What science knows about our daily dishwashing routine

2.1 Dishwashing in context of resource saving

One of the earliest studies regarding the comparison of manual and machine dishwashing is from 1956. Knowles Weaver et al. [5] investigated whether an electric dishwasher could save private household resources, such as time, water and detergent. They conducted a preliminary survey with 163 dishwasher users in the USA to determine the amount of time spent for the task of dishwashing before and after the installation of an electric dishwasher. After an education and accommodation period the participants were asked to use the electric dishwasher over the span of the next 30 days and to keep a detailed diary about it. One result was that the electric dishwasher reduced the amount of time needed for the task of dishwashing significantly. Depending on the household, participants only required one half to one fifth of the time in comparison to manually doing the dishes. The results for water and detergent usage, on the other hand, were not as clear. A lot of households required more water and energy than before. The authors explained this circumstance by the fact that the participants often additionally washed dishes by hand. These additional washing-up activities for individual load items consumed about as much water as the entire machine dishwashing process. The authors therefore pointed out that it should also be noted that despite the presence of a dishwasher, consumers will still wash some items by hand. Nevertheless, the researchers concluded that consumers – without knowing the study results – felt that the machine saved them a lot of work and time [5].

Today, the private household sector accounts for a large share of global water and energy consumption. According to the United States Energy Information Administration [7], about 17% of primary energy is consumed in private households. This is estimated to account for 25% of greenhouse gases in 2010 [8]. Influencing or changing consumer behaviour is therefore often seen as the key to saving energy and greenhouse gas emissions. Manufacturers of electrical household appliances are encouraged by energy labels to make their appliances more efficient. That is why in the years 2007–2015, the private household sector experienced the greatest improvement in energy efficiency (17%) [9].

A tendency regarding the improving energy efficiency was also observed by Stamminger [10] (cf. Figure 1a). He compared the resource consumption in manual and automatic dishwashing from different studies. This detailed analysis shows that with respect to water, energy and time consumption, automatic dishwashing is clearly superior to manual dishwashing. Furthermore, automatic dishwashers lead to hygienic cleaning results [10]. In addition, the author pointed out that manual dishwashing has become more expensive relative to machine dishwashing over the period under consideration (cf. Figure 2).

Figure 1:

Consumption of (a) energy and (b) water per place setting of a dishwasher programme for normally soiled dishes (tested by Stiftung Warentest) in the years 1975–2004 [10].

Figure 2:

Comparison of operating costs for dishwashing by hand and by machine (each based on 12 place settings) [10].

Vivian et al. [11] confirmed these findings with an analysis of data from six European countries (United Kingdom, Germany, Italy, Spain, Portugal and Poland). They stated that the shift from manual to mechanical dishwashing was beneficial for energy consumption, CO₂ emissions and cost savings. On average, relative differences of between 50% and 60% could be achieved in regard to all three parameters (cf. Figure 3) [11].

Figure 3:

Relative difference of electric dishwashing against manual dishwashing (manual dishwashing = 100%) [11].

In 2003 Stamminger ran a study with 113 participants from different European countries to describe regional differences in manual dishwashing [12]. Participants had to wash 12 place settings soiled according to EN 50242:2008 [13]. The observed washing behaviour showed huge differences, ranging from cleaning in sinks filled with very little water to using continuously running hot water. Overall, manual dishwashing did not achieve values as low as those of electric dishwashers in terms of water consumption [12].

This study was followed up by the laboratory study of Berkholz et al. [14] in 29 countries across the world which aimed to investigate individual attitudes towards manual dishwashing as well as the resource consumption and cleaning efficiency of the different dishwashing techniques. Two hundred and eighty nine participants were filmed while washing up 12 place settings and in addition, they were asked to answer a questionnaire. Dishes were soiled based on EN 50242:2008 [13] and the water, energy, detergent and time consumption were measured. In addition, the cleaning results were assessed after the dishwashing was finished. The average water consumption of the 29 countries ranged from 34.7 L up to 160.1 L and it took the participants 60 min on average to wash 12 place settings manually. Participants cleaning under running tap water had a high water consumption while the lowest water consumption was reached when participants used a filled sink to clean the dishes. The questionnaire confirmed the differences in attitudes and habits towards dishwashing between the countries and showed an overall lack of knowledge about the benefits of electric dishwashing. According to Berkholz et al. [15], this could also be due to the fact that the topic of dishwashing was not highly relevant to the participants. The goal of getting clean dishes was more important than the environmental impact [15].

