Food decay and offensive odorants: A comparative analysis among three types of food

In this study, a list of offensive odorants including reduced sulfur, carbonyls, nitrogenous, and volatile organic compounds was measured by the indirect (instrumental) method during the decay processes of three food types (snipe egg, mackerel, and squid). The strengths of the odor release were also quantified in terms of dilution-to-threshold (D/T) ratio based on the air dilution sensory test. To collect odor samples for each food type, decaying experiments were conducted in 100 mL throwaway syringes for 1 month. The results showed that ammonia had the largest mean ranging from 385 ppm (fish) to 554 ppm (egg). However, most odorants generally fell in the range of 0.01–10 ppm, regardless of food type. The odor strengths measured with the suprathreshold method in terms of average D/T values increased on the order of 33,520 (egg), 202,330 (fish), and 766,330 (squid). These results were highly comparable to the patterns of odor indices derived by empirical conversion of odorant concentration data. The overall results of this study thus suggest that a unique pattern of odor release develops among different odorants as well as food type.     

A mass transfer model of ammonia volatilisation from anaerobic digestate

Anaerobic digestion (AD) is becoming increasingly popular for treating organic waste. The methane produced can be burned to generate electricity and the digestate, which is high in mineral nitrogen, can be used as a fertiliser. In this paper we evaluate potential losses of ammonia via volatilisation from food waste anaerobic digestate using a closed chamber system equipped with a sulphuric acid trap. Ammonia losses represent a pollution source and, over long periods could reduce the agronomic value of the digestate. Observed ammonia losses from the experimental system were linear with time. A simple non-steady-state partitioning model was developed to represent the process. After calibration, the model was able to describe the behaviour of ammonia in the digestate and in the trap very well. The average rate of volatilisation was approximately 5.2 g N m^?2 week^?1. The model was used to extrapolate the findings of the laboratory study to a number of AD storage scenarios. The simulations highlight that open storage of digestate could result in significant losses of ammonia to the atmosphere. Losses are predicted to be relatively minor from covered facilities, particularly if depth to surface area ratio is high.     

Anaerobic co-digestion of food waste and landfill leachate in single-phase batch reactors

In order to investigate the effect of raw leachate on anaerobic digestion of food waste, co-digestions of food waste with raw leachate were carried out. A series of single-phase batch mesophilic (35 ± 1 °C) anaerobic digestions were performed at a food waste concentration of 41.8 g VS/L. The results showed that inhibition of biogas production by volatile fatty acids (VFA) occurred without raw leachate addition. A certain amount of raw leachate in the reactors effectively relieved acidic inhibition caused by VFA accumulation, and the system maintained stable with methane yield of 369–466 mL/g VS. Total ammonia nitrogen introduced into the digestion systems with initial 2000–3000 mgNH₄–N/L not only replenished nitrogen for bacterial growth, but also formed a buffer system with VFA to maintain a delicate biochemical balance between the acidogenic and methanogenic microorganisms. UV spectroscopy and fluorescence excitation–emission matrix spectroscopy data showed that food waste was completely degraded.We concluded that using raw leachate for supplement water addition and pH modifier on anaerobic digestion of food waste was effective. An appropriate fraction of leachate could stimulate methanogenic activity and enhance biogas production.     

Separation of polycarbonate and acrylonitrile–butadiene–styrene waste plastics by froth flotation combined with ammonia pretreatment

The objective of this research is flotation separation of polycarbonate (PC) and acrylonitrile–butadiene–styrene (ABS) waste plastics combined with ammonia pretreatment. The PC and ABS plastics show similar hydrophobicity, and ammonia treatment changes selectively floatability of PC plastic while ABS is insensitive to ammonia treatment. The contact angle measurement indicates the dropping of flotation recovery of PC is ascribed to a decline of contact angle. X-ray photoelectron spectroscopy demonstrates reactions occur on PC surface, which makes PC surface more hydrophilic. Separation of PC and ABS waste plastics was conducted based on the flotation behavior of single plastic. At different temperatures, PC and ABS mixtures were separated efficiently through froth flotation with ammonia pretreatment for different time (13 min at 23 °C, 18 min at 18 °C and 30 min at 23 °C). For both PC and ABS, the purity and recovery is more than 95.31% and 95.35%, respectively; the purity of PC and ABS is up to 99.72% and 99.23%, respectively. PC and ABS mixtures with different particle sizes were separated effectively, implying that ammonia treatment possesses superior applicability.     

