王志偉 戴若彬 張星冉 文越 陳妹 李佳藝
摘 要:在環(huán)境功能質(zhì)量提升和碳達(dá)峰、碳中和背景下,膜法污水處理發(fā)展面臨高標(biāo)準(zhǔn)和綠色低耗處理的雙重挑戰(zhàn)。如何突破現(xiàn)有膜技術(shù)水通量與污染物去除效率瓶頸,降低膜技術(shù)/工藝能耗與碳排放,支撐膜法污水處理技術(shù)未來可持續(xù)發(fā)展,是亟需關(guān)注和思考的問題。近年來,膜法污水處理在膜分離過程原理、抗污染膜材料制備、膜工藝創(chuàng)新及工程應(yīng)用等方面取得了長(zhǎng)足進(jìn)步,并逐步向材料設(shè)計(jì)精準(zhǔn)化、膜界面功能化、工藝綠色化的方向發(fā)展。圍繞高性能膜材料制備、膜技術(shù)/工藝的功能拓展、膜的可持續(xù)利用以及膜工藝綠色低耗發(fā)展等4個(gè)方面綜述了相關(guān)研究進(jìn)展,并對(duì)膜法污水處理技術(shù)可持續(xù)發(fā)展進(jìn)行了總結(jié)和展望。在未來多目標(biāo)需求背景下,膜法污水處理技術(shù)應(yīng)向多功能拓展、選擇性分離、定制化分離與綠色化發(fā)展等方向邁進(jìn),以實(shí)現(xiàn)膜法污水處理技術(shù)的可持續(xù)發(fā)展。
關(guān)鍵詞:污水處理;高標(biāo)準(zhǔn)處理;可持續(xù)發(fā)展;膜材料;功能拓展
中圖分類號(hào):TU703.1?? 文獻(xiàn)標(biāo)志碼:A?? 文章編號(hào):2096-6717(2022)03-0086-18
收稿日期:2021-07-05
基金項(xiàng)目:國(guó)家重點(diǎn)研發(fā)計(jì)劃(2019YFC0408200);國(guó)家自然科學(xué)基金(51838009)
作者簡(jiǎn)介:王志偉(1980- ),男,博士,教授,博士生導(dǎo)師,主要從事膜法污水處理與資源化研究,E-mail:zwwang@#edu.cn。
Received:2021-07-05
Foundation items:National Key R & D Program of China (No. 2019YFC0408200); National Natural Science Foundation of China (No. 51838009)
Author brief:WANG Zhiwei (1980- ), PhD, professor, doctorial supervisor, main research interests: membrane-based wastewater treatment and resource recovery, E-mail: zwwang@#edu.cn.
Recent advances and overview on sustainable development of membrane-based wastewater treatment technology
WANG Zhiweia,b, DAI Ruobina, ZHANG Xingrana, WEN Yuea, CHEN Meia, LI Jiayia
(a. School of Environmental Science and Engineering; b. Advanced Membrane Technology Center, Tongji University, Shanghai 200092, P. R. China)
Abstract: The development of membrane-based wastewater treatment technology faces two challenges, i.e.:(i) improvement in the environment quality requires high-standard treatment, and (ii) carbon emission peak and carbon neutrality calls for green and sustainable techniques. Under such circumstances, methods to break the bottleneck of water permeability and pollutant removal efficiency trade-off, to reduce the energy consumption and carbon emission of membrane technology, and to stimulate the sustainable development of membrane-based wastewater treatment are of great significance.Recently, remarkable progress in membrane-based wastewater treatment has been achieved in terms of membrane separation fundamentals, antifouling membrane preparation, membrane process innovations, and engineering applications. Current efforts have been dedicated to precise design of membrane materials, functionalization of membrane interface, and process greening transformation. In this work, recent research progress has been reviewed in terms of functional extension of membrane technology/process, design of high-performance membrane materials, sustainable utilization of membranes, and green development of membrane processes. Further analysis and discussion on future development are also presented.In the context of future multi-target needs, multi-functionalization, selective and customized separation, and green development are worthy of in-depth research for promoting the sustainable evolution of membrane-based wastewater treatment technology.
