Bacterial community patterns and thermal analyses of composts of various origins.

During composting, the degradation of organic waste is accompanied and driven by a succession of microbial populations exhibiting a broad range of functional capabilities. Detailed inventories of the microbial communities in mature compost, however, are not available. Mature composts, originating from biowaste as well as sewage sludge and anaerobic sludge, were studied by denaturing gradient gel electrophoresis-fingerprints after polymerase chain reaction (PCR) amplification of the 16S rRNA genes using three different universal primer pairs, as well as by differential scanning calorimetry and thermogravimetry. The composts of different origin had different bacterial communities. The influence of different 16S rDNA primer sets on the same batches of compost DNA was evaluated. The clearest separation of different compost types was obtained by using the PCR primer pair 338f + 518r which is suggested for future applications. Communities from the different biowaste compost samples clustered together and could be separated from sewage sludge communities indicating the establishment of different microbial consortia. A similar differentiation of composts was found with the thermogavimetric analyses. It may thus be concluded that the resulting humus quality is closely linked to the microbial communities involved.     

Sewage sludge composting: Influence of initial mixtures on organic matter evolution and N availability in the final composts

The influence of bulking agents on organic matter (OM) stability and nitrogen (N) availability in sewage sludge composts was investigated. The same sludge was composted on an industrial plant with different mixtures of bulking agents. The composting process included an active phase and a curing phase, both lasting 6 weeks, separated by the screening of composts. The OM evolution was characterised by carbon (C) and N mass balances in biochemical fractions. The OM stability and N potential availability of final composts were measured during soil incubations. During composting, the C and N losses reached more than 62% of the initial C and more than 45% of the initial N, respectively, due to C mineralisation or N volatilisation and screening. The bulking materials mostly influenced OM evolution during the active phase. They contributed to the mitigation of N losses during the active phase where N immobilisation through active microbial activity was favoured by bulking agents increasing the C:N ratio of the initial mixtures. However, the influence of bulking agents on OM evolution was removed by the screening; this induced the homogenisation of compost characteristics and led to the production of sludge composts with similar organic matter characteristics, C degradability and N availability.     

Soil bioassays as tools for sludge compost quality assessment

Composting is a waste management technology that is becoming more widespread as a response to the increasing production of sewage sludge and the pressure for its reuse in soil. In this study, different bioassays (plant germination, earthworm survival, biomass and reproduction, and collembolan survival and reproduction) were assessed for their usefulness in the compost quality assessment.Compost samples, from two different composting plants, were taken along the composting process, which were characterized and submitted to bioassays (plant germination and collembolan and earthworm performance). Results from our study indicate that the noxious effects of some of the compost samples observed in bioassays are related to the low organic matter stability of composts and the enhanced release of decomposition endproducts, with the exception of earthworms, which are favored. Plant germination and collembolan reproduction inhibition was generally associated with uncomposted sludge, while earthworm total biomass and reproduction were enhanced by these materials. On the other hand, earthworm and collembolan survival were unaffected by the degree of composting of the wastes. However, this pattern was clear in one of the composting procedures assessed, but less in the other, where the release of decomposition endproducts was lower due to its higher stability, indicating the sensitivity and usefulness of bioassays for the quality assessment of composts.     

Microbial biomass in a soil amended with different types of organic wastes.

Application of different types of organic wastes may have a marked effect on soil microbial biomass and its activity. The objective of this study was to quantify the amount of microbial biomass in a loamy-clayey soil, amended with different types of organic waste residues (composts of municipal solid waste of different ages, sewage sludge and farmyard manure) and incubated for 8 weeks at 25 degrees C and two-thirds of field capacity, using the fumigation-extraction method. Both microbial biomass-C and -N (BC and BN, respectively) appeared to be dependent on the type of organic waste residues, on their degree of stability, and on their chemical characteristics. In general, organic wastes increased the microbial biomass-C content in the soil and the microbial BC was positively correlated with the organic C content, the C/N, neutral detergent fibre/N (NDF/N) and acid detergent fibre/N (ADF/ N) ratios. The microbial biomass content decreased according to the period of incubation, especially when the compost used was immature. The microbial biomass-N was positively correlated with the total N and percentage of hemicellulose. The microbial biomass-C was linearly related with the microbial biomass-N and the ratio BC/BN was exponentially related with the BC.     

