Co-biodrying of sewage sludge and organic fraction of municipal solid waste: Role of mixing proportions

Analysis of the effect of intrinsic sludge properties on sludge drying characteristics from both sludge composition and type scales

Convective drying is an effective method for reducing the moisture content of the sludge. Fewer studies have discussed the effect of sludge physicochemical properties on drying compared to air parameters. Eleven types of sludge were collected, and ultimate analysis, proximate analysis, and heat value analysis were performed. Meanwhile, the maximum drying rate (umax) of sludge convection drying at 70 °C was determined. The results showed that the cumulative variance contribution of the two extracted principal components (PCs) was 92.5 %. Then, a regression model of umax was developed based on the extracted PCs. The coefficient of determination of this model was 0.788, and the difference was statistically significant, with a negative correlation between umax and PC2. Further, the principal component score plot enabled the traceability of the integrated sludge, and based on this classification results, the drying characteristics of various types of sludge were discussed, and a high correlation (R2 = 0.9590) between the initial moisture content of sludge and umax was found. Mathematical models between sludge physicochemical properties and drying characteristics can be effectively developed from both sludge composition and type scales. This exploration deepened the knowledge of sludge drying and facilitates the prediction of drying rate.

Inoculation with thermophiles enhanced the food waste bio-drying and complicated interdomain ecological networks between bacterial and fungal communities

Bio-drying is a practical approach for treating food waste (FW). However, microbial ecological processes during treatment are essential for improving the dry efficiency, and have not been stressed enough. This study analyzed the microbial community succession and two critical periods of interdomain ecological networks (IDENs) during FW bio-drying inoculated with thermophiles (TB), to determine how TB affects FW bio-drying efficiency. The results showed that TB could rapidly colonize in the FW bio-drying, with the highest relative abundance of 5.13%. Inoculating TB increased the maximum temperature, temperature integrated index and moisture removal rate of FW bio-drying (55.7 °C, 219.5 °C, and 86.11% vs. 52.1 °C, 159.1 °C, and 56.02%), thereby accelerating the FW bio-drying efficiency by altering the succession of microbial communities. The structural equation model and IDEN analysis demonstrated that TB inoculation complicated the IDENs between bacterial and fungal communities by significantly and positively affecting bacterial communities (b = 0.39, p < 0.001) and fungal communities (b = 0.32, p < 0.01), thereby enhancing interdomain interactions between bacteria and fungi. Additionally, inoculation TB significantly increased the relative abundance of keystone taxa, including Clostridium sensu stricto, Ochrobactrum, Phenylobacterium, Microvirga and Candida. In conclusion, the inoculation of TB could effectively improve FW bio-drying, which is a promising technology for rapidly reducing FW with high moisture content and recovering resources from it.

A review of application and development of combustion technology for oil sludge

Oil sludge is a typical hazardous waste in the petrochemical and electric power industry. It has complex components and special properties, and has serious hazards to humans, plants, water, and soil. Therefore, how to realize the effective disposal of oil sludge has become an urgent issue to be solved worldwide. Among the existing oil sludge treatment approaches, combustion has been considered to be a promising technology to realize the large-scale industrial application. In the present work, the characteristics of oil sludge were described in detail. The application and development of oil sludge combustion technology were critically summarized and discussed, including factors affecting combustion, drying process, combustion characteristics, synergistic treatment technology, and formation and control of secondary pollution. Besides, the development of combustion equipment, and integrated thermal treatment technology for oil sludge were prospected. This work can be used for guiding the industrial disposal of oil sludge.

Organic loading on biochemical fractions degradation pattern during food waste bioevaporation

More food waste (FW) is desired to be treated in a certain processing period, while the degradation pattern of biochemical fractions during FW bioevaporation was significantly influenced by the organic loading (OL). Lower OL facilitated the lipids degradation, while higher OL favored the protein degradation. It was the more porous structure and abundant oxygen accelerated the lipids degradation, and the rapid proliferation of aerobic microorganisms compensated for the low protein degradation in lower OL. Detailly, 76.8% of the lipids was degraded in the trial with OL of 1.04 kg VS_FW/kg TS_BS (Trial A), but in the trial with OL of 3.16 kg VS_FW/kg TS_BS (Trial C) it was only 0.5%. For protein, the degradation was different that 17.5% of the protein was degraded in Trial A, whereas 69.1% was degraded in Trial C. Lipids degradation contributed 63.0% to the metabolic heat in Trial A, but its contribution in Trial C was only 0.5%. For protein, it contributed 4.1% to the metabolic heat in Trial A, but in Trial C it accounted for 53.6%. In addition, the degradation of carbohydrates (71.6-80.8%) and their contribution to metabolic heat (32.8-45.9%) were comparable in all trials, thus OL had little effect on carbohydrates degradation. Results from this study could provide important guideline for FW practical disposal during their biological treatment.

