AND
COMPOSTING OF KITCHEN AND GARDEN WASTE
IN A COLD CLIMATE
Kerstin Grennberg
Göran Gabrielson
Dept. of Environmental Planning and Design
Luleå University of Technology
PREFACE
We are very grateful to COLDTECH for the financial support during two years which made this project possible.
The staff in the kitchen of the university restaurant is also gratefully acknowledged for collecting the kitchen waste.
GG participated in this project during the first season and PN during the second season.
Per Nilsson, Kerstin Grennberg, Göran Gabrielson
SUMMARY
Four prototypes of insulated, small compost containers were constructed with the following assumptions in mind:
1) The containers are intended to be used by families living in their own houses for composting kitchen and garden waste outdoors.
2) The degradation of the waste material will continue during periods with temperatures below freezing.
3) The volume of the container will be enough for additions of 15 litres of kitchen and garden waste per week during 8 months.
4) The container will be easy to handle, i. e. it will be easy to turn the waste in it and to empty it.
Comparisons of the prototypes with two commercial containers, the Hotter and Kåge containers, were made during two winter seasons: for 104 days 22/2/91 - 6/6/91 and for 193 days 28/11/91 - 9/6/92.
In order to compare the composting processes in the different containers, we carried out continuous measurements of the temperatures, regular measurements of dry weights, ash contents and volumes of the material in the containers, as well as of the added material. Observations concerning smell and the degree of decomposition of the material were also made. The reduction of the weight and volume of the added material was estimated at the end of the composting periods.
Of all the tested containers, the container which was found to function best , using the above mentioned criteria, was constructed in the following way. It was rectangular with the inner dimensions of 90 * 55 cm and a height of 72 cm. It was made of wood and insulated with 7 cm of styrofoam. The bottom was covered with 25 cm of haydite balls and a geotextile. One vent was located in the lid, and another was located in the wall close to the bottom and covered by haydite balls for warming incoming cold air. The free volume was about 200 litres.
INTRODUCTION
Nowadays the Government are supporting steps which lead to pre-separation of waste at its source. In the Environmental Bill of May 1990, there is a demand that all waste which is generated should be pre-separated at source from 1994 (Government Bill 1989/1990:100, Appendix 100). According to the proposal of the "Environmental Tax Committee" (SOU 1990:58), it should not be free of charge to leave mixed waste for any kind of final treatment. The Refuse Collection Act was changed in 1990:
§ 8 The Refuse Collection Act (changed 1990:235)
"If it is of importance from the point of view of recycling or environmental protection the Government may prescribe that a certain kind of waste, pending its removal, must be kept separate from other waste products, and communicate the regulations necessary for that purpose. The Government can delegate to an authority or to municipalities the issuing of such regulations."
Local composting in urban areas was not allowed before, according to the local public refuse regulations. Now, this is changed in many municipalities. The following excerpt is taken from the regulations in Sorsele:
§ 4.5 "Composting of compostable waste is allowed in the whole municipality if this can be achived without inconvenience to the surroundings."
There are roughly two big groups of waste. The first one contains organic material, which is possible to reintroduce to natural cycles, and the second one contains material, metals among other substances, that should be separated from these natural cycles.
Using local composting containers is one alternative method of taking care of organic waste, while another alternative is to transport it to large-scale, centralized composting plants. The latter alternative is, however, associated with several disadvantages compared to the former:
1) The number of transports will be the same as when collecting refuse without pre-separation at source. The only difference is that there will be a need to transport an additional fraction.
2) The working environment will be worse for the garbage collectors.
3) The final product can be contaminated with considerably higher concentrations of metals and other harmful substances.
Roughly, 50 per cent of the total quantity of waste from households is compostable. The rest of the waste can be stored if the compost fraction is sorted out and composted locally. It will therefore be possible for the public cleansing department to pick up this rest at longer intervals than nowadays. As a result, there will be fewer transports.
In order to achive an aerobic degradation of the waste material, there is a need for microorganisms, moisture, nutrients and oxygen. The temperature in the container is very important for good results in the composting process.