Recycling of waste biomass and mineral powder for preparation of potassium-enriched compost

An attempt was made to recycle waste biomass and mineral powder (waste mica) as an alternative source of potassium (K) through composting technology. Two different waste biomass, isabgol straw and palmarosa distillation waste along with two levels of waste mica (2 and 4% as K) were used for preparation of enriched composts. A notable decrease of C:N ratio was observed at the end of the composting (150 days) as an indicator of compost maturity. The mature composts were evaluated for K-supplying capacity through laboratory leaching and soil incubation study. Significantly higher water-soluble K released initially followed by a sharp decrease up to 21 days of leaching thereafter gradually decreased up to 35 days of leaching. Water-soluble K was released from K-enriched (mica charged) compost significantly higher than the ordinary compost throughout the leaching period. Soil incubation study also revealed that application of K-enriched compost greatly improved the available K (water soluble and exchangeable) pools in K-deficient soil which indicated that a considerable amount of K releases during composting. Therefore, K-enriched compost could be an effective alternative of costly commercial K fertilizer and eco-friendly approach to utilize low-cost waste mineral powder and plant residue.     

Effects of composted sewage sludge on microbial biomass, activity and pine seedlings in nursery forest

The investigation was carried out in a 2 year experiment to evaluate the benefits and hazards of the use of composted sewage sludge as a restoration agent for the soil of the nursery forest intended for growing Pinus sylvestris seedlings. The grey forest soil (Haplic Greyzem) was amended with compost at the 25, 50, 75, 100, 150 and 175 t ha(-1) application rates on a dry matter basis. The organic matter content increased with the increase in sludge amendment as well as the metal content. However, the concentrations of individual metals were below the current limits established for Russia and European countries. Sludge amendments enhanced the germination and decreased the mortality of the seedlings. The effects were more obvious for the soil with the highest sludge treatment. The beneficial effects on the biomass of seedlings and the height of the shoots as well as on the length of the roots of the pine seedlings were greater in plots with the highest rates of composted sludge. The application of composted sludge to soil was followed by an increase in microbial biomass and to a lesser extent in basal respiration. In the absence of any detrimental effect on microorganisms, this study lends support to using composted sewage sludge as the organo-mineral fertilizer for the soil of nursery forest.     

Influences of winery–distillery waste compost stability and soil type on soil carbon dynamics in amended soils

The application of organic materials to replenish soil organic matter and improve soil structure and fertility has become a common agronomic practice. This research deals with the effects of soil amendment with winery and distillery waste composts on organic carbon (C) mineralisation in two arable soils. A sandy-loam and clay-loam soil were treated and incubated with a number organic materials obtained from the co-composting of different proportions of grape stalk, grape marc, exhausted grape marc and vinasse, with sewage sludge or animal manure. Moreover, the effect of compost stability on C mineralisation dynamics was studied by applying organic materials from different stages of the composting process. The results obtained showed that the addition of exogenous organic matter stimulated microbial growth, enhanced soil respiration and increased water-extractable C contents in both soils, particularly in the days immediately following amendment. The initial composition of the different organic materials used, especially for the mature samples, and the texture of the receiving soil did not influence significantly the C mineralisation final values, with around 11–20% of the added organic C being mineralised over the first 140 days. However, the contribution of organic amendment to the labile organic C pool, maximum rates of soil respiration, as well as the extent of initial disturbance of the soil microbiota were all found to be related to the degree of organic matter stability. Moreover, irrespective of the type and stability of the organic amendment, the mineralogical composition of the receiving soil was found to significantly influence its resilience in such systems.     

