Multi-step sequential batch two-phase anaerobic composting of food waste

Composting Processes for Agricultural Waste Management: A Comprehensive Review

Composting is the most adaptable and fruitful method for managing biodegradable solid wastes; it is a crucial agricultural practice that contributes to recycling farm and agricultural wastes. Composting is profitable for various plant, animal, and synthetic wastes, from residential bins to large corporations. Composting and agricultural waste management (AWM) practices flourish in developing countries, especially Pakistan. Composting has advantages over other AWM practices, such as landfilling agricultural waste, which increases the potential for pollution of groundwater by leachate, while composting reduces water contamination. Furthermore, waste is burned, open-dumped on land surfaces, and disposed of into bodies of water, leading to environmental and global warming concerns. Among AWM practices, composting is an environment-friendly and cost-effective practice for agricultural waste disposal. This review investigates improved AWM via various conventional and emerging composting processes and stages: composting, underlying mechanisms, and factors that influence composting of discrete crop residue, municipal solid waste (MSW), and biomedical waste (BMW). Additionally, this review describes and compares conventional and emerging composting. In the conclusion, current trends and future composting possibilities are summarized and reviewed. Recent developments in composting for AWM are highlighted in this critical review; various recommendations are developed to aid its technological growth, recognize its advantages, and increase research interest in composting processes.

Effect of liquid digestate recirculation on biogas production and enzyme activities for anaerobic digestion of corn straw

To accelerate the degradation of substrate, 50% liquid digestate recirculation (LDR) was used in the anaerobic digestion (AD) of corn straw. The effects of recirculation on the enzyme activities and biogas production were investigated by comparing with control reactor (Reactor_CK). During the AD process, the fermentation system with 50% LDR was more stable. The average biogas and methane production in Reactor_LDR were 7,891 mL·d^-1 and 347 mL CH₄·g^-1 VS_added·d^-1 respectively. The total volatile fatty acids (TVFAs) concentration in the two reactors both increased at first and then decreased with time. The LDR made the VFAs accumulation significant, especially propionic acid accumulation in 4 ∼ 16 days. The maximum peak value of cellulase, xylanase, dehydrogenase and coenzyme F₄₂₀ activities in Reactor_LDR were 0.51 mg·g^-1·h^-1, 0.29 mg·g^-1·h^-1, 4.88 mL·g^-1·h^-1 and 6.69 μmol·L^-1, respectively, which were higher than that in Reactor_CK. With or without recirculation, the concentration of TVFAs was positively correlated with cellulase, xylanase and dehydrogenase activities, while was negatively correlated with coenzyme F₄₂₀ activity. Besides, a very significant correlation existed between hydrolase and dehydrogenase activities and daily biogas production in Reactor_CK. And the peaks of cellulase, xylanase and dehydrogenase activities appeared ahead of the peak of daily biogas production with the LDR.

An improved procedure to assess the organic biodegradability and the biomethane potential of organic wastes for anaerobic digestion

In this study a fractionation procedure was developed and applied to evaluate the potential of some organic wastes (two cattle manures and two catch crops, fresh and after ensiling) for anaerobic digestion. This procedure was based on water extraction of the raw sample, which enabled the evaluation of the contributions of water-soluble and particulate phases to the investigated properties. Biomethane potential (BMP) and chemical oxygen demand (COD) were determined and used to assess the anaerobic biodegradability of raw materials. Analysis of structural carbohydrates, total Kjeldahl nitrogen, water-soluble carbohydrates, volatile fatty acids and pH were also included to explain the main phenomena involved in methane production from the tested biomass. Results show that the origin and the preparation mode had a significant impact on BMP distribution. Based on a COD balance, the biodegradability of the various feedstocks ranged from 45% to 75%. Biodegradability of fresh materials was negatively correlated with the sum of structural carbohydrates and lignin content. Among the feedstock used, the water-soluble phase represented 8-69% of the total COD and 7-46% to the total BMP. Solubilization of organic matter during ensiling was due to the production and accumulation of organic acids from particulate carbohydrates and organic nitrogen. This procedure detects kinetic and biodegradability differences among biomass and thus it can be useful for the design of anaerobic digestion plants. Furthermore, it can be applied to evaluate the efficiency of biomass pretreatments.

