王佳妮,馬若男,唐若蘭,李麗瓊,彭麗娟,李國(guó)學(xué),林嘉聰,王定美,李勤奮,袁 京*

冷凝水回流對(duì)堆肥腐熟度和污染氣體排放影響

王佳妮1,馬若男1,唐若蘭1,李麗瓊1,彭麗娟1,李國(guó)學(xué)1,林嘉聰2,王定美2,李勤奮2,袁 京1*

(1.中國(guó)農(nóng)業(yè)大學(xué)資源與環(huán)境學(xué)院,農(nóng)田土壤污染防控與修復(fù)北京市重點(diǎn)實(shí)驗(yàn)室,北京 100193；2.中國(guó)熱帶農(nóng)業(yè)科學(xué)院環(huán)境與植物保護(hù)研究所,海南?？?571101)

為探究堆肥過程中高溫水蒸氣攜帶的冷凝水回流對(duì)腐熟度及溫室氣體排放的影響,本文以豬糞和廚余垃圾為堆肥原料,在60L密閉好氧堆肥反應(yīng)器中進(jìn)行48d的高溫好氧堆肥實(shí)驗(yàn).結(jié)果表明,堆肥高溫期產(chǎn)生的飽和水蒸汽因發(fā)酵罐封閉體系不能及時(shí)排出,會(huì)在發(fā)酵罐壁形成冷凝水回流入堆體中,整個(gè)堆肥過程冷凝水產(chǎn)率為物料濕重的18%.此外,回流冷凝水中主要成分為NH4+-N(11.40g/L)、溶解性有機(jī)碳(6.3g/L)以及無機(jī)鹽離子,具有較高的EC值(41.05mS/cm)和pH值(9.22),而較高的NH4+-N含量使冷凝水回流處理的堆肥產(chǎn)品種子發(fā)芽指數(shù)(GI)略達(dá)到腐熟(71.4%),與排出處理相比GI下降了25.5%.同時(shí),冷凝水回流使NH3和CH4排放量分別增加了55.2%的60.2%,但是CO2和N2O排放量分別降低了16.8%的34.8%.研究表明,冷凝水回流可降低17.8%總溫室效應(yīng).本文建議堆肥過程將高溫水蒸氣冷凝排出,一方面可提高堆肥腐熟度,降低產(chǎn)品含水率,另一方面可以減少NH3排放;進(jìn)一步地若能將冷凝水進(jìn)行收集,可對(duì)其中的氮素進(jìn)行回收再利用.本文為封閉式好氧堆肥工藝水分去除和腐熟度調(diào)控提供理論依據(jù)和數(shù)據(jù)支撐.

堆肥；冷凝水回流；腐熟度；氮素?fù)p失；溫室氣體

針對(duì)有機(jī)固體廢棄物產(chǎn)量大、資源化利用率低,隨意丟棄環(huán)境污染嚴(yán)重等問題,提高廢棄物的資源化利用率,減少環(huán)境污染迫在眉睫[1-2].好氧堆肥技術(shù)可將有機(jī)廢棄物在好氧微生物的作用下轉(zhuǎn)化為有機(jī)肥,是實(shí)現(xiàn)固體廢棄物肥料化利用的主要方式,具有無害化程度高、成本低等優(yōu)勢(shì)[1,3].堆肥工藝主要分為開放式和封閉式好氧發(fā)酵,密閉式堆肥反應(yīng)器是實(shí)驗(yàn)室規(guī)模和工廠化常用的堆肥方式[3],適宜于不同規(guī)模養(yǎng)殖場(chǎng)和堆肥廠,具備占地面積小、自動(dòng)化程度高、堆肥過程可調(diào)控且二次污染氣體易集中收集處理等優(yōu)勢(shì)[2].

