改性骨架材料應(yīng)用于污泥深度脫水的研究進(jìn)展

戈 拯,張義鑫,2,劉吉寶,魏源送

改性骨架材料應(yīng)用于污泥深度脫水的研究進(jìn)展

戈 拯1,張義鑫1,2,劉吉寶2*,魏源送2,3

(1.北京工業(yè)大學(xué),城鎮(zhèn)污水深度處理與資源化利用技術(shù)國(guó)家工程實(shí)驗(yàn)室,北京 100124；2.中國(guó)科學(xué)院生態(tài)環(huán)境研究中心,水污染控制實(shí)驗(yàn)室,北京 100085；3.江西省科學(xué)院能源研究所,江西 南昌 330096)

本文在原始骨架材料應(yīng)用于污泥脫水局限性的基礎(chǔ)上,重點(diǎn)綜述了改性骨架材料的類別?改性方法及其強(qiáng)化污泥脫水性能的作用機(jī)理,并對(duì)其未來應(yīng)用于污泥深度脫水的前景和存在問題進(jìn)行了展望.結(jié)果表明,改性后賦予了骨架材料更多的物化性質(zhì),在與污泥絮體接觸和擠壓過程中可發(fā)揮骨架支撐?電中和?胞外聚合物破解?物理吸附和催化作用等,以促進(jìn)污泥深度脫水.針對(duì)目前研究現(xiàn)狀,建議未來應(yīng)以工程實(shí)踐需要為導(dǎo)向開發(fā)更高效經(jīng)濟(jì)的改性方法,實(shí)現(xiàn)污泥的有效減量與資源化.

污泥脫水；骨架材料；改性方法；作用機(jī)制

污泥作為污水生物處理過程產(chǎn)生的副產(chǎn)物,其產(chǎn)量急劇增加[1].高含水率的污泥不利于后續(xù)的運(yùn)輸、填埋、干燥、焚燒和資源化利用[2].污水處理廠通常采用“污泥濃縮→絮凝調(diào)理→機(jī)械脫水”方式[3],脫水污泥含水率可達(dá)80%左右.但脫水后污泥體積仍然大,高含水率無法滿足后續(xù)污泥處置要求.污泥深度脫水作為污泥快速減量的重要手段,可以最大限度地減少污泥體積,降低運(yùn)輸和后續(xù)污泥處理處置成本[4].開發(fā)綠色?經(jīng)濟(jì)?高效的污泥深度脫水技術(shù)仍是污泥處理處置研究的熱點(diǎn),對(duì)污水處理廠節(jié)能降耗至關(guān)重要.

污泥深度脫水的限制因素主要包括污泥自身高壓縮性、高有機(jī)質(zhì)含量和特殊的膠體特性[5].其中,胞外聚合物(EPS)和微生物細(xì)胞形成凝膠狀的網(wǎng)絡(luò)結(jié)構(gòu),束縛大量的水分子[6].因此,EPS被認(rèn)為是影響污泥深度脫水的關(guān)鍵因素[7-9].為此,一些學(xué)者開發(fā)了多種化學(xué)預(yù)處理方法來破壞EPS,釋放結(jié)合水以提升污泥脫水性能[10-12].然而,單一預(yù)處理調(diào)理雖能破壞EPS和微生物細(xì)胞壁,卻容易引發(fā)污泥粘度增大、破碎的絮體堵塞壓濾過程中污泥內(nèi)部過水通道等問題,導(dǎo)致預(yù)處理調(diào)理中釋放出的自由水和束縛水仍難以排出[13].因此,單一預(yù)處理調(diào)理雖能改善污泥脫水性能,卻仍難以促進(jìn)污泥深度脫水[14].為了促進(jìn)污泥深度脫水,需要通過增加濾餅的孔隙率和降低濾餅的可壓縮性[15].基于此,研究者們發(fā)現(xiàn)以物理調(diào)理劑充當(dāng)骨架材料能發(fā)揮上述作用[16].目前,生石灰是實(shí)際中較為廣泛使用的骨架材料,它不僅可以加強(qiáng)金屬鹽的水解,促進(jìn)絮凝作用,還可以增加污泥絮體強(qiáng)度,降低可壓縮性[17-19].此外,化學(xué)藥劑和骨架材料的聯(lián)合調(diào)理,可減少單一化學(xué)藥劑使用的投加量[20].進(jìn)一步,對(duì)骨架材料改性可以豐富其化學(xué)特性,使其在發(fā)揮自身物理作用的同時(shí),亦發(fā)揮化學(xué)調(diào)理劑的作用,從而有望通過改性骨架材料的多功能性一步調(diào)理實(shí)現(xiàn)污泥深度脫水.

