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Organic loading rate (OLR) and hydraulic retention time (HRT) are important parameters, which can negatively affect both microbial community and reactor performance. The loading rate can be of two types - hydraulic load (HRT variation) and organic load (increase in feed strength at fixed HRT). In both ways same OLR can be applied to the reactor, but the effect on reactor performance in these two cases varies. In engineering point of view, organic load i.e an increase in feed results enhanced mass transfer of substrate, but in case of hydraulic load i.e keeping feed COD constant causes the driving force for mass transfer of substrate to remain same. Nachaiyasit and Stuckey (1997) studied the effect of organic and hydraulic load on an anaerobic baffled reactor and observed significant differences in reactor performance in both cases. While the increase in OLR can trigger microbial growth, low OLR can cause starvation. So, the increase in OLR can be used as a means to increase biogas production rate. However, the response of each trophic group to change in loading condition should be taken into account. For maintenance of reactor stability, proper balance between each trophic group of microbial communities is very important. The first hydrolytic step of anaerobic digestion producing VFA is very fast compared to methanogenesis, i.e. consumption of VFA. If the VFA produced is not removed from the system by methanogens, the accumulation of VFA can lower the pH leading to inhibition of methanogens. Moreover, it has been also reported that 4-5 mM acetate level can inhibit the acetoclastic methanogenic activity (Dogan et al., 2005). From another point of view, lower activity of hydrogenotrophic methanogenic due to low pH can be the caus... ... middle of paper ... ...ydraulic shock loads on archaeal community and performance of the reactor operated at mesophilic and thermophilic temperatures. At both the operating temperatures, significant changes in archaeal community structure were observed. They also observed better performance of the reactor having higher diverse methanogenic community at 37ºC than that operated at 55ºC. They concluded from the results that higher diversity plays an important role towards withstanding capability of anaerobic reactors towards shocks loads. Interestingly, the molecular fingerprints under organic and hydraulic shocks was different both in terms of band numbers and relative intensity. Hence, their results indicated that the microbial community responds differently towards hydraulic and organic shock leading to different level of tolerance of anaerobic reactors for two kinds of shock loads.

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