An accumulation of the propionic RO4929097 was found and it was accompanied with the increase of the hydrogen concentration. This is in accordance with the previous studies which found an increase in hydrogen partial pressure (HPS) along with propionic accumulation (Ariesyady et al., 2007). The hydrogen concentration under stable condition was within range of 250–310 ppm and subsequently increased to 350–470 ppm during shock loading period. This value (under stable condition) is higher than the theoretical threshold of HPS required for propionic acid degradation which is supposed to stand between range of 10−6–10−4 atm (1–100 ppm) (McCarty and Smith, 1986). Collins and Paskins (1987) found HPS in the digester headspace varied between 1.5 × 10−5 and 1.99 × 10−4 atm in 20 mesophilic sewage sludge digesters under normal operation. In view of this, hydrogen is not recommended for being used as a sole indicator for process imbalance, but rather in combination with other parameters such as VFA (propionic acid) and CH4 content in biogas. In addition to an elevated level of propionic acid and hydrogen concentration, while a decrease in alkalinity and pH was also observed during the first 15 days after the shock point (day 102–116). Bicarbonate alkalinity gradually decreased from 2.2 to 1.5 g/L, while the pH slowly decreased from 7.26 to 7.05 before finally dropped below 7.0 at day 117 (Fig. 2). During the same period of time, IA/PA ratio was found to increase from 0.7 to 1.2, while IA/TA ratio increased from 0.4 to 0.6. It shows that both pH and alkalinity ratio have a clear response towards the changes on loading rate. However, since the alkalinity produced by digester was high enough (>3.0 g/L) for buffering the drastic changes on digester’s acidity during shock loading, so that the pH was less affected by VFA accumulation and showed only a slight decrease ranging from 0.04 to 0.08/day. The effectiveness of pH as a control parameter is strongly dependent on the buffering capacity of the reactor, thereby it could be a useful parameter in a digester with low buffering capacity, but it would be less sensitive in a well-buffered system. In this matter, alkalinity ratio, especially IA/PA, is more effective for being used as parameter of system stability, since it is directly related to the consumption of bicarbonate alkalinity and showed higher sensitivity than IA/TA ratio. A small decrease in pH implies a large amount of consumption of alkalinity due to the logarithmic scale of pH and the linear scale of alkalinity.