Shi et al. [16] also investigated manual dishwashing behaviour. The authors categorised manual dishwashing techniques into three dishwashing methods based on detergent use and pointed out country-specifics. In the United Kingdom (UK), the United States of America (USA) and Germany it was found to be most common to clean in a full sink with detergent in the water. The application of detergent onto the dish was common in Japan and the application onto the cleaning utensil or the use of a pre-solution was common in Mexico [16].

A study by Bichler et al. [17] questioned 4000 households in Germany and thereby identified highly different ways of using electric dishwashers. It was shown that the use of the ECO programme, which saves the largest amount of water and energy, was often combined with a manual pre-rinsing step which reduced the overall resource savings. Furthermore, Richter [18] has shown that consumers tend to always select the same dishwasher programme and that programmes with higher temperatures are often preferred. A shift towards the ECO programme could help consumers save a lot of energy and water [18, 19]. Nevertheless, Richter [19] has shown in a four country consumer observation study that despite the suboptimal use of electric dishwashers, households owning an electric dishwasher still used only half the amount of water per dish item cleaned, compared to households without one. Among the different applications for water in household work, dishwashing was found to be responsible for over half of the tap water used in domestic kitchens [19]. Individual dishwashing behaviour is an undeniable factor in water consumption during household work. It is influenced by the demand of hygiene, time available and interest in sustainability.

However, because dishwashers are not affordable in most parts of the world or consumers do not see their benefits, the majority of dishes worldwide are washed by hand. Electric dishwasher penetration is less than 5% of households in Southeast Asia and Latin America, and around 10% in Africa, the Middle East and Eastern Europe. In Australia, New Zealand and Japan, where other household appliances such as refrigerators, washing machines and microwave ovens reach penetration levels of over 80%, electric dishwashers lag far behind at around 40% [20]. Therefore, manual dishwashing contributes significantly to global water consumption. Even in households where a dishwasher is available, manual dishwashing is always important because individual items, such as wooden items, thin glasses or large pots or pans, cannot or should not be loaded into the dishwasher [21].

Stamminger et al. conducted a non-representative study in 2007 with 113 study participants from seven countries/regions to investigate wash-up behaviour [22]. The approach was repeated in 2010 by Berkholz et al. [14] with a representative sample of 150 UK residents. The participants were asked to wash a full load of 140 individual items in a laboratory environment. The dishes were soiled according to EN 50242:2003 [23] and air-dried for 2 h beforehand. A dishwasher was run in a programme recommended for normally soiled dishes in parallel to the experiment. In 2013, a study with a similar design was conducted to test the results in a global context [15]. Taking into consideration the results of all three studies, it can be shown that there is a wide range of washing up behaviour within the consumer groups. The participants required different amounts of the resources time, energy and water, but on average performed poorer than the dishwasher in all points (cf. Table 1) [14, 15, 22].

The comparison of dishwashing behaviour of participants from 29 countries showed that the individual water consumption ranged from 18.3 L to 472.8 L for the same number of soiled dishes [15]. This demonstrated an enormous potential for saving water. In Spain, for example, inhabitants typically cleaned their dishes under running tap water [15]. If an average Spanish household were to save 225 L of water in one dishwashing session five times a week, it would add up to 58,500 L of water savings per year in one household. This again would lead to water savings of more than one trillion litres for the whole of Spain.

To further test consumer behaviour, the participants in the study from Berkholz et al. [14] that stated to own a dishwasher themselves at home were asked to load a full-size dishwasher in the laboratory up to a point where they felt its capacity was fully exploited. Therefore, the same 140 loading items were provided. The authors concluded that the participants utilised the capacity to its full potential (33% managed to put all items in the dishwasher; 57% omitted one to 10 items; 10% left 11–20 items) [14]. However, this result could also have been influenced by the conditions of the laboratory experiment. Indeed, Richter [18] and Brückner and Stamminger [24] found that in the domestic environment, the capacity of a dishwasher is usually not utilised to its fullest.

According to the accompanying survey of Berkholz et al. [25] the consumers who own a dishwasher in the UK also cleaned at least some dishes manually once per day. The survey also showed that the aspects “cleanliness” and “hygiene” as well as “time” were the most important factors UK consumers considered for the process of dishwashing. Ecological aspects like energy and water saving were considered to be less important [14].

