Abstract
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Our first group of articles focuses on anaerobic digestion (AD) of mainstream municipal wastewaters for both developed and developing world applications. AD is commonly applied to industrial wastewaters, biological sludges and livestock wastes; however, there has been a recent move toward using AD treatment of mainstream wastewaters. Hejnic et al. (2016) compared the performance of upflow anaerobic sludge blanket (UASB) and anaerobic membrane bioreactors (AnMBRs) for mainstream wastewater treatment under psychrophilic (15°C) conditions. Although the AnMBRs provided more stable effluent quality, the UASB required less energy input. Dolejs et al. (2016) introduced a contact stabilization with an enhanced accumulation (CoSEA) process to preconcentrate dissolved and colloidal organic compounds into sludge for subsequent AD. CoSEA utilizes bioflocculation–adsorption–sedimentation–stabilization in a sequencing batch reactor, allowing up to 55% of the incoming chemical energy to be recovered as methane. Sills et al. (2016) evaluated the economic and environmental sustainability of low-cost mainstream wastewater treatment technologies appropriate for developing countries. Life cycle and techno-economic assessment tools were used to compare results from a bench-scale anaerobic baffled reactor (ABR) with a conceptual design for a trickling filter (TF). The results showed that the ABR had the lowest net present value; however, post-treatment of effluent using an aerobic process, such as a TF, would reduce the environmental burden of ABR because of the high dissolved methane concentrations in the effluent.
Our next group of articles focused on AD of lignocellulosic waste materials, such as municipal solid wastes and crop residues. The lignin in these wastes acts as a barrier to cellulose hydrolysis, resulting in long required residence times for AD processing these waste streams. Hinds et al. (2016) addressed this challenge in a high solids AD process by inoculating yard wastes with acclimated microbial populations in granular sludge from AD of pulp and paper wastewater. A 73% enhancement in methane yield was observed with the pulp and paper sludge inoculum compared with a conventional inoculum. The authors also showed that the enhancement could be sustained by inoculating fresh yard waste with digestate from the initial digesters. Mancini et al. (2016) pretreated rice straw, cocoa shells, and hazelnut skins with the solvent, N-methylmorpholine-N-oxide (NMMO), to enhance methane yields. The NMMO pretreatment was particularly effective for rice straw, resulting in an 82% enhancement in methane production.
AD has also been used for treatment of human and livestock wastes in developing world contexts. The biogas produced can provide energy for cooking, heating, and electricity generation, while also recovering nutrients for use as fertilizers and reducing deforestation and greenhouse gas emissions. However, in countries where persistent pathogens are prevalent, AD can pose a public health risk if effluents are used to fertilize food crops. Forbis-Stokes et al. (2016) addressed this issue with fecal sludge in a peri-urban area in Kenya using an AD pasteurization latrine. In this system, AD of fecal sludge was used to generate biogas, which was then used as a source of heat to pasteurize effluents for use as fertilizer. Biogas production was sufficient to pasteurize digester effluent and no fecal coliforms were detected when the heater temperature was >65°C. Manser et al. (2016) combined bench-scale pathogen inactivation experiments and mathematical modeling to predict the inactivation of the very persistent and globally prevalent pathogen, Ascaris spp. (intestinal roundworms), during AD of swine waste at varying feeding intervals (FIs) and solids retention times (SRTs). The model predicted that under mesophilic conditions, Ascaris inactivation was weakly dependent on FI and strongly dependent on SRT. The authors also provide guidance for safe operation of household-scale ADs in regions where ascariasis is prevalent.
Finally, there has been a tremendous interest in production of algal biofuels over the past decade because of the high productivity of algae compared with other biomass crops, the ability of some algae to store lipids intracellularly, and the lack of competition of algae for arable land. However, life cycle assessment studies have shown that algal biofuel production can only be sustainable if waste nutrients and water (i.e., from wastewater) are used as the growth medium. Hwang et al. (2016) provide a review and synthesis of the literature on algae for wastewater treatment and bioenergy production. The authors outline the critical research needs for the field, including challenges in the cultivation, harvesting, and biofuel production stages.
The collection of articles in this special issue of EES represents breadth studies to develop and vet new systems to better recover underutilized energetic resources and incentivize treatment approaches. The versatility of AD processes included demonstrates biogas production across temperatures, settings, and waste and wastewater sources. Algal biofuel production could emerge as an additional tool in recovering energy from wastewaters. The diversity of strategies and approaches emphasizes the importance of context in making meaningful and appropriate advances.
Footnotes
Author Disclosure Statement
No competing financial interests exist.