When the microorganisms are active, heat is released. It is necessary that the temperature should lie above 0°C in the material that is to be composted. If it is not, the microorganisms are not active, and furthermore they cannot decompose the organic matter. The microorganisms can be divided into three groups based on the temperature intervals in which they are grown. Microorganisms that are active at low, medium and high temperatures are called psycrophiles, mesophiles and thermophiles respectively. If the temperature exceeds the maximum temperature for a certain group, the microorganisms will die. At a temperature below the lower limit, the microorganisms will not die but their activity will cease.
It is important that the temperature in compost should not fluctuate too much. A regular temperature level will make it possible to keep a certain group of microbes at their maximum activity. The prerequisites for this are many: right moisture, oxygen supply, C/N ratio, etc.
There are, on the market today, a few composting containers that have been especially constructed for a severe climate. Some of them have been tested under realistic conditions. Brink et al (1992) have published results from tests performed in Uppsala. However, these tests were done in a greenhouse at 17°C and in a cold room at 2°C. Containers which are intended to be used outdoors in Norrland (Northern Sweden) must be tested under considerably tougher conditions.
The climate in Norrland is severe and insulated composting containers must therefore be used outdoors. The purpose of this project has been to construct and test containers for composting of household waste in a cold climate, to be used by families living in their own houses.
MATERIAL AND METHODS (Forts)
The containers used
During the two composting periods, six containers were used. Four of them are of our own design. The other two are commercial ones.
The four containers that are of our own design were designed for households that generate approximately 10 litres of organic waste a week. This figure will give a total volume of about 400 litres during 8 months. Since the volumes of the waste in the containers are diminishing, a capacity of 200 litres for each container would be suitable.The volumes of the containers used are listed in Table 1
The containers were made out of wood with an insulation of styrofoam, which was 7 cm thick. In order to get a better distribution of air in the composted waste, a layer of haydite balls was placed at the bottom of the containers. Above this layer, a thin geomtextile was laid. The geomembrane was used in order to prevent the pore volume of the haydite balls from getting filled up with organic waste. All the vents were eqiuped with a net
in order to prevent problems with flies.
Three of the containers were of identical shape (see Figure 2). They were constructed with a large surface in relation to the material depth.
The only dimension that was not identical was the distance from the top of the container down to the layer of haydite balls. This distance was individually stated for the three containers shown in Figure 2. As can be seen in Figure 4, container 5 has been used without an underside. The layer of haydite balls was therefore placed directly on the ground. One of the results of this arrangement was that the volume in this container was considerably larger than in the other containers.
Container 1 (Figure 1 and Table 1) had a total volume which was equal to the other three. However, it had another shape. This container was 98 cm high compared to 72 cm for the others. It had a small cross-section area in relation to the material depth.
One of the two commercial containers was the Hotter container (see Figure 5). This type of container was made out of thin metal with an insulation of Frigolite. The volume of the container is 560 litres. It is possible to divide the container into two parts by using the special metal wall that was enclosed. We did not use the wall during this test.
The second commercial container was a prototype from AB Skellefteå Plastcisterner in Kåge (Figure 6). It is made of polyester reinforced with fiber glass and has an insulation of polyurethane. It has a cylindrical shape. The volume is 200 litres.
The organic waste used
The kitchen waste came from the kitchen of the restaurant at the university. The largest fraction of the waste consisted of parts of vegetables (and to a certain extent fruits). The vegetables were mainly cabbage, carrots and potatoes. Bread was also included, as well as smaller amounts of meat.
Another type of waste came from a garden and consisted to a great extent of twigs from bushes which were cut into pieces. It also consisted of ordinary waste such as potato haulm. This material was kept in a cold room in plastic bags until it was used. On one occasion (11/10/92), dry garden waste was used. It was a mixture of peat, sand and old compost in the weight ratio 1:2.55:0.1. The ratio between the volumes of peat and sand was 3:1.
The material was mixed and the largest parts were cut into pieces. All the containers were supplied with an equal quantity of waste. Garden waste was put in the containers when there was a need of dry material. The quantities of added waste as well as remaining quantities at different times are presented in Tables 1, 2 and 3.