Keywords:wastewater treatment; high-standard treatment; sustainable development; membrane materials; functional extension
污水處理與再生利用是全球應(yīng)對(duì)水資源危機(jī)、控制水體污染的重要途徑[1-3]。膜法污水處理技術(shù)由于具有出水水質(zhì)好、固液分離效率高、占地面積小等優(yōu)點(diǎn)[4-6],已成為污水處理與再生利用的主流技術(shù)之一[7-8],得到了廣泛研究與關(guān)注。據(jù)《中國(guó)水處理行業(yè)可持續(xù)發(fā)展戰(zhàn)略研究報(bào)告》,中國(guó)膜產(chǎn)業(yè)以每年20%以上的增長(zhǎng)率快速發(fā)展。預(yù)計(jì)2021年中國(guó)膜工業(yè)產(chǎn)值將達(dá)到3 300億元以上,帶動(dòng)相關(guān)膜技術(shù)工程總投資逾1萬億元。
當(dāng)前,水環(huán)境功能質(zhì)量提升的需求驅(qū)動(dòng)著高標(biāo)準(zhǔn)處理技術(shù)的持續(xù)應(yīng)用,為膜法污水處理技術(shù)帶來了快速發(fā)展機(jī)遇。但另一方面,碳達(dá)峰、碳中和導(dǎo)向下,污水深度處理與再生利用領(lǐng)域“呼喚”綠色低碳技術(shù),給膜法污水處理技術(shù)帶來了嚴(yán)峻挑戰(zhàn)。高標(biāo)準(zhǔn)處理與碳達(dá)峰、碳中和的交織倒逼污水處理技術(shù)創(chuàng)新與綠色發(fā)展。因而,如何在高標(biāo)準(zhǔn)、綠色低耗的要求下實(shí)現(xiàn)膜法污水處理關(guān)鍵理論與技術(shù)創(chuàng)新,支撐膜法污水處理技術(shù)未來可持續(xù)發(fā)展,是膜技術(shù)領(lǐng)域亟需關(guān)注和思考的問題。
本文基于過去10余年膜法污水處理技術(shù)研發(fā)與應(yīng)用進(jìn)展,圍繞高性能膜材料制備、膜技術(shù)/工藝的功能拓展、膜的可持續(xù)利用以及膜工藝創(chuàng)新發(fā)展等4個(gè)方面綜述了膜法污水處理技術(shù)相關(guān)研究與應(yīng)用動(dòng)態(tài)。在此基礎(chǔ)上對(duì)未來膜法污水處理技術(shù)發(fā)展趨勢(shì)與重點(diǎn)方向進(jìn)行分析和研判,提出膜法污水處理技術(shù)需要攻關(guān)的重點(diǎn)方向,以推動(dòng)膜法污水處理技術(shù)的可持續(xù)發(fā)展。
1 膜法污水處理技術(shù)相關(guān)研究與應(yīng)用動(dòng)態(tài)
在過去10余年,膜法污水處理技術(shù)得到了快速發(fā)展和應(yīng)用。一方面,膜法污水處理技術(shù)研究呈現(xiàn)蓬勃發(fā)展態(tài)勢(shì)。膜法污水處理技術(shù)集中在膜基礎(chǔ)理論、新型膜材料、膜污染及其控制、膜清洗、膜工藝等方面開展了大量研究(圖1),同時(shí),呈現(xiàn)出與材料、化學(xué)、
生物等學(xué)科深度交叉融合的特征,創(chuàng)新性成果不斷涌現(xiàn)。另一方面,膜法污水處理技術(shù)得到了快速規(guī)?;こ虘?yīng)用。據(jù)不完全統(tǒng)計(jì)[9],目前,中國(guó)膜生物反應(yīng)器污水處理工程(≥1萬m3/d規(guī)模工程)累計(jì)處理規(guī)模超過2 000萬m3/d。據(jù)《中國(guó)水處理行業(yè)可持續(xù)發(fā)展戰(zhàn)略研究報(bào)告》估算,工業(yè)廢水膜法處理工程累計(jì)處理規(guī)模逾3 600萬m3/d。中國(guó)一躍成為膜法污水處理技術(shù)研究最為活躍、應(yīng)用市場(chǎng)最為廣闊的國(guó)家。膜法污水處理技術(shù)在市政污水處理、工業(yè)廢水處理等領(lǐng)域扮演著重要角色。
1.1 高性能膜材料制備
膜材料是膜法污水處理工藝的核心,對(duì)于污水處理效果、工藝經(jīng)濟(jì)性能等具有重要影響。對(duì)于微濾和超濾(MF、UF)膜而言,抗污染性能優(yōu)越、壽命長(zhǎng)的膜材料是研究的重點(diǎn)。尤其是在膜制備工藝相對(duì)成熟、膜機(jī)械強(qiáng)度得到有效解決的情況下,膜材料的抗有機(jī)污染、抗生物污染及其長(zhǎng)期運(yùn)行穩(wěn)定性是關(guān)注的焦點(diǎn)(圖2)。對(duì)于高壓納濾、反滲透(NF、RO)膜而言,尋求打破過濾性能選擇性相互制約的膜材料始終是研究的熱點(diǎn)之一。此外,由于膜法污水處理中的高壓膜一般直接采用商業(yè)脫鹽膜,其設(shè)計(jì)標(biāo)準(zhǔn)面向鹽分的高效脫除[2],而不是微污染物去除,因而在微污染物去除率方面不一定理想,這就增加了有毒有害微污染物超量排放或水回用的風(fēng)險(xiǎn),對(duì)環(huán)境和人體健康造成嚴(yán)重威脅,因而研制面向微污染物截留的高壓膜是研究的重點(diǎn)方向之一。正滲透膜材料一度成為研究的熱點(diǎn)之一,研制具有低濃差極化特性的高通量、高截留正滲透膜材料是研究的重點(diǎn)。此外,電滲析技術(shù)中離子交換膜的設(shè)計(jì)除了常規(guī)的抗污染、抗結(jié)垢等要求外[10-11],其在水處理過程中對(duì)特征污染物的去除效果及對(duì)不同離子間的選擇性也逐漸受到關(guān)注[12-14]。膜蒸餾技術(shù)在過去十年間發(fā)展也十分迅速,研制抗污染、抗結(jié)垢、抗?jié)櫇竦哪げ牧鲜峭苿?dòng)膜蒸餾技術(shù)發(fā)展的關(guān)鍵。
1.1.1 抗有機(jī)污染與抗生物污染膜材料
抗污染膜材料研制在過去10余年快速發(fā)展。表面涂覆/接枝、功能材料摻雜/共混等抗污染改性方法得到了廣泛研究[15-17]。近年來,借鑒自然界中生物的功能、結(jié)構(gòu)和過程實(shí)現(xiàn)抗污染改性,通過仿生和生物啟發(fā)方法獲得具有抗污染物黏附、滋生的抗污表面是抗污染膜材料研制的熱點(diǎn)(圖2和表1),如受細(xì)胞膜非對(duì)稱結(jié)構(gòu)啟發(fā)的表面偏析法[18]、受荷葉效應(yīng)啟發(fā)的自清潔光滑抗污表面[19]、受沙漠昆蟲皮膚啟發(fā)構(gòu)建的親疏水梯度表面[20]等。以受荷葉和魚鰓結(jié)構(gòu)啟發(fā)的多級(jí)結(jié)構(gòu)PP膜制備為例,通過誘導(dǎo)微/納米SiO2在膜界面上有序生長(zhǎng),能夠有效增加膜表面湍流度、減低膜表面成核傾向,實(shí)現(xiàn)高滲透通量下(23.0 kg/(m2·h))長(zhǎng)期抗污染、抗?jié)櫇裥Ч鸞21]。此外,污染物與膜面的作用機(jī)制研究也為抗污染膜材料制備指明了方向。在官能團(tuán)與污染物相互作用研究中發(fā)現(xiàn),-COOH雖然親水、且?guī)ж?fù)電(在污水處理近中性pH值范圍內(nèi)),但在實(shí)際污染中由于與無機(jī)離子絡(luò)合以及與污染物的氧橋作用,并沒有表現(xiàn)出優(yōu)異的抗污染性能[22];而如果用無機(jī)離子將-COOH遮蔽,膜的抗污染性能則恢復(fù)[23],利用Ca2+屏蔽TFC膜面的-COOH可對(duì)海藻酸鈉提升約50%的抗污染效率[23]。此外,與-COOH相比,含有-OH官能團(tuán)的膜面展現(xiàn)出更加優(yōu)異的抗污染性能[24]。
