Structural Changes of Humic Acids During Olive Mill Pomace Composting

The aim of this work was to characterize the humic acids during composting using both chemical and spectroscopic analytical methods. Humic acids (HA) were extracted from olive mill pomace (OMP) after 5, 10, 21, 32, 48, 60, 84, 95, 109 and 153 days of composting and their elemental composition, functional groups, molecular weight, ¹³C CP-MAS NMR and FTIR spectra were determined. Elemental composition of HA showed a decrease of carbon content from 56.8% to 47.1% and an increase of oxygen content from 32.7 % to 43.3%. A slight increase of carboxylic groups was observed with no variation of phenolic groups. The molecular weight of HA determined by gel chromatography showed that after 95 days of composting the molecular weight of about 40% of the polymers was greater than 10² KDalton. The ¹³C CPMAS NMR and FTIR spectra of HA showed compositional and structural changes indicating a slight increase of polysaccharides and aromatic groups and a decrease of long aliphatic chains during the composting.     

Characterization of the organic fraction of an uncomposted and composted sewage sludge by isoelectric focusing and gel filtration

Summary Isoelectric focusing was used to characterize the organic matter of composted and uncomposted sewage sludge. The technique was applied to organic matter extracts and to three fractions, obtained by ultrafiltration, with different molecular weights (<10³, 10³–10⁴, >10⁴). The elution curves of the extracts through Sephadex G-50 revealed a loss in the proportion of organic matter of low molecular weight as composting progressed, together with an enrichment of the high-molecular-weight proportion. Separation into fractions by controlled ultrafiltration proved to be valid, as deduced from the chromatograms obtained by Sephadex G-50 filtration. The extracts of uncomposted sludge showed a greater number of bands with a low isoelectric point than the composted extract, because there were more acidic molecules in the samples that had not undergone humification. The spectrum corresponding to the extract of the 210-day compost showed greater homogeneity with a lower number of bands. A great part of the organic matter extracted with 0.1 M Na₄P₂O₇ at pH 7.1 corresponded to an intermediate molecular weight. The ampholytes at pH 4–6 gave better resolution than those at pH 3.5–10, in the focusing of fractions with the lowest and the greatest molecular weight. A more homogeneous spectrum was observed for the high-molecular-weight fractions from extracts of the 210-day composted sample; in addition, the bands were displaced towards higher isoelectric points, which indicated that the molecules were more condensed, with a minor content of negatively charged groups and a spectrum similar to that of relative fractions of true humic acids.     

Composting Olive Mill Waste and Sheep Manure For Orchard Use

An industrial-scale composting plant has been designed for producing organic fertilizers from olive mill waste using the windrow pile system. Materials to be composted, two phase olive mill waste (TPOMW) and sheep litter (SL), were characterized and made into three piles consisting of different proportions of each. Throughout the composting process, temperature (T), moisture (M), organic matter (OM), total organic carbon (C_org), total nitrogen (NT), germination index (GI), pH and electrical conductivity (EC) were monitored. The potential agronomic value of the final composts was ascertained by analyzing the bulk density, OM and Corg concentration, pH, EC, macro and micronutrient content (N, P, K, Ca, Mg, Fe, Cu, Mn, Zn, B), the concentration of humic and fulvic acids and inorganic nitrogen (NH₄⁺,NO₂^?,NO₃^?). Each compost was applied to an area of one hectare within a six year-old olive plantation. Four months after application, the soils showed an increased OM concentration and cationic exchange capacity (CEC).     

