J. Cegarra

Maturity and stability parameters of composts prepared with a wide range of organic wastes

Seven different composts were prepared ,(n a pilot plant by the Rutgers static-pile system using a wide range of wastes: sewage sludge, poultry manure, pig slurry, olivemill wastewater city refuse and the lignocellulosic wastes cotton waste, maize straw and sweet sorghum bagasse. Their chemical and biological properties were studied at four stages of the composting process: in the initial mixture, at the thermophilic phase, at the end of the active phase and after two months of maturation. The following maturity indices were established: CIN<12, C&1.7%, Cw/N0,<0.55 NHJNOs< 0.16, and NH4-N <0.04%, with a germination index greater than 50%. In addition, some carbon mineralization parameters could also be used as maturity indices: mineralized-c in 70 days (C,,,) ~3070, rapidly mineralizable-C (C,) < 7.270 and a slow mineralization rate (C, x KS) ~0.357~ day-‘. Maturation indices based on humification of the organic matter and the cation exchange capacity of different composts could not be found, since the values for mature compost depended on the wastes from which the composts were made. 0 1998 Elsevier Science Ltd.

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.

Relationships between organic matter and carbon contents of organic wastes

Abstract The values of organic matter of 38 organic wastes obtained by the loss-on-ignition method at 430°C have been correlated with both total organic carbon measured by elemental microanalysis and oxidable organic carbon determined by the traditional method of Walkley and Black, duly adapted to organic materials. Calcination at 430°C for 24 h is the most suitable method for determining the organic-matter content of organic wastes. Linear and quadratic equations for the correlations between organic matter and carbon were tested, and high values of r, significant at the 99·9% probability level, were found. The use of the linear equations is recommended to transform either carbon content into organic matter or vice-versa easily.

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.

Carbon and nitrogen transformation during composting of sweet sorghum bagasse

Two types of compost, consisting of sweet sorghum bagasse with either sewage sludge or a mixture of pig slurry and poultry manure, were studied in a pilot plant using the Rutgers system. The total degradation of the piles as determined by the weight loss of organic matter during the bio-oxidative and maturation phases accounted for 64% of the organic matter applied and followed a first-order kinetic function. Concentrations of total and organic N increased during the composting process as the degradation of organic C compounds reduced the compost weight. Losses of N through NH3 volatilization were low, particularly in the compost with sewage sludge due to pH values of <7.0 and the low temperatures reached in the compost during the first 2 weeks. The C:N ratio in the two composts decreased from 24.0 and 15.4 to values between 12 and 10. Increases in cation exchange capacity and in fulvic and humic acid-like C revealed that the organic matter had been humified during composting. The humification index, the C:N ratio, fulvic:humic acid-like C, and cation exchange capacity proved to be the most suitable parameters for assessing the maturity of these composts.

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.

Composts as Media Constituents for Vegetable Transplant Production

The use of compost with high salt concentration was evaluated, under commercial conditions, as a potential growing media constituent for vegetable transplant production. Two composts were prepared from sweet sorghum bagasse, pine bark, and either urea (compost A) or brewery sludge (compost B) as N source. Three vegetable species — broccoli (Brassica oleracea), tomato (Lycopersicum esculentum), and onion (Allium cepa) with different tolerance to salinity were used. Eleven substrates were formulated and tested: a control consisting of a moss peat-based commercial substrate; compost A; compost B; and, eight mixtures containing 33 or 67% by volume of each compost with either raw peat moss or commercial substrate as diluent. All the substrates prepared had suitable physical, physicochemical and chemical properties for use as growing media, except for the electrical conductivity (ranging from 3.20 to 13.21 dS m−1) which was above the reference levels for soilless cultivation. Broccoli was the least affected by substrate salinity whilst tomato was the most. Onion transplants had an intermediate response to saline conditions. Tomato seed germination was markedly reduced when compost A, with a higher salt concentration, was used at a rate higher than 67%. Media prepared with either of the composts, and mixed with either a commercial substrate or peat in a rate up to 67%, did not cause any detrimental effect on the growth and nutritional status of broccoli, tomato and onion transplants, despite the high initial salinity of the substrates. These composts appear to be acceptable substitutes for Sphagnum peat in seed sowing mixtures.

Chemical and structural evolution of humic acids during organic waste composting

The effects of the composting process on the chemical and structural properties of humic acids have been studied in seven different organic waste mixtures from different origin. Only slight changes in elemental composition have been found in the humic acids after the composting process pointing to a more aromatic structure with higher proportions of oxygen and nitrogen. Functional groups were the most sensitive to the changes caused by the composting process showing a marked increase in the total acidity and phenolic, carboxyl and carbonyl groups. Gel permeation chromatography showed a slight increase in the average molecular weight of the humic acids. Infrared spectroscopy did not show important differences in humic acid structure but a clear decrease in the intensity of the bands in the region 3000–2850 cm-1 corresponding to the aliphatic fractions. As a general result, the composting process yields humic acids in which the elemental and functional composition are closer to that of the more humified soil humic acids.