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        基于膜蒸餾的沼液資源化處理研究進(jìn)展

        2021-06-28 00:57:16賀清堯石明菲袁巧霞晏水平

        賀清堯,石明菲,馮 椋,艾 平,袁巧霞,晏水平

        基于膜蒸餾的沼液資源化處理研究進(jìn)展

        賀清堯,石明菲,馮 椋,艾 平,袁巧霞,晏水平※

        (1. 華中農(nóng)業(yè)大學(xué)工學(xué)院,武漢 430070;2. 農(nóng)業(yè)農(nóng)村部長(zhǎng)江中下游農(nóng)業(yè)裝備重點(diǎn)實(shí)驗(yàn)室,武漢 430070)

        沼液可占濕法厭氧發(fā)酵后發(fā)酵剩余物總質(zhì)量的80%以上,在農(nóng)田土地承載量和運(yùn)輸成本的雙重限制條件下,大型沼氣工程的沼液很難通過(guò)還田利用的方式進(jìn)行完全消納。對(duì)沼液實(shí)行資源化處理既能減少沼液體積和降低對(duì)環(huán)境的潛在威脅,還可實(shí)現(xiàn)高附加值的資源回收,促進(jìn)可持續(xù)的農(nóng)業(yè)循環(huán)經(jīng)濟(jì)發(fā)展。作為膜分離技術(shù)中的重要分支,膜蒸餾在沼液處理過(guò)程中具有適應(yīng)性強(qiáng)、膜污染程度低、避免發(fā)泡與快速脫氨等多方面的優(yōu)勢(shì)。在沼液處理與農(nóng)業(yè)廢棄物資源回收中具有廣闊發(fā)展前景。為此,該研究從介紹膜蒸餾的基本原理出發(fā),就膜蒸餾處理沼液過(guò)程中最核心的氨氮與水分回收部分進(jìn)行詳細(xì)的綜述,并針對(duì)沼液處理過(guò)程中的營(yíng)養(yǎng)物質(zhì)回收與減量化處理進(jìn)行了綜合分析,最后對(duì)膜蒸餾用于沼氣工程中的可行性進(jìn)行簡(jiǎn)要計(jì)算。相比于其他沼液處理技術(shù),膜蒸餾可在低成本與低碳足跡下實(shí)現(xiàn)沼液的資源回收與減量化處理,其處理沼液的成本與反滲透過(guò)程基本一致。在無(wú)外部能源供給的沼氣工程中,膜蒸餾更適用于高有機(jī)負(fù)荷沼液處理,或?qū)Ψ礉B透后剩余的高濃度沼液進(jìn)行處理。

        沼氣;膜;蒸餾;沼氣工程;沼液;資源回收;水回收

        0 引 言

        畜禽養(yǎng)殖污染物排放是導(dǎo)致農(nóng)業(yè)污染的重要源頭,亟需構(gòu)建與完善畜禽養(yǎng)殖污染物處理與資源化策略,推行先進(jìn)與可持續(xù)化的污染物處理技術(shù)[1]。在畜禽糞污處理主流技術(shù)中,厭氧發(fā)酵可同時(shí)處理固態(tài)糞污與液態(tài)廢水,殺滅病菌并將糞污轉(zhuǎn)化為沼氣與富含營(yíng)養(yǎng)物質(zhì)的沼肥,實(shí)現(xiàn)畜禽糞污處理的無(wú)害化、能源化與肥料化[2]。沼氣可為畜禽養(yǎng)殖場(chǎng)提供可再生能源,沼肥的還田利用可以實(shí)現(xiàn)營(yíng)養(yǎng)物質(zhì)循環(huán)并增加農(nóng)作物產(chǎn)量,促進(jìn)農(nóng)業(yè)循環(huán)經(jīng)濟(jì)的發(fā)展[3]。因此,厭氧發(fā)酵是畜禽糞污資源化處理的重要手段,但濕法厭氧發(fā)酵后沼液可占沼肥總質(zhì)量的80%以上,在農(nóng)田土地承載量和運(yùn)輸成本的雙重約束條件下,沼液很難通過(guò)還田利用的方式消納[4]?,F(xiàn)階段,沼液的安全高效處理與資源化回收已然成為限制糞污厭氧處理與沼氣工程發(fā)展的主要因素。因此,亟需開(kāi)發(fā)沼液綜合處理與資源化利用技術(shù)。

        沼液的理化特性與厭氧發(fā)酵原料及發(fā)酵濃度密切相關(guān),沼液含水率高達(dá)95%以上[5],pH值呈弱堿性(7.5~9.0)。沼液中總氮濃度約為500~5 000 mg/L,且主要以銨態(tài)氮(NH3和NH4+)形式存在,沼液中還富含磷元素(約20~300 mg/L)和鉀元素(500~3 000 mg/L)[6]。沼液的主要成分決定了其不適合采用高成本的市政污水處理技術(shù),更適合高附加值的開(kāi)發(fā)性處理,如果能對(duì)其中的主要營(yíng)養(yǎng)元素(N、P、K等)進(jìn)行合理的高附加值開(kāi)發(fā),可有效彌補(bǔ)沼液處理中的成本投入[7]?,F(xiàn)有的沼液資源化技術(shù)主要有化學(xué)沉淀法、微藻養(yǎng)殖、膜分離技術(shù)等[3]。其中,膜分離技術(shù)因?yàn)槠洳僮黛`活、設(shè)備簡(jiǎn)單且可用于多種資源的高效回收,備受關(guān)注[8-9]。膜蒸餾作為膜分離技術(shù)的一種,由于其分離過(guò)程中僅允許蒸氣透過(guò)膜,因此在處理高有機(jī)物濃度的沼液中具備適應(yīng)性強(qiáng)、膜污染程度低、避免發(fā)泡及快速脫氨等多重優(yōu)勢(shì)[10]。