Changes in organic matter composition during composting of two digested sewage sludges

Changes in the chemical and chemical-structural composition of the organic matter of two different sewage sludges (aerobic and anaerobic) mixed with sawdust (1:1 and 1:3, v/v) during composting were determined by monitoring chemical and microbiological parameters as well as by pyrolysis-gas chromatography. Composting was carried out in periodically turned outdoor piles, which were sampled for analysis 1, 30, 60 and 90 days after the beginning of the composting process. Both volatile organic matter and the water soluble C fraction decreased during composting, indicating that the more labile C fractions are mineralized during the process. Microbial activity as measured by microbial respiration (CO(2) evolved from compost samples during incubation) also decreased with composting, reflecting the more stable character of the resulting compost. No major differences were observed between the four composts studied as regards their chemical-structural characteristics. The acetonitrile, acetic acid and phenol pyrolytic fragment tended to increase with composting. Although the final composts were more aromatic in nature than the starting materials, a low degree of humification was observed in all four composts studied, as determined by their high proportion of polysaccharides and alkyl compounds. For this reason, the relationship between pyrolytic fragments, such as benzene/toluene or benzene+toluene/pyrrol+phenols, which are used as indices of humification for soil organic matter, are not of use for such poorly evolved sludge composts; instead, ratios that involve carbohydrate derivatives and aromatic compounds, such as furfural+acetic/benzene+toluene or acetic/toluene, are more sensitive indices for reflecting the transformations of these materials during composting. Both the chemical and microbiological parameters and pyrolytic analysis provided valuable information concerning the nature of the compost's organic matter and its changes during the composting process.     

Evaluation of an organic soil amendment generated from municipal solid waste seeded with activated sewage sludge

This study investigated the feasibility of using fresh activated sewage sludge as inoculum for the microbial valorization of segregated municipal solid waste and evaluated the quality of organic soil amendment generated. Organic fraction of municipal solid waste, which consisted of vegetative (vegetable, fruit and flower) wastes was seeded with activated sewage sludge and processed by rapid aerobic microbial treatment. Efficacy of microbial valorization process and quality of final product were assessed by physico-chemical analysis. Suitability of final product was assessed with regard to heavy metal content, pesticide residues, microbiological quality and phytotoxicity. Quality of the soil amendment generated was compared with the control product generated with a commercial microbial inoculum. Phytotoxicity experiments indicated the stimulatory effect of sewage sludge seeded soil amendment on plant growth but inhibition was observed in closed growth test due to the evolution of gaseous phytotoxic agents. The study suggests that segregated municipal solid waste can be effectively valorized with activated non-dewatered sewage sludge as inoculum and the quality of soil amendment generated was comparable to compost intended for unrestricted applications.     

Effect of composted sewage sludge application to soil on sweet pepper crop (Capsicum annuum var. annuum) grown under two exploitation regimes

The aim of this study is to monitor the effect of the application of three increasing amounts of composted sewage sludge (3, 6 and 9 kg compost m(-2)) on the physico-chemical properties of a horticultural calcareous soil where sweet pepper plants (Capsicum annuum var. annuum) cv. California were grown. A comparative study of two different exploitation regimes was carried out; the first was an open-air field-grown plot and the second plot was kept under controlled conditions in a greenhouse. Changes in physical and chemical properties measured in soil and sweet pepper crop were recorded during crop growth in order to measure the evolution of these properties as a consequence of increasing compost applications. Organic matter, total nitrogen Kjeldahl and available phosphorus contents increased in soil after composted sewage sludge applications. The 9 kg compost m(-2) application promoted the appearance of deleterious effects on the properties of soil, such as salt accumulation, a significant increase in the electrical conductivity and an input of heavy metals (Pb>Cr>Cd). The 6 kg compost m(-2) application provided a supply of nutrients necessary to grow peppers plants under both exploitation regimes. Pepper fruit biomass production under greenhouse was almost 60% higher compared to that of the open-air plot. Lower contents of Ca and increased levels of Cu in fruit under greenhouse growing conditions compared to those of open-air grown peppers seemed to promote the occurrence of blossom-end rot, affecting more than 10% of the harvested fruits.     

