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, DTGmax 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.

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.

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.

Performance of co-composting sewage sludge and organic fraction of municipal solid waste at different proportions.

In this study, the co-composting performance of sewage sludge (SS) and organic fraction of municipal solid waste (OFMSW) at different proportions was investigated. Cornstalk was added at 15% (of total wet weight) to improve the co-composting process. Results show that higher SS proportion could rapidly initialize the co-composting process; while increasing OFMSW percentage enhanced organic content for biodegradation, thus prolonging the thermophilic period and increasing the humification degree during co-composting. However, excessive OFMSW required longer co-composting period to ensure a desirable compost maturity and quality. Over 15days of rapid co-composting, adding 55-85% OFMSW aggravated compost quality by increasing the compost salinity (3.5-4.6mScm-1) and plant toxicity (indicated by the low germination index of 54.1-77.3%). Moreover, different proportions of SS and OFMSW varied gaseous emissions during co-composting. Our results suggested that OFMSW should be less than 55% for rapid co-composting with SS and cornstalk.

Effects of adding bulking agents on the biodrying of kitchen waste and the odor emissions produced.

The effects of adding a bulking agent on the performance and odor emissions (ammonia and eight sulfur-containing odorous compounds) when biodrying kitchen waste were investigated. Three treatments were considered: the addition of either cornstalks (CS) or wood peat (WP) to kitchen waste as a bulking agent before biodrying, and a control treatment (CK). The water-removal rates for CK, CS, and WP treatments were 0.35, 0.56, and 0.43kg/kg, respectively. Addition of bulking agents to kitchen waste produced less leachate, higher moisture-removal rates, and lower consumption of volatile solids. The CS treatment had the highest biodrying index (4.07), and those for the WP and CK treatments were 3.67 and 1.97, respectively. Adding cornstalks or wood peat decreased NH3 emissions by 55.8% and 71.7%, respectively. Total sulfur losses were 3.6%-21.6% after 21days biodrying, and H2S and Me2SS were the main (>95%) sulfur compounds released. The smallest amounts of sulfur-containing odorous compounds were emitted when cornstalks were added, and adding cornstalks and wood peat decreased total sulfur losses by 50.6%-64.8%.

Effects of the aeration pattern, aeration rate, and turning frequency on municipal solid waste biodrying performance.

Interactive influences of the aeration pattern, aeration rate, and turning frequency on municipal solid waste biodrying performance were investigated. Energy and water mass balances were used to identify the main water-removal routes and determine the amount of energy used and efficiency. Changing the aeration pattern and turning frequency did not significantly affect biodrying performance when the other conditions and total aeration volume were constant. The total aeration volume controlled the pile temperature and evaporation, making it the main factor affecting water loss during biodrying. A continuous aeration rate of 0.5 L kg-1 dry matter·min-1 gave the best biodrying performance (the highest water-removal rate, biodrying index, and sorting efficiency, 0.5 kg kg-1, 4.12, and 86.87%, respectively, and the highest lower heat value (LHV) and heat utilization rate, 9440 kJ kg-1 and 68.3%, respectively). There was an optimum aeration rate, water loss reaching a maximum at an aeration rate of 0.5 L kg-1 DM·min-1 and not increasing further as the aeration rate increased further. Lower aeration rates gave higher volatile solid degradation rates. The effects of turning could be achieved by increasing the aeration rate. The recommended biodrying parameters are continuous aeration at an aeration rate of 0.5 L kg-1 dry matter min-1 and one turn every 3 d.

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

This study investigated the performance of co-biodrying sewage sludge and organic fraction of municipal solid waste (OFMSW) at different proportions. Cornstalk was added at 15% (of total wet weight) as the bulking agent. Results show that increasing OFMSW percentage promoted the biodegradation of organic matter, thus enhancing the temperature integration value and water removal to above 75% during sludge and OFMSW co-biodrying. In particular, adding more OFMSW accelerated the biodegradation of soluble carbohydrates, lignins, lipids, and amylums, resulting in more organic loss and thus lower biodrying index (3.3-3.7 for 55-85% OFMSW). Water balance calculation indicated that evaporation was the main mechanism for water removal. Heat used for water evaporation was 37.7-48.6% of total heat consumption during co-biodrying. Our results suggest that sludge and OFMSW should be mixed equally for their efficient co-biodrying.

Seed germination test for toxicity evaluation of compost: Its roles, problems and prospects.

