The Extensions to the Davyhulme Sewage Works

7041 The extensions to theDavyhulme Sewage Works (1 957-67)

The Paper describes the design and construction of extensions to the Davyhulme Sewage Works which have been carried out over the past ten years at a cost of approximately E6 million.These extensions comprise inlet works, primary sedimentation plant, a 45 mgd Simplex aeration plant, final sedimentation tanks, storm sewage tanks, primary sludge digestion plant, secondary sludge thickening plant, power generation plant and jetties. A general accountof the whole plant is given with a more detailed description of those features which the Author considers to be of particular interest. A list of the contractors and a table of costs are appended.

Introduction In 1954, when the new extensions were first considered,the Davyhulme Sewage Works consisted of two separateplants with a common inlet works. An activated sludge plant completed in 1934, comprising a diffused air unit and small experimental unitson theSheffield Bio-aerationand the Simplex systems, treated a dry weather flowof 19 mgd and theremaining 35 mgd weretreated on primary and secondary contact beds completed in 1907. These beds were in very poor condition and the effluent quality was bad. 2. The City Council therefore decidedto retain theexisting activated sludge plant and to replace the contact beds by a new plant with a dry weather flow capacity of 45 mgd bringing the total capacity of the Works to 64 mgd. 3. In fact the rate of flow has risen more quickly than forecast and at the present time the dry weather flow is 68 mgd of which 24 mgd is trade waste. The old activated sludge plant has deteriorated rapidlyand is now capable of giving only partial treatment to about 10 mgd so that the new extensions were overloaded as soon as they were brought into operation. 4. It is now planned to replace the old activated sludgeplant by a new plant having a dry weather flow treatment capacity of 37 mgd which will bring the total Work's capacity to 82 mgd. These future extensions are not described in this Paper. 5. A general layout of the works is shown in Fig. 1. Inlet works 6. The existing inlet works consisted of six detritus channels emptied by a stream grab with coarse screens downstream. It was decided to use the existing channels but modify them to approximate to constant velocity conditions and install new coarse and fine screens upstream.

Ordinary Meeting: 5.30 p.m., 6 February, 1968. Written discussion closes 29 February, 1968, for publication after May, 1968. * Principal Assistant Engineer, Manchester Corporation City Engineer and Surveyor's De- partment. Downloaded by20 [ Paul Robert Matthew Hastings] on [13/07/18]. Copyright © ICE Publishing, all rights reserved.577 SYMES

