A system for separating gaseous mixtures from a biogas stream, the system comprising: at least one absorption riser having a first end and a second end, each absorption riser including: a first inlet operable to receive the biogas stream; a second inlet operable to receive a water stream; an outer pipe having a first end, a second end, and a first length, wherein the first end is in fluid communication with the second inlet to receive the water stream; a first dispersion element within the outer pipe and located in a first fluid flow path exiting the second inlet to distribute the water stream across an interior section of the outer pipe; a first inner pipe located within the outer pipe, the first inner pipe having a first end, a second end, a second length, and a wall extending for the second length between the first end and the second end of the first inner pipe, wherein the first end of the first inner pipe is proximate the first end of the outer pipe and is in fluid communication with the first inlet to receive the biogas stream, and the first inner pipe extends from the first end of the outer pipe and into the outer pipe for the second length to dispense the biogas stream from the second end of the first inner pipe within the absorption riser; a second inner pipe within the outer pipe, the second inner pipe having a first end and a second end, wherein: the first end of the second inner pipe is proximate the first end of the outer pipe and the second inner pipe extends from the first end of the outer pipe and into the outer pipe for a third length, the third length is less than the first length of the outer pipe and greater than the second length of the first inner pipe, and the second end of the second inner pipe is operative to receive a mixed water stream; a first outlet operable to deliver a purified biogas stream from the absorption riser; and a second outlet at the first end of the second inner pipe and in fluid communication with the second end of the second inner pipe, wherein the second outlet is operable to deliver the mixed water stream from the absorption riser, and wherein the first outlet and the second outlet are both located at the first end of the absorption riser. a first inlet operable to receive the biogas stream; a second inlet operable to receive a water stream; an outer pipe having a first end, a second end, and a first length, wherein the first end is in fluid communication with the second inlet to receive the water stream; a first dispersion element within the outer pipe and located in a first fluid flow path exiting the second inlet to distribute the water stream across an interior section of the outer pipe; a first inner pipe located within the outer pipe, the first inner pipe having a first end, a second end, a second length, and a wall extending for the second length between the first end and the second end of the first inner pipe, wherein the first end of the first inner pipe is proximate the first end of the outer pipe and is in fluid communication with the first inlet to receive the biogas stream, and the first inner pipe extends from the first end of the outer pipe and into the outer pipe for the second length to dispense the biogas stream from the second end of the first inner pipe within the absorption riser; a second inner pipe within the outer pipe, the second inner pipe having a first end and a second end, wherein: the first end of the second inner pipe is proximate the first end of the outer pipe and the second inner pipe extends from the first end of the outer pipe and into the outer pipe for a third length, the third length is less than the first length of the outer pipe and greater than the second length of the first inner pipe, and the second end of the second inner pipe is operative to receive a mixed water stream; the first end of the second inner pipe is proximate the first end of the outer pipe and the second inner pipe extends from the first end of the outer pipe and into the outer pipe for a third length, the third length is less than the first length of the outer pipe and greater than the second length of the first inner pipe, and the second end of the second inner pipe is operative to receive a mixed water stream; a first outlet operable to deliver a purified biogas stream from the absorption riser; and a second outlet at the first end of the second inner pipe and in fluid communication with the second end of the second inner pipe, wherein the second outlet is operable to deliver the mixed water stream from the absorption riser, and wherein the first outlet and the second outlet are both located at the first end of the absorption riser.