Combination of Alkaline and Heat Pretreatments with Zero-Valent Iron Application in Cassava Pulp and Wastewater for Methane Generation: Development from Batch to Continuous Systems

Abstract

Pretreatment with the addition of metals to anaerobic digestion in biogas production is crucial to address improper degradation of organic compounds with low methane production. Biogas production from a combination of cassava pulp and cassava wastewater in the batch system under the variation of alkaline and heat conditions as a pretreatment was investigated with the zero-valent iron (ZVI) addition after the pretreatment. It was found that alkaline pretreatment at pH 10 with the heat at 100 °C for 30 min combined with 50 g of ZVI kg of TVS−1 showed the highest methane production up to 4.18 m3 CH4 kg TVS−1. Nevertheless, chemical oxygen demand (COD) and volatile fatty acid (VFA) removals were slightly reduced when ZVI was added to the system. Furthermore, application in the continuous system showed increased COD and VFA removals after applying alkaline and heat pretreatments. On the other hand, additional ZVI in the substrate after the pretreatments in the continuous system increased the methane production from 0.58 to 0.90 and 0.19 to 0.24 of CH4 m3 kg TVS−1 in 20 and 60 days of hydraulic retention times (HRTs), respectively. Thus, a suitable combination of alkaline and heat pretreatments with ZVI is essential for increasing methane production in batch and continuous systems.

pH and Heat Pretreatments with Zero-Valent Iron Addition to Enhance Biogas Production from Cassava Pulp Wastewater: Optimization and Comparison of Mathematical Models

“…It was in line with the outcome of Puyol, et al [57], which showed that excess ZVI reduced methane production from the AD. Similarly, a previous study using similar wastewater showed that the optimum condition was 50-60 g•ZVI•kg•TVS −1 [48]. This phenomenon may be related to the high accumulation of Fe 2+ and Fe 3+ that inhibit the process of methanogenesis, as the excess attachment may be found in the methanogenesis with the iron ions [46].…”

“…ZVI (Fe 0 ) has been found predominantly with a small portion of iron-carbon (Fe-C) particles based on the peaks observed in the micrograph [47]. The XDR diffractogram was observed at 2θ (Coupled Two Theta/Theta) at the wavelength of 1.54060 in the range of 10-80 • and resulted in peaks that indicated the ZVI at 45 • and 65 • [48]. The structure of the wool was observed to have a diameter of 0.04 mm.…”

“…Previously, different heating times were reported from heat pretreatment studies. Thirty minutes was reported to be suitable for cassava pulp at 100 • C [48], 180 • C, and 60 min for cattle manure, corn silage, and sugar beet pulp residues [60], as well as 157.84 • C for 20.15 min to pretreat cassava residues [11]. Interestingly, a more extended heating period resulted in slightly lower biogas production.…”