Compost Production from Olive Tree Pruning Wastes Enriched with Phosphate Rock

Olive production is increasing very fast in the last 20 years in Al-Jouf region, northern region of the Kingdom of Saudi Arabia. There are now more than 5 million olive trees in the productive stage. A huge amount of waste is produced annually from this sector, including pruning waste of olive trees and solid waste from olive oil mills of three-phase system. The waste of olive used to produce compost could be of safe disposal technique. Disposal of such waste is expensive and it causes some environmental hazards. Rock phosphate (RP), which is available in the same region, can be used to improve the nutrient value of the produced compost. In this experiment olive pruning waste was composted alone and in a mixture with different RP in two different ratios, namely 5% and 10%, on the dry weight basis of olive waste. Compost processing took place on plies for 8 months. Temperature and moisture were monitored daily while periodical samples were taken from all piles for testing pH, EC, C/N, and germination index. Composts in all treatments reached the theomorphic phase, which killed weed seeds and pathogens. The produced compost from all treatments fulfilled the requirements of maturity according to the California Compost Quality Council after 8 months of composting. In addition, the RP enhanced composting efficiency and improved the quality of the produced compost. Compost parameters met the standard requirements for the compost that was suitable for agriculture purposes with average values of pH ranging from (7.02–7.65) and EC (2.20–3.94 dS m^?1). Nutrient concentrations in the produced compost were N (1.28–1.79%), P (0.23–2.15%), and K (2.59–4.22%).     

Agrochemical characterization,net N mineralization,and potential N leaching of composted olive‐mill pomace currently produced in southern Spain

In S Spain, the Andalusian olive oil industry generates annually 2.5–3.0 million tons of olive mill pomace, a by‐product which is comprised of the residues from the two‐phase oil‐extraction process. The agricultural policies of the EU have led to widespread interest in recycling these agricultural by‐products. Olive mill pomace might be evaluated as an organic fertilizer after composting, however, before wider use of composted olive mill pomace is advocated, characterization of the final product is needed. In this study, the physico‐chemical characteristics, net N mineralization, and the potential for N leaching of 7 out of the 11 olive‐mill‐pomace composts currently produced in the Andalusian olive mills were investigated. Compost of olive mill pomace differed in the proportions of raw materials co‐composted with the olive mill pomace, such as olive leaf material, manure, and straw. In all the composts tested, organic matter, total C and K were high with 60.5%, 30.7%, and 1.7% on average, respectively, whereas total P was low (0.4%) and with intermediate levels of N (1.5%). Compost pH (8.03), electrical conductivity (2.85 dS m^–1), and germination index (65% on average) were adequate for agricultural use. Furthermore, principal component analyses revealed a clear relationship between the quality of the composts and the proportion of manure mixed with the raw materials. Net N mineralization was negative on average (–20 μg IN g^–1) after 1 y, but positive after 2 y of incubation with up to 94% of available N from the total N added and the short‐term potential N leaching after compost application was negligible (less than 3.9% of added N) and much lower than the other N fertilizer with up to 80% added N leached. Overall, results of this study clearly show that these currently produced composts of olive mill pomace are suitable as soil improvers for agricultural purposes, but may not contribute significantly as a N fertilizer for up to 2 y after application.     

Assessment of Biotransformation of Sludge from Vegetable Oil Refining by Composting

The evolution of organic matter of sludge from vegetable oil refining (50%) mixed with turf (40%) and straw (10%) during 6 months of composting was evaluated by physicochemical and spectroscopy analysis. The intense microbial activity is characterized by a significant increase in temperature (over 67°C) during the thermophilic phase (7 days). The final product is characterized by a decomposition rate of 50, C/N ratio about 12, NH₄⁺/NO₃^? ratio less than 1, and a neutral pH. The lipid analysis showed that total lipids decreased by 83% as a result of biodegradation of lipid compounds of the composted substrate. The degree of polymerization during composting is of about 16%, which provides information on the success of the process. The spectroscopic analysis showed a decrease of the ratios E4/E6 and E2/E6, which clearly shows the humification of organic matter. The physicochemical and spectroscopy parameters of the mixture show the stability and maturity of the final compost, which is confirmed by the germination index (60% for lettuce and turnips, and 90% for cress and lucerna). The results of the evolution of sludge from vegetable oil refining mixed with green wastes produced a mature product that can be applied in agriculture.     

