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Publications
June 2003,
Environment Australia
As outlined already in Section 4.4.4, the collected organic material was taken to the selected farm for composting and beneficial reuse. Figure 29 provides a schematic overview of the farm area where the composting facility was established.
The selected composting area was located on unproductive grassland next to the main gravel road leading to the dairy shed. Access to the site through the fence line was levelled and covered with gravel to ensure all-weather access. The chosen land was sloping slightly towards a paddock of cultivated land. Uphill of the composting area was a gully and the permanent fence line contained a diversion ridge. Both features prevented the possible inflow of storm water. The two ponds that received effluent and wash water from the milking shed were approximately 30 m away from the designated composting area. When the effluent ponds were cleaned out, the pond sludge was stored along the permanent fence line, covering some of the grassland and some of the cultivated land. When necessary, a removable electric fence was used to keep cows out of the composting area when passing through this section of the farm on their way to the milking shed.
Due to the small scale of the trial and its limited time frame, a composting and processing system had to be devised that could be operated with existing machinery, required little additional investment and was simple to manage and operate.
This is in line with requirements by Durack (2003) for key performance criteria of waste management systems within agriculture:
To keep operating costs low, there was no intention to shred the delivered organic material prior to it being composted. To make this possible, residents were asked not to dispose of branches or wood with a diameter greater than 2.5 cm via the organics recycling bin. Participating residents complied very well with this request. No large pieces of wood were found in the incoming material.
The devised simple on-farm composting system consisted of a 'perpetual windrow' to which new material was added at one end and mature material was removed at the other end (see Figure 30). By the time the windrow reached the end of the composting area, the old compost had matured and had been removed, making place for the second windrow to be established in the same place the first one started.
Figure 30 Schematic view of 'perpetual' windrow including curing pile
The on-farm composting system comprised the following components and activities.
The collected organic materials were unloaded in such a way that the freshly delivered material extended the existing windrow. The collection vehicle backed up to the existing windrow and dumped the new material at that end of the windrow that contained the youngest material, thereby extending the windrow with each new delivery. This is depicted in Figure 31.
Figure 31 Unloading of newly delivered organic material to extend existing windrow
Visible impurities such as plastics, metal and glass were removed during or shortly after delivery of the new organic materials. Quick action was warranted, as the wind tended to disperse plastic bags and other lightweight packing and make the site unsightly.
Residents were asked to wrap kitchen scraps, particularly meat and fish leftovers in newspaper in order to reduce odour emissions from the kitchen tidy and the organics recycling bin. Some people used whole newspaper sections for wrapping. These thick layers of paper degraded relatively slowly. Against the darkening and maturing compost the light coloured paper appeared to be impurities, particularly from a distance, while in fact it was just newspapers that broke down slowly (see for example Figure 33 [foreground]). This was only a minor visual problem.
Turning of the windrows also often revealed impurities, which were removed subsequently. As plastic bags and bottles were not fractured and torn into small pieces by shredding or turning they could be removed relatively easily as whole items.
Once the kerbside collected organics were delivered and the impurities removed, the new material was covered with chicken manure (sawdust-based) and pond sludge so that no foodstuff was exposed. During the following two weeks the material was left to decompose before it was turned just prior to the next delivery. It was assumed that food scraps that might be of interest to animals were degraded to such a level during the initial two-week composting period that they were no longer attractive to animals.
These above measures were taken in response to concerns raised about the possibility of farm or wild animals picking up foodstuff and spreading diseases such as Foot and Mouth Disease or BSE. For more information on this matter see Appendix IV.
The windrow was turned every two weeks with the front-end loader of a tractor (Figure 32). The system was very simple in that in week two, the windrow was moved from Position 1 to Position 2 and in week four the windrow was moved back to Position 1. This system is demonstrated in Figure .
Figure 32 Turning of windrow by moving it back and forth with a front-end loader from Position 1 to Position 2
Figure 33 Use of tractor front-end loader to turn windrow
Once the composting process was nearing completion after some six months and temperatures within the older sections of the windrow dropped below 40-45 șC, those sections were pushed up into mounds for curing and maturing. The mounds were turned once or twice every six weeks before the compost was considered ready for use.
The organic material was composted and cured for a relatively long time, much longer than may be considered necessary in other circumstances for compost that is destined for agricultural use. However, as the compost was not to be screened before application, the aim was to break down as much as possible of the woody material (twigs and small branches) and the newspaper packs.
