Evaluation of “alperujo” composting based on organic matter degradation, humification and compost quality

Olive mill waste sludge: From permanent pollution to a highly beneficial organic biofertilizer: A critical review and future perspectives

Olive mill wastewater sludge (OMWS) is a by-product of the olive extraction process that is attracting substantial attention due to its extremely hazardous effects on aquatic and terrestrial ecosystems. OMWS is a product of the common disposal method of olive oil mill wastewater (OMWW) that accumulates in evaporation ponds. It is estimated that approximately 10 × 10⁶ m³ of OMWS is generated worldwide each year. OMWS is characterized by its significantly variable physicochemical properties and organic pollutant constituents, such as phenols and lipids, which are dependent upon the environmental features of the receiving ponds. Nonetheless, many related studies have recognized the biofertilizer potential of this sludge owing to its high mineral nutrient and organic matter load. OMWS exhibits promising valorization potential in several fields, including agriculture and energy production. Compared to those of OMWW, studies of OMWS are still lacking concerning its composition and characteristics, which are necessary for the future implementation of efficient valorization strategies. The main purpose of this review paper is to fill the gap that exists in the literature by providing a critical analysis of the available data on OMWS production, distribution, characteristics, and properties. Additionally, this work sheds light on important factors affecting OMWS properties, including the variability of the indigenous microbial communities regarding bioremediation. Finally, this review addresses the current and future valorization routes, from detoxification to the development of promising applications in agriculture, energy, and the environment, which could have significant socioeconomic implications for low-income Mediterranean countries.

Towards a circular economy in virgin olive oil production: Valorization of the olive mill waste (OMW) "alpeorujo" through polyphenol recovery with natural deep eutectic solvents (NADESs) and vermicomposting

Virgin olive oil (VOO) production generates large amounts of a harmful by-product, olive mill waste (OMW) or alpeorujo, which has a strong environmental impact and that must be recycled to adapt VOO production to a circular economy model. Here, the valorization of OMW was studied by considering three consecutive stages: Stage 1 involves the generation of OMW; Stage 2 the recovery of bioactive phenolic compounds from the fresh OMW using natural deep eutectic solvents (NADESs), generating a valuable phenolic extract and a new by-product, a dephenolized OMW named "alpeoNADES"; and Stage 3 involves vermicomposting alpeoNADES with Eisenia fetida earthworms. Six NADES were formulated and tested, selecting a NADES composed of citric acid and fructose (CF) derived from food grade and biodegradable substances. CF was the most effective solvent to obtain phenolic extracts for nutraceutical and agronomical purposes, extracting 3988.74 mg/kg of polyphenols from fresh OMW. This alpeoNADES is a non-palatable substrate for E. fetida earthworms, as the residual CF gives it an acidic pH (pH 2). Its palatability was improved by mixing it with horse manure and straw for vermicomposting, in a 1:1 and 3:1 dry weight ratio. When these substrates were precomposted for 3 weeks they reached pH 5.5-6 and they could then be vermicomposted for 23 weeks (using OMW as a control). The best substrate for vermicomposting was determined by the worm biomass, growth rate, carbon to nitrogen (C:N) ratio, and N and P content. AlpeoNADES and manure 3:1 produced the highest quality vermicompost in the shortest time, generating a product that complied with European standards for organic fertilizers. Hence, alpeoNADES was recycled to a low-cost, organic balanced fertilizer in Stage 3, enabling the olive oil industry to transition to sustainable production through this integrated circular economy design.

Field evaluation of industrial non-food crops for phytomanaging a metal-contaminated dredged sediment

Phytomanagement is a concept fit for a bio-based circular economy that combines phytotechnologies and biomass production for non-food purposes. Here, ten annual and perennial industrial non-food crops (Sorghum Biomass 133, Sorghum Santa Fe red, Linum usitatissimum L., Eucalyptus sp., Salix Inger, Salix Tordis, Beta vulgaris L., Phacelia tanacetifolia Benth., Malva sylvestris L., and Chenopodium album L.) were studied under field conditions for phytomanaging a metal (Cd, Cu, Pb, and Zn)-contaminated dredged sediment in the North of France. The crops were selected according to their relevance to pedoclimatic and future climatic conditions, and one or more non-food end-products were proposed for each plant part collected, such as biogas, bioethanol, compost, natural dye, ecocatalyst, and fiber. Based on the soil-plant transfer of metals, eight out of the crops cultivated on field plots exhibited an excluder behavior (bioconcentration factor, BCF < 1), a trait suitable for phytostabilization. However, these crops did not change the metal mobilities in the dredged sediment. The BCF < 1 was not sufficient to characterize the excluder behavior of crops as this factor depended on the total dredged-sediment contaminant. Therefore, a BCF group ranking method was proposed accounting for metal phytotoxicity levels or yield decrease as a complemental way to discuss the crop behavior. The feasibility of the biomass-processing chains was discussed based on these results and according to a survey of available legislation in standard and scientific literature.

Involvement of the metabolically active bacteria in the organic matter degradation during olive mill waste composting

RNA-based high-throughput sequencing is a valuable tool in the discernment of the implication of metabolically active bacteria during composting. In this study, "alperujo" composting was used as microbial model for the elucidation of structure-function relationships with physicochemical transformation of the organic matter. DNA and RNA, subsequently retrotranscribed into cDNA, were isolated at the mesophilic, thermophilic and maturation phases. 16S rRNA gene was amplified by quantitative PCR (qPCR) and Illumina MiSeq platform to assess bacterial abundance and diversity, respectively. The results showed that the abundance of active bacteria assessed by qPCR was maximum at thermophilic phase, which confirm it as the most active stage of the process. Concerning diversity, Proteobacteria, Firmicutes, Bacteroidetes and Actinobacteria were the main phyla presented in composts. Concomitantly, three different behaviours were observed for bacterial dynamics: some genera decreased during the whole process meanwhile others proliferated only at thermophilic or maturation phase. Statistical correlation between physicochemical transformations of the organic matter and bacterial diversity revealed bacterial specialisation. This result indicated that specific groups of bacteria were only involved in the organic matter degradation during bio-oxidative phase or humification at maturation. Metabolic functions predictions confirmed that active bacteria were mainly involved in carbon (C) and nitrogen (N) cycles transformations, and pathogen reduction.

