AEP  >> Vol. 9 No. 2 (April 2019)

    湿地水生植物厌氧发酵研究进展
    Research Progress of Anaerobic Fermentation of Aquatic Plants from Wetland

  • 全文下载: PDF(633KB) HTML   XML   PP.141-150   DOI: 10.12677/AEP.2019.92022  
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作者:  

田永兰:华北电力大学工程生态学与非线性科学研究中心,北京;
高明瑜:山东省烟台市环境监测中心站,山东 烟台;
侯成林:河北省北方工程设计研究有限公司,河北 石家庄;
徐 翔:山东大学环境科学与工程学院,山东 青岛

关键词:
水生植物厌氧发酵沼气重金属污染Aquatic Plants Anaerobic Fermentation Biogas Heavy Metal Pollution

摘要:

随着社会经济的发展,大量污染物如重金属等进入水体。人工湿地应运而生,在去除水体污染物方面表现出独特的优势,尤其适合发展中国家。湿地水生植物生物量大、有机物含量高,将其作为厌氧发酵的原料,一方面可以产出沼气作为生物能源,另一方面可以实现水生植物的有效处置,避免二次污染和生物质资源的浪费。基于前人关于水生植物厌氧发酵的研究结果,本文从湿地水生植物去污特点出发,总结水生植物生物质的特点,首次从是否受到金属污染的角度总结水生植物厌氧发酵产沼气方面的研究进展,归纳了不同水生植物厌氧发酵产气潜力,并对未来该技术的发展趋势进行了展望,为未来水生植物的处理处置和资源化提供参考。

With the rapid development of industries, heavy metals wastewaters are directly or indirectly discharged into the environment increasingly, especially in developing countries. The removal of heavy metals from contaminated environment has become an urgent issue. Phytoremediation, which is defined as the use of plants to remove contaminants from contaminated environment, has attracted extensive attention. Many aquatic plants have been found to be efficient in removing contaminants. Nevertheless, excessive growth of aquatic plants is conversely able to cause various problems such as interfering with releasing unwanted odors, blocking daylight to the organisms and deoxygenation of water leading to the death of fish and other aquatic life forms. Moreover, the second pollution caused by the decayed plants is also the negative impact that should be avoided. Thus, cost-effective disposal and bio-resource utilization are required to build a sustainable treatment system. Aquatic plants harvested from wetland have large biomass and high organic content. Those biomasses can be used as raw materials of anaerobic fermentation to produce biogas as bioenergy and achieve the effective disposal of residues as well as avoid the secondary pollution. Basing on the previous studies on the anaerobic fermentation with non-contaminated or contaminated aquatic biomass as feedstocks, this article reviewed the mechanisms of decontami-nation and characteristics of aquatic plant biomass. The progress of anaerobic fermentation, using the metal contaminated aquatic plant or not, was then summarized for the first time. The future development of the technology was prospected. This review is expected to provide reference for disposal and reuse of aquatic plants in the future.

1. 引言

环境污染是当代人类面对的重要挑战,受人类生产、生活的影响,城市污水、工业废水和农业污水大量产生,排放到水体环境中,使得大量湖泊、水库等受到污染。人工湿地因净化效果佳、经济效益合理、能量消耗低、工程操作简单,并能促进重建和恢复良好的水生生态系统,在水体修复中得到广泛应用,是最为环保、简单、经济和有效的处理废水方式之一 [1]。湿地植物对总氮、总磷的去除率可以达到80%以上 [2] ,可以使COD降低70%~80%,BOD降低60%~90% [3]。在水体水质恢复的同时,湿地水生植物因适应性强、繁殖速度快,会大量生长。然而,目前很多湿地系统缺乏对水生植物的有效管理,大量水生植物不进行收割,植物残体会分解继而腐烂,营养元素再次回到水体,引起二次污染 [4]。

水生植物经济价值不高,即便及时收割,大量残体的堆放处理也是难题 [5] ,日本最大的湖——琵琶湖,每年需要清理1200吨的水生植物。如何科学地、合理地处置这些植株亟待解决。以往的植物废弃物处置方法大多借鉴于城市固体垃圾的处置技术,比如焚烧法、高温分解法、压缩填埋法、堆肥法等 [6]。但是这些方法一般是基于减量化和无害化的原则,植物原有的资源并没有得到充分的发掘。此时,应转变思维方式,充分利用其生长迅速的特点,将其作为一种资源加以利用,通过资源化利用达到“变废为宝”的目的。