但是,改性骨架材料應(yīng)用于污泥深度脫水的研究主要處于實(shí)驗(yàn)室研究階段.并且,雖然骨架材料及其改性應(yīng)用于提升污泥脫水性能方面已有大量的研究,但有關(guān)骨架材料的選擇、改性方法以及作用機(jī)制等方面卻缺少系統(tǒng)性的綜述.此外,骨架材料改性方法的選擇存在一定的不確定性.基于此,本文在總結(jié)骨架材料種類和性質(zhì)基礎(chǔ)上,重點(diǎn)闡述了骨架材料的改性方法及其在污泥調(diào)理過程中的主要作用機(jī)制,展望了改性骨架材料在污泥脫水中的應(yīng)用前景和發(fā)展趨勢(shì).

1 骨架材料應(yīng)用于污泥脫水的研究現(xiàn)狀

1.1 骨架材料的來源和分類

現(xiàn)有研究的骨架材料通常選擇一些工農(nóng)業(yè)廢棄物,這些廢棄物來源廣、存量大,但較難資源化利用,將其應(yīng)用于污泥脫水調(diào)理,能達(dá)到“以廢治廢”協(xié)同處理的目的.骨架材料可以根據(jù)其性質(zhì)不同分為高纖維材料、富碳材料、礦物質(zhì)材料(如表1).高纖維骨架主要包括米糠[21]、稻殼[20,22-23]、核桃殼[24-25]、竹粉[22,26]、麥秸粉[27]、木屑[28-30]等,這些材料具有較低的灰分含量和較高的熱值,也有利于后續(xù)的污泥焚燒和堆肥等處理處置過程.富碳骨架主要包括稻殼炭[20,31-32]、秸稈炭[27,33]、污泥炭[34-37]、玉米炭[38]等,它們的比表面積和孔隙度較大,還可以作為土壤改良劑,對(duì)土壤中重金屬的固化具有作用[39-40].礦物質(zhì)骨架主要包括赤泥[41]、粉煤灰[42-43]、石膏[44-45]、硅藻土[46]、累托石[47]、蒙脫石[48]等,礦物質(zhì)材料剛性強(qiáng),但可能重金屬含量高,對(duì)后續(xù)的污泥資源化利用具有不利影響[49].

1.2 骨架材料應(yīng)用于污泥脫水的局限性

表1 不同骨架材料的分類和組成

骨架材料雖然利于污泥深度脫水,卻不適于作為單一調(diào)理劑(無任何化學(xué)藥劑),并且往往需要較大投加量才能發(fā)揮作用[5,15].例如,Guo等[50]以小麥秸稈粉、玉米秸稈粉和稻殼粉調(diào)理污泥,在70%污泥干重(DS)投加量下,泥餅的含水率由93%降至80%左右.Wang等[23]比較了稻殼粉、稻殼炭、稻殼-污泥生物炭對(duì)污泥脫水性能的影響,在70%DS投加量下,稻殼-污泥生物炭的效果最優(yōu).此外,骨架材料的脫水效果受污泥脫水方式的影響.與使用帶式壓濾機(jī)或離心機(jī)等設(shè)備相比,采用高壓脫水方式(如壓濾機(jī))時(shí)骨架材料能明顯降低污泥固體壓縮性[15].值得注意的是,現(xiàn)有研究中,評(píng)價(jià)污泥脫水性能的指標(biāo)并不統(tǒng)一(如表2),導(dǎo)致不同研究報(bào)道的結(jié)果較難對(duì)比.骨架材料本身作為一種固體材料,投加量過高時(shí),必然導(dǎo)致濾餅含水率的降低,一些易吸水的骨架材料會(huì)影響毛細(xì)細(xì)水時(shí)間(CST),無法用來準(zhǔn)確反映污泥脫水性能[51].因此,當(dāng)骨架材料應(yīng)用于污泥脫水時(shí),需避免通過單一指標(biāo)來評(píng)價(jià)污泥脫水性能,應(yīng)綜合考慮脫水率、結(jié)合水含量、污泥壓縮系數(shù)、污泥凈產(chǎn)率等指標(biāo)[16].