在實(shí)際長(zhǎng)期運(yùn)行過程中,微生物在膜表面的滋生所導(dǎo)致的生物污染是膜法污水處理工藝面臨的挑戰(zhàn),而制備抗生物污染膜是控制生物污染的核心手段之一。相對(duì)于物化防御和驅(qū)除機(jī)制,通過抗菌劑的釋放或接觸造成微生物損傷的主動(dòng)抗污染改性機(jī)制更加高效[15]。膜面或膜體中負(fù)載的金屬納米顆粒(如銀[33]、銅[34]和鋅[35]等)能夠通過釋放金屬離子改變細(xì)胞膜的滲透性,并可誘導(dǎo)胞內(nèi)的氧化應(yīng)激行為形成活性氧,造成細(xì)胞死亡和生長(zhǎng)抑制;而基于接觸損傷機(jī)制發(fā)揮抗菌效能的方法,可以有效避免釋放型抗菌劑依賴藥劑持續(xù)釋放的問題以及釋放可能引起的環(huán)境風(fēng)險(xiǎn),因而在抗生物污染膜制備方面具有顯著優(yōu)勢(shì)。Zhang等[36-38]基于主動(dòng)接觸殺菌策略,利用季銨鹽(QAC)對(duì)微生物的接觸損傷應(yīng)激誘導(dǎo)機(jī)制,制備了一系列基于QAC共混和表面接枝的抗污染QAC/PVDF MF膜,在處理市政污水的膜生物反應(yīng)器(MBR)系統(tǒng)中,其污染速率顯著降低(0.29 kPa/d),遠(yuǎn)低于對(duì)照PVDF膜的0.91 kPa/d[39]。同樣,自然界中精巧的抗菌結(jié)構(gòu)和方法也啟發(fā)了抗生物污染膜改性。一些昆蟲翅膀表面高長(zhǎng)寬比的納米柱能夠直接捕捉和殺死表面黏附細(xì)菌,類似形貌的納米材料已經(jīng)被應(yīng)用于膜抗污染改性[40];微生物種群間的信號(hào)傳遞是生物膜形成和生長(zhǎng)的關(guān)鍵因素,利用膜面負(fù)載的猝滅劑屏蔽信號(hào)能夠有效控制膜面生物膜的滋生[41]。
1.1.2 不斷突破水通量和截留率相互制約的高性能膜材料
水通量和截留率之間的制衡關(guān)系(Trade-off)嚴(yán)重制約了膜材料的性能提升(圖3(a)),也是膜法污水處理膜材料所面臨的共性問題,如何打破這一Trade-off現(xiàn)象是目前國(guó)際上的研究前沿和熱點(diǎn)。Tan等[42]在Science上發(fā)文報(bào)道,通過在水相中添加大分子的聚乙烯醇去結(jié)合水相哌嗪(PIP)單體,能調(diào)控水相單體擴(kuò)散的速率,從而形成表面具有圖靈結(jié)構(gòu)的聚酰胺薄膜復(fù)合(TFC)NF膜,水通量得到大幅提高,鹽截留率保持穩(wěn)定(圖3(b))。此外,近期Ma等[43]研究發(fā)現(xiàn),界面聚合過程中,界面局部溢出的納米氣泡(通常是CO2)會(huì)對(duì)反應(yīng)有顯著影響,是RO膜峰谷結(jié)構(gòu)的主要致因,通過調(diào)控納米氣泡的形成,能顯著提升TFC膜的分離性能。
多孔支撐層的結(jié)構(gòu)和表面特性也會(huì)顯著影響TFC膜表面聚酰胺層的形成和性能[44-45],通過在基膜和聚酰胺層之間引入中間層,既能通過影響界面聚合情況優(yōu)化聚酰胺結(jié)構(gòu),又能利用“溝壑”效應(yīng)增加聚酰胺層過水位點(diǎn)(圖3(c)),從而打破聚酰胺膜截留率與水通量的Trade-off[46]。最近,Karan等[47]在多孔支撐層上預(yù)覆氫氧化鎘納米線中間層,并在中間層上界面聚合制備了厚度為10 nm的聚酰胺層。這一均勻、平滑的納米線中間層為形成超薄、無缺陷聚酰胺層提供了良好的反應(yīng)場(chǎng)所,顯著提高了膜的溶劑通量。同時(shí),Wang等[48]采用纖維素納米晶作為中間層以制備TFC NF膜,這一親水中間層能儲(chǔ)存水相單體并調(diào)控界面聚合反應(yīng)速率,形成低交聯(lián)度的聚酰胺層。其制備所得的NF膜通量可達(dá)34 L/(m·h·bar),硫酸鈉截留率>97%。Yang等[49]將單寧酸鐵復(fù)合物作為預(yù)覆中間層制備高性能TFC NF膜,該中間層相比于原本的基膜具有更小的表面孔徑,能防止界面聚合形成的聚酰胺生長(zhǎng)進(jìn)入基膜膜孔,同時(shí),更光滑的中間層表面能更好地避免缺陷的產(chǎn)生。此外,還有研究者[50-51]將碳納米管作為中間層,制備了NF膜和正滲透膜,均取得了理想的水通量和鹽截留率。
另外,通過在界面聚合過程中引入納米顆粒,制備薄膜納米復(fù)合(TFN)聚酰胺膜,能在增加膜水滲透性的同時(shí)維持膜的鹽截留率,也可解決傳統(tǒng)TFC膜水滲透率和鹽截留率的Trade-off問題(圖3(d)、(e))[52-54]。一般而言,TFN膜制備的關(guān)鍵是納米顆粒的均勻分布,若納米顆粒分散不佳而團(tuán)聚,易使膜產(chǎn)生缺陷,制約TFN膜的分離性能。近些年,隨著新型納米材料的不斷研發(fā),為TFN膜的發(fā)展注入了新的動(dòng)力[55-56]。如源于細(xì)胞膜的水通道蛋白,其理論水通量比商業(yè)RO膜高2~3個(gè)數(shù)量級(jí),且對(duì)溶質(zhì)(包括H+)的截留率接近100%[57-59]。另外的一些合成通道或多孔材料也被用于合成高性能膜,如自組裝人工水通道[60]、碳納米管[61-63]、金屬有機(jī)框架化合物(MOFs)[64-67]、氧化石墨烯[68-70]、二硫化鉬納米片[71-72]等,其自身強(qiáng)大的過水性能有望大幅削減膜法水回用能耗[73]。
1.1.3 利用納米水通道調(diào)控膜的性能
對(duì)于具有孔道的納米材料,其內(nèi)部傳質(zhì)特性與聚酰胺膜完全不同[74]。以碳納米管為例,在碳納米管中,隨著水分子持續(xù)充滿碳納米管,會(huì)在管內(nèi)形成連續(xù)的一維水分子單鏈,而碳納米管內(nèi)表面是非極性的,水分子與管壁之間幾乎無摩擦作用力,因此水分子可以快速滑過碳納米管[75],且另有研究[76]發(fā)現(xiàn),隨著碳納米管直徑的減小,水分子的滑移距離成倍增加。可見,若能使多孔納米材料的孔道作為膜的主要過水通道,能顯著提升膜的水通量。并且,當(dāng)納米材料孔道成為膜的主要過水通道時(shí),膜對(duì)污染物的選擇性將取決于納米材料的孔道性質(zhì),即通過適當(dāng)選取納米材料能定制膜的選擇性。