Evolution of Organic Matter Within Sixty Days of Composting of Lignocellulosic Food Industry Waste in Malaysia

Empty fruit bunches (EFB), coffee grounds (CG), and palm oil mill sludge (POMS) were composted in the laboratory for 60 days in order to study the composting process of lignocellulosic food industry wastes. In the first part of the experiment, EFB, CG, and POMS were composted alone (composting of single lignocellulosic material), and in the second part, EFB was composted with CG (1EFB:1CG ratio) and POMS (1EFB:1POMS ratio). The effects of different turning frequencies on the physical and chemical properties of composting were observed and its relation with the degradation process was highlighted. Results showed that oil and grease were first degraded, followed by recalcitrant compounds like alpha-cellulose, hemicellulose, and lignin. Cellulose and hemicellulose were degraded mainly during the 60 days of composting, and the progressive reduction of the cellulose/lignin ratio proved that the main evolution of these wastes took place. It was observed that 3, 6, and 9 days of turning frequency did not affect the physicochemical properties of the compost. Composting EFB alone failed to achieve the required quality of maturity compost within 60 days, while CG and POMS recorded low in biological activity. Better results were shown in composting of EFB mixed with coffee grounds and POMS, the C/N ratio dropped to less than 20 by the 8th week of the composting period. Composting of mixed lignocellulosic materials showed larger changes compared to composting of single lignocellulosic material, reaching a C/N ratio below 20 within 8 weeks.     

Maturity indices for composted dairy and pig manures

Bulking agents and bedding materials used on farms for composting manures affect the time required for composts to mature. The effects of these materials on guidelines for the use of composted manures in potting mixes are not fully known. Several chemical and biological compost characteristics were mentioned and a cucumber plant growth greenhouse bioassay was performed on samples removed from windrows during composting of: (i) dairy manure amended with wheat straw; (ii) dairy manure amended with sawdust (mostly Quercus spp.); and (iii) pig manure amended with sawdust and shredded wood (mostly Quercus spp.). Dry weights of cucumber seedlings grown in fertilized and unfertilized potting mixes amended with composts (30%, v/v) having stability values of <1 mg CO₂-C g^-1 dw d⁻¹, did not differ significantly from those in a control peat mix. Only the most mature dairy manure-wheat straw compost samples consistently established sufficient N concentrations in cucumber shoots in unfertilized treatments. For the dairy manure-wheat straw compost, all possible subset regression analyses of compost characteristics versus cucumber plant dry weight revealed that any of several compost characteristics (electrical conductivity-EC, compost age, total N, organic C, C-to-N ratio, ash content, CO₂ respirometry, Solvita CO₂ index and the Solvita^® Compost Maturity Index) predicted growth of cucumber in the unfertilized treatments, and thus maturity. In contrast, at least two characteristics of the dairy manure-sawdust compost were required to predict growth of cucumber in the unfertilized treatments. Effective combinations were EC with compost age and the Solvita^® maturity index with total N. Even five compost characteristics did not satisfactorily predict growth of cucumber in the non-fertilized pig manure-wood compost. Nutrient analysis of cucumber shoots indicated N availability was the principal factor limiting growth in potting mixes amended with the dairy manure-sawdust compost, and even more so in the pig manure-wood compost even though the compost had been stabilized to a high degree (<1 mg CO₂-C g⁻¹ dw d⁻¹). Maturity of the composted manures, which implies a positive initial plant growth response of plants grown without fertilization, could not be predicted by compost characteristics alone unless the bulking agent or bedding type used for the production of the composts was also considered.     

Evaluation of a new composting method in terms of its biodegradation pathway and assessment of compost quality,maturity and stability

The need for scientific composting methods for effective utilization of organic waste is increasing day by day. In this respect, a new process called the Novcom composting method is being increasingly adopted by the organic tea planters of Assam and Darjeeling (India) for large-scale composting. Study of the biodegradation process under this method and quality evaluation of the end product was carried out at Maud tea estate (Assam) during 2008–2009 and 2009–2010. Generation of high temperatures (>65°C) within the compost heap during the biodegradation process provided an indication regarding the destruction of pathogens and weed seeds in the composted material. Samples collected on day 0, 7, 14, 21 and 30 of composting, were analyzed for physicochemical properties, nutrient status, microbial population, stability and phytotoxicity parameters. The most significant finding was the high microbial population (in the order of 10¹⁶ cfu g^?1) in the final product, which was generated naturally during biodegradation. Assessment of the maturity and stability parameters of the compost indicated that biodegradation was complete in ～3 weeks. The study provided an indication of the potential of the Novcom composting method for the production of good quality, stable and mature compost, within a short period.     