        沼液中能夠以蒸氣形式分離的主要成分為H2O和NH3,其次還有少量溶解性氣體和揮發(fā)性有機(jī)物[11]。若能實(shí)現(xiàn)沼液中H2O和NH3的高效分離回收,即可實(shí)現(xiàn)沼液的養(yǎng)分回收和減量化處理等目標(biāo)[12]。僅從沼液中回收H2O而氨氮留在沼液中,可實(shí)現(xiàn)沼液濃縮和水分回收,而僅從沼液中分離NH3也能實(shí)現(xiàn)沼液中氮素回收和降低沼液施用帶來(lái)的環(huán)境風(fēng)險(xiǎn)[13]。此外,從沼液中回收NH3還可用于沼氣提純或原料預(yù)處理,提升氨氮附加值。為此,本文以促進(jìn)沼液的循環(huán)利用、降低農(nóng)業(yè)面源污染為導(dǎo)向,首先介紹了膜蒸餾的基本理論,隨后綜述了多種膜蒸餾過(guò)程處理沼液的研究進(jìn)展,重點(diǎn)關(guān)注了沼液中NH3與H2O的分離與回收,文章還綜合分析了膜蒸餾用于沼氣工程中的可行性。最后,本文對(duì)不同沼液處理方法的優(yōu)缺點(diǎn)及成本進(jìn)行了總結(jié)。

        1 膜蒸餾的基本理論

        1.1 膜蒸餾的基本形式及熱質(zhì)傳遞機(jī)理

        膜蒸餾是一種熱驅(qū)動(dòng)型的膜分離過(guò)程,進(jìn)料液和滲透?jìng)?cè)的溫度差導(dǎo)致了膜兩側(cè)的蒸氣分壓差,在蒸氣分壓差的驅(qū)動(dòng)下,NH3和H2O等可揮發(fā)成分以氣體分子的形式從進(jìn)料側(cè)透過(guò)膜孔傳質(zhì)到滲透?jìng)?cè),溶液中不可揮發(fā)性成分被截留在進(jìn)料側(cè),從而實(shí)現(xiàn)進(jìn)料溶液的濃縮及相關(guān)資源的回收[13]。以水的傳質(zhì)為例,可將膜蒸餾傳質(zhì)簡(jiǎn)化為三個(gè)部分:首先,進(jìn)料側(cè)形成的水蒸氣從進(jìn)料邊界層傳遞到達(dá)膜表面;隨后,水蒸氣從膜表面以自由擴(kuò)散的形式穿過(guò)膜孔傳遞到達(dá)滲透?jìng)?cè);最后,穿過(guò)滲透?jìng)?cè)邊界層進(jìn)入滲透?jìng)?cè)主體。因此,水蒸汽的傳質(zhì)阻力主要包含進(jìn)料側(cè)傳質(zhì)阻力、膜傳質(zhì)阻力和滲透?jìng)?cè)傳質(zhì)阻力,它們之間對(duì)傳質(zhì)阻礙的關(guān)系與電阻串聯(lián)關(guān)系類似[14]。膜蒸餾的傳熱過(guò)程與傳質(zhì)過(guò)程類似,也分為三個(gè)部分:進(jìn)料側(cè)、膜和滲透?jìng)?cè)。其中,膜內(nèi)傳熱主要依靠多孔膜自身的熱傳導(dǎo)和蒸汽的汽化潛熱。在一個(gè)穩(wěn)定的狀態(tài)下,總傳熱通量和三個(gè)部分的傳熱通量相等[15]。

        1.2 膜材料特性對(duì)膜蒸餾的影響

        膜蒸餾過(guò)程中主要采用疏水膜,用于防止進(jìn)料側(cè)溶液直接滲透進(jìn)入滲透?jìng)?cè)。常用的有機(jī)膜有聚丙烯(Polypropylene,PP)、聚二甲基硅氧烷(Polydimethylsiloxane,PDMS)、聚偏氟乙烯(Polyvinylidene Fluoride,PVDF)和聚四氟乙烯(Polytetrafluoroethylene,PTFE)微孔膜,膜孔徑一般在0.1~1.0m,孔隙率30%至80%[16]。常見(jiàn)膜的結(jié)構(gòu)形式有平板膜、中空纖維膜、毛細(xì)膜及管狀膜等,可根據(jù)不同的應(yīng)用場(chǎng)景進(jìn)行選擇。一般情況下,膜孔越大越有利于減少膜內(nèi)傳質(zhì)阻力和提升傳質(zhì)系數(shù),但膜孔過(guò)大更易導(dǎo)致膜孔潤(rùn)濕,進(jìn)而導(dǎo)致膜蒸餾過(guò)程失敗,因此應(yīng)用中的膜孔徑一般不大于0.5m[14]??紫堵试礁吆湍ぴ奖?,膜傳質(zhì)阻力越低,但厚度降低,膜的機(jī)械性能也會(huì)減弱。在針對(duì)尿液的直接接觸膜蒸餾過(guò)程中,進(jìn)料液和滲透?jìng)?cè)溫度分別為70 ℃和20 ℃條件下,采用厚度為89m的PTFE膜的總通量可比厚度為140m的PVDF膜高1.74倍[17]。

        除有機(jī)膜被廣泛應(yīng)用于膜蒸餾外,無(wú)機(jī)陶瓷膜經(jīng)過(guò)疏水改性后也可用于膜蒸餾,與有機(jī)膜相比,陶瓷膜材料具備更好的耐高溫和耐機(jī)械沖擊等優(yōu)良性能,但其在組件總體積方面沒(méi)有優(yōu)勢(shì)[18]。為降低膜污染和膜潤(rùn)濕的風(fēng)險(xiǎn),研究人員在膜材料改性方面有大量的研究工作,通過(guò)對(duì)膜表面進(jìn)行改性,提高其疏水性能,增強(qiáng)抗污染性能并提升過(guò)程的傳質(zhì)通量[19]。