Dai等[77]在界面聚合制備過程中原位引入親水多孔的金屬有機(jī)框架材料MIL-101(Cr),構(gòu)建了親水納米水通道內(nèi)嵌的TFN聚酰胺NF膜。通過多種鹽截留、納米材料與膜表面電位、金納米顆粒過濾與透射電子顯微鏡(TEM)截面表征等手段,證明MIL-101(Cr)的親水通道主導(dǎo)了整體膜的傳質(zhì)。親水多孔納米材料的引入使其水通量相較于傳統(tǒng)NF膜提升了130%,且同時(shí)提升了其對(duì)疏水內(nèi)分泌干擾物的截留率,有效提升了其對(duì)水/內(nèi)分泌干擾物的選擇性,在污水處理與回用領(lǐng)域具有很好的應(yīng)用前景。進(jìn)一步地,在MIL-101(Cr)納米水通道的配位不飽和金屬中心接枝了乙二胺,使其具有外部羧基、內(nèi)部氨基的雙電性結(jié)構(gòu)[78]。雙電性納米水通道也能主導(dǎo)TFN膜的分離性能,改性膜對(duì)荷正電和荷負(fù)電藥物(PhACs)均有優(yōu)異的去除效果。納米水通道外側(cè)的羧基對(duì)負(fù)電PhACs的排斥能高于內(nèi)部孔道的氨基,而對(duì)正電PhACs的排斥能主要由孔道內(nèi)的氨基貢獻(xiàn)。
此外,納米水通道不僅可通過納米材料內(nèi)部孔道來構(gòu)造,非多孔納米材料也能在聚酰胺層內(nèi)形成納米水通道。通過在基膜上預(yù)先負(fù)載親水的銀納米顆粒,在界面聚合過程中,親水銀納米顆粒附近能吸附一層水相薄層,油相單體均苯三甲酰氯(TMC)在其附近易發(fā)生水解,從而在界面聚合過程中于銀納米顆粒和聚酰胺層之間形成納米級(jí)的空腔,即納米水通道[79]。銀納米顆粒周圍的納米水通道能顯著提升RO膜的水通量和對(duì)氯化鈉、硼和微污染物的截留效果。不過,若納米顆粒是疏水的,則難以在其周圍和聚酰胺之間形成納米水通道[80],因此,若欲在聚酰胺膜內(nèi)形成納米水通道,多孔的疏水/親水納米材料或無孔的親水納米材料均在選用之內(nèi)。
1.1.4 基于影像組學(xué)和人工智能的膜材料3D打印
傳統(tǒng)的膜材料制備方法通?;诶碚撏茰y(cè)和經(jīng)驗(yàn)觀察,難以實(shí)現(xiàn)其精準(zhǔn)設(shè)計(jì),近些年興起的3D打印技術(shù)為膜材料未來的精準(zhǔn)設(shè)計(jì)提供了廣闊空間。3D打印是以數(shù)字模型文件為基礎(chǔ),通過程序控制來進(jìn)行材料精細(xì)制造的技術(shù),有望在膜材料制造和人工智能之間搭建重要橋梁。
膜材料精準(zhǔn)設(shè)計(jì)的前提是對(duì)膜材料構(gòu)效關(guān)系具有深入的理解并建立可靠的物理、數(shù)學(xué)模型關(guān)系。例如,基于TEM圖像三維重構(gòu)能獲得聚酰胺膜皮層結(jié)構(gòu)的精細(xì)三維結(jié)構(gòu),通過反應(yīng)條件控制等能解析聚酰胺層表面結(jié)構(gòu)與其過水性能的關(guān)聯(lián)[81];采用金納米顆粒過濾聯(lián)合TEM能可視化膜截留層上的有效過水位點(diǎn)[82];抑或進(jìn)行嚴(yán)格的單因素變量試驗(yàn),探究膜內(nèi)單一結(jié)構(gòu)的改變(如基膜的孔徑)與膜過水、截留性能的關(guān)系[44]。
膜材料精準(zhǔn)設(shè)計(jì)的基礎(chǔ)是正確選擇底層化合物。通過現(xiàn)有化合物及其對(duì)應(yīng)的膜材料性能訓(xùn)練機(jī)器學(xué)習(xí)算法,能預(yù)測(cè)、篩選可打破分離膜性能上限的化合物[83],同時(shí),機(jī)器學(xué)習(xí)也能用于指導(dǎo)RO膜等其他膜材料的設(shè)計(jì)[84]。不過,目前在機(jī)器學(xué)習(xí)中用于訓(xùn)練的案例大多是實(shí)際的經(jīng)驗(yàn)案例,在利用其優(yōu)化一種膜材料前仍需進(jìn)行大量的前期學(xué)習(xí)案例積累。未來,待分子模擬的手段更加成熟、精度更高后,可將分子模擬的結(jié)果用于機(jī)器學(xué)習(xí)算法的訓(xùn)練,從而實(shí)現(xiàn)膜材料制備底層化合物的高效篩選??傊ㄟ^深度學(xué)習(xí)算法對(duì)高性能復(fù)合膜材料的頂層設(shè)計(jì),有望實(shí)現(xiàn)對(duì)高性能膜材料創(chuàng)新的智能化控制。
膜材料精準(zhǔn)設(shè)計(jì)的實(shí)施關(guān)鍵是3D打印設(shè)備的精度及方法。目前市面上的3D打印主要是運(yùn)用粉末狀金屬或塑料等可黏合材料來逐層打印構(gòu)造物體,商業(yè)化精度最高限制在微米級(jí)[85]。由此可知,目前用傳統(tǒng)3D打印思路僅能精準(zhǔn)設(shè)計(jì)微米級(jí)別的膜材料,例如,Lv等[86]用含有納米二氧化硅的聚二甲硅氧烷墨水打印多孔微濾膜,其孔徑為370 μm,水通量為23 700 L/(m2·h),油水分離效率為99.6%。此外,聚砜也可用于微米級(jí)別膜材料的3D打印,由此制備得到的超疏水膜(水接觸角161°)對(duì)正己烷/水的分離效率可達(dá)99%[87]。對(duì)于納米級(jí)別的膜材料,需要進(jìn)一步提高3D打印精度[88],或者轉(zhuǎn)換思路,采用其他3D打印策略進(jìn)行材料結(jié)構(gòu)構(gòu)筑。香港大學(xué)湯初陽(yáng)教授和美國(guó)康涅狄格大學(xué)McCutcheon課題組先后提出采用電噴涂反應(yīng)單體的手段從納米級(jí)別精確調(diào)控聚酰胺層的結(jié)構(gòu)[89-90],從而實(shí)現(xiàn)聚酰胺RO膜在納米尺度下的3D打印構(gòu)筑,為未來膜材料在納米尺度下的精細(xì)設(shè)計(jì)提供了借鑒。
1.2 膜技術(shù)/工藝的功能拓展
在膜法污水處理工藝中,膜往往起到的是固液分離作用,并不具備污染物轉(zhuǎn)化的功能。以MF膜和UF膜為例,盡管能實(shí)現(xiàn)對(duì)懸浮顆粒物、膠體、細(xì)菌等污染物的高效截留,但對(duì)有毒有害微污染物的截留極其有限[91]。因此,將膜分離技術(shù)與生物處理工藝、催化氧化工藝等工藝耦合可以拓展膜技術(shù)的功能,在固液分離(溶質(zhì)溶劑分離)過程中實(shí)現(xiàn)對(duì)物質(zhì)的轉(zhuǎn)化和去除(圖4)。尤其是在低壓膜領(lǐng)域,將膜分離技術(shù)與其他工藝結(jié)合具有巨大發(fā)展?jié)摿Α?/p>
1.2.1 膜生物耦合集成高標(biāo)準(zhǔn)處理工藝
在膜法污水處理中,最常見的膜功能拓展仍是與生物法耦合,如膜生物反應(yīng)器(MBR)技術(shù)?;谖⑸锏拇x特性,與微生物耦合能賦予膜分離體系一定的微污染物降解性能[92]。在MBR技術(shù)研究領(lǐng)域,中國(guó)在反應(yīng)器設(shè)計(jì)、運(yùn)行優(yōu)化、膜污染機(jī)理、膜污染控制等方面取得了豐碩成果[93-94],為MBR技術(shù)在污水處理領(lǐng)域的應(yīng)用提供了理論與技術(shù)支持。目前,有關(guān)MBR設(shè)計(jì)已經(jīng)納入新修訂的《室外排水設(shè)計(jì)標(biāo)準(zhǔn)》(GB 50014—2021),為進(jìn)一步規(guī)范MBR工藝設(shè)計(jì)提供了標(biāo)準(zhǔn)依據(jù)。
