start-up treatments 1 and 3 initially had low pH,increasing to neutral levels during the leachate exchange process. Treatment 3 pH increased more quickly than treatment 1. This was most likely a function of the more rapidly declining VFA concentration and higher leachate exchange rate between treatments 3 and 4. Plotting treatment 3 pH against acetic acid and the three highest intermediate VFA concentrations showed that pH changed irrespective of VFA concentration (Fig.6) During the initial digestion phases, pH increased with increasing VFA concentration and continued to do so until
reaching a plateau at day 35, corresponding to a pH slightly above 8. This indicated that there were other factors apart from VFA concentration involved in controlling pH in start-up reactors. This may be attributed to the highly buffered leachate from the mature reactor. At a pH of 7, VFA concentration started to decrease and VFA degradation became more rapid as the pH reached 8. There was also a strong association between pH and methane generation rate in the start-up reactors. Methane generation rate remained low in the initial digestion phase until pH reached neutral at day 24, where upon methane production increased dramatically. This indicated that there was a threshold limit for methane generation rate around pH 7. This is a similar finding to that determined by Vavilin et al. (2006), which showed that inhibition is related either directly or indirectly to low pH.The relation ship between methane rate and pH is stronger than that
observed between pH and VFA concentrations. Mature treatments 2 and 4 maintained leachate pH above 7 at all times due to a high buffering capacity. Small changes in pH occurred when there were large changes in process performance, which is in agreement with a number of other researchers that have shown pH not to be an effective measure of the stability of an anaerobic process when there is a high buffering capacity (Angelidaki and Ahring, 1994; Bjo¨rnsson et al., 2000).