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GRAPHICAL INTEGRATED

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1992-National-Waste-Processing-Conference-20-00011992-National-Waste-Processing-Conference-20-00021992-National-Waste-Processing-Conference-20-00031992-National-Waste-Processing-Conference-20-00041992-National-Waste-Processing-Conference-20-0005GRAPHICAL INTEGRATED WASTE MANAGEMENT MODEL FOR PUERTO RICO SAMUEL A. VIGIL Civil and Environmental Engineering Department California Polytechnic State University San Luis Obispo, California ABSTRACT The application of STELLA, a commercially available, graphically based simulation lang�age for the Macintosh computer, to solid waste facility planning is described. This paper discusses a STELLA model which was developed for the Commonwealth of Puerto Rico Solid Waste Management Authority (SWMA). The model simulates the existing solid waste management system in Puerto Rico and also the proposed system of regional landfills, transfer stations, and waste-to-energy plants. The model incorporates population projections for all 78 municipalities in Puerto Rico from 1990 to 2010, solid waste generation factors for all municipalities, data on remaining life for the existing landfills, and projected startup time for the new landfills and waste to energy plants. The model is being used as a planning tool by SWMA to manage time phased procurement of these new facilities. INTRODUCTION The Commonwealth of Puerto Rico Solid Waste Management Authority (SWMA) contracted with Brown and Caldwell Consultants to prepare a computer model for tracking solid wastes. The model is programmed in the STELLA computer language on the Macintosh microcomputer. As part of an earlier contract, Brown and Caldwell had assisted SWMA in preparing a Site Facility Plan which summarized the 185 HILARY M. THEISEN Brown and Caldwell Consultants Walnut Creek, California existing solid waste management system in Puerto Rico and presented a new system of regional landfills, transfer stations, and waste-to-energy plants. This paper describes a computer model based on that Plan. The model incorporates population projections for all 78 municipalities in Puerto Rico from 1990 to 2010, solid waste generation factors for all municipalities, data on remaining life for the existing landfills, and projected startup time for the new landfills and waste-to-energy plants. THE STELLA LANGUAGE STELLA is a commercially available programming language which runs on the Macintosh computer. It combines features of a computer aided design program, a spreadsheet, and a conventional programming language. STELLA is widely used in scientific applications to model complex systems. The most important feature of STELLA is that it is programmed graphically as a logic diagram. The program is generated by the logic diagram. Thus the diagram and the program are always in agreement. In conventional programming, the logic diagram (or flowsheet) is used by programmers as an outline. Since it is a paper document, the finished program may not actually match the logic diagram. Spreadsheets, although easy to construct, are difficult to correct since the programming logic is hidden behind the spreadsheet cells. With a STELLA program, thetime_in PopSJ MSWSJ MSW_facSJ FIG. 1 SAN JUAN LOGIC DIAGRAM (Existing System) logic diagram always accurately represents the structure of the model. The logic diagram can be easily understood by nonprogrammers. STELLA is programmed by constructing a flowsheet on the computer screen using graphical elements as shown in Fig. 1, the logic diagram for the municipal solid waste (MSW) flow in the City of San Juan. The rectangle represents a "stock" or accumulation of material; in this case, the total MSW deposited at landfill 51. The circles represent "converters", algebraic expressions or constants. The arrow with double lines represents a "flow" of materials. The associated circle contains algebraic logic to control the flow. Finally the single line arrows represent logical connections between the flowsheet elements. Thus, the population of San Juan (PopSJ) affects the yearly MSW generation in San Juan (MSWSJ). STELLA MODEL - EXISTING MSW SYSTEM The STELLA model for Puerto Rico was prepared in two parts: (a) a simulation of the existing system; and (b) a simulation of the proposed system. The STELLA logic diagram for the existing San Juan MSW system is shown in Fig. 1. It is typical of the other 78 municipalities modeled. San Juan Logic Diagram The converter (circle) labeled PopSJ is a graphical function which contains the population data for San Juan. It is linked to a "ghost" converter, or copy, labeled time.in. This provides time input to the model from 0 to 20 years in 1 year increments. The PopSJ function is saved in the STELLA model as an equation: PopSJ = graph(time_in) (0.0,415399), (5.00,404144), (10.0,393858), (15.0,382210) (20.0,370562) , 186 The equation shows that the population at year 0 = 415,399 and so on. STELLA interpolates the yearly values. Graphical functions can also be drawn freehand. STELLA automatically converts the graph into numerical data. The converter labeled MSWSJ, calculates the municipal solid waste (MSW) generated yearly in San Juan (in tons/year) using the equation below: MSWSJ = MSW_facSJ"(300/2000)"PopSJ The equation calculates MSWSJ by multiplying the population (PopSJ) by the daily MSW generation in lb/ cap-day (MSW JacSJ). The arithmetic equation (300/ 2000) converts lb/cap-day into tons/day (assuming 300 days/year production). MSW JacSJ is a constant, 4.9 lb/cap-day. (Note: These constants and assumptions were specified by the client, SWMA. Any user specified days production per year and MSW generation rates can be used.) The MSW "flows" to the landfill which is shown as the rectangle or stock labeled LF51SUM (for landfill #51). The flow is controlled by the converter LF51 which contains the equation: LF51 = IF (LFT_51 = 0) OR (LFT_51-TIME � 0) THEN 0 ELSE MSWSJ This is a logical


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