Characterization of odor emissions and microbial community structure during degradation of pig carcasses using the soil burial-composting method

A soil burial-composting method was proposed as a hybrid disposal method for infected carcasses. This is a modified soil burial technique that involves covering carcasses with compost to achieve a final compost bed of 1.0-1.2 m during the soil burial process. To evaluate the feasibility and applicability of the soil burial-composting method, a pilot-scale system was constructed to dispose of pig carcasses and monitored its performance for 346 days. Temperature around the pig carcasses in the compost bed increased gradually, and was in the range of 35-45 °C after 200 days. Mesophilic (Sporosarcina and Steroidobacter) and thermophilic (Truepera) bacteria were dominant in the compost bed. Based on odor gas profiling and the morphological properties of the carcasses excavated after 346 days, it was estimated that an advanced decay stage was reached after 243 days. Considering the results of previous studies, the carcass degradation rate achieved by soil burial-composting was faster than that of soil burial, but slower than that of the composting method. Sum of odor quotient (SOQ) in the upper soil bed was lower than the SOQ in the compost bed where the carcasses were buried. This result demonstrated that the upper soil bed functioned as a biofilter to mitigate odor gases emitted during degradation of the carcasses. The soil burial-composting disposal method is preferred over soil burial because the degradation of carcasses is faster, and over composting because odor complaints and compost usage can be minimized.

Effect of food to vegetable waste ratio on acidogenesis and methanogenesis during two-stage integration

The mixing ratio of food waste (FW) to vegetable waste (VW) (2:3 FW:VW ∼ 152.51 g VS and 2:1 FW:VW ∼ 137.03 gVS) was optimized using two-stage (LBR-UASB) experimental process depending upon volatile solid (VS) load. The effect of FW to VW ratio was studied in Leach Bed Reactor (LBR) towards leachate production. Results revealed that hydrolysis rate (73.11%), sCOD (3294.3 g/KgVS) and tVFA (2664 g/KgVS) yield was higher in 2:1 FW:VW ratio. Acetate, propionate, lactate and methane yield for 2:3 FW:VW (420 g/KgVS, 87 g/KgVS, 180 g/KgVS and 226.86 ml/gVS respectively) was different from 2:1 FW:VW (340 g/KgVS, 247 g/KgVS, 340 g/KgVS and 218.54 ml/gVS respectively). 2:3 FW:VW ratio depicted higher VS (53.96%) and COD (54.1%) removal than 2:1 FW:VW ratio 46.34% and 41.8% respectively. VW addition regulated pH, restricted propionate and lactate production with enhanced methanogenesis by improving acetate production in two-stage AD process which further boosted process stability and efficiency.

Effects of digestate recirculation on a two-stage anaerobic digestion system, particularly focusing on metabolite correlation analysis

Single-stage (S-N treatment) and two-stage anaerobic digestion with (T-R treatment) and without digestate recirculation (T-N treatment) for methane production using food waste (FW) were comparatively evaluated to examine the effects of digestate recirculation on anaerobic digestion (AD). Digestate recirculation positively affected the methane yield and organic loading rate (OLR). Metabolite correlation analysis revealed that a systematic hydrolysis degree of greater than 75% is crucial to achieve the complete recoverable yield of methane from FW. Digestate recirculation also markedly increased the system alkalinity, maintaining an optimum pH for methanogens. However, the ammonium accumulated by T-R treatment would destroy the metabolic balance between the hydrolytic bacteria and methanogens, especially at a critical OLR. Therefore, the appropriate control of two-stage AD systems with digestate recirculation is limited not only to OLR regulation but also to the prevention of ammonium accumulation.