Compost is commonly used for the growth of plants and the remediation of environmental pollution. It is important to evaluate the quality of compost and seed germination test is a powerful tool to examine the toxicity of compost, which is the most important aspect of the quality. Now the test is widely adopted, but the main problem is that the test results vary with different methods and seed species, which limits the development and application of it. The standardization of methods and the modelization of seeds can contribute to solving the problem. Additionally, according to the probabilistic theory of seed germination, the error caused by the analysis and judgment methods of the test results can be reduced. Here, we reviewed the roles, problems and prospects of the seed germination test in the studies of compost.

Effects of woody peat and superphosphate on compost maturity and gaseous emissions during pig manure composting.

This study investigated the effect of calcium superphosphate on compost maturity and gaseous emissions during pig manure composting with woody peat as the bulking agent. Two treatments were conducted with or without the addition of calcium superphosphate (10% dry weight of the composting mass), which were denoted as the control and superphosphate-amended treatment, respectively. Results show that the composting temperature of both treatments was higher than 50°C for more than 5days, which is typically required for pathogen destruction during manure composting. Compared to the control treatment, the superphosphate-amended treatment increased the emission of nitrogen oxide, but reduced the emission of methane, ammonia and hydrogen sulfide by approximately 35.5%, 37.9% and 65.5%, respectively. As a result, the total greenhouse gas (GHG) emission during manure composting was reduced by nearly 34.7% with the addition of calcium superphosphate. The addition of calcium superphosphate increased the content of humic acid (indicated by E4/E6 ratio). Nevertheless, the superphosphate-amended treatment postponed the biological degradation of organic matter and produced the mature compost with a higher electrical conductivity in comparison with the control treatment.

Effects of adding bulking agents on biostabilization and drying of municipal solid waste.

The influence of adding a bulking agent on the bio-stabilization and drying of municipal solid waste (MSW) was investigated. Three treatments were considered: the addition of either cornstalks or wood peat to MSW as a bulking agent before bio-drying and a control treatment that contained no bulking agent. Addition of bulking agents to MSW produced less leachate, higher moisture-removal rates, and consumed less volatile solids. Bulking with cornstalks achieved the highest water-removal rate (0.58-0.65kgkg-1). The extent of organic degradation was related to temperature integration during bio-drying. Lipids and cellulose were the main components of organic losses in all treatments and adding a bulking agent changed the sequence and extent of degradation of biochemical components. The bio-drying index values were 1.75, 3.18, and 2.64 for MSW alone, MSW with cornstalks, and MSW with wood peat, respectively. Evaporation heat was the main component of heat consumption, accounting for 58.1-60.7% of the total energy consumption.

Control of dimethyl sulfide and dimethyl disulfide odors during pig manure composting using nitrogen amendment.

Effects of nitrogen electron acceptors on dimethyl sulfide (Me2S) and dimethyl disulfide (Me2SS) odor emission during composting was investigated. The chemicals and doses used included sodium nitrate (NO3- at 10mM, 20mM and 40mM), sodium nitrite (NO2- at 10mM, 20mM and 40mM) and sodium nitrite (10mM, 20mM and 40mM) with hexaammonium heptamolybdate tetrahydrate (HHT). The results showed that the addition of these chemicals restricted the emission of Me2S and Me2SS. The emission reduction effect of NO2- was greater than NO3- at the same dosage. A greater reduction was observed when HHT was also added. With 2mM HHT+40mM NO2- addition, the emission of Me2S and Me2SS was reduced by 92.3% and 82.3%, respectively. Comparison of compost maturity indices for treated and untreated composts indicated that none of the additives adversely affected compost quality. These results indicate that nitrogen chemical addition may provide an efficient method to control sulfur odors during composting.

Influence of aeration on volatile sulfur compounds (VSCs) and NH3 emissions during aerobic composting of kitchen waste.

This study investigates the influence of aeration on volatile sulfur compounds (VSCs) and ammonia (NH3) emissions during kitchen waste composting. Aerobic composting of kitchen waste and cornstalks was conducted at a ratio of 85:15 (wet weight basis) in 60L reactors for 30days. The gas emissions were analyzed with force aeration at rates of 0.1 (A1), 0.2 (A2) and 0.3 (A3) L (kgDMmin)-1, respectively. Results showed that VSCs emission at the low aeration rate (A1) was more significant than that at other two rates (i.e., A2 and A3 treatment), where no considerable emission difference was observed. On the other hand, NH3 emission reduced as the aeration rate decreased. It is noteworthy that the aeration rate did not significantly affect the compost quality. These results suggest that the aeration rate of 0.2L (kgDMmin)-1 may be applied to control VSCs and NH3 emissions during kitchen waste composting.

