Sewerage and Sewage-Disposal Works at Withington.” by JOSEPHSWARBRICK, Assoc

332 SWARBRICK ON WITHINGTONSEWERAGE. [Selected

(Paper No. 2G63.) (Abridged.)

“ Sewerage and Sewage-disposal Works at Withington.” By JOSEPHSWARBRICK, Assoc. M. Inst. C.E. THE Withington main sewerage and sewage-disposal works have comprised the construction of about 7 miles of main intercepting- sewers, several miles of branch sewers, andthe preparation of about 78 acres of landas a sewage-farm. The workswere primarily designed to comply with the requirementsof the Rivers PollutionPrevention Act 1876, in regardto the district of Withington, which comprises the townships of Burnage, Chorlton- cum-Hardy andDidsbury, with most of the township of Withing- ton. The district covers an area of 5,728 acres, and at the census of 1891 had a population of 25,729. In November, 1877, a scheme prepared by Messrs. Cawley, Newtonand Smith, of Manchester, was adoptedfor conveying the sewage of the Withington district, the estimated population of which was 15,000, by gravitation to land within the district at Chorlton Ees, Chorlton-cum-Hardy. Theurgent necessityfor the extensions and improvement of some of the existing sewers, preparatory to the execution of street-improvement works, caused the Local Board to construct in 1878, at a cost of 510,600, about 2 miles of brick outfall-sewer, utilizing temporarily the outlet of theDidsbury sewer intothe River Mersey. This sewer ranged ih size from 3 feet by 2 feet to 4 feet by 2 feet 8 inches, and the gradient varied between 1 in 250 and 1 in 343. Its depth below thesurface of theground varied between 12 feet and 30 feet. This portion of the main sewerage-works was designed by and executed under the supervision of Messrs. Cawley, Newton and Smith almost immediately after the Author became Surveyor to the Board and before the sewerage-works were placed under his charge. Havingacquired about 78 acres of landat Chorlton Ees with a view of forming a sewage-farm, the Local Board decided in 1880 to consult thelate Mr. JohnBailey Denton in

Downloaded by [ York University] on [19/09/16]. Copyright © ICE Publishing, all rights reserved. Papers.] SWARBRICK ON WITHINGTON SEWERAGE. 333 reference to the under-drainage and preparation of the land and the mode of distributingthe sewage thereon. Aftercarefully considering his recommendations, the Board decided in the spring of 1881 toentrust the execution of the work at the sewage- farm to him, the Author acting as resident engineer. Mr. Bailey Denton and the Author were subsequently instructed to repGrt upon thequestion of thelevels of the proposed outfall-sewers. They concurred in recommending certain modifications of the original scheme, involving two additionalsewers for some distance co-extensive withthe existing mains, SO asto join them at a higherlevel and thereby to obtain an outfall-sewer of better gradient, having inclinations of 1 in 800 and 1 in 930 instead of 1 in 1,760 as originally designed. Experiments have shown that the surface-velocity of the sewage, where the sewer is laid to an inclination of 1 in 930, is 100 feet per minute, with the present maximum dry-weather flow. TheAuthor ascertained afterthe sewer had been in use for some time that this velocity had been sufficient to prevent any depositupon theinvert; though his observations convinced himthat it is theleast velocity which should at any timeexist in a conduit designedto convey unstrained sewage. Nearly the whole of the Withington drainage-area lies within the watershed of the Mersey on the New Red Sandstone formation. A small area on the north side is within the watershed of the Irwell, but no practical difficulty exists in conveying the sewage of that portion of the district into the southern outfall. Through- out almost the whole length of the drainage-area the Mersey is flanked on each side by a bank averaging 20 feet in height and distant therefrom in most places between 10 and 20 chains. The low land below these banks consists of a deposit of alluvium upon the bed of an estuary which existed before the river was confined by embankments toits presentchannel. The sewage-farm has been constructed upon a portion of this low land at the western extremity of the drainage-area. The watershed is comparatively level from north to south, but from the easterly side of Burnage to itswesterly extremity above the sewage-farm, a distance of 4$ miles, there is a fall of 85 feet; a fall of 65 feet occurs in a distance of 2 milesfrom the eastern boundary,therefore in the remaining 28 miles the fall is 20 feet. In the steeper portion of the drainage-area, the sewers, which are 12 feet to 24 feet deep, were mostly laid in tunnels through the sandstone. This rock is overlaid with seams of sand, mar1 and clay. On approaching the flatter portions of the drainage-area, the sandstone was found

