In recent years, there has been a huge interest from local communities in decentralized composting. Decentralized community composting refers to a community-scale network in a specific neighborhood that diverts and composts biowaste in a controlled operative environment. In fact, the lack of centralized composting facilities in small towns or rural areas can be supported by decentralized solutions. Decentralizing waste treatment facilities and thus creating local solutions to urban waste management strategies will help to achieve the resource recovery and valorization targets in line with the circular economy.
Considering the limitations of centralized waste treatment facilities originating from diverting food waste and increased costs for collecting and transporting waste in long distances, some of municipal composting programs may not be fully successful. In addition, high operational costs and operational complexity are other factors that should be taken into consideration for centralized systems [1][2][3]. At this point, alternative strategies must be identified and developed, such as decentralized collection and treatment. Decentralized composting, also known as community composting, refers to a community-scale network in a specific neighborhood that diverts and composts biowaste in a controlled operative environment [4]. The main advantages of decentralized composting over centralized systems are summarized in Table 1. In a broad perspective, decentralized composting can help to decrease the cost and effort for transportation of waste for processing and treatment, and further reduce the need to construct new disposal facilities, enable local reuse of organic matter, create local small-scale enterprises as well as reduce costs associated with commercial fertilizer purchase [4][5][6]. Furthermore, the final compost product is comparatively of higher quality due to efficient separation and less intercontamination of wastes [7][8]. Community composting is thus attracting some attention from policymakers, who consider this as a logical implementation [9]. However, some drawbacks are also faced during decentralized composting. The collection of organic waste in containers may result in an uncontrolled degradation of organic matter that leads to odor problems and leachate generation in the case of poor management [10]. Furthermore, logistic problems can lead to unsatisfactory implementations [4]. In this regard, new composting technologies should be well-addressed, and the information gathered from the operative environments should be thoroughly analyzed for a win-win situation for all stakeholders.
Main advantages of decentralized composting over centralized composting [8][11].
Centralized | Decentralized |
---|---|
Transportation costs relatively high | Transportation costs relatively low |
High operation and maintenance costs | Comparatively less maintenance costs |
A high degree of specialized skills to operate and maintain | Low level skills required |
Advanced technology with highly mechanized equipment | Simple technology with labor intense |
Large facilities | Small facilities |
Low quality of compost due to poor separation of wastes with high risk of contamination | High quality of compost since waste is efficiently separated and risks for contamination are minimized |
Final product transported to farms or regional markets | Final product to fields or local markets as soil conditioner |
When a decentralized composting system at the community-scale is demonstrated in a specific city or urban area, current and future proposed land use availability, and status of vacant land and community interest are initially considered within the regulatory frameworks. Once the location type and the individual site within each area are selected, the composting capacity is latter calculated within the city or specific region, based on the population size and waste generation trend [4]. The next step is then the decision on the composting technology. Community composting reactors can be different, in other words, “simpler”, than centralized composters. Plastic bins in any shapes (i.e., rectangular, cylindrical, conical) are often used for community composting reactors [8][12]. Plastic drum reactors were also recently reported [13]. These reactors can be operated in batch, semi-continuous or continuous mode, based on the sustainability of the wastes. The reactor capacity is usually between 100–1000 L [12][13]. In most cases, holes are constructed at the bottom or on the periphery for aeration and turning/mixing is applied manually. Some examples of decentralized composting practices in Europe are presented in Table 2. The biggest drawbacks of these bin-type reactors is the uncontrolled emission of GHGs, such as methane, ammonia or nitrous oxide [14][15], non-homogenous matrix of the final compost product due to inadequate mixing [16]; odor and leachate [10]. For instance, gas emissions (i.e., (CH
4, N
2O, NH
3and volatile organic compounds (VOCs)) of a bin-type composter were calculated in the range of 30–148 kg CO
2 eq/Mg leftovers of raw fruits and vegetables (Colón, et al., 2010).eq/Mg leftovers of raw fruits and vegetables [17].
Characteristics of selected decentralized composting systems in Europe.
