Rotary drum composting of different organic waste mixtures

A comprehensive review on the decentralized composting systems for household biodegradable waste management

Municipal solid waste primarily consists of household biodegradable waste (HBW). HBW treatment is a crucial step in many countries due to rapid urbanization. Composting is an effective technique to treat HBW. However, conventional composting systems are unable to produce matured compost (MC), as well as releasing huge amounts of greenhouse and odorous gases. Therefore, this review attempts to suggest suitable composting system to manage HBW, role of additives and bulking agents in composting process, identify knowledge gaps and recommend future research directions. Centralized composting systems are unable to produce MC due to improper sorting and inadequate aeration for composting substrate. Recently, decentralized compost systems (DCS) are becoming more popular due to effective solid waste reduction at the household and/or community level itself, thereby reducing the burden on municipalities. Solid waste sorting and aeration for the composting substrate is easy at DCS, thereby producing MC. However, Mono-composting of HBW in DCS leads to production of immature compost and release greenhouse and odorous gases due to lower free air space and carbon-to-nitrogen ratios, and higher moisture content. Mixing HBW with additives and bulking agents in DCS resulted in a proper initial substrate for composting, allowing rapid degradation of substrate due to longer duration of thermophilic phase and produce MC within a shorter duration. However, people have lack of awareness about solid waste management is the biggest challenge. More studies are needed to eliminate greenhouse and odorous gases emissions by mixing different combinations of bulking agents and additives (mainly microbial additives) to HBW in DCS.

Effect of diaper waste on composting of household wet biodegradable waste in a decentralized system

Due to changes in lifestyle and improved economic status, the use of diapers is also increasing in developing nations. Hence, there is a need to develop an eco-friendly system for the disposal of discarded diapers which is termed diaper waste (DW). In the present study, the co-composting of DW with household wet biodegradable waste (HWBW) was performed in a compartmentalized rotary drum (CRD) (total capacity = 160 L, number of compartments = 4) under passive aeration conditions. For the co-composting runs, 1 kg of HWBW and DW mixture (mass ratio = 100:0, 90:10, 85:15, and 80:20) was added in four individual compartments daily for 10 days. During the process, the highest temperature of ~ 50-56 °C could be achieved in different compartments for a duration of 2-8 days. The compost yield (i.e., below 4 mm size material) was ranged 10.4-13% after 55 days of composting and the mass of DW was reduced by ~ 61-68%. A mixture of 15% DW and 85% HWBW can be suggested as the best combination for the co-composting process. Based on the "Dewar test" results, the samples recovered after composting could be categorized as "stable". The pot results showed an improvement in the growth of Vigna Radiata when 10% compost sample was mixed with soil whereas with 20% compost in the soil-compost mixture, the plant growth was adversely affected. Hence, co-composting of DW with HWBW can be a feasible proposition which can produce a good quality compost.

Biowaste composting process - comparison of a rotary drum composter and open container

Composting is recognized as a sustainable waste management approach in which microorganisms treat and stabilize biodegradable waste under aerobic conditions to obtain compost as a final product. In this paper, composting of biowaste in a rotary drum composter (closed system) and an open container (open system) was compared. Temperature, pH, electrical conductivity, a carbon-to-nitrogen mass ratio (C/N ratio) and contents of moisture, carbon and dry and volatile matter, were measured during composting. Results showed decreasing profiles for moisture, volatile matter, and carbon contents, as well as for the C/N ratio, while increasing profiles for the dry matter content and electrical conductivity during composting in both systems. Leachates were formed only during the first three days of composting and were characterized with high organic loads, high ammonia concentrations, low pH, and high conductivity and turbidity. The organic matter content data during the composting process were analysed according to the first order kinetic model. Results suggested that there was a difference in the rate of organic matter decomposition, which was higher when composting in the open vessel than in the rotary drum composter.