to dip below the level of the sewer, which is 20 feet deep; and there the boulder-clay is thicker and is overlaid with fine grey sandand thin veins of gravel.The thickness of thesand in thelower portion of the watershedvaries between 1 foot and 17 feet. Thesurface of saturation lies 5 feet to 10 feet below the surface of the ground, and the depth of water in the working shafts varied between 2 feet and 9 feet. In several places large ‘‘ pot-holes ” of red sand, highly charged with water which ha filtered through the clay, were found, at depths varying between 2 feet and 12 feet below the top of the clay. In the grey sand- beds, the slope of theupper surface of the subsoil-water was 8 feet 9 inches in 1 mile. When the scheme was originally prepared in 1877 it was sug- gested that provision should be made for dealing with the sewage of a population of 50,000. The water-supply was estimated to be equivalent to 20 gallons per head per day, and the Withington andDidsbury outfall-sewer, below thestorm-water outlet, was designed todischarge 5,000,000 gallons of sewage daily;but above the storm-overflows intothe River Mersey, the sewer is capable, whenrunning full, of discharging 21,000,000 gallons per day. The Author, in designing the Withington outfall-sewer tothe modified gradients referredto, was authorized to make provisionfor the discharge of 5,000,000 gallonsder day. In 1880 the Local Board of the township of Levenshulme, which is situated on thenorth side of Withington, agreed withthe Withington Local Board to convey the sewage of Levenshulme, whichhad a population of 4,300 andan area of 605 acres, to the sewage-farm at Chorlton-cum-Hardy, for treatment thereon. Above the storm-wateroutlets, thenew main sewers varyin size between 30 inches by 20 inches and 48 inches by 32 inches. They are egg-shaped in section, constructed of brickwork in lias lime mortar and Portland cement mortar, consisting of one part of cement to one part of clean sharp sand. Most of the briok sewers arelined inside as high as the springing of thearch with smooth, highly salt-glazed fire-clay bricks. Throughout, the whole of thebricks are moulded to various radiito suit the form of the sewers. Thethickness of the mortar-joints was not allowed to exceed 4 inch,and the cement-joints in glazedwork were 4 inch to & inch. It is believed to be largely due to the impervious andsmooth character of the inverts that some of the sewers withbut slight inclinations are comparatively free from any coating of the slimy matter commonly found on ordinarybrickwork insewers; and its absence must

Downloaded by [ York University] on [19/09/16]. Copyright © ICE Publishing, all rights reserved. Papers.] SWARBRICK ON WITHINGTON SEWERAGE. 335 tendto minimize the generation of offensive emanations of sewer-air.Glazed inverts alsoadd greatlytothe facility with whichaccumulations adhering to the bottomand sides of sewers duringcontinued dry weather,can be removed by flushing. Thebranch sewers are constructed of selected salt- glazed circular socketedpipes, in thicknessone-tenth of their internal diameter, of sizes ranging from 12 inches to 18 inches in diameter. Cement, tarred gasket and other water-tight joints havebeen everywhere adopted. Theinclinations of thebranch sewers vary between 1 in 150 and 1 in 500. Below the storm- overflows the sewers arecircular, 18 inches to 30 inches in diameter. Theyare laid at inclinationsranging from 1 in 740 . to l in 1,700. Thelatter gradient was only adopted in cases where the sewershave occasionally to work under pressure. Fireclay and stoneware pipes have been adopted up to a diameter of 21 inches; but where the pipes exceed 15 inches in diameter andare laid atdepths exceeding 15 feet,through open-cut trenchesand under working-shafts, theyhave beenimbedded in 6 to 1 Portland cementconcrete. Similarprecautions have been takenin the case of alljunctions and connections with deepsewers. The sewers, whichare more than 21inches in diameter, are constructed either of brickwork, or, where near the surface and subject to hydrostatic pressure, of cast-iron socketed pipes Q inchthick, in 12-feetlengths, coated whilst hot with Dr. Angus Smith's composition. The pipes were subjected, before being coated, to hydrostatic pressure amounting to one-tenth of their estimated bursting-pressure, and whilst under pressure were rapped with a hammer. The iron pipes were well caulked with tarred gasket, the remainder of the joint being then run with molten lead 2 inches deep, well caulked afterwards. The Author's investigations have shown that many of the old sewers, made prior to 1877, are byno means watertight, but admit large volumes of subsoil-water,and, in fact, in some instances appear to have been designedto act as land-drains. Where sewage has to be purified at theoutfall, and especially if it has also to be pumped, too great stress cannot be laid upon the importance of adoptingevery possiblemeans of excludinglarge volumes of subsoil-water from sewers, in consequence of the increased annual expense whichis involved in pumpingand treating, whether through land or chemically, an unnecessary quantity of sewage. The highly-diluted character of the sewage, the probability of rapid increase in the population of the drainage-area, and the nature and limited area of the land available at the outfall, have caused

