Composting of limed fleshings generated in a tannery: sustainable waste management

In tanneries, limed fleshing is an unavoidable waste generated in beamhouse operation. Proper management of limed fleshing with protein, fat, lime, and sulfide will help to protect the natural environment and at least reduce the pollution that ends up in it. In this study, excluding any pretreatment, limed fleshing is used for compost production. Chopped and mixed limed fleshing with chicken manure, cow dung, and sawdust was heaped onto a horizontal bamboo frame. Three composting heaps were fabricated weighing 720, 700, and 760 kg. The turning of composting materials in the heaps causes temperature changes in the thermophilic range. The thermophilic temperatures in these heaps were 69.07 °C (heap 1), 69.9 °C (heap 2), and 69.19 °C (heap 3) which ensured the death of the pathogenic organism. The quality of compost was assessed based on the nutrients-nitrogen (N), phosphorous (P), potassium (K), and sulfur (S) content. NPKS in the compost fulfils the requirements of the investigated materials as compost. The largest amounts of metals- zinc (Zn), copper (Cu), chromium (Cr), lead (Pb), and nickel (Ni) of the compost detected in the heaps were, respectively, 200.3, 37.4, 20.3, 12.0, and 3.9 mg/kg. Cadmium (Cd) in the compost was below the detection limit. Scanning electron microscope (SEM) photographs show the decomposing of composting materials. This study indicates that limed fleshing can be converted into nutrient-enriched compost without any pretreatment. Using an easy, simple, and adaptable technique could reduce the volume of solid waste generated in the tannery to reduce environmental pollution.

Can the biological stage of a mechanical-biological treatment plant that is designed for mixed municipal solid waste be successfully utilized for effective composting of selectively collected biowaste?

Although the requirements for overall recycling rates can only be met when organic recycling is not overlooked, information is scarce regarding adaption to biowaste composting of existing mechanical-biological treatment (MBT) plants originally designed for stabilization of organic municipal solid waste (OFMSW). Thus, this study aimed to assess the suitability of the operational conditions in the biological part of a full-scale MBT plant now used for stabilization of OFMSW (working line: closed-module-covered-pile-open-pile) with a view to producing compost from biowaste. Temperatures above 75 °C were maintained in the closed module and reached again in the covered pile, indicating that intensive organic-matter mineralization occurred in both stages. In the covered pile, the temperature sharply decreased, indicating depletion of easily biodegradable organic matter. An aerobic 4-day respiration test (AT4) value below 10 mg O₂/g dry matter, the cut-off for assessing compost stability, was obtained after 8 weeks. However, a high content of humic substances (HS), reflecting compost maturity, was obtained only after 120 days. The increase in HS content proceeded in two phases. In the first phase (45-84 day), the rate constant and the rate of HS formation were lower than in the second phase (84-120 day) (0.072 vs. 0.087 day^-1, 1.97 vs. 3.06 mg C/(g organic matter·d)). All the above-mentioned indicators and the nutrient content (N, P, K, Mg, Ca) in the compost indicates that the biological stage of an MBT plant can successfully treat biowaste. This is in accordance with a circular economy and will contribute to increasing recycling rates.

Effect of initial moisture content and sample storage duration on compost stability using the ORG0020 dynamic respiration test

Biological tests are widely used to assess composting process status and finished material stability. Although compost stability is known to be influenced by moisture content (MC) and storage duration, there is a lack of data supporting boundary limits for standardised testing. Using the ORG0020 dynamic respiration test we assessed the stability of materials from different commercial composting sites processing only green waste or mixed green and food waste. Samples were tested at three different MC following adjustment with the 'fist' test within the range 40-60%. The results showed manipulation of MC within this range could have significant impact on measured stability for some but not all samples. Two samples reported significantly higher activity when MC was manipulated from ~50% to ~60%. For storage duration, samples showed significant decrease in measured activity over several weeks of cold storage. However, there was no significant difference in stability for samples tested up to nine days from receipt. The results of this research will support decisions relating to the boundary limits for moisture content and storage time for the ORG0020 test. The results will also provide insight to the wider range of biological tests used to assess compost stability.