厌氧发酵技术是废弃物资源化利用的有效途径 [7] [8] [9]。针对大量的水生植物残体,将其厌氧发酵制沼气能够充分利用植物所含的能量,有效地减轻水生植物废弃物的二次危害。这能够实现应对能源挑战和强化环境保护的有机统一,具有巨大的意义。目前,有关水生植物厌氧发酵方面的研究越来越多。有文献综述了藻类净化水体并用作发酵原料的潜力 [10] ,而对挺水植物和沉水植物这些大型水生植物的生物质特点、产气潜力方面缺少总结。本文着重总结水生植物,尤其挺水植物和沉水植物的厌氧发酵产沼气的研究进展,为未来湿地水生植物的管理和沼气化利用提供有益参考。

2. 水生植物生物质特点

水生植物生长速率快,光合作用能力强,可通过吸收和富集污染物,例如营养盐和重金属等,来降低水环境的营养负荷,实现湿地系统的修复,尤其适用于发展中国家 [11]。湿地水生植物主要包括芦苇、香蒲、菖蒲等挺水植物,水葫芦、槐叶萍等浮水植物和马来眼子菜、菹草、金鱼藻等沉水植物。高产凤眼莲(Eichhornia crassipes (Mart.) Solms)的氮吸收能力较强,可高达2000 kg/hm2/a,沉水植物的氮吸收能力较差,一般小于700 kg/hm2/a [12]。芦苇、香蒲等挺水植物的氮吸收能力约为200~2500 kg/hm2/a,而大薸(Pistia stratiotes)、香蒲(Typha latifolia)、慈菇(Sagittaria sagittifolia)、菖蒲(Acorus calamus)、水葱(Scirpus tabernaemontani)、凤眼莲等6种水生植物都能较好地吸收污染河流水中的营养物质,吸收贡献率分别可占氮、磷总去除率的11.7%~54.6%和17.6%~64.6% [13]。积累在植物体内的营养盐通过收割而最终从湿地中去除 [14]。

水生植物的种类不同,对湿地氮磷去除效果的影响不同,这可能与植物的耐受性、根区的氧化状态、微生物的种类和数量等多种因素有关 [15] [16]。挺水植物对硝态氮的去除效果比沉水植物和浮水植物都高 [17]。凤眼莲在修复水体氮污染方面表现突出,修复后,凤眼莲植株内的碳氮比(9.7~16.0)远低于正常水葫芦的碳氮比值(20~35) [11]。

利用水生植物产沼气具有很大优势:1) 水生植物生长速度快、生物量大,且可以多次收割,具有与玉米秸秆相似的热值 [18] ,厌氧发酵产沼气潜力巨大;2) 不存在争地的问题,水生植物多生长在湖泊或湿地,不占用陆地多余空间或陆地资源;3) 水体修复植物生长发育所需养分全部来自废水中氮磷等营养物质,不需要投入额外的肥料等化学品。不同水生植物碳氮比含量如表1所示。从表中可以看出,水生植物含水量大,有机质含量高,一般占总固体含量的80%以上。大部分水生植物的碳氮比较低,单独使用不适合厌氧发酵微生物的生存。因此,为更充分的利用水生植物,将其与不同种类生物质混合,通过不同配比组合,确定最佳比例,营造良好发酵环境,使产气效果达到最佳。

3. 水生植物的厌氧发酵产甲烷潜力

水生植物,含有大量的可分解有机物,可以被用于厌氧发酵产气 [11]。近年来,国内外有关水生植物厌氧发酵产沼气的研究逐渐增多,本文将从未受金属污染的水生植物厌氧发酵和受金属污染的水生植物厌氧发酵两个方面进行总结。

3.1. 未受金属污染水生植物厌氧发酵

由于水生植物品种、生长条件、收获时间等的不同,其有机成分含量不同,最后得到的产气量也不相同,如表2所示。在37℃条件下,香蒲(Typha orientalis Presl.)、香菇草(Hydrocotyle vulgaris)、再力花(Thalia dealbata)、菖蒲(Acorus calamus Linn)、美人蕉(Canna indica)、紫芋(Colocasia tonoimo)和梭鱼草