Downloaded by [ York University] on [19/09/16]. Copyright © ICE Publishing, all rights reserved. 336 SWARBRICK ON WITHINGTON SEWERAGE. [Selected theAuthor to adopt every available means to excludesub- soil-water inthe construction of themain intercepting-sewers, which, for a distance of 24 miles, had to be laid in wet ground. Great care wastaken to keep allworking-shafts and headings intunnelled work perfectlydry. The methodadopted was as follows : Throughthe wet sand the shafts were setout 9 feetby 7 feet. Theywere closely timberedwith 9-inch X %inch and 9-inch X 14-inch spruce poling-boards, driven down to the clay beneath. As the sinking progressed, 9-inch X 3-inch

and 1 11-inch X 4-inch walings wereintroduced, andtightly wedged about 2 feetapart. The working space withinthe timberingwas thus reduced toabout 8 feetby 6 feet. Upon reaching the water-line, the pumping commenced and continued constantly until the shaft-sinkers got down into good stiff clay. Atthis stage preparations were made for puddlingout the water, which would vary in height between 5 feet and 12 feet above the bottom of the excavation. Pumpinghaving been resumed and the water having beenlowered to a sump in the centre of theshaft, an inner lining of 9-inch X 3-inchspruce " puddle-planks" carefully bedded upon solid clay and secured together with iron dogs was inserted, reducing the working space within the timbering in the lower part of the shaft, to 6 feet by 4 feet. Theannular space between the puddle-planks andthe poling-boards wasthen filled with temperedclay-puddle well rammed and trodden. Other courses of 9-inch X 3-inch planks on edgewere then placed upon those already secured, and the puddling was proceeded with ina similar manner until the puddle had been carried a foot above the water-line. By exercising great care the workmen seldom failed to at once make the shafts perfectly watertight, if solid clay existed at the levelof the lower puddle-planks. By these means the headings, which were in stiff blue and brown clay, were kept perfectly dry during theconstruc- tion of the sewers, althoughthey were laid15 feet below the subsoil water-line. This methodwas adopted throughout 95 per cent. of this class of work, and in the remainder of the length, owing to localcircumstances which prevented its adoption, the sewers were laid in open cutting and otherprecautions were taken to render the work as watertight as practicable. All brickwork under working-shafts and for 2 feet on each side of them, and also thatin open cutting, was setin cement. Cementcollar-joints werealso introduced in some places undershafts and in the trenches. The cement work referred to was protected by a cover- ing of well-temperedpuddle, at least12 inches in thickness,

