S. Barrios, R. Fernández, F. Vázquez and X. Font
BioCycle February 2004, Vol. 45, No. 2, p. 64

Catalonia is an Autonomous Community in northeastern Spain with 6.3 million inhabitants and an area equivalent to that of The Netherlands. In 2001, 3.7 million tons of MSW were generated, 45 percent of which consisted of organics. In the past, the main destinations of this organic fraction have been landfilling and incineration.
However, since the mid-1990s, following European Union directives, the Catalonian government, Generalitat de Catalunya, has been promoting composting as the best process for treatment of the organic fraction of MSW. At the same time, the Generalitat has also been endorsing source separation of these residues. So far, 25 composting plants are operating in Catalonia and five are under construction. Following is a general overview of the present state of the operational plants.
METHODOLOGY
To fulfill the proposed objective, all composting plants were contacted and asked for permission to interview their technical supervisors. Supervisors were requested to answer a questionnaire that covered different issues concerning the running of the plant. Questions included general aspects such as ownership, nominal and operating capacity, type of residues processed, compost system used, etc., as well as characteristics of the process such as type of machinery used, treatment of emissions and characteristics of the final product. Afterwards, a small number of people living in the neighborhood of each plant were interviewed and asked for their opinion about the impact of the plant on its surroundings. Eighteen out of the 25 composting plants currently operating in Catalonia could be visited; the others were not for reasons such as being under refurbishment, maintenance or in the initial phases of start-up.
GENERAL INFORMATION
According to Catalonian legislation, towns of over 5,000 inhabitants are required to have selective collection of the organic fraction of the municipal solid wastes (OFMSW). This type of collection started in 1996. Figure 1 shows the number of publicly-owned composting plants that have been built. As seen in the figure, before 1997 there was only one composting plant operating in Catalonia, construction of new plants thrived that year mainly as a result of the decision of the Catalonian; government to promote this treatment. Increase in the number of composting plants has been accompanied by an increase in the selective collection of OFMSW.
Ten of the visited plants are publicly-owned, whereas eight are privately-owned. Nevertheless, all visited plants are under private management. Publicly-owned plants treated 105,230 tons of the OFMSW in 2002. Although selective collection of OFMSW has steadily been increasing every year, it is still far from the 1,248,205 tons that could be potentially recovered.
According to data provided by the different visited plants, 63 percent of the publicly-owned and 20 percent of the privately-owned plants are working below 50 percent of their nominal capacity. Publicly-owned plants work at an average of 54 percent of their design capacity whereas privately-owned work at an average of 89 percent. The low value for publicly-owned plants is mainly due to problems with feedstock. Publicly-owned composting plants in Catalonia have been designed under the assumption that their feedstock would be the organic fraction of MSW previously separated at source. However, the selective separation and collection of the organic fraction in Catalonia is still very limited and therefore, sometimes, there is not enough raw material for the plants to operate. In 2002, the total design capacity of public plants was 212,870 t/year, while 105,230 tons of OFMSW were selectively collected that year. These data confirm the information provided by the plants, indicating that the average operating capacity was around 50 percent. It must be also considered that some of the plants have just started their operation and therefore their operating capacity is still low.
In contrast, two of the 18 visited plants, one public and one private, are working over their design capacity. An expansion in the processing area is under consideration in both cases.
The organic fraction of municipal solid wastes is the major feedstock in the composting plants visited (61 percent) followed by agro-industrial residues (28 percent). There are also plants that process both residues (11 percent). About 67 percent of the plants use open windrows for the whole process, whereas 33 percent use in-vessel composting with forced aeration for the first stage of the process and generally, open windrows for the second stage – the curing phase. Open windrows are chosen mainly because of their lower cost. In-vessel composting is preferred because it occupies less space and problems such as odor can be better controlled. Nevertheless, this system is more expensive since it involves the construction of the container and the expense of energy to maintain aeration. The in-vessel process is mostly used in public plants.
PROCESS
Feedstock Conditioning
Feedstock entering a composting plant should mainly contain compostable organic matter (OFMSW, agro-industrial residues, sludge from wastewater treatment plants, etc.). However, it frequently still contains an important amount of noncompostable materials, around 14 percent for OFMSW. Therefore, screening is needed to separate as much noncompostable material as possible. This operation is carried out in all the plants. Most of them (81 percent) use a trommel for screening, while the rest combines trommels and sieves. Three of the visited plants also have a hand sorting system. However it is hardly used, since the percentage of noncompostable materials and water content of the MSW are too high to make the system feasible.
