Parameters affecting the composting process

The end product of a composting process is dependent on different parameters. In order to produce a suitable growing medium for containerised plants, it is essential to understand how the composting processes can be influenced, and how the different parameters interact.

C/N ratio is an important variable correlated to mass loss during composting (Eiland et al., 2001a). C/N ratios of approximately 25 have been suggested as optimal for composting (Bernal et al., 1996). If the C/N ratio is above this, a low initial decomposition rate is seen with low respiration rates and low microbial biomass. Most of the nitrogen seems to be immobilised initially when C/N ratio is high, resulting in no net mineralisation (Eiland et al.,

2001a). In contrast, if the initial C/N ratio is below 25 this results in a high decomposition rate with high microbial biomass and this will lead to net mineralisation (Eiland et al., 2001a). In general, a negative correlation is found between initial C/N ratio and N loss (Kirchmann and Witter, 1989).

Many waste materials such as household wastes have an initial low pH, often around 5, due to a high content of short chain fatty acids, and pH can decrease further due to release of organic acids during decomposition (Tuomela et al., 2000). During successful and fully developed composting, the pH often rises to 8-9 due to microbial respiration and loss of CO2. Presence of short chain fatty acids under acidic conditions and their absence when the compost turn alkaline indicate that they are a key factor regulating pH in compost (Beck-Friis et al., 2001). In the transition from mesophilic to thermophilic conditions, a lag phase of stagnation or decline in microbial activity is seen (Beck-Friis et al., 2001; Schloss et al., 2003). This has been noted to coincide with low pH, and Beck-Friis et al. (2001) observed a change in pH from acidic to alkaline conditions when the temperature rose from mesophilic to thermophilic conditions. An explanation for this lag phase could be that microbial respiration is seriously inhibited when the temperature rises above 40°C while the substrate was still acidic (Smars et al., 2002). In extension to previous studies, Sundberg et al. (2004) showed that the degradation rate of municipal waste differed only slightly with pH values ranging from 5 to 8 as long as the temperature was 36°C. However, if temperature was raised to 46°C, the degradation rate decreased at low pH, but increased if pH was raised to above 6.5. The differences between the degradation rates at 36 and 46°C can probably be explained by the sensitivity of the microbial communities to the combined effects of acidic conditions and temperature. One explanation is that microorganisms can withstand one extreme environmental factor at a time, either high temperature or low pH, but not both simultaneously. Another possibility is the existence of two different microbial communities, a mesophilic acid-tolerant community and a thermophilic community that does not tolerate acids. Fungi in general, are important during the initial mesophilic phase of the decomposition (Klamer and Baath, 1998) and since they are more tolerant towards acid and less tolerant towards temperature than bacteria, this supports the hypothesis that different microbial groups are active at the different temperatures. Additionally, Sundberg et al. (2004) found that the changes in pH during the experiment were different at different temperatures, indicating that fundamentally different metabolic paths were present at the different temperatures. When composting plant residues, the pH range was found to be much lower than for many waste types (Dresboll and Thorup-Kristensen, submitted A). The initial pH is generally higher in dried plant material, since the content of fatty acids is much lower than in waste materials. When using wheat straw and clover-grass hay as plant material, the pH initiated at 7 or more, and increased during the first 3-4 weeks. As the amount of fatty acids was low, this increase should be explained by mineralisation of organic N such as proteins during the initial part of the composting process. Mineralisation of organic N is a proton-assimilating process, resulting in the liberation of NH4+ and an increase in the pH (Beck-Friis et al., 2003; Tuomela et al., 2000). Towards the end of the composting period, a decrease in pH was seen, which coincided with NO3- production as the nitrification process releases protons. Thus, the pH variability is tightly connected to the substrate and the activity of the microbial communities. There is however a general overall pattern during a composting process with increasing pH initially and a decrease when nitrification occurs at the end of the process. Despite the final decrease, pH in compost is in general above 7 or even higher. When growing organic plants in compost, this might be suboptimal as it can be difficult to lower pH in organic systems, and high pH might inhibit the uptake of several micronutrients.

2.2.3. Moisture and oxygen content

Optimum air and moisture contents are important in keeping microbial populations active during composting. The dominant type of metabolism in composting is aerobic respiration rendering oxygen availability very important. Despite the intention of having aerobic decomposition processes, anaerobic conditions will almost inevitable occur in some small zones within the compost pile. Thus, aerobic and anaerobic conditions can co-exist during composting (Beck-Friis et al., 2003). The temperature increase is generated mainly by the aerobic metabolism of microorganisms, and is therefore affected by the availability of O2. Oxygen in a compost pile is replenished by air flowing into the pile, and hence affected by the air exchange that is dependent of the air-filled space within the heap (Sommer, 2001).

Water is essential to microbial activity and should be present in appropriate amounts throughout the composting cycle. It has even been suggested that moisture content had a greater influence on the microbial activity than the temperature when compost was incubated at different temperatures and moisture contents (Liang et al., 2003). An abundance of water would certainly ease microbial migration and colonisation as well as the diffusion of substrates and metabolic wastes. However, water tends to impede gas exchange as the pores are filled, and a balance should be maintained between the needs for available water and gas exchange. Although water is produced during decomposition airflow generated by the heat convection during composting evaporate significantly more water than is produced and tend to dry the material out (Kulcu and Yaldiz, 2004). The drying of the compost is very unevenly distributed, as the losses occur in vents created within the compost by the airflow. The compost around these vents will have high moisture contents, whereas the areas surrounding them will dry out. If moisture becomes below a critical level, microbial activity will decrease and the microorganisms become dormant. On the other hand, too high a moisture content can cause a lack of aeration creating anaerobic regions within the material (Agnew and Leonard, 2003; Tuomela et al., 2000). Still, the N mineralisation is independent of the moisture content over a wide range, with significant declines only under extreme dry or wet conditions (Amlinger et al., 2003).

