Quality of Reclaimed Domestic Water Irrigated Peppers-NPK Couple Model Based on Optimized Combination Technique

China has been utilizing wastewater to irrigate grain, cotton, and vegetable crops, as well as landscapes, for decades. Nowadays, about 1/3 of the reclaimed water is used for irrigation purposes in China. This study focuses on the coupling between the nitrogen (N), phosphorus (P), and potassium (K) content in the reclaimed domestic water irrigated peppers and capsaicin. A field experiment in the three-factor, five-level quadratic general revolving combination design was conducted for an in-depth analysis of the capsaicin content coupling model by testing the significance of the regression equation and coefficient of the regression equation. The test result shows that: (1) the factors affecting the content of capsaicin are in the following order: nitrogen fertilizer application level (x1) > phosphorus fertilizer application level (x2) > potassium fertilizer application level (x3). This arrangement is according to the effect analysis based on the established capsaicin-NPK coupling model. (2) The nitrogen-potassium interaction effectively improves the content of capsaicin. Theoretically, the content of capsaicin tends to be 0 g.kg−1 when both the nitrogen and potassium fertilizer application are at the lowest level, while the content of capsaicin increases with the increase of nitrogen and potassium fertilizer application. The nitrogen-potassium combination can achieve a high level of capsaicin content when both are taken on a medium scale, which can reach 0.068 g.kg−2 when the level of nitrogen and potassium fertilizers application reaches 120 g.kg−1 and 112.5 g.kg−1, respectively. Further, maximum capsaicin content of 0.21 g.kg−1 is achieved when the level of potassium and phosphorus fertilizer application reaches 120 kg.hm−2 and 60 kg.hm−2, respectively. (3) The range of the ideal target content of capsaicin in peppers irrigated with the reclaimed domestic water in China’s Ningxia Region and an optimized NPK-coupling model is developed. More specifically, based on the developed model the level of NPK fertilizers application would be 186.15 kg.hm−2, 71.17 kg.hm−2, and 122.02 kg.hm−2, respectively, under the condition that the content of capsaicin is greater and beyond 0.12 g.kg−1.