Effect of Woody Peat as an Additive on Maturity and Gaseous Emissions During Pig Manure Composting

Woody peat was used as an additive to compost with pig manure in 1.2 m³ composting reactors under aerobic conditions for a 77?days period to estimate the effect on the compost maturity and gaseous emissions (NH₃, N₂O, and CH₄). Pig manure was also composted with cornstalks (the traditional method) as a control treatment. The results showed that both cornstalks and woody peat composts reached the required maturity standard. Composting with woody peat as a bulking agent was found to reduced NH₃ emissions by 36% than the cornstalks amended treatment. Although CH₄ emission increased by adding woody peat, N₂O emission was considerably reduced, resulting in a slight decrease in total greenhouse gas emissions. More importantly, woody peat could reduce the losses of total carbon and total nitrogen, improve the compost quality as fertilizer.     

Temperature and Chemical Changes During Composting of Broiler Litter

Composting broiler litter (a mixture of manure, bedding material, and wasted feed) with commonly available high-C substrates may be a viable alternative to reduce current land disposal practices for litter. Broiler litter with wood shavings as a bedding material and broiler litter with peanut hulls as a bedding material were composted with wheat straw, peanut hulls, pine bark and paper mill sludge in 0.33 m³ batch reactors. Litters and C substrates were mixed to achieve C:N ratios of approximately 30:1. Dry weight, total N, total C, temperature, electrical conductivity and pH were determined at regular intervals. Maximum temperatures peaked near 70°C within 2.25 d after mixing peanut hulls with litter and within 2.58 d for pine bark and litter. Composts made from paper mill sludge approached 50°C within 3.71 d. Wheat straw composts never exceeded 40°C which could present potential health problems associated with pathogenic microorganisms. Mass loss and C:N ratio gradually declined and stabilized approximately 84 d after mixing. Mass loss averaged 73 percent for wheat straw compost, 33 percent for peanut hull composts, and 16 percent for the other mixes. Wheat straw compost C:N ratios stabilized near 14:1 and other mixes remained above 20:1, indicating N limited conditions for complete composting. Compost pH was 5.8 after 84 d from pine bark composted with wood shaving litter and was significantly lower than pH from paper mill sludge compost with an average pH of 6.9 but similar to all other compost mixes (pH 6.7). Electrical conductivity ranged from 0.35 S m^?1 for paper mill sludge composted with wood shaving litter to 0.91 S m^?1 from wheat straw composted with peanut hull litter. Composting temperature varied considerably among C sources and all required at least 72 d of curing to stabilize the C:N ratio. Composts made from wheat straw were most effective for waste reduction but temperatures were below the 50°C level generally considered necessary to kill pathogens.     

Application rate and composting method affect the immediate and residual manure fertilizer value in a maize–rice–rice–maize cropping sequence on a degraded soil in northern Vietnam