Abeliotis et al. [25] investigated the attitudes and behaviour of consumers regarding dishwashing in Patras, Greece. In this study also, despite the presence of a dishwasher, manual dishwashing was still frequently used. About 49.6% of those who owned a dishwasher washed their dishes manually at least once a day. Among those who did not own a dishwasher, the proportion rose to 86.9%. Most of the respondents (53%) washed their dishes under running, hot tap-water. The widespread use of solar-powered water heaters in Greece was seen as a positive impact on the environmental consequences of dishwashing. However, the common practice of rinsing dirty dishes before putting them in the dishwasher still led to unnecessary consumption of water and energy. According to the authors, Greek household culture lacked consumer education and training on resource-saving tips [25].

Maitra et al. [26] conducted a global web-based consumer survey analysing individual consumers’ attitudes towards the use of water in manual dishwashing in 2017. The aim was to estimate the probability that a change in daily manual dishwashing would lead to more economical water consumption in the household. This study investigated the manual dishwashing technique via online questionnaire survey in 5249 households in nine different countries – China (CN), Germany (DE), Italy (IT), Russia (RU), Indonesia (ID), Brazil (BR), India (IN), South Africa (ZA) and Argentina (AR). In addition to socio-demographic information, technical questions such as cleaning steps, hygiene questions, motivation questions, environmental awareness questions, questions about paying for water and electricity, and questions about sources of information about household chores were included in this study [26]. The results were also analysed by Belke et al. [9], who used the survey by Maitra et al. [26] to identify which behaviours mainly lead to water and energy waste.

A total of 3565 participants indicated ownership of a dishwasher and were asked, “How often do you normally use your automatic dishwasher?” The results show that some people do not use their dishwasher at all. From 14% to 15% of the participants in India, Indonesia and Argentina answered: “I never use the dishwasher.” It is common in Italy, Russia and Argentina to run the appliance once a day. The frequency is lower in the countries with smaller households, like Germany and China, with around one to four runs a week. The authors calculated the average number of dishwasher cycles per week per person to eliminate the influence of the household size. Accordingly, Germany was found to be the only country with more than two cycles per person per week, and participants in Indonesia and India run less than one cycle per week [26].

Possible reasons for the higher number of dishwashing runs were discussed to be that dishwashers were loaded differently, that more dishes were soiled because more meals were eaten at home, or that not only dishes but also cookware was put in the dishwasher. The last point was analysed by the questionnaire: “Which dishes do you prefer to clean by hand?” as a multiple-choice question. The items most frequently cleaned by hand in all countries were found to be pots and pans (55%), long glasses (51%) and wooden items (51%), probably due to either their size or the sensitivity of the items. However, the authors also found that even when a dishwasher was used, there were manual steps before and after the washing process. For example, pre-washing steps or additional drying with a towel after washing. How often these steps were performed was surveyed via multiple-choice question: “Which steps are included in your usual dishwashing process? (Includes before and after running the dishwasher)”, with six possible answers. Disposing of residues was found to be common across all countries. On average, 75% indicated to do so, with the lowest percentage in Germany (52%) and the highest in Indonesia (91%). However, these differences were also discussed to be caused by a non-comparability of conditions. The necessity of scraping depends strongly on the country-specific meals and also on etiquette. In Indonesia, for example, many fish and meat dishes are served with inedible parts such as bones. In comparison, in Germany there are many dishes that are free of residues, such as bread and salad [9]. In China, an empty plate is considered a sign that the host has not served enough food, whereas in Germany it is considered impolite not to finish a dish [27]. So, the authors concluded that not scraping the plate could also be due to the fact that it is not necessary to scrape the plate [9].

Belke et al. [9] pointed out that pre-rinsing before dishwashing is an unnecessary step. Regardless of whether the plates are to be washed manually or by machine, the soiling could be removed in the cleaning step. Thus, pre-rinsing was a waste of water [9]. In addition, as soon as hot water is used this habit is also correlated with a higher energy consumption.