        膜結(jié)構(gòu)失穩(wěn)主要包括膜孔徑尺寸的變化、膜孔潤(rùn)濕、膜孔堵塞、膜表面污染等[20]。膜孔徑變化主要是由于機(jī)械沖擊或膜材料性質(zhì)本身發(fā)生變化等造成的。膜孔潤(rùn)濕則是由于溶液的表面張力降低或者液相壓力差超過(guò)膜的最大浸潤(rùn)壓力。膜孔堵塞和膜表面污染均是由于污染物導(dǎo)致。膜污染可以通過(guò)一定的物理或化學(xué)方法進(jìn)行清洗,進(jìn)而實(shí)現(xiàn)膜通量的恢復(fù)[21]。膜蒸餾處理沼液過(guò)程中最常見(jiàn)的是有機(jī)物和無(wú)機(jī)物共同作用形成的污染層,此外還有少量的微生物參與和強(qiáng)化該污染層的形成[22-23]。

        1.3 操作參數(shù)對(duì)膜蒸餾性能的影響

        根據(jù)膜蒸餾基本原理,溫度差導(dǎo)致的蒸氣分壓差是膜蒸餾過(guò)程的主要驅(qū)動(dòng)力。進(jìn)料溫度直接影響被分離物質(zhì)的蒸氣分壓[14],根據(jù)安托尼方程,蒸氣分壓與進(jìn)料溫度呈指數(shù)曲線關(guān)系,因此,在滲透?jìng)?cè)溫度和壓力不變時(shí),提升進(jìn)料溫度,傳質(zhì)通量呈指數(shù)曲線上升[24]。相比之下進(jìn)料速率對(duì)整體傳質(zhì)效果影響較小,但增加進(jìn)料速率可降低流體邊界層的厚度進(jìn)而降低邊界層傳質(zhì)阻力,有利于提升膜通量[25]。采用直接接觸膜蒸餾處理沼液的研究發(fā)現(xiàn),跨膜溫差由20 ℃增加到60 ℃過(guò)程中,傳質(zhì)通量呈指數(shù)型增加,但流速為0.18和0.27 m/s下通量差異不顯著[26]。采用減壓膜蒸餾從沼液中進(jìn)行氨氮分離的研究發(fā)現(xiàn),沼液流量對(duì)氨通量和水通量的影響均呈現(xiàn)出顯著的線性關(guān)系[24]。

        1.4 極化現(xiàn)象

        極化現(xiàn)象是由于在進(jìn)料邊界層發(fā)生蒸發(fā)現(xiàn)象而導(dǎo)致邊界層側(cè)溫度降低和濃度增加,進(jìn)而出現(xiàn)和進(jìn)料側(cè)主體溫度和濃度不一致的情況,極化現(xiàn)象包括溫度極化和濃度極化,且兩種極化現(xiàn)象可同時(shí)發(fā)生[27]。其中溫度極化是指液相邊界層的溫度低于液相主體溫度,進(jìn)而阻礙傳熱和傳質(zhì)。濃度極化是指由于水分的蒸發(fā),導(dǎo)致進(jìn)料邊界層鹽分的濃度增加,而導(dǎo)致蒸汽分壓降低而減少傳質(zhì)的現(xiàn)象。通過(guò)在液相增加擾動(dòng)或降低傳質(zhì)通量等措施可緩解極化現(xiàn)象的發(fā)生[28]。雖然關(guān)于沼液膜蒸餾過(guò)程中降低極化現(xiàn)象的報(bào)道較少,但利用極化現(xiàn)象對(duì)促進(jìn)沼液膜蒸餾處理和氨氮回收有利。減壓膜蒸餾處理沼液過(guò)程中,可通過(guò)利用邊界層形成的pH極化現(xiàn)象來(lái)促進(jìn)氨氮回收,減少沼液pH值調(diào)節(jié)的化學(xué)品消耗,大幅降低沼液處理成本[29-30]。

        2 膜蒸餾回收沼液NH3與H2O研究進(jìn)展

        膜蒸餾處理沼液的基本形式如圖1所示,其基本原理為:加熱后沼液的揮發(fā)性組分通過(guò)膜孔自由擴(kuò)散到滲透?jìng)?cè),餾出物在滲透?jìng)?cè)冷凝并收集,實(shí)現(xiàn)沼液的資源回收與減量化處理[12]。沼液中的揮發(fā)性組分主要包括水分、氨氮(200~5 000 mg/L)[31]以及揮發(fā)酸等低含量的揮發(fā)性有機(jī)物等[11]。因此,根據(jù)從沼液中回收組分的不同,所采用膜蒸餾的形式也存在差異[32]。根據(jù)滲透?jìng)?cè)組分收集形式的不同,膜蒸餾可分為直接接觸膜蒸餾(Direct Contact Membrane Distillation,DCMD)、氣隙式膜蒸餾(Air Gap Membrane Distillation,AGMD)、氣掃式膜蒸餾(Sweep Gas Membrane Distillation,SGMD)與減壓膜蒸餾(Vacuum Membrane Distillation,VMD)(圖1)[33]。其中,相比于其他膜蒸餾過(guò)程,DCMD不需要額外的冷凝裝置,在沼液處理的試驗(yàn)初期研究中使用較多[18,34]。

