废物利用强化甲烷发酵过程的技术探究

A prerequisite realizing carbon neutral in wastewater treatment plants is to convert organics contained in excess activated sludge into energy. Due to energy efficiency and applicability, methane (CH4) has been considered as a choice ofappropriate energy forms. There are two processes in the methanogenic reactions: aceticlastic and hydrogenotrophic methanogeneses; at the same time homoacetogenesis is also involved in them. Aceticlastic methanogenesis is related to the amount of acetic acid converted from organics; hydrogenotrophic methanogenesis and homoacetogenesis depend on the intermediates of both CO2 and H2. If CO2 and/or H2 are directly or indirectly exerted into digesters, both hydrogenotrophic methanogenesis and homoacetogenesis could be in principle improved, which would make produced and/or exerted CO2 converted into CH4 to a large extent. CO2 from carbon capture and waste iron scrap (WIP) could be used for the two external media; CO2 could be directly effective after added; H2 formed during the iron corrosion could also function. Moreover, producing H2 is not the only function of WIP, and ferrous ions (Fe2+)formed after the iron corrosion could precipitate and even recover phosphates in a high concentration from cell lysis of sludge; WIP could also degrade some heavy metals and organic contaminant mixtures.

污水处理厂实现"碳中和"目标前提是将剩余污泥中的有机物转化为能源。着眼于能源转化效率及其实用性，把CH4确定为转化能源的终产物较为适当。污泥厌氧消化产甲烷主要涉及嗜乙酸异养产甲烷细菌和嗜氢自养产甲烷细菌两种代谢过程，同时也存在着同型产乙酸过程。嗜乙酸异养产甲烷过程与系统内被转化的乙酸多寡有关，而嗜氢自养产甲烷与同型产乙酸过程则与系统内中间产物- - H2和CO2相联系。如果直接或间接从外部向系统内投加H2和CO2，理论上有可能促进嗜氢自养产甲烷与同型产乙酸过程，使得中间产物或外加CO2最大程度转化为终产物CH4。"碳捕捉"后的CO2和废铁屑两种废物可以充当外加媒介；CO2直接通入系统即有可能奏效，而铁腐蚀后产生的H2亦可起到强化转化CH4的作用。铁屑介入系统不仅仅是单一的产H2作用，它腐蚀后形成的Fe2+还能沉淀/回收污泥裂解后产生的磷酸盐。此外，铁还能降解污泥中的一些重金属和有毒有机物。

有关全球气候变化的《巴黎协定》签署正推动着污水处理朝着“碳中和”方向迈进。其中，污水中有机物（或剩余污泥）中蕴含的巨大化学能唾手可得。尽管污水中有机物转化能源目前出现了像MFC/MEC这样的高技术，但其能量转化效率可能还不及传统厌氧消化。模型测算表明，我国污水水质因有机物（COD）含量低而难以满足碳中和运行的全能量需求（仅能达到约50%），且剩余污泥“细胞破壁”以及所含难降解木质纤维素成分“结构破稳”制约着有机物厌氧消化甲烷产率。预处理（酸、碱、热解与超声波）可实现“细胞破壁”和“结构破稳”的双重作用。预处理实验表明，热解为最佳预处理技术方式，最佳工况下可分别获得52.6%木质纤维素降解率和53.6%甲烷产率。以载体方式向厌氧消化过程投加富集微生物亦可有效促进污泥中木质纤维素的降解。. 实验证实，纳米铁（NZVI）和废铁屑（WIS）投入厌氧消化可明显增加表观甲烷产率，最大投加量时可增产60.5%和44.7%。铁促厌氧消化产甲烷途径研究初期认为，乃铁腐蚀产氢助力自养产甲烷菌之作用。遂采用通入外源H2之方式予以实验确认。实验显示，在系统H2分压（ ）为0.96 bar下，自养产甲烷菌与同型产乙酸菌对H2消耗产甲烷贡献率分别为60%和40%，但自养产甲烷菌具有H2消耗产甲烷的上限。系统 增加可强化同型乙酸菌转从H2转化为乙酸、继而产生甲烷的作用。这与常规厌氧消化中低 下仅25%同型乙酸菌转化率完全不同。同时，外源H2可强化上游有机物水解，继而增加甲烷气体产量。. 有关铁促厌氧消化产甲烷机理进一步实验揭示，铁腐蚀产氢对甲烷增产贡献率其实作用不大，还不及5%；铁诱氧化还原电位（ORP）降低而导致的严格厌氧环境也无助于甲烷增产；而铁刺激厌氧细菌酶活性方可能是铁促厌氧消化产甲烷的重要作用（95%）途径。