You cannot drain a sink into the ground. This will certainly be against the local regulations. The general rule for drainage is that rainwater can be and should be drained to the storm water drainage system of the city or municipality. All other waste, including those from utility sinks, should be drained only to the sewage water drainage system. You can drain rainwater to daylight. However, the output of a utility sink is certainly not clean and cannot be considered the equivalent of rainwater.