Typically plumbing requires a vent for a few purposes, among them: ensuring that sewer gases go through the roof, not into the living space, reducing the risk of bad siphoning, and ensuring optimal drain flow. A kitchen island, however, is not an ideal space for venting. You don’t want a pipe going up through your island and through the roof. One solution is a device called an Air Admittance Valve (AAV). This allows air in for good flow, but does not let sewer gases out. The P-trap of the sink ensures that no sewer gases exit through the sink.