Control and utilisation of anaerobic digestors
Utilization of anaerobic digestion in waste treatment is an extremely interesting method. Its main advantages are in its low energy requirement, a production of methane readily utilizable "in situ" and a very reduced production of microorganisms. The lack of knowledge concerning this process led formerly to more or less serious breakdowns owing mainly to the organic overload of various origins. They created a certain suspicion towards this process and delayed its development. For the past fifteen years, research has been carried out on the microbial ecosystem, its kinetics and its modelling. It has permitted to determine the different parameters involved and the values necessary to obtain good operating conditions. These parameters can be divided into two categories : those indicating whether the recommended operating conditions are fulfilled, and those enabling to control the microbial ecosystem. The former concern - in the effluent : the pH, the COD concentration, the suspended solid content and the flow rate; - the temperature, the liquid level, the recirculating flows and the volatile suspended solid (VSS) content (for the completely stirred tank reactor - CSTR -) in the digestor. The parameters thanks to which the kinetics of the microbial ecosystem can be controlled are : - for the liquid phase : the pH, the volatil fatty acids (VFA), the bicarbonate and the COD concentrations; - for the gas phase : the flow rate, the gas composition (CO2/ CH4) and the hydrogen content. Variations in certain of these parameters (VFA, H2) are direcly related to changes in the microorganism activity or result from the variation in factors such as the gas composition, the pH, or the bicarbonate. In the expression of the variations in the microbial ecosystem these parameters are less important. In most cases, these parameters can be measured by sensors which allow an optimal automatic control of the digestor. The development of an optimal automatic control should improve the performance of methane-producing conditions, especially at starting point. The choice of one or several parameters for an automatic control depends on several factors, such as its sensivity to variations and its response time to an organic overload. The last two factors were measured for different parameters after an experimental overload of a 6 liter anaerobic filter used in the treatment of wine vinasse. Partial hydrogen pressure and gas flow rate were the first parameters to change. The variation coefficients varied around 50 and 4 to 5 respectively. In liquid the variation in pH and total organic carbon (TOC), which, like COD, indicates the amount of organic matter, occured later. In our study, the pH varied little and TOC was multiplied by 3. The variations in gas composition, in VFA concentration and alkalinity due to bicarbonate were the last factors detected. Utilization of hydrogen as a controlling parameter is therefore interesting because its concentration varies rapidly and more largely than the one of the other parameters. Moreover, it is much involved in the microbial ecosystem equilibrium. When the controlling parameters reach critical values, operating conditions have to be modified to restore a good microbial activity. The process can be modified : - by the reduction or the stopping of the input rate; - by the addition of a base; - by the recycling of methane atone; - or by the increase in the microorganism concentration in CSTR. The nature and significance of these actions are defined on the basis of the data collected by the recording device. These values can be processes by black-box type algorithms, or by simple models derived from cognitive models. At present, research in adaptative control is in progress. In these algorithms, we consider the optimal criteria leading to a stable process with either a maximum methane production, or a minimal COD (or TOC) in the output, or with both.