Bio-process performance, evaluation of enzyme and non-enzyme mediated composting of vegetable market complex waste

Vegetable Market have become major sources of organic waste. Some of such waste when being diverted to landfills not only increase the landfill loading but also contribute to increase greenhouse gas emission. Of the many technologies available in handling such hugely generated waste, composting has proven very effective for decades. Enzyme and non-enzyme mediated aerobic composting of vegetable market complex waste (VMCW) have been investigated. Conventional composting technique though being capable of handling large quantum of waste are found to consume more time. Proven to be disadvantages factor. In the present investigation, the pre-cultured seed inoculums used for vegetable market complex waste, shortened the typical composting period from 45 to 9 days for the first time. Also, rapid size and volume reduction of VMCW was witnessed. The organic degradation of VMCW was observed as 42% (82 ± 2.83% to 40.82 ± 0.61%), with a volume reduction from 0.012m³ to 0.003 m³ within 9 days. An enriched nutrients NPK level of compost bio-fertilizer was recorded as 0.91% w/w, 0.5% w/w and 1.029% w/w respectively. Compost maturity observed through the X-ray diffraction (XRD) analysis of the manure confirmed the conversion of the crystal structure of the compost particle to amorphous form and the mineralization of organic matter during the composting. Thus, the fermented pre-cultured seed inoculums favored an enhanced nutrients level with shortened composting time.

Effect of neem leaf inclusion rates on compost physico-chemical, thermal and spectroscopic stability

Neem leaves possess antimicrobial, insecticidal and nitrification inhibitory biochemicals that may influence compost stability. However, if neem-based compost achieves stability, it can prove useful by providing fertilizer-cum-pesticide properties. Therefore, conventional (physico-chemical), thermogravimetric (TGA) and spectroscopic analyses (FTIR and ¹³C SS NMR) were used to evaluate the effects of neem leaf content on stability. Treatments included 0, 10, 20, 30, 40 and 50% neem leaves by volume combined with complimentary amounts of corn stover to form 50% of the substrate formulation (SF). Cow manure constituted the additional 50%. Despite all treatments reaching ambient temperature (32 °C ± 1 °C) by the 40th day, Solvita® results showed high CO₂ respiration, thereby classifying the compost treatments as active, whereas decreased C:N ratio, NH₄⁺: NO₃^- ratio and NH₄⁺ values among treatments indicated stability. Furthermore, TGA, FTIR and ¹³C NMR revealed degradation of labile organic matter and showed that complex aromatic and lignin compounds were also degraded, particularly when neem leaves were added to the mixture, suggesting that aromatisation does not always indicate stability in compost. Spearman's rank correlation showed that physico-chemical methods were poorly correlated to respirometric, thermal and spectroscopic methods. It also suggests that these respirometric and advanced methods are important in understanding the mechanisms affecting neem compost stability.

Towards optimized drum composting: evaluation of the radial mixing performance of a model substrate on the laboratory scale

The rotating drum composter (RDC) is one of the most widespread reactor systems for biowaste treatment, worldwide. Nevertheless, knowledge on optimum operating conditions including, e.g. fill level, turning frequency, and mixing tool configuration is sparse. This study investigated the effect of static mixing tools (SMTs) on mixing in a rotating drum at high fill levels (60-80%). The methodological approach encompassed mixing experiments in a laboratory RDC using soaked wheat grains as a model material. The temporal course of material blending was quantified in terms of the entropy of mixing using digital image analysis. Experiments without SMTs showed the evolution of unmixed cores. With a single SMT, mixing was superior even at fill levels >70% while peripheral unmixed zones persisted when overly long SMTs were used. The results of this study may help to derive optimal process conditions for RDCs operated at high fill levels.