Once the noncompostable material has been separated, feedstock is conditioned by mixing with bulking agents. The most commonly used are residues from municipal pruning of public trees and parks (71 percent), shredded pine bark is also used (11 percent) as well as residues from wooden chests (6 percent) and, mixtures of pruning and pine bark (6 percent). There is also one plant where selection of the feedstock is feasible, so that a high quality compost is obtained with no need for conditioning.
Process Time
Length of the composting process varies from plant to plant. Decomposition phase can last from 14 to 186 days depending on the plant, the average time being 63 days. Curing phase lasts longer and it varies from 42 to 280 days depending also on the plant, the average being 119 days. Two plants have a very long curing phase, because they use very big windrows where aeration seems to be defective, i.e. slowing the process down.
Process Control
Three out of the 18 visited plants monitor and control parameters such as O2 concentration, temperature, moisture, and pH. The rest rely only on temperature and moisture control. Nevertheless good quality compost is obtained in all cases.
Moisture Content
Leachates are used to adjust the moisture content of the windrows in most of the plants (89 percent). This procedure may contaminate the material since leachates could contain pathogens. However, this adverse effect does not seem to occur according to the supervisors of the plants. Only a few plants add water for the adjustment.
Treatment Of Emissions
Most plants (59 percent) do not have any system for treatment of emissions produced during composting, the most important being odorous substances. In these cases, it is argued that they are not needed since the processing plants are located far enough away to avoid odor problems to the population. Biofilters are the most common process used in the plants that include an emission treatment system (43 percent), while 14 percent have scrubbers and 43 percent a combination of both systems. Nevertheless, only 42 percent of the plants that have any system for the treatment of emissions actually use it.
FINAL PRODUCT
Quality Control
In all plants, operators use the moisture content as a preliminary indicator of composting activity since it is considered that a low water content suggests that the material is no longer active. In this case, moisture is roughly estimated using the squeeze test; 46 percent of the plants use the Dewar self-heating test to corroborate this estimate, while seven percent of the plants use the stability degree, 20 percent use only the squeeze test to estimate the maturity of the final product. Periodically, a complete characterization of final products, including dry weight, heavy metals content, pH, nitrogenous content, electric conductivity, etc. is routinely carried out by external laboratories.
Screening
A final screening is carried out to remove residual unwanted materials such as stones, glass and pits; 57 percent of the plants use a densimetric table to obtain the final product. Also, 21 percent of the plants use an air classifier to separate residual plastics, the most difficult contaminant to separate.
Final Product Storage
In 73 percent of the plants the final product is stored in closed warehouses before packing, three plants pack the product in waterproof plastic bags and then store it in open air storehouses. There is one plant that stacks the product in an open air warehouse before packing, with the consequent problems due to adverse weather conditions such as rain or wind.
Final Product
Particles of final product have a size between 10 and 20 mm. In some cases, it can be smaller than 10 mm because of commercial reasons; 47 percent of the finished compost is used in gardening, 24 percent goes to agricultural uses while the rest is used either in agriculture or gardening.
Public Perception
Public perception about the composting plants was obtained from a small sample of the population living in the surroundings of the plants (230 out of 434,796 inhabitants). The only purpose was to have an indication of the attitude of people about the plants – 73 percent of the interviewees were in favor of having the treatment plant close to their towns and only 9 percent were openly against them. The reason for this big acceptance may be due to a noticeable increase in the awareness of environmental problems. However a more important reason seemed to be the fact that having the plant close by means more job places for the local people.
However, only 59 percent of the people were aware of having the composting plant in the surroundings of their towns. This is probably because of the little environmental and visual impact that the plants seem to generate, since these are highly decisive factors when choosing the location of the plants. Regarding knowledge of the composting process, only 38 percent of the interviewees did have an idea of either what composting was or about the final use of the product. The rest of the interviewees did not know anything about the process.
CONCLUSIONS
It can be observed that older plants use windrows while more recent plants use an in-vessel process for the initial phase of the process. Plants that use in-vessel process are, in general, publicly-owned and they mostly process OFMSW. Plants that use windrows tend to process a wider range of residues. In-vessel processing is shorter and requires less labor. Operating capacity of windrow systems is closer to their nominal capacity, whereas in-vessel capacity is around 50 percent of their nominal one. Finally, the public does not oppose having the composting plants near their towns.
The authors are at Escola Universitària Politècnica del Medi Ambient (Universitat Autònoma de Barcelona) in Mollet del Vallès, Spain. They thank the facilities for the kindness and courtesy of the many people who allowed them to visit the plants and answered all questions.