表2 表征污泥脫水性能的評(píng)價(jià)指標(biāo)

2 針對(duì)污泥脫水調(diào)理的骨架材料改性策略

骨架材料性質(zhì)相對(duì)穩(wěn)定,未經(jīng)改性處理直接投加,較難改變污泥絮體結(jié)構(gòu),在污泥脫水中只能起骨架支撐作用(圖1).在投加量過低時(shí),不能在污泥絮體之間構(gòu)成完整的過水通道,投加量過高又會(huì)造成泥餅量增加等問題,不利于污泥后續(xù)處理處置.因此,對(duì)骨架材料進(jìn)行改性處理至關(guān)重要.骨架材料的改性方法主要包括浸泡、超聲、高溫、化學(xué)處理等.例如,通過浸泡將金屬鹽如鐵鹽[72]、鋁鹽[55,73]等負(fù)載到顆粒表面;采用酸[43,61,74]和堿[8]來活化材料表面化學(xué)特性;又如,通過表面活性劑[37,56,75-76]的醚化作用改變材料表面電荷性質(zhì).近年來有關(guān)骨架材料的改性方法和調(diào)理作用的研究情況如表3所示.

圖1 污泥壓濾脫水過程

2.1 高溫?zé)峤飧男?/h3>

高溫?zé)峤飧男灾饕蒙镔|(zhì)類材料在缺氧或無氧環(huán)境下,熱解炭化形成生物炭.與原始生物質(zhì)材料相比,生物炭的比表面積大、孔隙發(fā)達(dá),生物質(zhì)中的揮發(fā)性物質(zhì)被去除,而更具作用的剛性結(jié)構(gòu)被保留下來,從而利于降低骨架材料的投加量.同時(shí),生物炭本身是一種吸附材料,它具有吸附污泥中粘性大分子有機(jī)物的能力.但是,部分生物炭富含高度芳香化的石墨結(jié)構(gòu),其表面往往分布著負(fù)電荷[77],這不利于污泥脫水.例如,吳彥等[20]通過高溫?zé)峤庵苽涞牡練ぬ勘砻鎺в胸?fù)電荷,需要較大量的化學(xué)藥劑與之聯(lián)用才能達(dá)到更好的調(diào)理效果.因此,高溫?zé)峤飧男灾苽渖锾抗羌艿耐瑫r(shí),還需考慮化學(xué)改性以改變其表面負(fù)電性.此外,生物炭材料的吸附作用,還可降低污泥中重金屬的浸出毒性[69].高溫?zé)峤馇安捎肸nCl2預(yù)活化可以促進(jìn)炭骨架的芳香化,有利于孔隙結(jié)構(gòu)的形成[78].豐富的介孔結(jié)構(gòu)和較大的孔隙體積為污泥深度脫水提供豐富的過水通道[79].此外,污泥自身也可制備成生物炭材料,其中含有的硅鋁化合物會(huì)增強(qiáng)骨架材料的剛性,含有的可溶性陽(yáng)離子和有機(jī)官能團(tuán)可以中和污泥表面的負(fù)電荷,并表現(xiàn)出助凝劑的作用[79].

2.2 酸堿改性

生物炭和礦物質(zhì)骨架材料在酸性或堿性溶液中浸泡,可以提升其比表面積和孔隙率,其多孔結(jié)構(gòu)可以降低污泥可壓縮性,并吸附固定污泥中重金屬等有毒有害物質(zhì)[80].一些礦物質(zhì)顆粒如膨潤(rùn)土、高嶺土、粉煤灰等含有豐富的Al3+、Fe3+,這些離子通常以化合態(tài)形式存在,如Al2O3、Fe2O3.若未經(jīng)改性處理難以發(fā)揮混凝作用,而經(jīng)過酸溶或堿溶后,骨架材料表面可形成大量帶正電荷的硅、鋁活性點(diǎn)位,與污泥膠體顆粒進(jìn)行電荷中和作用.例如, Masihi等[61]比較了硫酸改性高嶺土、高嶺土和常規(guī)污泥調(diào)理劑(Al2(SO4)3+石灰),在相同投加量下,硫酸改性高嶺土提升污泥脫水性能作用最佳.酸改性可以使礦物質(zhì)骨架的比表面積和孔體積增大,其中含有的Al2O3以Al3+的形式溶出,SiO2與Al3+一起充當(dāng)助凝劑,而且酸化會(huì)在顆粒的表面產(chǎn)生正電荷,通過電荷中和、吸附橋接來改善絮凝體結(jié)構(gòu)[61].礦物質(zhì)骨架材料中往往含有大量的SiO2,其在改善污泥脫水性能中起著三方面重要作用:首先,SiO2與Al3+可以發(fā)揮助凝作用,使污泥形成大而密的顆粒;其次是增加污泥絮體表面的疏水性;此外,具有骨架支撐作用[43].因此,礦物質(zhì)骨架材料適合酸堿改性,既能充分利用自身的骨架支撐作用,亦有助凝、改善絮體結(jié)構(gòu)等作用.

