumber will be wasted. It is therefore a matter of prime importance that the principles governing work of this kind should be known to plumbers. Some may consider, perhaps, that laying earthenware drains does not pertain to the plumber's trade ; but master plumbers can avoid neither the work nor the responsibility which is connected with it. Cast-iron drains are largely used in America, and are being strongly advocated in this country. Wherever they are adopted the plumber must necessarily be prepared to supply and fix them, if he intends to continue to hold his position. The questions to be considered and settled in connection with any system of house drains will be taken in the following order: — The gradient and dimensions. The material. The construction and arrangement. 132 DOMESTIC SANITARY DRAINAGE AND PLUMBING. The question of proper fall or gradient for house drains is not sufficiently considered or understood. It is most important, as the proper discharge of the foul drainage and consequent purity of the house drain is mainly dependent on a proper falL In ordinary house drainage it is usual to specify for "the greatest fall attainable"; yet what a con- tractor may choose to understand as "the greatest fall attainable" may be quite insufficient to render the drain either satisfactory or safe. When possible, and when he is sufficiently paid for the time involved in the work, the sanitary engineer who specifies for the drain should ascertain the limits of the available gradient from summit to outfall, and lay down definitely the fall which the contractor shall be bound to secure, or, failing to secure, shall be required to report the difficulty to the engineer, or be held liable for consequences. The rule of thumb of "get all the fall you can" is often right, but one should know the point at which one fails to secure sufficient fall for efficient drainage, when special flushing arrangement becomes essential The velocity of flow in circular pipe-drains is the same, we may have seen, whether running full or half full; but in house drains we have the flow running intermittently, sometimes at the depth of one inch, and seldom more than quarter full, as house drains are generally provided much too large for their work. When drains run less than half full their velocity of flow decreases, and their efficiency in removing solid matters decreases in proportion. The best results would follow were it possible to arrange a drainage system of circular pipes, always flowing, about seven-eighths or three-quarters full in every part throughout. As this is not often attainable, it will be well to provide for fall and dimensions mutually adjusted to the work to be done, and to be guided in the determination of gradients SEWERAGE AND DRAINAGE. 133 for velocity by the requirements of drains when flowing quarter full, rather than when flowing quite full. There is a point of flow in all sewers and pipes at which they discharge a larger volume than when flowing quite full. When a circular pipe flows at seven-eighths or three-quarters, the velocity is greater than when flowing full ; when at two- thirds, the velocity is less than when flowing at seven- eighths or three-quarters, though still greater than when flowing full. When flowing half full, the velocity is exactly the same as when flowing full, because the contour and area assume the same proportions in each, and therefore the hydraulic mean depth is always the same in both cases. When at one-third, velocity reduces; and at a quarter full it becomes yet slower, as the following tables will show. The simplest formula for calculating the velocity of flow through sewers or drains under various heads or gradients is this : — V = 55.,/inr2F, in which V = velocity in feet per minute ; H = hydraulic mean depth ; F = fall in feet per mile. Multiply the hydraulic mean depth by twice the fall in feet per mile, find the square root of this product, and multiply it by the constant number 55 ; the result gives the velocity in feet per minute. Multiply this velocity again by the sectional area in square feet of water flowing ; the result gives the discharge in cubic feet per minute. Multiply this discharge by 6'24; the result gives the discharge in gallons per minute. Thus, to find the velocity in a 6-inch circular drain 134 DOMESTIC SANITARY DRAINAGE AND PLUMBING. running full and laid with a fall of one foot in sixty feet, or eighty-eight feet in a mile, find the hydraulic mean depth as hereafter explained, which is 125 ; multiply 88, the fall in feet per mile, by 2, and the product is 176 ; multiply 125 by 176, and the product is 2200 ; find the square root of 22, which is 469042, and multiply this by 55, and the product is 258, or 2579731 exactly, the velocity in feet per minute— 2x88 = 176x125 = 22; V22 = 469042 x 55 = 2579731 = V. Find the section area in square feet of water flowing as hereafter explained, which is '1963, and multiply this by the velocity, and the product is the discharge in cubic feet per minute — •1963 X 258 = 506454 cubic feet per minute ; and again — 506454 X 624 = 316 gallons per minute. The hydraulic mean depth or mean radius of pipe, drain, or sewer, no matter what shape its section may be, is found by dividing the sectional area in feet of the actual water flowing, by the length of the wetted perimeter in feet (the wetted perimeter being that portion of the conduitf in contact with the flowing water). The hydraulic mean depth can be found already cal- culated and' set out in published tables. It will be easier and safer, as a general rule, to depend on such tables for hydraulic mean depth, area, velocity, and discharge, than to work out the results ; but as the tables differ somewhat, owing to the use of different formula in their preparation, it will be well to be able to check such calculations for accuracy in important work. The writer hopes to explain the process clearly, and also to give many useful tables for plumbers in this volume. SEWERAGE AND DRAINAGE. 135 The table of co-efficients is here given for practical use for calculating hydraulic mean depth, as hereafter explained. XX. c. XX. c. XX. a XX. c. ■01 . . -00138 •26 . .. 162 •51 .. •403 •76 .. . 640 •02 . . -00376 •27 . .. -171 •52 .. •413 •77 .. . ^649 •08 . . -00685 •28 . .. -180 •58 .. •423 •78 ., . -657 •04 . . 01064 •29 . .. -189 •54 .. •433 •79 .. . -665 •05 . . -0147 •80 . .. •I 98 •56 .. •443 •80 .. . ^673 •06 . , -0192 •81 . .. -207 •56 .. •458 •81 .. . •681 •07 . . 0242 •82 . .. -217 •57 .. •462 •82 .. . -689 •08 . . -0295 '33 . .. '226 •68 .. •472 •83 .. . -697 •09 . . -086 •34 . .. ^235 •69 .. •482 •84 .. . -704 •10 . . -041 •85 . .. 246 •60 .. •492 •85 .. . -711 •11 . . -047 'Z^ . .. -254 •61 .. •602 •86 .. . -718 •12 . . 058 •87 . .. ^264 •62 .. •512 •87 .. . -726 •IS . . 059 •38 . .. -274 •63 .. •521 •88 .. . ^732 •14 . . -067 •89 . .. -284 •64 .. •531 •89 .. . -738 •15 . . -074 •40 . .. -294 •65 .. •541 •90 .. . -744 •16 . . •OBI •41 . .. -808 '66 .. '661 •91 .. . -750 •17 . . •088 •42 . .. •813 •67 .. •659 •92 .. . ^755 •18 . . •096 •48 . ..