During the composting indoors in the initial phase of the project, 333 litres of waste were added to a container which did not participate in the test later. Of this volume, 283 litres consisted of kitchen waste and 50 litres of garden waste. After a couple of weeks, only 160 litres of the material were left. The reduction of the volume was 52 per cent. The material left was divided into four parts and added to the containers outdoors as inocula of microorganisms.
Ten days before the composts were started outdoors in the second season, a compost was started indoors in order to get an inoculum of microorganisms. This culture was divided, in equal parts (4 kg of wet weight), between the containers outdoors.
The equipment for temperature measurements
The temperatures in the containers were measured continually during the testing periods. Each tenth minute, the temperatures were registered and mean values for six measurements were stored in a personal computer. The equipment consisted of thermoelements, a PC-logger and a PC. The error in these measurements is less than +/- 0.5°C and can be regarded as negligible.
Sampling and determination of dry weight and ash content.
Samples (about 250 g) were taken at random from 4 places in the added waste material and from the containers. They were mixed and cut to pieces in a mixer. From each mixed sample 25-50 g were weighed and dried to constant weight at 105°C. The ash content was determined from 2-3 g samples of the dried material at 550°C for 3 h. Mean values of dry weight and ash content for samples of the material added to each container are presented in the figures.
Determination of the weight and volume of the waste material
Before the material was added to the containers, as well as at the end of the experiments, the volume and weight of the material were noted.
The mean value of five measurements of the distance from the edges of the container down to the material surface was used for estimating the volume of the remaining material in the container.
RESULTS AND DISCUSSION OF THE COMPOSTING DURING THE FIRST SEASON
Variations in the temperature as a result of addition to and turning of the material.
The start of the composting process.
As seen in Figure 7, the contents of container 1 reached a temperature of 30°C, approximately two days after the start. The waste materials in the other containers did not reach their maximum temperatures during this period (23°C for container 3 and 18°C for container 2). These levels were achieved after two more days. The temperature in the Hotter container was decreasing. Outdoors, the lowest temperature was 13°C.
After seven days: Addition of kitchen and garden waste.
After one week of composting, more material was added. It consisted of both kitchen and garden waste. The fresh material was laid above the old composted mass. In container 3, the temperature almost reached 40°C after some days. Such a high level was not achieved in container 1 and 2. The temperature in the Hotter container had decreased to 0°C.
After two weeks: Addition of kitchen waste with subsequent turning.
After addition of the kitchen waste, which this time mainly consisted of macaroni, the composted material was turned. After two days, the composted mass in container 3 reached its maximum temperature. The contents of container 2 reached their maximum temperature level, more than 50°C, after four days, while the temperature in container 1 rose to 40°C.
After 20 days: Addition of kitchen and garden waste with subsequent
turning.
The temperature in container 1 and 2 increased while the temperature in container 3 decreased.
After 26 days: How turning affected the temperature in the material.
After approximately one more week, the composted mass was turned in containers 1,2 and 3, which did not affect the temperatures.
After 28 days: Addition of kitchen waste.
After 32 days: Addition of peat.
After 28 days, more material was added to each one of the containers and the composted masses were turned, with the exception of those in the Hotter container. Since the material was very wet, 30 litres of peat were added to each of the containers. About 50 per cent of the peat had a dry weight of approximately 50 per cent. The other half of the peat had a dry weight of about 35 per cent. As can be seen in Figure 7, the temperatures rose in all the containers except in the Hotter container. The composted mass in this container was still frozen.
The temperature outdoors began to rise and fluctuated around 0°C.
After 49 days: Addition of kitchen waste with subsequent turnings. .
The last addition of waste was done after 49 days. In Figure 7, it is shown that the composted mass froze after about four days in the Hotter container. The temperature in the material was around -2°C for four weeks. When the temperature outdoors had been above zero for approximately five days, the material thawed. Then the composting process started and the temperature in the middle of the material reached a level of 45°C. After ten days, it had almost decreased to zero again. The temperatures in containers 1, 2 and 3 were hardly affected by the additions of new material and turnings of the compost masses in the containers.