toprevent the possibility of subsoil-water(which, as soon as the timbering was renloved, filled the shafts to its normal level) from injuringthe new work andpercolating into the sewer beneath. All man-holes, lamp-eyes andshafts for private con- nectionswere made perfectlywatertight and surrounded with 12 inches of clay-puddle carried above the water-line. Inthe case of work in open cutting upon wet foundations, temporary subsoil-water drains, laid below the invert-level, surrounded with concrete, wereresorted to for keeping down the subsoil-water until the mortar or cement had set. These drainswere laid so as to allow of sumpsbeing placed atshort distances apart, to diminish the dangerof injury to the newly-constructedwork. The sewers lie in straight lines, with either a man-hole or a lamp-eye at each change of directionor inclination. The manholes are placed at distances varying between 80 and 120 yardsapart. An opening for ventilation is formed inthe side of each manhole, about 2 feet below the surface of the ground, communicating with a separate side chamber, the top of which is covered with an open grating, and in thebottom of which a small sump is formed toprevent road detritus from fallinginto the sewer. Atthe bottom of eachmanhole an inspection-chamber 4 feet 6 inches long by at least 2 feet 3 inches wide by 5 feet in height, is formed to afford facilities for examining the sewers. The Chorltonoutfall-sewer was siphoned underthe Chorlton Brook, to avoid interferingwith the level of the bed of the stream;the Withington outfall-sewer had also to besiphoned underan existing sewer in connection withthe Manchester Southern Cemetery. Thesiphons are both constructed of two lines of cast-iron spigot-and-socketpipes laid side by side. Im- mediately above and below the siphons, penstock-chambers admit of the sewage being shut off from either line of pipes, if necessary, for the inspection or cleansing of the pipes. Between the two penstock-chambers, inspection- and cleansing-chambers with a sump in one of them were formed over each end of the low portion of the pipes to allow of their being emptied bymeans of a hand-pump. Both of the siphons have been in operation eight years, and, so far, no obstruction has taken place which has Ten- dered it necessary to empty either of the four lines of pipes. In several instances, to preserve uniform gradients without interfering with the sectional form of conduits that had to be crossed, it was found desirable to remove the brick inverts or arches of existing gewers and to substitute iron ones, and also to flatten the section of the new outfall-sewers ; which in some cases was done by sub- [THE INST. C.E. VOL. CXVII.] Z

Downloaded by [ York University] on [19/09/16]. Copyright © ICE Publishing, all rights reserved. 338 SWABBRICK ON WITHINGTONSEWERAGE. [Selected stituting for the ordinary sectional form, two lines of iron pipes side by side, of equal discharging capacity to that of the ordinary section. SEWAGE-DISPOSALWORKS. The sewage-farm is situated at the confluenceof theRiver Mersey and the Chorlton Brook. Of the 78 acres acquired, 57 acres are available for the treatment of sewage. The particulars of the area of tbe farm are asfollows :- a. r. p. Filtration-area...... 49 0 22 Osier-beds ...... 8 0 3 Embankments, mater, roads, etc. ...15 3 25 Land outside the farm ...... 4 3 30 Total ....58 0 0

The subsoil throughout the farmconsists of a depositof alluvium, which, previously to the construction of the works herein referred to, had not been satisfactorily drained. The nature of the soil is variable. The subsoil of the northerly side of the land is much more impermeable than that on the south side, and it was deemed necessary, in the first instance, to place the under-drains on the north and south sides at 33 feet and 150 feet apart, respectively; it being anticipated that, as the volume of sewage to be dealt, with increased, additional under-drains would become necessary. The under-drainage of the subsoil has increased its porosity to a marked extent. Still, on some parts of the land, which consist of a plastic alluvial sediment, it was found, when trial-holes were made 10 feet distant from the drains where the latter were 4 feet 6 inches deep and 33 feet apart, that water rose in the holes to a height of l foot 9 inchesabove the level of thewater in the drains, and at a distance of 16 feet it rose 6 inches higher. , The close proximity of the subsoil-water to the surface had an injurious effect upon the first and second year’s crops. The filtration-areas were divided into beds, 9 feet wide, parallel with the drains, with surfacedistributing-grips, 2 feetwide, between them. In many instances the positions of the under-drains and the extent to which they were acting effectually were clearly indicated by the contrast between the condition of the crops on thecentral beds, under which the drains passed, and those without drains on each side of them. The filtration-area is divided by the osier-bed into two almost equal parts, which are subdividedon the north side intofive areas,