Composting of Olive Mill Pomace, Agro-Industrial Sewage Sludge and Other Residues: Process Monitoring and Agronomic Use of the Resulting Composts

The viability of co-composting of olive mill pomace added to sewage sludge with other organic residues was evaluated and the agronomic use of the final composts was investigated. Two composting piles at different carbon-nitrogen ratios were performed, in which olive mill pomace (OMP), sewage sludge from vegetable processing (SS), fresh residues from artichoke processing residues (AR), and wheat straw (WS) were used. The two composting piles were placed inside a specially built greenhouse and a turning machine pulled by a tractor was used for turning and shredding the organic matrix (every 6 days) during the process. The humidity and temperature of organic matrices have been monitored and controlled during the entire composting process, which lasted 90 days. The process was also monitored to evaluate the microbiological safety of the final compost. The humidity of both piles was always kept just above 50% until the end of the thermophilic phase and the maximum temperature was about 50 °C during the thermophilic phase. The carbon-nitrogen ratio decreased from 21.4 and 28.2, respectively (initial value at day 1 in Pile A and B), to values ranging from 12.9 to 15.1, both composts that originated from the two different piles were microbiologically safe. During a two-year period, the effects of different types of compost on the main qualitative parameters of processing tomato and durum wheat was evaluated. Five fertilization treatments were evaluated for tomato and durum wheat crops: unfertilized control (TR₁); compost A (TR₂); compost B (TR₃); ½ mineral and ½ compost A (TR₄); and mineral fertilizer commonly used for the two crops (TR₅). Concerning the processing tomato yield, TR₅ and TR₄ showed the best results (2.73 and 2.51 kg, respectively). The same trend was observed considering the marketable yield per plant. The only difference was related to the treatments that included the compost (2.32, 1.77, and 1.73 kg/plant for TR₄, TR₃, and TR₂, respectively). As regards the qualitative parameters of tomato, the highest average weight of the fruits was found in the TR₅, TR₄, and TR₃ treatments (respectively, 73.67 g, 70.34 g, and 68.10 g). For durum wheat, only the protein component was differentiated between treatments. Furthermore, wheat grain yield parameters generally increased by combined application of mineral fertilizer and compost.

Composting of swine production chain wastes with addition of crude glycerin: organic matter degradation kinetics, functional groups, and carboxylic acids

Little is known about the effect of adding crude glycerin (CG) as a carbon source during the composting of agro-industrial residues, such as those generated in the swine production chain, especially concerning the impact on organic matter humification. Therefore, the aim of this work was to study the effect of adding crude glycerin during the composting of organic swine waste, using appropriate analyses to determine the degree of maturation of the organic material. The experiment was performed using composters constructed from pallets. The variables considered were temperature, mass, volume, organic matter, functional groups, carboxylic acids, pH, electrical conductivity, total organic carbon, total Kjeldahl nitrogen, total phosphorus, potassium, basal respiration, and germination index. For all the CG concentrations tested, thermophilic temperatures were reached, while higher amounts of CG (4.5 and 6.0%) maintained temperatures above 55 °C for longer periods (28 days). Fourier transform infrared spectroscopy analysis showed the presence of an aromatic stretching vibration signal at 1620 cm^-1, confirming mineralization of the organic matter, while the decrease of carboxylic acids at the end of the composting period indicated stabilization. The organic composts presented high nutrient contents and absence of toxicity, indicating that they could be safely used in agriculture.

Sorption and dissipation of the allelochemicals umbelliferone and salicylic acid in a Mediterranean soil environment: Effect of olive-mill waste addition

Allelochemicals are receiving much attention as natural alternatives to synthetic pesticides. Very little is known, however, about the processes to which allelochemicals are subjected once they reach the soil environment, despite the fact that it is widely recognized that such processes can dramatically influence their bioactivity and applicability as eco-friendly pesticides. The objectives of this study were to characterize the sorption and dissipation of two phenolic allelochemicals, umbelliferone (UM) and salicylic acid (SA), after their simultaneous application to a Mediterranean agricultural soil and to assess to what extent sorption and dissipation were affected by amending the soil with an agro-industrial organic waste (olive-mill waste, OMW), as a common agronomic practice in Mediterranean agricultural systems. In experiments conducted under standard laboratory conditions, UM (pK_a = 7.5) showed greater sorption than SA (pK_a = 2.8) and both allelochemicals displayed very short half-lives in the tested soil (DT₅₀ < 1 day). Furthermore, the addition of OMW increased the sorption of UM and the half-lives of both SA and UM in the soil. A field experiment conducted on unamended and OMW-amended soil plots confirmed the ability of OMW to increase the persistence of SA and UM under a real Mediterranean soil environment and showed that, for all treatments, the allelochemicals displayed higher half-lives in the field than under standard laboratory conditions. This was attributed to reduced biodegradation of UM and SA under progressive soil drying, which was thus identified as a factor that can prolong the persistence of allelochemicals in semi-arid soil environments. We highlight the need to test the environmental fate of allelochemicals under specific agro-climatic scenarios and illustrate how management practices can help increase their soil persistence so that their bioactivity can be better expressed.

Waste Willow-Bark from Salicylate Extraction Successfully Reused as an Amendment for Sewage Sludge Composting

Due to the fact that compost is a valuable fertilizer that serves principally as a source of macronutrients, composting is one of the preferred methods of management of organic waste, including municipal sewage sludge. However, due to its high moisture content and low C/N ratio, sewage sludge cannot be composted alone. This study investigated the usefulness of waste willow-bark (WWB) (after salicylate extraction) as an amendment for municipal sewage-sludge composting in a two-stage system: an aerated bioreactor and a periodically turned windrow. Both organic matter (OM) removal and humification progress were monitored. It was found that the prepared feedstock (70% sewage sludge, 25% WWB, and 5% wood chips, w/w) enabled proper temperature profiles to be obtained, with a maximum temperature of 72.3 °C. The rate constant of OM degradation in the bioreactor was 0.25 d−1, almost 4-fold higher than that in the windrows. During composting, the concentrations of humic substances (HS), humic acids (HA), and the fulvic fraction (FF) changed. HS, HA, and FF formation proceeded according to 1. order kinetics, and their respective rates were 1.33 mg C/(g OM d), 1.03 mg C/(g OM d), and 0.76 mg C/(g OM d). However, in mature compost, FF predominated (ca. 70%) in HS. These results indicate that waste willow-bark, a product of salicylate extraction, can be successfully reused as an amendment during municipal sewage sludge composting. Both waste willow-bark reuse and sewage sludge composting are compatible with a circular economy.

Biocomposites: could agroindustrial wastes be used as reinforcements in polymers?

The use of agricultural by-products in the production of biocomposite materials is of growing interest worldwide. Mechanical properties and degradation temperature of a biocomposite depend strongly on the characteristics of the selected reinforcement. The present study focuses on the characterization of three lignocellulosic agro-industrial wastes: olive wet husk (OWH), olive pits (OP) and grape stalks (GS), generated by industries of Cuyo region in Argentina. Such characterization comprises proximate analysis, lignocellulosic composition, functional groups, crystalline phases, mineralogical and elemental composition, and thermal properties. The results obtained are of relevance for understanding the final properties of the biocomposites that will be prepared with these lignocellulosic particles, and will allow to determine which of them is the most appropriate for a specific application. This work suggests that OP could have better interfacial interaction with a polymeric matrix.

Olive mill wastewater-evaporation ponds long term stored: Integrated assessment of in situ bioremediation strategies based on composting and vermicomposting

During the last two decades, the method most widely used to manage olive mill wastewater (OMW) derived from olive oil production has been its disposal in evaporation ponds. Long-term storage of OMW leads to the accumulation of toxic sediments (OMWS) rich in recalcitrant compounds with phytotoxic and antimicrobial properties, which limit their use for agronomic purpose. The aim of this study was to compare the effect of two in situ bioremediation strategies (composting and a combination of composting followed by vermicomposting) to remove the potential toxicity of the sediments derived from long-term stored OMW. The results obtained showed that the composting method assisted with the earthworms enhanced the depletion of phenolic compounds and OMWS ecotoxicity more than composting, especially during the maturation stage. Moreover, vermicomposting was more effective in the reduction of the OMWS salinity. However, a pre-composting process to the OMWS is necessary prior to vermicomposting to provide the suitable conditions for earthworms survival and activity. Furthermore, the final compost showed a phytostimulating effect. Therefore, these in situ bioremediation strategies can be considered potential tools for decontamination and recovery of long-term stored OMWS in evaporation ponds, which currently poses an unsolved environmental problem.