As is shown in Figure 7, the temperatures in container 1, 2 and 3 did not drop below 0°C at any period during the composting season.
The temperatures in the containers fluctuated quite a lot, independent of the temperature outdoors. These fluctuations are probably due to the frequencies, quantities and kinds of additions. They may be avoided by more frequent additions of waste and regular turnings. A comparison between the three containers, with respect to the temperature in the middle of the composted mass, shows that the temperature In container 1 was higher than in the other two containers during the first composting phase. The temperature in the ventilated container 2 was lowest during this period.. Later on, when more material was added, the temperature in containers 1 and 2 became higher than in container 3. That shows the importance of ventilation.
The reduction of volume and weight
On the average, 13 litres of kitchen waste and 4 litres of garden waste were added to each one of the containers every week during the period of seven weeks when material was added. An addition of 30 litres of peat to each container was done on one occasion.
The reduction of the volumes of the material in the containers is shown in Figure 8. The reduction of the material was slow in the Hotter container owing to the freezing of the material. However, after 104 days of composting, the reductions of the volumes were approximately equal in all the four containers, about 70 per cent (Table 4).
At the emptying of the containers on 6/6/92, the remaining material in each one was weighed. The reduction, with respect to volume and with respect to weight, are shown in Tables 5 and 6.
The reduction of the material, with respect to weight, was equal for three of the containers, around 47 per cent. In the Hotter container, the reduction was only 32 per cent, which is a considerably smaller value. This was probably due to the very short composting period (from the 49th day) in this container.
The dry weight and ash content
As can be seen in Figure 9, the dry substance in the four containers did not fluctuate much. The dry substances were 20-27 per cent, which were quite low values. If the composted material contains too much water, it could lead to anaerobic conditions. That will cause a poorer degradation of the material. During the first three weeks, 80% of the material was added to the containers. The dry substance decreased during this period. When peat was added and when turnings were done, the temperature tended to rise.
The ash contents (Figure 10) of the samples showed a tendency to increase at the end of the compost period. This indicates that the composting process continued despite low temperatures in the containers. The ash content (in % of the dry weight) ought to increase during the composting process. Such a trend is shown in Figure 11. It is hard to draw any conclusions concerning which container functioned best from the results shown in Figure 10.
Subjective observations of the composts
During this composting period, notations were made concerning the moisture, the structure, the degree of degradation of and the smell from the composted masses. The material in all the containers was judged as very wet. The smell from container 1, 2 and 3 was not so strong, and there were not so many flies present in any of these containers. In the Hotter container, however, the wet and poor degraded material smelled. There were also fruit flies present in the Hotter container.
PRELIMINARY CLASSIFICATION OF THE CONTAINERS USED DURING THE FIRST SEASON
In containers 1, 2 and 3, the conditions for the microorganisms had obviously been good enough to allow a high activity in the composted mass, even if the temperature outdoors sometimes had decreased to -20°C. In the Hotter container, the material froze after about 5 days.
The problems which could be noticed were the following: generation of leachate and condensation water, and the lids becoming stuck in the bodies of the containers. The leachate and condensation water were a consequenze of the activity of the microorganisms. They cause a conversion of the organic material to, among other things, carbon dioxide and water. One way to decrease the generation of condensation water is to increase the ventilation. This can lead to: 1) an increase of the degradation of organic waste and an increase of the temperature if there is a lack of oxygen,. 2) a lowering of the temperature if it is cold outdoors and there is enough oxygen in the compost material.
In order to keep the temperature at a regular level, there are some steps which can be taken:
1) Material ought to be added to the containers at shorter intervals than during this test.
2) Turning the material should be done more regularly.
3) The quantity of added, relatively dry garden waste ought to be larger.
All the three containers of our own design functioned better in several respects than the commercial one, the Hotter container, which was used as a reference container.
One reason is that the volume of the Hotter container was too large. Another reason is that the walls of the container are made of a porous material, which allows air to pass. The problem with flies would be avoided if the lid were mounted tight to the body of the container.