Downloaded578 by [ Paul Robert Matthew Hastings] on [13/07/18]. Copyright © ICE Publishing, all rights reserved. EXTENSIONS TO DAVYHULME SEWAGE WORKS (1957-67) 7. In order that this could be done the flow had to be diverted to the existing storm water channel. This flowed through a grit pit and before diverting the flow an electric grab was installed over the pits. A coarse bar screen was installed in the channel upstreamof the pit. 8. The Works are fed by two main outfall sewers, one 13 ft dia. and one 10 ft dia. The maximum capacity of the combined sewers is 540 mgd which allowing for net infiltration water corresponds to eight times a dry weather flow of 76 mgd. It was decided to construct the new inlet works to deal with this maximum flow. 9. A new inlet carrier 16 ft 3 in. wide, maximum flow depth 10 ft 6 in. was constructed from a point adjacentto the outfall sewers to a distribution basin from which six culverts, each controlled by a 6 ft 6 in. X 4 ft 0 in. wide penstock, lead to the screenhouse. 10. The screenhouse is a steel framed building 199 ft long by 50 ft wide containing the six screening channels. Each channel was originally equipped with two coarsescreens with bars at 6 in. spacing. These were in pairs so that one screen could be lifted by the overhead crane for cleaning while the other remained in position. 1 1. These were followed by mechanicallyraked fine screens, 8 ft wide, with 5 in. clear space between bars. The screenings are deposited into a channel running the lengthof the screenhouse andare waterborn to three disintegrators which macerate the screenings and return them to the inlet channel. A large magnet extracts metal from the channel before it passes to the disintegrators. 12. The fine screens operate on an automatic timing cycle which can be adjusted both for the number of raking cycles per operation and the delay periodbetween operations. Originallydifferential float switches were incorporated across each screen but it was found difficult to keep the float chamber entrance from blocking with screenings, and they have now been replaced by switches mounted on the screen so that in times of storm theplant attendant can switch any or all the screens to continuous operation. 13. The gradient of the outfall sewers is rather flat (1 in 2264) and because, at low flows,the sewage is backed up tosome extent by the screens and detritus channels the hydraulic gradient is even less. At low flows the velocity in the sewers is only 1.5 ft/s, so that considerable deposition of grit and screenings takes place. These are picked up and arrive in large quantities at the Works with the first flush of a storm. The effect so far as,the screens are concerned wastwo-fold. In spite of the 6 in. bar spacing the coarse screens quickly becameblocked with rags requiring constant attention and far too many screenings passed through the fine screens causingchokage of the grit pumps, and other maintenance difficulties later in the Works. 14. Because of this it was decided to replace each pair of coarse screens by an additional mechanically raked fine screen with bars at 3 in. spacing and because the existing disintegrators would be unable to cope with the extra load band conveyors were installed to discharge into trailers which are towed to a tip on the site. 15. So that the head loss should be no greater than when the coarse screens wereused the raking mechanism of the replacement screens operates continuously. It is interesting to note that the amount of screenings caught on the second set of screens is over 50% of the amount caught on the first set. Blockage of the grit pumps is now rare. Downloaded by [ Paul Robert Matthew Hastings] on [13/07/18]. Copyright © ICE Publishing, all rights reserved.579 SY M ES 16. From the fine screens the flow passes into the detritus channels. These were formed by reducing the depthof the original channels from23 ft to 12 ft by filling in with quarry waste topped with 12 in. of cindercrete and 6 in. concrete. The channels are 16 ft wide and the base is divided into two V-shaped channels. The exit from each channel is controlled by a flume 2 ft 3 in. wide. The linear velocity in the channels varies from 0.8 ftjs at 20 mgd to 1.28 ft/s at 90 mgd. 17. Eachchannel is spanned by a travellingbridge carrying two 4 in. trunk-slung pumpswith flexible suctions extending intothe bottom of the ‘V’s. 18. The channels are 116 ft long which is shorter than desirable, but the maximum length which could be accommodated without far more extensive alterations. The speed of travel of the bridge is 4.35 ftlmin. 19. Heavy material tends to accumulate at the inlet end of the channel and this leads to blockage of the pumps. To rectify this the timing mechanism of the bridge is altered so that when the bridge arrives at the inlet end it stands for 15 min while the pumps continueto run. This practically eliminates blockage. 20. The mixture of grit and sewage (97.5% water) extracted by the pumps flows in channels to three grit classifiers of the reciprocating rake type which separate the grit from the organics which are returned to the Works inlet. The grit is mixed with clean effluent pumped from the main pumping station and pumped to underdrained beds for storage. 21. The number of screenings and detritus channels in use at one time is regulated automaticallyby the flow. This is measured at the flumes at the end of each detritus channel and summated on a recorder inscreenhouse the which is fitted with adjustable contacts to control the opening of the penstocks in the distribution chamber and downstreamof the flumes. Normally three channels are in use, the fourth is brought in when the flow reaches 80 mgd, the fifth at 260 mgd and the sixth at 350 mgd. If one or more channels are out of com- mission and the incoming flow exceeds the capacity of the available channels, emergency siphon overflows operate in the main inlet channel and discharge via the old stormwater channeldirect to the storm sewage tanks. 22. After the detritus channels the flow is split into three parts:- (a) via a 6 ft square Venturi meter to the new extensions; (6) via a 4 ft square Venturi meter to the old plant; (c) to the storm sewage tanks. 23. The flow is controlled by electrically operated penstocks immediately downstream of the Venturi meters. The hydraulic conditions are such that without control all the flow would pass through the larger Venturi. To obtain correct division, readings from the meters are fed into a controller which operates the penstock downstreamof the larger Venturi to divide the flow in a pre-set ratio, the penstock downstream of the smaller Venturi remaining fully open. When the flow reaches a pre-set quantity (normally 2.7 X dwf) both penstocks operate to prevent any additional flow passing, the excess being forced up over the storm weir. 24. Because of space restrictions the storm weir is crenellated. Its total length is 240 ft and ithas to pass a maximum quantity of 600 cusec, the head over the weir being just under 1 ft. It was thought that because of its shape a greater head would be required than for an equivalent straight weir but, perhaps because of the velocity of approach, this is not so. Downloaded by580 [ Paul Robert Matthew Hastings] on [13/07/18]. Copyright © ICE Publishing, all rights reserved. EXTENSIONS TO DAVYHULME SEWAGE WORKS (1957-67) 25. On completion of the inletworks the two outfall sewers had to be diverted into the newinlet channel. Diversion of the 10 ft dia. sewerwas comparatively easy as it could be shut off completely by a penstock in Trafford Park 2 miles upstream, the larger sewer being capable of carrying sufficient flow to prevent danger of flooding upstream. Diversion of this sewerwas more difficultbecause, although it also could beclosed by a penstock in Trafford Park, other sewers connected into it downstream and the penstock could be closed only when there was no danger of a storm. 26. To effect the diversion a 15 ft wide steel sheet pile channel with a concrete invertwas constructed, to by-pass the junction position. Where the channel joined the sewer the top of the sewer was broken out on one side carried down to springing level so that it could discharge to the channel. The sewer was then dammed downstream using an angle iron and timber stank covered with a plastic membrane and buttressed by shaped concrete blocks. The angle irons were bolted into the brickwork of the sewer. Another stank was placed in the main inlet channel to complete the isolation of the junction area.