表3 不同骨架改性方法對(duì)污泥脫水性能的影響

續(xù)表3

注:/表示該指標(biāo)未被研究,DS表示污泥干重(dry sludge),5‰表示海藻酸鈉基有機(jī)絮凝劑的含量.

2.3 金屬鹽/金屬氧化物改性

金屬鹽類可以在溶液中水解出陽(yáng)離子物種,如Al(OH)2+等.將金屬離子負(fù)載到骨架材料表面,可以電中和污泥膠體顆粒表面負(fù)電性,使污泥顆粒凝聚[6].因此,通過將金屬鹽負(fù)載在骨架材料上,在增加比表面積和表面官能團(tuán)的同時(shí),表面附著的金屬陽(yáng)離子可以起到電中和和壓縮雙電層的作用,從而減少骨架材料和脫水藥劑的投加量[20].例如,Wu等[72]采用FeCl3改性稻殼生物炭,將帶正電荷的Fe離子附著在稻殼炭上,破壞污泥膠體顆粒穩(wěn)定性,同時(shí)作為骨架降低了泥餅的可壓縮性系數(shù),保持一定的透氣性.Guo等[73]采用AlCl3改性的稻草生物炭,經(jīng)過改性后,生物炭表面含有帶正電荷的鋁系離子,破壞了污泥膠體穩(wěn)定性,在投加量為30%DS時(shí),調(diào)理后污泥含水率和CST分別下降到81.4%和38s,而未改性稻草生物炭調(diào)理后分別為92.1%和82s.除了金屬鹽改性,金屬氧化物改性可將骨架材料的助濾和過氧化作用結(jié)合在一起.例如,Wu等[81]將納米CaO2負(fù)載到蒙脫石上,蒙脫石上的Si基自由基催化CaO2產(chǎn)生×OH,在投加量10%DS、pH=5的條件下,污泥比阻降低了62.2%. Wu等[82]采用K2FeO4預(yù)處理后的摻鐵污泥炭負(fù)載納米CaO2作為調(diào)理劑,污泥炭可以催化CaO2生成·OH破壞污泥EPS,釋放束縛水.由于金屬鹽/金屬氧化物改性通常是將骨架材料與化學(xué)藥劑浸漬或與金屬共沉淀來實(shí)現(xiàn),因此骨架材料的比表面積和孔徑大小十分重要,在材料的選擇上應(yīng)盡量選擇多孔骨架材料來進(jìn)行改性,如生物炭等.

2.4 表面活性劑改性

高纖維骨架材料如玉米芯粉、馬鈴薯渣、殼聚糖等具有無毒、來源廣泛、環(huán)境友好、成本低廉等優(yōu)點(diǎn).因此,將天然高分子材料改性或合成為環(huán)境友好型絮凝劑,在環(huán)境保護(hù)領(lǐng)域具有重要意義[83].表面活性劑是一類能使液體表面張力顯著降低的物質(zhì),具有增溶、潤(rùn)濕、滲透等功能[84].在污泥脫水應(yīng)用中,環(huán)境友好的表面活性劑對(duì)污泥有機(jī)成分起分散、增溶和水解作用,通過促進(jìn)EPS的溶解來提高脫水性[85].多數(shù)研究者認(rèn)為,陽(yáng)離子型或兩性離子型表面活性劑相比于陰離子型或非離子型表面活性劑更利于提高污泥的脫水性能[86].例如,Guo等[75]采用NaOH和陽(yáng)離子表面活性劑-十六烷基三甲基溴化銨對(duì)玉米芯粉進(jìn)行堿化-醚化改性,使玉米芯粉攜帶正電荷,從而與污泥膠體顆粒發(fā)生電荷中和作用,同時(shí)表面活性劑中的季銨鹽基團(tuán)可以裂解細(xì)胞,釋放束縛水.Dai等[76,87]采用陽(yáng)離子表面活性劑-十八烷基二甲基芐基氯化銨對(duì)磷石膏進(jìn)行改性,改性后的磷石膏表面帶有正電荷,改性后骨架材料的加入使污泥顆粒的分形維數(shù)(DF)降低,污泥結(jié)構(gòu)松散,破壞了污泥EPS,有利于束縛水的釋放.Dai等[56]采用十六烷基三甲基溴化銨和丙烯酰胺對(duì)馬鈴薯渣進(jìn)行改性,改性后可以用于污泥脫水和污泥中有毒污染物的去除.