Waste paper and clinoptilolite as a bulking material with dewatered anaerobically stabilized primary sewage sludge (DASPSS) for compost production

Environmental problems associated with sewage sludge disposal have prompted strict legislative actions over the past few years. At the same time, the upgrading and expansion of wastewater treatment plants have greatly increased the volume of sludge generated. The major limitation of land application of sewage sludge compost is the potential for high heavy metal content in relation to the metal content of the original sludge. Composting of sewage sludge with natural zeolite (clinoptilolite) can enhance its quality and suitability for agricultural use. However, the dewatered anaerobically stabilized primary sewage sludge (DASPSS) contained a low concentration of humic substances (almost 2%), and the addition of the waste paper was necessary in order to produce a good soil conditioner with high concentrations of humics. The final results showed that the compost produced from DASPSS and 40-50% w/w of waste paper was a good soil fertilizer. Finally, in order to estimate the metal leachability of the final compost product, the generalized acid neutralization Capacity (GANC) procedure was used, and it was found that by increasing the leachate pH, the heavy metal concentration decreased. The application of the sequential chemical extraction indicated that metals were bound to the residual fraction characterized as a stabilize fractions.     

Potential of olive mill wastes for soil C sequestration

The present work deals with the potential of olive mill wastes as a C source for soil C sequestration strategy, which is based on the high lignocellulosic content that makes these wastes to degrade slowly during composting and after land application. A C balance was performed during the whole life cycle of two different two-phase olive mill wastes (TPOMW): C losses were calculated during the composting process and after soil application of the composting mixtures under laboratory conditions. The effect of the degree of stabilization of TPOMW on the overall C waste conservation efficiency was also evaluated. C losses after 34 weeks of TPOMW composting ranged from 40.58% to 45.19% of the initial C, whereas the amount of C evolved as CO2 after 8 months of incubation of soil amended with mature composts only represented between 20.6% and 21.9% of the added C. The total C losses considering the whole life cycle of the TPOMW showed lower losses compared to composts prepared with organic residues of different origin. Conversely to the typical behaviour of other organic wastes, the stabilisation degree of the TPOMW composting mixtures did not show any significant effect on the total C losses measured during composting and later land application. The low rate of degradation of TPOMW both during composting and after soil application makes the use of TPOMW as a C source an attractive strategy for soil C sequestration.     

Changes in cadmium mobility during composting and after soil application

The effect of twelve weeks of composting on the mobility and bioavailability of cadmium in six composts containing sewage sludge, wood chips and grass was studied, along with the cadmium immobilization capacity of compost. Two different soils were used and Cd accumulation measured in above-ground oat biomass (Avena sativa L.). Increasing pH appears to be an important cause of the observed decreases in available cadmium through the composting process. A pot experiment was performed with two different amounts of compost (9.6 and 28.8 g per kg of soil) added into Fluvisol with total Cd 0.255 mg kg^?1, and contaminated Cambisol with total Cd 6.16 mg kg^?1. Decrease of extractable Cd (0.01 mol l^?1 CaCl₂) was found in both soils after compost application. The higher amount of compost immobilized an exchangeable portion of Cd (0.11 mol l^?1 CH₃COOH extractable) in contaminated Cambisol unlike in light Fluvisol. The addition of a low amount of compost decreased the content of Cd in associated above-ground oat biomass grown in both soils, while a high amount of compost decreased the Cd content in oats only in the Cambisol.     

