BioCycle April 2005, Vol. 46, No. 4, p. 72
Since 1997, approximately 70,000 cubic meters of woody residuals, leaves and grass clippings have been processed annually at Migas Calientes.
Marta Benito, Alberto Masaguer, Roberto De Antonio and Ana Moliner

THE CITY of Madrid has more than 4,000 hectares of green areas and approximately 250,000 trees in the streets. Green waste (woody materials from pruning, leaves and grass were mostly discarded until 1997, when the composting facility of Migas Calientes was inaugurated. Residues had been placed in garbage dumps, causing settling, fires and unwanted fermentations. Just a small fraction was composted to produce ground leaf mulch that was used in municipal nurseries. The poor mechanization conditions, weed seeds, and pathogens created an inadequate product for later use. Now, the compost manufactured in this facility is being used again in gardens and municipal nurseries of Madrid. In addition to allowing a volume and weight reduction of treated materials, it constitutes a solution to the organic matter demand, nutritional imbalances and fertilizers and growing media costs of the nurseries and gardening department managed by Madrid’s City Hall. Residue valuation and its reutilization in municipal green areas is, no doubt, the best destination for garden trimmings generated in Madrid.
Composting plant residues from garden and tree trimmings without any other kind of residue yields a high quality organic amendment. However, the variability of plant residues produced, not only seasonally but within the same annual period due to plant diversity, makes necessary a chemical, physical-chemical and biological characterization of the compost to determine limitations to its use.
Characteristics of the final product depend basically on two factors: initial material and the composting process that it has undergone. Although there are some seasonal variations in the quantities and characteristics of green waste manufactured in this facility, approximately 60 percent to 70 percent of the waste volume is woody material; the rest is mainly grass clippings and leaves.
OPERATIONS AT THE COMPOSTING PLANT
Total surface of the plant is approximately 15,000 m2, where 70,000 m3 of green residues are processed annually into 9,000 m3 of compost. The plant is divided into six areas (See Figure 1): material reception, product preparation, active phase of the process, maturation phase, sieving, and storage.
The first stage of the process consists of weighing and stock piling incoming green residues on a large concrete platform of 1,100 m2. Starting mixture includes fresh and woody materials combined to adjust the C/N ratio.
Once the mixture of materials is determined, it is moved from the storage area to a contiguous space where the shredder is located. This machine (370 hp) has an output of 80 m3/h shredded material and includes two rotating tubes provided with metallic hammers.
Material is fed to the shredder balancing the amounts of woody and more biodegradable material such as leaves and grass clippings; chemical and physical optimization of the mixture is sought taking into account that the nature of the materials being treated is mainly woody.
Shredded material is placed in the area intended for the initial process, the thermophilic phase. During this stage, generally characterized by a higher microbial activity, the more readily biodegradable material is transformed and simultaneously pathogens and weed seeds are eliminated due to the high temperatures reached (>65°C).
This area resides in a 25 x 35 m rectangle with 10 underground perforated channels crossing through to provide air generated by five fan units located at the head. On top of two of these parallel channels, a 2.5 m high trapezoidal pile is formed.
Forced aeration is automatically controlled by means of two probes positioned in each of five initially formed composting piles. At the first one, temperature measurements are taken every 20 centimeters in depth, and the second one measures oxygen content and temperature at one metre depth. Measurements taken by both probes are registered by a computer which controls the corresponding fan. Fans turn on when temperature either exceeds 80°C or oxygen saturation descends below 82 percent (with respect to the atmospheric value, approximately 21 percent) and stops when it reaches 85 percent.
Forced aeration was used for the first eight weeks (thermophilic phase), followed by a ten-month maturation period during which the piles were turned periodically to maintain adequate oxygen levels.
The maturation area occupies 3,800 m2 of concrete floor. Piles are formed again in this area making a continuous plateau-like shape. Temperature is controlled by a manual probe. A mechanical turner is used to remix, aerate and crumble again the material, to create new degradable surfaces, as well as move the piles to leave space for newly formed piles.
The turning machine, with a 400-500 m3/h yield, is made essentially of a roller provided with “quills.” In each turn, it slices off a surface layer of the pile, introduces the material into a mill hopper where a helicoidal roller receives the material and throws it to the opposite side of the original pile. The composting maturation phase is considered complete when – following a turning – the pile temperature did not increase.
Once the maturation phase is complete (and with it, the composting process), the material moves to the next area where it is sieved. A trommel with interchangeable panels is used for the process. According to final destination of the compost, material is sieved through different calibers (10, 15, 20 mm).
As noted above, there are seasonal variations in the amounts and characteristics of plant residues generated in the city of Madrid that are processed at the Migas Calientes composting plant. Because of this, a physical, chemical and biological characterization of the final product is needed to predict use limitations. The objective of this paper is to present average characteristics of the compost from the Migas Calientes Plant. For this purpose, the final product was sampled periodically during 18 months, resulting in 12 samples.
ANALYTICAL METHODOLOGY AND COMPOST CHARACTERIZATION
The green waste compost (GWC) was analyzed for total organic matter (TOM) by the dry combustion method at 540°C for 4 h and total N (TN) by Kjeldahl digestion. Electrical conductivity (EC) and pH were analyzed in a 1:5 (v/v) water extract. After (1:5 v/v) water extraction, NO3-N and Cl- contents were evaluated with ion selective electrodes; K+, Na+ Ca2+ and Mg2+ by atomic absorption and P was determined The cation exchange capacity (CEC) was determined with 1M ammonium acetate at pH 7. Samples of air-dry media were passed through a series of sieves, from 8 mm to 0.1 mm, to determine particle size distribution.
Tables 1 and 2 show main chemical properties of GWC samples. Electrical conductivity values EC are low and pH values are high. Electrical conductivity presents an average value of 42 dS m-1. Average pH value in 1:5 (v/v) extract was 8.6. This value could be limiting if compost were to be used as growth media. Water soluble nutrients were low in general. These results indicate that utilization of this compost as substrate requires the addition of fertilizers in order to achieve an adequate plant growth. Total organic carbon (47.6 percent) and nitrogen (1.5 percent) contents were within levels considered to be adequate.
Average particle size distribution (Table 3) shows that the predominant fraction (26 percent of the total weight) has a particle size range between two and four mm. It is followed by th particle range between four and eight mm which represents 23 percent of the total weigh. This values imply an excessive aeration and low water retention capacity (Table 4). These results are in agreement with a high porosity (average values is 90 percent), excessive aeration capacity (44 percent), low values for easily available water (9.5 percent) and water reserve (4.2 percent)
With respect to seasonal variability, it can be concluded that, although samples were taken from piles established at different seasons, no significant differences were found in their chemical properties. However, water retention characteristics were affected by seasonal changes in components entering the facility. Compost piles established in the summer season had lower air space and higher available water.
After studying GWC properties, this material appears to be an acceptable component in soilless growing media. However, this material should be mixed with nutrient richer components to compensate for the low nutrient levels. The alkaline pH values should also be taken into account if the compost is used for plants sensitive to high pH. Composting of green waste provides not only a high quality and inexpensive substrate but it means an alternative for recycling residues and a solution for waste management in the city of Madrid.
Marta Benito is at Ciudad University in Madrid; Alberto Masaguer, Roberto De Antonio and Ana Moliner are at Universidad Politécnica de Madrid.