Storm sewage tanks 27. The existing Works included two blocks eachof eight rectangular tanks approximately 300 ft long with widths varyingfrom 95 ft to 120 ft. One block of eight had been used as primary settlement tanks and the other block as storm sewage tanks. Desludging was originally by hand and later by using a bladed tractor which was lowered into a tank by a crane. 28. It was necessary to use the whole of this tankage for storm sewage and to provide for mechanical desludging. If the original tanks had been used it would,because of the varyingwidths, have been necessary to provide 16 mechanical scrapers. On investigation the division walls showed settlements of up to 4 in. and it was considered that additional settlement might occur if they had to carry the additional load of the scrapers. 29. It was, therefore, decided to demolish the walls of the existing tanks and in place of the sixteen construct six new tanks with their axes at right-angles to those of the original tanks; three 846 ft long by 108 ft wide, two 811 ft by l 15 ft 6 in. and one 81 1 ft by 108 ft. The total capacity is 20 million gallons equivalent to a detention of 1.43 h on maximum flow. 30. The ground underlying these tanks is mainly silt and soft clay and the new walls were therefore carried on piles 16 in. dia. driven in situ. Concrete pileswere used, the lengths varyingbetween 20 ft and 30 ft. Theywere arranged in pairs along the wall the distance between the two piles being 5 ft and the spacing between pairs varying from 13 ft to 25 ft depending upon the depth of the tank at thatpoint. Above the piles the 1 ft 9in. thick wall is more strongly reinforced so that it supports the lengths of wall between pile points both horizontally and vertically. The piles are designed to carry a load of 50 tonsand to resist an upliftof 10 tons. The original tank floors were retained and screeded to new falls. Fig. 2 shows a cross section of the tanks. 31. Hoppers were constructed at one end of each tank and the floor is scraped to these hoppers by travelling bridges. These run on polyurethane tyred wheels and have horizontally mounted side wheels to prevent crabbing. Downloaded by [ Paul Robert Matthew Hastings] on [13/07/18]. Copyright © ICE Publishing, all rights reserved.581 SYM ES

LONGITUDINAL SECTION OF STORM TANKS 12”dia. sludge draw off pipe

CROSSSECTION OF STORM TANKS Fig. 2. Sections of storm tanks

Electrical pick-up is byTrolley Master equipment. Because of this it was necessary to construct the longwalls accurately to line, the allowable tolerance being k + inch. 32. The bridges are operated on an automatic cycle and may be pre-set for a full or a partial scrape. When set for ‘full scrape’ and actuated the machine lowers its sweeping blade and travels the length of the tank at a speed of 4 ft/min. The blade then lifts and the machine returns to the home position at a speed of 20 ft/min. When set for ‘partial scrape’ it travels a pre-set distance with its bladeraised at 20 ft/min, then lowers the blade and continues as for ‘full scrape’. 33. Each set of three tanks operates in series. When the first tank is full the sewage flows over weirsalong the length of the division wall into the second tank, thence into the third tank and, if this too becomes full, into an outlet channel leadingto the canal. Samples taken at the outlet from the third tank at times of maximum flow have suspendedsolids of the order of 60 ppm. 34. After a storm has subsided the tanks are emptied by five 8000 gpm pumpsin the adjacent main pumping station. The effluent is returned to either or both thenew or old plants. Sludge flowsby gravity to a sumpwhence it canbe pumped to either the primary or the secondarydigestion plant.

Primary sedimentation plant 35. From the Venturi meter the flow passes to the new works via twin culverts, 7 ft wide by 6 ft high by l150 ft long. The minimum velocity of flow is 0.8 ft/s and the maximum 2.7 ft/s. In spite of these low velocities, when the culverts were emptied recently they were found to be clean. No doubt some deposition of solids takes place at the lower flows but at higher flows the culverts are apparently self-cleansing. 36. The culverts discharge into a pre-aeration channel 35 ft wide by 11 ft water depth by 375 ft long. Surplus activated sludge from the aeration plant Downloaded by582 [ Paul Robert Matthew Hastings] on [13/07/18]. Copyright © ICE Publishing, all rights reserved. EXTENSIONS TO DAVYHULME SEWAGE WORKS (1957-67) is discharged into the channel at its upstream end. Nineteen 5 ft dia. Simplex aeration cones areinstalled each with individual motor drive and are mounted on pre-cast concrete bridges spanning the channel. The cones keep the solids in suspension and some flocculation takes place. 37. The pre-aeration channel runs alongside and forms one endwall of the three western storm tanks. The underlying ground is also silty and the channel is therefore founded on piles. These were arranged in rows of four capped with beams at 11 ft centres. 38. The flow from the pre-aeration channel passes through two 7 ft by 6 ft culverts each feeding one block of four primary sedimentation tanks. Pen- stocks are fitted to the entrances of the culverts and they are operated by a float mechanism to keep the water level in the channel within t.2 in. of the datum level, this being the operating tolerance of the aeration cones. A timer is incorporated to prevent the penstocks hunting. 39. The eight primary sedimentation tanks have an internal diameter of 146 ft with a floor slope of 73", the side wall depth being 8 ft 0 in. The total capacity is 9 680 000 gallons. The central hoppers are30 ft dia. and 15 ft deep and hold a day's make of sludge. The tanks are scrapedby trailing blades on a revolving half-bridge which also carries a scum blade. Scum is discharged at the periphery via a tilting trough. 40. The bridges are carried on a circular beam supported on columns. The beam is extended downwards to form a curtain wall to baffle the inlet. The inlet is a 48 in. dia. pipe discharging vertically upwards. As the bridge also carries a 'picket-fence' thickener in the sludge hopper the drive shaft passes down the centre of the inlet pipe and through agland in the 90" bend. 41. The tanks are founded on silt except the deep central hoppers which bear on firm boulder clay. It was decided therefore to pile the outer channel of the tanks so that it forms a separate structure. The central hopper and bridge support is also separate so that the floor can settle slightly without affecting the operation of the tanks. The actual settlement which has taken place does not exceed + in. and, because the scraper blades are trailing, has no effect on the scraping efficiency. 42. As the standing ground water level is 55 AOD and the tank floors fall from 47.72 to 40.30 AOD it is necessary to make provision against uplift when the tanks are empty. As the floor is mechanically scraped the normal type of pressure relief valve whichprotrudes abovethe floor cannot be used and if they are sunk into pocketsin the floor these may pack solid with sludge. 43. The method used was to place a 2 ft thick layer of gravel graded from 200 BS sieve to in. drained by radial porous pipes to eight flap valves per tank sited in thewall of the central hopper. These work efficiently to date andcare is taken to see that they are working each time the tank is emptied and that they are greased before the tank is refilled. 44. Sludge is withdrawn fromthe central hoppers through a24 in. dia. pipe controlled by an electrically operated penstockfitted with an automatic timer. The sludge then flows to the main pumping station where it is pumped to the digestion plant. Two loo0 gpmcentrifugal pumpsand one 500 gpm ram pump are provided. 45. Effluentis discharged over a scum boardprotected weir. The peri- pheral channel is made much deeper than is common in order to keep a low velocity and preserve head. Because of the low velocity some deposition of Downloaded by [ Paul Robert Matthew Hastings] on [13/07/18]. Copyright © ICE Publishing, all rights reserved.583 SYMES solids takes place in the channel and to prevent this the channel is swept by brushes trailing from the scraper bridge. 46. The effluent is collected by a distribution channel and is fed through four 54 in. dia. Dall tubes to the aeration plant. Butterfly valves downstream of the meters enablethe flow to be correctly apportioned to the aeration units.