Alcaligenes aquatilis GTE53: Phosphate solubilising and bioremediation bacterium isolated from new biotope "phosphate sludge enriched-compost"

The isolation and identification of beneficial bacteria from the active phase of composting is considered to be a key bio-quality parameter for the assessment of the process. The aim of this work was the selection and identification of beneficial bacteria from a co-composting experiment of vegetable waste (VW), olive oil mill waste (O₂MW), and phosphate sludge (PS). Phosphate-solubilizing strains were isolated from the thermophilic phase using Pikovskaya (PVK) solid medium supplemented with tricalcium phosphate Ca3(PO4) (TCP) as the sole source of phosphorus (P). Therefore, the selected isolate Alcaligenes aquatilis GTE53 was tested to tolerate abiotic stresses (different levels of temperature, variable pH, high salinity and water stress). The isolate was also assessed for indole acetic acid (IAA) and siderophores synthesis, nitrogen fixation, phenol degradation and pathogens inactivation. The quality of the co-composting process was also investigated by monitoring the physico-chemical parameters. The obtained results showed that A. aquatilis GTE53 displayed a higher solubilization index of 2.4 and was efficiently dissolved, up to 162.8 and 247.4 mg·mL^-1 of inorganic phosphate from PS and phosphate rock (PR), respectively. A. aquatilis GTE53 exhibited siderophores and IAA release, along with atmospheric nitrogen fixation. In addition to that, A. aquatilis GTE53 showed a high resistance to heat and tolerance to acidic and alkaline pH, high salinity and water stress. Moreover, A. aquatilis GTE53 could degrade 99.2% of phenol from a high-concentrated medium (1100 mg·L^-1 of phenol) and can inactivate the most abundant pathogens in industrial wastes: Escherichia coli, Streptococcus sp., Salmonella sp., and Fusarium oxysporum albedinis. Analysis of temperature, pH, electrical conductivity, carbon/nitrogen (C/N) ratio, indicated successful co-composting. An efficient transformation of P to the available form and a great abatement of polyphenols, were also recorded during the process. The findings of this study will help to advance the understanding of A. aquatilis GTE53 functions and will facilitate its application to promote beneficial microbial organisms during composting, thus obtaining a high-quality product.

Industrial composting of low carbon/nitrogen ratio mixtures of agri-food waste and impact on compost quality

The agri-food waste (AW) require amendments for composting to adjust nutritional and physicochemical deficiencies. The theoretical mixtures formulation is difficult to reach on an industrial scale. The main objective of this work was to evaluate to what extent the composition of AW-based mixtures determines the quality of the final compost produced at the industrial scale. Raw materials having the same AW share characteristics, irrespectively of the amendments added, but their compost were different. All the materials were biological stable at the cooling phase, and mature enough at the end, although the degree of humification did not match with the absence of phytotoxicity. The final compost had sufficient quality even though the AW-based raw materials have a low C/N ratio (<20) and other characteristics such as high electrical conductivity (13 mS·cm^-1) and pH (<8.5) that are unfavorable for composting. The management operations during industrial composting correct the deficiencies of raw materials.

Assessment of the diversity and abundance of the total and active fungal population and its correlation with humification during two-phase olive mill waste (''alperujo") composting

Metagenomic and transcriptomic techniques applied to composting could increase our understanding of the overall microbial ecology and could help us to optimise operational conditions which are directly related with economic interest. In this study, the fungal diversity and abundance of two-phase olive mill waste ("alperujo") composting was studied using Illumina MiSeq sequencing and quantitative PCR, respectively. The results showed an increase of the fungal diversity during the process, with Ascomycota being the predominant phylum. Penicillium was the main genera identified at the mesophilic and maturation phases, with Debaryomyces and Sarocladium at the thermophilic phase, respectively. The fungal abundance was increased during composting, which confirms their important role during thermophilic and maturation phases. Some Basidiomycota showed an increased during the process, which showed a positive correlation with the humification parameters. According to that, the genus Cystofilobasidium could be used as a potential fungal biomarker to assess alperujo compost maturation.

Enzymatic profiles associated with the evolution of the lignocellulosic fraction during industrial-scale composting of anthropogenic waste: Comparative analysis

In the new European Waste Law, composting is proposed as one of the best options to properly manage organic waste of anthropogenic origin. Currently, the massive generation of this type of waste, as well as its heterogeneity, makes difficult in many cases control this process of degradation on an industrial scale. In this work, 15 facilities were selected based on 5 types of organic waste: Urban Solid Waste, Vegetable Waste, Sewage Sludges, Agrifood Waste and "Alpeorujo". The samples were collected in different thermal phases. The results revealed very different physicochemical and enzymatic profiles, as well as different degrees of humification depending on the process and the raw materials. However, parameters such as β-glucosidase, amylase, lignin/holocellulose ratio and humification rate showed similar trends in all cases. All of them could act as important indicators to evaluate the quality of a composting process, despite the heterogeneity of the starting materials.

Stabilisation of municipal solid waste after autoclaving in a passively aerated bioreactor

Autoclaving of unsorted municipal solid waste is one of the solutions in waste management that maximises the amount of waste for recycling. After autoclaving, however, a large part of the waste is composed of unstabilised biodegradable fractions (organic remaining fraction, ORF), which may comprise up to 30% of autoclaved waste and cannot be landfilled without further stabilisation. Thus, the aim of this study was to investigate the effectiveness of aerobic stabilisation in a passively aerated reactor of organic remaining fraction after full-scale autoclaving of unsorted municipal solid waste. The organic remaining fraction had a volatile solids content of ca. 70%, a 4-day respiration activity test (AT4) of ca. 26 g O₂ kg^-1 total solids and a 21-day gas formation test (GP21) of ca. 235 dm³ kg^-1 total solids. Stabilisation was conducted in a 550 L reactor with passive aeration (Stage I) and a periodically turned windrow (Stage II). The feedstocks consisted entirely of organic remaining fraction, or of organic remaining fraction with 10% inoculum (ORF + I). Inoculum constituted product of stabilisation of organic remaining fraction. During stabilisation of organic remaining fraction and ORF + I, thermophilic conditions were achieved, and the decreases of volatile solids, AT4 and GP21 could be described by 1 order kinetic models. The rate constants of volatile solids removal (k_VS) were 0.033 and 0.068 d^-1 for organic remaining fraction and ORF + I, respectively, and the thermophilic phase was shorter with ORF + I (25 days vs. 45 days). The decrease in GP21 corresponded to volatile solids decrease, but AT4 decreased sharply during the first 10 days of waste stabilisation in the reactor, indicating that the content of highly biodegradable organic matter decreased during this time.