為進(jìn)一步保障工藝出水質(zhì)量,在市政污水深度處理與回用中,MBR常和高壓膜聯(lián)用。其中,MBR發(fā)揮預(yù)處理的功能,而高壓膜實(shí)現(xiàn)污水的深度處理?,F(xiàn)有研究[95]表明,與傳統(tǒng)活性污泥法——MF分置式預(yù)處理相比,MBR作為預(yù)處理能使后續(xù)高壓膜得到更為穩(wěn)定的出水水質(zhì),且高壓膜運(yùn)行膜通量提高了30%。在高壓膜的選擇上,相比于NF,RO雖然理論上出水各類物質(zhì)去除率更高,但存在過度凈化的風(fēng)險(xiǎn),使其產(chǎn)水作為飲用水之前可能還需額外添加礦物質(zhì)。此外,由于RO對(duì)鹽去除率顯著高于常規(guī)NF,使RO膜兩側(cè)滲透壓差較大,于熱力學(xué)角度限制了RO的最低能耗[17]??梢姡谖鬯幚砼c回用中,基于鹽截留率低、微污染物截留效率高的選擇性NF膜[96],采用MBR-NF串聯(lián)體系是保障污水處理品質(zhì)和效率的有效手段。
1.2.2 膜分離耦合催化
在膜的功能拓展上,耦合電催化在污水處理中具有較大的實(shí)施潛力。制備復(fù)合電極材料的導(dǎo)電低壓膜分離材料,利用電化學(xué)高級(jí)氧化所產(chǎn)生的活性氧物質(zhì)降解有毒有害污染物,能使低壓膜也具備較好的微污染物去除能力[97-98](圖5)。Zheng等[98]基于相轉(zhuǎn)化法制備了PVDF不銹鋼絲網(wǎng)導(dǎo)電復(fù)合膜,構(gòu)建了陰極電化學(xué)膜過濾系統(tǒng)(EMF),利用電促界面鐵循環(huán),實(shí)現(xiàn)了抽吸傳質(zhì)增強(qiáng)的對(duì)氨基苯磺酸高效降解。此外,還可考慮將電極內(nèi)置于低壓過濾膜組件中,
保護(hù)電極不受污水中膠體物質(zhì)的污染。Zheng等[97]進(jìn)一步制備了TiO2@SnO2-Sb陽(yáng)極,與鈦網(wǎng)陰極內(nèi)置于陶瓷膜組件中,構(gòu)建了電極內(nèi)置式EMF,實(shí)現(xiàn)了抽吸傳質(zhì)增強(qiáng)的對(duì)氯苯胺電催化降解,陶瓷膜同時(shí)有效保護(hù)電極不受膠體等污染。Fan等[99]將含有CNTs和PANI的混合液真空抽濾至中空纖維Al2O3陶瓷膜表面,進(jìn)行高溫灼燒后得到CNTs@Al2O3電催化無機(jī)MF膜,發(fā)現(xiàn)外加電場(chǎng)的引入可不同程度地增強(qiáng)膜的抗有機(jī)污染和抗生物污染性能,并提高膜通量。Li等[100]制備了陽(yáng)極管式MF碳膜,在處理模擬含油廢水過程中,該膜展現(xiàn)出良好的抗污染性能以及對(duì)含油廢水良好的脫色效率。Bani-Melhem等采用1 V/cm的間斷直流電場(chǎng)(供電∶斷電為15 min∶45 min),在ZeeWeed-1中空纖維UF膜組件外側(cè)設(shè)置鐵絲網(wǎng)作為陰極,考察了電化學(xué)膜生物反應(yīng)器在處理模擬生活污水過程中的抗膜污染性能,與控制組相比,實(shí)驗(yàn)組的膜污染速率下降了16.3%[101]。
除針對(duì)性去除小分子難降解有機(jī)污染物外,EMF還能與工業(yè)廢水中某些污染物產(chǎn)生“聯(lián)動(dòng)”,達(dá)到自強(qiáng)化催化的效果。Li等[102]發(fā)現(xiàn),當(dāng)用EMF處理含銅絡(luò)合物(Cu-EDTA)的模擬工業(yè)廢水時(shí),利用陽(yáng)極生成的·OH攻擊穩(wěn)定的銅絡(luò)合物會(huì)生成具有催化活性的降解中間體,中間體進(jìn)一步與陰極產(chǎn)生的H2O2發(fā)生類芬頓反應(yīng)生成大量·OH,誘導(dǎo)形成鏈?zhǔn)阶詮?qiáng)化破絡(luò)過程,同時(shí),中間體和銅離子在陰極導(dǎo)電膜發(fā)生還原沉淀,進(jìn)而實(shí)現(xiàn)銅回收。此外,由于系統(tǒng)的鏈?zhǔn)椒磻?yīng)特征,EMF系統(tǒng)能耗顯著低于文獻(xiàn)中處理相同重金屬絡(luò)合物廢水的其他方法的能耗。由此可見,EMF系統(tǒng)不僅能實(shí)現(xiàn)城市污水中微污染物的去除,也能在一些工業(yè)廢水處理中發(fā)揮突出的處理效果。
膜的功能復(fù)合設(shè)計(jì)不局限于電化學(xué),耦合光催化、酶催化等亦是較為熱門的研究方向[103-104]。例如,Ni等[105]將CdS/MIL-101(Cr)負(fù)載于PVDF膜上構(gòu)建了光催化膜,在可見光照射下,通過光催化作用產(chǎn)生活性物種(·OH、e-以及h+)原位清洗膜面污染物并局部殺菌,從而實(shí)現(xiàn)CdS/MIL-101(Cr)光催化PVDF膜在MBR中的長(zhǎng)效運(yùn)行;Lee等[106]將群感效應(yīng)猝滅酶固定在膜上,利用其對(duì)微生物群感效應(yīng)的抑制,從而顯著延長(zhǎng)了膜在MBR中的清洗周期。此外,也有研究發(fā)現(xiàn),在膜上針對(duì)性負(fù)載催化酶,能實(shí)現(xiàn)對(duì)水中微污染物的催化降解[107]。
1.2.3 電化學(xué)膜生物反應(yīng)器
EMF賦予傳統(tǒng)低壓膜降解小分子污染物的功能,而MBR作為污水處理領(lǐng)域低壓膜常用工藝,也可考慮引入EMF系統(tǒng)進(jìn)行功能強(qiáng)化。在MBR中引入具備電化學(xué)高級(jí)氧化與物理分離雙重功能的復(fù)合低壓膜,不僅活性氧對(duì)膜具有原位清洗作用,保障膜組件的穩(wěn)定高效運(yùn)行,電化學(xué)與微生物降解還能協(xié)同作用高效降解污染物[108]。Chen等[109]在MBR中嵌入EMF并同步加入導(dǎo)電填料,構(gòu)建了電化學(xué)膜生物反應(yīng)器(EMBR),在外加電場(chǎng)情況下,導(dǎo)電懸浮顆粒、微生物和電極協(xié)同強(qiáng)化了磺胺甲惡唑的去除,同時(shí),緩解了膜污染問題,抑制了抗性微生物的產(chǎn)生。Chen等[110]進(jìn)一步設(shè)計(jì)制作了中試陰陽(yáng)極內(nèi)置式一體化電化學(xué)PVDF膜組件,搭建了中試EMBR,用以強(qiáng)化市政污水中微污染物的去除,實(shí)現(xiàn)了EMBR從小試向中試的應(yīng)用推廣及其處理對(duì)象由模擬污水向市政污水轉(zhuǎn)變的突破。由于EMBR結(jié)構(gòu)緊湊、易于模塊化組裝、出水質(zhì)量穩(wěn)定、污染清洗周期長(zhǎng),且對(duì)小分子難降解有機(jī)物的去除效果好,在小規(guī)模污水處理領(lǐng)域具有較大的應(yīng)用潛力。Li等[111]結(jié)合電化學(xué)高級(jí)氧化與無泡曝氣膜生物反應(yīng)器技術(shù)構(gòu)建了一種新型的電化學(xué)膜生物膜反應(yīng)器(EMBfR),用于處理含磺胺嘧啶的模擬微污染地表水。在電場(chǎng)作用下,能夠有效提高磺胺嘧啶的去除效果,同時(shí),電化學(xué)和生物之間的協(xié)同作用能顯著抑制抗生素耐藥性基因的產(chǎn)生與潛在橫向遷移,降低了二次污染風(fēng)險(xiǎn),為水環(huán)境的抗生素污染控制提供了一種高效的處理方案。