        2.1 膜蒸餾回收沼液中的水

        膜蒸餾可從沼液中直接回收獲得水分,而其余大部分物質(zhì),如有機(jī)物、氮、磷、鉀等營(yíng)養(yǎng)元素可被疏水膜截留在濃縮沼液中,實(shí)際應(yīng)用和試驗(yàn)研究均重點(diǎn)關(guān)注沼液膜蒸餾過(guò)程中的水質(zhì)和水通量。對(duì)從沼液中獲得的回收水的水質(zhì),主要影響因素為沼液中揮發(fā)性有機(jī)物的成分與含量、沼液的pH值等。而對(duì)沼液水分回收通量影響較大的因素有:進(jìn)料溫度、進(jìn)料速度、膜污染及膜材料特性等。

        將沼液pH值調(diào)節(jié)至酸性是最常用的提高回收水分水質(zhì)的方法,其可最大限度的將氨氮在內(nèi)的營(yíng)養(yǎng)物質(zhì)截留濃縮在進(jìn)料側(cè),從而有利于營(yíng)養(yǎng)物質(zhì)的回收。Yan等[23]的研究結(jié)果表明,將沼液從pH值為8.5酸化至pH值為5.0可使氨氮截留率從66%增加至99%。酸性條件下沼液膜蒸餾滲透?jìng)?cè)回收水的電導(dǎo)率可保持在100S/cm以下,另外,酸性條件下可大幅降低水回收過(guò)程中的膜污染[35],降低鈣、鎂、硅等無(wú)機(jī)元素在膜表面或膜孔內(nèi)的沉積,從而減少膜污染[22]。因此,將沼液酸化不僅可以提升對(duì)營(yíng)養(yǎng)元素的截留率,同時(shí)能夠降低膜污染程度,是目前可以用于沼液減量化處理且同時(shí)回收營(yíng)養(yǎng)元素和水分的重要途徑。但Kim 等[26]的研究發(fā)現(xiàn),DCMD運(yùn)行72 h后,由于膜表面污染物的形成,其對(duì)總氮(Total Nitrogen,TN)的截留率在pH值為8.5時(shí)可達(dá)90%以上,遠(yuǎn)高于理論值65%。該研究結(jié)果與超濾(Ultrafiltration,UF)處理湖水時(shí),隨著膜污染程度的增加而氨的截留率也相應(yīng)增加的規(guī)律一致[36]。其主要原因在于膜污染的形成,既阻礙了水分的傳質(zhì)也阻礙了氨氮的傳質(zhì)。說(shuō)明膜蒸餾過(guò)程中污染物的形成在一定程度上可提升氨氮的回收性能,但其過(guò)程機(jī)理和實(shí)際運(yùn)行可行性還有待深入研究。

        增加跨膜溫差、增加流速、采用低傳質(zhì)阻力的膜并保持膜結(jié)構(gòu)特性的穩(wěn)定是獲得沼液膜蒸餾過(guò)程中高水分回收通量的主要措施。當(dāng)跨膜溫差在50 ℃,采用厚度89m、膜孔0.45m和孔隙率為73%的PTFE平板膜處理尿液時(shí),水回收通量可高達(dá)65.84 L/m2·h[17]。通過(guò)對(duì)比DCMD、VMD和SGMD三種結(jié)構(gòu)形式的膜蒸餾發(fā)現(xiàn),VMD具有最大的跨膜驅(qū)動(dòng)力,因此能夠獲得最大的膜通量,而DCMD獲得的膜通量最小[37]。沼液本身的特性也會(huì)影響膜蒸餾回收水的通量,Jacob等[38]報(bào)道了DCMD處理沼液時(shí)滲透通量隨溫度和進(jìn)料流量的關(guān)系,同時(shí)闡明了沼液中有機(jī)物含量增加會(huì)大幅增加膜污染進(jìn)而降低滲透通量。

        工程上采用氣隙式膜蒸餾(AGMD)的方式處理廢水時(shí),滲透?jìng)?cè)設(shè)置有一層空氣間隙,以便于餾出物透過(guò)空氣間隙后在冷凝板上冷凝液化后進(jìn)行收集[37],相比較DCMD,AGMD過(guò)程可有效減少熱傳導(dǎo)帶來(lái)的能量損失。在沒(méi)有熱量循環(huán)利用的情況下,AGMD的熱能消耗為900~1 300 kW·h /m3,低于DCMD的熱能消耗(1 600~2 000 kW·h /m3)[23,39]。具備熱能回收利用的AGMD過(guò)程的熱能消耗可降低至66~170 kW·h /m3[39],用于沼液和接收液循環(huán)泵的電能消耗僅為1.22 kW·h /m3,遠(yuǎn)低于反滲透過(guò)程的3~4 kW·h /m3[23]。該過(guò)程既可實(shí)現(xiàn)對(duì)沼液中水分的回收,也能對(duì)氨氮進(jìn)行回收,同時(shí),該方法可降低接收液的使用[39]。雖然AGMD相比于DCMD有更多優(yōu)點(diǎn),但AGMD在沼液處理中的研究還較少。

        除膜蒸餾外,還可采用正滲透、反滲透、納濾和超濾等過(guò)程實(shí)現(xiàn)對(duì)沼液中水分的回收。但是,由于沼液富含氨氮,簡(jiǎn)單的超濾及納濾過(guò)程對(duì)氨氮截留效果不佳,需與生化反應(yīng)過(guò)程結(jié)合,形成膜生物反應(yīng)器[38]。本文總結(jié)了可直接用于沼液水分回收的三種膜分離過(guò)程的水回收通量(圖2)。顯然,采用壓力驅(qū)動(dòng)的反滲透過(guò)程具有最高的水回收通量,處理效率最高[40]。而采用滲透壓力驅(qū)動(dòng)的正滲透和采用熱驅(qū)動(dòng)的膜蒸餾的水回收通量均較低[41-42]。通過(guò)條件優(yōu)化與材料改性,膜蒸餾可獲得高達(dá)115 kg/m2·h的水回收通量[12]。正滲透和反滲透工藝均可實(shí)現(xiàn)對(duì)沼液濃縮至5倍左右,而膜蒸餾過(guò)程還可實(shí)現(xiàn)對(duì)反滲透濃縮液進(jìn)行處理和深度濃縮。因此,膜蒸餾具備單獨(dú)運(yùn)行并處理高濃度沼液的優(yōu)點(diǎn)。