Co-composting of Olive Mill Waste and Wine-Processing Waste: An Application of Compost as Soil Amendment

In order to decrease the environmental harm produced by the agro industries’ wastes’, an investigation of the co-composting of olive mill waste (olive mill wastewater (OMW), olive mill sludge (OMS)) and wine by-products (grape marc and winery wastewater) was done. Three aerated windrows of variable compositions were performed; these windrows differ in terms of their initial composition and the liquid used for their humidification; OMW and wastewater winery were used for humidification to replace water for windrow moistening. Moreover, the main physicochemical parameters (temperature, pH, electrical conductivity, and C/N) were monitored to evaluate the co-composting process. The latter lasted around three months. The elaborated composts were characterized by low C/N ratio, and they were rich in fertilizing and nutriment elements and of low heavy metal contents. The humidification of the windrows with OMW showed effectiveness in improving the windrows temperature, reflected by the high temperatures monitored during the composting process in comparison with the windrow humidified with winery wastewater. Furthermore, a longer thermophilic phase was held in windrows carrying OMS. The valorization of the produced composts for soil amendment significantly improved the soil fertility. Indeed, field experiments showed an increase in radish yield by 10%, the composts were harmless and did not have any phytotoxic effect on radish growth.

Effect of barley straw and coniferous bark on humification process during sewage sludge composting

This study investigated how different amendments (barley straw, coniferous bark) influence organic matter (OM) removal kinetics and humification during sewage sludge composting. With bark, high temperatures, intensive OM degradation and humification were achieved later than with straw. The rate of OM degradation was lower with bark than with straw (15.18 g/kg OM·d vs 24.07 g/kg OM·d) and the time needed for intensive HS formation was longer with bark (140 days vs 60 days). The kinetic constants for humic substances (HS) and humic acids (HA) formation were lower with bark than with straw (k_HS 0.025 d^-1 vs 0.047 d^-1, k_HA 0.022 d^-1 vs 0.044 d^-1). With bark, however, the increase in HS concentration during composting (C_max,HS) was higher (178 mg C/g OM vs 84 mg C/g OM), and the fulvic fraction predominated in HS (80%), whereas with straw, humic acids predominated (82% of HS).

Valorization of a pharmaceutical organic sludge through different composting treatments

Nowadays, the agricultural reuse of pharmaceutical sludge is still limited due to environmental and agronomic issues (e.g. low stabilization of the organic matter, phytotoxicity). The aim of the present study was to evaluate the characteristics of a pharmaceutical sludge derived from the daptomycin production and to study the possibility of improving its quality through composting. The pharmaceutical sludge showed high content of macronutrients (e.g. total Kjeldahl N content was 38 g kg^-1), but it was also characterized by high salinity (7.9 dS m^-1), phytotoxicity (germination index was 36.7%) and a low organic matter stabilization. Two different mixtures were prepared (mixture A: 70% sludge + 30% wood chips w/w, mixture B: 45% sludge + 45% wood chips + 10% cereal straw w/w) and treated through static composting using two different aeration systems: active and passive aeration. The mixtures resulted in the production of two different compost, and the evolution of process management parameters was different. The low total solids and organic matter content of mixture A led to the failure of the process. The addition of cereal straw in mixture B resulted in increased porosity and C/N ratio and, consequently, in an optimal development of the composting process (e.g. the final organic matter loss was 54.1% and 63.1% for the passively and actively aerated treatment, respectively). Both passively and actively aerated composting of mixture B improved the quality of the pharmaceutical sludge, by increasing its organic matter stabilization and removing phytotoxicity.

Role of biochar on composting of organic wastes and remediation of contaminated soils-a review

Biochar is produced by pyrolysis of biomass residues under limited oxygen conditions. In recent years, biochar as an amendment has received increasing attention on composting and soil remediation, due to its unique properties such as chemical recalcitrance, high porosity and sorption capacity, and large surface area. This paper provides an overview on the impact of biochar on the chemical characteristics (greenhouse gas emissions, nitrogen loss, decomposition and humification of organic matter) and microbial community structure during composting of organic wastes. This review also discusses the use of biochar for remediation of soils contaminated with organic pollutants and heavy metals as well as related mechanisms. Besides its aging, the effects of biochar on the environment fate and efficacy of pesticides deserve special attention. Moreover, the combined application of biochar and compost affects synergistically on soil remediation and plant growth. Future research needs are identified to ensure a wide application of biochar in composting and soil remediation. Graphical abstract ᅟ.

Growth and reproductive potential of Eisenia foetida (Sav) on various zoo animal dungs after two methods of pre-composting followed by vermicomposting

Disposal of animal manure without treatment can be harmful to the environment. In this study, samples of four zoo animal dungs and one horse dung were pre-composted in two ways: (a) traditional composting and (b) bokashi pre-composting for 1month, followed by vermicomposting for 3months. The permanence (PEf) and reproductive potential (RP) of Eisenia foetida as well as the quality of vermicompost were evaluated. The PEf values and RP index of E. foetida were higher for samples pre-composted using the traditional composting method (98.7-88% and 31.85-16.27%, respectively) followed by vermicomposting (92.7-72.7% and 22.96-13.51%, respectively), when compared with those for bokashi pre-composted samples followed by vermicomposting, except for the horse dung sample (100% for both the parameters). The values of electrical conductivity (EC), cation exchange capacity (CEC), organic C, total N, available P, C/N ratio, and pH showed that both treatments achieved the norms of vermicompost (<4mScm^-1, 40cmolkg^-1, 20-50%, 1-4%, ≤20, 5.5-8.5, respectively). However, the maturity indices of vermicompost, namely, organic matter loss, N loss, and CEC/organic carbon (OC) ratio indicated that bokashi pre-composting followed by vermicomposting produced the highest values (98.7-70.7%, 97.67-96.65%, and 2.7-1.97%, respectively), when compared with the other method adapted in this study. Nevertheless, further studies with plants for plant growth evaluation are needed to assess the benefits and limitations of these two pre-composting methods prior to vermicomposting.

Evolution of bacterial diversity during two-phase olive mill waste ("alperujo") composting by 16S rRNA gene pyrosequencing

Microorganisms are the main contributing factor responsible for organic matter degradation during composting. In this research, the 454-pyrosequencing of the 16S rRNA gene was used to elucidate evolution of bacterial diversity during mesophilic, thermophilic and maturation composting stages of the two-phase olive mill waste ("alperujo"), the main by-product of the Spanish olive oil industry. Two similar piles were performance composting AL with sheep manure as bulking agent. Actinobacteria, Bacteriodetes, Firmicutes and Proteobacteria were the main phyla found in genomic libraries from each composting phase. Shannon and Chao1 biodiversity indices showed a clear difference between the mesophilic/thermophilic and maturation phases, which was mainly due to detection of new genera. PCA analysis of the relative number of sequences confirmed maturation affected bacterial population structure, and Pearson correlation coefficients between physicochemical composting parameters and relative number of genera sequences suggest that Planomicrobium and Ohtaekwangia could be considered as biomarkers for AL composting maturation.