While turning with a front end loader is advantageous with regard to the removal of impurities, use of a bucket loader is not ideal for mixing of various feedstock components and pockets or lumps of materials tend to remain unchanged for a long time. The breaking-up of twigs and branches is also not aided greatly by this turning method. The use of a dedicated windrow turner would certainly deliver satisfactory results on both aspects.
The collected materials contained predominantly garden organics with a relatively low proportion of grass clippings or other soft materials with high moisture content. Consequently, the incoming material tended to be relatively dry, a situation that was exacerbated by drought conditions. This lack of rainfall and the fact that moisture was lost during the composting process made it necessary to irrigate the composting material.
The selected composting area was particularly attractive since it offered the opportunity to use dairy effluent to irrigate the composting material. Consequently, the farmer installed a valve and connected a diversion pipe to the main effluent pipe in order to direct effluent from the milking shed to the composting pile. However, as effluent transfer from the dairy shed to the ponds was driven only by gravity and as the diversion pipe had a smaller diameter than the main pipe, the diversion pipe tended to block up frequently. The farmer did not own a suitable pump for solving this problem. There was also no pump at hand that could have been used to extract effluent from the ponds. It did not seem justified to purchase a new pump for the remainder of the trial period.
Consequently, a 3,000L trailer mounted tank with a small pump was used to cart water to the composting area for irrigation. This made irrigation of the windrows laborious and subsequently supply of water tended to be neglected. Consequently, the composting material was often relatively dry and not in a condition that would be described as ideal for composting. This fact would have also contributed to the prolonged composting period and the slow breakdown of newspaper packs.
When the project began, the farm employed four people for farm work and had part-time administrative support. One of the farm staff was employed part-time to produce, bag and deliver lucerne mulch. It was agreed that this person would also look after the composting operation. It was estimated that running of the composting operation would require between one and two hours per fortnight, plus time for irrigation. Turning of the windrow with the tractor bucket took between 30 and 45 minutes. Segregation of impurities took only a few minutes each time, provided it was done regularly each time new material was delivered and after each turning. Including filling of the tank (3,000L), irrigation of the windrow took between approximately 30 minutes (one tank) and 90 minutes (three tanks).
Communication between the farmer and his employee turned out to be a major problem. The farm worker would perform compost related tasks only after being asked explicitly to do so by the farmer. It was not sufficient to explain regularly recurring tasks and tell him to perform them for example 'each time new material is delivered'. This resulted in many compost related tasks not being performed during the first eight weeks or so. The situation improved only when responsibilities were transferred to another farm worker.
Even though the farmer fully supported the project, composting was not his core business and it did not have a very high priority. Understandably, staff and machinery were allocated first of all to accomplish farm related tasks and if there was time to spare, to undertake compost related tasks. The limited scale of the trial fostered this approach as it provided only little additional income for the farm.
A temperature probe (1.5 m length) with data logger capabilities was used to record temperatures within the windrow. As the recording interval could be set between once every five seconds and once per day, the probe was used to measure the temperature regularly along the windrow and also to monitor temperature development over prolonged periods.
Generally, records showed that temperatures reached during the composting process were sufficient to eliminate pathogens and weed seeds. However, even though temperatures reached sufficient levels (> 55 °C for more than three days) conditions for pathogen and weed seed elimination may not have been ideal at all times. It has been shown that the elimination and inactivation of pathogens during composting is a complex process that is influenced by many factors, moisture content being one of them (Marciniscyn and Gottschall, 1995).
Figure on the following page, which shows continuous temperature recordings over a six week period, demonstrates that temperatures ranged mainly between 60 and 70 °C for the first two weeks of composting (A and B). This is well above the required threshold of 55 °C for three days and should ensure elimination of pathogens. However, after the first turn (after two weeks), temperatures dropped and ranged only from 45 to 55 °șC during weeks three and four. After the second turn, temperatures increased slightly and ranged largely between 55 and 60 °C. The temperature range between 45 and 55 °C is ideal for biological activity and decomposition but not necessarily for pathogen elimination.
The perpetual windrow contains organic material in all stages of decomposition and pasteurisation. This is reflected in the temperature gradient along the perpetual windrow where sections with fresh material exhibit high temperatures and older sections show lower temperatures (Figure 34). Working with this type of system, or with any other one for that matter, care should be taken not to contaminate pasteurised product with new, non-pasteurised material. Therefore, turning of windrows or other work should always start with the old, pasteurised materials.
(Temperatures represent the average of two measurements, one on either side of the windrow; temperatures in the curing pile represent average of three measurements)
Figure 34 Temperature gradient in 'perpetual windrow', including curing pile, measured over a two-week period
Figure 35 Temperature profile during six week composting period (27.8.-8.10.02)