Some people just let it gravity drain to another container (inside or out) and then use it for outdoor watering. The flow rate & pressure is typically very low

Some people just let it gravity drain to another container (inside or out) and then use it for outdoor watering. The flow rate & pressure is typically very low for the drain water with these systems, so you'd need another setup to handle pumping it to your toilet, etc. Sending it outside is probably the easiest way to immediately use it.