Seafood-Processing Sludge Composting: Changes to Microbial Communities and Physico-Chemical Parameters of Static Treatment versus for Turning during the Maturation Stage

In general, in composting facilities the active, or intensive, stage of the process is done separately from the maturation stage, using a specific technology and time. The pre-composted material to be matured can contain enough biodegradable substrates to cause microbial proliferation, which in turn can cause temperatures to increase. Therefore, not controlling the maturation period during waste management at an industrial level can result in undesired outcomes. The main hypothesis of this study is that controlling the maturation stage through turning provides one with an optimized process when compared to the static approach. The waste used was sludge from a seafood-processing plant, mixed with shredded wood (1:2, v/v). The composting system consists of an intensive stage in a 600L static reactor, followed by maturation in triplicate in 200L boxes for 112 days. Two tests were carried out with the same process in reactor and different treatments in boxes: static maturation and turning during maturation when the temperature went above 55°C. PLFAs, organic matter, pH, electrical conductivity, forms of nitrogen and carbon, hydrolytic enzymes and respiratory activity were periodically measured. Turning significantly increased the duration of the thermophilic phase and consequently increased the organic-matter degradation. PCA differentiated significantly the two treatments in function of tracking parameters, especially pH, total carbon, forms of nitrogen and C/N ratio. So, stability and maturity optimum values for compost were achieved in less time with turnings. Whereas turning resulted in microbial-group stabilization and a low mono/sat ratio, static treatment produced greater variability in microbial groups and a high mono/sat ratio, the presence of more degradable substrates causes changes in microbial communities and their study during maturation gives an approach of the state of organic-matter degradation. Obtaining quality compost and optimizing the composting process requires using turning as a control mechanism during maturation.

End-product quality of composts produced under tropical and temperate climates using different raw materials: A meta-analysis

A meta-analysis on end-product quality of 442 composts was performed to assess the effects of climate and raw materials on compost quality. The analysis was performed using an ANOVA including a mixed model with nested factors (climate, raw material and publication effect). Tropical composts presented lower carbon, nitrogen, potassium and soluble-carbon contents, and higher electrical conductivity. The results suggest that compost quality in the tropics was affected by weather conditions during composting (e.g. high temperature and rainfall), which induced high losses of carbon and nutrients. For most properties, industrial, sewage sludge and manure-based composts displayed the highest quality under both climates, while the contrary was found for household and municipal solid waste-based composts. The publication effect represented >50% of total variance, which was mainly due to the heterogeneity of the composting procedures. The meta-analysis was found to be a helpful tool to analyse the imbalanced worldwide database on compost quality.

Evolution of microbial dynamics during the maturation phase of the composting of different types of waste

During composting, facilities usually exert greater control over the bio-oxidative phase of the process, which uses a specific technology and generally has a fixed duration. After this phase, the material is deposited to mature, with less monitoring during the maturation phase. While there has been considerable study of biological parameters during the thermophilic phase, there is less research on the stabilization and maturation phase. This study evaluates the effects of the type of starting material on the evolution of microbial dynamics during the maturation phase of composting. Three waste types were used: sludge from the fish processing industry, municipal sewage sludge and pig manure, each independently mixed with shredded pine wood as bulking agent. The composting system for each waste type comprised a static reactor with capacity of 600L for the bio-oxidative phase followed by stabilization and maturation phase in triplicate 200L boxes for 112days. Phospholipid fatty acids, enzyme activities and physico-chemical parameters were measured throughout the maturation phase. The evolution of the total microbial biomass, Gram + bacteria, Gram - bacteria, fungi and enzymatic activities (β-glucosidase, cellulase, protease, acid and alkaline phosphatase) depended significantly on the waste type (p<0.001). The predominant microbial community for each waste type remained present throughout the maturation process, indicating that the waste type determines the microorganisms that are able to develop at this stage. While fungi predominated during fish sludge maturation, manure and municipal sludge were characterized by a greater proportion of bacteria. Both the structure of the microbial community and enzymatic activities provided important information for monitoring the composting process. More attention should be paid to the maturation phase in order to optimize composting.

Dissipation and effects of tricyclazole on soil microbial communities and rice growth as affected by amendment with alperujo compost

The presence of pesticides in surface and groundwater has grown considerably in the last decades as a consequence of the intensive farming activity. Several studies have shown the benefits of using organic amendments to prevent losses of pesticides from runoff or leaching. A particular soil from the Guadalquivir valley was placed in open air ponds and amended at 1 or 2% (w/w) with alperujo compost (AC), a byproduct from the olive oil industry. Tricyclazole dissipation, rice growth and microbial diversity were monitored along an entire rice growing season. An increase in the net photosynthetic rate of Oryza sativa plants grown in the ponds with AC was observed. These plants produced between 1100 and 1300kgha(-1) more rice than plants from the unamended ponds. No significant differences were observed in tricyclazole dissipation, monitored for a month in soil, surface and drainage water, between the amended and unamended ponds. The structure and diversity of bacteria and fungi communities were also studied by the use of the polymerase chain reaction denaturing gel electrophoresis (PCR-DGGE) from DNA extracted directly from soil samples. The banding pattern was similar for all treatments, although the density of bands varied throughout the time. Apparently, tricyclazole did not affect the structure and diversity of bacteria and fungi communities, and this was attributed to its low bioavailability. Rice cultivation under paddy field conditions may be more efficient under the effects of this compost, due to its positive effects on soil properties, rice yield, and soil microbial diversity.

Kinetics of organic matter removal and humification progress during sewage sludge composting

This study investigated the kinetics of organic matter (OM) removal and humification during composting of sewage sludge and lignocellulosic waste (wood chips, wheat straw, leaves) in an aerated bioreactor. Both OM degradation and humification (humic substances, HS, and humic acids, HA formation) proceeded according to 1. order kinetics. The rate constant of OM degradation was 0.196 d(-1), and the rate of OM degradation was 39.4 mg/g OM d. The kinetic constants of HS and HA formation were 0.044 d(-1) and 0.045 d(-1), whereas the rates of HS and HA formation were 3.46 mg C/g OM d and 3.24 mg C/g OM d, respectively. The concentration profiles of HS and HA indicated that humification occurred most intensively during the first 3 months of composting. The high content of HS (182 mg C/g OM) in the final product indicated that the compost could be used in soil remediation as a source of HS for treating soils highly contaminated with heavy metals.

Biochar improves N cycling during composting of olive mill wastes and sheep manure

The use of biochar has been revealed to have beneficial effects during the composting of manures and other N-rich materials by reducing N losses and enhancing the rate of the process. However, the impact of biochar has not been explored in other complex organic matrices with low N nitrogen that may hinder the composting process. The main novelty of this work was to study the impact of a small amount of biochar (4%) on the composting process of olive mill wastes, which are characterised by a recalcitrant lignocellulosic composition with reduced nitrogen (N) availability. Two treatments: (i) control (olive mill waste 46%+sheep manure 54%, dry weight) and (ii) the same mixture treated with biochar (4%), were composted during 31 weeks. The incorporation of a small amount of biochar improved N cycling by increasing NO3(-)-N content, indicating a higher nitrifying activity, and reducing N losses by 15% without affecting the amount of N2O released. The use of biochar as an additive for composting could improve the value of olive mill waste composts by reducing N losses and increasing N availability in lignocellulosic and N-poor materials.

