Effect of horticultural waste composting on infected plant residues with pathogenic bacteria and fungi: integrated and localized sanitation

The aim of this work was to study the effect of composting on the viability of plant pathogenic fungi and bacteria. The research consisted of pilot-scale composting of horticultural waste in compost windrows. Studies were carried out on vegetable residues infected with plant pathogenic microorganisms included by either integrated or localized infection. In the first case, the plant pathogen viability was investigated when infected material was mixed throughout compost, while the localized infection was used to study the effect of the composting process on plant waste spot-inoculated with pathogenic microorganisms. Results for localized sanitation showed the total elimination of all tested phytopathogens between 48 and 120 h after composting began. In this case significant differences were observed in relation to 9 different zones in the pile. The disappearance of these microorganisms was similar when all plant waste included in the windrow was infected (integrated infection). Additionally, the results obtained confirmed that the bacteria showed a greater capacity to persist during composting than the fungi. Composting is therefore considered a useful method for recycling horticultural waste and eliminating phytopathogenic bacteria and fungi that inhabit this kind of residue.     

Mass and element balance in food waste composting facilities

The mass and element balance in municipal solid waste composting facilities that handle food waste was studied. Material samples from the facilities were analyzed for moisture, ash, carbon, nitrogen, and the oxygen consumption of compost and bulking material was determined.Three different processes were used in the food waste composting facilities: standard in-vessel composting, drying, and stand-alone composting machine. Satisfactory results were obtained for the input/output ash balance despite several assumptions made concerning the quantities involved. The carbon/nitrogen ratio and oxygen consumption values for compost derived only from food waste were estimated by excluding the contribution of the bulking material remaining in the compost product. These estimates seemed to be suitable indices for the biological stability of compost because there was a good correlation between them, and because the values seemed logical given the operating conditions at the facilities.     

LCA comparison of windrow composting of yard wastes with use as alternative daily cover (ADC)

This study compared the environmental impacts of composting yard wastes in windrows with using them in place of soil as alternative daily cover (ADC) in landfills. The Life Cycle Assessment was made using the SimaPro LCA software and showed that the ADC scenario is more beneficial for the environment than windrow composting. ADC use is also a less costly means of disposal of yard wastes. This finding applies only in cases where there are sanitary landfills in the area that are equipped with gas collection systems and can use yard wastes as alternative daily cover. Otherwise, the environmentally preferable method for disposal of source-separated yard wastes is composting rather than landfilling.     

Aerobic composting of waste activated sludge: kinetic analysis for microbiological reaction and oxygen consumption

In order to examine the optimal design and operating parameters, kinetics for microbiological reaction and oxygen consumption in composting of waste activated sludge were quantitatively examined. A series of experiments was conducted to discuss the optimal operating parameters for aerobic composting of waste activated sludge obtained from Kawagoe City Wastewater Treatment Plant (Saitama, Japan) using 4 and 20 L laboratory scale bioreactors. Aeration rate, compositions of compost mixture and height of compost pile were investigated as main design and operating parameters. The optimal aerobic composting of waste activated sludge was found at the aeration rate of 2.0 L/min/kg (initial composting mixture dry weight). A compost pile up to 0.5 m could be operated effectively. A simple model for composting of waste activated sludge in a composting reactor was developed by assuming that a solid phase of compost mixture is well mixed and the kinetics for microbiological reaction is represented by a Monod-type equation. The model predictions could fit the experimental data for decomposition of waste activated sludge with an average deviation of 2.14%. Oxygen consumption during composting was also examined using a simplified model in which the oxygen consumption was represented by a Monod-type equation and the axial distribution of oxygen concentration in the composting pile was described by a plug-flow model. The predictions could satisfactorily simulate the experiment results for the average maximum oxygen consumption rate during aerobic composting with an average deviation of 7.4%.     

Assessment of microbiological and parasitological quality of composted wastes: health implications and hygienic measures.