Container 1 (High ventilated): The container functioned well. The amount of condensation was less than in the unventilated container 3.
Container 2 ( Low ventilated): The container functioned like container 1, but it took a longer period of time to raise the temperature in this container. The amount of condensation water was less than in container 1.
Container 3 (Low unventilated): The container functioned like container 1 with respect to the temperature level. The amounts of condensation water were largest in this one.
RESULTS AND DISCUSSION OF THE COMPOSTING DURING THE SECOND SEASON
With the results of the previous season in mind, the tests of the second season started with containers 4, 5 and 6, and a new container called Kåge. (See Figures 2, 4 and 6).
The intentions of the tests during this period were the following.
1) To extend the test period and add more material to the containers than the previous season.
2) To compare containers 4, 5 and 6 with the Kåge container concerning the temperatures (in the middle of the added material and at the inside walls), the size, the insulation and ventilation, as well as possible problems with condensation and leachate water.
3) To examine the effects of addition of more garden waste than the previous season.
Temperatures and ventilation in the containers.
The temperatures in the middle of the added waste material and outdoors from the 60th day (28/1/92) to the 110th day (18/3/92) of composting are shown in Figures 11, 12 and 13. A comparison between the containers shows that containers 6 and 4 had the highest temperatures, followed by the Kåge container and container 5. The same order was observed for temperatures at the inner walls of the containers (not shown here).
The temperatures in the middle of the material and at the inner walls of containers 4, 5 and 6 and the Kåge container are shown in Figures 11, 12 and 13. The fluctuations of the temperature curves can be explained by the opening of lids and addition and turning of the waste material.
The temperatures in container 6 were almost the same in the middle of the material as at the inner wall and at the vent in the wall. This vent was opened after 76 days. The temperature increased for a few days afterwards, but the temperature decreased faster and to a lower value than in container 4. In container 4, the temperature in the corner was low, despite the fact that the temperature in the middle of the material was the same as in container 6. This can be explained by the fact that the temperature was measured where there was a chink between the styrofoam disks.
During the first season of this investigation, it was found not surprisingly, that the temperatures differed depending on whether the containers were ventilated or not (Figure 7). The amount of condensed water inside the lids differed also between the containers. In order to get as equal temperatures as possible during the start period of season 2, all the vents were closed when the compost material was added the first time. During the period 11/12/91 - 27/12/91 both vents of container 4 were open. Then they were closed again. The vent in the lid of container 4 was opened on 11/12/91 and the side vent was opened on 13/2/92. One vent of the Kåge container was opened on 12/12/91 and closed on 27/12/91.
Different amounts of leachate water from the containers were noticed during the compost period. When the temperatures were high in the compost material and no vents were opened, the leachate and condensation water appeared (cf Compost diary, Table 4 and temperature curves, Figures 11, 12 and 13). The amount of condensed water was considerable on the lid of container 4, in which the temperature was relatively high during most of the composting period. In addition, the vents of that container were closed during most of the time.
The reduction of volume and weight
The changes of the volumes of the material in the four containers are shown in Figures 13 and 14. The reduction was approximately the same in all the containers at the end of period. However, during the compost period the fastest reductions of the volumes were achieved in containers 4 and 6 (Figure 14).
The volumes and weights of the remaining material in the containers were measured when they were emptied on 9/6/92 (Table 7 and 8).
As can be seen in Table 7, the reduction of the volumes was the same for containers 2, 3 and 4. The lower figure for the material in the Kåge container can be explained by a poorer decomposition of the material than in the other containers. The high values for the reduction of the volumes are due to the packing of the material in the containers.
The reduction of the weights is presented in Table 8. The Kåge container shows the lowest figure, 59%, while reductions in the others were about 70%. All these figures are high considering that the composting process took place outdoors during winter in a cold climate, and the temperature in the composts did not reach the optimum value of 55°C mentioned by Biddlestone and Gray (1985).
