M.P. Bernal

Composting of animal manures and chemical criteria for compost maturity assessment. A review.

New livestock production systems, based on intensification in large farms, produce huge amount of manures and slurries without enough agricultural land for their direct application as fertilisers. Composting is increasingly considered a good way for recycling the surplus of manure as a stabilised and sanitised end-product for agriculture, and much research work has been carried out in the last decade. However, high quality compost should be produced to overcome the cost of composting. In order to provide and review the information found in the literature about manure composting, the first part of this paper explains the basic concepts of the composting process and how manure characteristics can influence its performance. Then, a summary of those factors such as nitrogen losses (which directly reduce the nutrient content), organic matter humification and compost maturity which affect the quality of composts produced by manure composting is presented. Special attention has been paid to the relevance of using an adequate bulking agent for reducing N-losses and the necessity of standardising the maturity indices due to their great importance amongst compost quality criteria.

Nitrogen transformation during organic waste composting by the Rutgers system and its effects on pH, EC and maturity of the composting mixtures.

The evolution of the different forms of nitrogen during the composting of several wastes was studied, as well as its relation to the pH, electrical conductivity and parameters of maturity of the composts obtained. Four mixtures were prepared from different organic materials: sewage sludge, municipal solid waste, brewery sludge, sorghum bagasse, cotton waste and pine bark. The evolution of the different forms of nitrogen during composting depended on the material which supplied the nitrogen to the mixtures and the organic matter (OM) degradation rate during composting. The greatest concentration of ammonium was observed during the first weeks of composting, coinciding with the most intense period of OM degradation, and ammonium then decreased gradually to reach final values of below 0.04%. The use of urea as a nitrogen source in the mixtures led to high ammonium levels during the first weeks as a result of its rapid hydrolysis. The nitrification process began only when the temperature of the mixtures had dropped below 40 degrees C and its intensity depended on the quantity of ammonium present when the process began. The highest concentrations of NO3-N were always produced at the end of maturation, reaching values of 0.52%, 0.53%, 0.12% and 0.20% in the four mixtures studied. Nitrogen losses during composting depended on the materials used and on the pH values of the mixtures. Mixtures with the highest lignocellulose content showed the lowest losses (below 25%), while those containing municipal solid waste lost more than 40% of the initial content. Statistically significant correlations at a high probability level were found between the NO3-N concentration and pH and electrical conductivity. confirming that nitrification was responsible for the falling pH values and increasing electrical conductivity. The ratio of NH4-N and NO3-N concentrations was shown to be a clear indicator of the maturity of the mixtures during composting, the final values of 0.08, 0.04, 0,16 and 0.11 for the four mixtures being equal to, or below the maximum value established as a maturity index in other materials.

Evolution of organic matter and nitrogen during co-composting of olive mill wastewater with solid organic wastes

Abstract Four olive mill wastewater (OMW) composts, prepared with three N-rich organic wastes and two different bulking agents, were studied in a pilot plant using the Rutgers system. Organic matter (OM) losses during composting followed a first-order kinetic equation in all the piles, the slowest being the OM mineralisation rate in the pile using maize straw (MS). The highest N losses through NH3 volatilisation occurred in the mixtures which had a low initial C/N ratio and high pH values during the process. Such losses were reduced considerably when MS was used as the bulking agent instead of cotton waste (CW). N fixation activity increased during the bio-oxidative phase before falling during maturation. This N fixation capacity was higher in piles with a lower NH4+-N concentration. Only the composts prepared with OMW, CW and poultry manure or sewage sludge reached water-soluble organic C (CW) and NH4+-N concentrations and CW/Norg and NH4+/NO3– ratios within the established limits which indicate a good degree of compost maturity. Increases in the cation-exchange capacity, the percentage of humic acid-like C and the polymerisation ratio revealed that the OM had been humified during composting. The germination index indicated the reduction of phytotoxicity during composting.

