Water conservation Recycling Outline strategy for black-water recycling at Wood Wharf
Technology Proven method for sludge treatment
Membrane bioreactor (MBR) technology provides combined activated sludge treatment with membrane filter cassettes, which sit in the sludge and have wastewater drawn through them using vacuum pumps. This greatly reduces the footprint of the system. Within an MBR, wastewater is first screened
>
infrastructure would be needed; and that, as it was no longer restricted to wastewater from baths, showers and basins, far bigger volumes of recycled water could be produced to satisfy a larger proportion of the development’s non-wholesome water needs, including cooling tower top-up on the office buildings. The conclusion of this study was that a centralised
black-water recycling plant was the best fit for the aims of the development. Buildings require compact high- density wastewater treatment technologies. We found that the most widely used technology in this application is the membrane bioreactor (MBR), which has a smaller footprint than a traditional sewage treatment plant (see Technology box). Table 1 shows the proposed volumes and uses for
recycled black water per building type, together with an estimate of the black water discharged by each type of building. It should be noted that the cooling tower make-up of 100 to 140 cu m /day represents 25% to 35% of the average annual requirement to allow mixing with wholesome water to avoid the risk of dissolved solids build-up in the towers, as experienced at the Solaire scheme. Based on these estimates it was proposed that a 500 cu m/day MBR should be considered, with any excess black water being discharged (untreated) to the local utilities sewer.
Whole-life costing Indicative costs for the black-water solution for Wood Wharf were worked out as: • Capital cost of plant: £600,000 • Civil and infrastructure works: £1,100,000 • Preliminaries, contingency and O&M: £1,150,000 Using this information, together with annual energy
and maintenance costs obtained from GE Power & Water, and the estimated savings on Thames Water wastewater treatment costs – and assuming an income based on Thames Water charges for selling the recycled water to each parcel – a basic 25-year whole-life costing analysis was performed. This indicated a payback period of between 11 and 18 years.
42 CIBSE Journal October 2010
much as in a traditional plant. The wastewater then enters the bioreactor where high levels of aeration promote the growth of an active sludge. This sludge feeds on the organic content of the wastewater, significantly reducing its biological oxygen demand (BOD). The wastewater is then abstracted via membrane filtration units with a pore size of 1x10-7 m, which remove the majority of pathogens and discoloration. The water leaving the unit is of a high standard, but may need polishing to kill any remaining viruses and remove discoloration. Outside the UK there are examples where this
technology has been used successfully in buildings, predominantly in Australia and North America. The Council House 2 scheme in Melbourne, Australia, treats 100 cu m per day and pipes treated water off site for use in other buildings, street cleaning and irrigation. The Solaire residential apartment scheme in
New York treats 95 cu m per day and supplies water for toilet flushing, irrigation and cooling tower top-up. This scheme achieved the LEED Gold environmental standard. Other schemes using MBR technology include the Vancouver Convention and Exhibition Centre, the mixed-use Dockside Green development in Victoria (both in Canada), and the Palm Jumeirah Development in Dubai.
However, it must be stressed that this was a
simplified exercise which ignored the complexities of the development’s phased nature; this could have a significant impact on the real payback period because it would be many years before the plant operated at full duty. However, it is reasonable to say that, in terms of
both technology and cost, it has been established that a centralised black-water recycling scheme is feasible for Wood Wharf. What is much harder to establish are the wider issues, such as acceptance, perceived risk and the commercial issues surrounding a scheme such as Wood Wharf. At the time of writing, Wood Wharf awaits the revival of the property market, in particular residential property values. So sadly, the opportunity to work through these issues with the stakeholders is on hold – but if water really is the new carbon, then black- water recycling may well be a solution in waiting. l
Peter White is public health principal at Hoare Lea consulting engineers
www.cibsejournal.com
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84