Feedstock and compost samples were collected from twenty composting plants and analysed from the microbiological point of view. Faecal indicator organisms were determined in order to evaluate the efficacy of processes for the removal of pathogenic micro-organisms with similar survival characteristics and to verify their suitability as appropriate markers of microbial quality of composted products. In addition to the classical bacterial indicators, selected organisms, such as Salmonella, Giardia, Cryptosporidium, Clostridium spores and helminth ova, were investigated. Statistically significant differences in the removal of the different micro-organisms were observed with regard to both the different composting plants (P < 0.05) and feedstock composition (P < 0.05). In fact, compost obtained by feedstock containing sewage sludge was shown to have a better hygienic quality in comparison with compost containing green discards and municipal solid waste as raw matter. Giardia cysts, Cryptosporidium oocysts and helminth ova were not effective indicators of hygienic quality of compost, whereas Clostridium perfringens spores, because of their high resistance to treatments, could be considered as an additional model for assessing the composting process, especially with regard to more resistant pathogen reduction.     

Modeling of substrate degradation and oxygen consumption in waste composting processes

A multi-component modeling system was developed to simulate substrate degradation and oxygen consumption in waste composting processes. Levels of soluble substrate (Ss), insoluble substrate (Si), active biomass (X), inert material, moisture, temperature, and oxygen concentration were considered as state variables. The relationships among these variables were also incorporated within the modeling framework. Three conversion reactions, including growth of aerobic biomass, decay of aerobic biomass, and solubilisation of insoluble substrate, were considered in the simulation system. The modeling inputs included temperature, moisture, oxygen concentration, and initial conditions of the state variables, while the outputs included oxygen uptake accumulation (OUA), oxygen uptake rate (OUR), Ss, Si, and X for representing the substrate degradation and oxygen consumption status. The effectiveness of the developed model was demonstrated through its application to a case study in a 30L vessel over 200h. Through verification-based composting experiments, it was shown that the modeling solutions were consistent with the experimental results with an acceptable accuracy level. Sensitivity analyses of the model showed that an increased maximum microbial growth rate would result in raised OUA, OUR, Ss, and X levels; a decreased biomass decay rate constant would help enhance the composting process. Moreover, variations in the maximum growth rate would affect the composting process more significantly than those of the biomass decay rate constant.     

Microbiological assessments of compost toilets: in situ measurements and laboratory studies on the survival of fecal microbial indicators using sentinel chambers

Compost toilet systems were assessed for their ability to reduce microbial indicators and pathogens. Bacterial pathogens were not detected in any samples indicating a low survival rate in composting feces and/or an initial low occurrence. Indicator bacteria showed large variations with no clear trend of lower bacterial numbers after longer storage. In controlled composting experiments, thermophilic conditions were only reached when amendments were made (grass and a sugar solution). Even then it was impossible to ensure a homogenous temperature in the composting fecal material and therefore difficult to achieve a uniform reduction and killing of indicator organisms. Presumptive thermotolerant coliforms, Salmonella typhimurium Phage 28 B and eggs of Ascaridia galli, proved useful as indicators. However, regrowth was detected for enterococci and total numbers of bacteria grown at 36 degrees C. These indicator parameters may therefore overestimate the level of other (pathogenic) bacteria present in the material and can not be recommended for use as reliable indicator organisms in composting toilet systems. The addition of indicator bacteria to fecal material contained in semi-permeable capsules proved to be a useful technique to ensure that microorganisms were contained in a small test volume.     

Sanitary aspect of using partially treated landfill leachate as a water source in green waste composting

Shredded green wastes were composted in windrows, at the Harewood Whin landfill, near the city of York, in West Yorkshire, UK. Landfill leachate were added twice during the second and fourth week of the process in two piles. One pile was turned once every week for eight weeks and the other was turned twice, during the same period. Each time approximately, 2 m3 of leachate was added, into each pile. The two piles each contained about 45 m3 of shredded green waste. The effect of adding leachate on the sanitisation of the green waste during composting, was evaluated based on the changes in the levels of faecal coliforms and faecal streptococci. The results suggested that using leachate as the moisture source had no significant effect (tested with two factors ANOVA test) on the sanitisation process when compared with two similar piles, used as the control, for which tap water was used for moisture addition. In all four piles sanitisation was almost complete and below the acceptable levels. Additionally, the results indicated that there was no significant effect on the sanitisation process of the turning frequency.     