Aeration plant 47. There are eight aeration units each consisting of a tank 35 ft wide and 385 ft long with a maximum water depth of 10 ft. This is divided partially by baffle walls and boards intoeleven 35 ft square pocketsin each of which a 6 ft dia. Simplex modified high intensity cone operates. The floor is level over the central 23 ft square sloping up to a height of 3 ft at the boundaries of each pocket (see Fig. 3). 48. Each line of eleven cones is driven by three motors. A 75 hp motor at each end of the line drives three cones and the central five cones are driven by a 125 hp motor giving 25 hp per cone. The cones, the gearboxes and the line shafts are carried in concrete channels 5 ft wide by 3 ft deep which span the 35 ft between the baffle walls. The channels are dowelled on to one wall and free to slide at their other end. 49. The units are arranged in two blocks of four. The cones in the four outside lines can be reached by a travelling crane and to provide a similar facility for the four inner lines an 11 ft wide roadway was cantilevered off both sides of the central walls. 50. Four feed channels, fed from the Dall tubes, each feed two units and provision is made to feed into any of the first seven pockets to allow for re- activation of the sludge or step aeration. 51. Activated sludge is returned via two 42 in. pipes which run underneath the feed channels. Each pipe distributes to four chambers and the quantity returned to each aeration unit is measured over a weir. 52. Theaeration period is just over three hours on dry weatherflow. Previous experimentalwork at Davyhulme hadindicated that thesewage could be treated satisfactorily with such a short detention peri0d.l 53. The outlet from the tanks is controlled by electrically operated adjust-

, WALKWAY

CROSS SECTION OF AERATION UNITS,

CULVERT LONGITUDINAL SECTION OF AERATION UNITS Downloaded by584 [ Paul Robert Matthew Hastings] on [13/07/18]. Copyright © ICE Publishing, all rights reserved. EXTENSIONS TO DAVYHULME SEWAGE WORKS (1957-67) able double-sided weir troughs which may be altered to control the depth of immersion of the cones. 54. Foam dousing equipment is installed in the basement of the adjoining substation and consists of eight 500 gallon tanks each carrying two 12-point Delvac lubricators. Ten miles of & in. id nylon tubing carries the oil to each corner of each aeration pocket. By distributing the oil in this way in measured quantities it is possible to obtain efficient foam control with only 1 ppm of anti-foam oil. 55. Twelve final tanks are provided, each set of three taking the flow from two aeration units. The feed to each setis via a culvert terminating in a vertical cast iron bellmouth which changes in cross section from 5 ft square to 10 ft dia. This is encased in a circular concrete distribution chamber 14 ft dia. equidistant from the tanks it serves. Each tank is fed through a 39 in. dia. pipe controlled by a penstock in the distributionchamber. By this means it is possible to obtain a reasonably accurate distribution of flow between the tanks without resorting to head consuming measuringdevices. Very little spare head is available and thefeed culverts and pipes are therefore made as large as possible subject to the minimum velocity of flow being 14 ft/s. The minimum flow is equivalent to 1.) dwf made up of 3 dwf settled sewage, .) dwf return activated sludge and + dwf recirculation, or alternatively .) dwf settled sewage and 1 dwf return activated sludge. The maximum design flow is 4+ dwf made up of 3 dwf settled sewage and 13 dwf return activated sludge. 56. A cross section of a final tank is shown in Fig. 4. The totalweir length per tank.is 933 ft and consists of a stainless steel strip continuously formed with‘V’-notches 1: in. deep and 3 in.across. This gives a very uniform distribution over the weir and a very low approach velocity. 57. Toform the insetweirs thestrips are bolted to cast iron troughs supported ontwelve 9 in. dia. cast iron pipes which carry the effluent into the outer channel. 58. The tanks have a detention period of 4.92 h on dry weather flow and the upward Bow velocity at maximum flow is 6.6 ft/h. This is higher than normally recommended but, perhaps because of the great weir length,the tanks function satisfactorily unless overloaded. 59. Each tank is continuously desludged by a three-armed scraper having

