Does the Shul need to have two separate dishwashers for meat and milk or can one suffice if different racks are used?
There are two basic ways that a utensil or appliance may become non-kosher: through coming into direct contact with food (“be’ein” – “בעין”) or through transfer of taste (“ta’am” – “טעם”) without any actual food being present. Both of these concerns are present in a dishwasher – let us explain why.
The reason we have separate utensils generally for meat and milk is due to the second issue – that of taste transfer. When one uses a clean pot in which meat was cooked, there is a concept of halachic taste which is transferred into the walls thereby making this pot “meaty”. If that pot is then used for milk then the halachic taste is infused into the food meaning that there is now a mixture of milk and meat together which is not kosher. Although, as a general rule, this taste is only transferred through heat, since a dishwasher uses hot water, this issue is present in our case.
Nevertheless, if one puts a plate that has been rinsed into a dishwasher, one could argue that although the “taste” from the plate goes into the walls and will then be transferred back to the dishes on the next cycle, this secondary infusion of “taste” (called “nat bar nat” in halachic terminology) is considered weaker and would not create a halachic problem. This is the argument of the great Rabbi Moshe Feinstein (see, for instance, Igrot Moshe YD 2:28), who first wrote about dishwashers in 1956.
However, many notable poskim have disagreed with Rav Moshe (Rav Feivel Cohen, Rav Binyamin Forst, Rav Y.D. Soloveitchik, among others), noting that dishes are often put in a dishwasher with residue still on them. For instance, when dishes with meat residue go in the dishwasher, the meat “taste” would be transferred into the walls directly (not just from the “taste” on the plate), thereby making the dishwasher “meaty”. If milk dishes were put in for the next cycle, this would now infuse meat “taste” into the milk dishes.
Furthermore, the filter in a dishwasher keeps larger food items from being drained away meaning that, unless one cleans the filter between cycles, there can be actual meat (“be’ein”, as above) pushed onto the milk dishes with hot water jets, not just “taste”, creating a more serious halachic concern.
There are many more nuances in the discussion surrounding dishwashers (including the even more lenient opinion of Rav Ovadia Yosef), getting into halachic topics of bitul (annulment), noten ta’am lifgam (negative “taste”), kli rishon/sheni (primary/secondary utensils) and more which could keep a person occupied with studying for many years.
When there is a legitimate difference of opinion in halacha, one opinion tends to naturally come to the fore to be more widely practiced – in this case, it is clear that standard Ashkenazi practice is to use separate dishwashers for meat and milk. In short, while individual homes, in consultation with their Rabbi, may follow the lenient opinion and have one dishwasher, an Ashkenazi shul should certainly uphold standard practice, in its role as a leading religious institution.