Environmental and economic impacts of implementing tighter effluent standards at municipal wastewater treatment plants in Yiwu City, China
PAN Yirong1,2,3,, LUO Yuli1,2, LIU Junxin1,2,3, WANG Xu1,4,, 1.State Key Laboratory of Environmental Aquatic Chemistry, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China 2.University of Chinese Academy of Sciences, Beijing 100049, China 3.Yangtze River Delta Branch, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Yiwu 322000, China 4.School of Civil and Environmental Engineering, Harbin Institute of Technology (Shenzhen),Shenzhen 518055,China
Abstract:Water utilities in Yiwu City, Zhejiang Province, China, has devoted substantial efforts to satisfy ever-tightening effluent discharge regulations and standards since 2014. Based on the operational data and life-cycle inventory data (including pollutant emission, energy and chemicals consumption) at 9 wastewater treatment plants (WWTPs) in Yiwu, this study simulates and analyzes the change in the overall environmental and economic impacts of implementing stricter standards in 2017 than a baseline in 2015. Results show that tightening discharge levels indeed help reduce waterborne pollutants. However, an excessive amount of chemicals are consumed for the enhanced treatment processes, especially owing to insufficient influent carbon sources in most existing WWTPs. In addition, these intensive practices are also found to cause detrimental effects in terms of climate warming, terrestrial acidification and ecotoxicity. Finally, this work puts forward prospects of future efforts for establishing cost-effective, green and low carbon wastewater treatment paradigm in Yiwu and even China, which can meet tightening effluent standards, reduce unintended negative effects, and increase economic revenue of wastewater treatment services. Key words:wastewater treatment/ environmental impact/ economic potential/ life-cycle impact assessment/ techno-economic analysis/ effluent standard.
图1义乌市污水处理厂主要污染物的年度处理排放情况 Figure1.Annual inflows, reductions, and discharges of main pollutants of all WWTPs of Yiwu City
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1.State Key Laboratory of Environmental Aquatic Chemistry, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China 2.University of Chinese Academy of Sciences, Beijing 100049, China 3.Yangtze River Delta Branch, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Yiwu 322000, China 4.School of Civil and Environmental Engineering, Harbin Institute of Technology (Shenzhen),Shenzhen 518055,China Received Date: 2020-08-26 Accepted Date: 2020-12-11 Available Online: 2021-04-23 Keywords:wastewater treatment/ environmental impact/ economic potential/ life-cycle impact assessment/ techno-economic analysis/ effluent standard Abstract:Water utilities in Yiwu City, Zhejiang Province, China, has devoted substantial efforts to satisfy ever-tightening effluent discharge regulations and standards since 2014. Based on the operational data and life-cycle inventory data (including pollutant emission, energy and chemicals consumption) at 9 wastewater treatment plants (WWTPs) in Yiwu, this study simulates and analyzes the change in the overall environmental and economic impacts of implementing stricter standards in 2017 than a baseline in 2015. Results show that tightening discharge levels indeed help reduce waterborne pollutants. However, an excessive amount of chemicals are consumed for the enhanced treatment processes, especially owing to insufficient influent carbon sources in most existing WWTPs. In addition, these intensive practices are also found to cause detrimental effects in terms of climate warming, terrestrial acidification and ecotoxicity. Finally, this work puts forward prospects of future efforts for establishing cost-effective, green and low carbon wastewater treatment paradigm in Yiwu and even China, which can meet tightening effluent standards, reduce unintended negative effects, and increase economic revenue of wastewater treatment services.