Characterization of olive mill wastewater (alpechin) and its sludge for agricultural purposes

Abstract Ten samples of olive mill wastewater (OMW) taken from different mills in southern Spain and other ten of OMW sludges from evaporation ponds were analysed. The aim was to study the composition of these wastes and to find relationships which would make it possible to use easily determinable parameters to ascertain their composition. Compared with other organic wastes, these materials had a high potassium concentration, a similar organic matter content and notable levels of nitrogen, phosphorus, calcium, magnesium and iron. The highest potassium concentrations were observed in the OMWs, while the sludges showed higher levels of the other nutrients, especially iron. The dry matter of the OMWs was significantly correlated with most of the parameters studied but, in the sludges, the only correlation was between the ash content and the total organic carbon and total nitrogen concentrations. The regression equations obtained permitted a rapid characterization of the OMWs from their dry matter content.

Bio-degradation of olive mill wastewater sludge by its co-composting with agricultural wastes

The use of maize straw (MS) or cotton waste (CW) as bulking agents in the composting of olive mill wastewater (OMW) sludge was compared by studying the organic matter (OM) mineralisation and humification processes during composting and the characteristics of the end products. Both composts were prepared in a pilot-plant using the Rutgers static-pile system. The use of CW instead of MS to compost OMW sludge extended both the thermophilic and bio-oxidative phases of the process, with higher degradation of polymers (mainly lignin and cellulose), a greater formation of nitrates, higher total nitrogen losses and a lower biological nitrogen fixation. The CW produced a compost with a more stabilised OM and more highly polymerised humic-like substances. In the pile with CW and OMW sludge, OM losses followed a first-order kinetic equation, due to OM degradation being greater at the beginning of the composting and remaining almost constant until the end of the process. However, in the pile with MS and OMW sludge this parameter followed a zero-order kinetic equation, since OM degraded throughout the process. The germination index indicated the reduction of phytotoxicity during composting.

Growth of ornamental plants in two composts prepared from agroindustrial wastes

Two composts prepared from agroindustrial wastes were assayed as substrates: C1 from brewing waste (yeast and malt) plus lemon tree prunings; and C2 from the solid fraction of olive mill wastewater plus olive leaves. Sixteen substrates were prepared by combining each compost with Sphagnum peat or a commercial substrate (CS) in different proportions. The nutrients (N and K) provided by the composts, which acted as slow-release fertilisers, influenced especially the development of calendula, although the physical and physico-chemical properties such as total pore space and electrical conductivity (EC) were also relevant. On the other hand, in the salt-sensitive calceolaria hybrid, EC and chloride concentration were the main factors influencing growth. Adequate substrates for the development of calendula can be prepared by mixing C1 at up to 75% with peat or at up to 50% with CS, and C2 at up to 50% with peat or CS. For calceolaria, the substrate should have a lower proportion of compost, C1 at up to 50% and C2 at up to 25%, both mixed with peat or CS. Therefore, composts of agroindustrial origin such as these can be used as an alternative to peat and CSs for growing ornamental plants. provided the mixture contains at least 25% peat or CS.

Co-composting of distillery wastes with animal manures: carbon and nitrogen transformations in the evaluation of compost stability.

The aim of this work was to study the viability of recycling the solid wastes generated by the winery and distillery industry by means of co-composting with animal manures, as well as to evaluate the quality of the composts obtained. Two piles, using exhausted grape marc and cattle manure or poultry manure, respectively (at ratios, on a fresh weight basis, of 70:30), were composted by the Rutgers static pile composting system. Throughout the composting process, a number of parameters were monitored, such as pH, electrical conductivity, organic matter, water-soluble carbon, water-soluble polyphenols, different forms of nitrogen (organic nitrogen, ammonium and nitrate) and humification indices (humification ratio, humification index, percentage of humic acid-like C, polymerisation ratio and cation exchange capacity), as well as the germination index. Organic matter losses followed first-order kinetics equation in both piles, the highest organic matter mineralisation rate being observed with exhausted grape marc and cow manure. On the other hand, the mixture with the lowest C/N ratio, using exhausted grape marc and poultry manure, showed the highest initial ammonium contents, probably due to the higher and more labile N content of poultry manure. The increase in the cation exchange capacity revealed the organic matter humification during composting. In contrast, other humification parameters, such as the humification ratio and the humification index, did not show the expected evolution and, thus, could not be used to assess compost maturity. Composting produced a degradation of the phytotoxic compounds, such as polyphenols, to give composts without a phytotoxic character. Therefore, composting can be considered as an efficient treatment to recycle this type of wastes, due to composts presented a stable and humified organic matter and without phytotoxic effects, which makes them suitable for their agronomic use.