A field experiment was carried out in northern Vietnam to investigate the effects of adding different additives [rice (Oriza sativa L.) straw only, or rice straw with added lime, superphosphate (SSP), urea or a mixture of selected microorganism species] on nitrogen (N) losses and nutrient concentrations in manure composts. The composts and fresh manure were applied to a three-crop per year sequence (maize–rice–rice) on a degraded soil (Plinthic Acrisol/Plinthaquult) to investigate the effects of manure type on crop yield, N uptake and fertilizer value. Total N losses during composting with SSP were 20% of initial total N, while with other additives they were 30–35%. With SSP as a compost additive, 65–85% of the initial ammonium-N (NH₄-N) in the manure remained in the compost compared with 25% for microorganisms and 30% for lime. Nitrogen uptake efficiency (NUE) of fresh manure was lower than that of composted manure when applied to maize (Zea mays L.), but higher when applied to rice (Oriza sativa L.). The NUE of compost with SSP was generally higher than that of compost with straw only and lime. The mineral fertilizer equivalent (MFE) of manure types for maize decreased in the order: manure composted with SSP?>?manure composted with straw only and fresh manure?>?manure composted with lime. For rice, the corresponding order was: fresh manure?>?manure composted with SSP/microorganisms/urea?>?manure composted with lime/with straw alone. The MFE was higher when 5 tons manure ha^?1 were applied than when 10 tons manure ha^?1 were applied throughout the crop sequence. The residual effect of composted manures (determined in a fourth crop, with no manure applied) was generally 50% higher than that of fresh manure after one year of manure and compost application. Thus, addition of SSP during composting improved the field fertilizer value of composted pig manure the most.     

MSW Compost Reduces Nitrogen Volatilization During Dairy Manure Composting

Manures lose N through volatilization almost immediately after deposit. Attempts to control losses include the addition of a C source to stimulate nitrogen immobilization. Composting is a treatment process that recommends the addition of carbonaceous materials to achieve a C:N ratio of 30:1 to stimulate degradation and immobilize nitrogen. Dairies near cities may be able to reduce N loss from manures by composting with urban carbonaceous residues such as municipal solid waste (MSW) or MSW compost that, by themselves, have little agronomic value. Studies were conducted using a self-heating laboratory composter where dairy solids were mixed with MSW compost to determine the reduction of N loss during composting. One-to-one mixtures (v/v) of dairy manure solids and MSW compost were composted and NH₃ volatilization, CO₂ evolution and temperatures were compared to composting of manure alone. Addition of MSW compost resulted in increased CO₂ evolution and reduced N loss. Nitrogen loss from composting dairy manure alone was four to ten times greater than that from composting dairy manure mixed with MSW compost. Adjustment of the C:N ratio to 25 by adding MSW compost to manure appeared to be the major factor in reducing N losses.     

Determining Optimal Maturity of Compost Used for Land Application

A stable compost is needed in plant growth media. However, when compost is land applied, its effect is through the initiation and acceleration of microbial processes leading to the production of soil stabilizing agents.

It was proposed that there is an optimum degree of maturity of compost used for land application, a degree characterized by the reduction of the labile organic matter to a point when the material is relatively stable yet is still active enough to support an increased microbial activity in the soil.

The effect of composting time on the efficiency of municipal solid waste compost (MSWC) to improve soil structural properties, evaluated through laboratory indexes, is presented in this work. MSWC was sampled immediately following 24 hours precomposting in a Dano drum. Samples were taken throughout 60 days of windrow composting. The compost samples were mixed with a structure-impaired loess soil and incubated aerobically for 21 days. Hydraulic conductivity and residual turbidity, a measure of microaggregate stability, were measured in solutions of two SAR levels, five and 20. Compost application had a positive effect on these soil structural properties. The optimal activity was obtained for the compost sampled following seven to 14 days of windrow composting. Polysaccharide concentrations also followed a similar optimum curve. The peak concentration was found following 14 to 30 days old compost application.

The determination of the optimal maturity of composted municipal solid wastes is essential toward an efficient utilization of the compost, toward satisfaction of environmental constraints and for a cost-efficient operation of composting plants. The present preliminary study calls for more research in this field.     