3 改性骨架材料應(yīng)用于污泥脫水的主要作用機(jī)制

影響污泥脫水的主要因素包括污泥絮體特性(固體濃度、絮體尺寸、絮體強(qiáng)度和表面電荷)、溶液化學(xué)性質(zhì)和污泥EPS等[6].由于污泥自身的絮體強(qiáng)度小,在機(jī)械脫水過程中,破碎的污泥絮體會(huì)堵塞泥餅和濾布的通道,惡化污泥脫水性能.污泥中含有的羧基、氨基和磷酸基等基團(tuán)還會(huì)發(fā)生水解電離,產(chǎn)生帶負(fù)電荷的離子基團(tuán),導(dǎo)致污泥表面帶有負(fù)電荷[62],影響污泥顆粒的聚集和沉降性能.針對(duì)上述限制污泥脫水的主要因素,化學(xué)和物理改性使骨架材料具有更多的理化特性,從而影響污泥絮體特性?EPS結(jié)構(gòu)等,以提高污泥脫水性能.總體上,改性骨架材料在污泥脫水中的作用主要包括骨架支撐作用、電中和作用、胞外聚合物破解、物理吸附和催化作用等.

3.1 骨架支撐作用

機(jī)械壓濾脫水是污水處理廠中最常用的脫水方式,脫水過程可以分為過濾階段和壓縮階段,如圖2所示[97].在過濾階段,污泥顆粒被濾布攔截并逐漸形成污泥餅.當(dāng)污泥絮體之間的間隙水開始承受壓力時(shí),脫水過程進(jìn)入壓縮階段.隨著間隙水的排出,污泥內(nèi)部的排水通道關(guān)閉,內(nèi)部已經(jīng)分離的自由水無法排出[98].在此情況下,提高機(jī)械壓力或延長(zhǎng)壓縮時(shí)間都無法提高脫水能力.骨架材料的支撐作用可以使污泥結(jié)構(gòu)變的松散,污泥固體的機(jī)械強(qiáng)度和滲透性增加,使其在高壓下仍能保持多孔結(jié)構(gòu),從而使水在壓濾過程中通過微小孔隙流出,提高機(jī)械脫水能力[50].骨架材料也可以在污泥絮體內(nèi)形成排水通道,使污泥餅呈現(xiàn)出多層結(jié)構(gòu),利用管網(wǎng)效應(yīng)和層間通道效應(yīng)使水順利排出[16].例如,利用污泥炭作為骨架時(shí),其中的SiO2、Al2O3、CaO、MgO組分的剪切模量均高于所施加的壓力,在污泥體系中形成了高抗壓強(qiáng)度的多孔網(wǎng)狀結(jié)構(gòu),提高了污泥脫水性能[99].郭俊元等[100]采用玉米秸稈生物炭作為骨架材料,泥餅剖面出現(xiàn)比較明顯的大孔,可壓縮性系數(shù)從2.05降至1.14,污泥餅內(nèi)部保持一定的滲透性.許欣欣[30]研究了木屑與不同絮凝劑聯(lián)合調(diào)理,使用壓汞儀測(cè)定泥餅孔隙率,發(fā)現(xiàn)添加木屑骨架后,泥餅孔隙率明顯增大,有效增加了污泥過水通道,提升污泥脫水性能.

圖2 板框式壓濾機(jī)的原理示意圖[97]

