Improving the Anaerobic Treatment of Sludges and High-Strength Wastewaters through Addition of Electrically-Conductive Particles

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Title: Improving the Anaerobic Treatment of Sludges and High-Strength Wastewaters through Addition of Electrically-Conductive Particles
Author: Call, Douglas; de los Reyes, Francis; Cheng, Qiwen
Abstract: The growing list of high-strength wastewaters across North Carolina (NC) are well suited for treatment and energy recovery using anaerobic digestion (AD). These include livestock wastes, food wastes, and wastewater-derived sludges that if left untreated, pose risks to NC’s water resources and public health. AD, however, is largely underutilized to treat these wastes. This can be attributed to both economic and technology performance barriers. With respect to performance, one limitation is the operational instability that can arise when syntrophic relationships between key groups of microorganisms in the digester are disrupted; another is the incomplete conversion of organics to methane gas (CH4). To overcome these limitations, we proposed augmenting digesters with electrically conductive microscale particles. These particles have proven effective as conduits for syntrophic microbe-to-microbe direct electron transfer, resulting in improved CH4 generation and waste degradation rates using pure microbial strains in the lab. We hypothesized that supplementing digesters with these materials would result in a similar effect and could provide operators a means to stabilize and improve performance. Our objectives were to experimentally determine the impact of 1) material properties (type, conductivity, size) and 2) particle loading on AD performance. We assessed performance as CH4 generation rates, CH4 recoveries, and organic matter destruction [measured as chemical oxygen demand (COD) removal]. To fulfill our objectives, we supplemented lab-scale, serum bottlebased digesters with different conductive particle types (graphite, biochar, activated carbon) or non-conductive glass (a surface area control) and compared all particle amended bottles with controls lacking particles. We used swine wastewater collected from the NC State Swine Educational Unit as a representative high-strength wastewater. Our results indicate that across all particle types tested, graphite was the most consistent in improving digester performance. Both CH4 production rates and recoveries strongly correlated with the graphite loading rate, reaching a maximum production rate of 30 ± 2.5 mL-CH4/(gVSseed day), which was 34 ± 11% higher than the no-particle control. The other material types did not show clear trends with loadings, and in most cases led to reduced performance relative to the no-particle control. Material electrical conductivity was not found to be a decisive factor for predicting CH4 generation rates. This result was contrary to our expectations. A primary reason was that biochar, and more significantly, activated carbon, strongly adsorbed organic matter from the wastewater. Adsorption was found to have a negative impact on performance. Graphite exhibited little to no adsorptive behavior, resulting in a larger conversion of the initial COD into CH4 gas. Under our conditions tested (single batch cycle; 19 day retention time), the additional CH4 generated with graphite would not recuperate the cost of graphite. Our recommendations moving forward are threefold. First, fundamental investigations of microbial community structure and mechanisms in the presence of conductive versus non-conductive material are needed to determine if the improvements are associated with direct interspecies electron transfer (DIET). Second, alternative means of providing electrically conductive surfaces, such as graphitic brushes or cloths, should be explored, so as to avoid the costs associated with continuous particle amendments. Finally, a broader suite of high-strength wastewaters need to be studied to determine if similar responses as reported here are observed.
Publisher: NC WRRI
Date: 2017-07
Series/Report No.: UNC-WRRI;469
WRRI Project no. 15-01-W

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