Definition
A dry well is a covered pit, usually filled with loose stone, designed to receive and disperse excess water, whether drainage from a roof or wastewater from a septic tank or other wastewater system. The dry well may be lined with perforated material. The pit is open on the sides and bottom to let the wastewater seep into the ground. Some dry wells are concrete containers with openings for the water to escape. Dry wells must be built on permeable soil and can't be built near industrial waste sites because of the possibility of groundwater contamination.
No Food
You can't put food down the sink's drain if you connect it to a dry well instead of to your home's drainage system. Food particles that go down a drain to a dry well sit in the dry well rather than going into a sewage system. The particles will rot, eventually causing the dry well and the sink itself to smell. If you know no food will go down the sink's drain, a dry well may work for you.
Laws and Codes
Laws in your jurisdiction are a major factor determining whether you can use a dry well for your outdoor sink. Building codes in many jurisdictions do not allow connection of outdoor sinks to dry wells. If this is the case where you live, you must connect your outdoor sink to your home's plumbing system. Contact your city's zoning department to find out about the laws and obtain any necessary permits.
Alternative
If you can't connect your outdoor sink to a dry well, you might connect it to your home's plumbing. You can also place a bucket under the sink to collect waste water. This solution can be messy, as you have to remember to empty the bucket. If you're looking for a green solution, however, this may be the best option, as you can recycle the waste water by using it in your garden.
Considerations
The solution that will work best for you depends on the location of your sink as well as the laws in your area. If your sink is not close to your home, connecting it to your home's plumbing isn't a potential solution. If you're able to connect the sink to your home's plumbing, you can use it just as you would use your indoor sinks. If the sink is remote, however, your choices are a bucket, a septic tank or a dry well. Soil conditions also impact whether you can use a dry well; if the soil doesn't percolate sufficiently to drain off waste water within 72 hours, the dry well will create hazards.