將有二次使用價值之物品回收再利用，促使上游製程母料之消耗減少，達到源頭減量之觀念普遍存在於現今社會當中，但各國間常因國內回收人力成本高升，選擇將可回收物品販賣至人力成本較為低廉的開發中國家處理，此舉造成跨國運輸中額外排放的二氧化碳增加。物品運輸中造成的環境衝擊，回收可達到的二氧化碳減量及回收業者存在的永續性，漸漸在廢棄物資源回收過程中受到重視。
本研究旨在利用線性模式評估物質跨界式回收策略，並以寶特瓶在台灣、日本及中國間為案例。案例研究以溫室氣體排放最小化為目標函數，並考慮兩種寶特瓶回收方式，分別為物理性回收寶特瓶取代紡織用聚酯粒及化學性回收方法。以化學性方式回收之寶特瓶，將著重於將寶特瓶投入煉焦爐中取代原物料之方式，計算其在煉焦爐中取代燃料以及煉焦煤所獲得的二氧化碳減量，並評估在最低環境衝擊下，模擬三個國家、兩種回收方式寶特瓶的處理流向。
案例中顯示，在適當假設下，跨界式回收不失為一可行方法，二氧化碳排放將比各個自行回收處理為低。研究發現以二氧化碳排放最小化為目標當中，物理式寶特瓶回收將優於化學式寶特瓶回收。
本模式未預期以經濟成本為目標函數來達到成本最小化之原因有二，政府對資源回收業者提供補助是在各國間常見之政策，經濟、社會和環境之間複雜的關係要如何平衡將倍受檢視；此外，當此模式考慮成本最小化時，環境衝擊的外部成本應建立於廣泛考慮各個國家背景之一致性。
本研究尚需蒐集更多資料於模式模擬，以取得更詳實之回收管理策略。模式建立後，可選擇其他環境衝擊指標，例如酸雨及優氧化甚至混合性指標，蒐集適當限制因子配合下進行目標最佳化。此模式亦可評估其他化學性寶特瓶回收方式，如取代焦炭在鼓風爐中還原性的角色，或是將寶特瓶以熱回收方式處理在三個國家間最低環境衝擊下應有的最佳流向。更甚者，可以將其他鄰近國家例如韓國或菲律賓等回收寶特瓶加入到此模式當中。

Recycling is primarily considered as a method to save resource by replacing operation of upstream processes that consume virgin materials with a recovery processes that use materials utilized at least once. As the same time, it is expected that recycling achieve reductions in environmental impacts associated with the production of the virgin material. Nowadays, many post-consumer products are seen as economically competitive resources and traded across the national boundaries. However, the trans-boundary trades of post-consumer resources have not been assessed comprehensively from the social and environmental points of views. Namely, environmental impacts associated with transportation, differences in emission reductions achieved by the recycling, and sustainability of recyclers in each country is raised as concerns associated with the trans-boundary flows of post-consumer resources.
To assess above issues, a platform equipped with a tool that can generate trans-boundary treatment scenarios that achieves environmentally preferable treatment is needed. Such as platform also needs a database that provides the characteristics and performances of varied technologies and inventories of life-cycle wide background industrial activities in the multiple countries. To develop such a platform, in this study, PET bottles was chosen as a target material, and 1) a linear programming model was constructed to explore management strategies of post-consumer PET bottles, including the possibilities of trans-boundary trades on Japan, China and Taiwan, and 2) data was collected to demonstrate the use of the constructed model.
In the case study, green-house gas (GHG) emission was used as the environmental impact to be minimized. The constructed model can choose any other environmental objective functions, including minimization of indicators such as acidification and eutrophication, or even mixture of the several indicators. Various assumptions on difference in the product that could be replaced by the recycled material could be incorporated as well if data were available.
Economic objectives are not intended to be used in the model, although it is technically possible to do so. This is because recycling of post-consumer products receive subsidies in many countries, thus, cost minimization does not make sense when discussing how a sustainable economic and social system should be designed. Use of minimization of cost including external cost could be explored in the future, if it becomes possible to do so in a comprehensive and consistent manner across countries.
Despite efforts paid for data collection, the case study used some fictitious data due to lack of data. The case study compared the CO2 emission reduction induced by collected PET bottles in coke oven utilization and fiber product substitution. The result revealed in some cases, minimum CO2 emission was achieved when trans-boundary flows were allowed, rather than enforcing domestic treatment. Furthermore, sensitivity of the flows to fluctuation of some of the parameters was assessed in this case study. Such assessment allows discussions on sustainability of recyclers in respective countries, because if flows fluctuate as a result of implementation of a social and economic system that minimize environmental impact, recyclers cannot play their role as modeled due to unstable supply of materials and demands.
Refinement of models and further data collection is suggested to utilize the developed model for discussions on trans-boundary resource management in general. Application of our model to other post-consumer materials is also a potential research in the future. The case studies on PET bottles could be extended by further collecting data on other recycling methods such as thermal recycling and recycling in blast furnace as reducing agent. Consideration of more neighbor countries such as Korea and Philippines is also recommended.

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