nl .INNER WE1

Fig. 3 (left). Cross section of the aeration units

Fig. 4 (above). Cross section 0f.a final tank

Downloaded by [ Paul Robert Matthew Hastings] on [13/07/18]. Copyright © ICE Publishing, all rights reserved.585 SY M ES fixedblades. To ensure correct levels the floorswere screeded after the machines had been erected. 60. Sludge is withdrawn fromthe central hoppers via 18 in. dia. pipes which run toa central pumping stationwhere they dischargeinto individual chambers. Each pipe is controlled by a penstock. The sludge rises in the chamber and flows overa measuring weir into acentral sump. From here sludge isreturned to the aeration units by three 27 000 gpm mixed flowpumps (oneof which is a standby) drivenby variable speed motors. The speed is controlled automatically by the level in the sump. It is therefore possible to control the rate of sludge return by adjusting the penstocks on the sludge pipes from the tanks. The rate of return can be varied between 3 dwf and l+ dwf. 61. Surplus sludge is drawn off by an automatically regulatedvalve into an adjacent sump and returnedby centrifugal pumps to the pre-aeration channel and thence into the primary sedimentation tanks. Two 1000 gpm and two 2000 gpm pumps are installed. These pumps can also be used to empty the final tanks (via the sludge pipes) and the aeration tanks. 62. The effluent from the final tanks flows via concrete culverts to the final aeration channel. This is 150 ft long by 35 ft wide with a 6 ft water depth and carries five bridges supporting conessimilar to those used in the main aeration plant but driven individually by 35 hp motors. The level of the effluent in the channel is controlled at the outlet by a 177 ft long weir. From this the effluent flows to the Manchester Ship Canal. 63. In the final aeration channelthe level of dissolved oxygen in the effluent is raised to about 70% of saturation. This is done by arrangement with the Mersey and Weaver River Authority in lieu of producing a stable effluent which is impracticable because of the nature and quantity of the trade waste in the sewage. 64. The whole of the aeration plant and final tanks are carried on piles, the ground consisting of a mixtureof silt, gravel and clay. Before the construction of the Manchester Ship Canalthe River Irwell flowedthrough the site and trees up to 3 ft dia. and 50 ft long were discovered 30 ft below the original ground level. Four piling lirms were invited to drive test piles and then tender on a fixed price per pile basis. On receipt of a bona fide tender each firm was paid E500 as a contribution to the cost of the test piles. The piles used were 17 in. nominal diameter ‘Holmpress’ Patent Redrivenpiles designed to carry a load of 65 tons and an uplift of 10 tons-the latter condition being necessary to prevent flotation of the units due to the ground water table which is above the top water level in the final tanks. The length of the piles varied between 20 ft and 30 ft. Random piles were tested to 150% of working load and nofailures were experienced. Each final tank is carried on 132 piles connected by ring beams between which the floor spans. Under the aeration units the piles are arranged in rows spanned by the tank walls except in the centre of the units where the piles are connected by a beam. 65. In order to shorten the combined design and construction period for the aeration plant and final tanks the contracts were let in three stages. First, the site was excavated (300 000 cu. yd) and blinded with 6 in, of consolidated ashes to the general formation level. The next stage was to place the 3000 piles and finally the main construction work was started. The detailed design of the plant was done and tenders obtained during the excavation and piling period. Downloaded by586 [ Paul Robert Matthew Hastings] on [13/07/18]. Copyright © ICE Publishing, all rights reserved. EXTENSIONS TO DAVYHULME SEWAGE WORKS (1 957-67) Sludge treatment 66. Sludge is treated in three stages:- (a) primary digestion, (b) secondary consolidation and dewatering, (c) transport to sea. 67. The object of the first two stages is mainly to reduce the bulk of the sludge before transportation tosea. 68. However,when, due to adverse weather conditions or the need for repairs, the sludgevessel is unable to transport the sludge to sea and the accumulation exceeds the capacity of the storage tanks, sludge is pumped a distance of about 3 miles to lagoons at Flixton. Because of the odour nuisance which would otherwise result it is essentialthat only digested sludgeis pumped to these lagoons. A digestion plant is therefore essential. 69. Because of staff shortage in the City Engineer’s Department at that time, a consultant, Dr D. Matthews, was employed to design the concrete structures of both the primary digestion and the secondary consolidationtanks.