and on the south side into three areas. All the area is laid out in horizontal beds, tofacilitate the more perfect filtration of the sewage byinsuring its filtrationthrough the subsoil, and to preventits merely passing over the surface of theland. The areas were designedto treat the dry-weathersewage, and the osier- bed, the surface of which is two feet below the remainder of the farm, affords a means of treating by simple subsidence excessive volumes of storm-water. Theunder-drainage of thefarm has been accomplished withbutt-jointed and socketed drain-pipes, the former 4 inchesand 6 inches in diameter,and the latter varying between 6 inches and 18 inches in diameter. The drains were laid at an average depth of 5 feet below the surface of the ground. The main effluent outfall-drain is 930 yards in length, 30 inches in diameter, and has an inclination of 1 in 2,000. The drain was, in order to avoid interference with the levelof the bed of the Chorlton Brook, siphonedfor a length of 60 feetwith cast-iron pipes embedded in concrete. Penstocksare fixed upon the lines of the main under-drains to afford means of flushing the drains and the inverted siphon under thebrook. Enteringthe farm at its north-eastern and south-eastern ex- tremities, the outfall-sewersdischarge into two separate sets of tanks, in each of which the sewage is strained through cast-iron gratings with vertical bars 1 inch apart. The gratings are easily kept open by one of the labourers occasionally clearing them with a rake. The sewage is conveyed through underground conduits, constructed of 18-inchcircular socketedfire-clay pipes with cement joints, and is discharged from one or more distributing- chambersupon each filtration-area. Provision is also made at each of these chambers and at the tanks, to allow any excess of storm-water sewageto overflow intothe osier-bed. The sewage is distributed upon the land ina crude state. Occasionally, during storms, road-detritus and solid matter out of old drains is deposited in the tanks; but beyond that, the quantity of solid matter inter- cepted is comparatively small anddoes not necessitate very frequent emptying of the tanks. The cost o€ the outfall-sewers amounted to .S28,670. The average cost of the land (inclusive of Compensation) was 5157 3s. 7d. per acre, andthe cost of thefarm was equal to 51 2s. 9d. per head of the population of the joint area in1885. The average cost for under-drainage of the filtration-area was S35 per acre. The average cost of the sewaged land for tanks, distributing-carriers,and chamberswas 518 5s. per acre. The average cost of the sewage-farm for the whole system of under- 22

Downloaded by [ York University] on [19/09/16]. Copyright © ICE Publishing, all rights reserved. 340 SWARBRICKWITHINGTON ONSEWERAGE. [Selected drainage,effluent-outfall drain,preparation of the surface, construction of roads, tanks, and distribution-works was S115 10s. per acre. Dry-Weather Flow.-In 1887 theactual dry-weather flowof sewage (which was highly diluted) oftenreached 1,800,000 gallons daily. EfJEuent.-The Author has on several occasions, under varying conditions, endeavoured to ascertain by experiments conducted at ChorltonEes, therelative quantities of the volume of sewage distributed on the farm and that yielded as effluent. It is clear that the ratio will vary almost at every farm, and in fact upon different parts of the same farm, according to the character of the soil andother localconditions. In the case in question it was found that the quantity of effluent water was mainly governed by (1) The normal flow of subsoil-water from the filtration-areas ; (2) The volume of sewage distributed upon the land; (3) The rainfall upon the farm ; and (4) Excessive rainfall in the higher and more distant partsof the watershed, which causes considerable variation in thelevel of the adjacent river. The River Mersey, at Chorlton-cum-Hardy, is subject to a variation in level of about 17 feetbetween the lowest dry-weather flow and the highest-flood level. The average height of the farm above the normallevel of the river is 7 feet. The drains being 5 feet below the surface of the land, when the water-level of the river is high, during wet seasons, it has a tendency to increase the volume of water passed through the drains under the belt of poroussubsoil. In 1884, during a period of thirteendays in which 2.08 inches of rain fell, the Authorfound that 48 per cent. of the average daily flow of sewage upon the farm-plus the rain-water upon the filtration-areas-was yielded by the effluent- drain.Again, in 1885, aftera week of dry weather, duringa period of six dayswhen no rainfell, 47 per cent. of the flow upon the areas was yielded by the effluent drain. The worksdescribed were completed in 1885. The Author ceased his official connection withthe district in 1888. The Paper wascompiled immediately afterthe completion of the works, but its presentation to the Institution has been deferred pending thesettlement of certainnegotiations between neigh- bouring authorities.

The Paper is accompanied by a sheet of drawings giving details of the siphons and manholes, and a plan of the sewage-disposal works.