Chemical oxygen demand (COD) is one of the most representative indicators in water quality monitoring. Among numerous methodologies of COD measurement, optical method has received increasing interests as it can avoid secondary contamination and the frequent replacement of electrodes. However, different with the evolving progress of Infrared (IR) based turbidity sensor, the optical measurements of COD with ultraviolet (UV) absorption still rely on the complex high-end hardware architecture to achieve high accuracy, which prevent its wide deployment in the large scale monitoring of fresh waters. Nonetheless, the essential reason is the weak and in-stable UV signals after absorption. This motivates numerous method in optic domain to enhance the Signal-to-Noise Ratios (SNR) of UV signal, which, as a trade off, have to involve the complex and precise hardware architecture. This paper chooses to ease this challenge from the digital domain, where the Chirp modulation has been applied to both UV and IR signals to form a UV-IR dual wavelength COD measurement system. With Chirp modulated signals, the effective SNRs have been effectively improved due to the convolution gain, while the cross interference of dual-wavelength has been avoided as well with the higher time resolution. Benefited by this simple architecture, a low cost, low power, and high-precision COD sensor has be designed to support the large scale in-Situ deployment.