Solar drying of granulated waste blends for dry biofuel production

In the paper, results of drying biofuels from sewage sludge using solar energy are presented. Drying rates of biofuels made from sewage sludge and coal slime (PBS), sewage sludge and meat and bone meal (PBM), and sewage sludge and sawdust (PBT) with 15-mm and 35-mm granule particle size were studied. Tests were performed in a solar greenhouse dryer equipped with a specially designed mixing system. Experiments were aimed at determining the drying time of biofuels under various weather conditions in the southwestern part of Poland. In summer, in order to determine the best conditions for drying, tests were performed using various parameters, i.e., layers of various thickness, such as 5, 10, and 20 cm, and various mixing intensity (no mixing, mixing 3 and 5 times/day). In spring and the fall, 10-cm thick layers combined with 5 times mixing of fuels per day were used. The performed tests demonstrated that it is beneficial to dry fuels in 10-cm thick layer. In spring and the fall, PBS and PBM biofuels laid out in layers with just such thickness showed moisture content reduced to less than 10% after 8 days, while the PBT biofuel reached the same level after 14 days. In summer, the same result may be obtained for all the biofuels after 4 days on average. The presented original method of solar drying of biofuels obtained from sewage sludge and other waste may be used in wastewater treatment plants which process sewage sludge into fuels without incurring any additional costs for supplying heat.

Biomass fuel production from cellulosic sludge through biodrying: Aeration strategies, quality of end-products, gaseous emissions and techno-economic assessment

This study assesses the technological, environmental and economic feasibility of biodrying to valorise cellulosic sludge as a renewable energy source. Specifically, three different aeration strategies were compared in terms of biodrying performance, energetic consumption, gaseous emissions, quality of end-products and techno-economic analysis. These strategies were based on different combinations of convective drying with biogenic heat produced. Two innovative biodrying performance indicators (Energetic Biodrying Index and Biodrying Performance Index) were proposed to better assess the initial and operational conditions that favour the maximum energy process efficiency and the highest end-product quality. The end-products obtained consistently presented moisture contents below 40% and lower heating values above 9.4 MJ·kg-1. However, the best values achieved were 32.6% and 10.4 MJ·kg-1 for moisture content and lower heating value, respectively. Low N₂O and CH₄ emissions confirmed the effective aeration of all three strategies carried out, while NH₄ and tVOCs were related either to temperature or biological phenomena. A techno-economic analysis proved the economic viability and attractiveness of the biodrying technology for cellulosic sludge in all the strategies applied.

Evolution of enzyme activity, heavy metals bioavailability and microbial community in different temperature stages of the co-bioevaporation process

Laboratory investigations documented enzyme activity, heavy metals' bioavailability and the bacterial community during co-bioevaporation treatment of food waste and landfill leachate. The activities of dehydrogenase, protease, urease and phosphatase were sensitive to the changes in operating temperature inherent in co-bioevaporation. The maximum dehydrogenase activity was appeared at warming 30 °C. The maximum hydrolytic activity of the microorganisms on protein, urea and phosphorus-containing organic compounds appeared at warming 50 °C. The bacteria mainly gathered on the surface and in the pores of the sludge particles used as a bulking agent. Bacterial abundance reached its maximum at warming 50 °C. Firmicutes, Actinobacterica and Proteobacterica were the dominant bacterial phyla involved. Even though co-bioevaporation concentrated the heavy metals in the leachate, their bioavailability was substantially reduced during the process.

Using of indigenous bulking agents (IBAs) in complementary stabilization and enhancing of dewatered sludge class B to class a on a full scale

BACKGROUND

Different bulking agents are used in the compost of dewatered sludge (DWS). The aim of this study has been using of indigenous bulking agents (IBA_s) in the enhancing of the DWS class of municipal wastewater from class B to class A and complementary stabilization of it for production of green manure in Sari city, Iran.