Effect of temperature on VFA’s and biogas production in anaerobic solubilization of food waste

The effectiveness of methane fermentation treatment used in food waste processing is currently limited by solubilization and acidogenesis. In efforts to improve the treatment process, this study examined the effects of temperature on solubilization and acidogenesis. The solubilization rate of food waste, which was based on suspended solid removal, was 47.5%, 62.2%, 70.0%, 72.7%, 56.1% and 45.9% at 15 °C, 25 °C, 35 °C, 45 °C, 55 °C and 65 °C, respectively. Solubilization rate was accelerated from the middle to late experimental periods under mesophilic (35 °C and 45 °C) conditions. In contrast, overall solubilization rate was significantly lower under thermophilic (55 °C and 65 °C) conditions than under mesophilic conditions, although solubilization occurred rapidly in the early experimental period. The production of biogas was high under mesophilic conditions of 35 °C and 45 °C, at 64.7 and 62.7 mL/g-VS, respectively, while it was scarce under thermophilic conditions. Solubilization of food waste was accelerated under both mesophilic and thermophilic conditions; however, solubilization rate was observed to be particularly high under mesophilic conditions, and a shortening of the hydraulic retention time is expected under thermophilic conditions.     

Temperature and deactivation of microbial faecal indicators during small scale co-composting of faecal matter

Small scale co-composting of faecal matter from dry toilet systems with shredded plant material and food waste was investigated in respect to heat development and deactivation of faecal indicators under tropical semiarid conditions. Open (uncovered) co-composting of faecal matter with shredded plant material alone did not generate temperatures high enough (<55 °C) to reduce the indicators sufficiently. The addition of food waste and confinement in chambers, built of concrete bricks and wooden boards, improved the composting process significantly. Under these conditions peak temperatures of up to 70 °C were achieved and temperatures above 55 °C were maintained over 2 weeks. This temperature and time is sufficient to comply with international composting regulations. The reduction of Escherichia coli, Enterococcus faecalis and Salmonella senftenberg in test containment systems placed in the core of the compost piles was very efficient, exceeding 5 log₁₀-units in all cases, but recolonisation from the cooler outer layers appeared to interfere with the sanitisation efficiency of the substrate itself. The addition of a stabilisation period by extending the composting process to over 4 months ensured that the load of E. coli was reduced to less than 10³ cfu^?g and salmonella were undetectable.     

Tank-connected food waste disposer systems – Current status and potential improvements

An unconventional system for separate collection of food waste was investigated through evaluation of three full-scale systems in the city of Malmö, Sweden. Ground food waste is led to a separate settling tank where food waste sludge is collected regularly with a tank-vehicle. These tank-connected systems can be seen as a promising method for separate collection of food waste from both households and restaurants. Ground food waste collected from these systems is rich in fat and has a high methane potential when compared to food waste collected in conventional bag systems. The content of heavy metals is low. The concentrations of N-tot and P-tot in sludge collected from sedimentation tanks were on average 46.2 and 3.9 g/kg TS, equalling an estimated 0.48 and 0.05 kg N-tot and P-tot respectively per year and household connected to the food waste disposer system. Detergents in low concentrations can result in increased degradation rates and biogas production, while higher concentrations can result in temporary inhibition of methane production. Concentrations of COD and fat in effluent from full-scale tanks reached an average of 1068 mg/l and 149 mg/l respectively over the five month long evaluation period. Hydrolysis of the ground material is initiated between sludge collection occasions (30 days). Older food waste sludge increases the degradation rate and the risks of fugitive emissions of methane from tanks between collection occasions. Increased particle size decreases hydrolysis rate and could thus decrease losses of carbon and nutrients in the sewerage system, but further studies in full-scale systems are needed to confirm this.     

