«Dimambro ME, Lillywhite RD & Rahn CR Warwick HRI, University of Warwick, Wellesbourne, Warwick, CV35 9EF Corresponding author: ...»
Organic matter The organic matter (loss on ignition) of the composts tested varied from 20% (compost E) to 69% (compost L; Table 3.3). Since intensive agriculture can reduce soil organic matter (SOM), so the addition of composts can enhance SOM. The low cellulose and lignin content in composts E and F is reflected by the low organic matter content. In the other composts, more woody green waste present could result in higher cellulose and lignin concentrations and thus a higher organic matter content.
The high cellulose concentrations in the MSW compost (compost G) could be due to the high paper and cardboard content. This is reflected in the high organic matter content of this compost.
Results so far have indicated that compost L, which was composted for two weeks in-vessel, is not mature. Thus the high organic matter content and high cellulose content of compost L could be due to incomplete degradation, as mixed MSW can take up to six months to reach maturity (Francou et al. 2005).
Ash Ash in compost is due to the presence of minerals, soil and sand. A study comparing six composts found that the ash content varied considerably 31-67% (Garcia et al. 1992). These results compare well with results from this study, where ash contents ranged from 31% (compost L) to 80% (compost E). Composts E and F had a high ash content of nearly 80%.
These two composts had a high bulk density and low moisture content.
3.1.6 Carbon and nitrogen content and the C:N ratio
The carbon content ranged between 12% (compost F) and 37% (compost L). In general terms, composts containing source materials low in cellulose and lignin should have low carbon contents with the opposite also being true, so composts containing wood waste and cardboard as feedstocks should have high carbon contents. Our results do not always support this view.
The three composts (E, F, I) with the lowest carbon contents ( 20%) contained cardboard. In contrast, compost B had a high carbon content ( 30%) possibly due to over 12% wood, present in the 15 mm particle size fraction.
The nitrogen content ranged from 1.0% (compost F) to 2.2% (compost K). The nitrogen content of composts is dependent on the quantity of protein in the original feedstock, and the organic nitrogen, nitrate and ammonium in plant materials. High protein and therefore high nitrogen materials, e.g. kitchen and pet food waste, should lead to higher nitrogen content while low nitrogen materials, e.g. straw, cardboard and wood, should result in composts with lower nitrogen content. This view is supported by the results.
The C:N ratio of the composts ranged between 12:1 (composts E & F) and 32:1 (compost G).
The C:N ratio can be used as an indicator of the speed of compost decomposition in the soil.
Composts with low ratios will break down relatively quickly whilst those with high ratios will take longer. If composts are to have a fertilising value then the nitrogen content and C:N ratio are important.
There will be a C:N ratio above which the composts will use soil nitrogen to aid compost decomposition and will therefore immobilise soil nitrogen. At low C:N ratios of 10 or less, nitrogen will be released from composts. C:N ratios tend to be higher in “brown” BMW such as paper, card, bark and wood, and lower in “green” BMW such as green waste, vegetable waste and grass cuttings. The mixed waste composts had a higher proportion of “browns” (paper and cardboard) than the source segregated composts. With the exception of compost G, all C:N ratios are below the 20:1 level recommended by PAS 100 and Apex.
3.1.7 Nutrients Essential macronutrients Nitrate (NO3 -N) concentrations ranged between 0.4 mg kg-1 (compost G) and 518 mg kg-1 (compost F). Composts E, F and H had levels greater than 299 mg kg-1 which we attribute to the original feedstocks containing vegetable wastes, which themselves contain high levels of nitrate. There is one exception, compost K, where the inclusion of pet food waste may have resulted in the high nitrate concentration.
Ammonium (NH4 –N) concentrations ranged from 15 mg kg-1 (compost E) to 1510 mg kg-1 (compost K). However, this wide range is misleading since ten out of the twelve composts showed levels below 200 mg kg-1 with just two composts (K and L) having concentrations greater than 1300 mg kg-1. As was the case for nitrate we think the inclusion of pet food waste accounts for the high level in compost K while compost L was immature.