1.2.4 膜生物耦合技術(shù)的新發(fā)展
目前對(duì)MBR整體研究偏向于工程上的工藝組合。未來隨著生物技術(shù)發(fā)展,膜生物耦合技術(shù)將實(shí)現(xiàn)功能的更大突破。例如,從合成生物學(xué)角度,可考慮采用模塊化的合成生物學(xué)策略對(duì)MBR中關(guān)鍵微生物的核心代謝路徑進(jìn)行系統(tǒng)的優(yōu)化與重構(gòu)[112],從源頭優(yōu)化MBR對(duì)污染物的降解性能,并利用MBR長(zhǎng)泥齡的特性實(shí)現(xiàn)對(duì)目標(biāo)微生物菌群的富集。此外,還可考慮對(duì)膜表面的優(yōu)勢(shì)污染菌群進(jìn)行基因重構(gòu)[113]:例如,插入綠膿桿菌las群感系統(tǒng)基因片段,借助LasI/LasR蛋白的表達(dá)及信號(hào)分子反饋機(jī)制,實(shí)現(xiàn)污染生物膜生長(zhǎng)自控;引入枯草桿菌的bNos基因片段,通過樹膠醛糖誘導(dǎo)基因表達(dá)生成NO合成蛋白Nos,分泌NO,控制膜污染;在bNos基因片段連續(xù)放射性土壤桿菌的環(huán)氧化物酶合成基因,使細(xì)菌分泌環(huán)氧化物水解酶,降解環(huán)氧氯丙烷。
1.3 膜的可持續(xù)利用
商用聚合物膜材料在歷經(jīng)多個(gè)污染清洗周期后,不可避免地會(huì)達(dá)到壽命終點(diǎn)(RO膜3~7 a,低壓膜5~10 a)[114-115],必須更換新膜才能維持工藝正常運(yùn)行[116-117],這樣也會(huì)產(chǎn)生大量的廢棄膜。到2020年,全球每年將會(huì)產(chǎn)生超過30 000 t的聚合物廢棄膜[118]。通常情況下,廢棄膜需要按照各個(gè)國(guó)家/地區(qū)的法律法規(guī)進(jìn)行處理,但幾乎所有舊膜都會(huì)在垃圾場(chǎng)進(jìn)行填埋[119]。盡管膜材料的處理處置通常不被認(rèn)為屬于污水處理領(lǐng)域,但水處理膜材料的使用是膜法污水處理碳排放的重要一環(huán),若能有效優(yōu)化膜材料的處理處置路徑,有望進(jìn)一步提升膜法污水處理技術(shù)在碳達(dá)峰、碳中和背景下的競(jìng)爭(zhēng)力。因此,為增強(qiáng)膜材料的可持續(xù)利用,應(yīng)從報(bào)廢膜的循環(huán)利用進(jìn)行著手(圖6)。
報(bào)廢膜的再生循環(huán)利用可分為3類:1)報(bào)廢高壓膜的“降級(jí)”再生;2)報(bào)廢低壓膜的“升級(jí)”再生;3)報(bào)廢高壓膜或低壓膜的“平級(jí)”再生。國(guó)際上環(huán)境、材料領(lǐng)域的科學(xué)家已經(jīng)開始嘗試在經(jīng)過適當(dāng)?shù)奶幚砗髮?bào)廢的RO/NF膜降級(jí)為NF/UF膜。例如,利用聚酰胺不耐次氯酸鈉清洗的特性,采用自由氯攻擊法直接降級(jí)再生報(bào)廢RO膜,當(dāng)采用6 200 mg·L-1·h的次氯酸鈉濃度時(shí),能將報(bào)廢RO膜降級(jí)再生為NF膜,而當(dāng)清洗強(qiáng)度達(dá)300 000 mg·L-1·h時(shí),則能將報(bào)廢RO膜的聚酰胺層完全剝離,從而將報(bào)廢RO膜降級(jí)再生為UF膜[120]。受次氯酸鈉攻擊后的報(bào)廢RO膜也可用作膜曝氣生物膜反應(yīng)器中的氣膜,膜的氣體滲透率可通過自由氯濃度和攻擊時(shí)間控制[121]。此外,還有研究者[119]提出,在自由氯攻擊后通過層層組裝法再生制備NF膜。
然而,對(duì)于到達(dá)壽命終點(diǎn)的低壓膜(UF/MF),可考慮采用“升級(jí)”的方法來對(duì)其進(jìn)行再生循環(huán)利用。在生物多聚物污染的PES MF膜上直接界面聚合,能得到高性能的NF膜,其水通量和硫酸鈉截留率均高于在未污染MF膜上界面聚合得到的NF膜(最高水滲透率可達(dá)30 L/(m2·h·bar),硫酸鈉截留率約95%)。基于此,在實(shí)際嚴(yán)重污染的PVDF MF膜上,也能通過界面聚合直接制備升級(jí)得到NF膜,其硫酸鈉截留率接近90%,這也是將實(shí)際污染低壓膜升級(jí)制備高壓膜的首次研究報(bào)道[122]。從工程實(shí)施角度考慮,直接在污染低壓膜上界面聚合需預(yù)先摸索條件,且因污染層組成不同,制備得到的NF膜性能也不同,為保障升級(jí)NF膜的性能穩(wěn)定性,筆者研究團(tuán)隊(duì)提出,通過化學(xué)清洗聚多巴胺修復(fù)界面聚合的三步法策略升級(jí)制備NF膜[123],升級(jí)NF膜性能顯著高于直接界面聚合法得到的升級(jí)NF膜,經(jīng)三步法制備的NF膜,其硫酸鈉截留率>93%,水滲透率達(dá)20 L/(m2·h·bar)以上。
實(shí)際而言,在污水處理工程中,報(bào)廢膜的平級(jí)再生與循環(huán)利用的需求較大,如膜生物反應(yīng)器中低壓膜到達(dá)壽命終點(diǎn)時(shí)面臨更換的問題,此時(shí)可以考慮采用較為綠色的手段對(duì)膜材料進(jìn)行再生延壽。若將延壽年化成本控制在新膜更換成本的一定比例之下(如50%),則能在有效提升膜法污水處理技術(shù)可持續(xù)性的同時(shí),充分調(diào)動(dòng)企業(yè)循環(huán)利用膜材料的積極性。然而,目前市面上尚無有效的膜材料平級(jí)再生循環(huán)利用技術(shù)。筆者研究團(tuán)隊(duì)近期在膜材料平級(jí)再生循環(huán)利用技術(shù)方面取得了一定進(jìn)展:針對(duì)某污水廠膜生物反應(yīng)器中的報(bào)廢中空纖維PVDF超濾膜,通過深度清洗結(jié)構(gòu)轉(zhuǎn)化再生修復(fù)策略,能將報(bào)廢PVDF膜成功延壽,其水通量能恢復(fù)至新膜水平(新膜水通量約為400 L/(m2·h·bar),再生膜水通量約為420 L/(m2·h·bar),報(bào)廢膜水通量約為40 L/(m2·h·bar)),且再生PVDF膜出水質(zhì)量與新膜相當(dāng),再生膜的抗污染性能與通量清洗恢復(fù)率也均優(yōu)于新膜,經(jīng)初步核算,該報(bào)廢PVDF超濾膜平級(jí)再生策略的年化成本小于9元/m2,并節(jié)省了報(bào)廢PVDF膜的處理處置費(fèi)用。
