Adrie Veeken

Moisture Relationships in Composting Processes

Moisture is a key environmental factor that affects many aspects of the composting process. Biodegradation kinetics are affected by moisture through changes in oxygen diffusion, water potential and water activity, and microbial growth rates. These relationships are made more complex by the dynamic nature of the composting process, with changes in particle size and structure occurring over time. A deductive model of the effects of moisture on composting kinetics has defined these relationships based on fundamental physical properties and biological mechanisms. This study applies this model to experimental data from a manure and papermill sludge composting system. The results demonstrate that the optimum moisture content for biodegradation can vary widely for different compost mixtures and times in the composting process, ranging from near 50 to over 70% on a wet basis. While there is a significant reduction in biodegradation rate when operating outside the optimum range, the results also suggest opportunities to mitigate this effect through manipulation of substrate density and particle size. This framework for engineering analysis demonstrates the importance and challenges of maintaining optimum moisture content in dynamic composting systems, where biological drying, metabolic water production, and changes in compaction and porosity are all occurring over time.

Characterisation of NaOH-extracted humic acids during composting of a biowaste

Changes in amount and characteristics of humic acids (HA) were studied during the composting of a biowaste in an 80 l composting device. HA was extracted by NaOH, purified with HCl–HF and dialysed. Freeze-dried HA was characterised by UV-285, elemental analysis, 13C NMR and pyrolysis-GC/MS. Composting of biowaste resulted in the degradation of 65% of the organic matter. The amount of HA decreased in the initial stage of composting but started to increase again after 20 days. At the start, HA was mainly composed of aliphatic compounds which were replaced by aromatic compounds during composting. Characterisation of HA revealed that the contribution of the condensation route was significant for HA formation during composting. Maturity indexes based on NaOH-extracted HA were inappropriate and moreover, beneficial effects of compost amendments to soils cannot be studied by organic matter analysis of bulk compost, but the amount and characteristics of available HA should be determined.

Sources of Cd, Cu, Pb and Zn in biowaste.

Biowaste, the separately collected organic fraction of municipal solid waste, can be reused for soil conditioning after composting. In this way, environmentally harmful waste management strategies, such as landfilling or incineration, can be reduced. However, frequent application of composts to soil systems may lead to the accumulation of heavy metals in soils, and therefore legal criteria were laid down in a decree to guarantee the safe use of composts. The heavy metal content of biowaste-composts frequently exceeds the legal standards, and thus raises a conflict between two governmental policies: the recycling of solid waste on the one hand, and the protection of natural ecosystems and public health on the other hand. In this study, the heavy metal content (Cd, Cu, Pb and Zn) of biowaste was compared with the natural background content of Cd, Cu, Pb and Zn in the different constituents of biowaste. For this, the physical entities of biowaste were physically fractionated by wet-sieving and subsequent water-elutriation. In this way, organic and inorganic fractions of different particle sizes were obtained and the content of Cd, Cu, Pb and Zn and the organic matter content of the different fractions were determined. On the basis of particle size, density and visual appearance, the particle-size fractions were assigned to various indoor and outdoor origins of the biowaste. It was found that a large amount of biowaste was not organic, but over 50% was made up of soil minerals due to the collection of biowaste constituents from gardens. The heavy metal content of the various fractions in biowaste was compared with the natural background contents of heavy metals in the constituents of biowaste, i.e. food products, plant material, soil organic matter and soil minerals, by collecting literature data. The heavy metal content in the fractionated physical entities of biowaste corresponded with the natural background concentration of its constituents and indicated that biowaste was not contaminated by other sources. However, the natural background content of biowaste constituents will result in heavy metal contents for biowaste-compost that will exceed the legal standards. It is advised that the legal standards for composts should be critically re-examined. The protection of soil systems could be better guaranteed if the input of heavy metals was evaluated for all inputs of fertilisers and soil conditioners, i.e. animal manures, various types of compost and artificial fertilisers.

Air-filled porosity and permeability relationships during solid-state fermentation.

