biological treatment is the next step. the water moves to a special tank where tiny bugs that you can't see with your eyes live. the bugs eat the waste in the water (like human wastes). they are hungry bugs and there is a lot to eat. because they are eating so much,they reproduce and make more bugs to eat more of the waste (their food). whene they eat the waste, they digest it and instead of creating waste like humas do. they create oxygen. this puts more oxygen back in the water.