One-Step Extraction Procedure as an Easy Tool for Monitoring a Full-Scale Composting Plant

ABSTRACT. Composting is an efficient technique for managing the increasing production of organic wastes since it transforms them into a valuable product that can be applied to soil to improve it. However, it is necessary to ensure the adequate properties of this product for a proper agricultural or environmental application. Various parameters have been proposed as estimators of the evolution of the composting process, nevertheless there is a lack of easy techniques with a wide application range for monitoring it. This study evaluated some parameters that can be used to test compost evolution in samples of composted sewage sludge under full-scale conditions. Total extractable carbon (TEC) by alkaline solution of sodium pyrophosphate, humic acids (HA), fulvic acids (FA), and the ratio of them (HA/FA) were obtained by a one-step extraction procedure. Results were compared with those obtained after an official extraction method for the same parameters. Total organic matter was calculated by calcination (TOM%). The objective was to assess the usefulness of the one-step extraction as an easy and cheap tool for monitoring the composting process. The results indicate that both extraction procedures show a statistically significant correlation between them and with TOM%. Therefore, TEC obtained by the one-step extraction procedure can provide useful information about the evolution of the composting process of compost samples with the easiest methodology.     

Relating Compost Measures of Stability And Maturity to Plant Growth

Assessment of compost maturity is important for successful use of composts in agricultural and horticultural production. We assessed the “maturity” of four different sawdust-based composts. We composted sawdust with either cannery waste (CW), duck manure (DM), dairy (heifer) manure (HM) or potato culls (PC) for approximately one year. Windrows were turned weekly for the first 60 days of composting, covered for four winter months and then turned monthly for six more months. We measured compost microbial respiration (CO₂ loss), total C and N, C:N ratio, water soluble NO₃-N and NH₄-N, dissolved organic carbon (DOC), pH and electrical conductivity at selected dates over 370 days. Compost effects on ryegrass biomass and N uptake were evaluated in a greenhouse study. We related compost variables to ryegrass growth and N uptake using regression analysis. All composts maintained high respiration rates during the first 60 days of composting. Ammonium-N concentrations declined within the first 60 days of composting, while NO₃-N concentrations did not increase until 200+ days. After 250+ days, DM and PC composts produced significantly more ryegrass biomass than either CW or HM composts. Total C, microbial respiration and water-extractable NO₃-N were good predictors of compost stability/maturity, or compost resistance to change, while dissolved organic carbon, C:N ratio and EC were not. The compost NO₃-N/CO₂-C ratio was calculated as a parameter reflecting the increase in net N mineralization and the decrease in respiration rate. At ratio values >8 mg NO₃-N/mg CO₂-C/day, ryegrass growth and N uptake were at their maximum for three of the four composts, suggesting the ratio has potential as a useful index of compost maturity.     

Carbohydrates as Chemical Constituents of Biowaste Composts and their Humic and Fulvic Acids

The decomposition of organic matter of source-separated biowaste during composting was followed during 18 months. Compost samples were fractionated into three parts: (i) hot water soluble extract (HWE) (ii) bitumen fraction and (iii) humic substances (humic acids (HA) and fulvic acids (FA)). Original compost samples and the HA and FA fractions were hydrolyzed with sulfuric acid for hexoses and pentoses. Quantitative spectrophotometric and qualitative GC/MS analyses of monosaccharides as trimethylsilyl ethers of the corresponding alditols were carried out.

During composting, the amount of HA in the organic matter of the compost increased, the amounts of HWE and bitumen decreased and the amount of the FA fraction changed only a little. Carbohydrates were found to be important constituents of biowaste composts and their HA and FA fractions. Elemental analysis (C, N and H) of compost and HA samples showed an increase in the C:H ratio and in unsaturation of compounds during composting. The decrease in the C:N ratio was marginal.

The amounts of hexoses and pentoses in original compost samples and the HA and FA fractions decreased during composting. The sugar alcohols erythritol, xylitol, L-arabitol, ribitol, L-rhamnitol, L-fucitol, D-mannitol, D-glucitol and galactitol were identified in both the HA and FA fractions. 2-Deoxy-D-erythro-pentitol was identified in one HA fraction and inositol in two FA fractions. An analysis of gas chromatographic data for relative abundances showed that, in every sample except one and in every stage of composting D-glucitol was the main sugar alcohol. In general, the relative amount of D-glucitol decreased during composting, while the relative amounts of all other sugar alcohols increased.

As chemical indicators of compost maturity, carbohydrates would appear to be a important group of compounds. Most informative as a general indicator would be the ratio of the amount of HA to the amount of organic matter in the total compost samples.