METHODS

Three IBAs including the Saccharum Wastes (SW), Citrus Purning Wastes (CPW) and Phragmites Australis (PA) from eight IBAs were selected to be compared with the sawdust (SD) that was as a control bulking agent. Five turned windrow piles were constructed on a full scale and on base of optimal C/N equal 25.All experiments were performed on the base of the standard methods on initial mix and final compost.

RESULTS

Among five windrow piles, P5 was been the best pile with a weighting ratio of DWS to IBAs (DWS: SW: CPW: PA) equal 1: 0.2: 0.24: 0.28. Pile P1 with weighting ratio DWS: SW equal 1: 0.6, Pile P3 with weighting ratio DWS: PA equal 1: 0.84, Pile P2 with weighting ratio DWS: CPW equal 1: 0.73 and Pile P4 with weighting ratio DWS: SD equal 1: 0.57 were placed in the next rounds. The results showed that the class of DWS enhanced to Class A for about 80 to 97 days and complementary stabilization of DWS by IBAs was done well and produced green manure in term of organic matter, potassium, germination index, PH, C/N and electrical conductivity had reached to the Grade 1 of Iran's manure 10716 standard and in term of phosphorus and moisture had reached to the Grade 2 of this standard. Also heavy metals were below the maximum permissible of standards.

CONCLUSION

Using of IBAs, had a higher efficiency than the control bulking agent (sawdust) in enhancing sludge class and its stabilization, so that using of them in combination (mix of IBA_s) had the highest efficiency and respectively, Saccharum Wastes (SW), Phragmites Australis (PA), Citrus pruning wastes (CPW) were placed in the next round, and sawdust was placed after them. By adding suitable IBA_S, with an optimal ratio in turned windrow method, the class of DWS of sari WWTP enhanced to Class A and complementary stabilization of DWS has been well done and the produced green manure has been reached to agricultural standards and can be safely used in agriculture.

Biosolids and sludge management

The advancements in the field of sludge and biosolids have been made over the past year. This review outlines the major contributions of researchers that have been published in peer-reviewed journals and conference proceedings throughout 2018. The review is organized in sections including regulatory developments and market analysis; analysis and quantification of characteristics including microconstituents and metals; treatment advances for the conversion of sludge to biosolids including pretreatment and sludge minimization, conditioning and dewatering, digestion, composting, and innovative technologies; product development and reuse including adsorbents and thermal products, agricultural and other uses, and innovative uses; odor and air emissions; and energy factors. PRACTITIONER POINTS: Summary of advances in the field of residuals and biosolids research in 2018. This review outlines the major contributions of researchers that have been published in peer-reviewed journals and conference proceedings. Topics covered range from regulation to innovation.

Co-biodrying of sewage sludge and organic fraction of municipal solid waste: A thermogravimetric assessment of the blends

This study investigated the thermogravimetric properties of sewage sludge and organic fraction of municipal solid waste (OFMSW) during their co-biodrying at different fractions. Sewage sludge and OFMSW were co-biodried at the mass proportion of 0%, 42.5% and 85% (of the total wet weight), respectively, with 15% cornstalk as the bulking agent. Results show that of these three raw materials, OFMSW exhibited the lowest ignition temperature and the highest burnout temperature. Moreover, OFMSW had a better comprehensive combustion performance (S) than sewage sludge. Blending OFMSW, sewage sludge and cornstalk showed the highest S value (4.0 × 10^-7%² min^-2 °C^-3). In addition, there existed certain interactions between the co-combustion process, especially at high temperature stage. The burning characteristics, including ignition performance, burnout efficiency, DTG_max and S increased with fluctuations in the first 6-9 days of co-biodrying process, and then declined in all treatments. Hence, 15-day of biodrying made the product with poor burning behavior (S value of 1.0 × 10^-7-1.4 × 10^-7%² min^-2 °C^-3). More importantly, the optimal combustion performance was observed when co-biodrying the same amount (42.5%) of sewage sludge and OFMSW with the peak of 8.3 × 10^-7%² min^-2 °C^-3 achieved on day 9. In addition, the blends were easier to burn after the biodrying process.

Biodrying performance and combustion characteristics related to bulking agent amendments during kitchen waste biodrying

Biodrying of kitchen waste amended with different bulking agents was carried out to evaluate performance, heat values and combustion characteristics. Results showed that adding bulking agents produced higher water removal rates of 55.6%-65.4%. Addition of bulking agents also yielded higher volatile solid contents related to slower degradation. Lower heat values of bulking agent treatments increased from 2000-3218 kJ·kg^-1 to 8544-9849 kJ·kg^-1. Organic degradation did not influence ignition or combustion temperatures during the second combustion stage, but did influence combustion rate. Maximum combustion rate and temperature of the third combustion stage remained stable. Bulking agents produced higher apparent activation energies compared with the control, although biodrying made combustion of kitchen waste easier, because of an overall decrease in apparent activation energy.