3.2 電中和作用

圖3 改性骨架材料與污泥膠體的電中和作用機(jī)制

污泥的膠體顆粒表面含有大量的負(fù)電荷,電荷特性通常用Zeta電位來描述.Zeta電位是指在嚴(yán)密層和擴(kuò)散層之間的靜電電位[97],其絕對(duì)值越高,膠體顆粒之間的排斥力就越大,會(huì)影響到污泥顆粒的聚集和沉降.未改性的骨架材料表面往往含有負(fù)電荷,直接投加到污泥中會(huì)導(dǎo)致污泥Zeta電位降低,惡化脫水性能.研究者往往通過酸堿洗滌、金屬鹽改性、離子交換等方式使骨架材料表面帶有正電荷,與污泥顆粒發(fā)生電中和作用,破壞污泥膠體的穩(wěn)定性,其作用機(jī)制如圖3所示.污泥焚燒渣作為骨架時(shí),污泥膠體的負(fù)電荷可以被骨架材料表面上的硅酸鋁、偏硅酸鐵和硅酸鈣的正電荷中和[101].在各種稻殼基骨架材料中,稻殼-污泥生物炭是最優(yōu)的骨架材料,通過表征發(fā)現(xiàn)其表面富含F(xiàn)e元素并帶有正電荷,可以與帶負(fù)電的污泥膠體發(fā)生電荷中和,使其嵌入絮凝體中[23].淀粉經(jīng)過醚化反應(yīng)與接枝共聚改性后,其表面含有大量的陽(yáng)離子基團(tuán),可以與帶負(fù)電的污泥顆粒發(fā)生電中和作用,有效壓縮顆粒表面雙電層[102].因此,通過改變骨架表面的電荷性質(zhì),發(fā)揮電中和作用,可以提升骨架材料的脫水調(diào)理作用.

3.3 胞外聚合物破解

如圖4[103]所示,根據(jù)EPS與微生物細(xì)胞的結(jié)合能力,EPS可以被分為可溶型EPS(SEPS)、松散結(jié)合型EPS(LB-EPS)和緊密結(jié)合型EPS(TB-EPS)[6].EPS的濃度和內(nèi)部組成都會(huì)影響污泥脫水性能[38].為了破壞污泥EPS結(jié)構(gòu),提高污泥脫水性能,研究者們開發(fā)了多種骨架改性方法使骨架表面的化學(xué)基團(tuán)更加豐富,在與污泥顆粒接觸過程中破壞污泥的內(nèi)部結(jié)構(gòu),使污泥中結(jié)合水釋放出來.骨架材料表面負(fù)載的金屬鹽等可以與EPS中的蛋白質(zhì)、多糖、腐殖酸等有機(jī)物發(fā)生化學(xué)反應(yīng)(圖3).骨架材料表面的物理性質(zhì)也會(huì)對(duì)污泥EPS產(chǎn)生作用,例如,木屑粗糙和不規(guī)則的表面可以剝離污泥EPS,穿刺細(xì)胞,使污泥顆粒均勻粘附在骨架上[28].改性玉米芯粉結(jié)構(gòu)上的季銨鹽基團(tuán)會(huì)使細(xì)胞破裂,LB-EPS被破壞后,SEPS中的蛋白質(zhì)、多糖、腐殖酸和磷酸根的濃度明顯增加,破壞了污泥膠體的穩(wěn)定性[75].改性污泥炭可以使SEPS、LB-EPS和TB-EPS中的蛋白質(zhì)含量分別減少72.53%、18.71%和61.12%[78].改性薯渣(D-MPR)與污泥顆粒作用過程中,可以通過氫鍵作用和酶的抑制作用破壞TB-EPS層,增加細(xì)胞壁的滲透性,最終導(dǎo)致TB-EPS的溶解和細(xì)胞死亡[56].因此,改性骨架材料對(duì)污泥EPS的裂解作用可以充分發(fā)揮其改性作用的優(yōu)勢(shì).

圖4 EPS的組成和細(xì)菌結(jié)構(gòu)[103]

3.4 物理吸附

骨架材料,特別是改性骨架含有豐富的吸附位點(diǎn)和官能團(tuán),其主要吸附機(jī)理如圖5[104]所示.改性骨架材料具有更大的比表面積,其吸附性能更強(qiáng),可以吸附污泥中的有機(jī)物和無機(jī)物.污泥EPS和細(xì)胞壁被破壞后,溶出的有機(jī)物帶有負(fù)電荷會(huì)與污泥膠體發(fā)生靜電斥力,使污泥粘度增大,惡化污泥的過濾能力.投加的骨架材料可以吸附去除蛋白質(zhì)等粘性親水物質(zhì),削弱污泥絮體的持水能力,降低液相的粘度,從而改善了污泥的脫水性能[78].因此,污泥膠體失穩(wěn)、EPS的溶解和持水物質(zhì)的去除在削弱水分子和固相之間的親和力過程中起到了協(xié)同作用[78].例如,鐵修飾污泥生物炭可以吸附SEPS中的高分子物質(zhì)、膠體和黏液,對(duì)LB-EPS和TB-EPS也有一定的吸附作用[105].這種對(duì)S-EPS和LB-EPS的吸附有助于形成緊湊的絮狀結(jié)構(gòu),從而降低泥餅的壓縮性.改性污泥炭會(huì)嵌入到污泥絮體內(nèi)部,與污泥絮體充分接觸,促進(jìn)了EPS的溶解,并通過非共價(jià)相互作用吸附粘性大分子,從而降低污泥粘度,改善了污泥的脫水性能[78].骨架材料的吸附作用還表現(xiàn)在對(duì)污泥濾液水質(zhì)的改善上.沸石和改性沸石都能一定程度上降低污泥上清液的濁度[106].核桃殼可以減少污泥中的細(xì)菌總數(shù)[107].此外,骨架的吸附作用還可以使污泥濾液中的重金屬含量略有下降[33].由此可知,骨架材料的物理吸附作用對(duì)于改善污泥脫水性能和凈化濾液水質(zhì)起重要作用.

