An approach to the design of sustainable systems for meeting the basic urban services, as well as the food and energy requirements, of communities of different sizes is discussed, using integrated systems as a common design feature.  The presentation begins with a definition of a sustainable system and includes discussions on integrated solid waste management and defining the system boundary.  Some components of the overall system (solid waste, wastewater treatment, food production, energy production, and others) are described, along with the characteristics that are common among two or more of the components, thus enabling their meshing into integrated systems. 

Examples of integrated and sustainable systems are presented in the form of material and energy balances, including one integrated system composed of solid waste management, wastewater treatment, energy production, and food production facilities.  In this example, nutrient and energy requirements are used as the common bases of design and integration. 

Key Words:  sustainable integrated solid waste management,  process models, community systems, recycling, system modeling, unit operation, energy conservation, system optimization, resource conservation 
 

Introduction  

In terms of resource conservation, sustainability can be defined as a means of using resources such that the resources are not depleted nor permanently damaged.  The meaning of sustainability has been evolving in the field of solid waste management during the last 20 years.  In the 1970s, sustainability was closely associated with resource conservation, in particular material and energy conservation.  In the late 1980s and early 1990s, the emphasis (and interpretation) of sustainability seems to have shifted from resource conservation to final disposition and the difficulties associated with final disposal of environmentally persistent wastes.  Most recently, sustainable solid waste management is described as a method or series of methods (technical, political, social, environmental, etc.) which reflects the needs of the community and are sufficiently flexible to provide environmentally sound solutions for the longest practical period of time. 

Integrated solid waste management, on the other hand, is the application of various methods, systems, or approaches for the management of solid wastes in an environmentally sound manner, without negatively impacting the public health, and at an acceptable cost.  Although a popular emphasis has been placed on sustainability, very little work has been carried out on sustainable systems for the management of solid wastes.  The work reported in this paper describes the quantification and development of models which can be used to design and analyze sustainable systems. 

Model Development  

The initial step in the development of a sustainable system is the definition of the "system boundary."  The system boundary is an arbitrary line of demarcation used to limit not only the scope of the analysis but also to measure and define the inputs and outputs of the system.  In this particular case, the system boundary is a community which has the following facilities: a power generating station, a conventional wastewater treatment facility, animal production, open field agriculture, and greenhouse production of vegetables.  The requirement is to develop a solid waste treatment system which will be integrated into the other services and economic activities of the community. 

Once the system boundary has been established, the next step of the development of the model is the quantification of the inputs and outputs in order to arrive at mass and energy balances.  Typical inputs that were quantified in this work included: 

• 1) electrical and mechanical energy; 
• 2) nutrients (either as part of a waste stream or of a primary material in a usable form); 
• 3) radiated and convected heat energy applied to maintain a desired temperature level; 
• 4) drinking and process water; and 
• 5) oxygen and carbon dioxide. 

• 1) water loss (transpiration, evaporation, or throughput); 
• 2) heat loss (conductive, radiative, convective, and evaporative); 
• 3) solid wastes (manure, urban wastes, vegetation slash, and others); 
• 4) gases (oxygen, methane, carbon dioxide, etc.); 
• 5) dissolved solids (salts, BOD, COD); and 
• 6) food products (quantity per unit area). 

The characterization of the input-output streams was based on extensive information from various scientific and engineering disciplines.  Consequently, a thorough literature search was carried out, and quantitative data were collected on the various agricultural, biological, and energy and waste management processes under consideration.  The nature of available information, which included wide ranges of reported values and a lack of data on certain parameters, directed the course of the work.  In order to validate mass balances and to develop integrated systems models, it was necessary to establish a process unit to which each input and output could be related.  The unit could be expressed as per heat dissipation unit, as per animal or per area, or any other unit considered appropriate to the overall system.  Lifetime average values typically were used for animal and plant production systems. 
 