Assessment of compost quality for its environmentally safe use by means of an ecotoxicological test on a soil organism

A research project was carried out to evaluate toxicological effects of compost addition to agricultural soil using the earthworm Eisenia foetida (Annellida) as a representative organism of the soil fauna. Moreover, the correlation between compost biochemical stabilization and toxicity at different phases of the composting process was assessed. Samples were collected from three composting plants at three different maturation levels (beginning of the composting process, intermediate compost after bio-oxidation, and mature refined compost). Two tests were performed: a standard chronic solid-phase test and an acute solid-phase test (developed originally by the authors). In the first test, the measured end-points were mortality, growth and reproduction; while in the second test earthworms’ behavior was evaluated. The chosen compost concentrations in soil ranged from 2.5 to 100 %, with the aim of obtaining the toxicological parameters (LC50) and to mimic real agricultural dosages for the lower concentrations. Results indicated an increase in compost toxicity with greater compost concentrations; in particular, agricultural compost dosage below 10 % showed no toxicity. Moreover, toxicity did not decrease during composting; intermediate compost showed the highest LC50 values. As a consequence, no correlation was ascertained between the results of ecotoxicological analysis and waste biochemical stability parameters during the composting process.     

Composting anaerobic and aerobic sewage sludges using two proportions of sawdust

Sawdust has been proven to be a good bulking agent for sludge composting; however, studies on the most suitable ratio of sludge:sawdust for sludge composting and on the influence of the sludge nature (aerobic or anaerobic) on the composting reaction rate are scarce. In this study two different sewage sludges (aerobic, AS, and anaerobic, ANS) were composted with wood sawdust (WS) as bulking agent at two different ratios (1:1 and 1:3 sludge:sawdust, v:v). Aerobic sludge piles showed significantly higher microbial activity than those of anaerobic sludge, organic matter mineralization rates being higher in the AS mixtures. The lowest thermophilic temperatures during composting were registered when the anaerobic sludge was mixed with sawdust at 1:1 ratio, suggesting the presence of substances toxic to microorganisms. This mixture also showed the lowest decreases of ammonium during composting. All this matched with the inhibitory effect on the germination of Lepidium sativum seeds of this mixture at the first stages of composting, and with its low values of microbial basal respiration. However, the ANS+WS 1:3 compost developed in a suitable way; the higher proportion of bulking agent in this mixture appeared to have a diluting effect on these toxic compounds. Both the proportions assayed allowed composting to develop adequately in the case of the aerobic sludge mixture, yielding suitable composts for agricultural use. However, the ratio 1:1 seems more suitable because it is more economical than the 1:3 ratio and has a lower dilution effect on the nutritional components of the composts. In the case of the anaerobic sludge with its high electrical conductivity and ammonium content, and likely presence of other toxic and phytotoxic substances, the 1:3 ratio is to be recommended because of the dilution effect.     

Changes in soil chemical and microbiological properties during 4 years of application of various organic residues

A 4-year field trial was established in eastern Sweden to evaluate the effects of organic waste on soil chemical and microbiological variables. A simple crop rotation with barley and oats was treated with either compost from household waste, biogas residue from household waste, anaerobically treated sewage sludge, pig manure, cow manure or mineral fertilizer. All fertilizers were amended in rates corresponding to 100kgNha(-1)year(-1). The effects of the different types of organic waste were evaluated by subjecting soil samples, taken each autumn 4 weeks after harvest, to an extensive set of soil chemical (pH, Org-C, Tot-N, Tot-P, Tot-S, P-AL, P-Olsen, K-AL, and some metals) and microbiological (B-resp, SIR, microSIR active and dormant microorganisms, PDA, microPDA, PAO, Alk-P and N-min) analyses. Results show that compost increased pH, and that compost as well as sewage sludge increased plant available phosphorus; however, the chemical analysis showed few clear trends over the 4 years and few clear relations to plant yield or soil quality. Biogas residues increased substrate induced respiration (SIR) and, compared to the untreated control amendment of biogas residues as well as compost, led to a higher proportion of active microorganisms. In addition, biogas residues increased potential ammonia oxidation rate (PAO), nitrogen mineralization capacity (N-min) as well as the specific growth rate constant of denitrifiers (microPDA). Despite rather large concentrations of heavy metals in some of the waste products, no negative effects could be seen on either chemical or microbiological soil properties. Changes in soil microbial properties appeared to occur more rapidly than most chemical properties. This suggests that soil microbial processes can function as more sensitive indicators of short-term changes in soil properties due to amendment of organic wastes.     

