Estimating VFA concentrations in prefermenters by measuring pH

We present a fundamental relationship between the pH, volatile fatty acid (VFA) concentration and alkalinity in prefermenters. Once the relationship is established, wastewater treatment plant operators can estimate on-line the VFA concentration in a prefermenter at any time from pH measurements. Parameters in the model can either be measured at some reference point for the wastewater in question, such as the prefermenter inlet, or be established by measuring the pH and VFA values at two different points (e.g. influent to and effluent from a prefermenter). Using this relationship, the VFA concentration can then be estimated for any measured pH value, without knowing the alkalinity in the system. The analysis relies on a number of simplifying assumptions, which can be relaxed at the expense of a more complicated predictive model and additional chemical analyses. We have collated pH and VFA data from four different prefermentation systems and show that the correspondence between model prediction and measured values is good. The measured pH values and VFA concentrations are related to each other via the alkalinity of the wastewater. The relationship can be used to monitor prefermenter performance (with respect to VFA production rates) from pH measurements alone, without the need to measure VFA concentrations. It can also be applied to simple or model-based control strategies for prefermentation processes.