The dry weight and ash content
The dry weights and ash contents of samples from the containers, as well as of samples from the waste material added, were measured. The results are shown in Figures 16 and 17. Both the dry weights and ash contents varied during the period when material was added. The dry weights of the added material varied between 6 and 60 per cent of wet weight, depending on the origin of the added material. However, after addition of very wet material to the containers, the dry substance of samples from the containers was considerably higher compared to samples from added material. After 118 days the additions were finished. After that time the dry weights increased somewhat in all containers.
The differences between the dry weights of samples from the four containers were small. The dry weights varied between 26 and 35 % with the highest value for samples from container 4 at the end of the composting period. Biddlestone and Gray (1985) give 40-50% as an optimum value for the moisture content, which gives the a weight of 40-60%, which is much higher than the results presented here.
In Figure 17 the ash contents of samples from the containers and from added material are shown. The ash content, which is a measure of the degree of mineralization of the added material, varied in the same way as the dry substances. The ash contents were low in the beginning of the compost process in all the containers. After the start of the degradation process and the addition on day 43 of old garden compost material containing sand, the ash content increased considerably.
The ash content increased after day 133. This shows that the compost process continued despite the decreasing temperature in the containers. When the additions were finished the ash content had increased to about 35 % of dry weight in containers 4 and 6, and to about 25 % in container 5. In the Kåge container the corresponding figure was 17 %. Of the two parameters dry weight and ash content, the latter gives more information about the composting process, since it reflects the amount of decomposed material in the containers.
CLASSIFICATION OF THE CONTAINERS USED
Characterization of the composts
In order to characterize the composts and to estimate which container the best compost was produced in, the following methods were used:
1) Measurements of the temperature, dry substance, ash content and reduction of volume and weight of the waste material in the containers.
2) Using the senses of smell and sight, some subjective observations of the composts were made.
The most efficient degradation will be attained if there is a regular, and not so high temperature level in the containers. Too high a temperature will kill the mesofils among the microorganisms (Golueke, 1986, Fogarty & Tuovinen, 1991). Observations made show that the amount of leachate water increased considerably when the temperature in the composts reached 45-50°C (containers 4 and 6). However, this could be regulated by ventilation. The amount of condensation water was highest in container 5, which was not ventilated.
The ash content of the compost material, which is a measure of the degree of mineralization of the added material, showed that the degradation was best in containers 4 and 6. Also the dry weight values showed the same tendency.
The reduction of the weight of the material in the containers differed depending on the construction of the containers. The reduction was 32% in the Hotter container and 45-50% in containers 1, 2 and 3. The reduction at the end of the second season was 57% for the Kåge container, 67% for container 5, and 72-73% for containers 4 and 6.
Using the senses of smell and sight some subjective observations of the composts were made. Our observations of the smell, humidity and structure of the material, and the presence of fruit flies, moulds and fungi in the containers have also given information about the decomposition process of the material. The growth of moulds and other fungi is natural since the added garden waste contained spores of fungi. When the right conditions are found, the spores germinate. Judging from these criteria, containers 4 and 6 functioned best.
When the containers were emptied the composts were not ready for use. They needed to mature for some months.
Properties of the containers
The properties of the containers can roughly be divided into two categories:
1) Those which are vital for the composting process: size, geometrical shape, ventilation, and thickness and type of insulation material.
2) Those which are vital for the management of the container (ergonomical properties): the construction of the lid, the arrangement for filling and emptying the container, and the geometrical shape of the container.
Construction features which affect the composting process
Size of the containers
The containers used have a volume of 200 litres, except the Hotter container which has a volume of 560 litres and container 5 which has a volume of about 270 litres. About 15 litres of waste were composted, on average, each week (28/11/91-9/6/92). This is quite a big volume. A single household can be presumed to generate 5-10 litres of kitchen waste a week. During the autumn, the total volume of waste will of course be larger if garden waste is included. Despite the fact that 15 litres of waste a week is quite a large quantity, none of the containers were filled up, or even close to being filled up, at any moment during the two composting periods.
