人工湿地污水生态处理技术研究现状、挑战与展望
Research status, challenges and prospects of constructed wetland technology for wastewater ecological treatment
收稿日期: 2021-08-7
基金资助: |
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Received: 2021-08-7
Constructed wetlands(CWS) technology has been adopted as one of the green ecological water treatment technologies in the world due to its significant reduction of energy demand, economic cost and environmental pollution. Constructed wetland wastewater treatment technology has high pollutant removal efficiency and ecological benefits. However, constructed wetland wastewater treatment technology still faces many problems and challenges. This paper first summarized the research status of constructed wetland wastewater ecological treatment technology, focusing on matrix selection, plant optimization and hydraulic control, summarized the current research progress and existing problems of related technologies. At the same time, the research and development focus of constructed wetland function enhancement technology were discussed. Finally, combined with the goals and requirements of the ecological environment protection plan of the fourteenth five year plan and the rural vitalization strategy of China, the important role of constructed wetland wastewater treatment technology in the coordinated development of urban and rural water environment, water ecology and water resources in the future and the direction of future scientific and technological research were discussed and prospected.
Keywords:
本文引用格式
杨长明, 张翔, 郝彦璋, 杨阳.
Yang Changming.
1 人工湿地对污染物的去除机理
1.1 有机物的去除
湿地中大部分植物的地上部分能吸收氧气并向根部传输,为促进植物根系的发育以及通过根释放氧气,人工湿地床体深度一般为0.6~0.8 m。但许多研究表明,根系释放的氧气远不能满足好氧反应需求,因此在水平潜流人工湿地系统中,缺氧和厌氧对有机物的代谢也起到了重要作用〔18〕。
1.2 氮的去除
图1
图1
氮在湿地系统中的转化过程
Fig.1
Transformation process of nitrogen in constructed wetland system