Analysis of the data was also impacted by the nature of the data.  Thus, it was necessary to make several assumptions.  These assumptions were as follows: 

• 1) It is valid to use average values for some variables regardless of geographical, seasonal, lifecycle, and observable differences. 
• 2) Biological processes can be characterized by only a few of their inputs and outputs. 
• 3) Published data can be reasonably adjusted within limits of reported ranges to reconcile the analyses without significantly affecting the validity of the overall models. 
• 4) All processes analyzed are at steady-state conditions; consequently, for any given process, inputs and outputs should agree.

The procedure is described in Table 1. 
 
 

Selection of Unit Processes  

Several unit processes were selected that would lend themselves to a variety of integrated systems.  Some of these included: chicken, cattle, and swine rearing; aquaculture; crop production (open field and greenhouse); wastewater treatment; and anaerobic digestion. 

Due to space limitations, examples of only two unit processes are provided: chicken production and anaerobic digestion.  A brief description of the models follows. 

Chicken Production:  Chicken raising, whether it be for meat or for egg production, has been developed to the point that it is a very systematic process geared to a factory-style operation.  A representative mass balance diagram for chicken rearing in large-scale operations is presented in Figure 2.  The data in the figure correspond to an operation in which the temperature of the chicken house was maintained at 25^oC.  Body temperature of individual birds was assumed to be at 41^oC. 

Anaerobic Digestion:  This process was considered because of its application in the stabilization of various waste streams, particularly wastewater solids (biosolids) and the organic fraction of urban waste (biowaste).  The process has been thoroughly described in the literature and need not receive detailed attention in this presentation.  A mass diagram of key inputs and outputs of the anaerobic digestion process that are of significance to the analysis is presented in Figure 3. 
Once the mass and energy balances were completed, the next step in the work was to integrate the selected processes into what was considered feasible systems or complexes.  The potential roles of various unit processes are presented in Table 2. 
 

 Table 2.  Potential Roles of Unit Processes in Integrated Complexes 
 

Unit Process	Primary Role in System	Use of Waste Heat	Possible Recycled Inputs	Possible Recycled Outputs
Chicken	Meat Production	Housing	Manure, CH4 for Heating, Slaughter Wastes, Crop Residues, Algae	Manure, Slaughter Wastes, Litter
Cattle	Meat Production	Housing	Manure and Bedding, Water Hyacinths, Crop Residues, Algae	Manure, Slaughter Wastes
Swine	Meat Production	Housing	Manure, Water Hyacinths, Crop Residues, Algae	Manure, Slaughter Wastes
Catfish	Fish Production	Water Warming	Feedlot Manure, Algae	Slaughter Wastes, Fish Wastes
Tilapia	Fish Production	Water Warming	Algae, Slaughter Wastes	Slaughter Wastes, Fish Wastes
Greenhouses	Plant Production (Vegetables, Flowers)	Maintenance of Optimum Temperature	CH4 for Heating, CO2 Enrichment, Composted Sludge and Crop Residues	Crop Residues
Open Field Agriculture	Plant Production (Grain, Vegetables)	Soil Warming, Grain Drying	Sludge, Supernatants, Composted Crop Residues	Crop Residues, Grain
Water Hyacinths	Water Treatment	Maintenance of Optimum Temperature	Organic Wastewater, CO2	Feed for Livestock or Fish
Algae	Water Treatment	Maintenance of Optimum Temperature	All Types of Organic Wastes, CO2	Feed for Livestock or Fish
Anaerobic Digestion	Water Treatment, Methane Production	Maintenance of Optimum Temperature	All Types of Organic Wastes	CH4, CO2, Supernatant, Sludge
Sewage Treatment	Water Treatment	Maintenance of Optimum Temperature	All Wastes	Supernatant, Sludge

 
 

Integration of the Solid Waste Processing System  

The final step in the study was the development of a solid waste processing system which could be incorporated into the general schemes discussed previously.  Some of the requirements of the processing system included: capability of processing mixed residential solid waste, maximum recovery of resources (both materials and energy), minimum quantities requiring final disposal, minimum negative impact on the environment, public health protection (including industrial health), and acceptable cost. 