        2.2 膜蒸餾分離沼液中的氨氮

        與酸性條件下將氨氮截留在濃縮液中不同,堿性條件下主要以氨氮回收為主。例如DCMD回收沼液氨氮過(guò)程中,一般調(diào)節(jié)沼液pH值至9~11,此時(shí)氨氮以自由氨的形式透過(guò)膜孔,被滲透?jìng)?cè)的酸液接收[16],該過(guò)程也被稱為透氣膜回收氨氮過(guò)程[43],或者液-液膜接觸器過(guò)程[44]。在沼液pH值呈中性偏堿性的條件下采用DCMD用于沼液氨氮脫除是目前最接近實(shí)際應(yīng)用的一項(xiàng)技術(shù)[45],主要?dú)w功于其穩(wěn)定性、高污染物耐受性、低操作成本與簡(jiǎn)單的操作流程,針對(duì)該過(guò)程已經(jīng)有多篇綜述從不同角度進(jìn)行了報(bào)道[16, 46]。

        膜蒸餾分離沼液氨氮的研究集中在沼液pH值提升、膜傳質(zhì)通量的穩(wěn)定、氨氮選擇性分離特性提高與接收液選取等四個(gè)方向。除直接向沼液中添加堿性物質(zhì)外[47],還可通過(guò)液相氣體吹脫[48]、施加真空[49]、耦合電化學(xué)過(guò)程等方式提升沼液pH值[50-51]。其中液相直接氣體吹脫和施加真空促進(jìn)自由氨的形成更加適合于沼液處理,可大幅降低堿性化學(xué)品的消耗。在DCMD長(zhǎng)期運(yùn)行后,由于膜表面受無(wú)機(jī)物結(jié)垢以及有機(jī)物污染的影響,導(dǎo)致膜表面疏水性和整體通量降低[10],可通過(guò)對(duì)沼液進(jìn)行過(guò)濾預(yù)處理結(jié)合受污染膜的清洗策略,穩(wěn)定膜過(guò)程的正常運(yùn)行[52]。除膜污染物控制外,膜材料的改性也被用于強(qiáng)化廢水氨氮回收,其中,通過(guò)在膜上固定功能性碳納米管等材料,可強(qiáng)化膜對(duì)氨氮的吸附作用,進(jìn)而增加氨氮傳質(zhì)通量和分離因子[53]。等溫膜蒸餾配合酸吸收過(guò)程也被用于發(fā)酵過(guò)程中沼液氨氮的原位脫除,進(jìn)而解除氨氮抑制[54-55]。水、碳酸、有機(jī)酸、無(wú)機(jī)強(qiáng)酸等用于氨氮接收均有報(bào)道[56-57],其中硫酸用于氨氮的接收液最為常見(jiàn)[58]。但是,目前報(bào)道的氨氮接收液僅能一次使用,而關(guān)于可循環(huán)利用的氨氮吸收液的報(bào)道還較少[59]。

        已有多種形式的膜蒸餾過(guò)程用于沼液氨氮回收。除最常見(jiàn)的DCMD用于沼液氨氮回收外,采用SGMD也可實(shí)現(xiàn)對(duì)沼液氨氮脫除[32]。相比于DCMD,SGMD對(duì)于氨氮分離的傳質(zhì)系數(shù)和分離因子均較低[37],但SGMD在回收沼液氨氮過(guò)程中,可采用類似于氣體吹脫氨氮脫除的結(jié)構(gòu),更加方便的用于厭氧發(fā)酵的中期和后期沼液氨氮回收[60]。SGMD處理沼液時(shí),必須配合吸收塔對(duì)尾氣進(jìn)行凈化,不如其他膜蒸餾過(guò)程精簡(jiǎn)。

        將沼液調(diào)節(jié)至堿性后,采用減壓膜蒸餾(VMD)的方式在膜的滲透?jìng)?cè)施加真空,可以獲得更高的氨氮傳質(zhì)系數(shù)[37],同時(shí)得到低濃度的可再生氨水[61-63]。相比于以氮肥形式回收氨氮,氨水具有更高的工業(yè)價(jià)值,如氨水是重要的化工原料[64],氨水可用于沼氣CO2分離[24,65-66],用于厭氧發(fā)酵原料預(yù)處理等[67]。膜蒸餾回收氨過(guò)程中,氨氮分離傳質(zhì)系數(shù)與水通量呈現(xiàn)出正向關(guān)系,有研究表明可采用等溫膜蒸餾的方式回收氨氮,實(shí)現(xiàn)降低水通量的同時(shí)進(jìn)而提升氨氮分離因子[57]。值得注意的是,目前關(guān)于沼液膜蒸餾過(guò)程中膜污染與膜結(jié)垢的研究大多基于直接接觸膜蒸餾進(jìn)行開(kāi)展,關(guān)于SGMD、AGMD和VMD過(guò)程中的穩(wěn)定性運(yùn)行研究還較少,其中,堿性沼液在膜蒸餾過(guò)程中的污染物沉積與結(jié)垢機(jī)理還需要深入解析[66]。

