CORRESPONDENCE/REBUTTAL pubs.acs.org/est
Comment on “Parameter Identification and Modeling of the Biochemical Methane Potential of Waste Activated Sludge”1 ppels et al.1 measured the biochemical methane potential (BMP) of 29 thickened waste activated sludge (TWAS) samples from 12 wastewater treatment plants. BMP values were then correlated against 19 measurements (dry matter, carbohydrates, lipids, COD, etc.) in a linear multivariate model. It was concluded that acetate, caproate, carbohydrates, and protein had a positive impact on BMP, whereas organic and total solids fraction (dry matter), sulfur, and iso-valerate had a negative impact. A reduced model was applied to predict methane potential based on these chemical measurements. The anaerobic degradability of activated sludge is of strong concern, given the financial and environmental risks and costs associated with inappropriate biosolids handling, as well as development of pretreatment technologies to improve WAS digestibility, often essential to enable digestion. However, there are several issues with this publication: (a) Incorrect experimental procedure; (b) Stochastic modeling which ignores the microbial and biochemical structure of the anaerobic digestion process; and (c) Interpretation and overgeneralization not supported by the authors findings, all of which have led to erroneous conclusions. With respect to the experimental procedure, the major issue is that the BMP assay was conducted with a relatively low inoculum:sludge ratio (20%). An inoculum:substrate volumetric ratio of at least 50% is required for sludge BMP assays, and 80% is preferable.24 Our experience with multiple dilutions is that a ratio of 50% is adequate for BMP testing of WAS if the inoculum is highly active. When a low inoculum:substrate ratio is used, the methane production proceeds, although excessive acidification occurs, and a low methane yield is observed. In order to avoid this underestimation of the methane potential, a series of substrate loading relative to the inoculum should be assessed, in order to avoid possible inoculum overloading.2 The Appels et al.1 paper states that a low inoculum ratio was used to avoid inoculum masking, but this can be readily addressed by documenting and subtracting no-food, inoclum only blanks contribution.2 The major implications of using a low inoculum:substrate ratio for the BMP assay would be that where the test material was more concentrated, the inoculum would have been overloaded, produced excessive organic acids, and resulted in a false low BMP value. It should be noted that chemical analyses were not conducted at the end of the 21-days incubation period, which would not only have examined the possibility of acidification as noted above, but would also have allowed COD balance calculations. Time-series methane production profiles can also identify overload, and in our opinion should be provided as supporting material. The final experimental issue is that the BMP period was relatively short (