In addition, the process had to be composed of available and tested subsystems and needed to have sufficient flexibility to compensate for periods of unfavorable market conditions.  A basic design premise was that a solid waste processing system cannot be closed down when a market for a particular material (e.g., paper) has been curtailed.  The system that eventually was developed is capable of recovering about 60% (by weight) of the incoming solid waste in the form of recyclable secondary materials.  In addition, the system is capable of increasing the recovery rate to 80% using other subsystems [2]. 
The waste processing system is of modular construction.  Modularity allows for the combination of several subsystems.  The basic module is the mechanical processing plant.  This plant has been designed to provide intermediate products for further processing and beneficiation.  In the plant, ferrous metals, a clean paper fraction (containing a certain amount of light plastics), and organic materials (light and heavy) are recovered. 

 The ferrous metals are sufficiently clean to be marketed as ferrous scrap.  In the design, glass and non-ferrous metals are not recovered.  The rationale behind this decision is that these materials are more efficiently segregated at the point of generation.  Thus a curbside collection program is suggested for the management of these materials. 

The light fraction (mixture of paper and light plastics) can be used without any additional processing as a refuse-derived fuel in the community's power plant.  The underlying assumption is that the power plant would be properly outfitted to accept solid fuels.  The alternate use for the light fraction is to become an input to a paper mill.  The typical composition of the light fraction is presented in Table 3 and is compared to the typical composition of feedstock to a wastepaper mill.  The data in the table show that the quality of both residential and commercial screened light fraction is equal to or exceeds that of typical wastepaper mill furnish.  In addition, the results of analyses of key properties of fiber recovered from the light fraction are presented in Table 4.  The processing system can be designed to recover high and low grades of secondary fiber.  For example, as indicated by the data in Table 4, commercial waste can be processed to recover a higher quality fiber than that recovered from residential waste. 

Finally, the highly putrescible organic fraction is introduced into the anaerobic digester and the rest of the organic fraction is composted.  Individual subsystems for this process have been tested at either the full-scale or the pilot-scale basis utilizing mixed waste collected in communities from the San Francisco Bay Area. 

Models for most of the unit processes have been developed.  Eventually, the solid waste processing option will be incorporated into the overall sustainable community system for the purpose of conducting optimization of the system and of conducting sensitivity analyses. 

Table 3.  Composition of Screened Light Fraction 
Compared to Feedstock to a Wastepaper Mill (%) 
 

	Corrugated	Newsprint	Other Paper	Plastics	Other Contam.
Average Residential Waste	25.6	52.9	52.9	7.7	13.8
Average Commercial  Waste	44.7	3.0	42.2	5.7	4.4
Typical Feedstock to Wastepaper Mill	75.3			14.1	10.6

 

Conclusions  

The main value of the models developed in this research lies in their application for conducting engineering analyses of completely integrated, sustainable systems aimed at maximum resource conservation, minimum environmental impact, and acceptable cost.  Integrated mass and energy balances for the entire system are accomplished by means of an iterative process.  This involves first making estimates of sizes of subsystems and flows, and then making appropriate adjustments to achieve balance of the various inputs and outputs for each subsystem. 
The models that were developed make it possible to plan new systems or to plan for the retrofitting of existing facilities for the purpose of optimizing material and energy recovery, thus supporting the objective of developing sustainable communities.  The integrated solid waste management system was designed based on actual data from various subsystems tested by the authors in order to conserve resources and reduce the quantities of materials requiring disposal on the land. 

 References  

1. CalRecovery, Inc. Input-Output Analysis of Various Elements of an Energy-Agro-Waste Complex, prepared for the U.S. Department of Energy, Oak Ridge National Laboratory, Oak Ridge, Tennessee, November 1979. 
2. Diaz, L.F., G.M. Savage, and C.G. Golueke, "An Integrated Resource Recovery System," BioCycle, November/December, 1987.