The rapid increase in the population is expected to result in the approaching of design capacity for many US wastewater treatment plants (WWTPs) over the next decade. WWTPs treat both municipal and industrial wastewater influents, resulting in the production of biosolids after digestion. Biogas, a potential recovered alternative energy source, is also produced as an output from successful anaerobic digestion. More than 7M of dry tons/year of biosolids produced in the US are most often disposed in either landfills or land-applied (~80%). These options are becoming more challenging to implement due to increases in transportation costs and tipping fees, decreases in the availability of landfill/landfarm space, and most importantly, increased regulations. This situation is strongly encouraging WWTPs to find alternatives for the disposal of biosolids. Developing alternative management/disposal options for biosolids are evolving. One of the most attractive alternative option from a sustainability perspective are biorefineries (converts waste to commercial products), which are a fast-growing option given the push toward circular urban source economies (little to no waste generation). Anaerobic digestion has been widely applied in WWTPs to reduce the volume of activated sludge due to its low energy requirements, effective handling of fluctuations due to organic loading rate, relative flexibility with temperature and pH changes, and since biogas is produced that can be transformed into energy. Various pretreatment methods for waste sludges prior to digestion that have been studied to reduce solids production and increase the energetic content of the biogas are presented and discussed. Solids handling and management, which comprises ~60% of the operational cost of a WWTP, is estimated to save more than $100 M annually by achieving at least 20% reduction in the annual production of biosolids within the US. This review incorporates an assessment of various pretreatment methods to optimize the anaerobic digestion of waste sludges with a focus on maximizing both biosolids reduction and biogas quality.

Background: Waste activated sludge (WAS) has recently gained attention as a feedstock for resource recovery. The aim of this study is to investigate the corrosion inhibition efficiencies of extracellular polymeric substances (EPS) extracted from WAS. Methods: The studied corrosion inhibitors were tested with carbon steel in 3.64% NaCl saturated with CO₂ at 25 °C, which is the typical oilfield environment. They were first prepared by EPS extraction (heating at 80 °C), followed by centrifugation for solid and liquid separation, then the supernatant was freeze-thawed five times for sterilization of microorganisms in WAS to terminate metabolic activities in the test inhibitors to ensure consistency in corrosion inhibition. The EPS mixture (supernatant) was then deemed as the test corrosion inhibitor. The inhibition performance was determined using potentiodynamic polarization scans. Results: Waste activated sludge alone showed unsatisfactory inhibition. However, EPS extracted from WAS showed an optimum inhibition of approximately 80% with 1,000 mg/L of inhibitor. The average total solid (TS) and EPS contents of the WAS were 7,330 mg TS/L WAS and 110 mg EPS/g TS, respectively. Three sets of extracted EPS were scanned with fourier-transform infrared spectroscopy (FTIR) and showed almost overlapping curves, yielding the consistent inhibition performance. Discussion: The potentiodynamic polarization results indicated that EPS acts as a mixed-type inhibitor which inhibits corrosion on both anode and cathode sites of metal surfaces. Based on the FTIR results, it was assumed that major chemical groups O–H, N–H, C–N, C=O, and C–H contributed to the inhibition by adsorbing on the metal surface, forming a biofilm that acts as a protective barrier to isolate the metal from its corrosive environment. Results show that WAS EPS corrosion inhibitors have inhibition performance comparable to commercial products, signifying their potential in commercialization. This corrosion inhibitor is renewable, biodegradable, non-toxic, and free from heavy metal, making it a superior green corrosion inhibitor candidate. Additionally, turning biomass into value-added product can be beneficial to the environment and, in this case, deriving new materials from WAS could also transform the economics of wastewater treatment operations.

Background: Extracellular polymeric substances (EPS) extracted from waste activated sludge (WAS) have previously shown its potential in corrosion inhibition. The aim of this study is to design a synthetic EPS formulation as a surrogate of natural WAS EPS to overcome the corrosion inhibition inconsistency in WAS EPS. The adsorption behavior of the designed inhibitor was studied by kinetic and thermodynamic analyses. Methods: Synthetic EPS is a bio-inspired material that was formulated based on the most typical chemical compositions of natural WAS EPS, that is, proteins, carbohydrates, humic substances, nucleic acids, and uronic acids, which was not optimized for corrosion inhibition performance. It is a mixture of glutamic acid, carboxymethylcellulose, humic acid, thymine, and alginic acid. Its corrosion inhibition performance was tested with carbon steel in 3.64% NaCl saturated with CO₂, using the potentiodynamic polarization scanning technique. The resulted electrochemical parameters were used to evaluate the empirical corrosion kinetic and thermodynamic adsorption parameters. Results: Addition of synthetic EPS showed significant decrease in corrosion rate as compared to the control. The inhibition efficiency improved with increasing inhibitor concentration and temperature. The optimum performance was 94% with 204 mg/L of inhibitor applied at 70 °C (343 K). The inhibition performance was controlled by both the concentration of inhibitor and temperature. Chemisorption of the inhibitor molecules contributed to the overall inhibition performance, reducing the contact of metal with the corrosive environment, thus, slowing down the overall corrosion rate.

Biomass gasification is one of the most promising routes to produce green hydrogen, power, fuels, and chemicals, which has drawn much attention as the world moves away from fossil fuels. Syngas produced from gasification needs to go through an essential gas cleanup step for the removal of tars and particulates for further processing, which is one of the cost-inducing steps. Existing hot gas cleanup strategies involve the particulate removal step followed by catalytic tar reforming, which could be integrated into a single unit operation using porous ceramics owing to their advantages including high-temperature resistance, high corrosion resistance, flexibility, and robust mechanical integrity. Ceramic filters have proven to be effective at filtering particulates from hot gas streams in various applications including combustion, incineration, gasification, and pyrolysis. These materials have also been evaluated and used to an extent as catalyst support to remove contaminants such as nitrogen oxides (NO_x), volatile organic compounds (VOC), and in particular, tars, however, the use of these ceramic materials to remove both tars and particulates in one unit has not received much attention, although it has a promising potential to be a cost-effective hot gas cleanup strategy. Thus, this review presents the ability of catalytic ceramic filters to boost energy efficiency by converting unwanted byproducts while simultaneously eliminating PM in a single unit and is shown to be valuable in industrial processes across the board. This article presents a comprehensive and systematic overview and current state of knowledge of the use of porous ceramics for catalytic hot gas filtration applications with an emphasis on biomass syngas cleanup. In addition, a similar strategy for other applications such as combustion exhaust streams is presented. Prospects and challenges of taking this approach, and the necessary research and development to advance the novel use of reactive ceramic filters within biomass-fed thermal systems are presented. Major challenges include the low surface area of the ceramic filter media and high-pressure drop across the filter media, which can be overcome by wash coating or dip coating mechanisms and porosity tailored to meet the requirements. Owing to limited R&D efforts in this area, a systematic approach toward developing these integrated hot gas filtration systems is much needed, which will ultimately contribute to cost-effective green hydrogen production.