An experimental apparatus was constructed to measure the structural parameters of organic porous media, i.,e. mechanical strength, air‐filled porosity, air permeability, and the Ergun particle size. These parameters are critical to the engineering of aerobic bioconversion systems and were measured for a straw‐manure mixture before and after 13 days of in‐vessel composting. Porosity was measured using air pycnometry at four (day 0) and five (day 13) moisture levels, with each moisture level tested at a range of different densities. Tested wet bulk densities varied with moisture level, but dry bulk densities generally ranged from 100 to 200 kg m−3. At each moisture/density combination, pressure drop was measured at airflow rates ranging from 0.001 to 0.05 m sec−1, representing the range of airflow rates found in both intensive and extensive composting. Measured air‐filled porosities were accurately predicted from measurements of bulk density, moisture, and organic matter content. Reductions in air‐filled porosity at increasing moisture content were accompanied by an increase in permeability, apparently due to aggregations of fines. This aggregation was quantified by calculating an effective particle size from the Ergun permeability relationship, which increased from 0.0002 m at 50% moisture to 0.0021 m at 79% moisture. The range of airflow velocities reported in composting systems requires consideration of the second‐order drag force term, particularly at velocities approaching 0.05 m s−1 for the higher moisture treatments tested. Calculated permeabilities for the matrix ranged from 10−10 to 10−7 m2, varying with both air‐filled porosity and moisture. Mechanical strength characterization provided a means to predict the effects of compaction on air‐filled porosity and permeability of porous media beds. The results of this investigation extend porous media theory to the organic matrices common in solid‐state fermentations and help build a framework for quantitative and mechanistic engineering design.

Passively Aerated Composting of Straw-Rich Pig Manure : Effect of Compost Bed Porosity

Straw-rich manure from organic pig farming systems can be composted in passively aerated systems as the high application of straw results in a compost bed with good structure and porosity. The passively aerated composting process was simulated in one-dimensional reactors of 2 m3 for straw-rich manure with compost bed densities of 1100, 700 and 560 kg m−3. Temperature profiles over the reactor height were monitored online and ammonia emissions were measured periodically. The composition of the compost bed over the reactor height was determined at the end of the composting process. The composting process strongly depends on the density of the compost bed. At a density of 1100 kg.m−3, the porosity of the bed is too low to initiate natural convection, and aerobic degradation fails and anaerobic conditions may lead to emissions of methane and odorous compounds. At a density of 560 kg.m−3, the porosity of the bed is high and the high rate of natural convection will keep the temperature low thereby preventing the removal of pathogens and weeds. Best results were observed at a density of 700 kg.m−3 for which aerobic degradation and drying were adequate and temperatures were high enough to kill pathogens and weeds. On basis of the Ergun equation, which describes the airflow in porous media with internal heat generation, this corresponds to a compost bed permeability of 7×10−8 m2. It was also shown that it is possible to compost animal manures with a low C/N ratio without significant emissions of ammonia. This can be established by trapping the initial ammonia emissions in a straw filter, which is placed on top of the compost bed. Ammonia absorbed in the straw filter and in the compost bed were removed by nitrification and denitrification. The passively aerated composting system results in a compost bed which is highly heterogeneous with respect to temperature, oxygen level and its composition. It is proposed that in this way a highly diverse microbial community in the compost bed is established which can perform various microbial conversions. The extensive composting system is most promising for on-farm production of an organic fertilizer from straw-rich manure, since the costs of the process and the level of ammonia emissions were low.

Qualitative modifications of humic acid-like and core-humic acid-like during high-rate composting of pig faeces amended with wheat straw

Abstract Humic acid-like (HA-like) and core-humic acid-like (core-HA-like) were characterized during the high-rate composting process by CP-MAS 13C NMR, pyrolysis-gas chromatography (GC)/mass spectrometry (MS), and elemental analysis. Results obtained indicated that humification proceeded through a relative concentration of aromatic fractions due to the faster degradation of the O-alkyl and alkyl fractions. Core-HA-like, after purification of the parent material, showed a large reduction of the O-alkyl fraction in terms of HA-like. We concluded that HA-like consisted of refractory organic molecules, such as lignin and biopolymers, which formed a stable structure (core-HA-like) coated with degradable material associated with the core by weak physical association, ether or ester bounds.

