Composting optimization: Integrating cost analysis with the physical-chemical properties of materials to be composted
Introduction
The production of compost has long been the object of great interest to researchers, since compost may be used as a fertilizer or as a material for producing plant nursery substrates. From an economic point of view, composting can therefore bring reductions in the cost of disposing of organic residues, as well as providing an income, by virtue of compost being used as a substitute for other materials (chemical fertilizers and peat) that may be quite expensive. In addition, composting can have a strong ecological-environmental value, allowing organic by-products to be subtracted from the disposal cycle and put back into the production cycle, enhancing it and closing the organic carbon cycle, while also being a tool for the economic and social sustainability of production activities in rural areas (Scarpato and Simeone, 2013, Simeone et al., 2015), also in accordance with the European strategy “Horizon 2020″ for rural sustainable development.
Composting can be performed by using a variety of organic materials, such as: agricultural by-products (prunings, straw, other crop residues, etc.), agro-industry by-products (pomace, marc and stalks, etc.), livestock waste, sewage sludge and the organic fraction of municipal solid waste. Generally, a given type of organic putrescible material has to be supplemented with combinations of bulking materials (Gigliotti et al., 2012, Nolan et al., 2011), to ensure that in the mixture all the process parameters are in the optimum range in order to support the process properly and to obtain a good quality compost. Often, at a given time and place, different materials are available simultaneously; the need then arises to achieve the best mixture of raw materials to be used in composting, with the aim of minimizing costs as well as producing a high quality compost, in terms of stability, maturity and with good agronomic and environmental parameters. This means that a significant number of variables have to be considered simultaneously.
Trémier et al. (2009) conducted a study to investigate the effects and interactions between certain features of the starting mixture, such as humidity and particle size, on the process of composting sewage sludge and plant residues. This allowed them to produce an empirical method to optimize the combinations of the two starting materials according to the above parameters.
Bongochgetsakul and Ishida (2008) developed an analytical method that integrates models in the exchange and distribution of heat and humidity and the oxygen concentration with other models of microbiological activity and physical parameters, such as turning the mixture.
Bueno et al. (2009) linked certain chemical-physical parameters (aeration, humidity, particle size, time) during the composting of pruning residues, evaluating the influence that each parameter has on the process and obtaining a correlation between these parameters, in order to define the optimal ranges within which to produce a compost with certain defined characteristics.
Barrena et al. (2011) tried to optimize the mixing of different raw materials to be composted, evaluating the biological activity potential of the mixture, by measuring the respirometric indices, related to physical-chemical parameters such as humidity, C/N ratio and free air space (FAS).
Some studies have dealt with the economic aspects of compost production, analyzing production costs and possible returns, both direct and indirect. Such studies, in general, have dealt with the economic problems a posteriori, i.e. after having defined the production process from the technical point of view and the materials to be used.
Wei et al. (2001) analyzed the building and operating costs of a series of processing plants, used for composting sludge from urban waste water treatment plants, and found that the percentage of moisture is the factor that has most influence on both types of cost, especially because it determines the size of the plant; moreover, even the bulking material used is a determining factor for annual operating costs, representing 64%–86% of operating costs, which, in turn, represent the majority of the total costs: 69%–87%.
In an area of Spain where a large number of vegetables are produced in greenhouses, Parra et al. (2008) developed a decision model to evaluate the feasibility of a plant to compost the crop residues, analyzing costs and benefits including the environmental benefits as well.
Alfano et al. (2008) conducted a study on composting wastewater from oil mills, using a composter prototype, assessing the chemical, physical and microbiological characteristics of the raw materials used, the mixture to be composted, and the compost obtained, monitoring also the development of the various parameters during the process. They also produced a concise analysis of investment and operating costs of the proposed method.
Ruggieri et al. (2009a) analyzed the technical and economic feasibility of composting waste from wine making, assessing the environmental impact and energy performance using the LCA methodology. The study compared the costs of the composting system designed with those of disposing of the waste. The economic result was very positive. The environmental impact of composting is positive for the majority of composting systems evaluated; the energy balance, when comparing compost with chemical fertilizer, is also favorable.
In a survey of 25 Japanese composting facilities, Zhang and Matsuto (2011) correlated certain physical-chemical characteristics of the composts obtained with the characteristics of the starting materials, considering the process parameters and operating costs; in this way broad indications were obtained on the factors that most influence the make up of the compost and the related costs (with particular reference to energy costs), including the size of the plant. This study also highlighted the great burden of the cost of bulking materials compared to the total operating costs.
Nolan et al. (2012), investigated the costs of disposing of pig slurry, in Ireland, comparing the investment and operating costs to calculate the break-even point of various alternative destinations for the waste, including composting the solid fraction.
Recently, Tan et al. (2014), in a study for optimizing the management of municipal solid waste in Malaysia (including composting), conducted a review of the optimization models proposed for this issue. Of the 14 models identified and ranked in their review, 7 models used Linear Programming or L.P. Mixed Integer methods and the other 7 used nonlinear, stochastic or fuzzy methods. The study concludes that few models simultaneously address all the technical, environmental, energy and economic problems, while it would be of great interest to integrate these issues in modeling.