Effects of inoculation amount and application method on the biodrying performance of municipal solid waste and the odor emissions produced

The effects of inoculation amount and application method on the biodrying of municipal solid waste (MSW) was investigated in this study. Results showed that a low level (5%) of inoculation with mature compost significantly improved the biodrying index (4.96), while adding greater amounts decreased the biodrying performance by increasing the volatile solid degradation rate. Covering the pile with inoculation material resulted in the highest water removal (72.7%) and greatest water content reduction (from 60.2% to 17.7%). Meanwhile, first covering and then incorporating the inoculation material into the biodrying pile did not improve biodrying performance. Clearly, addition of varying amounts of inoculation material via different application methods enhanced cellulose degradation rates by 2.3-14.2%. Using 10% inoculation material reduced the NH₃ emissions by 39.1-54.3% regardless of inoculation method, inoculation amount had a greater effect on NH₃ emissions than that of inoculation method. The covering inoculation material could reduce 65.08% H₂S emission, the inoculation method had a greater effect on H₂S emissions than that of inoculation amount. Given the comprehensive considerations of emission reduction and biodrying performance, a covering of 10% inoculating material is a suitable approach to improve biodrying performance and mitigate odorous gases emissions.

Energetic enhancement of thermal assistance in the cooling stage of biodrying by stimulating microbial degradation

In this study, thermal assistance was employed in the cooling stage of conventional biodrying. The results indicated that thermal assistance greatly enhanced water removal with improved vapor-carrying capacity of air-flow, and rapidly decreased moisture contents (MCs) from 45.15% to 49.42% to 15.20-25.85% in 6 days, which were much lower than those of conventional biodrying (CB, 34.90-40.85%). More importantly, a synergistic enhancement of physical and biological effects was observed in thermally assisted biodrying (TB) in terms of stimulated enzymes activity and microbial metabolism (higher oxygen uptake rate and degradation coefficient k). Among the degraded organics, lignocellulose was noted to be important for bio-heat generation in cooling stages, especially for straw as bulking agent. Heat balance results suggested that small fractions of thermal heat (19.76-24.73%) were required to upgrade CB processes for water evaporation with higher energy efficiency. Based on economic viability analysis and with consideration of the further drying for CB products, thermally assisted biodrying presented more economic benefits with less investment and shorter payback period. This research provided an efficient engineering approach to upgrade the cooling stage of conventional biodrying with low external heat cost.

Thermally assisted bio-drying of food waste: Synergistic enhancement and energetic evaluation

Recently, bio-drying is becoming a promising method to treat the slurry-type food waste together with recovering refused derived fuels (RDFs). In practice, however, conventional process frequently encountered low temperature and inefficient drying performance due to the low microbial activity and organics degradability. In order to improve bio-drying performance, in this study, an externally thermal assistant strategy was proposed to increase water evaporation and stimulate microbial degradability. Based on this idea, a series of experiments were conducted to establish, evaluate and optimize the thermally assisted bio-drying system. It was found that staged heating acclimation was an effective strategy to obtain a superior thermophilic inoculum with high metabolic activity and microbial consortia. In thermally assisted bio-drying process, an extremely high metabolic activity [cumulative OUR, 38.98 mg/(g TS·h)] was obtained, which was greatly higher than that of conventional bio-drying [19.74 mg/(g TS·h)]. Furthermore, thermally assisted bio-drying exhibited a high water-evaporation capacity as thermal drying (157.9 g vs. 147.8 g), which was 3-fold higher than conventional bio-drying. Heat balance calculation indicated that externally supplying a small fraction (12.94%) of thermal energy triggered conventional bio-drying, thus greatly promoting water removal with high energy utilization efficiency as conventional bio-drying (Q_evapo 60.30% vs. 64.62%). In addition, the increased air-flow rates greatly accelerated water removal with high bio-energy efficiencies, especially at 0.8 L·min^-1·kg^-1. The drying effect after 4 days was close to that of 20 days in conventional bio-drying. This research suggests that thermally assisted bio-drying is a promising approach to upgrade conventional bio-drying with high efficiency and low energy cost.