Earth Notes: Efficient Dishwashing

How to be green and idle, save money and energy, and have sparkling dishes!
full dishwasher

I can't offer a masterclass, but I can tell you what has worked for us, a family of four, with not-very-smart slimline domestic dishwashers as of November 2009 (and now 2017 on the ZDS2010)!

(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.)

Basics

The basics are very simple:

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 Loading Up...

Before putting the plates, pans, etc, in:

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:

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):

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:

If you are in the market for a new dishwasher, eg because yours is beyond repair:

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

  • 2015/03/10: 7 common dishwasher-loading mistakes that may surprise you.
  • 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.
  • Energy-saving tips from "Time to Change".
  • 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:
    ApplianceLitres of water usedCost 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...