結合水質模式與成本效益分析評估水環境品質提升策略之研究

Abstract

本研究結合水質模式與成本效益分析，評估一條河川在多個污染整治策略情境下水質改善之情形，與相對應的環境經濟價值，提供決策者更多量化的資訊。本研究建置台灣重點河川二仁溪與其支流三爺溪QUAL2Kw河川水質模式，模擬溶氧(DO)、生化需氧量(BOD)、懸浮固體(SS)、氨氮(NH3-N)河川水污染指標，搭配HEC-RAS水理模式計算水質模式所需之相關參數，並假設海水與河水均勻混合下計算河口感潮對水質的影響，並蒐集環保署過去在二仁溪流域推估的每日污染產量，與計算污染流達率等方法，改善河川水質模擬精度。二仁溪水質模式之校正、驗證平均絕對誤差百分率為25.30%與29.07%；三爺溪水質模式之校正、驗證平均絕對誤差百分率為11.62%與15.57%。 針對污染削減策略採用公共污水下水道系統建置、現地處理設施與畜牧養豬業設置沼氣發電設備做為污染削減手段，並參考環保署歷年報告中的規劃報告，做為基線情境比較，針對污水下水道系統設計了另外九種替代方案，綠能養豬則是假設符合政府補助規定的業者，設置沼氣發電設備。在成本效益的計算中，將初期設置費用與營運維護費用當作社會成本，而社會效益的部分則是考量BOD污染削減、廢水回收再利用、沼氣發電等，在折現率為1.98%與計算時間為65年的條件下，分析其成本效益之淨現值大小，並利用線性規劃求解不同情境之最佳淨現值。 從水質模擬結果顯示，污水下水道建設對於三爺溪水質能有明顯的改善，在不考量氨氮的狀況下，三爺溪下游水質能改善至輕度污染或未受污染標準，而氨氮濃度從原本高出嚴重污染標準的6至10倍降低至接近嚴重污染標準。在綠能養豬推廣對於二仁溪水質改善的情境中，當只考慮符合補助對象的業者設置設備時，僅能微幅改善二仁溪的污染程度，但在假設流域中的畜牧廢水皆採用沼氣發電方式處理後，二仁溪可以達到全河段不發生嚴重污染。在計算不同情境之成本效益分析時，利用線性規劃調整污水下水道系統建設的規模，並且依照每年最多推動兩個污水下水道系統的方式時，能夠比基線情境的規劃還要增加26億的淨現值，是所有替代方案中最佳的整治策略，而綠能養豬業者按照現況投資方式，大約需要20年的還本期，如果想要全面推動綠能養豬，還需要增加更多的優惠方案降低設置門檻。

The purpose of this study is to quantify the potential strategic benefits by applying the water quality modelling integrated with cost-benefit analysis to simulating scenarios based on regional development planning. We established the QUAL2Kw models of Erhjen River and Sanye Creek and simulated the river pollution indices of dissolved oxygen (DO), biochemical oxygen demand (BOD), suspended solids (SS) and ammonia nitrogen (NH3-N). In the water quality simulation, HEC-RAS was employed to calculate the hydraulic parameters and dilution impact of tidal effect in the downstream section. Daily pollution loadings were obtained from the Water Pollution Control Information System maintained by Taiwan EPA, and the wastewater delivery ratios were calculated by comparing the estimated pollution loadings with the monitoring data. In model calibration and verification, the mean absolute percentage errors (MAPE) of Erhjen River were calculated to be 25.30% and 29.07%, respectively, which met the prescribed acceptable criteria of 50%. The MAPEs of Sanye Creek were calculated to be 11.62% and 15.57%. This model was applied to simulating water quality based on constructing public sewage systems, on-site treatment facilities and promoting biogas-based power generation in the Erhjen River watershed. Using the Taiwan EPA's annual report as a baseline scenario, we designed nine different scenarios for pollution control simulation. Pig husbandry assumes the establishment of biogas-based power generation in the conformity of government subsidy policy. In the cost-benefit analysis, we adopted the market valuation method, choosing a period of 65 years for analysis and the discount rate of 1.98%. Capital investments included the costs of design, construction, operation and maintenance for each project in Erhjen River catchment. In combination of a variety of proposed strategies, we solved the maximum net present value of different scenarios by linear programming. As a consequence, the water quality of the Sanye Creek is obviously improved after the construction of the sewage systems. By the negligence of ammonia nitrogen, the water quality of the lower reaches of the Sanye Creek can be improved to slightly-polluted or non-polluted standards, and the ammonia nitrogen concentration reduced from the 6 to 10 times severely-polluted standards to merely-above severely-polluted standards. In the scenario of biogas-based power generation promotion, if all pig farms with livestock number >2000 installed the on-site power generation equipment, water quality can be improved slightly. If all the manure waste from pig-farms is collected for subsequent electricity generation, the river pollution index is estimated to improve to moderately-polluted category for all the length of Erhjen River. In calculating the cost-benefit analysis of different scenarios, if the scale of public sewage systems were adjusted by linear programming and two sewage systems were the maximum promoting number every year, this scenario is recommended to increase a net present value of 2.6 billion more than the baseline scenario. Pig husbandry in accordance with the current investment for biogas power generation needs about 20 years for capital recovery. If government plans to fully promote the biogas-based power generation, more promotional programs and reduction in the administrative constraints should be considered.