Relationships between water-soluble carbohydrate and phenol fractions and the humification indices of different organic wastes during composting

Abstract The present work dealt with the relationships between the degradation and humification processes which the organic matter underwent during the composting of six different organic-waste mixtures. Four of them were prepared by the Rutgers forced-ventilation composting system and the other two by the mobile (turn over) pile system. The main components were: sewage sludge, sorghum bagasse and municipal solid waste. Different degradation rates were observed for the three main components (cellulose, hemicellulose and lignin) of the organic matter during composting. In the case of the first two components, the degree of degradation ranged from 70 to 85% during the whole process, depending on the starting mixture, whereas only 30–50% of the initial concentration of lignin was degraded in the mixtures prepared with municipal solid wastes (MSW) and lignocellulosic materials. Water-soluble carbohydrate and phenol degradation were studied because they have been proposed as precursors of the humification processes. In the experiment described, they had different degradation rates during composting depending on the starting mixture and the composting system used. The water-soluble carbohydrate was the most intensely degraded fraction in the piles prepared with urban refuse although no appreciable degradation was measured in the other three mixtures, whereas there was an appreciable reduction in the water-soluble phenol fraction of all six mixtures during composting, values of less than 0.1% being reached in the mature composts. Humification processes were studied by quantification of the extractable humic-like substances and the generally accepted humification indices: extractable carbon to total organic carbon (EXC/TOC), humic acid carbon to total organic carbon (HAC/TOC), humic acid carbon to extractable carbon (HAC/EXC) and the humic acid carbon to fulvic acid carbon (HAC/FAC) and by determining the cation exchange capacity of the mixture during composting. All the indices increased during composting and followed a similar trend. The humic-like acid fraction was mainly responsible for these changes, showing that the composting involves a process of humification. The cation exchange capacity to total organic carbon ratio showed itself to be a useful humification index during composting since this index clearly increased more than the others. Correlations between some of the above humification indices and the concentrations of water-soluble and less polymerised carbohydrates and phenols indicates the possible influence of these fractions on the humification processes. Significant correlations were found between the phenols and the HAC/TOC and the HAC/EXC ratios, whereas no significant correlations were recorded with the carbohydrate fraction.

Composting of the solid fraction of olive mill wastewater with olive leaves: organic matter degradation and biological activity.

The flocculated solid fraction of olive mill wastewaters, obtained from two different olive oil extraction systems (FOMW1 and FOMW2) was composted, with olive leaves (OL) as bulking agent, by the static pile system (Rutgers). The dynamic of organic matter (OM) degradation during composting and its relationship with the basal respiration and fluorescein diacetate (FDA) hydrolytic activity, as indicators of biological activity, were studied. Two mixtures were prepared: C1, from 65% FOMW1 plus 35% OL; and C2, from 74% FOMW2 plus 25% OL and 1% urea. The biooxidative phase of composting in C1, which had a high initial C/N ratio, was long, leading to a high OM degradation, mainly of the lignocellulosic compounds. The water-soluble organic carbon content, C/N ratio and the urea supplied as a N source for the C2 compost make this mixture more adequate for composting, as it had a shorter composting time than C1, and developed a microbial population with a high metabolic activity. The results for basal respiration in C1 and C2 were correlated at a high probability level with those of FDA hydrolysis, and both parameters can be used for establishing the degree of biological stability of the composting material.

An automatic microanalysis method for the determination of organic carbon in wastes

Abstract An automatic microanalysis method for the determination of total organic carbon (TOC) in organic wastes is proposed. This method is quick, reliable, accurate, and able to overcome the interference of carbonate‐carbon. The obtained values for TOC in 38 samples consisting of city refuse, sewage sludges, manures, and plant residues are correlated with those of oxidable organic carbon (OXC) measured by the traditional method of Walkley and Black duly adapted to organic materials, and the linear, power, and exponential regression equations are calculated. The use of the equations easily allows to transform the values of OXC into the corresponding values of TOC, the former being within the range of 74.59% to 97.38% of the latter depending on the kind of waste.