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Water efficiency in the south east of England retrofitting existing homes (2007)

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This study commissioned by the Environment Agency explores effective ways of increasing water efficiency in existing homes in South East England, building on findings of an earlier study. Following a pre-selection process, five demand management measures were analysed in-depth. Results indicate that average incremental social costs of retrofitting water efficiency measures compare favourably with the costs of traditional resource development schemes. The highest water savings would spring from a combination of different measures. The study also highlights that water savings increase significantly with only a small increase in cost if full subsidies and free installation are provided as part of the ‘package’.

Executive summary

We previously commissioned environmental and engineering consultancy Entec to assess the viability of different options for retrofitting water efficiency into existing homes in the South East. Entec was subsequently commissioned by the Environment Agency to undertake a more in-depth exploration of effective ways of increasing water efficiency in existing homes, building on the findings of the earlier study.

The objectives of the study were:

  • to review the range of water efficiency measures identified in the previous study, ‘Water Efficiency Analysis of South East England Rollout Options (Entec, September 2005)’;
  • to review any other similar studies and assess whether to consider other measures;
  • using the best options and the most up to date information, to assess the scope for reducingwater use in existing homes;
  • to consider the most effective ways of implementing the efficiency measures, for instance bylinking with other ongoing programmes;
  • using up to date information, to fully assess the costs and benefits and identify the pros andcons of these approaches.Having identified and reviewed all potential demand management measures, five were selected for further analysis. These were:
    • ultra low flush toilet replacement scheme
    • variable flush retrofit devices
    • low flow showers
    • metering
    • range of low water use fittings.
      Potential strategies were identified and average incremental social costs (AISC) in pence per cubicmetre and yield benefits in megalitres per day (Ml/d) were calculated. Results were calculated for:

• the total domestic housing population in the Greater London Authority (GLA) and the South East England Regional Assembly (SEERA).

and also as worked examples for three case study areas:

  • all Housing Association households in the South East;
  • a water resource zone with a tight supply-demand balance;
  • typical population and demand of a medium sized water company in the South East.These worked examples were assessed together with the South East of England as a whole to confirm that the results are realistic on a local scale.Finally two implementation strategies consisting of combinations of schemes were assessed. This was to examine the savings that could be made by implementing water efficiency measures together as part of an overall strategy. Two combinations were explored:
    • metering combined with variable flush retrofit devices and low flush replacement WCs;
    • metering combined with low use fittings.Results and conclusionsIn general, the average incremental social costs of retrofitting water efficiency measures compare favourably with the costs of traditional resource development schemes.The highest water savings come from the combined implementation strategies. Compulsory metering combined with fitting of variable flush retrofit devices and subsidising the end of life replacement of toilets with low flush models returns yields of 77.2 Ml/d (+/-25.3) for 136 p/m3 (+/- 39). The same scheme, but with metering on change of occupancy, can save 31.9 Ml/d (+/-10.5) for 115 p/m3 (+/-30).

Savings from compulsory metering, combined with a range of low water use fittings, were just as high but cost more: 77.5 Ml/d (+/-25.8) for 162 p/m3 (+/-49). Low use fittings combined with metering on change of occupancy is estimated to save 22.4 Ml/d (+/-6.6) for 150 p/m3 (+/-42).

Probabilistic modelling using a Monte Carlo simulation estimates that there is a 95 per cent chance of achieving savings of approximately 65 Ml/d from each of the combined schemes when implemented with compulsory metering and a 75 per cent chance of achieving approximately
70 Ml/d.

The relatively high savings from these schemes are based on the assumption that households switching to a metered supply will be more interested in water efficiency. Metering on change of occupancy may be more likely as compulsory metering currently depends on applying for water scarcity status.

Improving water efficiency in existing homes can offset increased demand in growth areas like Ashford and the Thames Gateway. Increase in demand caused by these growth areas is predicted to be around 50 – 70 Ml/d, which is less than the potential demand management savings from implementation of combined strategies. The total increase in demand for the entire South East region is estimated to be approximately 160 Ml/d by 2015. So, introducing the combined strategy measures in existing homes could reduce this increased consumption by almost 50 per cent.

Individual, wide-scale schemes across the GLA and SEERA regions that could also achieve significant savings are:

  • compulsory metering – 51.6 Ml/d (+/-19.5) for 176 p/m3 (+/-59)
  • variable flush retrofit devices – 9.9 Ml/d (+/-2.5) for 102 p/m3 (+/-19)
  • low use fittings – 8.4 Ml/d (+/-2.0) for 113 p/m3 (+/-20)The results show that water savings increase significantly with only a small increase in cost if full subsidies and free installation are provided as part of the ‘package’. This is because greater incentives encourage more people to take up the schemes, which increases savings. Also, up front costs are discounted over the life of the scheme.The implementation of individual demand management measures at a local level is likely to achieve limited savings. However, compulsory metering is estimated to provide useful savings in water resource zones with limited available resources, particularly if combined with water efficiency measures such as low use fittings or toilet replacement/retrofit. The combined implementation schemes can also achieve significant savings even at resource zone level.Significant uncertainty remains around many of the inputs used for calculating scheme savings. This uncertainty will reduce as new and ongoing studies are completed creating higher confidence levels for the calculation of updated results.