Inside the small-scale composting of kitchen and garden wastes: Thermal performance and stratification effect in vertical compost bins

Decentralized composting has been proposed as a best available practice, with a highly positive impact on municipal solid wastes management plans. However, in cold climates, decentralized small-scale composting performance to reach thermophilic temperatures (required for the product sanitization) could be poor, due to a lack of critical mass to retain heat. In addition, in these systems the composting process is usually disturbed when new portions of fresh organic waste are combined with previous batches. This causes modifications in the well-known composting evolution pattern. The objective of this work was to improve the understanding of these technical aspects through a real-scale decentralized composting experience carried out under cold climate conditions, in order to assess sanitization performance and to study the effects of fresh feedstock additions in the process evolution. Kitchen and garden organic wastes were composted in 500 L-static compost bins (without turning) for 244 days under cold climate conditions (Bariloche, NW Patagonia, Argentina), using pine wood shavings in a ratio of 1.5:1 v: v (waste: bulking agent). Temperature profile, stability indicators (microbial activity, carbon and nitrogen contents and ratio) and other variables (pH and electrical conductivity), were monitored throughout the experience. Our results indicate that small-scale composting (average generation rate of 7 kg d^-1) is viable under cold weather conditions, since thermophilic sanitization temperatures (> 55 °C) were maintained for 3 consecutive days in most of the composting mass, according to available USEPA regulations commonly used as a reference for pathogens control in sewage sludge. On the other hand, stability indicators showed a differentiated organic matter degradation process along the compost bins height. Particularly, in the bottommost composting mix layer the process took a longer period to achieve compost stability than the upper layers, suggesting that differential organic matter transformation appears not to be necessarily associated to the order of the waste batches incorporation in a time line, as it could be expected. These findings suggest the need to discuss new ways of studying the composting process in small-scale compost bins as well as their commercial design.

Study and assessment of segregated biowaste composting: The case study of Attica municipalities

This work aims to assess the operation of the first large scale segregated biowaste composting scheme in Greece to divert Household Food Waste (HFW) from landfill and produce a material which can be recovered and used as compost. The source separation and collection of HFW was deployed in selected areas in Attica Region serving about 3700 households. Sorted HFW is collected & transported to the Mechanical and Biological Treatment (MBT) plant in Attica Region that has been designed to produce Compost Like Output (CLO) from mixed MSW. The MBT facility has been adjusted in order to receive and treat aerobically HFW mixed with shredded green waste in a dedicated composting tunnel. The composting process was monitored against temperature, moisture and oxygen content indicating that the biological conditions are sufficiently developed. The product quality was examined and assessed against the quality specifications of EU End of Waste Criteria for biowaste subjected to composting aiming to specify whether the HFW that has undergone recovery ceases to be waste and can be classified as compost. More specifically, the heavy metals concentrations (Cr, Cu, Ni, Cd, Pb, Zn and Hg) are within the set limits and much lower compared to the CLO material that currently is being produced at the MBT plant. In regard to the hygienic requirements of the product it has been found that the process conditions result in a pathogen free material (i.e. E. Coli and Salmonella) which does not favor the growth of viable weeds and plant propagules, while it acquires sufficient organic matter content for soil fertilization. Noticeable physical impurities (mainly fractions of glass) have been detected exceeding the quality control threshold limit of 0.5% w/w (plastics, metals and glass). The latter is related to the missorted materials and to the limited pre-treatment configurations prior to composting. The above findings indicate that effective source separation of biowaste is prerequisite for good quality production and marketing of compost and special consideration should be made to minimize glass impurities prior composting (i.e. awareness raising and pretreatment stage). Therefore, it is feasible to gradually replace the production of questionable quality CLO in MBTs with biowaste compost which is in line with the required quality control standards especially when heavy metals concentrations is concerned.

