Use of modelling to evaluate best control practice for winery-type wastewaters

Winery wastewaters are high strength, and readily biodegradable, making them perfect for application of anaerobic digestion. However, inherent buffering against pH changes also requires some process knowledge and monitoring. They are therefore an important target for anaerobic process control. This has had limited application, but is emerging as an important research and development area. In this paper, we evaluate the use of model-based control analysis of a heavily loaded vinasse-fed reactor. Two controllers--both proportional-integral (PI)--on total volatile fatty acids (VFA) (< 500 mg/L), alkalinity (VFA/Total Alkalinity < 0.2) were evaluated in a fitted model, against the actual control mechanism used, which was model-based adaptive control. The two controllers were both less aggressive than the adaptive controller, producing less gas overall, but also using more caustic for pH control. The controllers were also evaluated against their ability to deal with noise, as PI controllers are relatively poor for controlling non-linear processes. The VFA controller was very poor with noise added, proving difficult to tune, and oscillatory. The alkalinity controller was effective with moderate detuning. This emphasizes the need to use effective controller inputs, when applying simple, linear controllers. Overall, use of a model was an effective method to evaluate the different controllers in a competitive way, in a standardized environment.