圖5 改性污泥炭的吸附機(jī)理[104]

3.5 催化作用

圖6 AOP調(diào)理污泥脫水作用機(jī)制[103]

催化氧化過程產(chǎn)生的不同類型自由基,包括羥基自由基(OH×)、硫酸根自由基(SO4×-)、超氧自由基(O2×-)、鐵酸鹽自由基(FeO42-)和臭氧自由基(×O3),可以使污泥EPS有效裂解,釋放污泥結(jié)合水和胞內(nèi)水[103].同時(shí),自由基的氧化作用有利于重金屬和微污染物的去除[108].催化氧化調(diào)理污泥脫水的作用機(jī)制如圖6[103]所示,自由基可通過催化劑活化的方式產(chǎn)生,而金屬基改性骨架材料本身就可以作為非均相催化劑,無需外源投加Fe2+,極大降低了處理成本.此外,生物炭能產(chǎn)生環(huán)境持久性自由基(PFRs),可以與氧氣反應(yīng),生成羥基自由基(OH×),而不需要添加外部氧化劑[109].例如,Guo等[38]采用電子順磁共振光譜儀觀察到玉米秸稈炭有明顯的PFRs信號(hào),可以活化過硫酸鹽生成SO4-,通過強(qiáng)氧化作用將EPS中的部分疏水有機(jī)物有效地破壞和分解成小分子單體物質(zhì),從而促進(jìn)了結(jié)合水的釋放.Yang等[110]制備的MnFe2O4-生物炭可以活化過硫酸鹽產(chǎn)生SO4-,破壞污泥的蛋白質(zhì)結(jié)構(gòu),提高污泥的脫水性能.Tao等[111]利用富鐵生物炭作為Fenton反應(yīng)的鐵源和催化劑,與常規(guī)Fenton調(diào)理相比,污泥處置的總運(yùn)行成本降低28.39%.因此,利用骨架材料的催化性能可以與催化氧化過程相結(jié)合,減少藥劑投加量.

綜上所述,改性骨架材料在污泥脫水中的作用機(jī)制受改性方式的影響明顯,對(duì)污泥脫水性能的提高是幾種機(jī)制共同作用的結(jié)果.如表4所示,選擇合適的改性方法才能最大限度的發(fā)揮骨架材料的多功能性.對(duì)于富碳骨架材料,其表面往往分布著負(fù)電荷,這對(duì)于污泥脫水是不利的.但是,其自身的高比表面積和多孔結(jié)構(gòu)是金屬鹽良好的載體,二者結(jié)合可以改變富碳骨架表面的電荷性質(zhì),同時(shí)發(fā)揮電中和和絮凝作用.高纖維顆粒骨架材料自身含有豐富的纖維素、淀粉等組分,具有高熱值及易降解的特性,其作為綠色的骨架材料,可以選用環(huán)境友好的表面活性劑進(jìn)行改性,有助于后續(xù)污泥的資源化利用.礦物質(zhì)顆粒由于其自身就含有大量的硅、鋁、鐵元素,適合選用酸堿改性來活化其絮凝作用.總之,改性骨架材料應(yīng)用于污泥脫水時(shí),盡量使其具有大的比表面積、強(qiáng)的剛性結(jié)構(gòu)和表面正電荷.改性后與催化氧化作用結(jié)合,能進(jìn)一步破壞污泥EPS,釋放污泥結(jié)合水,利于污泥深度脫水.