Application of compost for effective bioremediation of organic contaminants and pollutants in soil

Soils contaminated with hazardous chemicals worldwide are awaiting remediation activities; bioremediation is often considered as a cost-effective remediation approach. Potential bioapproaches are biostimulation, e.g. by addition of nutrients, fertiliser and organic substrates, and bioaugmentation by addition of compound-degrading microbes or of organic amendments containing active microorganisms, e.g. activated sludge or compost. In most contaminated soils, the abundance of the intrinsic metabolic potential is too low to be improved by biostimulation alone, since the physical and chemical conditions in these soils are not conducive to biodegradation. In the last few decades, compost or farmyard manure addition as well as composting with various organic supplements have been found to be very efficient for soil bioremediation. In the present minireview, we provide an overview of the composting and compost addition approaches as 'stimulants' of natural attenuation. Laboratory degradation experiments are often biased either by not considering the abiotic factors or by focusing solely on the elimination of the chemicals without taking the biotic factors and processes into account. Therefore, we first systemise the concepts of composting and compost addition, then summarise the relevant physical, chemical and biotic factors and mechanisms for improved contaminant degradation triggered by compost addition. These factors and mechanisms are of particular interest, since they are more relevant and easier to determine than the composition of the degrading community, which is also addressed in this review. Due to the mostly empirical knowledge and the nonstandardised biowaste or compost materials, the field use of these approaches is highly challenging, but also promising. Based on the huge metabolic diversity of microorganisms developing during the composting processes, a highly complex metabolic diversity is established as a 'metabolic memory' within developing and mature compost materials. Compost addition can thus be considered as a 'super-bioaugmentation' with a complex natural mixture of degrading microorganisms, combined with a 'biostimulation' by nutrient containing readily to hardly degradable organic substrates. It also improves the abiotic soil conditions, thus enhancing microbial activity in general. Finally, this minireview also aims at guiding potential users towards full exploitation of the potentials of this approach.

Mathematical modeling of olive mill waste composting process

The present study aimed at developing an integrated mathematical model for the composting process of olive mill waste. The multi-component model was developed to simulate the composting of three-phase olive mill solid waste with olive leaves and different materials as bulking agents. The modeling system included heat transfer, organic substrate degradation, oxygen consumption, carbon dioxide production, water content change, and biological processes. First-order kinetics were used to describe the hydrolysis of insoluble organic matter, followed by formation of biomass. Microbial biomass growth was modeled with a double-substrate limitation by hydrolyzed available organic substrate and oxygen using Monod kinetics. The inhibitory factors of temperature and moisture content were included in the system. The production and consumption of nitrogen and phosphorous were also included in the model. In order to evaluate the kinetic parameters, and to validate the model, six pilot-scale composting experiments in controlled laboratory conditions were used. Low values of hydrolysis rates were observed (0.002841/d) coinciding with the high cellulose and lignin content of the composting materials used. Model simulations were in good agreement with the experimental results. Sensitivity analysis was performed and the modeling efficiency was determined to further evaluate the model predictions. Results revealed that oxygen simulations were more sensitive on the input parameters of the model compared to those of water, temperature and insoluble organic matter. Finally, the Nash and Sutcliff index (E), showed that the experimental data of insoluble organic matter (E>0.909) and temperature (E>0.678) were better simulated than those of water.

Crop residue stabilization and application to agricultural and degraded soils: A review

Agricultural activities produce vast amounts of organic residues including straw, unmarketable or culled fruit and vegetables, post-harvest or post-processing wastes, clippings and residuals from forestry or pruning operations, and animal manure. Improper disposal of these materials may produce undesirable environmental (e.g. odors or insect refuges) and health impacts. On the other hand, agricultural residues are of interest to various industries and sectors of the economy due to their energy content (i.e., for combustion), their potential use as feedstock to produce biofuels and/or fine chemicals, or as a soil amendments for polluted or degraded soils when composted. Our objective is review new biotechnologies that could be used to manage these residues for land application and remediation of contaminated and eroded soils. Bibliographic information is complemented through a comprehensive review of the physico-chemical fundamental mechanisms involved in the transformation and stabilization of organic matter by biotic and abiotic soil components.

Shifts in soil chemical properties and bacterial communities responding to biotransformed dry olive residue used as organic amendment

Dry olive residue (DOR) is a waste product derived from olive oil extraction and has been proposed as an organic amendment. However, it has been demonstrated that a pre-treatment, such as its transformation by saprophytic fungi, is required before DOR soil application. A greenhouse experiment was designed where 0 and 50 g kg(-1) of raw DOR (DOR), Coriolopsis floccosa-transformed DOR (CORDOR) and Fusarium oxysporum-transformed DOR (FUSDOR) were added to soil. Analyses of the soil chemical properties as well as the structure and relative abundance of bacterial and actinobacterial communities were conducted after 0, 30 and 60 days following amendment. The different amendments produced a slight decrease in soil pH and significant increases in carbon fractions, C/N ratios, phenols and K, with these increases being more significant after DOR application. Quantitative PCR assays of the 16S rRNA gene and PLFA analyses showed that all amendments favoured bacterial growth at 30 and 60 days, although actinobacterial proliferation was more evident after CORDOR and FUSDOR application at 60 days. Bacterial and actinobacterial DGGE multivariate analyses showed that the amendments produced structural changes in both communities, especially after 60 days of amendment. PLFA data analysis identified changes in soil microbial communities according to the amendment considered, with FUSDOR and CORDOR being less disruptive than DOR. Finally, integrated analysis of all data monitored in the present study enabled us to conclude that the greatest impact on soil properties was caused by DOR at 30 days and that soil showed some degree of resilience after this time.

Biodiversity and succession of mycobiota associated to agricultural lignocellulosic waste-based composting

A comprehensive characterization of the culturable mycobiota associated to all stages of lignocellulose-based composting was achieved. A total of 77 different isolates were detected, 69 of which were identified on the basis of the 5.8-ITS region sequencing. All the isolates were assigned to the phyla Ascomycota and Basidiomycota, with prevalence of the Sordariomycetes (19) and Eurotiomycetes (17) classes. Penicillium was the most represented genus (11 species), while the species Gibellulopsis nigrescens and Microascus brevicaulis were detected at all the composting stages and showed the highest relative abundances. Fungal diversity decreased as the process proceed, while similarity between fungal communities associated to different samples were maximal for those phases closely connected chronologically and showing similar biological activity degree. Thus, the structure of the lignocellulose-based composting mycobiota can be divided into two major stages corresponding to bio-oxidative phase and maturation phase together with the final product, with a transitional cooling stage joining both of them.