Optimization of micro-aeration intensity in acidogenic reactor of a two-phase anaerobic digester treating food waste

Micro-aeration is known to promote the activities of hydrolytic exo-enzymes and used as a strategy to improve the hydrolysis of particulate substrate. The effect of different micro-aeration rates, 0, 129, 258, and 387 L-air/kg TS/d (denoted as LBR-AN, LBR-6h, LBR-3h and LBR-2h, respectively) on the solubilization of food waste was evaluated at 35 °C in four leach bed reactors (LBR) coupled with methanogenic upflow anaerobic sludge blanket (UASB) reactor. Results indicate that the intensity of micro-aeration influenced the hydrolysis and methane yield. Adequate micro-aeration intensity in LBR-3h and LBR-2h significantly enhanced the carbohydrate and protein hydrolysis by 21–27% and 38–64% respectively. Due to the accelerated acidogenesis, more than 3-fold of acetic acid and butyric acid were produced in LBR-3h as compared to the anaerobic treatment LBR-AN resulting in the maximum methane yield of 0.27 L CH₄/g VS_added in the UASB. The performance of LBR-6h with inadequate aeration was similar to that of LBR-AN with a comparable hydrolysis degree. Nevertheless, higher aeration intensity in LBR-2h was also unfavorable for methane yield due to significant biomass generation and CO₂ respiration of up to 18.5% and 32.8% of the total soluble hydrolysate, respectively. To conclude, appropriate micro-aeration rate can promote the hydrolysis of solid organic waste and methane yield without undesirable carbon loss and an aeration intensity of 258 L-air/kg TS/d is recommended for acidogenic LBR treating food waste.     

High solids co-digestion of food and landscape waste and the potential for ammonia toxicity

A pilot-scale study was completed to determine the feasibility of high-solids anaerobic digestion (HSAD) of a mixture of food and landscape wastes at a university in central Pennsylvania (USA). HSAD was stable at low loadings (2 g COD/L-day), but developed inhibitory ammonia concentrations at high loadings (15 g COD/L-day). At low loadings, methane yields were 232 L CH₄/kg COD fed and 229 L CH₄/kg VS fed, and at high loadings yields were 211 L CH₄/kg COD fed and 272 L CH₄/kg VS fed. Based on characterization and biodegradability studies, food waste appears to be a good candidate for HSAD at low organic loading rates; however, the development of ammonia inhibition at high loading rates suggests that the C:N ratio is too low for use as a single substrate. The relatively low biodegradability of landscape waste as reported herein made it an unsuitable substrate to increase the C:N ratio. Codigestion of food waste with a substrate high in bioavailable carbon is recommended to increase the C:N ratio sufficiently to allow HSAD at loading rates of 15 g COD/L-day.     

Metal toxicity assessment of mobile phone parts using Milli Q water

Environmentally safe disposal of end-of-life (EoL) or discarded mobile phone is a serious problem on account of their ever increasing number and toxic metals contents. In the present work, metal toxicity of mobile phone plastics, printed wire boards (PWBs) and batteries were assessed through dynamic batch leaching using Milli Q (MQ) water. Phone plastics failed Toxicity Characterization Leaching Procedure (TCLP) and Waste Extraction Test (WET) for Pb as the cumulative amount of Pb leached from plastics (5.33 mg/l) exceeded the regulatory limits (5.0 mg/l) used in characterizing a waste as hazardous. Similarly, the average cumulative amount (21.83 mg/l) of Ni leached from PWBs exceeded the regulatory limit of 20 mg/l and thus PWBs failed WET. Metals leached from batteries in small amounts (Cr: 0.40 mg/l and Ni: 0.15 mg/l). The presence of Fe in the batteries and its precipitation as oxides/hydroxides in the leaching solution hindered the leaching of other metals in MQ water. Both plastics and PWBs should be treated as hazardous waste and should not be disposed in open landfills. Further, MQ water leaching could provide good simulation of metals leaching from the mobile phones disposed at landfill sites.     

