Anaerobic co-digestion of sewage sludge with cellulose, protein, and lipids: Role of rheology and digestibility

An accurate and robust method for intensification of wastewater sludge pipe flow

Globally, environmental impacts and population growth are driving the process intensification of wastewater treatment plants (WWTPs) via transition from conventional (2-3 wt% solids) to highly concentrated (4-6 wt% solids) wastewater sludges (HCWS). This presents an industrial challenge as HCWS are complex, non-Newtonian materials whose viscosity increases nonlinearly with solids concentration. This viscosity increase is particularly relevant for sludge pipe flow as it leads to considerable pumping pressure that ultimately limits the feasibility of pipe flow transportation. Hence, process intensification demands accurate prediction of HCWS turbulent pipe flow to design and optimise pumping infrastructure and piping systems. Such prediction requires accurate rheological characterisation of HCWS and numerical prediction of HCWS turbulent pipe flow, neither of which has been achieved to date due to respective limitations associated with benchtop rheometry and numerical turbulence models. We address these challenges by first developing accurate methods for rheological characterisation of HCWS via laminar flow of digested sludge at various solids concentrations (2-5 %) in a fully instrumented pipe loop facility at a large-scale WWTP. These rheological parameters are used in direct numerical simulation (DNS) computations (that avoid turbulence models) of turbulent pipe flow of HCWS. These predictions are then validated against turbulent flow pipe loop data. This method yields accurate (2-15 % error) predictions of HCWS turbulent pipe flow, compared with up to ∼75 % error for conventional pipe flow correlations. This validation highlights the need for accurate rheological characterisation and numerical simulation to predict HCWS pipe flow and provides a sound basis for the intensification and optimisation of WWTP pipeline systems.

Recovery of nutrients from fish sludge to enhance the growth of microalga Chlorella sorokiniana CMBB276

Intensive aquaculture production generates large amounts of sludge. This waste could be considered as a potential source of nutrients that can be recovered and utilized. Little attention has been paid to nutrient recovery from fish sludge. In this study, bioconversion of sludge was evaluated in lab scale under anaerobic (AN), facultative anaerobic (FA) and aerobic (AE) conditions. After 40 days of fermentation, AN recovered the highest values of dissolved total nitrogen (82.7 mg L-1), while AE showed the highest dissolved total phosphorus (11.8 mg L-1) and the highest reduction of total suspended solids (36.0 %). Microbial analysis showed that AN exhibited a distinct bacterial community than that of FA and AE. Furthermore, C. sorokiniana grown in AN effluents collected after 12 days of fermentation achieved the highest biomass production (1.96 g L-1). These results suggest that AN has the best potential to recover nutrients from sludge for production of C. sorokiniana.

Physico-chemical and rheological properties datasets related to batch mesophilic anaerobic digestion of waste activated sludge, primary sludge, and mixture of sludge with organic and inorganic matter

The datasets included in this paper provide periodically measured physico-chemical and rheological properties of mesophilic batch anaerobic digesters' content for 30 days biochemical methane potential tests (BMP). Waste activated sludge (WAS) and primary sludge (PS) were the main substrates and digested sludge from a large scale mesophilic anaerobic digester was the inoculum. The substrates (F) and inoculum (I) were fed into the BMP rectors at different ratios of feed to inoculum (F/I = 1:1, 1:2, and 1:3). Experimental data on co-digestion of WAS with inorganic and organic additives were also reported. The reported characteristics such as total solids, volatile solids, total and soluble biochemical oxygen demand, ammonia, pH, as well as rheological properties over the duration of the BMP test could be used for analysing the changes in digestate properties as the anaerobic digestion process proceeds. The discussion and interpretation of the data have been provided in previous publications [[1], [2], [3]].