Emmel et al. [28] also discussed the habit of pre-rinsing before putting dishes into an electric dishwasher with regard to consumers in the USA. They found out that a pre-rinsing cycle of a dishwasher uses around 4 L of water while manual pre-rinsing before an electric dishwasher run usually consumes about 100 L. The authors trace this behaviour back to the energy crisis of the 1970s and 1980s, where washing up had to be done with colder water, which is why cleaning results that came out of an electric dishwasher were often unsatisfactory. However, since the beginning of the 2000s at the latest, this is no longer a problem, according to Emmel et al. [28]. They recommend to reassure consumers about good dishwashing results without manual pre-rinsing and to encourage them to adopt water- and energy-saving behaviour instead [28].

A further multiple-choice question was included by Maitra et al. [26] to investigate whether the participants adjusted their dishwashing behaviour to different types and amounts of soil. The goal was to examine whether the consumers changed the amount of detergent or water used or if they increased the water temperature. The results show that almost half of the participants said that they dose at their own discretion. If an adjustment was made to the detergent dosage, water temperature or quantity, it was most often due to very greasy dishes – on average 38% used more detergent in this case. Adjustments to water temperature, on the other hand, were only made by an average of 34%, with Argentina leading the way at 54%. The authors attributed lower percentages, such as 30% in Germany, to the fact that people generally wash dishes with warm water. This means that the temperature does not have to be raised as much as in other countries. Another reason for a low percentage for increasing the water temperature was seen for countries that do not have a hot water supply at the sink. However, especially in these countries, increasing water temperature could help save detergent, according to Maitra et al. [26].

Belke et al. [9] investigated the barriers to purchase an electric dishwasher on the basis of the multiple-choice question: “Why do you not have a dishwasher in your household?”. Figure 4 displays the responses of consumers who did not own a dishwasher, separated by countries with low, medium and high penetration of dishwashers.

Figure 4:

Reasons for not owning a dishwasher reported in countries with low, medium and high dishwasher penetration (created according to ref. [9]).

In this survey, an average of 85% of respondents who did not own a dishwasher stated that they often clean items individually – either whenever there was a single item in the sink or before, during and after cooking. The only country that was not close to this average was Germany, where 60% of participants reported that they collect dishes for a larger dishwashing cycle, which, according to the authors, is considered to be much more water-efficient [9].

Stamminger et al. [21] investigated the question why consumers in Europe were not using electric dishwashers to clean their dishes in a semi-representative online survey. The aim of the survey was to assess the behaviour of more than 5000 households in 11 European countries (Czech Republic, Finland, France, Germany, Hungary, Italy, Poland, Romania, Spain, Sweden and the United Kingdom). The participants were asked about their electric dishwasher and their dishwashing behaviour, their opinion regarding energy-saving issues and their awareness of information reported on the energy label [21].

This research by Stamminger et al. [21] shows that in the EU sample not all items were indicated to be loaded into the dishwasher when one was available. About 36.7 items were cleaned per day according to the calculated EU average. Of these, 5.5 were not cleaned in an electric dishwasher but by hand. This corresponds to 15%. However, the results show that the fraction of items cleaned manually was different depending on the type of kitchenware. While the majority of plates and cutlery were cleaned in an electric dishwasher, 41% of pots and pans were cleaned manually [21].

Various reasons for the manual cleaning of certain items were given. Overall, the three most frequently chosen options were the “need to have some items immediately” (56%), that “some items take up too much space in the dishwasher or are too bulky” (52%) and that some items were considered “not suitable for the dishwasher” (51%). There were some differences in the frequency of responses between the countries observed, but all arguments were mentioned everywhere. For example, overall, only 6% of respondents said that they “wash everything in the dishwasher”. However, 9% of respondents in one-person households gave this reason, while only 42% said that the “items take up too much space or are too bulky”. The argument that there are not enough dishes for the dishwasher, on the other hand, surprisingly did not differ between single and multi-person households [21].

The authors also investigated why participants did not own an electric dishwasher by asking a multiple-choice question with nine predefined arguments. Overall, panellists on average selected two arguments. The argument most frequently mentioned (51% as the European average) was that “there is not enough space in the kitchen” to place an electric dishwasher, followed by the argument that the panellist was happy without owning a dishwasher (45%). Not washing enough dishes to justify buying a dishwasher was the reason for 29% of the panellists and 27% argue that the initial costs were too high [21].

The comparison of the number of tableware items used and cleaned by households owning and not owning an electric dishwasher showed that households not owning a dishwasher used a significant lower average number of tableware items per day. This was found to be independent of the household size [21].