供氧量会限制硝化反应的进行。美国环保署经验指出,当进水中氨氮的质量浓度达到1.0 mg/L时,为保证硝化过程的顺利进行,湿地中的含氧质量浓度需要大于4.6 mg/L。当进水负荷高,硝化反应进行不完全时,水平潜流湿地出水DO小于0.5 mg/L。国外研究的运行经验指出:对于一般的水平潜流人工湿地,在TN的负荷不超过73 g/(m2·a)时,才能满足完全硝化的条件〔25〕。这对那些通常以BOD和SS去除率为设计标准的系统而言难以达到。根茎区域氧气含量较高,因此,硝化反应一般发生在根茎区域,氧气含量低的区域则进行反硝化反应,硝态氮的浓度限制了反硝化反应的进行〔26-27〕。但对城市污水厂尾水来说,关键在反硝化,因为硝化基本在污水厂好氧生化阶段就已经完成了〔28〕。
1.3 磷的去除
图2
图2
磷在人工湿地系统中的转化过程
①—矿化;②—植物和微生物吸收;③—脱附和溶解;④—沉积、吸附和沉淀;⑤—产生磷化氢气体;⑥—与周围水体(如地下水)交换;⑦—降水降尘中带来的磷。注:图中的不溶性磷酸盐包括磷、黏土、金属含水氧化物复合体和离散相的磷酸盐矿物
Fig.2
Transformation process of phosphorus in constructed wetland system
1.4 重金属的去除
人工湿地去除重金属主要通过物理沉淀、过滤、化学沉淀、吸附、微生物交互作用以及植物的吸收。人工湿地植物的吸收和生物富集作用、填料的吸附沉淀作用和金属离子与S2-形成硫化物沉淀是去除重金属的主要方式〔37〕。
(1)基质的去除作用。物理吸附和化学作用是实现基质去除重金属的主要途径。重金属进入湿地后,进行一系列物理、化学反应。在水平潜流或垂直潜流型人工湿地中,重金属与填料发生化学反应而留在填料中,最后被人工湿地植物吸收,或者在更换填料时被去除。
另外,人工湿地基质可强化对重金属的去除,使其转化为难迁移转化的形态。在厌氧环境下,硫酸盐在微生物作用下转化成S2-从而使得重金属沉淀为较为稳定的硫化物〔38〕。
(2)植物的去除作用。人工湿地植物主要通过吸附、挥发和吸收去除重金属。总的来说,植物对金属离子的摄取量很少。植物对重金属的去除主要是调节痕量金属在固相和液相中的分布。可分为两个过程:植物表面的快速吸附和生物质中缓慢的沉积和迁移〔39〕。
湿地植物将空气中的氧气传入根部,在一定范围内形成有氧区域,其中一部分氧气向外扩散,使原本沉淀的硫化物重新氧化,重金属得以释放。湿地植物还可以释放出有机碳至重金属沉淀物表面使其变成还原状态。因此,湿地植物加强了硫的循环和金属在氧化态和还原态间的转化。湿地植物还可为微生物提供场所,植物代谢产物和残体及溶解的有机碳为人工湿地中的硫酸盐还原菌和其他细菌提供食物源。
(3)微生物的作用。主要包括:①对重金属的吸收或吸附作用;②微生物分泌蛋白对可溶性重金属的螯合沉淀作用;③对重金属形态转化的间接作用,硫酸盐还原菌在厌氧条件下将硫酸盐还原成硫化氢,重金属便与硫化氢反应生成沉淀而被去除〔40〕。
1.5 对病原微生物的去除和综合毒性的削减
2 影响人工湿地污水处理的主要因素
2.1 湿地植物的种类及其配置
2.2 基质类型
2.3 水力条件
以上3种人工湿地各有特点,结构特点、水力负荷、占地面积、运行管理等方面的比较见表 1。
表1 不同水流类型人工湿地处理效果比较
Table 1
人工湿地类型 | 表面流人工湿地 | 潜流型人工湿地 | 垂直流人工湿地 |
结构特点 | 水流流态单一,基质较单一,适合生长的植物类型多 | 水流流态复杂,基质类型多,适合生长的植物类型单一(挺水植物较多) | 水流流态复杂,基质类型多,适合生长的植物类型单一(挺水植物较多) |
水力负荷 | 较低 | 较高 | 较高 |
占地面积 | 较大 | 较小 | 较小 |
受气候影响 | 较大 | 较小 | 较小 |
建设成本 | 较小 | 较大 | 较大 |
运行管理 | 较简单 | 较复杂 | 较复杂 |
主要用途 | 适合处理只经过简单沉淀或一级处理的受污水体,处理农村生活、养殖污水等 | 适合用于二级污水处理,处理二级城市污水、垃圾渗滤液等 | 适合处理氨氮含量较高的污水,城市生活污水的深度处理等 |
图3
图3
垂直复合流人工湿地结构与水流示意
Fig.3
Structure and flow diagram of vertical composite flow constructed wetland