TP是淡水水体富营养化的主控元素，也是2017年义乌市污水处理厂提标处理的首要目标。对该市污水处理厂生命周期过程的淡水水体富营养化潜能进行了模拟，结果如图2所示。整体而言，污水处理厂尾水排放仍然是其富营养化潜能的主要产生过程，2015年和2017年的贡献比例分别达到69%和61%；2017年污水处理厂尾水排放贡献的淡水富营养化潜能约为37 t P-eq，较2015年污水处理未提标时有所降低。因此，提标改造有利于控制尾水排放直接产生的富营养化潜能。然而，在污水处理厂进行提标改造之后，污水处理生命周期过程中产生的富营养化潜能则较2015年增加了5 t P-eq。实际上，电力生产(尤其是火力发电)和水处理药剂制备的过程也会产生和排放含磷废液[7]，从而影响水体富营养化程度。基于污水厂实际运行数据(表3)，进一步对比提标前后义乌市污水处理厂全年电耗和药耗总量发现，污水提标处理过程中电耗和药耗均有增加；其中电耗由52 333 MW·h增至56 064 MW·h，PAC和醋酸则分别由7 780 t和1 839 t增至13 193 t和4 180 t。相应地，电耗和药耗对总体富营养化潜能的贡献比例由29%上升至38%，这是2017年该市污水处理生命周期过程富营养化潜能总体增加的主要原因。综上所述，提标前后，义乌市每排放1 t尾水产生的全生命周期淡水水体富营养化潜能，从0.34 g P-eq增至0.37 g P-eq(见图2)。从工业生命周期链条来看，现有污水二级处理技术普遍仰赖外部动力驱动或需辅以化学药剂进行过程强化，由此表明，污水被过度处理或未能真正有效地解决水体的富营养化问题。
对义乌市每座污水处理厂生命周期过程的淡水水体富营养化潜能进行单独分析(见图2(b))发现：2号和4号污水处理厂每排放1 t尾水产生的生命周期富营养化潜能分别从提标前的0.37和0.31 g P-eq增至提标后的0.46和0.45 g P-eq，增幅明显，达23%和47%；提标后，2号和4号污水处理厂因电耗和药耗而间接产生的富营养化潜能占全生命周期潜能的47%和62%。进一步分析可发现，使用除磷药剂PAC间接产生的富营养化影响占2号污水处理厂电耗和药耗影响总和的49%，而4号污水处理厂的同等影响主要来自作为补充碳源的醋酸(见图2(b)中的饼状图)。除排放标准以外，水处理药剂的消耗量还与污水处理厂进水条件密切相关，尤其是污水的可生化性和碳氮比。 BOD/COD是评价污水可生化性的常用指标。对义乌市9座污水处理厂进水BOD/COD进行分析(见图3)，得到义乌市污水处理厂进水BOD/COD的平均值为0.32，低于我国城镇污水处理厂进水BOD/COD的平均水平(约为0.4)[11]。其中，2号和4号污水处理厂的BOD/COD分别仅为0.33和0.30。另一方面，污水碳氮比低于5，通常被认为不利于生物脱氮除磷[12]；义乌市污水处理厂进水碳氮比平均水平为2.27，2号和4号污水处理厂进水碳氮比分别为1.90和1.86(见图3)，是9座污水处理厂中最低的2座。由此可见，对于2号和4号污水处理厂而言，自身碳源不足是既有工艺提升处理标准的核心难点，而依赖外加药剂则是其处理污水过程中产生环境负效应的主因。
在污水处理过程中，部分污染物经微生物作用会转化为二氧化碳、甲烷和氧化亚氮等温室气体，其无组织排放会对气候变暖产生影响；同时，在生命周期的视角下，污水处理厂上游的电力生产和水处理药剂制备等工业生产过程也会产生碳排放，从而可能间接导致污水处理对气候变化造成不利影响。经分析发现，2017年义乌市污水处理厂处理每吨污水排放1.2 kg CO2-eq，较2015年未提标处理时增幅接近20%。其中，污水处理所耗电力和化学药剂的生产过程是最主要的排放来源，水中污染物降解过程的碳排放次之，二者排放量分别达到0.59和0.46 kg CO2-eq(见图4(a))。除碳排放外，电力和药剂生产过程还会排放硫化物等酸性气体，这些酸性物质经大气沉降后在土壤中积累，从而使土壤酸化[19]。2015年义乌市污水处理厂生命周期过程处理每吨污水排放2.11 g SO2-eq，2017年提标处理后上升了20%，达到2.54 g SO2-eq。虽然电力生产过程排放的酸性气体是主要来源，但提标后酸性气体排放量主要来自PAC制备过程，相比2015年其增幅接近70%(见图4(b))。PAC等药剂生产设施的建设过程还会向环境排放铜、锌、镍等重金属物质，从而给生态系统带来毒性风险[7]。模拟结果表明，药剂生产过程的环境排放是义乌市污水处理厂形成淡水和土壤生态毒性潜能的重要来源；2017年污水提标处理后，义乌市污水处理厂处理每吨污水造成的淡水和土壤生态毒性风险分别增加了50%和44%(见图4(c)~(d))。造成以上结果的主要原因包括PAC在内的药耗增加。进一步对比提标前后9座污水处理厂PAC使用量可发现：提标后各厂的吨水PAC用量均有不同程度的增加(见表3)，2015年9座污水处理厂的平均吨水PAC用量为0.05 kg，而2017年该用量为0.08 kg，为提标前的1.5倍；提标前后PAC投加摩尔比的平均水平由3.3上升至4.0，其中2号污水处理厂2017年的PAC投加比最高，达到6.4，该值远超一级A稳定达标的经验范围(2~3)[20]。 综上所述，以化学强化为策略的污水提标处理技术路径，将会对宏观生态系统产生不利影响，故探索少药剂、零药剂的绿色方案是核心突破口。近来有研究[21]表明，采用新型膜材料等先进分离技术从化学污泥中回收铝用作二次絮凝剂，减少絮凝剂消耗的同时可缓解化学污泥处理处置的压力。低环境影响的药剂开发和应用，也有利于减少水处理药剂使用对环境产生的不利影响[22]。另一方面，将人工湿地、组合生态塘、生态浮岛等生态处理技术作为污水处理厂的深度净化单元，充分利用生态系统的净化功能，有望减少污水处理过程对外部资源和能源的过度依赖，为少药剂、零药剂的绿色处理方案提供了可能[23-24]。
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