An additional difference found by Stamminger et al. [21] was that panellists who did not own an electric dishwasher were more likely to wash up in a sink filled with water (46%) compared to those households who did own an electric dishwasher (38%). This was particularly noticeable in households in the United Kingdom, Finland, the Czech Republic and Italy. Here, the practice of washing dishes under running tap water was more pronounced when an electric dishwasher was available [21].

These data are consistent with the results of Stamminger and Streichardt [29]. The authors speculate that this may reflect different cooking and eating habits: households in which people do not cook frequently may tend not to have an electric dishwasher. First results showed that there were no differences between the group of households with or without owning a dishwasher and the sum of meals per week with an average of 22 meals a week each [21].

These findings are also in line with the research of Richter [18, 19] who has also noticed that the washing-up behaviour is different for owners and non-owners of dishwashers.

A closer look at the manual dishwashing practices of those households owning an electric dishwasher in the study of Stamminger et al. [21] also showed a significant relation between practice and age of the owners. A higher likelihood of washing up under running tap-water was observed for younger panellists while older panellists used a sink or bowl. Also, there was a small significant relation between the country of the participant and the manual washing-up behaviour. While more than 50% of British and German panellists stated that they used a sink or bowl filled with water for washing up, only about 10% of Romanian panellists indicated that [21].

The differences between countries found by Stamminger et al. [21] are consistent with the research of Stamminger et al. [22, 30] and Berkholz et al. [14, 15], who observed consumers from various European and other countries in laboratory studies. Maitra et al. [26] also confirmed these differences in dishwashing behaviour and attributed them to a mixture of reasons, one being the awareness to conserve domestic water usage.

In addition, the Stamminger et al. [21] questionnaire included the question “Why don’t you have a dishwasher in your household?”. According to the authors, it is particularly interesting that on average 16% of participants answered that the energy and water consumption of an electric dishwasher was too high. This indicates that they did not know that electric dishwashers are more water and energy efficient compared to manual dishwashing. The country differences in respondents’ answers to this question are large. While 21% of the Spanish panellists and 20% of the German and British panellists agreed with the incorrect statement that the energy and water costs for machine dishwashing are too high, only 7% of the Finnish panellists agreed with this statement. Overall, this response behaviour indicates in the best scenario that in all countries the majority of respondents actually understood the connection between resource savings and electric dishwashing. This would also be consistent with the research of Abeliotis et al. [25] and Emmel et al. [28]. The alternative explanation would be that this cost item simply did not matter to the respondents and that was the reason for them to not select the answer. Nevertheless, according to Stamminger et al. [21], it should not be ignored that in some countries up to one fifth of the respondents seem to assume a false connection.

Porras et al. [31] agree with the recommendations of other studies in their Life Cycle Analysis (LCA) of dishwashing in private households in the USA. They state that consumers in the USA could significantly reduce their emissions when switching from typical to recommended or BPT. In case of consumers who wash their dishes manually, the authors estimate a saving of 71% of emissions. Consumers who use an electric dishwasher can save an estimated 11% [31].

2.2 Dishwashing in context of hygiene

One of the earliest studies in terms of hygiene during the dishwashing process was an investigation by Kleinfeld and Buchbinder [4]. They examined the microbiological contamination associated with the dishwashing method used in about 1000 restaurants in New York City. They collected 3657 samples using swab-rinse test to record and evaluate the hygienic efficiency of the dishwashing process within the restaurants. In accordance with the regulations of the U.S. Health Department the detection of <100 colony-forming units (CFU) was set as the threshold for hygiene acceptability. The results show that manual dishwashing was the much more common method, at 88%. Only 17.5% of restaurants used a machine for some or all dishwashing tasks. Hygiene was found to be very poor. In the case of glasses and cups washed by hand, only 10.5% of the samples met the set limit. For machine-washed glasses and cups, the percentage was significantly higher, at 35.8%. The authors conclude that machine cleaning is clearly superior to manual cleaning in terms of hygiene. Nevertheless, there was still a clear need for innovation even for electric dishwashers [4].

In later years, the aforementioned study by Knowles Weaver et al. [5] confirmed the better performance of electric dishwashers in regard of hygiene in comparison to manual dishwashing. During the intervention study, the 10 participating households were visited and swab-rinse tests were conducted at four different time points to check for bacterial contamination. As in Kleinfeld and Buchbinder [4], a count of 100 CFU was set as an acceptable limit. The evaluation was carried out by the statistical laboratory of the Ohio State University. It was shown that again in this study, the dishes washed by hand had significantly more bacterial contamination than those washed by machine. The bacterial counts on the four groups, i.e. plates, cups, glasses and forks ranged from 1 to 6000 CFU for the items washed by hand. The items washed in an electric dishwasher almost all fell below the limit of 100 CFU [5].