Sewage sludge composting in a two-stage system: carbon and nitrogen transformations and potential ecological risk assessment

The study examined how aeration rate (AR) in bioreactor (1.0 and 0.5l/min kg dm) at low C/N ratio (ca. 15-16) affected kinetics of organic matter (OM) removal, i.e. rate constant of OM removal (k) and maximum degradation of OM (A) and nitrogen evolution during sewage sludge composting. Moreover, potential ecological risk (Er) based on metal (Cd, Cr, Cu, Hg, Ni, Pb, Zn) content was evaluated. The process involved a two-stage system (bioreactor and windrow). In the bioreactor, at higher AR, k and A equaled 0.34 d(-1) and 101.9 g/kg dm, respectively; at lower AR k was 0.38 d(-1), however A 1.4-fold lower. Interestingly, in the windrow, k was much higher (0.086 d(-1)) for the biomass subjected to a lower AR, compared to 0.026 d(-1) at higher AR. Moreover, although at lower AR, k in the windrow was 4.4-fold lower than in the bioreactor, A was 1.15-fold higher. Total N content in mature compost was on the level 23.51-22.35 g/kg dm and metal concentration showed low ecological risk (Er < 16).

Enzymatic characterization of microbial isolates from lignocellulose waste composting: chronological evolution

Successful composting is dependent upon microbial performance. An interdependent relationship is established between environmental and nutritional properties that rule the process and characteristics of the dominant microbial communities. To reach a better understanding of this relationship, the dynamics of major metabolic activities associated with cultivable isolates according to composting phases were evaluated. Ammonification (72.04%), amylolysis (35.65%), hemicellulolyis (30.75%), and proteolysis (33.61%) were the more frequent activities among isolates, with mesophilic bacteria and fungi as the prevalent microbial communities. Bacteria were mainly responsible for starch hydrolysis, while a higher percentage of hemicellulolytic and proteolytic isolates were ascribable to fungi. Composting seems to exert a functional selective effect on microbial communities by promoting the presence of specific metabolically dominant groups at each stage of the process. Moreover, the application of conglomerate analysis led to the statement of a clear correlation between the chronology of the process and characteristics of the associated microbiota. According to metabolic capabilities of the isolates and their density, three clear clusters were obtained corresponding to the start of the process, including the first thermophilic peak, the rest of the bio-oxidative stage, and the maturation phase.

Coriolopsis rigida, a potential model of white-rot fungi that produce extracellular laccases

In the last two decades, a significant amount of work aimed at studying the ability of the white-rot fungus Coriolopsis rigida strain LPSC no. 232 to degrade lignin, sterols, as well as several hazardous pollutants like dyes and aliphatic and aromatic fractions of crude oil, including polycyclic aromatic hydrocarbons, has been performed. Additionally, C. rigida in association with arbuscular mycorrhizal fungi appears to enhance plant growth, albeit the physiological and molecular bases of this effect remain to be elucidated. C. rigida's ability to degrade lignin and lignin-related compounds and the capacity to transform the aromatic fraction of crude oil in the soil might be partially ascribed to its ligninolytic enzyme system. Two extracellular laccases are the only enzymatic components of its lignin-degrading system. We reviewed the most relevant findings regarding the activity and role of C. rigida LPSC no. 232 and its laccases and discussed the work that remains to be done in order to assess, more precisely, the potential use of this fungus and its extracellular enzymes as a model in several applied processes.

Effects of dry olive residue transformed by Coriolopsis floccosa (Polyporaceae) on the distribution and dynamic of a culturable fungal soil community

Dry olive residue (DOR) is an abundant waste product resulting from a two-phase olive oil extraction system. Due to its high organic and mineral content, this material has been proposed as an organic soil amendment; however, it presents phytotoxic and microtoxic properties. Thus, a pretreatment is necessary before its application to soil. Among the strategies for the bioremediation of DOR is treatment with ligninolytic fungi, e.g. Coriolopsis floccosa. This work aimed to assess the diversity of culturable fungi in a soil of the southeast Iberian Peninsula and to evaluate the short-term impact of untransformed and C. floccosa-transformed DOR on soil mycobiota. A total of 1,733 strains were isolated by the particle filtration method and were grouped among 109 different species using morphological and molecular methods. The majority of isolates were ascomycetes and were concentrated among three orders: Hypocreales, Eurotiales and Capnodiales. The soil amendment with untransformed DOR was associated with a depression in fungal diversity at 30 days and changes in the proportions of the major species. However, when C. floccosa-transformed DOR was applied to the soil, changes in fungal diversity were less evident, and species composition was similar to unamended soil.

Impact assessment of intermediate soil cover on landfill stabilization by characterizing landfilled municipal solid waste

Waste samples at different depths of a covered municipal solid waste (MSW) landfill in Beijing, China, were excavated and characterized to investigate the impact of intermediate soil cover on waste stabilization. A comparatively high amount of unstable organic matter with 83.3 g kg(-1) dry weight (dw) total organic carbon was detected in the 6-year-old MSW, where toxic inorganic elements containing As, Cd, Cr, Cu, Mn, Ni, Pb, and Zn of 10.1, 0.98, 85.49, 259.7, 530.4, 30.5, 84.0, and 981.7 mg kg(-1) dw, respectively, largely accumulated because of the barrier effect of intermediate soil cover. This accumulation resulted in decreased microbial activities. The intermediate soil cover also caused significant reduction in moisture in MSW under the soil layer, which was as low as 25.9%, and led to inefficient biodegradation of 8- and 10-year-old MSW. Therefore, intermediate soil cover with low permeability seems to act as a barrier that divides a landfill into two landfill cells with different degradation processes by restraining water flow and hazardous matter.

Dynamics of microbial communities related to biochemical parameters during vermicomposting and maturation of agroindustrial lignocellulose wastes

Scarce information is available on the changes in abundance of microbial taxa during vermicomposting. Quantitative PCR and DGGE analysis were used to monitor variations in the microbial structure, relative abundance of four bacterial classes and fungi over the vermicomposting and maturation period of wet olive cake (O) and vine shoots (W). Multivariate correlation analysis between microbial structure and abundance, earthworm biomass and enzyme activities revealed similar and divergent interactions in both processes. Although Eisenia fetida development was different, significant correlations were found with β-glucosidase activity and with bacterial and fungal structure. In the vermicomposting period of O and W, a decline was found in bacteria (94% and 77%), fungi (93% and 94%), and Gammaproteobacteria (56% and 71%) but an increase in Betaproteobacteria and Actinobacteria (62-79%). Alphaproteobacteria increased only in O (26%). Despite the different initial lignocellulose wastes, the mature vermicomposts were similar in microbial and biochemical properties.

Composted versus raw olive mill waste as substrates for the production of medicinal mushrooms: an assessment of selected cultivation and quality parameters

Two-phase olive mill waste (TPOMW, “alperujo”) is a highly biotoxic sludge-like effluent of the olive-oil milling process with a huge seasonal production. One of the treatment approaches that has so far received little attention is the use of TPOMW as substrate for the cultivation of edible mushrooms. Fifteen fungal strains belonging to five species (Basidiomycota), that is, Agrocybe cylindracea, Pleurotus cystidiosus, P. eryngii, P. ostreatus, and P. pulmonarius, were evaluated for their efficacy to colonize media composed of TPOMW, which was used either raw or composted in mixtures with wheat straw in various ratios. Qualified strains exhibited high values of biological efficiency (e.g., 120–135% for Pleurotus spp. and 125% for A. cylindracea) and productivity in subsequent cultivation experiments on substrates supplemented with 20–40% composted TPOMW or 20% raw TPOMW. Only when supplementation exceeded 60% for raw TPOMW, a negative impact was noted on mushroom yields which could be attributed to the effluent's toxicity (otherwise alleviated in the respective composted TPOMW medium). Earliness and mushroom size as well as quality parameters such as total phenolic content and antioxidant activity did not demonstrate significant differences versus the control wheat-straw substrate. The substrates hemicellulose content was negatively correlated with mycelium growth rates and yields and positively with earliness; in addition, cellulose: lignin ratio presented a positive correlation with mycelium growth and mushroom weight for A. cylindracea and with earliness for all species examined. TPOMW-based media revealed a great potential for the substitution of traditional cultivation substrates by valorizing environmentally hazardous agricultural waste.