Site | Population | Demographic Characteristics | Waste Origins | Bulking Agent | Reactor Type/Model | Waste/Reactor Volume |
Leachate/Gas Collection | Aeration | Mixing | Operation Mode | Composting Duration | Reference |
---|---|---|---|---|---|---|---|---|---|---|---|---|
Allariz (Spain) | 5982 inhabitants, density of 70 inhabitants/km2 | Residential area with shops and an industrial estate | Yard waste and kitchen waste | Shredded wood | Modular composter made of recycled plastic slat | 1000 L | - | - | - | Continuous | - | (Comesaña, et al., 2017)[12] |
Ballymun (Dublin-Ireland) | 89 apartments | Apartment complex | Household kitchen waste | Wood pellet | In vessel technology (Big Hanna T-120) | 26 t/y | Biofilter to treat exhaust gases from the Big Hanna | - | - | Continuous | - | (Miller et al., 2013)[18] |
Dublin (Ireland) | - | Residential area | Organic waste, primarily catering waste | - | In vessel technology (Big Hanna T-120) | 2 m3 | Biofilter | Rotating cylinder | Rotating cylinder | Continuous | - | (O’Sullivan and Curran, 2011)[19] |
Lithuania | - | Catering company | Catering, biodegradable waste | - | Batch reactor (Oklin GG 10s composting machine) | 10 t/y | Activated carbon filter | Forced ventilation system | - | Batch | 24 h inside the machine plus maturation time outside | (Kliopova et al., 2019)[20] |
Barcelona (Spain) | - | Universitat Autònoma de Barcelona | Leftovers of raw fruit and vegetable and pruning wastes | - | Bin-type composter Model 400 RRR Compostadores SL | 0.5 m3 | - | Holes on the periphery | Mixing tool: Compostadores SL. Shredding tool: electric garden chipper (BOSCH AXT 2500 HP) |
Continuous | 12 weeks | (Colón et al., 2010)[21] |
Considering the negative impacts of conventional composting reactors on the environment, new generation composting reactors are therefore highly promising. The greatest advantages of these reactors are: pre-treatment units, biofilters, automatic mixing, leachate collection reservoirs and aeration modules. For instance, a community-scale novel drum bioreactor was reported to be a promising system for efficient pre-treatment of organic household wastes [10]. Noteworthy to mention is that the composting process can last between nine and eleven weeks if mechanical mixing is applied, and up to fifty weeks under static conditions. Currently promoted electromechanical reactors can be generally divided in two main categories: systems with only one chamber and systems with a double chamber. In the first case, the composter constitutes a rotating cylinder without any mechanical tool inside, the waste is introduced, and the rotation allows mixing, aeration and advancement of the material up to the exit point [19]. The two chamber machines work in a different way, and these systems are usually equipped with a shredder and a mechanical mixing tool in both of the chambers; waste is introduced in the first chamber where it is continuously mixed, and when the chamber is full, the second starts to be filled, while the first is closed in order to complete the composting process.
In most agri-environmental programs, the lack of participation of interested stakeholders in designing frameworks, the poor information basis to support policy formulation and the failure to consider local specificities in the scheme design are reported to be the main reasons for low success achievements [22]. In a recent survey [23], the farmers’ perception of compost production was found to be 83.9%, in which the participants showed also a high, yet lower, willingness level (63.6%) of the more salient option to produce compost themselves and use it in agriculture. In another survey, 67% of respondents indicated that they are interested or very interested in community composting systems [24]. Without a doubt, public acceptance and encouragement are the key factors for a successful decentralized composting implementation. As the actual processing volume is dependent on the participation of residents in a community, low participation rates can be a major challenge in such cases [4]. By community composting, local resources community participation can be established [25] and people may be more motivated to reduce their food waste when they see it separated out from the rest of their waste [24]. In a common sense, decentralized composting systems should be inexpensive, require low maintenance and easy handling [13]. Identifying a suitable location in a city/region is critical and logistical characteristics such as the distance from waste sources, need/use of compost, demographic characteristics, and environmental characteristics such as drainage, potential or existing environmental conditions, should be all considered during the identification. A lack of technical support in operating and building community composting facilities has also been a critical challenge in maintaining decentralized composting systems [4]. Hence, training and navigating the community within the specific region is crucial.