A comparative study by Blackmore et al. [32] compared the bacteria contamination of dishes that were washed manually and dried with a tea towel with the one from dishes cleaned by a dishwasher. Therefore, they took samples of the bacteria load and counted the individual colonies after incubation. The observation of three subsequent manual dishwashing sessions revealed that the tea towel used for drying quickly got contaminated throughout the process. In the first session the bacteria load could be reduced in comparison to before the wash. However, in the later sessions, the number of microorganisms counted was about seven times higher. The cleaning of dishes with an electrical dishwasher run in a 65 °C programme was shown to be very effective in removing bacteria. This was proven both with naturally occurring bacteria as well as with bacteria artificially placed onto the investigated dishes. The electric dishwasher decreased the bacteria contamination to a level below what the researchers found on clean plates that were stored in a cupboard [32].

According to Stamminger [10], however, resource consumption does not necessarily correlate with the cleaning result. In this study, the cleaning evaluation was performed according to the six-point scale of the EN 50242:2003 (with zero being the worst and five being the best possible result) [23]. Good cleaning results were reached with small as well as large amounts of water, energy or detergent. Overall, the participants achieved scores in the range between 2.5 and 4.5, whereby, according to the German Stiftung Warentest, values above 4.0 can be classified as “very good” and below 3.5 as “no longer acceptable” [10].

This was confirmed by Stamminger et al. [22] and Berkholz et al. [14, 15] who showed that with a water consumption ranging from 34.7 L up to 160.1 L in the 29 investigated countries no big differences in the cleaning results could be observed (cf. Table 2) [14, 15, 22].

Stamminger et al. [21] showed that the reputation of electric dishwashing in terms of hygiene is very high. An average of only 10% of their respondents not owning an electric dishwasher thought that it would not fulfil their expectations in terms of cleaning performance. On the other hand, satisfaction with the cleaning results of manual dishwashing observed by Stamminger et al. [21] was also quite high. On a five-point “smiley scale” 47% of all participants rated it with the highest grade and 41% with the second best. Nevertheless, a portion of 12% of the consumers reported that they were not happy with the results of their manual dishwashing. The more detailed analysis showed that there was a relation between the satisfaction with the result of manual washing dishes and owning a dishwasher. Households with a dishwasher were more satisfied than households without one. A significant relation between the age of the panellist and the satisfaction with the cleaning results of manual dishwashing was found for owners of a dishwasher and participants without an automatic dishwasher [21].

However, the higher cleaning efficiency of electric dishwashers regarding the microbial load could constitute a disadvantage. Hesselmar et al. [33] suspect a connection between the dishwashing method in the household and the occurrence of allergies in children living in the household. In an observational, cross-sectional, questionnaire-based study in Sweden with 1029 returned questionnaires (56% response rate), the authors found indications that the use of automatic dishwashing could be associated with an increased incidence of allergies. The medical theory that microbial contact in early childhood can reduce or prevent the risk of allergy development supports the thesis of Hesselmar et al. [33]. However, the authors themselves restrict that their conclusions are still based on too little data [33]. Furthermore, the study design chosen by the authors cannot be considered suitable for establishing a causal relationship. Only a correlation of circumstances can be determined. It is also susceptible to falsification due to false statements by the study participants in the questionnaire (e.g. due to recall bias). Since the process of allergy development is very complex and depends on many factors, it is very difficult to isolate individual influencing factors. Nevertheless, Hesselmar et al. [33] consider this connection to be plausible.

3.1 Manual dishwashing

Fuß and Stamminger [34] did a broad review on dishwashing recommendations available on European websites (different blogs, forums and websites of household organisations such as the German “Forum Waschen”). Most of the dishwashing recommendations found were household tips, which were structured by the author according to different aspects or individual steps of dishwashing (collecting, soaking, washing, rinsing, drying, detergent dosage etc.) and in the next step linked to the necessary scientific background. Based on this review a multi-step research was set up by Fuß and Stamminger [34] to identify the optimal manual dishwashing technique. This was supposed to enable resource-saving manual dishwashing without compromising the cleaning result [34].

