Earth Notes: Low-Carbon Efficient Dishwashing
Updated 2020-12-22 17:57 GMT.
Summary Questions and Answers
How do I eco-dishwash?
Run when the machine is full. Usually not at the hottest setting. Usually at night when grid demand and carbon intensity are low. (Or when you have full sun to cover most or all of the load, if you have solar PV.)
Why dishwash at 2am?
When electricity demand is highest the transmission system is under strain. And there may well be dirtier or higher-carbon fuels generating the electricity to run your dishwasher. In the UK peak is typically early evening 4pm to 7pm. Don't run anything that you can do without then, including the dishwasher! Conversely, if you can run the dishwasher in the wee hours, maybe 1am to 4am, less CO2 is likely to belched out to run your wash, in future maybe even zero when wind (etc) is meeting demand.
(Cold rinses take so little energy that you should probably do them when you need them.)
When did 16WW start to eco-dishwash?
Since at least November 2009!
Basics
The basics are very simple:
- Run your dishwasher only when it is full if possible. You get more for the detergent and energy that goes in.
- Run the coolest/quickest programme that will do the job. Heating water uses almost all of the wash energy.
- Choose a programme that lets your dishes dry in the air. A 'thermal dry' uses a fair bit of energy and is usually not needed. Especially if you have time and/or a dish rack for tricky items.
These last two steps save us about one third of our total energy consumption per load. So now we use 0.86kWh instead of 1.27kWh. I was using 'Quickwash' at 65°C in our old machine which was too ancient to have cooler washes. In our new machine the "ECO" 50°C cycle generally works very well. You probably can't use water much above 45°C for hand-washing. So anything hotter than that in the dishwasher is likely to give a better wash, and kill more bugs...
Before Loading Up...
Before putting the plates, pans, etc, in:
- Scrape off any excess food before loading the dishwasher (maybe for your compost bin). Now the detergent is washing the dishes not your discarded food!
Rinsing dishes under a tap is probably unnecessary and wasteful of water. The wash programme on our ancient (Zanussi DW-24 slimline) dishwasher that I normally used ('heavy soil') starts with a prewash (essentially a cold rinse with some detergent). That is followed by a full hot wash, hot rinse and thermal dry.
Instead maybe:
- Run the cold rinse programme with a little detergent (or soda crystals or non-foaming washing-up liquid to be effective cold). Prevent food caking on, especially dairy products, if a full wash is not going to be run for a while.
- Clean the filters (if bunged up) before the main wash. This makes sure that the water pumping, etc, is as effective as possible, not merely spreading old food around the machine! Your dishes will be cleaner for it.
A cold rinse can get a lot of dirt off the dishes and uses virtually no energy. I can't even measure the consumption on my plug-in meter. So it's way less than making a cup of tea, and no more than a few percent of the full wash. If you are in an area short of water then you may skip this step. In rainy London I sometimes go wild and do a rinse in the morning to get the milk/porridge/etc off the breakfast dishes. Then again in the evening and empty the filter before the main wash!
For extra greeny points (and possibly extra savings depending on your tariff):
- Run the main wash on a delay/timer at 2am or as near as you can get. Just before you go to bed is good. (Or, if you have solar PV, when it's sunny to directly cover most or all of the load. Your PV will typically need to be putting out 2kW or so to cover water heating.) Avoid right after the evening meal!
- Turn off the dishwasher at the wall when done. Avoid any 'vampire' losses from the dishwasher or timer.
Also, I like to alternate large and small plates in the dishwasher, to avoid big ones pressing together and getting insufficient water flow to get them fully clean.
Maybe once per week, or if the machine seems to be clogging up or ineffective, or for/after a particularly greasy or dirty load:
- Clean the filters. In our case that means rinsing and brushing them under a running tap.
- Run a 'maintenance' wash with the machine empty or at least not jammed full. Use a hotter wash and more detergent than usual to shift any grease and gunge that has been accumulating, and that might even bung up your drains or damage the machine. If showing signs of difficulty pumping/draining, bring forward and repeat the maintenance wash to try to clear the problem.
2020: I may reduce maintenance washes to once per month in winter, eg on particularly sunny days where our PV can carry at least half the load.
If you are in the market for a new dishwasher, eg because yours is beyond repair:
- Buy the most energy-efficient model that you can. It will probably pay itself back in saved energy.
- Don't buy a bigger model than you need.
There's a lot of speculation on the IntarWebs either way. But it seems likely that if you follow the steps above then you may have a lower environmental impact than washing by hand, while saving money and a dull chore...
'Eco'/Zero-Phosphate Dishwasher Detergents
The complex phosphates in conventional/cheap dishwasher detergents do a good job of cleaning. But they come from a diminishing resource. They are also difficult to remove at wastewater treatment plants. They cause algal growth and oxygen depletion in rivers and lakes, to the extent that some places have banned their use.
'Green' or 'eco-friendly' dishwasher detergents may be labelled as such for a number of reasons. They may work well at lower temperatures, saving energy, And for being 'zero-phosphate'.
We do now use such low-temperature phosphate-free tablets in most washes. But they do not do quite as good a job, typically leaving a very slight residue. This most obviously builds up on glassware over time.
To fix that, during the typically-once-per-week hotter maintenance wash I use a phosphate-based tablet. For the other washes the prewash detergent is still phosphate-based. (I can't find a 'green' powder anyway.) In any case, I add only a little of it along with the main-wash tablet.
So we are now still using some phosphates in the dishwashing. But probably a lot less than before.