Efficient Dishwashing

I can't offer a masterclass, but I can tell you what works for us, a family of four, with a creaking ~10-year-old slimline simple domestic dishwasher as of November 2009.

(Shortly after writing this our old DW24 dishwasher packed up (2009/12/12), and given that it had already been repaired at least once, and had shown signs of going dangerously insane, we ordered a replacement ZDS2010 which we have reviewed.)

The basics are very simple:

Run your dishwasher only when it's full if possible because you get more for the detergent and energy that goes in.
Run the coolest/quickest programme that will do the job because heating water uses almost all of the energy consumed.
Choose a programme that lets your dishes dry in the air since a 'thermal dry' uses a fair bit of energy and is usually not needed if you have time and/or a dish rack for recalcitrant items.

These last two steps for us save us about 1/3rd (0.86kWh instead of 1.27kWh) of our total energy consumption per load, 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). Note that you probably can't use water 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 putting the plates, pans, etc, in:

Scrape off any excess food before loading the dishwasher (maybe for your compost bin) so that 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') is a prewash (essentially a cold rinse with some detergent) 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) to prevent food caking on, especially dairy products, and/or if the full wash is not going to be run for a while.
Clean the filters (if bunged up) before the main wash so that the water pumping, etc, is as effective as possible, and again 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 (say) 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, and 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, for example just before you go to bed, not right after the evening meal!
Turn off the dishwasher at the wall when done to avoid any standby losses from the dishwasher or timer.

Why 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 one peak is typically early evening. Conversely, if you can run the dishwasher in the wee hours, maybe 1am to 4am, much 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 do them when you need them.)

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 which in our case is brushing them and/or rinsing under a tap.
Run a 'maintenance' wash with the machine empty or at least not jammed full, and/or using 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.

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 which 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 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 are from a diminishing resource and are difficult to remove at wastewater treatment plants thus causing algae growth and oxygen depletion in rivers for example, 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, such as working well at lower temperatures allowing energy saving, 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 that most obviously builds up on glassware over time.

To counteract that, during the typically-once-per-week hotter maintenance wash I use a phosphate-based tablet, and for the other washes the prewash detergent is still phosphate-based (I can't find a 'green' powder anyway) and I put a very little of it with the main-wash tablet.

So while we are now still using some phosphates in the dishwashing, it is probably a lot less than before.

Sources/Links

2010/08/19: What's the carbon footprint of ... doing the dishes?
Consumer Guide to Home Energy Savings: Condensed Online Version: US-oriented but still very much the same story.
The Which? guide to a greener dishwash.
The Guardian 2005 answers Is it OK ... to use a dishwasher? with a "yes" citing Rainer Stamminger, professor of household and appliance technology at the University of Bonn in 2004. For 12 place settings the average human washer used 103l and 2.5kWh, though the best were pretty close to (or beating) the dishwasher at 15l and 1kWh using the following techniques: "scraping, not rinsing, food scraps from dishes; not pre-rinsing all the dishes under a running tap; using two sinks, one filled with hot water and detergent, the other with cold for rinsing; and not over or under using detergent." My money would still be on the machine being more consistent and removing more detergent residues!
Do 'eco' (non-phosphate) dishwasher detergents work? Spokane (WA, US) residents smuggle suds over green brands; a US industry site explains clearly the dishwashing process. (Our initial experience with Lidl's "W5" non-phosphate dishwasher tablets 2010/04 seems OK and they leave no landfill waste at all. "Greenforce" tablets are OK too, but they seem to gradually let residues build up so a 65°C cycle with a 'normal' phosphate tablet from time to time, ie a 'maintenance' wash, seems like a good idea. I now tend to add a small amount of phosphate powder when using a 'green' tablet as the residues are noticably less; not zero-phosphate, therefore, but less.)
The carbon footprint of 1l of UK potable mains water is estimated at 0.298g of CO2 circa 2008. In private correspondence my local water company said:
The figure quoted by Thames Water for our Green House Gas Emissions is calculated using a water industry standard Carbon accounting tool. A third party, the Water Research Council, has developed this tool. In calculating the figure we take account of the electricity used to treat and pump water. The average figure across the Thames Water region for Green House Gas Emissions is 0.3 grams/litre.

The Thames Water 2008/2009 metered charges leaflet includes the following guideline table:

Appliance	Litres of water used	Cost of water used
Shower (non-Power)	30--60	4.8--9.5p
Bath	50--170	8.0--27p
Toilet flush	7.5--9.0	1.2--1.4p
Washing machine	60--100	9.5p--15.9p
Dishwasher	20--40	3.2--6.4p
Garden hose per hour	500	79.5p

20100510, Canada: Do Household Tasks While Asleep To Save Energy. Well, the energy consumption is the same, but many other things are saved...

Earth Notes: Efficient Dishwashing

'Eco'/Zero-Phosphate Dishwasher Detergents

News

Sources/Links