Lactic acid production with undefined mixed culture fermentation of potato peel waste

Potato peel waste (PPW) as zero value byproduct generated from food processing plant contains a large quantity of starch, non-starch polysaccharide, lignin, protein, and lipid. PPW as one promising carbon source can be managed and utilized to value added bioproducts through a simple fermentation process using undefined mixed cultures inoculated from wastewater treatment plant sludge. A series of non-pH controlled batch fermentations under different conditions such as pretreatment process, enzymatic hydrolysis, temperature, and solids loading were studied. Lactic acid (LA) was the major product, followed by acetic acid (AA) and ethanol under fermentation conditions without the presence of added hydrolytic enzymes. The maximum yields of LA, AA, and ethanol were respectively, 0.22 g g^?1, 0.06 g g^?1, and 0.05 g g^?1. The highest LA concentration of 14.7 g L^?1 was obtained from a bioreactor with initial solids loading of 60 g L^?1 at 35 °C.     

Membrane bioreactor technology: A novel approach to the treatment of compost leachate

Compost leachate forms during the composting process of organic material. It is rich in oxidizable organics, ammonia and metals, which pose a risk to the environment if released without proper treatment. An innovative method based on the membrane bioreactor (MBR) technology was developed to treat compost leachate over 39 days. Water quality parameters, such as pH, dissolved oxygen, ammonia, nitrate, nitrite and chemical oxygen demand (COD) were measured daily. Concentrations of caffeine and metals were measured over the course of the experiment using gas chromatography – mass spectrometry (GC/MS) and inductively coupled plasma – mass spectrometry (ICP–MS) respectively. A decrease of more than 99% was achieved for a COD of 116 g/L in the initial leachate. Ammonia was decreased from 2720 mg/L to 0.046 mg/L, while the nitrate concentration in the effluent rose to 710 mg/L. The bacteria in the MBR system adjusted to the presence of the leachate, and increased 4 orders of magnitude. Heavy metals were removed by at least 82.7% except copper. These successful results demonstrated the membrane bioreactor technology is feasible, efficient method for the treatment of compost leachate.     

Optimization of thermo-chemical pretreatment and enzymatic hydrolysis of kitchen wastes

The use of abundant waste materials with high carbohydrate content may contribute substantially to reduction of biofuels production cost. The present study aimed at optimizing the combined effect of thermo-chemical pretreatment and enzymatic hydrolysis of kitchen wastes (KW) for maximizing the production of fermentable soluble sugars. To this end, acid pretreatment of KW samples was performed with hydrochloric acid (0–3% HCl) at 30–100 °C for 0–120 min treatment time. Alternatively, alkaline pretreatment of KW samples was performed with potassium hydroxide solution (0–11%) at constant temperature and time (0 °C and 20 min, respectively). KOH pretreatment at such conditions targets to degrade the resistant starch of KW samples. Both acid and alkaline pretreatments were followed by addition of variable levels of enzyme dosage (0–3.6% v/v α-amylase and 0–3.2% v/v amyloglucosidase-AMG) at constant pH, temperature and time (pH = 5, T = 50 °C and t = 30 min, respectively). Based on our results, glucose concentration increased by ～300% after pretreatment with either acid or KOH in combination with enzymatic hydrolysis (2% HCl, 85 °C, 80 min, 0.1% α-amylase, AMG, and 1% KOH, 0 °C, 20 min, 1.1% α-amylase, 0.4% AMG) compared to raw (untreated) KW. Estimating the different Y_G yields at KW loading of 5%, an increase of 192% and 121% for total soluble monosugars and total soluble sugars, respectively, was succeeded compared to untreated KW. The effect of solids loading on the obtained sugar yields using the optimum conditions for thermo-chemical pretreatment followed by enzymatic hydrolysis was also tested resulting to 27.5% increase of the soluble glucose yield when half of the solids loading (2.5%) was used. A decrease of total soluble sugars yield by 32.2% was observed when solely acid hydrolysis at optimum conditions from our previous study was applied at 30% solids loading.     

