Wash dishes by hand or in the machine? Which option uses less water? In recent years, the efficiency of washing machines has increased so much that modern machines use less water than a common hand wash.
The savings are basically due to two reasons: the machine is able to use water at high temperatures, reducing the amount of liquid used in washing, and more modern models quickly see if the dishes are clean — avoiding more scrubbing.
A very efficient machine uses up to 8 liters per cycle—capable of washing 60 items.
According to Sabesp, washing the dishes with the faucet half open for 15 minutes consumes 117 liters of water at home and 243 liters in apartments (due to the stronger water pressure).
A survey commissioned by a dishwasher manufacturer tested washers and volunteers to wash dishes by hand. When washing 45 items —three dishes, three cups, three saucers, etc.—, the result obtained by the washers in relation to the volunteers was a water saving of 4 to 6 times (they spent between 44 and 105 liters with manual washing) , depending on the model and cycle used.
“The water is very hot in the washer [cerca de 60°C] and it’s already mixed with that soap, which is stronger and more concentrated than the soap we use”, explains Marcelo Oliveira, professor of design at the University of São Paulo. “When we wash with a sponge, most of the time, it’s with cold water and it’s not the same thing.”
How does the machine know it is clean?
Oliveira also remembers that the most modern washers have the so-called turbidity sensor, which identifies whether the water is cloudy or not. This sensor, created by the food industry, works by shining a light on water.
If it reflects at the expected angle, it means it’s clean. If it reflects at another angle, it means another wash is needed. Everything is evaluated using a microprocessor.
Another survey carried out by the Falcão Bauer Laboratory, at the request of another manufacturer, revealed that the replacement of manual washing by a dishwasher in a family of four people can generate annual savings of up to 27 thousand liters of water — equivalent to 55 boxes of water. 500 liters of water.
The recommendation of the United States Environmental agencies is for people not to prewash and then use the machine, as this uses more water than washing by hand. Just get rid of excess food. The pressure of the water jets in modern machines is enough to clean the dishes.
In the future, who knows, washing machines could become even more efficient, envisions Oliveira. If it were possible to reuse the water used by the washing machine with the filter developed by NASA for trips to Mars, the water could be reused infinitely.
Can you be economical with manual washing?
“What makes the machine good is that the water flow is restricted there in a confined environment, so you don’t spend as much because it jets in a targeted way, with a high temperature.” One of the only ways to surpass the efficiency of the machines is to fill the sink with water and wash and rinse the dishes with that same water, according to the professor.
According to Sabesp, in this way it is possible to reduce consumption from 117 to 20 liters in the same 15 minutes, warns Sabesp, which recommends removing food scraps before starting the wash.