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.

Stabilisation of spent mushroom substrate for application as a plant growth-promoting organic amendment

Over three million tonnes of spent mushroom substrate (SMS) are produced in Europe every year as a by-product of the cultivation of Agaricus bisporus. The management of SMS has become an increasing challenge for the mushroom production industry, and finding environmentally and economically sustainable solutions for this organic residue is, therefore, highly desirable. Due to its physical properties and nutrient content, SMS has great potential to be employed in agricultural and horticultural sectors, and further contribute to reduce the use of non-renewable resources, such as peat. However, SMS is often regarded as not being stable and/or mature, which hampers its wide use for crop production. Here, we demonstrate the stabilisation of SMS and its subsequent use as organic fertiliser and partial peat replacement in horticulture. The stabilisation was performed in a laboratory-scale composting system, with controlled temperature and aeration. Physical and chemical parameters were monitored during composting and provided information on the progress of the process. Water soluble carbohydrates (WSC) content was found to be the most reliable parameter to predict SMS stability. In situ oxygen consumption indicated the main composting phases, reflecting major changes in microbial activity. The structure of the bacterial community was also found to be a potential predictor of stability, as the compositional changes followed the composting progress. By contrast, the fungal community did not present clear successional process along the experiment. Maturity and quality of the stabilised SMS were assessed in a horticultural growing trial. When used as the sole fertiliser source, SMS was able to support Lolium multiflorum (Italian ryegrass) growth and significantly improved grass yield with a concentration-dependent response, increasing grass biomass up to 300%, when compared to the untreated control. In summary, the results indicated that the method employed was efficient in generating a stable and mature product, which has a great potential to be applied in horticulture. This study represents a step forward in the management of SMS residue, and also provides an alternative to reduce the use of peat in horticulture, alleviating environmental impacts to peatland ecosystems.

Composting technology in waste stabilization: On the methods, challenges and future prospects

Composting technology has become invaluable in stabilization of municipal waste due to its environmental compatibility. In this review, different types of composting methods reportedly applied in waste management were explored. Further to that, the major factors such as temperature, pH, C/N ratio, moisture, particle size that have been considered relevant in the monitoring of the composting process were elucidated. Relevant strategies to improve and optimize process effectiveness were also addressed. However, during composting, some challenges such as leachate generation, gas emission and lack of uniformity in assessing maturity indices are imminent. Here in, these challenges were properly addressed and some strategies towards ameliorating them were proffered. Finally, we highlighted some recent technologies that could improve composting.

Static, dynamic and inoculum augmented respiration based test assessment for determining in-vessel compost stability

The purpose of this work was to evaluate compost (and related industry) stability tests given recent large-scale changes to feedstock, processing techniques and compost market requirements. Five stability tests (ORG0020, DR4, Dewar self-heating, oxygen update rate (OUR) and static respiration) were evaluated on composts from ten in-vessel composting sites. Spearman rank correlation coefficients were strong for the ORG0020, OUR and DR4 (both CO2 and O2 measurement), however, OUR results required data extrapolation for highly active compost samples. By comparison the Dewar self-heating and static respiration tests had weaker correlations, in part the result of under reporting highly active, low pH samples. The findings suggest that despite differences in pre-incubation period both dynamic respiration tests (ORG0020 and DR4) are best suited to deal with the wide range of compost stabilities found.

Composting of waste algae: a review

Although composting has been successfully used at pilot scale to manage waste algae removed from eutrophied water environments and the compost product applied as a fertiliser, clear guidelines are not available for full scale algae composting. The review reports on the application of composting to stabilize waste algae, which to date has mainly been macro-algae, and identifies the peculiarities of algae as a composting feedstock, these being: relatively low carbon to nitrogen (C/N) ratio, which can result in nitrogen loss as NH3 and even N2O; high moisture content and low porosity, which together make aeration challenging; potentially high salinity, which can have adverse consequence for composting; and potentially have high metals and toxin content, which can affect application of the product as a fertiliser. To overcome the challenges that these peculiarities impose co-compost materials can be employed.