Primary digestion plant 70. There are four primary digestion tanks each 87 ft id with a maximum sludge depth of 62 ft 6 in. and a total capacityof 8+ million gallons. The lower 30 ft is below ground level and the base is founded on sandstone which occurs at that depth. The walls (15 in. thick below ground level and 10 in. thick above) arepost-stressed both horizontally and vertically using 12/0.276 cables, and are hinged at the junction with the base. The base is constructed in reinforced concrete and is in the form of an annular ‘V’-shaped channel, the internal sides of which form a central cone point uppermost. At the top of the walls a post-stressed ring beam supports areinforced concrete dome(see Fig. 5). 71. The four digestors are arranged in the formof a square and thecentral area is enclosedby walls joiningthe digestors. There is, therefore, earth , pressure onthree quarters of the circumference of each digestor so that below ground level the tank walls are designed as arches and the junction walls are thickened to act as buttresses. 72. A 12 ft id tower occupies the centre of the space between the digestors. At its lowest levelit forms a sump intowhich the digestors may beemptied and pumped to thesecondary plant. This sumpis also used to receive sludge from the storm tanks which can then be pumped to the top of the central tower which forms a sump from which the digestors can be fed by gravity. Sludge fromthe primary settlement tanks is also pumped to this point. Sludge entering the digestors raises the level inside the tank and a float control operates hydraulicplug valves which draw sludge out from three points in the bottom of the digestors to correct the level. This sludge flows into a mid-level sumpin the central tower andthence bygravity to the secondary plant. Measurements of flow are by magnetic flow meters. 73. Floors are provided at four levels in the central area:

(a) the basement which houses two 550 gpm ram pumps and a 200 gpm high head centrifugal pump providingeffluent for washing down,

Downloaded by588 [ Paul Robert Matthew Hastings] on [13/07/18]. Copyright © ICE Publishing, all rights reserved. EXTENSIONS TO DAVYHULME SEWAGE WORKS (1957-67) (b) the ground level Aoor housing the switchgear, (c) the first floor housing the heat exchangers and their pumps, (d) the roof on which are situated the valves controlling the feed and discharge.

74. The floors are connected by a staircase and a 2 ton capacity goodslift. 75. Access to the digestors is provided by three manholes in each roof and by a 48 in. dia. cast iron pipe fitted with a blank flange passing through the wall at the base level. Because of the depth of the tanks an oxygen deficiency may exist after a tank hasbeen emptied and purged in the normalway and an air pipe connexion is fitted to the blank flange to assist in overcoming this. 76. Thecontents of the digestors areturned over by 12 screw pumps, three per tank, each pump running for 20 minutes in each hour. The action of these pumps effectively breaks up the scum. 77. Digestion is maintained at 95°F by utilizing the waste heat from the generating station. Water, ata maximum temperature of 165°F is fed through eight Rosenblad spiral heat exchangers. Sludge is withdrawn from two points at approximately half depth in the digestors, passed through the heat exchangers and returned with the incoming sludge thereby providingthe necessary seeding. 78. To provide additional heat the incoming sludge can be fed through four additional Rosenblad heat exchangers where the temperature is raised about 8°F by passing outgoing sludge through the other side of the exchangers. 79. Gas production from the digestors normally averages about 500 000 cu. ft/day butoccasionally digestion is affectedby certain of the trade effluents when gas production is reduced drastically. 80. Considerable difficulty has been experienced in making the roofs gas- tight. The combination of vibration (from the screw pumps) and temperature has produced a number of fine cracks resulting in gas leakage.Once the digestors.were in use it was impracticable to attempt to seal these cracks from the inside and various methods andmaterials were tried to seal the cracks on the top surface of the dome. The most successful seal wasformed by sticking aluminium foil over the crack with an epoxy resin.

Secondary treatment plant 81. After digestion the sludge flows by gravity through 1200 yd of 16 in dia. pipe to an overhead carrier formed by precast ‘V’-shaped channels’sup- portedon piledcolumns. Thesecondary consolidation tanks are fed by gravity from this channel. 82. There are six tanks each 100 ft id with a side wall depth of 14 ft and a floor slope of 1 in 10. The capacity of each tank is 750 000 gallons. The tanks are prestressed horizontally only and have a sliding joint at the junction with the base. Each tank is equipped with a revolving half-bridge carrying a thickener of the picket fence type and echelon scrapers. Swivelling decanting arms are provided to remove bands of liquor. This liquor is extremely foul and is treated by settlement in four tanks formedby converting the old experimental digestion tanks. After settlement the liquor is diluted with effluent and treated on two 70 ft dia. percolating filters before it is returned to the Works’ inlet. Downloaded by [ Paul Robert Matthew Hastings] on [13/07/18]. Copyright © ICE Publishing, all rights reserved.589 .SYMES 83. Theconsolidation tanks are operated on a fill and draw, principle. After thickening, sludge is withdrawn via 18 in. pipes controlled by electrically operated penstocksto the secondary sludge pumping station. This houses two 2000 gpm ram pumps which can pump the sludge either to storage tanks adjacent to the jetty or direct to the ships. A 550 gpm high head ram pumpis also installed to pump sludge to the Flixton lagoons when necessary. Two 1000 gpm centrifugal pumps pump the decanted liquor to the settlement tanks. 84. When the tankswere filledfor testing three of them started to tilt. They were immediately emptied andan investigation showed that there were lenses of silt in the clay sub-strata and that some of the silt had been removed by dewatering during the constructionof a deep culvert nearby. Two of these tanks were situated within former rectangular tanks and,to avoid interference with operation until the last possible moment, boreholes had been sunk only outside the existing tanks. To stabilize the tanks holes were drilled at approximately 14 ft centres and 36 3 in. dia. pipes were inserted through the floor of each tankpassing, in the case of the two tankswithin the old tanks, through the old tank floor. The holes were grouted in fours, using a 10 gallon water to 1 cwt cement mix, slowly and uniformly until a pressure of 5 lb/sq. in. was reached and could be maintained. The tanks were then refilled and found to be stable. However, during the construction of the final settlement tanks dewatering was again necessary and the tanks began to tilt once more. They were emptied and left empty during the period of dewatering. 85. On completion of the final tanks the ground water table rose to its original level and the tanks were again brought into use and have remained stable since. 86. The tilt of the tank worst affected is 10 in. but fortunately this does not interfere with its operation. In the circumstance it was decided not to attempt to right the tanks.

