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Anaerobic digestion of cheese whey in an upflow anaerobic sludge blanket reactor Yan, Jing-Qing


The anaerobic digestion of cheese whey was studied in an upfiow anaerobic sludge blanket reactor for its start-up characteristics, the effects of various process parameters, the effect of sulfate addition and the determination of optimal operating conditions. Start-up of an UASB reactor treating cheese whey was extremely difficult due to its tendency to acidify. Various start-up strategies were tested to facilitate start-up and to ensure stable operation. Among the operating parameters, sludge loading rate was the most critical for proper start-up of the UASB reactor. The initial sludge loading rate during start-up period should not exceed 0.25 g COD/g VSS. The response of whey digestion to several process parameters was investigated. Without pH-control, over 97% COD removal was obtained for influent concentrations from 5 to 28.8 g COD/1 and HRT of 5 days. However, instability was observed when the influent concentration was increased to 38.1 g COD/1. Gas production from whey is affected by organic loading rate (OLR). At an OLR less than 4 g COD/l-d, higher influent strength resulted in a higher methane production rate. When the OLR was greater than 6, higher strength feed or shorter hydraulic retention time (HRT) produced less methane. From the profiles of substrate concentration measured at various levels above the bottom of the reactor, two reaction stages, acidogenesis and methanogenesis were distinguished. It was experimentally illustrated that the rate of acidogenesis is much faster than the rate of methanogenesis in a whey anaerobic digestion system. The accumulation of VFAs in the first stage being faster than its assimilation in the second stage creates a distinct acidogenic phase in the bottom of the reactor. The instability caused by high influent concentration could be attributed to the accumulation of VFAs beyond the assimilative capacity of the methanogenic stage. A set of empirical models for accumulation and degradation of VFAs was developed using linear regression analysis. The requirement for maintaining this system in a dynamic balance was that the degradation capacity for VFA in the second stage be greater than the accumulation of VFA in the first stage. Based on this idea, the optimal influent concentration was given as between 25 to 30 g COD/1 for system stability. A hypothesis was proposed in this study that a proper amount of sulfate may be applied to moderate the detrimental influence of excess hydrogen on a stressed anaerobic reactor. The effect of sulfate was tested to study the biochemical mechanism. The permissible influent COD concentration was increased from 30 g COD/1 to 50 g COD/1 by using sulfate addition. The pH in the reactor was on the average 0.8 units higher and the concentration of butyric acid in the acidogenic phase much lower with added sulfate than without sulfate addition. The significant improvement of process stability and treatment efficiency made by the addition of sulfate clearly illustrated that sulfate acted like a stimulator which helped to maintain conditions favorable to methanogenesis. The mechanism of this stimulation is explained according to thermodynamics and hydrogen regulation which suggested that sulfate is able to promote the β-oxidation of VFAs by consuming hydrogen. A two-stage inhibition mechanism was proposed to explain the inhibition of high VFA concentrations and the stimulation of sulfate. Higher hydrogen pressure is the cause of preliminary inhibition, resulting in the accumulation of VFAs, which subsequently inhibit the activity and growth of methanogens in the second inhibition stage. The mechanism of inhibition of methanogens from VFAs was interpreted as being caused by the acidification of the internal cytoplasm and destruction of the pH gradient by non-ionized acids based on the theory of bacterial membrane transport. A new control strategy for stabilization of an anaerobic system is recommended. Under the optimal operating conditions based on the results in the first three steps, over 97% reduction of COD was achieved when the influent COD was 30 g /l using an HRT of 2 days, an OLR of 16.61 g COD/l-d and sulfate concentration of 0.2 g/1.

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