This paper describes the valorization of onion peel biowaste following the principles of green chemistry by transforming it into a green colorimetric agent for both spectrophotometric and smartphone-based detection of Al(III) in water samples. One of the advantages of this study is the ability to utilize two portable devices for on-site analysis, particularly the possibility of using a smartphone, which is less expensive and more easily accessed by everyone than complicated instruments. The sustainable sensing method relies on the increase in color intensity of the Al(III)flavonoid complex, which is directly proportional to the Al(III) concentration using flavonoids extracted from onion peels. Under optimal conditions, a remarkable LOD value of 0.033 mg L-1 (1.22 mu M) was obtained using the spectrophotometric method, and 0.027 mg L-1 (1.00 mu M) using the smartphone-based method. These values are lower than the WHO and US-EPA permissible limit (0.2 mg L-1 or 7.41 mu M) for Al(III) in drinking water. Various water ions were examined for possible interference effects and interferences caused by Cu2+, Pb2+, Fe2+, and Fe3+ were easily eliminated using the required amount of Na2EDTA as a masking agent without changing the absorption of Al(III)-flavonoid complex. The testing and validation of both developed methods were conducted on actual water samples, achieving a 95% confidence level when compared to the ICP-OES reference method. In addition, the generation of Al(III)-flavonoid complex was proved with a visible spectrophotometer and FTIR-ATR. Also, the polyphenol content of extraction obtained from onion peel was calculated by the Folin-Ciocalteu method as gallic acid equivalent.