Co-treatment of fruit and vegetable waste in sludge digesters. An analysis of the relationship among bio-methane generation,process stability and digestate phytotoxicity

The co-digestion of a variable amount of fruit and vegetable waste in a waste mixed sludge digester was investigated using a pilot scale apparatus. The organic loading rate (OLR) was increased from 1.46 kg VS/m³ day to 2.8 kg VS/m³ day. The hydraulic retention time was reduced from 14 days to about 10 days. Specific bio-methane production increased from about 90 NL/kg VS to the maximum value of about 430 NL/kg VS when OLR was increased from 1.46 kg VS/m³ day to 2.1 kg VS/m³ day. A higher OLR caused an excessive reduction in the hydraulic retention time, enhancing microorganism wash out. Process stability evaluated by the total volatile fatty acids concentration (mg/l) to the alkalinity buffer capacity (eq. mg/l CaCO₃) ratio (i.e. FOS/TAC) criterion was <0.1 indicating high stability for OLR <2.46 kg VS/m³ day. For higher OLR, FOS/TAC increased rapidly. Residual phytotoxicty of the digestate evaluated by the germination index (GI) (%) was quite constant for OLR < 2.46 kg VS/m³ day, which is lower than the 60% limit, indicating an acceptable toxicity level for crops. For OLR > 2.46 kg VS/m³ day, GI decreased rapidly. This corresponding trend between FOS/TAC and GI was further investigated by the definition of the GI ratio (GIR) parameter. Comparison between GIR and FOS/TAC suggests that GI could be a suitable criterion for evaluating process stability.     

Simultaneous recovery of vanadium and nickel from power plant fly-ash: Optimization of parameters using response surface methodology

Simultaneous recovery of vanadium (V) and nickel (Ni), which are classified as two of the most hazardous metal species from power plant heavy fuel fly-ash, was studied using a hydrometallurgical process consisting of acid leaching using sulfuric acid. Leaching parameters were investigated and optimized in order to maximize the recovery of both vanadium and nickel. The independent leaching parameters investigated were liquid to solid ratio (S/L) (5–12.5 wt.%), temperature (45–80 °C), sulfuric acid concentration (5–25 v/v%) and leaching time (1–5 h). Response surface methodology (RSM) was used to optimize the process parameters. The most effective parameter on the recovery of both elements was found to be temperature and the least effective was time for V and acid concentration for Ni. Based on the results, optimum condition for metals recovery (actual recovery of ca.94% for V and 81% for Ni) was determined to be solid to liquid ratio of 9.15 wt.%, temperature of 80 °C, sulfuric acid concentration of 19.47 v/v% and leaching time of 2 h. The maximum V and Ni predicted recovery of 91.34% and 80.26% was achieved.     

Performance evaluation of a completely stirred anaerobic reactor treating pig manure at a low range of mesophilic conditions

Many Chinese biogas plants run in the lower range of mesophilic conditions. This study evaluated the performance of a completely stirred anaerobic reactor treating pig manure at different temperatures (20, 28 and 38 °C). The start-up phase of the reactor at 20 °C was very long and extremely poor performance was observed with increasing organic loading rate (OLR). At an OLR of 4.3 g ODM L^?1 d^?1, methane production at 28 °C was comparable (3% less) with that at 38 °C, but the risk of acidification was high at 28 °C. At low OLR (1.3 g ODM L^?1 d^?1), the biogas process appeared stable at 28 °C and gave same methane yields as compared to the reactor operating at 38 °C. The estimated sludge yield at 28 °C was 0.065 g VSS g^?1 COD_removed, which was higher than that at 38 °C (0.016 g VSS g^?1 COD_removed).     

Sulfide emissions from different areas of a municipal solid waste landfill in China

Degradation of municipal solid waste in landfills generates sulfide compounds, which are considered one of the main sources of odor emissions. Field sampling was conducted at surfaces of operating, inoperative, and soil-covered areas of a landfill site in northern China to characterize the sulfide compounds. The results showed that dimethyl disulfide dominated the sulfide compounds, accounting for up to 73.6% of the total detected sulfide. With the biggest odor concentration of 365, diethyl sulfide was the most significant sulfide compound. The estimated sulfide emission rates at surfaces of operating and soil-covered areas were similar, and the emission rate of dimethyl disulfide at Surface of Operating Area was up to 345.9 μg/m³ h. Dimethyl disulfide could be released from the fresh waste, and its normalized concentration at 0.2 m beneath operating surface was 10.4 times that at 0.4 m.     