Effect of Pretreatment by Freeze Vacuum Drying on Solid-State Anaerobic Digestion of Corn Straw

As a common agricultural waste, corn straw (CS) has a refractory structure, which is not conducive to anaerobic digestion (AD). Appropriate pretreatment is crucial for addressing this problem. Thus, freeze vacuum drying (FVD) was proposed. In this study, fresh CS (F-CS) pretreated (5 h, −40 °C) by FVD and naturally dried CS (D-CS) were compared. Differences in substrate surface structure and nutrient composition were first investigated. Results show that a loose and porous structure, crystallinity, and broken chemical bonds, as well as higher proportions of VS, C, N, cellulose, hemicellulose, and crude proteins in F-CS show a potential for methane production. Besides, process performance and stability were also examined in both high (4, VS basis) and low (1, VS basis) S/I ratio AD. A higher degradation ratio of hemicellulose as well as richer dissolved microbial metabolites, coenzymes, tyrosine-like proteins, and hydrolysis rate of particulate organic matter in the F-CS system enhanced the efficiency of methane conversion. The cumulative methane yield increased from 169.66 (D-CS) to 209.97 (F-CS) mL/gVS in the high S/I ratio system (p = 0.02 < 0.05), and 156.97 to 171.89 mL/gVS in the low S/I ratio system. Additionally, 16S-rRNA-gene-based analysis was performed. Interestingly, the coordination of key bacteria (Clostridium_sensu_stricto_1, Bacillus, Terrisporobacter. Clostridium_sensu_stricto_7, Thermoclostrium, UCG-012, and HN-HF0106) was more active. Poorer Methanosarcina and Methanomassiliicoccus as well as richer Methanobrevibacter and Methanoculleus stimulated the co-relationship of key archaea with diverse methanogenesis pathways. This study aims to verify the positive effect of FVD pretreatment on AD of CS, so as to provide a reference for applications in waste management.

Environmental management of industrial decarbonization with focus on chemical sectors: A review

A considerable portion of fossil CO₂ emissions comes from the energy sector for production of heat and electricity. The industrial sector has the second order in emission in which the main parts are released from energy-intensive industries, namely metallurgy, building materials, chemicals, and manufacturing. The decarbonization of industrial wastes contemplates the classic decarbonization through optimization of conventional processes as well as utilization of renewable energy and resources. The upgrading of existing processes and integration of the methodologies with a focus on efficiency improvement and reduction of energy consumption and the environment is the main focus of this review. The implementation of renewable energy and feedstocks, green electrification, energy conversion methodologies, carbon capture, and utilization, and storage are also covered. The main objectives of this review are towards chemical industries by introducing the potential technology enhancement at different subsectors. For this purpose, state-of-the-art roadmaps and pathways from the literature findings are presented. Both common and innovative renewable attempts are needed to reach out both short- and long-term deep decarbonization targets. Even though all of the innovative solutions are not economically viable at the industrial scale, they play a crucial role during and after the energy transition interval.

Effect of mixed primary and secondary sludge for two-stage anaerobic digestion (AD)

As an energy-efficient and eco-friendly sludge treatment process, two-stage anaerobic digestion (AD) is widely employed to recovery biomass energy from waste sludge. However, the effect of primary and secondary sludge for two-stage AD was not clear. In this study, two-stage AD of mixed sludge in different volume ratio was investigated. The maximum cumulative H₂ yield (100.5 ml) and CH₄ yield (2643.6 ml) were obtained in volume ratio of 1:3 (primary sludge: secondary sludge). In two-phase AD, mixed sludge could induce positive effect on both organics releasing in extracellular polymeric substances (EPS) and the utilization of volatile fatty acids (VFAs). By investigating the compositional characteristics of dissolved organic matters (DOM) through excitation-emission matrix (EEM) coupling with fluorescence regional integration (FRI), it revealed more degradable substances utilization in mixture of sludge. Results from this work suggest that two-phase AD with mixed sludge is efficient for renewable energy recovery.

Can nitrifiers from the sidestream deammonification process be a remedy for the N-overload of the mainstream reactor?

The combination of sidestream deammonification and bioaugmentation of the mainstream reactor using ammonia oxidizers from partial nitritation (PN) was not achieved before. This novel solution not only enables the efficient sidestream nitrogen removal, but also improves mainstream resistance to stress situations such as biomass washout or nitrogen overload. This feature is important for wastewater treatment plants (WWTPs) equipped with reject water deammonification as its implementation leads to lower nitrifier mass in the mainstream reactor and therefore diminish ability to cope with rapid increase in the loading rate (i.e. due to sidestream process failure). The proposed approach presents the use of the excess sludge from a modified PN process to boost the mainstream nitrification in unfavourable conditions. In a long-term laboratory experiment, the operation of an existing WWTP at low temperature was simulated in two reactors using real wastewater fluxes. One of them was augmented with the excess sludge from a PN reactor that treats reject water containing 20% of the WWTP N-load. The treatment efficiency in both reactors was tested under different nitrogen loading rates, as well as in the case of the of biomass loss. The bioaugmentation intensity was set according to the actual nitrogen load balance of the modelled WWTP, resulting in a daily seed volume only equal to 0.28% of the reactors' influent. Two incidents were simulated, where the nitrogen load increased by about 24.5% and 34%. In both cases, the nitrification efficiency in the non-augmented reactor dropped by about 45%, while the augmented reactor maintained efficient ammonium removal. The bioaugmentation effect was also noticeable during biomass washout - only in the non-augmented reactor nitrification was insufficient for over 60 days. These results undoubtedly show the possibility of combining two different approaches for sidestream nitrogen removal into one technology demonstrating the advantages of both component solutions.

