A device for optical inspection of a waste liquid including a water content affecting a quality of the waste liquid and for determining a value of the waste liquid, comprising: a transparent part; a first pipeline connected to the transparent part; a draining device configured to drain the waste liquid into the transparent part via the first pipeline; a first container; a second pipeline provided on the other side of the transparent part to the first pipeline; a second container; an illumination device for illuminating the transparent part; an image acquiring device configured to acquire light from the waste liquid passing through the transparent part and generate at least one image signal with respect to the transparent part; a signal processing module configured to receive the image signal and determining a quality of the waste liquid according to the image signal; a flow rate measuring unit disposed adjacent to the transparent part and configured to measure a flow rate of the waste liquid passing through the second pipeline and determine a volume of the waste liquid according to the flow rate of the waste liquid; and a calculating unit electrically coupled to the flow rate measuring unit and the signal processing module to calculate a value of the waste liquid according to the measured volume of the waste liquid and the determined quality of the waste liquid; whereby the flow rate measuring unit is outside an illuminating area of the illumination device, wherein the draining device further includes a draining nozzle inserted into the top of the first container, along a depth direction of the first container, into the waste liquid in the first container, and at a distance away from contacting the bottom surface of the first container, the first pipeline is between the draining nozzle and the transparent part and configured to guide the waste liquid from the draining nozzle to the transparent part, the second pipeline is between the transparent part and the second container and configured to guide the waste liquid from the transparent part to the second container. a transparent part; a first pipeline connected to the transparent part; a draining device configured to drain the waste liquid into the transparent part via the first pipeline; a first container; a second pipeline provided on the other side of the transparent part to the first pipeline; a second container; an illumination device for illuminating the transparent part; an image acquiring device configured to acquire light from the waste liquid passing through the transparent part and generate at least one image signal with respect to the transparent part; a signal processing module configured to receive the image signal and determining a quality of the waste liquid according to the image signal; a flow rate measuring unit disposed adjacent to the transparent part and configured to measure a flow rate of the waste liquid passing through the second pipeline and determine a volume of the waste liquid according to the flow rate of the waste liquid; and a calculating unit electrically coupled to the flow rate measuring unit and the signal processing module to calculate a value of the waste liquid according to the measured volume of the waste liquid and the determined quality of the waste liquid; whereby the flow rate measuring unit is outside an illuminating area of the illumination device, wherein the draining device further includes a draining nozzle inserted into the top of the first container, along a depth direction of the first container, into the waste liquid in the first container, and at a distance away from contacting the bottom surface of the first container, the first pipeline is between the draining nozzle and the transparent part and configured to guide the waste liquid from the draining nozzle to the transparent part, the second pipeline is between the transparent part and the second container and configured to guide the waste liquid from the transparent part to the second container.