但因密閉堆肥反應(yīng)器具備較好的保溫和發(fā)酵條件,密閉反應(yīng)器堆肥溫度在較短時(shí)間內(nèi)易實(shí)現(xiàn)堆肥高溫(50～65℃),甚至超高溫(80～90℃).高溫堆肥氣流攜帶大量的飽和水蒸汽,當(dāng)堆體溫度為55℃時(shí),氣體飽和蒸汽含水量可達(dá)116.28g/kg干空氣,隨著溫度的升高飽和蒸汽含水量呈現(xiàn)指數(shù)級(jí)上升趨勢(shì).堆肥過程是一個(gè)利用微生物降解有機(jī)質(zhì)產(chǎn)熱的發(fā)酵過程,堆肥過程微生物降解有機(jī)質(zhì)產(chǎn)生的熱量中60%用于堆體中水分蒸發(fā)[4].因此,密閉式反應(yīng)器堆肥高溫期通風(fēng)氣流中攜帶大量的飽和水蒸汽,若不能及時(shí)排出,易在反應(yīng)器表面形成冷凝,以冷凝水形式回流到堆體中,進(jìn)而影響堆體的理化性質(zhì).研究表明,堆肥過程中冷凝水產(chǎn)量與溫度呈顯著正相關(guān)關(guān)系[5],堆肥高溫期(>50℃)的積溫可達(dá)到2300～ 4000℃·h,該階段水分損失占堆肥過程中總損失的89%[6].

密閉式堆肥體系冷凝水回流一方面會(huì)影響堆肥物料含水率,使得堆肥過程含水率不易降低,甚至增加[7].另一方面堆肥過程中的冷凝水不僅是純水分的蒸發(fā),而是溶解了CO2、NH3等氣體的復(fù)雜組分[8],冷凝水的回流會(huì)使大量的溶解性碳、氮和離子等進(jìn)入堆體,改變堆肥的理化性質(zhì),進(jìn)而影響堆肥腐熟度和堆肥過程污染氣體排放.目前大多數(shù)研究集中在通過工藝參數(shù)調(diào)節(jié)去除堆肥物料中的水分,研究快速生物干化過程堆肥腐熟度變化.但是,目前還未有研究系統(tǒng)分析封閉式反應(yīng)器高溫好氧堆肥過程造成的冷凝水回流對(duì)堆肥腐熟度和溫室氣體排放的影響.

因此,本文在實(shí)驗(yàn)室規(guī)模的密閉式好氧堆肥反應(yīng)器中設(shè)置冷凝水回流和排出兩個(gè)處理,通過負(fù)壓外排泵冷凝回收系統(tǒng)排出和收集高溫飽和水蒸汽,研究高溫堆肥過程飽和水蒸汽冷凝回流對(duì)堆肥過程腐熟度和溫室氣體排放量的影響,為封閉式好氧堆肥工藝水分去除和腐熟度調(diào)控提供理論依據(jù)和數(shù)據(jù)支撐.

1 材料與方法

1.1 試驗(yàn)材料與試驗(yàn)裝置

堆肥試驗(yàn)在中國(guó)農(nóng)業(yè)大學(xué)上莊實(shí)驗(yàn)站進(jìn)行,堆肥主料為豬糞和廚余垃圾,輔料為玉米秸稈.豬糞取自中國(guó)農(nóng)業(yè)科學(xué)院畜牧研究所昌平基地,廚余垃圾取自北京市海淀區(qū)大工村廚余垃圾處理廠,玉米秸稈取自中國(guó)農(nóng)業(yè)大學(xué)上莊實(shí)驗(yàn)站,經(jīng)自然風(fēng)干后粉碎成2～5cm小段.原料及原始混合物料的基本理化性質(zhì)見表1.

表1 堆肥原料基本理化性質(zhì)

注:a,基于濕基;b,基于干基.

