Abstract: Take a model sewage treatment plant as an example, the mathematic model of sewage acid/alkali deviation is figured out oriented by combined oxidation ditches. The relationship between pH measuring value and acid/alkali deviation is built up by means of non-linear transformation. According to these results, the adaptive algorithm is given out for digital discharge valve control. The algorithm adopted Restricted Least Square Method to estimate the parameter θ0 and realized estimating on-line. The control system has been used in that model plant and the controlling results are proved good.