Biosorption of lead using Bacillus badius AK strain isolated from compost of green waste (water hyacinth)

The bacterial strain Bacillus badius AK isolated from water hyacinth compost was investigated for biosorption characteristics in Pb(II) removal. Batch mode experiments depicted the optimum conditions for biosorption as pH at 4, the temperature of 30°C, 150 rpm of the rotational speed at biomass concentration of 20 mL with 1.7 × 10¹⁶ colony forming unit per milliliter (CFU/mL) value, at 100-150 mg/L concentration of Pb(II). The bacterial biomass was used in its native and non-pretreated state, unlike the dried, freeze-dried or chemically treated biomass. The biosorption followed pseudo-second-order kinetics and isotherm fitted well to the Langmuir model. Maximum Pb(II) biosorption was observed at 1.7 × 10¹⁶ CFU/mL. Influence of Pb(II) on the growth of bacterial biomass was examined by fitting the monod's model. Specific growth rate and maximum specific growth rate of B. badius AK was observed as 0.05 and 2.54 h^-1, respectively; biomass yield coefficient was 11.81. The results indicated that bacterial biomass was efficient, robust and cheaper biosorbent for removal of Pb(II).

Optimization of waste combinations during in-vessel composting of agricultural waste

In-vessel composting of agricultural waste is a well-described approach for stabilization of compost within a short time period. Although composting studies have shown the different combinations of waste materials for producing good quality compost, studies of the particular ratio of the waste materials in the mix are still limited. In the present study, composting was conducted with a combination of vegetable waste, cow dung, sawdust and dry leaves using a 550 L rotary drum composter. Application of a radial basis functional neural network was used to simulate the composting process. The model utilizes physico-chemical parameters with different waste materials as input variables and three output variables: volatile solids, soluble biochemical oxygen demand and carbon dioxide evolution. For the selected model, the coefficient of determination reached the high value of 0.997. The complicated interaction of agricultural waste components during composting makes it a nonlinear problem so it is difficult to find the optimal waste combinations for producing quality compost. Optimization of a trained radial basis functional model has yielded the optimal proportion as 62 kg, 17 kg and 9 kg for vegetable waste, cow dung and sawdust, respectively. The results showed that the predictive radial basis functional model described for drum composting of agricultural waste was well suited for organic matter degradation and can be successfully applied.

Effects of composting process on the dissipation of extractable sulfonamides in swine manure

Effects of composting on the fate of sulfonamides (SAs) in the manure-straw mixture were explored through a simulation of aerobic composting process. Additionally, factors of temperature and coexistence of heavy metal Cu that might influence the removal efficiency were particularly investigated. As shown in the results, the extractable SAs dissipated rapidly during the composting process. The coexistence of Cu in the composting process might have delayed the decline of SAs, but the drugs could still be completely removed by the end of the composting. In contrast to the thermophilic aerobic composting, extractable SAs in air-temperature-placed mixture dissipated much slower and 1.12-1.56mg/kg could be detected after 35days of incubation. The results confirmed that temperature could influence the dissipation of SAs, which was identified as a more important factor than Cu-coexistence. Hence, thermophilic aerobic composting is an effective process to eliminate VAs before manure land application.

Effects of Cu exposure on enzyme activities and selection for microbial tolerances during swine-manure composting

A simulated experiment of aerobic composting was conducted on swine manure to evaluate the effects of Cu at two exposure levels (200 and 2000 mg kg(-1), corresponding to low-Cu and high-Cu treatments, respectively) on the activity of microorganisms. In addition, the microbial pollution-induced community tolerance (PICT) to Cu and co-tolerance to selected antibiotics (tylosin and vancomycin) in the composted products were also investigated using the Biolog Ecoplates™ method. It was demonstrated that the enzymatic activities were significantly inhibited by the high-Cu treatment, with maximal inhibition rates of 56.8% and 65.1% for urease and dehydrogenase, respectively. In response to the PICT test, the IC50 (half-maximal inhibition concentrations) values on the microorganisms in the high-Cu-treated composts were clearly higher than those in the low-Cu-treated and control composts, for the toxicity tests on both Cu and antibiotics, including tylosin and vancomycin. The data demonstrated that high-Cu exposure to the microbial community during the composting not only selected for Cu resistance but also co-selected for antibiotic resistance, which was of significance because the tolerance might be transferred to the soil after the land application of composted manure.