The most apparent consequence of the large volumes of some of the containers was the difficulty of keeping the temperature at a high enough level. This difficulty was most obvious for container 5 and for the Hotter container. Of course, the lack of a bottom in container 5 is also of importance. A large amount of relatively cold air was introduced into container 5 through the bottom. Even if the insulation is relevant, it is very hard to maintain a high temperature if there is too much air to be warmed up by the activities of the microorganisms in the container. For the purpose mentioned here, a container with a maximum volume of 200 litre is to be prefered.
Material in the walls and the ventilation
The oxygen supply to the containers is a function of the properties of the material in the walls and the ventilation of the containers.
Styrofoam was used as the insulation material in the containers of our own design. Frigolit has been used to insulate the Hotter container. The Kåge container is made of polyester reinforced with fiber glass and with an inner insulation layer consisting of polyurethane. The k-values for Frigolit and styrofoam do not differ so much. It is important for the ventilation that the wooden containers are not completely tight. There are chinks in the walls, between the styrofoam plates in the corners, where air can slip through. The walls of the Hotter container are porous and allow penetration of air. In the Kåge container, however, the walls are hermetic.
Frigolite absorbs water and there can therefore be a loss of heat through a frigolite wall. Styrofoam and reinforced polyester do not have these properties and can be considered as more convenient materials for insulation in a composting container.
Even if the containers are constructed with a large cross-section area and a relatively small material-depth, there is often a need for further oxygen supply. The comparisons between container 2 and 3 and between container 4 and 6 showed that, among other things, oxygen supply through the vents was of great importance for the temperature and assists the degradation in the containers. Containers 6 and 4 functioned better than the others. In the Kåge container, anaerobic conditions were achieved. One reason for this is probably the placing of the two vents in the lid and the absence of any vent at the side of the container. The small holes at the bottom did not seem to work especially well as oxygen suppliers.
The containers ought to be constructed with at least two vents, one at the lid and one at a wall close to the bottom. These vents should be regulated to achieve the best effect.
The shape of the container
The most natural form of compost is achieved when the relative area of the composted material exposed to air is large. This will facilitate the oxygen supply and assist the degradation of the material. A small relative area exposed to air could lead to insufficient oxygen supply and, if the worst comes to the worst, anaerobic conditions, as in the Kåge container.
Container 1 did not function well compared to container 2 and 3. The material in the high containers 1 and Kåge tended to be very compressed at the bottom. As a function of this, the oxygen supply was obstructed even more.
Containers 2, 3, 4 and 6 have better geometrical proportions than the other composting containers used in this test
Ergonomical parameters.
Besides the composting parameters, which mainly have an influence on the biological processes in the containers, the ergonomical properties of the containers are also important. When people pre-separating their waste at source and start composts, it is essential that the management of the containers is easily accomplished. If not, the composting period will not last long. During this test, some vital deficiencies of the constructions of the containers have been observed.
The lid
None of the containers have an arrangement to fasten the lid when it is opened. If it is windy outdoors, there will be an obvious risk for accidents. In a cold climate, it could also be an advantage if the lid were constructed with snow-loads in mind. Therefore, the lid ought to have a surface made of some glossy material and lean to some angle.
The shape and the weight of the containers and the arrangements for emptying them
The composted material in the high and narrow containers 1 and Kåge was not easy to access. A lot of effort was required to turn wet, heavy material through the top opening. To empty these containers it was necessary to turn them upside down. Down on the wall of the Kåge container, there is an opening covered with a lid, which should be used when emptying the container. However, the lid was difficult to open and the opening was too small. Nor were the other containers easy to empty. One solution to this problem ought to be a detachable bottom for the containers. A prerequisite for this is that the containers must not be too heavy and must be supplied with handles at the right height.
Of all the tested containers, the container which functioned best was constructed in the following way. It was rectangular with the inner dimensions of 90 * 55 cm and a height of 72 cm, and was made of wood and insulated with 7 cm of styrofoam. The bottom was covered with 25 cm of haydite balls and a geotextile. One vent was located in the lid, and another was located in the wall close to the bottom and covered by haydite balls for warming incoming cold air. The free volume was 200 litres.
REFERENCES
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Brink, N., Gärdedal, L., Hansson, Y. & Robertsson, M. 1992. Provning av kompostbehållare. REFORSK 65 .
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