Effects of aeration rate on maturity and gaseous emissions during sewage sludge composting.

This study investigated effects of aeration rate (AR) on maturity and gaseous emissions during sewage sludge composting, sewage sludge and corn stalks as the bulking agent were co-composted at different ARs (0.1, 0.2, 0.3L·kg(-1) dry matter (DM)·min(-1)). The thermophilic phase for the low and moderate AR treatments was able meet sanitation requirements, but too short to meet sanitation requirements in the high AR treatment. The high AR treatment was significantly different from the other treatments, and had the lowest electrical conductivity and highest E4/E6(absorbance ratio of wavelength 465 and 665nm). The AR influences the nitrogen variations; high AR compost had the highest NH4(+)-N content and lowest NOx(-)-N content. The AR was the main factor influencing compost stability, but the AR had little impact on pH and the germination index. The moderate AR treatment had the highest NH3 emissions during composting, while the low AR treatment had the highest CH4 and N2O emissions. Based on our comprehensive investigation, the recommended AR for sludge composting is 0.2L·kg(-1) DM·min(-1).

Effects of mix ratio, moisture content and aeration rate on sulfur odor emissions during pig manure composting.

Sulfur compounds in swine manure can cause odor emissions during composting if conditions are not conducive to their rapid oxidation and degradation. In this study, the effects of controllable composting process variables on sulfur odor emissions were investigated. These included pig manure to corn stalk mix ratio (0.7:1, 1.5:1 and 2.2:1dw basis), initial moisture content (60%, 65%, 70% and 75%) and aeration rate (1.0, 2.0, 3.0 and 4.0m(3)m(-3)h(-1)). The compounds measured were carbonyl sulfide, carbon disulfide, hydrogen sulfide, methyl mercaptan, ethyl mercaptan, diethyl sulfide, dimethyl sulfide (Me2S) and dimethyl disulfide (Me2SS). The results showed that total sulfur losses ranged from 3.9% to 18.3% after 26days of composting. Me2S and Me2SS were the primary (>59.61%) sulfur compounds released during this period. After turning, emission rates of both Me2S and Me2SS increased. Emissions of the other six sulfur compounds were low and inconsistent during composting. Within the compost, feedstock mix ratio significantly influenced the concentration of Me2SS, while aeration rate significantly affected Me2S concentration (p<0.05). Moisture content did not have a significant effect on the concentrations of either of these two compounds. Concentrations of sulfur odor compounds were the lowest at the highest aeration rate. Therefore, high aeration rates during the thermophilic phase, especially after turning, are recommended to minimize sulfur odors produced during swine manure composting.

Solid state anaerobic co-digestion of tomato residues with dairy manure and corn stover for biogas production.

Solid-state anaerobic co-digestion of tomato residues with dairy manure and corn stover was conducted at 20% total solids under 35°C for 45days. Results showed digestion of mixed tomato residues with dairy manure and corn stover improved methane yields. The highest VS reduction (46.2%) and methane yield (415.4L/kg VSfeed) were achieved with the ternary mixtures of 33% corn stover, 54% dairy manure, and 13% tomato residues, lead to a 0.5-10.2-fold higher than that of individual feedstocks. Inhibition of volatile fatty acids (VFAs) to biogas production occurred when more than 40% tomato residues were added. The results indicated that ternary mixtures diluted the inhibitors that would otherwise cause inhibition in the digestion of tomato residues as a mono-feedstock.

Use of additive and pretreatment to control odors in municipal kitchen waste during aerobic composting.

The effects of adding a bulking agent and chemically pretreating municipal kitchen waste before aerobic composting were studied using a laboratory-scale system. The system used 20-L reactors and each test lasted 28days. The objective was to decrease NH3 and H2S emissions during composting. The bulking agent, dry cornstalks, was mixed with the kitchen waste to give a mixture containing 15% (wet weight) bulking agent. A combined treatment was also conducted, in which kitchen waste mixed with the bulking agent was pretreated with ferric chloride (FeCl3). Less leachate was produced by the composted kitchen waste mixed with bulking agent than by the kitchen waste alone, when the materials had reached the required maturity. The presence of cornstalks also caused less H2S to be emitted, but had little impact on the amount of NH3 emitted. The FeCl3 was found to act as an effective chemical flocculant, and its presence significantly decreased the amounts of NH3 and H2S emitted. Kitchen waste mixed with cornstalks and treated with FeCl3 emitted 42% less NH3 and 76% less H2S during composting than did pure kitchen waste.