Evolution of humic acid-like and core-humic acid-like during high-rate composting of pig faeces amended with wheat straw

Abstract The changes in the absolute amounts and in the composition of humic acid-like (HA-like) and core-humic acid-like (core-HA-like) were monitored for the high-rate composting process (4 weeks) of wheat straw-amended pig faeces. Absolute amounts of HA-like and coreHA-like were obtained by extraction of HA-like and core-HA-like from the compost bed once a week. The decrease in the amount of HA-like (350 g kg−1) took place during the first week of composting and was due to the degradation of organic molecules co-extracted with HA-like, the so-called interference or coating material. The coating material was mainly composed of fatty acids. Core-HA-like, after an initial decrease, showed an increase in the total amount after 2 weeks of composting. This was due to the partial solubilization of the humin fraction, which started when lignin-degrading fungi became active at the mesophilic stage of the composting process. We concluded that HA-like consisted of refractory organic molecules, probably lignin and biopolymers, which formed a stable structure (core-HA-like) coated with the degradable material associated with the core by weak physical association, ether or ester bounds. The ratio of HA-like and core-HA-like, the so-called Organic Matter Evolution Index (OMEI), was found to be useful to quantify the evolution of organic matter during composting, as it takes into account the biodegradability of all types of organic compounds and humification processes. OMEI increased from 0.26 to 0.41 during the highrate composting process. This value was lower than that for well-developed composts (OMEI > 0.6) but the presence of degradable organic matter could promote the stabilization of soil aggregates and soil biological activity.

Passively Aerated Composting of Straw-Rich Organic Pig Manure

Composting of animal manures using forced aeration generates high ammonia emissions. Although passively aerated composting results in lower ammonia emissions, there are concerns it may lead to high methane emissions and poor organic matter degradation. This study examined these issues using pig manure originating from organic farming systems, which could be directly composted by passive aeration because it contains high amounts of straw, which served as a bulking agent. Experiments were performed in 2-m3 reactors (2 m in height and closed walls) with a bottom grid providing vertical aeration of the compost pile. A series of experiments evaluated the effects of the compost bed structure and monthly turning of the piles on the composting process and on the emissions of ammonia, nitrous oxide and methane. Effectiveness of the composting process strongly depended on the density of the compost. Above a critical maximum density natural convection would not be initiated, aerobic degradation would fail, and anaerobic conditions would lead to emissions of methane and odorous compounds. Below a critical lower density the high rate of natural convection would keep the temperature low, thereby preventing the destruction of pathogens and weeds. Best results were observed at a density of 700 kg m−3, where both aerobic degradation and drying were adequate and temperatures were high enough to kill pathogens and weeds. Monthly turning shortened the process from 8 to 3 months, as it provided a better compost bed structure. Moreover, turning gave a more homogeneous end product. Composting did not result in significant ammonia emissions (13% of total nitrogen initially present) and nitrous oxide emissions were low (2%) for turned piles but significant (7%) for undisturbed piles. Anaerobic regions were present inside the pile but methane emissions were not observed as methane was oxidised in the top layer of the bed. Mass balance analysis indicated that the major part of nitrogen (50%) was emitted as dinitrogen, presumably a result of simultaneous nitrification-denitrification. Surprisingly, nitrification also took place at temperatures higher than 50 °C, where thermophilic methanotrophs may have been responsible for oxidation of ammonia.

Reduction of Ammonia Emission and Waste Gas Volume by Composting at Low Oxygen Pressure

This chapter reports first results on the performance of a composting reactor with off-gas oxygen levels ≤10%. The low oxygen level is the result of uncoupling the two functions of the airflow: heat removal and oxygen supply. Uncoupling these functions is achieved by cooling and recirculating the gas, in this way reducing off-gas flow and emission of gaseous ammonia. The objective of the research was to identify a control strategy ensuring reactor performance stability, and a sufficient compost quality.