According to our studies, the carbohydrates in composts are covalently bound to the structures of FA and HA. Carbohydrate determination clearly deserves more attention in the structural elucidation of FA and HA.     

Comparison of humic acids derived from city refuse with more developed humic acids

Abstract

The humic acids (HA) from composted and uncomposted city refuse (CR) were characterized by degradative (oxidation with persulphate and permanganate) and non-degradative techniques (FT-IR and ¹³C-NMR) in order to analyze the effect of the composting process on these HA. They were also compared with commercial HA extracted from leonardite. The carboxyl and carbonyl group content of the HA from CR increased slightly during composting. Since the HA from the composted CR showed a lower N and H content, the FT-IR spectra showed a lower intensity in the bands corresponding to peptides and carbohydrates. Differences were revealed when the HA from both CR were compared with those from leonardite which showed a much lower N and H content and a less aliphatic character. The percentage of degraded products by persulphate was higher for the HA from uncomposted CR. For the HA from both CR the major components among the oxidation products were dicarboxylic acids and normal fatty acids. In the leonardite HA, the major components consisted of benzene polycarboxylic acids. ¹³C-NMR revealed an attenuation of the aliphatic character of the HA from CR with composting.     

Maturity and Stability Evaluation of Composted Yard Trimmings

The objective of this research was to evaluate a variety of stability and maturity indices for yard trimmings compost produced in the Puget Sound region of western Washington State. Compost samples were collected periodically during a 133-d composting cycle at a commercial composting facility, showing that indices of compost respiration rate were sensitive indicators of compost quality. All respiration rate indices identified a period of high respiration rates during active composting (first 27 d), and a period of relatively stable respiration rates during the latter part of curing (70 to 133 d). Chemical tests of compost solids showed less promise as maturity indicators, but provided valuable information on final compost quality. Mature yard trimmings compost had a C:N of 12, an NH₄-N to NO₃-N ratio of less than 4, a cation exchange capacity (CEC) of 400 cmol per kg of compost-C, and a pH between 6.5 and Seed germination tests and sensory tests (color and odor) were of limited value in assessing compost maturity. Fully-cured compost produced with forced aeration had a Solvita CO₂ test value of 6 to 7 and a respiration rate via the alkaline trap method of 2 mg CO₂-C g compost-C^?1 d^?1. It reheated less than 2°C in an insulated Dewar flask in a 7 d incubation. Further evaluation and calibration of respiration test protocols for compost quality assurance testing programs are recommended.     

Stability Evaluation of Mixed Food Waste Composts

As interest in food waste composting grows, so does the need for proven composting methods. Stability testing has been proposed as a compost quality assurance tool. We conducted this study to: (i) to evaluate the efficacy of simple outdoor composting methods in producing a compost with a low, stable decomposition rate, and (ii) to determine the reliability of simple, 4-h compost stability evaluation methods. Composting was conducted outdoors in winter and spring in Eugene, Oregon without moisture addition. Mixed food waste was combined with screened dairy solids and ground yard trimmings. Sawdust was used to cover windrows for the first 27 d of composting. Compost windrow temperatures remained above 55°C for 30+ d. Carbon dioxide evolved with several 4-h test methods was strongly correlated (r² > 0.7) with CO₂ evolved using a 48-h test. A limited-turn windrow (LTW) composting system produced compost with slightly greater stability than a passively aerated windrow (PAW) composting system. Food waste compost samples had a low CO₂ evolution rate after 71 to 99 d using either composting system. Compost CO₂ evolution rate at 25°C decreased with composting time, reaching approximately 1 to 4 mg CO₂-C g compost C^?1 d^?1 for the PAW method and 0.5 to 2 mg CO₂-C g compost C^?1 d^?1 for the LTW method. Putrescible organic matter in food waste was effectively decomposed in outdoor windrows using composting methods that did not employ forced aeration, self-propelled windrow turners, or manufactured composting vessels. Several 4-h stability tests showed promise for implementation as quality assurance tools.