Microbially-enhanced composting of olive mill solid waste (wet husk): bacterial and fungal community dynamics at industrial pilot and farm level

Bacterial and fungal community dynamics during microbially-enhanced composting of olive mill solid waste (wet husk), used as a sole raw material, were analysed in a process carried out at industrial pilot and at farm level by the PCR-DGGE profiling of the 16 and 26S rRNA genes. The use of microbial starters enhanced the biotransformation process leading to an earlier and increased level of bacterial diversity. The bacterial community showed a change within 15 days during the first phases of composting. Without microbial starters bacterial biodiversity increased within 60 days. Moreover, the thermophilic phase was characterized by the highest bacterial biodiversity. By contrast, the biodiversity of fungal communities in the piles composted with the starters decreased during the thermophilic phase. The biodiversity of the microbial populations, along with physico-chemical traits, evolved similarly at industrial pilot and farm level, showing different maturation times.

Composting versus vermicomposting: a comparative study of organic matter evolution through straight and combined processes

Changes in physical and chemical characteristics of an organic waste (tomato crop waste) throughout composting (COM), vermicomposting (VER) and the combination of both processes (C+V) were assessed at five selected stages. COM consisted of a combination of the static Rutgers system with forced aeration and pile turning. For VER Eisenia worms were fed with the raw material. For C+V pre-composted material was added to the worms. Particle size decreased during COM, yet it increased during VER and C+V due to the amalgamation of small particles. pH was alkaline throughout the processes. Heavy irrigation during vermicomposting resulted in greater decrease of EC and greater leaching of organic matter, total nitrogen and most macronutrients in VER and C+V than in COM. Final materials were not phytotoxic but compost could have salinity related problems. Thus, COM, VER and C+V produced treated organic material, which could be suitable for horticultural purposes.

Composting in small laboratory pilots: performance and reproducibility

Small-scale reactors (<10 l) have been employed in composting research, but few attempts have assessed the performance of composting considering the transformations of organic matter. Moreover, composting at small scales is often performed by imposing a fixed temperature, thus creating artificial conditions, and the reproducibility of composting has rarely been reported. The objectives of this study are to design an innovative small-scale composting device safeguarding self-heating to drive the composting process and to assess the performance and reproducibility of composting in small-scale pilots. The experimental setup included six 4-l reactors used for composting a mixture of sewage sludge and green wastes. The performance of the process was assessed by monitoring the temperature, O(2) consumption and CO(2) emissions, and characterising the biochemical evolution of organic matter. A good reproducibility was found for the six replicates with coefficients of variation for all parameters generally lower than 19%. An intense self-heating ensured the existence of a spontaneous thermophilic phase in all reactors. The average loss of total organic matter (TOM) was 46% of the initial content. Compared to the initial mixture, the hot water soluble fraction decreased by 62%, the hemicellulose-like fraction by 68%, the cellulose-like fraction by 50% and the lignin-like fractions by 12% in the final compost. The TOM losses, compost stabilisation and evolution of the biochemical fractions were similar to observed in large reactors or on-site experiments, excluding the lignin degradation, which was less important than in full-scale systems. The reproducibility of the process and the quality of the final compost make it possible to propose the use of this experimental device for research requiring a mass reduction of the initial composted waste mixtures.

Soil mineralization of two-phase olive mill wastes: effect of the lignocellulosic composition on soil C dynamics

The low degradation rate of two-phase olive mill wastes (TPOMW) during composting and after soil application is a characteristic feature of these materials. The aim of this work was to evaluate the relationship between the lignocellulosic fraction of TPOMW and the organic matter (OM) degradation rate in three agricultural soils amended with four TPOMW composting mixtures at different degree of stabilisation and prepared with different bulking agents and N sources. The mineralisation kinetics of TPOMW composting mixtures in soil reflected a large amount of slowly mineralisable C even in the starting mixtures (I and T1) where this fraction represented up to 85% of the total potentially mineralisable C pool. The effect of rich lignocellulosic composition was confirmed by the study of the DTS (50% dry TPOMW + 50% sheep manure) mixtures prepared with dry TPOMW, which had undergone partial degradation in a storage pond for one year before composting. These DTS samples showed a more similar kinetic behaviour in soil than the more transformed composting mixtures as reflected in the principal component analysis (PCA) diagram, where they were grouped in the same quadrant dominated by the lignin/holocellulose ratio. Soils amended with mature composts evolved very low amounts of C (between 2 and 6% of the added C) after two months of incubation, which highlights the suitability of these materials as a suitable C source for the soil to promote long term soil C stabilisation.

Organic matter transformations and kinetics during sewage sludge composting in a two-stage system

The use of different proportions of rape straw and grass as amendments in the composting of dewatered sewage sludge from a municipal wastewater treatment plant was tested in a two-stage system (first stage, an aerated bioreactor and second stage, a periodically turned windrow). The composition of feedstock affected the temperature and organic matter degradation in the bioreactor and the formation of humic substances, especially humic acids (HA), during compost maturation in the windrow. The total HA content (the sum of labile and stable HA) increased according to first-order kinetics, whereas labile HA content was constant and did not exceed 12% of total HA. ΔlogK of 1.0-1.1 indicated that HA was of R-type, indicating a low degree of humification. Temperature during composting was the main factor affecting polymerization of fulvic acids to HA and confirmed the value of the degree of polymerization, which increased only when thermophilic conditions were obtained.

Effect of the herbicides terbuthylazine and glyphosate on photosystem II photochemistry of young olive (Olea europaea) plants

The purpose of this study was to understand the effect produced by the addition of the herbicides terbuthylazine (N(2)-tert-butyl-6-chloro-N(4)-ethyl-1,3,5-triazine-2,4-diamine) and glyphosate (N-(phosphonomethyl)glycine) on photosystem II photochemistry of young plants of Olea europaea L. under greenhouse conditions. The effect of soil amendment with an organic residue from olive oil production was also assessed. Terbuthylazine reduced the efficiency of photosystem II photochemistry of plants due to chronic photoinhibition, and this effect was counterbalanced by soil amendment with the organic waste, whereas the photosystem II photochemistry of olive plants was not affected by glyphosate or by glyphosate and organic waste addition. In this study, we have shown that the soil application of terbuthylazine is a source of indirect phytotoxicity for olive plants. We have also observed that the olive plants were not affected by higher amounts of glyphosate in the soil.