        圖3所示的膜分離過(guò)程對(duì)氨氮的回收率均高于80%,但由于氨氮分子量與水分子接近,采用壓力驅(qū)動(dòng)型膜分離技術(shù)不能實(shí)現(xiàn)對(duì)沼液氨氮達(dá)到100%截留。而在酸性條件下對(duì)沼液進(jìn)行膜蒸餾,由于氨氮以離子態(tài)形式存在而不會(huì)揮發(fā),因此可實(shí)現(xiàn)近100%的氨氮截留。

        2.3 膜蒸餾過(guò)程的能耗與成本

        沼液膜蒸餾過(guò)程中主要靠熱能驅(qū)動(dòng)揮發(fā)性組分的跨膜傳質(zhì),在單級(jí)膜蒸餾且不具備熱能回收的系統(tǒng)中,DCMD的熱能耗可高達(dá)2 000~3 500 kW·h/m3,VMD的熱能耗可高達(dá)1 100 kW·h/m3[68]。雖然膜蒸餾過(guò)程中的熱能耗可采用低品位熱源如沼氣發(fā)電機(jī)或沼氣鍋爐的余熱、太陽(yáng)能、甚至空氣源熱泵等提供[69-71],但其能源消耗量巨大,遠(yuǎn)高于多效蒸餾過(guò)程的熱能消耗(30~120 kW·h/m3)和反滲透過(guò)程的電能消耗(4~6 kW·h/m3)。為降低膜蒸餾的能源消耗,研究者開(kāi)發(fā)了多級(jí)膜蒸餾、多效膜蒸餾并配合余熱回收等技術(shù)來(lái)實(shí)現(xiàn)膜蒸餾過(guò)程中的能源多級(jí)循環(huán)利用(圖4)[72]。

        對(duì)于單級(jí)膜蒸餾的熱能回收利用,主要是針對(duì)濃縮液和滲透液回流對(duì)進(jìn)料液進(jìn)行加熱(圖4a)。當(dāng)采用熱能回收技術(shù)后,單級(jí)減壓膜蒸餾的熱能耗可降低至800 kW·h/m3左右[73]。多級(jí)與多效膜蒸餾的主要工作形式如圖4b和4c所示,多級(jí)膜蒸餾主要是在單級(jí)的基礎(chǔ)上串聯(lián)、并聯(lián)或者串聯(lián)并聯(lián)混合的方式組合多個(gè)膜組件進(jìn)行工程應(yīng)用[74]。多級(jí)膜蒸餾與多效膜蒸餾過(guò)程降低熱能耗的實(shí)質(zhì)均是熱能的多級(jí)利用,不同的是多級(jí)膜蒸餾主要通過(guò)外部換熱器對(duì)熱量進(jìn)行回收和再利用,而多效膜蒸餾是在膜組件內(nèi)部對(duì)熱量進(jìn)行直接回收和再利用[75-76]。一般情況下,多效膜蒸餾過(guò)程中膜組件的級(jí)數(shù)越多,熱能回用效率越高,進(jìn)而獲得更低的比熱能消耗。例如,多效減壓膜蒸餾淡化海水過(guò)程中,內(nèi)部膜組件級(jí)數(shù)為6時(shí),比熱能消耗為200 kW·h /m3[73]。另外,進(jìn)料液溫度、進(jìn)料流量、膜孔隙率和膜孔徑的增加均會(huì)降低膜蒸餾過(guò)程中的熱能消耗。多效減壓膜蒸餾過(guò)程中,當(dāng)采用廢熱作為膜蒸餾熱源時(shí),其產(chǎn)水成本可降低至0.59 $/m3,與反滲透過(guò)程的產(chǎn)水成本相當(dāng)(0.63 $/m3)[74,77]。

        3 膜蒸餾處理沼液過(guò)程中水分和能量平衡

        在具備熱電聯(lián)產(chǎn)的沼氣工程中,膜蒸餾可利用發(fā)電余熱對(duì)沼液進(jìn)行處理,可大幅降低膜蒸餾處理沼液的成本。以沼氣產(chǎn)量10 000 Nm3/d的沼氣工程為例,若采用如圖5所示的工藝流程圖對(duì)沼液進(jìn)行膜蒸餾處理,厭氧發(fā)酵中產(chǎn)生的沼氣視為100%的化學(xué)能,熱電聯(lián)產(chǎn)過(guò)程的發(fā)電效率為35%,產(chǎn)熱效率為45%。

        將所產(chǎn)熱能全部用于驅(qū)動(dòng)膜蒸餾處理沼液,并提供對(duì)應(yīng)的電能,則沼氣工程還能輸出占沼氣化學(xué)能33.45%的電能。因此,當(dāng)采用膜蒸餾處理沼液時(shí),在不影響沼氣工程對(duì)電能輸出的前提下,還能獲得循環(huán)用水。由于厭氧發(fā)酵原料和發(fā)酵濃度不同,所產(chǎn)生的沼液體積也存在差異[78]。根據(jù)不同原料的VS (Volatile Solids)產(chǎn)氣率,可計(jì)算不同原料和發(fā)酵濃度下圖5所示的工藝流程中水的回收率。根據(jù)第2節(jié)的分析,膜蒸餾過(guò)程的熱能耗假定為100 kW·h/m3,電能耗假定為1.22 kW·h/m3。沼液中水分回收率如圖6所示,當(dāng)原料TS(Total Solids)濃度為4%時(shí),采用豬糞、雞糞和牛糞為原料時(shí),沼液中僅有30%左右的水分可回收,而采用餐廚垃圾為原料時(shí),可實(shí)現(xiàn)50%至100%的水回收。因?yàn)椴蛷N垃圾的VS產(chǎn)氣率較高,相同TS濃度下達(dá)到同樣的沼氣產(chǎn)量所產(chǎn)生的沼液較少。水回收率隨發(fā)酵原料的TS濃度的增加而升高,當(dāng)發(fā)酵罐中TS濃度提升到8%時(shí),基本可實(shí)現(xiàn)60%以上的水分回收率,當(dāng)發(fā)酵TS濃度提升到10%則基本上可實(shí)現(xiàn)沼液中水分的全部回收。顯然,在無(wú)外部熱源供給的情況下,膜蒸餾適合于較高濃度下厭氧發(fā)酵后沼液的處理與水回收利用。另外,當(dāng)采用高濃度厭氧發(fā)酵的沼氣工程也更適合選用膜蒸餾作為處理沼液的主要技術(shù),因?yàn)樵诟邼舛劝l(fā)酵條件下,沼液中有機(jī)物濃度和鹽分均較高,不利于反滲透技術(shù)或其他生化處理技術(shù)的應(yīng)用。