Evaluation of laboratory-scale in-vessel co-composting of tobacco and apple waste

Efficient composting process requires set of adequate parameters among which physical–chemical properties of the composting substrate play the key-role. Combining different types of biodegradable solid waste it is possible to obtain a substrate eligible to microorganisms in the composting process. In this work the composting of apple and tobacco solid waste mixture (1:7, dry weight) was explored. The aim of the work was to investigate an efficiency of biodegradation of the given mixture and to characterize incurred raw compost. Composting was conducted in 24 L thermally insulated column reactor at airflow rate of 1.1 L min^?1. During 22 days several parameters were closely monitored: temperature and mass of the substrate, volatile solids content, C/N ratio and pH-value of the mixture and oxygen consumption. The composting of the apple and tobacco waste resulted with high degradation of the volatile solids (53.1%). During the experiment 1.76 kg of oxygen was consumed and the C/N ratio of the product was 11.6. The obtained temperature curve was almost a “mirror image” of the oxygen concentration curve while the peak values of the temperature were occurred 9.5 h after the peak oxygen consumption.     

Implementing separate waste collection and mechanical biological waste treatment in South Africa: A comparison with Austria and England

The degradation of organic compounds found in municipal solid waste (MSW) under the anaerobic landfill conditions produces gas and liquid emissions that can protract well into the landfill after-care period. The European Landfill Directives regulate the amount and nature of the organic compounds disposed into landfills. In South Africa and other developing countries, MSW is still landfilled without any kind of pre-treatment. This paper presents a pilot project of mechanical biological waste treatment (MBWT) in South Africa implemented at municipal level in the city of Durban using passively aerated open windrows. Based on case studies from Austria, England and South Africa, a waste minimisation model which can facilitate full-scale implementation of MBWT in developing countries is presented. MSW was treated in open windrows for 8 weeks. Composting temperature reached a maximum of 65 °C in less than 10 days. The results of eluate tests on waste samples from the windrows at the end of composting show a reduction of BOD₅ and BOD₅/COD ratios equal to 35.7% and 16.7%, respectively. The percent waste composition of the treated MSW was 28.3% putrescibles, 17.4% garden refuse, 13.3% plastic, 12.4% fabrics, 12% paper and other elements. The waste composition shows that more than 40% of un-treated organic material and also more than 40% non-biodegradable and recyclable materials are still landfilled without any form of biological treatment or resource recovery. A simple wet and dry waste collection model can promote recycling, treatment of biological waste before landfilling, resource recovery, labour intensive jobs and hence sustainable landfilling in the South African scenario as well as in similar developing countries.     

Depletion of selective serotonin reuptake inhibitors during sewage sludge composting

Sewage and sewage sludge is known to contain pharmaceuticals, and since sewage sludge is often used as fertilizer within agriculture, the reduction of the selective serotonin reuptake inhibitors (SSRIs) Citalopram, Sertraline, Paroxetine, Fluvoxamine and Fluoxetine during composting has been investigated. Sewage sludge was spiked with the SSRIs before the composting experiment started, and the concentration of the SSRIs in the sludge during a 21 day composting period was measured by liquid phase microextraction (LPME) and high-performance liquid chromatography–mass spectrometry. All the SSRIs had a significant decrease in concentration during the composting process. The highest reduction rates were measured for Fluoxetine and Paroxetine and the lowest for Citalopram. In addition three out of four known SSRI metabolites were found in all the samples, and two of them showed a significant increase in concentration during the composting period.     

Open windrow composting of polymers: an investigation into the operational issues of composting polyethylene (PE)

This paper investigates the operational issues surrounding the open windrow composting of degradable polyethylene sacks. Areas for consideration were the impact of degradable polyethylene sacks on the composting process, the quality of the finished compost product, and how the use of sacks influenced the on-site processing. These factors were investigated through determining the amount of polymer residue and chemical contaminants in the finished compost product and the daily monitoring of windrow temperature profiles. Site and practical handling considerations of accepting an organic waste contained within PE sacks are also discussed. Statistical analysis of the windrow temperature profiles has led to the development of a model that can help to predict the expected trends in the temperature profiles of open compost windrows where the organic waste is kerbside collected using a degradable PE sack.     