1.4 膜工藝低耗綠色發(fā)展
隨著膜材料性能不斷提升、膜組件迭代升級(jí)、膜污染機(jī)理認(rèn)識(shí)深化、膜污染控制技術(shù)發(fā)展以及工藝運(yùn)行優(yōu)化,膜法污水處理工藝運(yùn)行能耗快速降低,其技術(shù)經(jīng)濟(jì)性能不斷提升。以膜生物反應(yīng)器技術(shù)為例,十年前的水處理能耗在0.6~0.7 kWh/m3,而目前其處理能耗降至0.4 kWh/m3以下(國(guó)際上運(yùn)行能耗最低可以達(dá)到0.3 kWh/m3以下),膜生物反應(yīng)器技術(shù)在污水高標(biāo)準(zhǔn)處理領(lǐng)域展現(xiàn)了非常大的競(jìng)爭(zhēng)力。
與此同時(shí),在資源能源短缺以及碳達(dá)峰、碳中和的大背景下,研究者們[124-125]開發(fā)出諸多新型低耗MBR工藝,包括厭氧MBR(AnMBR)、厭氧氨氧化MBR等新型組合MBR工藝,以實(shí)現(xiàn)膜工藝的低耗綠色發(fā)展。在捕獲市政污水中碳源的前提下,構(gòu)建主流污水AnMBR處理技術(shù)是一種綠色處理工藝[126]。運(yùn)行溫度是AnMBR處理市政污水效果面臨的一大挑戰(zhàn),在低溫下(<20 ℃),厭氧微生物代謝活性會(huì)顯著降低,影響反應(yīng)器處理效能[127]。不過,隨著技術(shù)革新(如接種耐寒微生物、優(yōu)化反應(yīng)器構(gòu)型等),研究者們[128-129]逐漸挖掘出了AnMBR在低溫下的處理潛力。此外,溶解性甲烷收集問題也是市政污水處理AnMBR的一大難點(diǎn),在市政污水低有機(jī)物濃度的進(jìn)水下,產(chǎn)生的甲烷相當(dāng)一部分會(huì)溶解在水中,隨出水流失,在30 ℃左右時(shí),約有>40%的甲烷隨出水流失,且溫度越低,流失越嚴(yán)重[130],此時(shí)甲烷作為溫室氣體會(huì)對(duì)環(huán)境產(chǎn)生不利影響。Li等[131]基于氟化二氧化硅納米顆粒構(gòu)建了超疏液表面的微孔膜,在15、25、35 ℃下均能實(shí)現(xiàn)甲烷飽和進(jìn)水中90%以上甲烷的回收。
此外,可從削減MBR中膜成本、降低抽吸壓力等方面降低系統(tǒng)能耗,如采用大孔廉價(jià)材料(如滌綸網(wǎng)、尼龍網(wǎng)等),利用微生物、膠體等污染物在其上生長(zhǎng)形成動(dòng)態(tài)膜,利用泥餅層進(jìn)行過濾。由于動(dòng)態(tài)膜的高滲透性,其抽吸所需能耗低于傳統(tǒng)的MF膜[132]。據(jù)報(bào)道[133],處理垃圾滲濾液時(shí),在低能耗情況下,中試厭氧動(dòng)態(tài)膜生物反應(yīng)器(AnDMBR)對(duì)13 000 mg/L COD的進(jìn)水去除率能達(dá)62.2%,有機(jī)負(fù)荷達(dá)4.87 kg COD/(m3·d),甲烷產(chǎn)率平均為0.34 L/g COD去除。同時(shí),動(dòng)態(tài)膜的污泥持留性能可以與傳統(tǒng)MF膜相比擬,故可考慮利用DMBR培養(yǎng)長(zhǎng)世代周期的微生物菌群。此外,AnDMBR也可用于污泥的處理,由于膜污染原因,傳統(tǒng)基于MF或UF的AnMBR很難用于污泥的處理,而AnDMBR通過固體停留時(shí)間和水力停留時(shí)間的分離,可實(shí)現(xiàn)污泥的同步厭氧消化和濃縮,實(shí)現(xiàn)揮發(fā)性固體消解率和沼氣產(chǎn)率的顯著提升[134]。
除傳統(tǒng)MBR技術(shù)的節(jié)能降耗外,正滲透等新型膜分離技術(shù)的涌現(xiàn),也為膜工藝綠色低耗發(fā)展注入了新動(dòng)力。正滲透技術(shù)曾在脫鹽領(lǐng)域被寄予厚望,但由于汲取液再生能耗等問題逐漸淡出人們視線。不過,在污水處理領(lǐng)域,正滲透技術(shù)仍具有較大潛力[135]。采用肥料作為汲取液的正滲透過程[136]能以低能耗的方式濃縮污水,稀釋后的汲取液無需再生,可直接用于灌溉,而濃縮后的污水可采用厭氧生物法進(jìn)行低耗處理。為進(jìn)一步簡(jiǎn)化該工藝流程,可將正滲透膜與厭氧生物法直接耦合,構(gòu)建厭氧正滲透膜生物反應(yīng)器,對(duì)污水進(jìn)行綠色低耗處理[137]。此外,膜法污水處理的能耗還能在“供給側(cè)”進(jìn)行改革,如采用可再生能源驅(qū)動(dòng)膜法污水處理過程,包括太陽(yáng)能、風(fēng)能等,目前文獻(xiàn)中已有采用太陽(yáng)能驅(qū)動(dòng)的膜蒸餾用于污水低耗綠色處理[138]。
2 未來可持續(xù)發(fā)展的思考與展望
在環(huán)境功能質(zhì)量提升、碳達(dá)峰、碳中和的背景下,膜法污水處理應(yīng)向多功能拓展、選擇性分離、定制化分離和綠色化發(fā)展方向持續(xù)邁進(jìn)。生物、信息、材料、人工智能、3D打印等科技的快速發(fā)展為膜法污水處理技術(shù)革新帶來無限可能,未來需要進(jìn)一步強(qiáng)化膜法污水處理技術(shù)綠色發(fā)展思維,堅(jiān)持技術(shù)創(chuàng)新驅(qū)動(dòng),強(qiáng)化學(xué)科交叉融合,從而支撐構(gòu)建高品質(zhì)出水、工藝過程低碳排的膜法污水處理技術(shù)。
1)多功能拓展。將膜與電催化、光催化、酶催化、先進(jìn)生物處理技術(shù)等耦合,可在分離基礎(chǔ)上賦予膜技術(shù)污染物轉(zhuǎn)化、降解功能,從而拓寬膜分離應(yīng)用領(lǐng)域與應(yīng)用范圍。膜體內(nèi)具有發(fā)達(dá)的微納米級(jí)別通道,在膜孔限域空間內(nèi)引入非均相催化劑,從而構(gòu)建納米限域強(qiáng)化的高效膜反應(yīng)器,實(shí)現(xiàn)水中污染物的快速去除,是目前膜分離多功能拓展的研究熱點(diǎn)。而將納米限域膜反應(yīng)器用于實(shí)際污水處理時(shí),膜內(nèi)部污染對(duì)膜孔內(nèi)反應(yīng)位點(diǎn)的屏蔽效應(yīng)是納米限域催化膜實(shí)際應(yīng)用需要解決的問題。合成生物學(xué)、基因編輯等生物科技的快速發(fā)展為膜生物反應(yīng)器技術(shù)的迭代升級(jí)提供了強(qiáng)有力的技術(shù)支持,尤其是在特定場(chǎng)合的應(yīng)用(如典型工業(yè)廢水處理、污染物定向資源化等)可能是未來重點(diǎn)發(fā)展的方向。