表4 不同骨架材料的改性方法及主要作用機(jī)制

4 結(jié)語(yǔ)

本文從骨架材料的種類?改性方法?作用機(jī)制等方面,針對(duì)改性骨架材料應(yīng)用于污泥深度脫水的相關(guān)研究進(jìn)行了系統(tǒng)性的綜述和分析,以期為選擇適宜的骨架材料和改性方法提供更多的理論依據(jù)和參考.改性骨架材料提升了污泥的脫水性能,其作用機(jī)制主要包括骨架支撐作用、電中和作用、EPS破解、物理吸附和催化作用.但是,改性骨架材料強(qiáng)化污泥脫水仍處于實(shí)驗(yàn)室研究探索階段,為了推進(jìn)該技術(shù)在污泥處理處置中的應(yīng)用,提出以下4 個(gè)建議.

4.1 在骨架材料的選擇上,應(yīng)盡量選擇富含硅、鋁、鐵的原材料,這樣的骨架結(jié)構(gòu)硬度大、剛性強(qiáng)、表面的活性位點(diǎn)更豐富,可以更大程度的發(fā)揮骨架在污泥中的作用.同時(shí),要結(jié)合當(dāng)?shù)氐纳a(chǎn)方式,選擇產(chǎn)量大、亟需資源化利用的固體廢棄物作為骨架材料,降低運(yùn)費(fèi)成本,達(dá)到“以廢治廢”的目的.

4.2 現(xiàn)有的骨架改性方法和評(píng)價(jià)脫水性能指標(biāo)還存在局限性,不同研究中評(píng)價(jià)污泥脫水性能指標(biāo)不統(tǒng)一.同時(shí),未來研究中,應(yīng)嘗試更多的改性方法,賦予骨架更多的物化特性,使其在污泥體系中發(fā)揮更大作用,并建立統(tǒng)一的改性骨架材料應(yīng)用污泥脫水的評(píng)價(jià)指標(biāo).

4.3 在改性方法的選擇上,酸堿改性比較適合礦物質(zhì)骨架材料;金屬鹽/金屬氧化物改性,適用于多孔的生物炭骨架材料;高溫?zé)峤夂捅砻婊钚詣└男赃m用于選擇無毒、無害的生物質(zhì)原料.

4.4 改性骨架材料應(yīng)用于污泥脫水的研究尚停留在實(shí)驗(yàn)室研究階段,其大規(guī)模應(yīng)用的實(shí)踐較少,仍需中試規(guī)模以上的實(shí)驗(yàn)驗(yàn)證,并降低改性骨架材料投加量.

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Research progress on the application of modified skeletal materials for sludge deep dewatering.

GE Zheng1, ZHANG Yi-xin1,2, LIU Ji-bao2*, WEI Yuan-song2,3

(1.National Engineering Laboratory for Advanced Municipal Wastewater Treatment and Reuse Technology, Beijing University of Technology Beijing 100124, China；2.Laboratory of Water Pollution Control Technology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China；3.Institute of Energy, Jiangxi Academy of Sciences, Nanchang, Jiangxi 330096, China)., 2023,43(11)：5787～5799

Regarding the limitations of using raw skeleton materials, this literature reviewed the categories of skeletal materials, modification methods, and the mechanisms of enhancing sludge dewatering. In addition, the prospects and problems of their future application in sludge deep-dewatering were also discussed. The results showed that modified skeletal materials presented better physical and chemical properties which could play vital roles with multiple functions, including mechanical supporting, electrical neutralization, disruption of extracellular polymeric substances, physical adsorption and catalysis, etc., thus to promote deep dewatering. In terms of current state of research, to realize the reduction and utilization of sludge, development of efficient and economical modification methods for practical engineering applications is recommended.

sludge dewatering；skeleton material；modification method；interaction mechanism

X703.5

A

1000-6923(2023)11-5787-13

戈 拯(1986-),男,湖北十堰人,副教授,博士,主要研究方向?yàn)槲鬯?污泥處理及資源化.發(fā)表論文20余篇.gezheng@bjut.edu.cn.

戈 拯,張義鑫,劉吉寶,等.改性骨架材料應(yīng)用于污泥深度脫水的研究進(jìn)展 [J]. 中國(guó)環(huán)境科學(xué), 2023,43(11):5787-5799.

Ge Zheng1, Zhang Yi-xin1,2, Liu Ji-bao, et al. Research progress on the application of modified skeletal materials for sludge deep dewatering [J]. China Environmental Science, 2023,43(11):5787-5799.

2023-03-31

國(guó)家重點(diǎn)研發(fā)計(jì)劃項(xiàng)目(2019YFC1906502)

* 責(zé)任作者, 助理研究員, jbliu@rcees.ac.cn