Table 1. C/N ratios of different aquatic plants

表1. 不同水生植物碳氮比含量

NR, not reported。

(Pontederia cordata)七种挺水植物产气率分别为513.23、539.09、577、508.95、555.05、629.41、473.09 mL/g∙VS−1 [21]。马来眼子菜(Potamogeton malaianus)、金鱼藻(Ceratophyllum demersum)等八种沉水植物在中温发酵条件下的产甲烷量在275~418 mL/g∙VS−1 [5]。水葫芦、水盾草和槐叶萍三种浮水植物的产气潜力分别是267、221、155 mL/g∙VS−1 [22]。董诗旭等 [23] 以滇池新鲜蓝藻(Cyanophyta)为发酵底物,在平均温度为20.2℃的发酵环境下,蓝藻的发酵潜力为491 mL/g∙VS−1。南美入侵物种伊乐藻(Elodea nuttallii)的产气量为333 mL/g∙VS−1 [24]。凤眼莲以及其与蓝藻混合为底料的总产气量分别是113.43 L和153.19 L,各自对应的甲烷含量分别为54.3%和71.2%,可以看出混合发酵产气效果明显好于水葫芦单独发酵 [25]。将凤眼莲和巴参菜(Talinum triangulare)分别按100:0、70:30、50:50和30:70的比例混合进行厌氧发酵,结果表明凤眼莲和巴参菜的混合比凤眼莲单独发酵提高了产气量,混合比例为30:70产气量最高,是凤眼莲单独发酵产气量的7.2倍 [26]。以餐厨、粪便及芦苇3种物料为发酵底物,研究发现餐厨加入量对混合厌氧发酵影响显著,适当的增加餐厨比例,有利于提高产气 [27]。大量研究结果表明,水生植物具有良好的产气潜力,将其用于厌氧发酵是可行的,甚至某些水生植物产气量高于一般农业秸秆 [28]。因此,厌氧发酵可作为水生植物资源化利用的一种方式。

Table 2. Biogas potential of anaerobic biogas production of aquatic plants non-contaminated by metals

表2. 未受金属污染的水生植物厌氧发酵产气潜力

3.2. 受金属污染水生植物厌氧发酵

水生植物在修复重金属污染物中起重要作用,其中含有的重金属会影响后续发酵过程。用絮凝铝的藻类(Scenedesmus spp. + Chlorella spp.)生产沼气的实验发现这些含铝的藻类能够产生更多的沼气,而不影响气体的组成 [32]。水生植物满江红(Azolla pinnata R. Br)和浮萍(Lemna minor L.)可以吸附水体中的Fe、Cu、Cd、Ni、Pb、Zn、Mn和Co,用这些植物厌氧发酵产沼气,结果表明,Fe、Mn对满江红和浮萍的发酵没有表现出任何毒性作用,而Cu、Co、Pb和Zn表现出毒性作用 [33]。低浓度Cd、Ni污染的水生植物满江红和浮萍厌氧发酵产气量和CH4组成均高于对照组,促进厌氧发酵的进行,并且沼气产量随着Cd浓度的变化而变化 [33]。富含Fe的浮萍(Lemnaceae)作为原料添加到发酵体系中,可以显著提高微生物的演替速率,加快有机废物的快速稳定,促进沼气的产生,同时可以缩短发酵时间 [34]。研究发现凤眼莲、苦草(Vallisneria spiralis)和菱角(Trapa bispinnosa)作为修复工业废水的植物时,污水处理后的植物与没有污水处理的植物相比,能够产生更多的沼气 [35] [36] ,而苦草的产气量比水葫芦(9~12天内最大)的产气量更大、产气更快速(6~9天内最大) [36]。用凤眼莲作为发酵底物,在厌氧发酵试验中,添加一些金属元素如Fe3+、Zn2+、Ni2+、Co2+和Cu2+,不仅提高产气量,还可以提高气体中甲烷含量,而且增强了系统运行的稳定性 [37]。将水生植物凤眼莲和大薸用于Cd (0.8 mg/L)污染修复实验,再将受污染的两种植物用于产甲烷,发现植物中的Cd没有影响产气效率 [38]。关于受金属污染的水生植物厌氧发酵的研究总结见表3

Table 3. Biogas potential of anaerobic biogas production of aquatic plants contaminated by metals

表3. 受金属污染的水生植物厌氧发酵产气潜力

NR, not reported.