Land application of biosolids. Soil response to different stabilization degree of the treated organic matter

The effect of land application of biosolids on an agricultural soil was studied in a 2-month incubation experiment. The soil microbial biomass and the availability of heavy metals in the soil was monitored after the application of four different composting mixtures of sewage sludge and cotton waste, at different stages of composting. Land application caused an increase of both size and activity of soil microbial biomass that was related to the stabilization degree of the composting mixture. Sewage sludge stabilization through composting reduced the perturbance of the soil microbial biomass. At the end of the experiment, the size and the activity of the soil microbial biomass following the addition of untreated sewage sludge were twice those developed with mature compost. For the mature compost, the soil microbial biomass recovered its original equilibrium status (defined as the specific respiration activity, qCO2) after 18 days of incubation, whereas the soil amended with less stabilized materials did not recover equilibrium even after the two-month incubation period. The stabilization degree of the added materials did not affect the availability of Zn, Ni, Pb, Cu, Cr and Cd in the soil in the low heavy metal content of the sewage sludge studied. Stabilization of organic wastes before soil application is advisable for the lower perturbation of soil equilibria status and the more efficient C mineralization.     

Diversity of bacterial isolates during full scale rotary drum composting

Bacterial diversity of full scale rotary drum composter from biodegradable organic waste samples were analyzed through two different approaches, i.e., Culture dependent and independent techniques. Culture-dependent enumerations for indigenous population of bacterial isolates mainly total heterotrophic bacteria (Bacillus species, Pseudomonas species and Enterobacter species), Fecal Coliforms, Fecal Streptococci, Escherichia coli, Salmonella species and Shigella species showed reduction during the composting period. On the other hand, Culture-independent method using PCR amplification of specific 16S rRNA sequences identified the presence of Acinetobacter species, Actinobacteria species, Bacillus species, Clostridium species, Hydrogenophaga species, Butyrivibrio species, Pedobacter species, Empedobactor species and Flavobacterium species by sequences clustering in the phylogenetic tree. Furthermore, correlating physico-chemical analysis of samples with bacterial diversity revealed the bacterial communities have undergone changes, possibly linked to the variations in temperature and availability of new metabolic substrates while decomposing organics at different stages of composting.     

Influence of aeration rate and biodegradability fractionation on composting kinetics

The influences of aeration rate and biodegradability fractionation on biodegradation kinetics during composting were studied. The first step was the design of a suitable lab-reactor that enabled the simulation of composting. The second step comprised of composting trials of six blends of sludge (originating from a food processing effluent) with wood chips using aeration rates of 1.69, 3.62, 3.25, 8.48, 11.98 and 16.63 L/h/kg DM of mixture. Biodegradation was evaluated by respiration measurements and from the analysis of the substrate (dry matter, organic matter, total carbon and chemical oxygen demand removal). Continuous measurement of oxygen consumption was coupled with the analysis of initial substrate and composted product for chemical oxygen demand (in the soluble and non-soluble fractions), which enabled an evaluation of the organic matter biodegradability. Oxygen requirements to remove both the easily and slowly biodegradable fractions were determined. Dividing the substrate into different parts according to biodegradability allowed explanation of the influence of aeration rate on stabilization kinetics. Considering that the biodegradation kinetics were of the first-order, the kinetic constants of the easily and slowly biodegradable fractions were calculated as a function of temperature. The methodology presented here allows the comparison of organic wastes in terms of their content of easily and slowly biodegradable fractions and the respective biodegradation kinetics.     

Estimating fugitive bioaerosol releases from static compost windrows: feasibility of a portable wind tunnel approach

An assessment of the fugitive release of bioaerosols from static compost piles was conducted at a green waste composting facility in South East England; this representing the initial stage of a programme of research into the influence of process parameters on bioaerosol emission flux. Wind tunnel experiments conducted on the surface of static windrows generated specific bioaerosol emission rates (SBER2s) at ground level of between 13 and 22 x 10(3) cfu/m2/s for mesophilic actinomycetes and between 8 and 11 x 10(3)cfu/m2/s for Aspergillus fumigatus. Air dispersion modelling of these emissions using the SCREEN3 air dispersion model in area source term mode was used to generate source depletion curves downwind of the facility for comparative purposes.     