        注:SM、CHM、CM、FW分別表示豬糞、雞糞、牛糞和餐廚垃圾,數(shù)字代表平行試驗(yàn)次數(shù)。

        4 不同沼液處理技術(shù)與成本對(duì)比

        現(xiàn)階段,可用于沼液處理的技術(shù)如表1所示,分為氣體吹脫、膜分離、化學(xué)沉淀和生化反應(yīng)處理等。其中,生化反應(yīng)處理沼液比較徹底,可實(shí)現(xiàn)沼液的達(dá)標(biāo)排放處理,如硝化-反硝化技術(shù)、厭氧氨氧化技術(shù)等,但生化反應(yīng)過(guò)程無(wú)法回收資源,且更加適合低濃度養(yǎng)殖廢水處理,對(duì)于高濃度沼液處理還需要進(jìn)行深入研究。化學(xué)沉淀法可方便用于營(yíng)養(yǎng)物質(zhì)回收,操作簡(jiǎn)單且對(duì)于高污染物的耐受性好,其用于沼液處理還需要和其他技術(shù)配合使用。氣體吹脫法是近期可用于沼液氮磷回收的重要手段,操作簡(jiǎn)單且適用于高污染物含量的沼液,但該技術(shù)不能實(shí)現(xiàn)沼液達(dá)標(biāo)排放。膜分離技術(shù)如超濾、反滲透等技術(shù)配合使用可從沼液中回收水分和營(yíng)養(yǎng)物質(zhì)豐富的有機(jī)肥,但膜容易受污染,反滲透后剩下的濃縮沼液體積大,還需進(jìn)一步處理。膜蒸餾是膜分離技術(shù)的一種,可直接用于沼液氨氮回收、沼液水回收,也可用于反滲透后剩下的濃縮沼液處理,其對(duì)于污染物的耐受性要顯著高于壓力驅(qū)動(dòng)下的膜分離過(guò)程。因此,近年來(lái)膜蒸餾技術(shù)用于沼液處理的研究也日益增加[12]。

        表1 不同沼液處理技術(shù)對(duì)比

        5 結(jié)論與展望

        本文通過(guò)對(duì)膜蒸餾技術(shù)處理沼液的基本原理、過(guò)程、系統(tǒng)構(gòu)建、經(jīng)濟(jì)分析等方面的研究進(jìn)展進(jìn)行了總結(jié),主要結(jié)論如下:1)相比于其他沼液處理技術(shù),僅采用膜蒸餾技術(shù)可實(shí)現(xiàn)對(duì)沼液中營(yíng)養(yǎng)物質(zhì)和水分的回收,進(jìn)而實(shí)現(xiàn)沼液資源化與減量化處理,膜蒸餾在處理不同濃度的沼液時(shí)適應(yīng)性強(qiáng),在簡(jiǎn)化操作過(guò)程方面具備優(yōu)勢(shì)。2)沼液資源化處理時(shí),可采用膜蒸餾技術(shù)直接從沼液中提取氨氮等營(yíng)養(yǎng)元素,也可采用膜濃縮的方式實(shí)現(xiàn)資源回收。3)在利用余熱驅(qū)動(dòng)水回收時(shí),膜蒸餾與反滲透處理沼液的成本基本一致,且膜蒸餾處理沼液時(shí)具有更高的水分回收率。4)膜蒸餾可在低品位熱源驅(qū)動(dòng)下實(shí)現(xiàn)沼液氨氮回收和減量化處理,降低沼液處理的碳足跡。

        現(xiàn)階段膜蒸餾處理沼液的研究主要集中在操作性能優(yōu)化、膜污染特性以及與其他膜過(guò)程結(jié)合應(yīng)用研究。但因?yàn)槠錈崮芎妮^高,運(yùn)行通量較低等因素,導(dǎo)致其中試研究和工程化應(yīng)用還較少。未來(lái)需要在以下幾個(gè)方面加強(qiáng)研究:1)開(kāi)展膜蒸餾處理沼液的中試研究,并在長(zhǎng)期運(yùn)行過(guò)程中積累實(shí)踐數(shù)據(jù),用于指導(dǎo)沼液膜蒸餾處理的基礎(chǔ)研究和工程實(shí)踐。2)開(kāi)發(fā)專門針對(duì)高有機(jī)物含量沼液處理的膜材料,提升過(guò)程的穩(wěn)定性并減少膜污染。3)氨氮作為沼液中可回收的重要資源,針對(duì)氨氮回收的高性能膜材料尚需進(jìn)一步開(kāi)發(fā);此外,可循環(huán)使用的沼液氨氮接收液還有待篩選。4)基于余熱或可再生能源驅(qū)動(dòng)型的低成本沼液膜蒸餾技術(shù)值得深入研究;采用多效膜蒸餾過(guò)程處理沼液的報(bào)道還較少,多效膜蒸餾過(guò)程對(duì)沼液營(yíng)養(yǎng)物質(zhì)及水分的回收特性還有待開(kāi)展。5)不同形式的膜蒸餾過(guò)程在處理沼液時(shí)各有優(yōu)缺點(diǎn),將不同形式的膜蒸餾過(guò)程組合使用進(jìn)行沼液處理值得進(jìn)一步研究。6)基于膜蒸餾技術(shù)構(gòu)建沼液處理中資源回收與溫室氣體減排系統(tǒng),對(duì)減少農(nóng)業(yè)源溫室氣體排放有重要意義和現(xiàn)實(shí)價(jià)值,值得深入研究。