The first step was an experimental laboratory study, aiming to assess technical influences on the cleaning result. Tested variables were different detergent concentrations and different frequencies for water changes in the sink for soaking and cleaning as well as for rinsing [34]. Thirty plates were soiled with five different soils in reference to EN 50242:2008 [13] and cleaned afterwards by a manual dishwashing apparatus. This apparatus consisted of a heatable sink filled with water and a device that moved a sponge over the plate to be cleaned in a reproducible way. The cleaning took place in an inclined position of the dish pieces (hold by a metal plate in the sink) to mimic the consumer habit of submerging dishes partly into the water. An additional sink was used to rinse the plates after the washing process. With this construction it was possible to clean dishes with specified rotation and force of the cleaning utensil and with a consistent water temperature [34]. The cleaning results were assessed according to EN 50242:2008 [13, 34]. One important finding was that the amount of detergent did not correlate linearly with the cleaning result. Up to a certain amount the cleaning result was improved, but too much detergent caused a decrease of cleaning performance. The authors explained that more detergent led to more foam, which stuck to the dishes. This remaining foam transferred soil particles to the rinsing water which led to a less effective rinsing finish [34].

According to these findings, a basic method was constructed, where dishwashing was carried out in three consecutive steps in two sinks. The first sink was for soaking and the main washing of dishes whereas the second sink was for rinsing. The authors summarised their recommended two-sink method in six steps, the so-called “BPT” for manual dishwashing [34]. In 2011, the applicability of the BPT for both small and large amounts of dishes was tested in a laboratory set-up with 53 participants from seven different European countries [35].

The following BPT formulated in six major tips, were tested [35, p. 196]:

1. “Wash up in sinks filled with water. Avoid washing up under running tap water.

2. Wash up in three steps: Soaking and main washing in one bath, rinsing in a second bath. If you do not have a second sink, you can replace it by a basin.

3. Fill two sinks half with water: The sink for soaking and main washing with hot water, the sink for rinsing with cold water.

   • Step 1: Soaking (Hot water)

   • Step 2: Main washing (Hot water)

   • Step 3: Rinsing (Cold water)

4. Detergent: Add the detergent AFTER water let in. Dilute detergent, but avoid foam production because the foam restrains soil residuals and a fat film.

5. Washing up process.

   • Step 1: Dispose of food leftovers into the bin.

   • Step 2: Soaking: In the sink with hot water; the harder the soil, the longer the soaking time.

   • Step 3: Main wash: While some of the hard soiled items are soaking, start cleaning lighter soiled ones.

   • Step 4: Rinsing: Short dunking in second sink with cold water.

   • Step 5: Drying: Place the dishes in a dish rack to let them air dry. You do not need to towel dry.

6. Change the water if you feel it is too dirty. For example, if the foam emerging while washing up is collapsing again.”

In this study the participants first had to wash dishes as they were used to. The work station for dishwashing consisted of a double-bowl sink with a two-handled tap. The detergent concentration and amount were not specified and different cleaning utensils were available to choose from. After receiving a BPT training, participants had to wash 12 place settings again, applying the newly learnt method [35].

By using the BPT, significant resource savings were achieved in this study. On average, 60% less water, 70% less energy and 30% less detergent were used by the participants and a slightly better cleaning result was achieved. However, the authors noticed that the savings were much greater when a large number of dishes were cleaned in one session. The conclusion was to extend the BPT with the recommendation to collect the dishes instead of washing them in single pieces in order to fill the sink efficiently with water [35].

Another issue that stood out was that participants stated that they liked the BPT but expressed concerns about its application in their daily lives [35]. Therefore, in a third step of the validation, the applicability of the BPT was tested in real private households. They were tested in 20 kitchens of households in Germany and Spain respectively, in order to check the potential of saving resources by applying the BPT in daily life. The participants were asked about their current dishwashing behaviour and then washed their dishes as usual for a fortnight. After a training session on the BPT, another two-week period followed. Throughout the entire four weeks, a dishwashing diary had to be filled out, the sink was photographed with a webcam, and water consumption, water temperature and detergent consumption were measured. In this way, the dishwashing behaviour of the participants was evaluated. The average resource savings (cf. see Table 3) confirmed the results of the previous laboratory studies. The majority of households showed lower resource consumption during the period they applied BPT. However, the authors point out that at the individual level, savings varied widely. While some participants were able to save enormous amounts of water, others now consumed more resources. The latter was especially the case for participants with very few dirty dishes [36].