Jetties 87.Sludge is transported to sea by a 1450 ton capacity vessel, the MV Mancunium. Another similar ship is on order. The sludge is dumped in a spoil ground approximately 20 miles from the shore in theIrish Sea. 88. To accommodate the present and the future ship one jetty has been built and a similar one is at present under construction (see Fig. 6). 89. To construct the jetty two rows of Frodingham No. 4 octagonal piles were driven, 10 piles at 20 ft centres in each row andthe rows 25 ft apart. At this point the sandstone outcrops at canal bed level. The banks of the canal are pitched and divers were employed to form holes in which the piles were positioned beforedriving. The pileswere capped withreinforced concrete beams and the deck formed by pre-cast tee-beams filledin with in situ concrete. Bollards are provided and are fastened by bolts passing through the main beams. Nineteen timber fenderpiles fitted with rock shoeswere drivenin front of the jetty and fastenedby wrought iron straps tothe longitudinal beam form- ing the face of the jetty. Floating timber fenders areprovided to transmit the impact to the fender piles at the waterline. 90. The ship is loaded by a 24 in. dia. pipe hinged on the jetty deck so that it may move in a vertical plane. It is lowered from its normal near-vertical Downloaded by590 [ Paul Robert Matthew Hastings] on [13/07/18]. Copyright © ICE Publishing, all rights reserved. EXTENSIONS TO-DAVYHULME SEWAGE WORKS (1957-67)

1-.36’cf

Bollard ,, Precast T Beams 1 C)J In situ concrete

Main cross beam’

Octagonal steel piles

Piles at ZO’crr

Fig. 6. Cross section of a jetty

position so that it discharges into the hold of the ship. The ship can be filled in 65 minutes and emptied in 11 minutes.

Power generation 91. The whole of the power for the worksis generated by six 1500 hp EnglishElectric seven-cylinder supercharged engines directly coupled to 1225 kW, 6600 volt alternators. A workshop is built on tothe power station so that engine parts may betransferred direct from the engines tb the workshop by an overhead travelling crane. 92. Originally the engines were designed to run on normal fuel oil (35 S viscosity), medium/heavy fueloil (220 to 950 S viscosity) or a mixture of one of these oils with sludge gas,and for thefirst few yearsafter installation they were normally run on a mixture of 950 S oil and gas. However, it is no longer possible to purchase, economically, heavy oil with a sufficiently low sulphur content and to prevent air pollution the 35 S oil plus gas is now used. 93. The engine cooling water is on a closed system passing through the exhaust gas boilers and theengine jackets. Heat is transferredin anexchanger to another closed system supplying the digestor heat exchangers. 94. The engines are seated on concrete blocks 34 ft X 14 ft X 9 ft deep which are insulatedfrom thefloor by 3 in. of cork. To reduce noise further thewalls and ceiling of the power station are lined with acoustic tiles. Downloaded by [ Paul Robert Matthew Hastings] on [13/07/18]. Copyright © ICE Publishing, all rights reserved.591 SY M ES 95. Electricity is generated at 6600 volts and transmitted through a ring main to five sub-stations at various pointson the site where it is transformed to 415 volts. The consultants for the power generation and all the electrical work were Messrs Kennedy and Donkin.

Acknowledgements The Author wishes to thank Mr John Hayes, City Engineer and Surveyor, and Mr G. Ainsworth, General Manager of the Rivers Department, for their permission and encouragement to prepare this Paper.

Reference 1. MCNICHOLASJ. Some developments in the activated sludge process. Proc. Znstn civ. Engrs, 1961, 20 (Sept.) 19-38.

