Dishwasher or Hand-washing? Which is better for the environment?

 

The internet has been awash (pun intended) with the revelation dishwashers are better for the environment than handwashing your pots and pans in the sink. This 'news' also made it onto one of my favourite Radio 4 programmes: More or Less. It seems the 'study' revealing that even a part-loaded dishwasher is more efficient resource-wise than handwashing came from a division of Proctor & Gamble that makes dishwasher consumables. This does not necessarily mean the report is untrustworthy but it should be treated with some skepticism until its claims can be justified or validated.

My first port of call for information on carbon footprints was Mike Berners-Lee's book "How Bad are Bananas?" I have used the data from the 2020 edition which is different to that given in the 2013 edition.

Things we might want to consider:

1. Carbon footprint: This is mainly down to the embodied carbon in manufacturing/maintaining the dishwasher plus the energy needed to heat the water. Dishwashers have an embodied carbon dioxide equivalent (CO2e) = 100 g per wash but they do use less, though hotter, water than handwashing. 

(i) Handwashing in warm water (30-40 degC): 360g to 3000g CO2e depending on whether you use the water sparingly (bowl) or extravagrantly (under a running tap). Most people will be somewhere in-between these extremes.

(ii) Dishwasher in eco-mode (50 degC) = 470g CO2e. Dishwasher in professional mode (65-70 degC) = 600g CO2e. These data include the embodied carbon.

Conclusion: If you are extremely frugal with the amount of water you use then the carbon footprint of  handwashing could be a little better than the dishwasher but I suspect for most people it isn't. These carbon footprint calculations assume a relatively long lifetime for the dishwasher (12.5 years) so buy a machine that is efficient and well-built and look after it.

2. Energy use: Predominantly the energy needed to heat the water. Dishwashers use hotter water (50-70 degC) but much lower volumes (50 - 90% less depending on the energy rating of the machine) and they only heat the water they need. Handwashing typically uses water from the hot water cylinder (probably at 50-60 degC) so the volume of water heated is much more than that needed for washing the dishes. Conclusion: Dishwashers, on average, use about 1 kWh per wash so are very economical to run. If you were to boil water in a kettle for handwashing, the same 1 kWh would boil about 15 litres, enough for 5 washing up bowls (see Water usage below) of warm water. Running a full dishwasher likely uses less energy but the difference will not be large. 

3. Water usage: As noted above, handwashing uses considerably more water than a dishwasher to clean the same number of dishes assuming you change the dishwater before it starts recoating the dishes with grease and food particles! According to Which, you can wash 2 standard meal sets (12 pieces of crockery plus associated cutlery) by hand in a bowl containing 9 litres - it is not clear whether this includes rinsing. A full-size and filled dishwasher will wash between 9 and 19 standard meal sets, depending on its efficiency, in the same amount of water (9 litres) and definitely includes rinsing.

Conclusion: The dishwasher wins hands-down. No contest! Important note: I know some people who pre-wash or rinse plates before putting them in the dishwasher. Never do this - just scrape the plates into the kitchen compost caddy.

4. Cleanliness/hygiene: Handwashing in warm water is less hygienic than a dishwasher (400 times higher bacterial count in the case of the former) because of the higher water temperatures, longer contact times and natural air-drying in the latter. Drying with a tea towel is only going to make things worse.

Conclusion: The dishwasher wins by a country mile.

5. Cost: A reasonable quality and efficient dishwasher can be had for £400 though it is possible to spend much more. The performance and energy efficiencies appear to be quite similar (e.g. using less than 10L water and around 0.75 kWh/wash in the Eco-mode) so you may be paying for longevity and ease of repair. Even if you use the dishwasher everyday for 10 years that still works out at 11p per wash for the £400 machine assuming no repairs. This will be much more expensive than buying a few washing-up bowls and pan scrubbers.

Conclusion: Even when taking in the lower running costs of a dishwasher, handwashing is clearly the cheaper option.

6. Time/Convenience: Handwashing may take six times longer than loading & unloading the dishwasher! That could be an hour a day spent on more enjoyable or productive activities. Most people I know do not go back to handwashing after regular use of a dishwasher. It generally takes several hours to run a standard wash programme on a dishwasher so you do need to make sure you have enough crockery and cutlery.

Conclusion: Dishwasher every time!

In summary, the dishwasher comes out on top especially if you only run it when full and use the eco-mode function. Handwashing is the cheaper option overall but loses out on energy use per wash, total water usage, cleanliness/hygiene, convenience and time. Potentially, the carbon footprint of handwashing could be lower largely because of the embodied carbon in the manufacture of the dishwasher; in practice, this advantage is lost by the way most people handwash. Basically, just choose whichever option best suits your lifestyle.

Finally, I devised a frugal method for handwashing, first used when camping, and now used at home also. Camp-site washing-up facilities typically comprise a row of large stainless steel sinks. We tend to wash-up as we go along so there are only the pots, pans and cutlery for two people/one meal to consider. For the washing-up liquid, I use an old spray deodorant bottle filled with eco-friendly washing-up liquid diluted 50:50; this makes it less viscous for ease of spraying and reduces the likelihood of overuse.

After placing the plug into the plug-hole, briefly pre-wet the plate/cup/pan under the tap and then spray with a couple of shots of detergent. Scrub with scrubber of your choice, rinse and repeat if necessary (e.g. for greasy frying pans). The water that collects in the sink can be used to pre-soak items. The concentrated washing-up liquid (c.f. that diluted in a sink full of water) cuts through grease and grime easily and is applied only where required. On completion of the washing-up, there is usually about one inch of dirty water in the sink and very little in the way of suds from overuse of washing-up liquid. This makes cleaning the sink, ready for the next user, quick and simple. It is not uncommon to see fellow campers filling a nearby sink (30 litres or more), adding a 'too large' squeeze of washing-up liquid and then leaving a sink full of suds for the next person!

In the end, whichever method you choose, you can sleep easy at night provided you do your best to minimise the resources you use. We use a dishwasher (full, eco-mode with environmentally-friendly dishwasher tabs) plus occasional frugal handwashing as that suits our lifestyle best.