The savings due to the application of BPT were significantly higher for the Spanish participants than for the Germans. The authors attribute this to the fact that the Spanish participants had higher consumption values from the outset and thus a higher savings potential [36].

The dishwashing diaries provided additional information on how the BPT were received by the participants. In general, the participants of the study rated the BPT as easy to understand and easy to apply. However, many found the tips not very up-to-date but rather old-fashioned. Overall, nevertheless, more participants said that they found the BPT beneficial than not. When looking at the individual tips, the acceptance for the recommendation “air-drying dishes” was highest in both countries. The German participants gave the best rating for the tip to produce little foam and to wash in the sink. The Spanish participants showed different preferences and gave the best rating for the tip to soak plates and add the dishwashing detergent only after the water has run in [36].

The German “Forum Waschen” summarised these findings in 2019 as six golden rules for manual dishwashing (cf. Figure 5). Additional information to the studies by Fuß et al. [34], [35], [36] is to follow the dosage instructions from the detergent manufacturer and to regularly clean auxiliary washing-up materials such as towels and sponges [37].

Figure 5:

Six golden rules for manual dishwashing (translated according to [37]).

3.2 Electric dishwashing

Richter [18] reconfirmed in his non-representative study that electric dishwashing is significantly more resource-efficient than manual dishwashing. However, he also found that the extent of savings is strongly dependent on consumer behaviour. In a non-representative study with 200 households from four European countries (Germany, Italy, Sweden, United Kingdom), the author investigated the consumer behaviour of dishwasher owners and non-owners as well as the resulting savings. Participants were asked to keep a detailed daily diary with photos of their dishwashing activities. Furthermore, a flow metre at the tap and a camera with a view of the sink and dishwasher were installed. In this way, the dishwashing behaviour of the participants could be documented for the test period of two weeks [18].

The scientific evaluation showed that the recorded habits of the participants were not optimal in terms of resource consumption. For example, although many consumers reported using the full capacity of their dishwasher, the analysis of the footage showed a different result. On average, the full capacity was only used in 33% and 34% of the dishwashing cycles (upper and lower rack respectively). While the German and Swedish samples used it in almost 50% of the cycles, the British sample used the full capacity only up to 10% of the time. The author concludes from these findings that a saving of about one in 10 dishwashing cycles would be possible if every cycle was operated at full capacity [18].

In addition, the study showed that participants tended to choose dishwasher programmes with higher temperatures. In some of the investigated countries more than half of the observed dishwasher runs were set at 65 °C and above (cf. Table 4). Since more energy is consumed at an average programme temperature of 59 °C than is indicated on the energy label of the tested dishwasher with the ECO programme, the author considers this extra consumption to be wasteful [18].

Another point considered in this study was the pre-treatment of the dishes, which influences the water consumption. While in Germany only 10% of the load items were pre-treated, in Italy almost 50% of the items were pre-washed with water and/or detergent. The author’s recommendation, on the other hand, is to only wipe the dishes dry. In his eyes, pre-rinsing is only permissible if wiping is not sufficient for a satisfactory cleaning result In this case, however, an appropriate rinse programme should be used, as this is more resource-efficient than washing by hand [18].

Another important aspect regarding electric dishwashing is the use of detergent, salt and rinse aid. Stamminger and Streichardt [29] found in their analysis of 2599 questionnaires that the majority of consumers (76.2%) use detergent tablet formulations for electric dishwashing. According to the authors, a preference for multifunctional tablets was found. However, 60.7% of multifunctional tablet users also reported using rinse aid in addition. Based on the questionnaire results, this could not be linked to a lack of satisfaction with the dishwashing result [29]. It can therefore be assumed that consumers do this out of ignorance and thus waste resources unnecessarily.

The German “Forum Waschen” summarised these findings in 2019 as six golden rules for electric dishwashing (cf. Figure 6). Supplementing the studies by Richter [18] and Stamminger and Streichardt [29], it is recommended to follow the dosage instructions of the detergent manufacturers, to load the dishwasher in such a way that the load does not block each other in front of the water jets and to use the ECO programme with regular selection of a high-temperature programme to ensure sufficient hygiene [37].

Figure 6:

Six golden rules for electric dishwashing (translated according to [37]).