Leachate flush strategies for managing volatile fatty acids accumulation in leach-bed reactors

In anaerobic leach-bed reactors (LBRs) co-digesting an easily- and a slowly-degradable substrate, the importance of the leachate flush both on extracting volatile fatty acids (VFAs) at the beginning of newly-started batches and on their consumption in mature reactors was tested. Regarding VFA extraction three leachate flush-rate conditions were studied: 0.5, 1 and 2Lkg^-1TSd^-1. Results showed that increasing the leachate flush-rate during the acidification phase is essential to increase degradation kinetics. After this initial phase, leachate injection is less important and the flush-rate could be reduced. The injection in mature reactors of leachate with an acetic acid concentration of 5 or 10gL^-1 showed that for an optimized VFA consumption in LBRs, VFAs should be provided straight after the methane production peak in order to profit from a higher methanogenic activity, and every 6-7h to maintain a high biogas production rate.

Anaerobic co-digestion of spent coffee grounds with different waste feedstocks for biogas production

Proper management of spent coffee grounds has become a challenging problem as the production of this waste residue has increased rapidly worldwide. This study investigated the feasibility of the anaerobic co-digestion of spent coffee ground with various organic wastes, i.e., food waste, Ulva, waste activated sludge, and whey, for biomethanation. The effect of co-digestion was evaluated for each tested co-substrate in batch biochemical methane potential tests by varying the substrate mixing ratio. Co-digestion with waste activated sludge had an apparent negative effect on both the yield and production rate of methane. Meanwhile, the other co-substrates enhanced the reaction rate while maintaining methane production at a comparable or higher level to that of the mono-digestion of spent coffee ground. The reaction rate increased with the proportion of co-substrates without a significant loss in methanation potential. These results suggest the potential to reduce the reaction time and thus the reactor capacity without compromising methane production.

Upflow Anaerobic Sludge Blanket Reactor Codigestion of Algae and Acetate to Produce Methane

  Algae grown in wastewater treatment lagoons are a potentially important substrate for biofuel production. The feasibility of using upflow anaerobic sludge blanket (UASB) reactors in anaerobic digestion of algae to produce methane was investigated. A favorable carbon to nitrogen (C/N) weight ratio of 21/1 was determined in batch reactor experiments in which the ratio was adjusted by blending algal biomass with sodium acetate as a carbon source. This blend of algae and acetate was used in the feedstock applied to the UASB reactors. Duplicate, 34-L, UASB reactors initially received an organic loading rate (OLR) of 0.9 g chemical oxygen demand (COD)/L.d at a 7.2-day hydraulic retention time (HRT). The OLR was gradually increased to 5.4 g/L.d and the HRT was decreased to 5.5 days resulting in a methane production increase from 247 to 298 mL/g COD biodegraded. The COD removal efficiency was 80% with a biogas methane composition of 90%.

An Overview of the Control of Bacterial Pathogens in Cattle Manure

Cattle manure harbors microbial constituents that make it a potential source of pollution in the environment and infections in humans. Knowledge of, and microbial assessment of, manure is crucial in a bid to prevent public health and environmental hazards through the development of better management practices and policies that should govern manure handling. Physical, chemical and biological methods to reduce pathogen population in manure do exist, but are faced with challenges such as cost, odor pollution, green house gas emission, etc. Consequently, anaerobic digestion of animal manure is currently one of the most widely used treatment method that can help to salvage the above-mentioned adverse effects and in addition, produces biogas that can serve as an alternative/complementary source of energy. However, this method has to be monitored closely as it could be fraught with challenges during operation, caused by the inherent characteristics of the manure. In addition, to further reduce bacterial pathogens to a significant level, anaerobic digestion can be combined with other methods such as thermal, aerobic and physical methods. In this paper, we review the bacterial composition of cattle manure as well as methods engaged in the control of pathogenic microbes present in manure and recommendations that need to be respected and implemented in order to prevent microbial contamination of the environment, animals and humans.