As a matter of fact, despite extensive research in literature, it has not been possible to find any study that takes into account both the chemical-physical and economic issues in formulating the composting mixture. It should also be noted that on the internet (see list in Web References) there are already a number of practical “compost calculators” available, that attempt to optimize the mixture of raw materials to be composted; some of these also consider some economic aspects. All these calculators, however, show deficiencies which means they can only be used in a very approximate way: the calculation method is either greatly simplified or not documented at all, some also use wrong calculation methods, or do not take into consideration the actual biodegradability of the materials, or it is not possible to consider calculation parameters other than the few defaults.
This paper aims to analyze the costs of managing individual composting plants, depending on the raw material to be treated and the technical objectives, taking into account that the economic margins of this type of activity are generally modest and subject to significant variations due to the variability of the characteristics of the raw materials used and, consequently, those of the compost obtained. The predictability and consistency of the compost characteristics are very important for both the producer and the user. The consistency of the compost characteristics is even more important if the composting activity is entrepreneurial, also considering that in many countries, including Italy, compost has to be of a very high quality and all the parameters must comply with the strict legal limits in order to be marketed (Del Buono et al., 2011, Emino and Warman, 2011).
For these reasons, it was considered interesting to identify a method for optimizing the mix of raw materials used in composting, according to the chemical and physical characteristics required for properly starting and developing the process, and also the supply costs of these raw materials and other processing costs. The method focused on in this study would make it possible to identify which raw materials to use and in what proportions, taking into account their availability, chemical-physical characteristics, supply costs and management costs of the plant, in order to ensure the economic sustainability of the entire process and to obtain a product with certain characteristics, optimizing all the variables before starting the process.
In the agricultural and environmental sectors, it is known that mathematical models for optimizing the production processes are employed in various forms and continuously upgraded (Valipour and Montazar, 2012, Valipour, 2016, Sole-Mauri et al., 2007, Manos et al., 2013, Minh et al., 2007).
Since this is the optimization of a single process, a Linear Programming model was considered appropriate, the objective of which is to minimize costs in relation to a range of technical and economic constraints. A method such as Linear Programming is essential in cases such as this, because it is able to consider simultaneously a multitude of variables and to identify the best result from among the many possible outcomes (Cheng et al., 2003, Minh et al., 2007, Giasson et al., 2002, Weintraub et al., 2007). Moreover, in the course of the research, attempts were made to make it easy to apply the mathematical model defined, studying the characteristics that solver software should have to enable the input and an output of simple data, via an intuitive and easy to understand user interface; this interface should make it possible for these algorithms to be used by non-experts as well, who do not know their working mechanism, avoiding having to input data according to strict rules dictated by a mathematical language, as in all software dedicated to the resolution of Operations Research models (see appendices for examples).
This objective implies that the user interface is concerned with the mathematical formalization of the model, that is to say it automatically builds the mathematical model in the language required by the solver software, from the data entered each time by the user. To make this automatism possible, the various types of cost and constraint must be identified and classified, so that the interface software can be programmed to construct the calculation model automatically, starting from any given input.
By doing this, the intention was to create the conditions for simplifying the implementation of this method in real manufacturing situations, through the creation of appropriate software. All this is also designed to provide an organic and solid theoretical foundation for the implementation of more effective “compost calculators”.
In short, the goals of the research are:
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to identify in detail the types of constraint that recur in composting mixture design, to generalize their mathematical formulation and to define a calculation model;
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to design the functional characteristics of a software program to resolve the calculation model and to consider other useful parameters to correctly set the composting start, a software program which has a user interface that allows an intuitive input of the model calculation parameters, permits the input of any number and kind of parameters, translates the input data into the language used by the calculation procedure, and lastly presents the calculation results in an immediately understandable form.
Section snippets
Theoretical approach and methods
The basic aim of the method proposed is to determine the quantities of different raw materials to be mixed to obtain the mixture subjected to composting; these quantities must be able to minimize the costs of the process, in relation to the chemical and physical characteristics that the mixture must possess for allowing a correct start of the composting process and proper biodegradation of the chemical compounds.
The “quantity” is therefore the basic unit of measurement for all the parameters
Application examples
This section reports some examples of model and software applications, in order to explain logic and operation. The examples use data from previous studies of several authors to validate the method proposed in this study.
Conclusions
The study undertaken has made it possible to identify the types of constraints that can occur in preparing the mixture to be composted and to formulate them in general mathematical terms; each type of constraint has been discussed in depth and generalized; in this way a strong scientific basis has been obtained for the development of useful applications in concrete cases of composting plants. Also, the theoretical approach that was discussed and the mathematical formalization of the model could
Acknowledgements
This research was funded partly by the Italian Ministry of Agricultural, Food and Forest Policies (grant numbers DM 2597/7643/12 and DM 11060/7643/09) and partly under the EU Rural Development Plan 2007–2013 (Action 1.2.4. of Umbria Region, grant number 94751901433).
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