We only have gray water since we have a composting toilet, but we still have a traditional RV sewer line that we run into the connection at our Vintage trailer/tiny home park.
I’ll be paying attention to other tiny-housers’ answers on this one because we STRUGGLE with this! We also only have grey water because we have an incinerating toilet. At our first location, we basically dug a big hole using an auger well below the frost line (we live in upstate NY). We added some gravel, ran a PVC pipe into it, and filled it back up with dirt. It worked fine. At our current location, we dug a much bigger hole (again, below the frost line), put a huge steel drum in it upside down, made a hole in the top, added some gravel, and ran piping to it. We are having problems with it though, as the ground where we are parked is just really saturated and it doesn’t drain well. We need another solution!
As of right now, we are tankless. We hook directly to the park’s sewage system like a traditional RV would via the "Stinky Slinky". This will change when we move to @shellmateisland, but I must say it’s been pretty nice. I know this issue can stump a lot of potential tiny homies depending on what's available, because hard piping is not always an option. We’ll soon have switch to a composting toilet, and will be getting a gray water tank in the near future.
We have a composting toilet, which takes care of whatever blackwater we create. We clean out the bucket probably every four months on average? After we switch it out, the waste sits and decomposes for a minimum of six months before we then bury it somewhere in our yard. Grey water is a lot easier. We have a pipe that drains from our sink(s) and shower to a drainage ditch in our yard where it naturally disperses. We are very careful to only use biodegradable soaps, and we never put anything (cleaners, chemicals, etc.) down the drain that isn’t safe to ingest. So far this has worked for us! If we weren’t in such a rural area, we may have had to make some adjustments to this way of dealing with our waste water, but so far so good!
I wanted to keep my waste water situation as simple as possible for my tiny house. It's already difficult enough to find a parking spot, not to mention one with a water and electric hookup. Adding a septic hookup to that list was something I really wanted to avoid if I could. It would also challenge me to really consider my environmental impact in a way I hadn't before.
First, I went with a Nature's Head composting toilet instead of a traditional flush toilet connected to a septic tank, or a black water tank. The idea of dealing with a stinky septic hose and all of that waste water grossed me out so much more than the idea of a system that would just turn #2 into soil. I empty the liquids tank of my toilet once every week or two, picking a spot that is far enough away from any water source, preferably with some trees and without much ground cover plant life. The nitrogen in urine is actually good for plants, but the salt can burn grass or less hardy ground cover. My solids tank only needs to be emptied once every 4-6 months, and is dumped in a compost pile on the property, which will be used for non-edible plants.
To make my gray water as easy to deal with as possible, I converted all of my toothpaste, shampoo, and soaps to biodegradable options, so it can safely run off straight into the ground. Right now I just have it connected to a hose to direct it a little farther away from the base of my house, but soon it will be connected to a dry well that I am planning to dig. The dry well will keep bugs away, and allow the water to soak into the ground from beneath the sod.
While the Dream Team have given some great insight into their reasoning for how they deal with waste water, you will definitely have to decide for yourself how to deal with yours based on your lifestyle!
The most important factors contributing to this decision will be whether or not you plan to park in a spot with infrastructure already built-in (like an RV park), whether you plan to park in a spot long-term, or park in a spot short-term. Once you decide where to park or build your home, you will have a much better idea of how to proceed with building out the utility infrastructure, or altering the design of your home to accommodate a more off-grid disposal of waste.
While gray water disposal seems to be the "gray area," black water is less common as most tiny-housers use composting toilets to avoid dealing with this altogether!