Enhancement of substrate solubilization and hydrogen production from kitchen wastes by pH pretreatment

Pretreatment at different pHs was adopted in this study to enhance the substance solubilization and hydrogen production from kitchen wastes through anaerobic digestion. After a pretreatment set at pH = 13, solubilization of kitchen wastes improved substantially as the concentration of soluble carbohydrate, soluble protein, lipids and soluble chemical oxygen demand increased by 283.1%, 203.2%, 259.1% and 108.2%, respectively, as compared with those of the control. The maximum hydrogen production potential reached 105.38 mL/g VS after the pretreatment, which was 2.66 times that of the control. Furthermore, butyric acid and acetic acid were the major components in the total metabolites after fermentation, while propionic acid had a relatively low concentration. Finally, the concentration of exoprotein and exopolysaccharide within extracellular polymeric substances (EPS) kept increasing during the initial 14 and 9 hours, respectively, then decreased afterwards. However, the concentration of DNA increased throughout the whole stage. The total EPS might indirectly indicate the anaerobic digestion process. These findings may represent a feasible method for high-quality treatment of kitchen wastes.

Enhanced biomethanation of kitchen waste by different pre-treatments

Five different pre-treatments were investigated to enhance the solubilisation and anaerobic biodegradability of kitchen waste (KW) in thermophilic batch and continuous tests. In the batch solubilisation tests, the highest and the lowest solubilisation efficiency were achieved with the thermo-acid and the pressure-depressure pre-treatments, respectively. However, in the batch biodegradability tests, the highest cumulative biogas production was obtained with the pressure-depressure method. In the continuous tests, the best performance in terms of an acceptable biogas production efficiency of 60% and stable in-reactor CODs and VFA concentrations corresponded to the pressure-depressure reactor, followed by freeze-thaw, acid, thermo-acid, thermo and control. The maximum OLR (5 g COD L(-1) d(-1)) applied in the pressure-depressure and freeze-thaw reactors almost doubled the control reactor. From the overall analysis, the freeze-thaw pre-treatment was the most profitable process with a net potential profit of around 11.5 € ton(-1) KW.

Digestion of frozen/thawed food waste in the hybrid anaerobic solid-liquid system

The hybrid anaerobic solid-liquid (HASL) system, which is a modified two-phase anaerobic digester, is to be used in an industrial scale operation to minimize disposal of food waste at incineration plants in Singapore. The aim of the present research was to evaluate freezing/thawing of food waste as a pre-treatment for its anaerobic digestion in the HASL system. The hydrolytic and fermentation processes in the acidogenic reactor were enhanced when food waste was frozen for 24h at -20 degrees C and then thawed for 12h at 25 degrees C (experiment) in comparison with fresh food waste (control). The highest dissolved COD concentrations in the leachate from the acidogenic reactors were 16.9 g/l on day 3 in the control and 18.9 g/l on day 1 in the experiment. The highest VFA concentrations in the leachate from the acidogenic reactors were 11.7 g/l on day 3 in the control and 17.0 g/l on day 1 in the experiment. The same volume of methane was produced during 12 days in the control and 7 days in the experiment. It gave the opportunity to diminish operational time of batch process by 42%. The effect of freezing/thawing of food waste as pre-treatment for its anaerobic digestion in the HASL system was comparable with that of thermal pre-treatment of food waste at 150 degrees C for 1h. However, estimation of energy required either to heat the suspended food waste to 150 degrees C or to freeze the same quantity of food waste to -20 degrees C showed that freezing pre-treatment consumes about 3 times less energy than thermal pre-treatment.

Comparison of cellulose solubilisation rates in rumen and landfill leachate inoculated reactors

The aim of this study was to conduct a number of controlled digestions to obtain easily comparable cellulose solubilisation rates and to compare these rates to those found in the literature to see which operational differences were significant in affecting cellulose degradation during anaerobic digestion. The results suggested that differences in volumetric cellulose solubilisation rates were not indicative of the true performance of cellulose digestion systems. When cellulose solubilisation rates were normalised by the mass of cellulose in the reactor at each time step, the comparison of the rates became more meaningful. Cellulose solubilisation was surface area limited. Therefore, changes in the loading rate of cellulose to the reactor altered the volumetric solubilisation rate without changing the mass normalised rate. Comparison of mass normalised solubilisation rates from this study and the literature demonstrated that differences in reactor configuration and operational conditions did not significantly impact on the solubilisation rate whereas the difference in composition of the microbial communities showed a marked effect. This work highlights the importance of using appropriately normalised data when making comparisons between systems with differing operational conditions.