Biomethane production from anaerobic co-digestion at wastewater treatment plants: A critical review on development and innovations in biogas upgrading techniques

Anaerobic co-digestion (AcoD) can utilise spare digestion capacity at existing wastewater treatment plants (WWTP) to generate surplus biogas beyond the plant's internal energy requirement. Data from industry reports and the peer-reviewed literature show that through AcoD, numerous examples of WWTPs have become net energy producers, necessitating other high-value applications for surplus biogas. A globally emerging trend is to upgrade biogas to biomethane, which can then be used as town gas or transport fuel. Water, organic solvent and chemical scrubbing, pressure swing adsorption, membrane separation, and cryogenic technology are commercially available CO₂ removal technologies for biogas upgrade. Although water scrubbing is currently the most widely applied technology due to low capital and operation cost, significant market growth in membrane separation has been seen over the 2015-2019 period. Further progress in materials engineering and sciences is expected and will further enhance the membrane separation competitiveness for biogas upgrading. Several emerging biotechnologies to i) improve biogas quality from AcoD; ii) accelerate the absorption rate, and iii) captures CO₂ in microalgal culture have also been examined and discussed in this review. Through a combination of AcoD and biogas upgrade, more WWTPs are expected to become net energy producers.

Integrated ABR and UASB system for dairy wastewater treatment: Engineering design and practice

This work designed and assessed the engineering performance of dairy wastewater treatment by an integrated system consisting of an anaerobic baffled reactor (ABR) and an upflow anaerobic sludge blanket (UASB). With fats adsorbed and decomposed, proteins were first denatured coagulated into solids in the ABR treatment process, and this process created suitable conditions for sludge retention in the sludge bed of the UASB. As a result, the combined system achieved a substantial reduction in excess sludge from 3 to 5 t/d to 3 t/m, notable biogas generation, and 98% COD removal, while the other pollutants in the effluent met relevant standards. In addition, the system attained an excellent performance in terms of the energy consumption and water treatment agent amount. Two active plants achieved operation costs lower than 0.5 kW h/t, while stable operations under ambient temperature conditions lasted longer than three years. Engineering practices both technically and economically affirmed the potential of the proposed system for dairy wastewater treatment.

Effect of coupling alkaline pretreatment and sewage sludge co-digestion on methane production and fertilizer potential of digestate

This study aims at investigating how organic waste co-digestion coupled with alkaline pretreatment can impact the methane production and agronomic value of produced digestates. For this purpose, sludge alone and mixed with olive pomace or macroalgal residues were subjected to anaerobic digestion with and without alkaline pretreatment. In addition, co-digestion of pretreated sludge with raw substrates was also carried out and compared to the whole mixture pretreatment. KOH pretreatment enhanced methane production by 39%, 15% and 49% from sludge, sludge mixed with olive pomace and sludge mixed with macroalgal residues, respectively. The digestates were characterised according to their physico-chemical and agronomic properties. They were then applied as biofertilizers for tomato growth during the first vegetative stage (28 days of culture). Concentrations in chlorophyll a and carotenoids in tomato plants, following sludge digestate addition, rose by 46% and 41% respectively. Sludge digestate enhanced tomato plant dry weight by 87%, while its nitrogen content increased by 90%. The impact of nitrogen and phosphorus contents in the digestate was strongest on tomato plant dry weight, thus explaining the efficiency of sludge digestate relative to other types of digestate. However, when methane production is considered, the combination of pre-treatment with co-digestion of macroalgal residues and sludge appears most beneficial for maximizing energy recovery and for biofertilizer generation.