Compost structure and water content change dynamically during the composting process. Because moisture affects material and compost properties as well as microbial activity, it has important implications for both the physical and the biological aspects of the composting process (Richard et al., 2002). High moisture contents can affect the strength of the composted material, allowing it to be compressed more easily. Additionally, the decomposition process reduces particle size and increases compost dry bulk density, leading to a reduction in total porosity, which also can lead to production of anaerobic regions (Kulcu and Yaldiz, 2004). The combination of moisture filled pores and compression of the material will also result in an increase in thermal conductivity allowing heat to be conducted more easily away from the compost at thermophilic temperature levels. However, the airflow will be suppressed by the compression, reducing the heat lost this way. In addition, excessively high moisture contents result in free water draining through the compost as leachate (Agnew and Leonard, 2003).

Maintaining an optimum moisture content in dynamic composting systems, where evaporation, metabolic water production and changes in compaction and porosity are all occurring over time, is important and challenging (Richard et al., 2002). Moisture management requires a balance between two functions, encouraging microbial activity and permitting adequate oxygen supply. Moisture is thus a key variable that affects many aspects of the composting process, from the initial mixture to the mature compost. Generally, a moisture content of 50-60% is considered to be optimal for composting (Agnew and Leonard, 2003). If the moisture content of the material is below 50%, moisture must be applied when initiating the composting process and should be applied whenever necessary during composting. Higher water contents can also result in successful composting as reported by Vallini and Pera (1989), who composted vegetable wastes with an initial moisture content of 82%. Alternatively, high moisture contents could be regulated by adding structural materials of lower water contents to the compost, altering the overall structure and porosity.

2.2.4 Measuring compost process parameters

During the composting process, compost is a very heterogeneous material. The processes are conducted by different microorganisms controlled by competitive advantages of one community over another, and even small changes in the conditions may change the microbial community and thereby the composting processes. Thus, compost can be considered as consisting of several small micro-sites varying in temperature, pH, water content, aeration, nutrient availability and hence microbial communities. During the high activity phase, from the mesophilic to the thermophilic phase, the heat convection generates areas containing more moisture, and can create large vents of several centimetres in the compost piles. These 'hot spots' of high activity and heat loss will be significantly different from other sites in the compost and especially from the edges where temperature and water content in general are much lower. Hence, taking representative samples in compost to study compost processes is difficult and in many cases an average will not describe the actual processes adequately.

Compost sampling has been conducted in many different ways, in order to reduce heterogeneity. Eklind and Kirchmann (2000a) pooled 10 subsamples of 250 ml and took samples from the pooled material for further analyses. Sánchez-Monedero et al. (2001) collected one representative sample by mixing six subsamples taken from six sites of the mixture from the whole profile. Before analyzing the samples Sommer (2001) finely chopped samples of 2-l and subsamples was taken from this. In search for a suitable sampling method, sampling was also conducted in different ways by Dresboll and Thorup-Kristensen, (submitted A) and Dresboll and Thorup-Kristensen (submitted B). Sample size was kept low in order to reduce disturbance of the composting processes. Initially, 5 samples were collected at 40 cm depth and pooled before subsamples were taken for the various analyses. This method revealed large differences between replicate composting boxes. One explanation for the differences could be that the samples were not mixed thoroughly when pooled, resulting in subsamples, which did not represent the entire box. Thus, in the following experiment sampling was conducted by taking 10 small samples distributed randomly in the boxes for each analysis performed (Dresboll and Thorup-Kristensen, submitted A). However, this did not reduce the variability between the boxes. It was noted that especially the water content and thereby the dry weight was determined with great variation, affecting many of the other measurements, as these are dependent on dry weight dimensions (Dresboll and Thorup-Kristensen, submitted A). In the study of Dresboll and Thorup-Kristensen (submitted B) samples were only collected at initiation, after turning and at the end of the composting period, collecting samples at all three occasions right after mixing the entire composting box thoroughly. This should ensure a more representative estimate of water content of the material and did reduce the variation within the boxes although, interestingly, the variation was still significant between replicate boxes. Thus, this makes the outcome of composting in small scale more uncertain as even small differences can have an immense effect on the outcome, which in larger scale composting experiments might be equalised throughout the pile.

Alternatively, composting in smaller scale can be conducted in compost reactors where process parameters can be controlled leading to more homogeneous compost (Beck-Friis et al., 2001; Smárs et al., 2001; Eiland et al., 2001b). However, the normal dynamics of the composting process will not be expressed and controlling all parameters might affect the microbial succession and provide insight but not necessarily comparable results to large scale composting. Mixing the compost in order to obtain the most homogenous material for analysis was found to be the most effective way of reducing variability. Still, this might restrict sampling frequency when disturbance of the processes by turning the material too much is undesirable (Dresboll and Thorup-Kristensen, submitted A).

Temperature changes is generally monitored from the middle of the compost material always showing some of the highest temperatures, as temperature in the periphery do not reach the same high temperatures (Dresboll and Thorup-Kristensen, submitted A; Eiland et al., 2001). Thus, it is important to notice that the temperatures monitored are maximal temperatures and only some of the material will reach such high temperatures.

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