随着人工湿地在污水处理方面的应用和推广,芦苇和香蒲成为分布最普遍的从废水中吸收并积累金属的湿地挺水植物 [39]。采用从人工湿地收集的含有一定浓度的Ni和Fe元素的芦苇秸秆进行厌氧发酵续批实验,结果发现,这些含有重金属的芦苇秸秆可以被用于发酵 [40] [41] ,并且对金属离子有更高的需求,当向发酵体系添加额外的Fe2+时,酶活性提高,产气量可以得到进一步的促进 [40]。此外,一定浓度的Cu和Cr离子可以提高芦苇秸秆厌氧发酵产气量 [42] [43]。在瘤胃微生物液体环境下,以香蒲为实验材料,添加三种低浓度金属盐Cr6+、Cu2+、Cd2+ (分别对应是4、1.6、2.4 mg/l)可以促进香蒲降解,提高甲烷产量 [44]。芦苇和香蒲等大型挺水植物产量巨大、应用普遍,其厌氧发酵再利用过程有待进一步研究。

除了向发酵系统直接添加金属元素外,也有添加矿物质材料来研究其对厌氧发酵所产生的影响。添加一定量的矿物材料可以在一定程度上降低厌氧消化过程中的抑制性物质的浓度,对厌氧消化有一定的促进作用 [46]。以芦苇、粪便和餐厨为发酵基质,10%沸石作为添加物,结果表明10%沸石的添加使甲烷含量从44.1%增加到了65.3% [47]。使用菹草分别与针铁矿、赤铁矿和磁铁矿三种不同铁矿石一起发酵,结果表明三组加入铁矿石的菹草水解速率在49.1%~74.1%的范围内,比控制组31.4%要高,累积产气量和产气速率也有明显差别 [48]。可见,矿物材料的添加,不但可以使其主要成分如铁、钙、镁、钠等来促进微生物生长,也可以调节碳氮比,这些都有利于提高产气量。此外,还可以添加其他催化剂如蛋白酶等酶类、活性炭等吸附剂,改变发酵环境。

4. 结论与展望

水生植物具有很强的污染物去除能力,同时具有巨大的厌氧发酵潜力。利用沼气发酵技术处理收割后的水生植物,可以产生清洁能源,还能控制水生植物过度生长和繁殖,减轻植株腐烂和任意堆放对环境造成的危害,有利于水体生物修复技术的推广应用 [49] [50]。同时,与畜禽粪便混合发酵可以提高水生植物的发酵效率。在实践过程中仍然需要解决工艺、工程、经济等问题来确保该技术的可行性和普适性 [51]。结合本文综述内容,笔者认为可从以下几个方面入手:

1) 水生植物中污染物的释放、形态、价态、活性等,需结合植物组分和结构、底物中其他元素以及发酵条件综合考虑 [51]。厌氧发酵体系中金属的形态受到多种因素的影响,发酵工艺的运行参数也会改变金属的形态。因此,需要开展水生植物厌氧发酵过程中的金属释放研究。

2) 受污染植物的厌氧发酵结果与外源添加实验应相互辅证又有所区别,二者在机制研究方面应注意区分。受金属污染的水生植物生物质用于厌氧发酵时,金属进入发酵体系是一个逐渐释放的过程,金属的形态也与外源添加的离子态金属有很大的区别。厌氧发酵体系中的总金属浓度不能充分表明金属的活性和毒性,难以阐释污染植物发酵过程中金属的作用及影响。

3) 从厌氧发酵沼液沼渣中回收重金属,可以通过使之形成碳酸盐、硫酸盐或者其他聚合物沉淀的方式 [52]。经过发酵后生物质的体积大大缩小,便于后续处理以及回收重金属 [53]。

4) 与供热、交通运输、发电和最终处理处置相结合,确保该水生植物发酵技术的安全性和经济性。从全生命周期的角度考虑水生植物厌氧发酵的技术经济性,并与传统技术进行对比,确保技术的经济性和可用性 [54] [55] [56]。

5) 防止水生植物再利用过程的二次污染,包括运输、发酵、处理处置等 [57] [58]。在实际应用之前,应该形成相关的处理处置法规和标准。

基金项目

本研究受到国家水体污染控制与治理科技重大专项(2017ZX07101003)、中央高校基本科研业务费专项资金(2018MS051)资助。

参考文献

NOTES

*通讯作者

文章引用:
田永兰, 高明瑜, 侯成林, 徐翔. 湿地水生植物厌氧发酵研究进展[J]. 环境保护前沿, 2019, 9(2): 141-150. https://doi.org/10.12677/AEP.2019.92022

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