A state estimator and various model-based control systems have been designed for a real anaerobic digestion (AD) pilot reactor fed with dairy manure. The model used is a modified Hill model which is a relatively simple dynamical AD process model. The state estimator is an Unscented Kalman Filter (UKF) which uses only methane gas flow measurement to update its states. The model and the state estimates are used in different control systems. One of the control systems aims at controlling the methane gas flow to a setpoint. Simulations indicate that the setpoint tracking performance of a predictive control system is considerably better comparing with PI control, while disturbance compensation is not much better. Consequently, assuming the setpoint is constant, the PI controller competes well with the predictive controller. A successful application of predictive control of the real reactor is presented. Also, three different control systems aiming at retaining the reactor at an operating point where the volatile fatty acids (VFA) concentration has a maximum, safe value are designed. A simulation study indicates that the best control solution among the three alternatives is PI control based on feedback from estimated VFA. 1. Introduction Anaerobic digestion (AD) of organic substrates can produce biogas which consists mainly of methane and carbon dioxide, [1–3]. In a well-operated AD reactor, the methane content is sufficiently large to make the biogas combustible; that is, the AD process produces applicable energy. Moreover, the reactor digestate is often high in nutrients and can be used in fertilization. Animal waste, in many cases combined with, for example, food waste, is a typical feedstock of AD reactors. A presentation of AD of animal wastes, from dairy, beef, poultry, and swine, is provided, for example, in [4]. UASB (upflow anaerobic sludge blanket) type reactors are effective AD reactors as they allow for relatively high load rates (feed rates) and/or small reactor volumes, [1, 5]. The effectiveness is due to relatively large solids retention time (SRT), which is the retention time of the microorganisms which degrades the substrate and generates, for example, methane, compared with the hydraulic retention time (HRT) of the reactor. The AD reactor studied in the present paper is a UASB reactor. Anaerobic digestion is a complex and nonlinear dynamic process and most plants suffer from a lack of robust online-measurement systems for online process monitoring [3]. Therefore, automatic plant control is a challenging task. The present paper presents an
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