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biogas HANDBOOK substrate, would require a long retention time of the substrate inside the digester and a correspondingly large digester size. In practice, the choice of system design (digester size and type) or of applicable retention time is always based on a compromise between getting the highest possible biogas yield and having justifiable plant economy. In this respect, the organic load is an important operational parameter, which indicates how much organic dry matter can be fed into the digester, per volume and time unit, according to the equation below: BR =m*c/VR BR organic load [kg/d*m3] m mass of substrate fed per time unit [kg/d] c concentration of organic matter [%] VR digester volume [m3] 3.4.2 Hydraulicretentiontime(HRT) An important parameter for dimensioning the biogas digester is the hydraulic retention time (HRT). The HRT is the average time interval when the substrate is kept inside the digester tank. HRT is correlated to the digester volume and the volume of substrate fed per time unit, according to the following equation HRT=VR /V HRT hydraulic retention time [days] VR digester volume [m3] V volume of substrate fed per time unit [m3/d] According to the above equation, increasing the organic load reduces the HRT. The retention time must be sufficiently long to ensure that the amount of microorganisms removed with the effluent (digestate) is not higher than the amount of reproduced microorganisms. The duplication rate of anaerobic bacteria is usually 10 days or more. A short HRT provides a good substrate flow rate, but a lower gas yield. It is therefore important to adapt the HRT to the specific decomposition rate of the used substrates. Knowing the targeted HRT, the daily feedstock input and the decomposition rate of the substrate, it is possible to calculate the necessary digester volume. 3.4.3 Parameterlist A number of parameters (Table 3.6) can be used for evaluation of biogas plants and for comparing different systems. In literature two main categories of parameters can be found: - Operating data, which can be determined by measurement - Parameters, which can be calculated from the measured data In order to evaluate the performance capabilities of a biogas plant a multi-criteria analysis should be performed. Evaluations based on a single parameter can never do justice to the process. In order to determine if a biogas plant can provide a return on investment, in an acceptable time frame, economic parameters must always be included. 28PDF Image | biogas HANDBOOK
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