OptimalpHandtemperature
Both pH and temperature influence the selection of the micro- bial community, and mediate the biochemical reactions that the community performs. It follows that both factors were significant and the interaction of pH and temperature was near significant (p = 0.08) as shown in Table 3. Additional experimentation could prove the interaction to be statistically significant. These results are supported by the fermentation of food waste only, which also demonstrated both factors and their interaction to be significant [12].
A narrow pH range of 5–6 was used in order to minimize demand for buffer and identify an optimum that could inform practical process scale up. The screening experiments in batches 1 and 2 identified an optimum within these bounds but did not preclude the possibility of additional optimums outside this pH range. The optimal pH of 5.5 identified is distinct from the pH of 7–8 identi- fied to maximize lactate production in the digestion of food waste only [12]. Additional experimentation is needed to clarify whether this difference is an artifact of co-digestion with primary sludge, smaller pH intervals tested, or whether there are multiple pH values at which this system can be optimized for total lactate concentra- tion. Nevertheless, the difference constitutes a sizable reduction in chemical demand for pH buffering which bolsters the feasibility of scaling up the process.
The identification of 41 ◦ C as maximizing lactate is supported by optimums previously reported. Lactate dehydrogenase, the
enzyme that converts intermediate pyruvate to lactate, was found to have more than 37% higher activity at 35 ◦C than at 20 ◦C or 50 ◦C in the co-digestion of food waste with sludge [15]. Further, between 35 ◦ C and 45 ◦ C has been observed to have the highest rate of food waste solubilization during fermentation [4]. Therefore it is reason- able that an optimum temperature would be identified within the range of 35–55 ◦ C tested in this experiment, and that 41 ◦ C would balance the first and last biochemical reactions in the conversion of food waste to lactate.