It may sound a bit funny, but I'm stunted in the area of dish washing. The learning curve of having the dishes all washed, dried, and stacked neatly in our cabinets by the end of the night is one that has taken me quite a bit of time. More just always seem to appear, and then there is dinner and all its dishes, and just when I think all is completed I'll discover another glass in Jules' Man Room. It was enough to drive me completely insane. Along with this came much frustration from Jules that has a rather large issue with dishes and is convinced that every fly on earth must just know when there are dishes in our sink. Thankfully, with the expansion of our rain barrel water usage, the dishes have been converted into a very deliberate task during the day and have become much more manageable.
With the exclusion of about three months a year, we are one of the rainiest places in the continental United States. This leaves us with much water falling from the sky and quite a wonderful resource for harvesting to later return to the water table. With the rainy season returning (at least supposed to be returning) we are once again at a place to begin discussing the usage of our rain water. This last year, our toilet was flushed with rain water and with some treatment it can now be used for dish washing as well.
There is absolutely no treatment necessary for water if you are using it for irrigation, toilet flushing, or car washing. To use it inside as any kind of cleaning agent or use with human contact does require further considerations. The treating of rainwater really doesn't require much unless you are wanting to drink or cook with it. I don't think we will ever get to the point where our water is being used for direct consumption (however, I also never thought I would refill the back of my toilet after every flush as well). Two things need to be considered 1) the presence of bacteria or algae which could make you sick 2) heavy metals or pollution run-off from the rain collection sight. You can pay for an fancy (and expensive) water test, but ultimately don't need to if you know even a little bit about where you are collecting the water.
Living in an older house, both of these were concerns for us. The only portion of our roof which runs into our rain barrels is from our porch. The old fiberglass/asbestos sheeting which covered the porch has been swapped out for high impact plastic alleviating the worry of harmful chemical runoff from the sheeting. We have not replaced the gutter, however. This leaves some heavy metals as a possibility in our rainwater.
Secondly, the plastic we now use to cover our porch is clear and allows us to see the amount of accumulation of bird droppings, air pollution, or mold which might be growing. We get significant enough amounts of rain and the surface has a large enough slant, that it is fairly self cleaning.
The third consideration, is the area in which you live. We do not have a significant air pollution problem in our area. Sometimes in the summer there are air advisories, but mainly due to all the rain, it isn't an issue. If you are living places with acid rain, I would contact the local environmental studies/sciences department of your local university for advice.
Once you are sure of what might be contaminating your water, you can treat it accordingly. Water for dish washing needs to be treated much like potable (drinking) water because you are, after all, eating out of your dishes. But dishwater doesn't need to be taken to quite the measurable extreme as drinking water since poisoning yourself with the treatment chemicals isn't as much of a threat.
There are many different ways to disinfect water, but household bleach works for us. Make sure you are using plain bleach, no smells, or other foofy stuff. Straight bleach. Texas did quite a bit of research and came up with this list. Clorox is great for us. To sterilize water, you need 8 drops of bleach for each gallon. This means for a 55 gallon drum, you need to add 440 drops of bleach. Colorado State University recommends 1/2 teaspoon for every five gallons of water. Rather than count them all out, I compromised and just added 1/2 cup of bleach directly into the top of the barrel. This is more than 440 drops by quite a bit, but we are not drinking this water, just using it as a sterilized rinse to clean our dishes.
The bleachy water was left in the barrel for two days to thoroughly mix, combine, and have contact with any algae that might be stuck to the walls of the barrels. At this point, the water is all set to use for dish cleaning. When it comes out of the spigot, there is enough bleach that you can smell it (like a swimming pool) but not so much that it will take any color out of your clothes. One of our rain barrels is now filled with bleach water at all times and is used solely for dishwashing as the bleach would probably kill or severely damage any plants.
Warning: Do not drink water which has had bleach added, unless you are VERY careful about precision measurement. Any chemicals added to water without extreme precaution can make you very sick. We are NOT drinking this water, merely using it to clean with.
The dishwasher in our house is used as a holding tank for the dishes before I can get around to washing them by hand during the day. This way Jules doesn't ever have to see them and the flies which are lurking everywhere can't find them either. I do thirty minutes worth of dishes a day and then stop. This is normally enough to get the dishwasher completely empty, and results in me doing dishes usually only every other day because there aren't enough to justify using up the water.
Doing the dishes now is fairly quick and painless, with the added perk of Jules approving over the level of cleanliness now presiding in the kitchen.The stopper is put in the sink, criss-crossed with dish soap and then I go out and fill one of the old Tidy Cats liter containers (which has been cleaned thirty million times!) we use to transport most all of our household rainwater. By dumping the water into the sink, the water gets all soapy and the dishes are done one by one starting with the cleanest and ending with the dirtiest. All the dishes then get an incredibly quick rinse in tap water due to a small concern over possible heavy metals from our gutters. They are then dried really well and put away.
The left over water in the sink is usually kinda gross. This water is used to soak any dishes which I will be washing the next day or any dishes that have stuck on food which needs to soak. The junk is all rinsed or soaked off of them and then they go back in the dishwasher to be formally cleaned the next day. It may seem like extra work, but they aren't scrubbed much and by taking the food bits off of them in the leftover water that is already disgusting, the dishwasher tomorrow won't get disgusting right away.
Once all the dishes have either been put away or placed back in the dishwasher, I rinse out the dishwasher with my washcloth and the extra sink water. By pre-rinsing the dishes, the water isn't anywhere near as bad as it used to be when I washed dishes and can be used really well to wash out the inside of the dishwasher so it doesn't get stinky. A stinky dishwasher might alert the flies as to where the dishes are being kept. We can't have that!