After nitrogen, phosphorus (P) is the second most limiting element in soils, followed by potassium (Salisbury & Ross 1992). P concentrations ranged from 23 mg kg-1 (compost F) to 247 mg kg-1 (compost J). In general, concentrations of potassium (K) in the composts were higher than for all other nutrients, and varied three-fold from 1852 mg kg-1 (compost H) to 6615 mg kg-1 (compost J).
Secondary macronutrients Calcium (Ca) and Magnesium (Mg) concentrations were considerably higher in the two mixed waste composts (G and L) compared to the 10 source segregated composts. Ca ranged between 219 mg kg-1 (compost I) and 10082 mg kg-1 (compost L), with all source segregated composts below 535 mg kg-1. Mg content varied from 54 mg kg-1 (compost B) to 890 mg kg-1 (compost L). Again, MSW composts were at the higher end of the range (above 330 mg kg-1), with source segregated composts all containing Mg contents below 130 mg kg-1.
Sulphur (S) concentration ranged from 175 mg kg-1 (compost B) to 3327 mg kg-1 (compost G). Sodium (Na) concentration ranged between 239 mg kg-1 (compost D) and 3176 mg kg-1 (compost G). In all secondary nutrients, compost G had either the highest or second highest concentration of the 12 composts.
Micronutrients Iron (Fe) content ranged from 32 (compost H) to 366 mg kg-1 (compost E). Manganese (Mn) and copper (Cu) concentrations were highest in the two mixed waste composts (G and L).
Zinc (Zn) concentration varied from 0.64 (compost H) to 39.19 mg kg-1 (compost L).
Compost G had the second highest Zn concentration of 8.5 mg kg-1. The 10 source segregated composts had Zn levels of 5.3 mg kg-1. Concentration of boron (B) ranged between 0.59 mg kg-1 (compost H) and 6.14 mg kg-1 (compost A). The concentration of chloride (Cl) ranged four-fold from 1.0 g kg-1 (compost H) to 4.7 g kg-1 (compost K).
In summary, all composts contained all of the plant nutrients. The 100% source segregated composts contained higher levels of NO3 and Fe than the composts containing mixed MSW.
The two MSW composts contained levels of some plant nutrients which were considerably higher than the source segregated composts, including Ca, Mg, Mn, S and Na, in addition to the PTEs Cu and Zn. These elements are required by plants in small amounts, although high concentrations can be toxic.
Total nutrients The composts containing meat either from kitchen waste (A, G, J, L) or pet food waste (K) contained over 9 g kg-1 nutrients (Table 3.9). These composts had the highest EC levels (Table 3.1). Of these, the two mixed waste composts (G and L) had in excess of 15 g kg-1 nutrients.
The addition of composts with high soluble salts can be harmful to seed germination and plant growth, by causing water stress and ion toxicities (Recycled Organics Unit 2003).
In contrast, compost H contained less than a third of this (4.5 g kg-1 nutrients), and had low EC. Low levels of nutrients in composts may indicate low amount of nutrients in the original feedstock. This could be due to the potato waste in the feedstock in compost H, which is relatively nutrient poor.
Large variations in nutrients have been observed in a number of studies. A study of over 200 composts in USA revealed between 5-fold and 100-fold differences in nutrient levels (Mamo et al. 2002), such as NO3-N ranging from 2 to 1419 mg kg-1 and NH4-N ranging from 1 to 3220 mg kg-1.
Essential plant nutrients were present in all the composts. However, high nutrient levels do not necessarily infer that the compost will be suitable for application to agriculture. Indeed, it is not just the total nutrient level that must be considered, but the concentration of individual nutrients. For example, high levels of nitrate will be beneficial to the crop, whereas high levels of sodium or boron may reduce germination and growth.
Timing of compost application is important, as is the case with inorganic fertilisers and organic manures. Incorporating the compost into the soil (Mamo et al. 2002) and delaying of planting (O’Brien & Barker 1996) will allow ammonium and soluble salts to dissipate, thus reducing effects of salts on germination. However, to minimise leaching of valuable nutrients, application should be restricted just before sowing. WRAP (2004) advises a two week time gap between incorporating the compost into the soil and sowing.