Modelling response patterns of physico-chemical indicators during high-rate composting of green waste for suppression of Pythium ultimum

High-rate composting studies on green waste, i.e. banana leaves (BL) and lawn clippings (LC), were conducted in 0.25-m3 rotary barrel composters to evaluate and model changes in key physico-chemical parameters during composting. Time to compost maturity and antagonistic effects and relationships of composts against Pythium ultimum were also investigated. Higher temperatures were achieved in LC compost (LCC), which did not translate to higher total organic carbon (TOC) loss but resulted in lower carbon to nitrogen ratio (C:N) and a more mature compost. With the exception of electrical conductivity (EC), net decreases were observed in pH, TOC and C:N across compost types. Total Kjeldahl nitrogen (TKN) showed a net increase in LCC and a net decrease in BLC. With the exception of TOC and pH, the results showed that compost type and time had a significant effect on the respective TKN, EC and C:N models. Compost temperature and TOC were best described by the critical exponential and rectangular hyperbola functions, respectively. Whereas TKN, C:N and pH were described using double Fourier functions and EC using Fourier functions. Composts achieved maturity within 19 days and significantly inhibited the growth of P. ultimum. Bacterial population was positively related to growth inhibition (GI) across compost types, whereas total microbial population had a positive relationship with GI in LCC. Evidence suggests that multiple groups of microorganisms contributed to GI through antibiosis and competition for resources. Composts were determined to be suitable for use as components of plant growth substrates based on compost maturity indices.

Solid waste generation and decomposition using compost bin technique in Pulau Pinang, Malaysia

Solid waste comprised of a grass clippings mixture was decomposed using a locally-made compost bin in Pulau Pinang, Malaysia, to eliminate challenges associated with improper waste disposal. Bulk density, pH, moisture content, nutrients content, nitrogen (N), phosphorus (P), potassium (K), iron (Fe), zinc (Zn), copper (Cu) and carbon/nitrogen (C/N) ratio were determined over 77 days. A 34% reduction in compost bin volume was observed and bulk density and pH were also reduced from 732 to 482 kg m(-3) and 7.82 to 8.41, respectively, indicating fairly good performance. The final moisture content and C/N ratio were 44.06% and 14 : 1, respectively, and the results also showed that the presence of nutrients and heavy metals in the final compost were within acceptable limits for use as a soil conditioner. Final concentrations of N, P and K were 347 mg kg(-1), 510 mg kg(-1) and 14.8 g kg(-1) and for heavy metals, Fe, Zn and Cu were 5308, 300 and 20 mg kg(-1), respectively, which considerably assisted in the decomposition process. Processed waste materials from the bin were shown to be excellent organic fertilizers with over 75% germination index for seeds grown into bean sprouts in 72 h. An improved bin design to eliminate greenhouse gas emission into the environment is suggested.

The effect of storage and mechanical pretreatment on the biological stability of municipal solid wastes

Modern mechanical-biological waste treatment plants for the stabilization of both the source-separated organic fraction of municipal solid wastes (OFMSW) and the mixed stream of municipal solid wastes (MSW) include a mechanical pretreatment step to separate recyclable materials such as plastics, glass or metals, before biological treatment of the resulting organic material. In this work, the role of storage and mechanical pretreatment steps in the stabilization of organic matter has been studied by means of respiration techniques. Results have shown that a progressive stabilization of organic matter occurs during the pretreatment of the source-separated OFMSW, which is approximately 30% measured by the dynamic respiration index. In the case of mixed MSW, the stabilization occurring during the reception and storage of MSW is compensated by the effect of concentration of organic matter that the pretreatment step provokes on this material. Both results are crucial for the operation of the succeeding biological process. Finally, respiration indices have been shown to be suitable for the monitoring of the pretreatment steps in mechanical-biological waste treatment plants, with a strong positive correlation between the dynamic respiration index and the cumulative respiration index across all samples tested.