The Effect of Feed Mix Porosity in Composting

 

AASEN, ROALD, Lystad, Henrik, S. rheim, Roald and Bergersen, Ove

 

Jordforsk: Norwegian Centre for Soil and Environmental Research, Norway

 

Structural material such as wood chipping, grounded yard waste and bark are used as bulking agents mainly to increase the porosity and absorb water from the biowaste when composting source separated biowaste from households. A proper feed mix prevents odor and help to achieve optimum facility capacity and to improve the quality of the finished compost. Determination of dry matter in the structural material and in the biowaste is a commonly used procedure to define the correct feed mix. However, dry matter alone provides little information about the porosity. Porosity improves air exchange, increase surface for microbial growth and the enhance the ability to absorb water from the biowaste. Furthermore, measurements of dry matter do not include the effect of compression in the compost that leads to decreased porosity when reactor bins and static windrows can reach heights up to 4 m. Decreased porosity can cause slow anaerobic processes with odor problems and water run off. The objective of this study has been to determine the effect of porosity on the composting process. Composting experiments with feed mix of different porosity have been carried out in 200 l static bins with forced aeration. The compost process was monitored by chemical and physical parameters as well as an chemical characterization of the odor. Results from porosity and water capacity measurements in pressurized conditions up to 4 m height (500 kg/m3) show that porosity measurements is an alternative to dry matter for deciding on feed mix and in compost process monitoring. Results will be discussed in relation to the compost processes.

 

The Determination of Free Air Space and Bulk Density of Compost Mixtures with Compressive Loading

 

AGNEW,JOY (1), Leonard, J.J. (1)

 

(1) Department of Agriculture, Food, and Nutritional Science, University of Alberta, Canada

 

The physical properties of compost materials such as bulk density, porosity and free air space (FAS) play an important role in every stage of compost production as well as the handling and utilization of the product.  Current methods of determining bulk density and FAS are cumbersome and time consuming.  In addition, large errors and inconsistencies can arise from the use of disturbed or compacted samples and the non-homogeneity of small samples.  A method of measuring bulk density and FAS that is quick, accurate and simulates compressive loading will make possible the complete characterization of compost material. The modified air pycnometer included two large PVC vessels for pressure difference determination and a 10cm, 1.7MPa air cylinder with piston to simulate the compressive loading conditions found at all pile depths.  Manure compost, municipal solid waste (MSW) and biosolids materials were characterized at a variety of moisture contents and compressive loads.  Bulk density and FAS profiles were plotted and the relationship between bulk density and FAS was found to be linear for most material with R&2 values of 0.8 and greater.  The pycnometer was a useful tool for laboratory measurement and characterization, giving FAS readings in less than 5 minutes, but is too large and complex to be useful in the field.

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Effect of Free Air Space On Process Performance

 

Grant Eftoda &DARYL MCCARTNEY

 

Department of Civil Engineering, University of Manitoba, Winnipeg, Manitoba R3T 5V6 CANADA, Canada

 

The presentation will discuss the relationship between free air space (FAS), and windrow composting performance, e.g. odour generation, process temperature, pathogen inactivation, and volatile solids removal. The results of a full-scale trial using municipal biosolids and woodships will be presented. The objective of the trial was to develop a bench-scale FAS test that would predict the performance of the full-scale windrows. Biosolids to woodchip ratios investigated ranged from 1:1 to 1:4 (biosolids:woodchips; volumetric) using four 60 m3 windrows. Composting performance indicators monitored included: temperature, pore space oxygen, odour generation, volatile solids removal, and pathogen inactivation.

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The Effect of Bed Structure On Composting Start-Up

 

RUDRUM, DALE, Rulkens, Wim,  and Hamelers, Bert

 

Dept. Environmental Technology; Wageningen University, The Netherlands

 

An appropriate composition and structure of the composting material is a prerequisite for a successful composting operation. The question of improved feed conditioning and bed construction is widely dealt with in literature. Optimal bed construction is treated in terms of moisture and Free Air Space (FAS). Optimal moisture content and optimal FAS are treated as independent parameters. Recently a mechanistically model has been developed by Hamelers and Richard describing the relation between Oxygen Uptake Rate (OUR) and moisture. The model also incorporates the effect of dry bulk density. The model allows to study the interaction of the effects of moisture and FAS on OUR. The start-up is a critical phase during the composting process as unsuitable start-up can lead to unnecessary long process time. During initial experiments with separated pig manure the temperature sometimes failed to rise to thermophilic levels. The objective of this paper is to study the effect of bed structure on the start-up behavior of a composting reactor treating pig manure. The effect is studied experimentally and theoretically using the recently developed mechanistic model. A well-insulated 2000 litre reactor equipped with air recirculation was used, reducing gradients within the reacting bed. During start-up the oxygen was kept above 10%  (v/v) and temperature was allowed to rise autonomously. The feed consisted of a mixture of separated pig manure and cut wheat straw. The OUR was estimated from the rate of temperature ascent, at mesophilic conditions and at thermophilic conditions. The mechanistically model was linearized to enable easier parameter estimation. A free air space of 10% or more seems critical for a speedy start-up. The linear relation predicted by the model was confirmed. The results agree well with the model in the mesophilic temperature range. At higher temperatures the model is somewhat less. It will be discussed how the model can be used to improve the start-up for this type of materials.