3.1.8 Potentially toxic elements
PTEs (potentially toxic elements) in MSW can be due to a number of components including batteries, solder, wine bottle caps, old circuit boards and fishing weights. In addition, pigments and stabilisers in plastics may contain PTEs (Richard & Woodbury 1992). As source segregated composts should not contain these waste materials, PTEs should be considerably lower than in mixed MSW composts.
This is borne out by the results, which show that the concentrations of all PTEs in the 10 source segregated composts, plus compost L were lower than the upper limits specified by PAS 100. The PTE concentrations of 100% MSW compost G were generally much higher than for the other 11 composts, with Cu, Ni, Pb and Zn concentrations exceeding the PAS 100 upper limits (Tables 3.10 and 3.11).
PTE concentrations have been shown to be higher in mixed MSW composts compared to source segregated BMW composts (Richard & Woodbury 1992, Ciavatta et al. 1993). In a Europe-wide study, PTE concentrations were 2 to 10 times higher in mixed MSW compost as compared to compost from source segregated household waste (Amlinger et al. 2004). This compares well with the results from this study, where total PTEs were 1.5 g kg-1 in mixed MSW compost G, compared to 0.5 g kg-1 or less for the 10 source segregated composts (Table 3.9).
Interestingly, compost L, which contained 72% MSW also had a low total PTE concentration of 0.4 g kg-1. The high level of physical contaminants (22%) including metal, plastic and glass, could imply that PTEs would be higher in compost L. However, as previously discussed (section 3.1.5), compost L was not mature. As composts undergo degradation and maturation WSCs and cellulose are broken down. During this process PTEs become more concentrated as organic matter decreases. In addition, organic acids which are produced during composting cause leaching of metals from waste (Petruzzelli 1996). We suggest that if compost L was matured, PTE concentration would increase.
The behaviour of PTEs in the soil is influenced by a number of interactive biotic and abiotic processes, which determine chemical speciation and bioavailability. Soil pH, cationexchange-capacity and redox potential drive the biogeochemical processes in soils. Thus it is not merely the concentration of the PTEs in the compost which must be considered, but also any changes the compost may exert on the soil properties. High pH and high organic matter content of MSW composts minimise the availability of PTEs to plants. Indeed, a study in USA found the levels of PTEs in compost derived from MSW to be much lower than the Environmental Protection Agency maximum levels and safe for crop production (Shelton 1991).
Microbial contamination of MSW is mainly of faecal origin (Deportes et al. 1998). Sources of micro-organisms in MSW include nappies, pet litter and food. As many microbes are heat sensitive, they are normally killed during the first stage of composting, where temperatures in excess of 60oC are achieved.
In this study none of the composts contained Salmonella. However, coliforms and E. coli were present in all 12 composts (Table 3.13), with levels of E. coli below the PAS 100 limit of ≤1000 in seven composts (B, C, E, F, G, K and L). Compost H contained the highest levels of coliforms and E. coli. This is probably due to the feedstock containing 3% manure.
3.2 Physical and chemical characterisation of batch 2 composts Compost G was light brown and had a slight smell of silage. The other four composts were dark brown/black in colour, having a slight woody smell.
3.2.1 pH & conductivity Compost pH ranged between 7.67 (compost F) and 8.68 (compost A). Electrical conductivity ranged between 0.85 (compost B) and 2.25 mS cm-1 (compost F; Table 3.20).
3.2.2 Bulk density and moisture content Compost G had the lowest bulk density and the lowest moisture content. Compost F had the highest bulk density. Compost B had the highest moisture content and available water content (Table 3.21).
3.2.3 Particle size distribution No compost had particles greater than 30mm in size (Table 3.23). Compost A had the most small particles, with 38% less than 1mm in diameter.
3.2.4 Physical contaminants Compost G, the only compost from mixed MSW, had a greater proportion of physical contaminants than the four source segregated composts. PAS 100 standards state that total glass, metal and plastic should not exceed 0.5%, as is the case with composts A, F and J. The PAS 100 upper limit for stones is less than 8%, as it is for all five composts.
3.2.5 Water soluble carbohydrates, cellulose, lignin, organic matter and ash Compost G had the highest concentration of WSC and cellulose, with compost F having the lowest. Lignin content was highest in composts B and J, and lowest in compost F (Table 3.25). Organic matter was lowest, and ash highest in compost F