2)選擇性分離。膜的選擇性是衡量膜材料先進(jìn)程度、技術(shù)競(jìng)爭(zhēng)力的一個(gè)重要指標(biāo),尤其是在水和廢水的高標(biāo)準(zhǔn)處理方面。以RO膜為例,目前商用膜對(duì)水中小分子微污染物的截留效果仍然有限;在集成電路行業(yè)水循環(huán)利用方面,現(xiàn)有高壓膜分離精度仍然不能滿足水中污染物去除要求,往往需要流程很長(zhǎng)的制水過程;在特定污染物資源回收方面,往往要求膜的選擇性超出現(xiàn)有商用膜的性能。因此,在特定應(yīng)用場(chǎng)合,對(duì)水質(zhì)的高標(biāo)準(zhǔn)要求驅(qū)使膜的選擇性仍需不斷提升。然而高選擇性又帶來對(duì)膜過濾性能的影響,水通量和截留率之間的制衡關(guān)系(Trade-off)仍是未來高壓膜發(fā)展面臨的挑戰(zhàn)[74],也是未來研究需要持續(xù)突破的重點(diǎn)。
3)定制化分離。在實(shí)際污水處理中,面臨著分離的多樣化需求。實(shí)際污水處理時(shí)往往存在特定的、更具有經(jīng)濟(jì)性的分離精度要求。如紡織染整行業(yè)的染鹽分離、能夠選擇性透過Ca2+、Mg2+的納濾過程、特定污染物富集回收等,其并非要求膜具有很高的選擇性,而是要求膜具備高效分離特定污染物的“定制化”功能。例如,揮發(fā)性脂肪酸(VFA)是厭氧生物處理(非甲烷化)的另一重要生物質(zhì)資源,基于膜法的NF、滲透汽化和膜蒸餾技術(shù)均具有回收VFA的潛力,但如何進(jìn)行污水中VFA膜材料的定制化設(shè)計(jì)以實(shí)現(xiàn)VFA的定制化高效分離即是膜分離材料制備需要考慮的問題。通過膜結(jié)構(gòu)微納米級(jí)別的精細(xì)調(diào)控,設(shè)計(jì)選擇性NF膜對(duì)水中微污染物進(jìn)行有效去除,同時(shí)讓Ca2+、Mg2+等離子透過NF膜,是目前水處理NF膜材料的研究前沿。此外,針對(duì)特種廢水的處理,研發(fā)特種膜材料,實(shí)現(xiàn)經(jīng)濟(jì)高效分離處理也屬于“定制化”分離的研究范疇。
4)綠色化發(fā)展。膜法污水處理技術(shù)的綠色發(fā)展應(yīng)打破傳統(tǒng)僅關(guān)注節(jié)能降耗的單一視角,系統(tǒng)考慮整體膜法污水處理工藝流程的碳排放、膜材料的環(huán)境影響等??傮w而言,膜法污水處理技術(shù)的綠色發(fā)展依賴于工藝節(jié)能降耗、再生水循環(huán)利用、污染物資源化能源化、膜材料的循環(huán)利用等幾個(gè)方面。在工藝層面,膜分離技術(shù)在污染物富集方面具有廣泛應(yīng)用潛力(尤其是低濃度市政污水),可以為污染物的資源能源轉(zhuǎn)化提供有力的技術(shù)支撐[139-140]。此外,膜的選擇性分離、定制化分離可以在特定物質(zhì)的資源回收方面發(fā)揮重要作用(如典型工業(yè)廢水處理),也是膜法污水處理技術(shù)綠色發(fā)展的重要方向。在膜材料層面,如何通過低成本的手段使膜具有優(yōu)異的抗污染性能是膜材料長(zhǎng)效使用的關(guān)鍵,同時(shí)也是降低膜材料全生命周期環(huán)境影響的重要手段。與此同時(shí),尋找綠色替代性材料也是膜材料研發(fā)的方向之一。而對(duì)于膜材料的“末端處理”,如何綠色低碳地循環(huán)利用報(bào)廢膜材料,對(duì)“壽命終點(diǎn)”的膜材料進(jìn)行延壽處理,是降低膜材料碳排放的重要途徑。
5)智慧化運(yùn)維。膜法污水處理系統(tǒng)的智能化運(yùn)行管理技術(shù)是未來的重點(diǎn)研究方向之一。研究基于人工智能的多源數(shù)據(jù)融合的精準(zhǔn)化運(yùn)管技術(shù),建立膜污染與膜運(yùn)行的可視化監(jiān)測(cè)與信息化模擬系統(tǒng),構(gòu)建復(fù)雜應(yīng)用場(chǎng)景下的運(yùn)管策略,降低膜法污水處理系統(tǒng)能耗,提升系統(tǒng)運(yùn)行效能,是推進(jìn)膜法污水處理技術(shù)可持續(xù)發(fā)展的重要內(nèi)容。
未來在膜法污水處理研究中,還應(yīng)基于真實(shí)水環(huán)境與復(fù)雜污水體系進(jìn)行研究[140],需重點(diǎn)關(guān)注的關(guān)鍵科技問題包括:1)復(fù)雜環(huán)境背景下污染物在液功能材料膜材料多界面、多過程轉(zhuǎn)移轉(zhuǎn)化機(jī)制;2)多污染物共存條件下膜微納結(jié)構(gòu)驅(qū)動(dòng)的水/污染物分離與轉(zhuǎn)化機(jī)制;3)復(fù)雜水環(huán)境條件下膜表面/基體結(jié)構(gòu)性質(zhì)與過濾性能/選擇性制衡關(guān)系;4)新型膜材料結(jié)構(gòu)效能“組學(xué)”及其綠色化設(shè)計(jì)、制備與循環(huán)利用方法;5)膜工藝中物質(zhì)/能量/電子流的選擇性調(diào)控與分配機(jī)理。在上述關(guān)鍵科技問題研究基礎(chǔ)上,不斷推動(dòng)新型關(guān)鍵膜法污水處理技術(shù)突破,實(shí)現(xiàn)膜法污水處理技術(shù)的可持續(xù)發(fā)展。
3 結(jié)論
膜分離技術(shù)在污水處理與再生利用中發(fā)揮著重要作用??刮廴竟δ苣そ缑鏄?gòu)筑是高性能膜材料設(shè)計(jì)的重要一環(huán),改善膜材料表面/基體結(jié)構(gòu)性質(zhì)、引入中間層、調(diào)控膜中納米水通道,從而突破水通量/選擇性的Trade-off是高壓膜材料選擇性分離、定制化分離的關(guān)鍵所在,基于影像組學(xué)和人工智能的膜材料3D打印是膜材料精準(zhǔn)設(shè)計(jì)的未來。通過關(guān)鍵性功能材料設(shè)計(jì)與工藝集成耦合,在單一膜分離功能基礎(chǔ)上耦合污染物轉(zhuǎn)化、降解的功能拓展,是低壓膜分離技術(shù)出水滿足高標(biāo)準(zhǔn)的重要方向,而傳統(tǒng)的膜生物耦合工藝應(yīng)考慮在生物研究上尋求進(jìn)一步突破。膜材料到使用終點(diǎn)時(shí)的處理、處置是膜法水處理技術(shù)面臨的嚴(yán)峻挑戰(zhàn),通過報(bào)廢膜再生實(shí)現(xiàn)膜材料的循環(huán)利用,是膜法污水處理技術(shù)可持續(xù)發(fā)展的關(guān)鍵。在膜工藝綠色發(fā)展方面,可以將膜與綠色低耗生物處理技術(shù)進(jìn)行耦合,發(fā)揮技術(shù)的協(xié)同優(yōu)勢(shì),推進(jìn)膜技術(shù)持續(xù)節(jié)能降耗與污水資源化、能源化。在生物、信息、材料、人工智能、3D打印等科技快速發(fā)展的背景下,通過深化交叉融合與創(chuàng)新,未來膜法污水處理技術(shù)將持續(xù)在多功能拓展、選擇性分離、定制化分離和綠色化發(fā)展方向不斷革新與迭代升級(jí)。參考文獻(xiàn):
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(編輯 王秀玲)