Appendix I 1. Main civil engineering contractors Earth & General Contracts Ltd Part of inlet works before going into liquidation G. Percy Trentham Ltd (a) Inlet works, including rectangular sedimentation tank, mainpumping station (b) Power station John Laing Construction Ltd Primary sludge digestion plant A. Monk & Co. Ltd Secondary sludge treatment plant Lehane, Mackenzie & Shand Ltd Outfall and jetty Hussey, Egan & Pickmere Ltd (U) Pre-aeration and primarysedimen- tation plant (b) Workshop and laboratory (c) Storm water tanks George Wimpey & Co. Ltd Aeration plant and final tanks Holmpress Piles Ltd Piling to aeration plant, and final tanks and storm tanks Megan & Co. (Civil Engineers) Ltd Sludge main to Flixton

2. Specialist civil contractors B. Alexander & Sons Ltd Painting D. Anderson & Son Ltd Roofing Thos. Blackburn & Sons Ltd Structural steelwork Brown’s of Gildersome Ltd Handrailing Cochrane’s (Middlesbrough) Foundry Ltd Cast iron pipes Fencelines Ltd Concrete fencing Franki Compressed Pile Co., Ltd Piling G. A. Harvey & Co. (London) Ltd Rainwater goods Insulatall Services Ltd Acoustic tiling Liverpool Artificial Stone Co., Ltd Precast concrete Prodorite Ltd Flooring Quickset Water Sealers Ltd Floor and waterproofing Redpath, Brown & Co. Ltd Structural steelwork Robertson Thain Ltd Roofing Saxon Engineering Ltd Weirs Raymond Sleigh & Co. Ltd Handrailing Stanton & Staveley Ltd Cast iron and concrete pipes Stefanutti Terrazzo (Manchester) Ltd Terrazzo J. Thompson (Beacon Windows) Ltd Staircases and handrails 592 Downloaded by [ Paul Robert Matthew Hastings] on [13/07/18]. Copyright © ICE Publishing, all rights reserved. EXTENSIONS TO DAVYHULME SEWAGE WORKS (1957-67) Limmer & Trinidad Lake Asphalts Co. Ltd Asphalting Turners Asbestos Cement Co. Ltd Asbestos pipes Robert Watson & Co. Ltd Structural steelwork Wettern Bros. (Manchester) Ltd Concrete fencing

3. Machinery contractors Ames Crosta Mills & Co. Ltd Screens, digesters,screw pumps, second- arysludge thickeners, pre-aeration cones,aerating cones, final tank scrapers, final aerationcones, storm tank scrapers J. Blakeborough & Sons Ltd Valves G. Brady & Co. Ltd Roller shutter doors R. & J. Dempster Ltd Gasholders Dorr-Oliver Co. Ltd Grit classifiers, heat exchangers English Electric Co. Ltd Dual fuel alternator sets William E. Farrer Ltd Valves and penstocks,primary sedimentation scrapers (circular and rectangular tanks) Foxboro-Yoxall Ltd Magnetic flow recorders Glenfield & Kennedy Ltd Valves and penstocks Allen Gwynnes Pumps Ltd Pumps Hartleys (Stoke-on-Trent) Ltd Penstocks, filter distributors W. C. Holmes & Co. Ltd Pre-aeration blower George Kent Ltd Flow measurement Lea Recorder Co. Ltd Flow measurement Henry Lowe (Lifts) Ltd Goods lifts Petrol Pump Supplies Ltd Foam dousing equipment Pulsometer Engineering Co. Ltd Pumps John D. Scholes & Partners Overhead cranes Sigmund Pumps Ltd Pumps Sturtevant Engineering Co. Ltd Ventilation fans Sulzer Bros (London) Ltd Pumps Wallwin (Pumps) Ltd Pumps Whitehead & Poole Ltd Pumps, sludge loadingarm, troughs and weirs, grit disposal equipment

4. Electrical contractors N. G. Bailey Ltd Cables Brush Electrical Engineering Co. Ltd Switchgear Communication Systems Ltd Telephones English Electric Co. Ltd Switchgear Electric Construction Co. Ltd Switchgear Denis Ferranti Ltd Transformers W. J. Furse Ltd Lightning protection General Electric Co. Road lighting K. S. Construction Co. Ltd Cabling . Laurence Scott & Electromotors Switchgear and variable speed motors London Transformer Products Ltd Transformers W. T. Parker Ltd Cables Penmar Ltd Cables A. Reyrolle & Co. Ltd Switchgear South Wales Switchgear Ltd Switchgear C. Stanley Tagg & Co. Ltd Cables Bertram Thomas Ltd Switchgear Watford Electric & Mfg. Co. Ltd Switchgear F. H. Wheeler & Co. Ltd Cables Downloaded by [ Paul Robert Matthew Hastings] on [13/07/18]. Copyright © ICE Publishing, all rights reserved.593 SYMES Appendix II Table of costs Inlet works including associated pumping stations €l 020 000 Generation plant and building M37 000 Primary digestion plant €592 000 Secondary sludge consolidation plant E514 000 Primary sedimentation plant €783 000 Aeration plant and final tanks fl 535 OOO Workshop and laboratory €140 000 Outfall €63 000 Jetties €90 000 Additional sludge vessel €460 000 Storm tanks €493 000 Roads, landscaping, etc. €84 000 Totalexcluding staff costs and consultants’ fees €6 211 000