The Extensions to the Davyhulme Sewage Works
Total Page:16
File Type:pdf, Size:1020Kb
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 sedi- mentation 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 exist- ing 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 exist- ing 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 pen- stock, 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 in- corporated 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 was two-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 were used the raking mechanism of the replacement screens operates con- tinuously. 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. con- crete. 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.