It is estimated by Green Thing  that although the carbon footprint of unheated tap water is relatively small at around 0.0003kg per litre, when it is multiplied by 45 million toilets and 2 billion litres of water every day, it is actually quite significant. As the average toilet uses between 7 to 13 litres of water each flush, then by fitting a Cistern Displacement Device (CDD) such as a hippo bag or a “Save-a-Flush”, approximately 1.64kg of carbon dioxide could be saved per year. Collectively, this could save 600 million litres of water a day in the UK, or over 65,000 tonnes of carbon dioxide each year. As toilets use about 30% of the total water used in a household, fitting a CDD will save you money on your water bill and they are generally free from most water companies.
Who should use CDDs?
Generally, cistern devices are recommended for use in older toilets which have a greater maximum flush. Toilets installed before 1993 normally have a flush of 9 litres or more in volume, and those installed from 1993-2001 normally have a flush of 7.5 litres in volume. Cisterns installed post-2001 have a maximum of 6 litres therefore CDDs are not recommended, however only around 5 million toilets in the UK are these type. Consequently, there is the opportunity for 40 million toilets to fit a CDD, reduce their carbon footprint and their water bill.
Which CDD is suitable for me?
Hippo the Water Saver
The Hippo bag opens up into a box which sits beneath the cistern float and fills with water. Whenthe toilet is flushed water is saved in the bag, although it does have a small hole at the base to prevent the water becoming stagnant. Approximately 3 litres of water is saved using the Hippo which could provide a saving on your water bill of around £20 a year . Even if your water company does not provide the Hippo for free, you can expect a pay back on your initial investment within 8-12 weeks. Unfortunately, Hippos may only be used for tanks of 9 litres or more.
Freddie the Frog
Freddie the Frog contains special crystals that absorb water and one placed in the toilet cistern swells over a 5-6 hour period to displace 1 litre of water per flush. As an alternative to the Hippo, the Freddie the Frog can be used in toilets with lower tank volumes and has the advantage of being bio-degradable and recyclable . Unfortunately, Freddie the Frogs are only provided free of charge to Anglian Water customers however they are available as “Save-a-Flush” other water companies
Toilet Tank Bank
The Toilet Tank Bank is a simple displacement device which consists of a hot-water shaped bag to be filled with water. When placed inside your cistern it should save up to 2 litres per flush or 20% of the water originally used when flushing . Due to the amount of water it saves, the Toilet Tank Bank is only suitable for cisterns with a large volume.
CDDs from Water Companies 
|Anglian Water||Freddie the Frog displacement device|
|Southern Water||Save-a-Flush bags|
|Essex and Suffolk Water||Save-a-Flush bags|
|Portsmouth Water||Save-a-Flush bags|
|South Staffs Water||Hippo bags|
|Sutton and East Surrey Water||Hippos and Save-a-Flush bags|
|South West Water||Free Hippo bag or Hog bag for larger cisterns|
|Veolia Water||Hippo bag|
|Thames Water||Hippo bag or Save-a-Flush bag (to all UK residents)|
|United Utilities||Save-a-Flush bag|
|Welsh Water/ Dwr Cymru Cyf||Hippo bag|
|Wessex Water||Save-a-Flush bag|
|Yorkshire Water||Flush Saver devices|
Although there is a brief overview of CDDs provisions in each area the best way to find out what is available to you is to contact your local water supplier or council. CDDs are a simple cost efficient way to decrease your water bill and the production of carbon dioxide by your house and consequently the UK.
 www.dothegreenthing.com (2007)
 www.hippo-the-water-saver.co.uk (2010)
 www.anglianwater.co.uk (2010)
 www.reducereuserecycle.co.uk (2010)