Composting of a solid olive-mill by-product ("alperujo") and the potential of the resulting compost for cultivating pepper under commercial conditions

A pollutant solid material called "alperujo" (AL), which is the main by-product from the Spanish olive oil industry, was composted with a cotton waste as bulking agent, and the compost obtained (ALC) was compared with a cattle manure (CM) and a sewage sludge compost (SSC) for use as organic amendment on a calcareous soil. The experiment was conducted with a commercial pepper crop in a greenhouse using fertigation. Composting AL involved a relatively low level of organic matter biodegradation, an increase in pH and clear decreases in the C/N and the fat, water-soluble organic carbon and phenol contents. The resulting compost, which was rich in organic matter and free of phytotoxicity, had a high potassium and organic nitrogen content but was low in phosphorus and micronutrients. The marketable yields of pepper obtained with all three organic amendments were similar, thus confirming the composting performance of the raw AL. When CM and SSC were used for soil amendment, the soil organic matter content was significantly reduced after cultivation, while it remained almost unchanged in the ALC-amended plots.     

Effect of compost treatment of sewage sludge on nitrogen behavior in two soils

This work aims to evaluate the effects of compost treatment of digested sewage sludge on nitrogen behavior in two soils, a Spodosol and an Oxisol soil. Digested sewage sludge was composted with sawdust and woodchips, diluting the total nitrogen to one-fourth (dry mass basis) of its original value. Then, sludge and compost were added to the two soils on an equivalent dry weight basis to consider the risk of NO3- -N leaching. Compost treatment of sewage sludge has slowed down the release of mineral-N to half in the Spodosol and to one-third in Oxisol soil. As a result, NO3- -N concentrations in soils incubated with compost were less than half of the amounts found from soils incubated with digested sludge. Estimates were made of the maximum monthly nitrate to leach from the four combinations of soil and sludge treatment. Application of digested sludge, at a higher nitrogen application rate, resulted in a higher nitrate leaching potential than application of the compost product. Soil type also played an important role, with the Oxisol having slightly higher estimated leaching potential than the Spodosol. The higher nitrate release rate in the Oxisol is counterbalanced by its higher field capacity to lessen the expected difference between the two soils.     

Effect of sewage sludge or compost on the sorption and distribution of copper and cadmium in soil

The application of biosolids such as sewage sludge is a concern, because of the potential release of toxic metals after decomposition of the organic matter. The effect of application of sewage sludge (Sw) and compost (C) to the soil (S) on the Cu and Cd sorption, distribution and the quality of the dissolved organic matter (DOM) in the soil, was investigated under controlled conditions. Visible spectrophotometry, infrared spectroscopy, sorption isotherms (simple and competitive sorption systems), and sequential extraction methods were used. The E4/E6 (lambda at 465 and 665 nm) ratio and the infrared spectra (IR) of DOM showed an aromatic behaviour in compost-soil (C-S); in contrast sewage sludge-soil (Sw-S) showed an aliphatic behaviour. Application of either Sw or C increased the Cu sorption capacity of soil. The Cd sorption decreased only in soil with a competitive metal system. The availability of Cu was low due to its occurrence in the acid soluble fraction (F3). The Cu concentration varied in accordance with the amounts of Cu added. The highest Cd concentration was found in the exchangeable fraction (F2). The Sw and C applications did not increase the Cd availability in the soil.     

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.