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        Research progress of biogas slurry resourceful treatment by membrane distillation

        He Qingyao, Shi Mingfei, Feng Liang, Ai Ping, Yuan Qiaoxia, Yan Shuiping※

        (1.,,430070,;2.-,,430070,)

        Biogas slurry can account for more than 80% of the total mass of anaerobic digestates in biogas production. A large amount of biogas slurry has posed a great challenge on the carrying capacity of farmland and transportation cost. Particularly, returning to the farmland cannot completely consume such a great amount of incurred biogas slurry in a large-scale plant. The resourceful treatment is widely expected to reduce the volume of biogas slurry, and the potential threat to the agro-ecological environment for high value-added resource recovery in the sustainable development of the agricultural circular economy. For instance, membrane distillation serves as an important branch of membrane separation available for the resourceful treatment of biogas slurry in recent years. Excellent performance of membrane distillation has been achieved, including strong adaptability, rapid ammonia removal, as well as less membrane fouling and foaming. However, the high heat consumption and low flux have confined to the more efficient application of membrane distillation, compared with other technologies of membrane separation. In this study, a special process of membrane distillation was firstly introduced to systematically review the ammonia nitrogen and water recovery from biogas slurry. Water can normally be recovered from the acidified biogas slurry, while the nutrients were retained, including nitrogen, phosphorus, and potassium in the concentration phase. The water recovery can also be promoted, because the acidified biogas slurry can be utilized to suppress the ammonia volatilization, while relieving the membrane fouling. Typical reverse and forward osmosis concentrated the biogas slurry up to about 5 times than before, meaning that about 20% concentrated biogas slurry was left. The thermal-driven membrane distillation can even be used for the resourceful treatment of concentrated biogas slurry after reverse osmosis, where little biogas slurry was left. Nevertheless, membrane distillation presented a relatively low water flux for water recovery, compared with the typical reverse osmosis. Conversely, ammonia can be recovered from the biogas slurry, and then serve as ammonium fertilizer or aqueous ammonia solution for CO2absorption. Consequently, the resulting biogas slurry was more suitable for agricultural utilization after ammonia removal. To date, membrane distillation behaved the highest ammonia recovery ratio of about 99%, compared with the reverse and forward osmosis. Meanwhile, the membrane used for ammonia recovery was a benefit to control the greenhouse gas emission. In addition, the multi-stage and multi-effect membrane distillation was introduced to reduce heat consumption. The reason is that the huge heat consumption can inevitably result in the high operation cost for the treatment of biogas slurry in a single membrane distillation. The heat consumption for water recovery was reduced from 2 000-3 500 kW·h/m3to 100-200 kW·h/m3. Finally, the feasibility of membrane distillation was briefly evaluated for the biogas slurry treatment in a large-scale plant. The treatment cost of biogas slurry can even be much lower than that of a typical pressure-derived membrane process, where the heat and power were used from the Combined Heat and Power (CHP) in a biogas plant. Membrane distillation can efficiently realize resource recovery of biogas slurry in a facile, cost-saving, and environment-friendly way. Specifically, the cost of membrane distillation for biogas slurry was basically consistent with that of reverse osmosis. Consequently, membrane distillation was suitable for the treatment of high organic load or high residual concentration of biogas slurry after reverse osmosis, without any supplement of external heat source in a biogas plant.

        biogas; membrane; distillation; biogas plant; biogas slurry; resources recovery; water recovery

        賀清堯,石明菲,馮椋,等. 基于膜蒸餾的沼液資源化處理研究進(jìn)展[J]. 農(nóng)業(yè)工程學(xué)報(bào),2021,37(8):259-268.doi:10.11975/j.issn.1002-6819.2021.08.030 http://www.tcsae.org

        He Qingyao, Shi Mingfei, Feng Liang, et al. Research progress of biogas slurry resourceful treatment by membrane distillation[J]. Transactions of the Chinese Society of Agricultural Engineering (Transactions of the CSAE), 2021, 37(8): 259-268. (in Chinese with English abstract) doi:10.11975/j.issn.1002-6819.2021.08.030 http://www.tcsae.org

        2020-12-10

        2021-03-20

        國(guó)家自然科學(xué)基金(32002222,52076101);湖北省自然科學(xué)基金(2020CFB209,2020CFA107);中央高?;緲I(yè)務(wù)經(jīng)費(fèi)(2662018QD028,2662018PY046)

        賀清堯,講師,研究方向?yàn)榛谀し蛛x技術(shù)的農(nóng)業(yè)資源回收與溫室氣體控制。Email:qingyao.he@mail.hzau.edu.cn

        晏水平,教授,博士生導(dǎo)師,研究方向?yàn)榈湍芎腃O2化學(xué)吸收技術(shù)、沼氣提純技術(shù)與裝備、CO2植物與土壤固定技術(shù)。Email:yanshp@mail.hzau.edu.cn

        10.11975/j.issn.1002-6819.2021.08.030

        S216.4

        A

        1002-6819(2021)-08-0259-10

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