(2)水力负荷。水力负荷是指单位面积单位时间能够消化的污水体积,是人工湿地设计、管理的最重要指标。目前人工湿地水力负荷在0.2~0.4 m3/(m2·d)的范围内,欧洲、北美及澳大利亚,人少地多,其水力负荷大都低于0.1 m3/(m2·d)。我国人多地少,其水力负荷较高,一般高于0.2 m3/(m2·d),但考虑到冬季低温的影响,我国北方水力负荷为0.2~0.5 m3/(m2·d),南方为0.4~0.8 m3/(m2·d)。水力负荷的确定对湿地类型的选择及其尺寸(占地面积)的确定至关重要,并且直接关系着人工湿地对污染物的净化效果〔68〕。因此,宜根据不同水质及水量的实际情况,合理确定人工湿地的水力负荷。
2.4 碳源添加对人工湿地脱氮的影响
反硝化作用是在无氧或缺氧环境中进行,在这一过程中需要有机碳作为电子供体,将NO3--N和NO2--N还原为N2。由此可见,碳源对于维持反硝化过程的进行十分重要,充足的碳源才能保证反硝化反应的顺利完成〔72〕。人工湿地中有机碳源分为:(1)内部碳源,主要包括植物根系释放、死亡植物分解、微生物分解、湿地内部沉积物的缓慢释放产生的有机碳源;(2)外部碳源,即湿地进水中的有机碳源。虽然湿地内部碳源的来源较多,但无法满足生物反硝化的碳源数量,而作为湿地进水的污水厂二级出水中,有机物大部分在好氧时被去除,使外部碳源含量低,且多难以被降解,导致人工湿地反硝化速率降低,影响脱氮效果〔73〕。因此,外部添加碳源是非常必要的。
外加碳源分为传统和新型碳源,传统的碳源大多采用低分子有机物类和糖类物质作为液体碳源;新型碳源包括含纤维素类物质的天然植物等。传统碳源虽然有较高的脱氮效率,但也存在着一些不容忽视的缺点〔74〕,首先,本身毒性会对环境造成潜在的危险;其次,液态碳源消耗快,要不断补充,导致运营成本高;最后,出水的有机物含量超标。另外,糖类物质会使微生物过量生长,导致湿地堵塞,且低分子糖类易随水流流失,造成碳的利用率较低。
3 人工湿地技术的应用及发展趋势
3.1 人工湿地技术应用现状
3.2 人工湿地技术发展趋势与未来突破
人工湿地污水处理系统出水水质稳定,对氮、磷和有机物的去除能力强,投资低,易于维护,耐冲击负荷强,并具有美学价值,未来在我国城乡水环境、水生态和水资源统筹协调发展中的应用前景十分广阔。但由于湿地系统的影响因素众多,受地域及气候条件影响较大,人们对污水进入湿地系统后污染物的迁移转化机理与过程的认识尚不充分〔16〕,导致人工湿地的发展受限。因此,深入探索人工湿地处理机理,提升处理效率,实现我国水环境、水生态和水资源协同发展是未来的人工湿地技术研究趋势。同时,未来还要解决技术上存在的一些瓶颈和问题,应从以下几点进行关键技术的攻关和突破:
(1)需要进一步研究可持续的基质和植物种类,用于具有长期性能的人工湿地处理,较高地保留有机替代品和去除重金属和营养物质。
(2)在“碳中和碳达峰”背景下,进一步研究人工湿地植物与温室气体的关系,选择具有显著低温室气体排放和环境退化趋势的人工湿地大型植物和具有高效固碳和碳捕集功能的人工湿地基质,以有效提升人工湿地的碳汇功能〔82〕。
(3)人工湿地污水处理系统的效能评估还需要进一步加强。为了获得更好的人工湿地处理效果和生态环境效益,迫切需要对人工湿地进行全生命周期的设计-应用管理策略〔83〕,最大限度地降低目前人工湿地所面临的诸如堵塞和运行不稳定的问题。
4 总结与展望
人工湿地污水生态处理新技术具有污染物处理效率高、投资低、运行成本低等优点,在尾水深度处理中具有很大的优势。人工湿地应用生态系统中的物种共生、物质循环再生原理、结构与功能协调原则在促进废水中污染物质良性循环的前提下,充分发挥了资源的生产潜力,防止了环境的再污染,获得了污水处理与自然资源相协调的最佳效益。通过人工湿地对城市污水处理厂的中水进一步处理,出水水质达到地表水Ⅳ类,可以直接排入附近河道作为环境用水,有利于提升城市的水环境质量。
我国在短时间内提高污水处理达标排放存在很大的困难,但人工湿地系统投资低,不产生污泥,出水水质稳定,经济环保,具有良好的社会效益。同时人工湿地还能起到改善地表水整体水质的作用。随着我国污水产生量的逐年增加以及对污水深度处理的紧迫性加大,人工湿地污水处理技术的发展潜力巨大,特别是在未来我国乡村振兴和城乡水环境、水生态和水资源“三水”统筹协调发展中将发挥重要作用。在我国,乡村人口密度远低于城市,乡村相比于城市更适合建立人工湿地,并且乡村污水中重金属等有害物质含量低,因此,乡村人工湿地植物的选择可以结合当地的气候、地形、产业等,选择既有处理效果,又有经济价值的植物。
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