Introducing mechanical biological waste treatment in South Africa: a comparative study

This paper presents the results of the first pilot project on mechanical biological waste treatment (MBWT) in South Africa. The study has shown that biological waste treatment in windrows using a passive aeration system that utilises thermal convection to drive the aeration process within a windrow of waste is appropriate for South Africa, in relation to low capital costs, low energy inputs, limited plant requirements and potential for labour-intensive operations. The influence of climate, waste composition and operational facilities was evaluated to optimise the treatment technique to local conditions. The maximum temperatures reached during the intensive thermophilic stage were effectively equivalent to the German experience. The lower CO2 production experienced in the South African trials was attributed to a different waste stream (high presence of plastics) due to the absence of a proper source separated waste collection system. An accurate adjustment of the input material (structural matter in particular) to the specific ambient conditions and irrigation during composting should result in higher organic carbon degradation efficiency in equivalent timeframes. This preliminary experience suggests that the applicability of MBWT in emerging countries, such as South Africa, is directly dependant on the mechanical treatment steps, available operational facilities and nature of the input material.     

Characteristics of municipal solid waste and sewage sludge co-composting

The purpose of this work is to study the characteristics of the co-composting of municipal solid waste (MSW) and sewage sludge (SS). Four main influencing factors (aeration pattern, proportion of MSW and SS, aeration rate and mature compost (MC) recycling) were systematically investigated through changes of temperature, oxygen consumption rate, organic matters, moisture content, carbon, nitrogen, carbon-to-nitrogen ratio, nitrogen loss, sulphur and hydrogen. We found that a continuous aeration pattern during composting was superior to an intermittent aeration pattern, since the latter delayed the composting process. A 3:1 (v:v) mixture of MSW and SS was most beneficial to composting. It maintained the highest temperature for the longest duration and achieved the fastest organic matter degradation and highest N content in the final composting product. A 0.5L/minkgVS aeration rate best ensured rapid initiation and maintained moderate moisture content for microorganisms. After the mature MC was recycled to the fresh materials as a bulking agent, the structure and moisture of the initial materials were improved. A higher proportion of MC resulted in quicker decrease of the temperature, oxygen consumption rate and moisture. Therefore a 3:1:1 (v:v:v) proportion of MSW: SS: MC is recommended.     

Enhanced degradable yard waste collection bag behavior in a field-scale composting environment

The degradation of four formulations of yard waste-filled collection bags was evaluated in a field-scale test of 15.5- or 31-m-long windrows at a community yard waste composting site. Variables of bag contents, bag chemical composition, and length of exposure were evaluated. Chemical compositions of the bags included (1) low-density polyethylene (LDPE) + 6% cornstarch + 2 levels of prooxidant, (2) LDPE + 9% cornstarch + prooxidants, and (3) LDPE without cornstarch but with photooxidation enhancers. Results showed that all products weakened and/or disintegrated to some extent. However, the bags with 6% starch disintegrated too slowly to allow timely processing of the compost. The bags with 9% starch and other additives to promote multiple degradation mechanisms degraded at the fastest rate of those evaluated here. The photodegradable bags with solar exposure during composting disintegrated rapidly, but when turned to expose new surfaces to light, further strength losses occurred slowly.Paper presented at the Biodegradable Materials and Packaging Conference, September 22–23, 1993, Natick, Massachusetts.     

Life Cycle Assessment of Poly(Lactic Acid) (PLA): Comparison Between Chemical Recycling,Mechanical Recycling and Composting

This paper presents a life cycle assessment (LCA) comparing three forms of poly(lactic acid) (PLA) disposal: mechanical recycling, chemical recycling and composting. The LCA data was taken from lab scale experiments for composting and hydrolysis steps. Polymerization data in chemical recycling was obtained from computer simulation. Mechanical recycling data from lab scale were combined with the data from a plastics commercial mechanical recycling plant. The analysis considered two different product systems based on the input of the recycled PLA in the product system. Considering the categories: climate change, human toxicity and fossil depletion, the LCA showed that mechanical recycling presented the lowest environmental impact, followed by chemical recycling and composting. Among the forms of recycling, the most important input was the electricity consumption.