Esher WwTW Weys up the benefits
As part of the £5M upgrade to Esher WwTW, Thames Water has invested in shaftless conveyor technology to improve the quality of wastewater discharged into the rivers Wey and Ember.The dry weather flow of Esher WwTW is 434l/sec, with a maximum flow to treatment of 892l/sec and a peak flow of 1,579l/sec. Average screenings loading is 3.7m3/day with a screenings peak loading factor equating to 13.9m3/h (peak factor 90). The population equivalent of the site is 123,106.
The installation is part of a £5M upgrade to Esher WwTW to improve the quality of wastewater discharged into the River Ember and River Wey.
Thames Water project manager Ann Weston says: "The changes we have made will be a major boost for the local environment and bring the works in line with new Environment Agency treatment standards for discharge of sewage effluent to rivers."
Three 6mm perforated plate screens feed into a horizontal U320 Spirac shaftless screw conveyor. The screenings are then passed via a drop chute into a 5m3/h capacity Spirac VRW 280 Megawasher, installed in July 2005.
The Megawasher washes out organic matter from sewage screenings by means of two slowly rotating spirals, reducing the volume and weight of the screenings by up to 70%. It then compacts them to a high-dryness screenings plug, whose weight allows it to fall directly into the skip and not be prone to any wind-blown effects.
Subsequently, the flow to Esher WwTW increased because the Claygate and Thames Ditton foul water flood alleviation scheme was implemented to reduce the risk of catchments flooding. This was achieved with new larger inlet pumps, combined with uprated inlet works, storm-tank cleaning and better storm return valving.
Thames Water added a new channel and Spirac extended the existing horizontal conveyor as well as installed a new VWR Megawasher last April. The second unit is used as a duty assist in the case of high flows, programmed with an automatic switch-over during high loads. There is also a bypass directly to a chute that discharges into a skip, meaning that screenings continue to be removed from the screens regardless of solids loading.
This prevents the screens from blocking and ensures maximum availability to handle storm-flow events. The screenings are transported by means of Spirac's shaftless screw conveyors.
These conveyors use only shaftless spirals and their primary material is Spirac high tensile micro-alloyed steel (HTMAS), a special steel from Sweden. The mechanical properties of HTMAS mean that the spiral has a very high tensile strength, making it both wear and corrosion resistant, as well as being highly flexible - ideal to handle difficult material such as sewage screenings.
Spirac's proprietary liner, Duraflo SPX, is a specially formulated high-density polyethylene that has been designed specifically to ensure low wear and long life.
The Duraflo SPX liner comes in a two-colour format for visual identifications of wear and has a snap-in, snap-out feature enabling easy replacement.
The conveyor drive is 3kW only with the trough and lids all manufactured in wear-resistant stainless steel.
By using shaftless conveyors, Thames Water is saving power and money because it does not require any wash water. Also, the reduced friction means that the conveyors have low maintenance requirements and longer life making them cost efficient, with low operator intervention.
"We are delighted with the efficiency and performance of the Spirac installation at Esher WwTW and are pleased to report zero failures or problems with the equipment on site," says Alaistair Smith, principal engineer from Black & Veatch working in a Thames Water alliance.