Nitrogen removal from landfill leachate via ex situ nitrification and sequential in situ denitrification

Ex situ nitrification and sequential in situ denitrification represents a novel approach to nitrogen management at landfills. Simultaneous ammonia and organics removal was achieved in a continuous stirred tank reactor (CSTR). The results showed that the maximum nitrogen loading rate (NLR) and the maximum organic loading rate (OLR) was 0.65 g N l^?1 d^?1 and 3.84 g COD l^?1 d^?1, respectively. The ammonia and chemical oxygen demand (COD) removal was over 99% and 57%, respectively. In the run of the CSTR, free ammonia (FA) inhibition and low dissolved oxygen (DO) were found to be key factors affecting nitrite accumulation. In situ denitrification was studied in a municipal solid waste (MSW) column by recalculating nitrified leachate from CSTR. The decomposition of MSW was accelerated by the recirculation of nitrified leachate. Complete reduction of total oxidized nitrogen (TON) was obtained with maximum TON loading of 28.6 g N t^?1 TS d^?1 and denitrification was the main reaction responsible. Additionally, methanogenesis inhibition was observed while TON loading was over 11.4 g N t^?1 TS d^?1 and the inhibition was enhanced with the increase of TON loading.     

Full-scale blending treatment of fresh MSWI leachate with municipal wastewater in a wastewater treatment plant

Fresh leachate, generated in municipal solid waste incineration (MSWI) plants, contains various pollutants with extremely high strength organics, which usually requires expensive and complex treatment processes. This study investigated the feasibility of blending treatment of MSWI leachate with municipal wastewater. Fresh MSWI leachate was pretreated by coagulation–flocculation with FeCl₃ 2 g/L and CaO 25 g/L, plate-and-frame filter press, followed by ammonia stripping at pH above 12. After that, blending treatment was carried out in a full-scale municipal wastewater treatment plant (WWTP) for approximately 3 months. Different operational modes consisting of different pretreated leachate and methanol addition levels were tested, and their performances were evaluated. Results showed that throughout the experimental period, monitored parameters in the WWTP effluent, including COD (<60 mg/L), BOD₅ (<20 mg/L), ammonium (<8 mg/L), phosphorus (<1.5 mg/L) and heavy metals, generally complied with the Chinese sewage discharged standard. Under the experimental conditions, a certain amount of methanol was needed to fulfill TN removal. An estimation of the operation cost revealed that the expenditure of blending treatment was much lower than the total costs of respective treatment of MSWI leachate and municipal wastewater. The outcomes indicated that blending treatment could not only improve the treatability of the MSWI leachate, but also reduce the treatment cost of the two different wastewaters.     

Optimization of thermo-chemical hydrolysis of kitchen wastes

Municipal Solid Wastes (MSWs) in Greece consist mainly of fermentable organic material such as food scraps (～50%) and paper residuals (～20%). The aim of this work was to study the thermo-chemical pretreatment of the kitchen waste (KW) fraction of MSW focusing on biotechnological exploitation of pretreated wastes for biofuel production. A representative sample of municipal food residues was derived by combining weighted amounts of each individual type of residue recognized in daily samples obtained from the University of Patras’ students restaurant located at the Students Residence Hall (Greece). Chemical pretreatment experiments of the representative KW sample were performed using several types of chemical solutions (i.e. H₂SO₄, HCl, NaOH, H₂SO₃) of different solute concentration (0.7%, 1.5%, 3%) at three temperatures (50, 75, 120 °C) and a range of residence times (30–120 min). Optimized results proved that chemical pretreatment of KW, using either 1.12% HCl for 94 min or 1.17% HCl for 86 min (at 100 °C), increased soluble sugars concentration by 120% compared to untreated KW. The increase of soluble sugars was mainly attributed to the mono-sugars glucose and fructose.