1.2 試驗(yàn)方法

圖1 密閉式強(qiáng)制通風(fēng)好氧堆肥試驗(yàn)裝置示意

1.篩板;2.通氣口;3.冷凝水收集瓶;4.抽氣泵;5.通風(fēng)自動(dòng)化控制系統(tǒng);6.冷凝管;7.制冷冰箱;8.氣體樣采集口;9.溫度自動(dòng)采集計(jì)算機(jī);10.絕熱層;11.溫度傳感器;12.固體樣采集口;13.堆肥原料;14.滲濾液收集口

試驗(yàn)共設(shè)置2個(gè)處理,包括冷凝水回流(W)和冷凝水排出(WR)兩個(gè)處理.如圖1所示,冷凝水排出(WR)處理在堆肥罐出氣口安裝一套冷凝裝置,將堆體產(chǎn)生的飽和水蒸汽通過等流量的外排泵抽出,與袁京等[6]報(bào)道的試驗(yàn)裝置相同.冷凝水回流處理為全密閉好氧發(fā)酵反應(yīng)器,發(fā)酵罐頂蓋僅留了1個(gè)用于取氣的直徑為1cm的取氣口.選取豬糞和廚余垃圾以濕基1:1進(jìn)行混合,添加玉米秸稈調(diào)節(jié)初始C/N為18,設(shè)置初始含水率為65%,將充分混合后的物料置于60L的不銹鋼密閉發(fā)酵罐中進(jìn)行48d的高溫好氧堆肥試驗(yàn).通風(fēng)方式為連續(xù)通風(fēng),高溫期(1～21d)通風(fēng)速率為0.38L/(kg DM·min),降溫期(22～27d)為0.25L/(kg DM·min),腐熟期(28～48d)為0.13L/(kgDM·min).每日同一時(shí)間讀取溫度、采集氣體樣品以及收集冷凝水.除堆肥起始和結(jié)束日外,每7d進(jìn)行人工翻堆,物料混勻后進(jìn)行多點(diǎn)取樣,所收集的樣品一部分于4℃保存,用于含水率、pH值、EC、NH4+-N、NO3--N、種子發(fā)芽指數(shù)(GI)等理化指標(biāo)的測(cè)定;另一部分自然風(fēng)干后過0.5mm篩,用于碳(C)、氮(N)元素含量的測(cè)定.

1.3 測(cè)定指標(biāo)與分析方法

堆肥溫度由溫度傳感器(175-T3,Testo,德國(guó))測(cè)定,自動(dòng)通過紅外裝置進(jìn)行數(shù)據(jù)監(jiān)控.O2、CO2含量由便攜式沼氣分析儀(Biogas 5000,Geotech,英國(guó))測(cè)定.含水率采用烘干稱重法測(cè)定.元素含量(TC、TN)使用元素分析儀(vario MACRO cube,Hananu,德國(guó))測(cè)定.pH值、EC的測(cè)定采用去離子水,按照1:10(m:v)浸提,震蕩30min,靜置10min,過濾后取濾液,使用多參數(shù)分析儀(DZS-706-A,雷磁,上海)進(jìn)行測(cè)定. NH4+-N、NO3--N的測(cè)定采用2mol/L 的KCl溶液,按照1:10(:)浸提,震蕩30min,靜置10min,過濾后取濾液經(jīng)流動(dòng)分析儀(Auto Analyzer 3,Seal,德國(guó))測(cè)定.

種子發(fā)芽率(GI)的測(cè)定取5mL水浸提液于帶濾紙的培養(yǎng)皿中,均勻放置10粒蘿卜種子,于(25±1)℃培養(yǎng)箱(SHP-250,精宏,上海)中恒溫避光培養(yǎng)48h,記錄種子發(fā)芽數(shù)及發(fā)芽種子根長(zhǎng),采用公式,計(jì)算GI值.

溫室氣體N2O、CH4采用氣袋收集,使用安裝有火焰電離檢測(cè)器、電子捕獲檢測(cè)器的氣相色譜(Trace 1300,賽默飛世爾科技有限公司,美國(guó))測(cè)定. NH3使用裝有2%的硼酸吸收變色后使用0.01mol/L標(biāo)準(zhǔn)硫酸滴定測(cè)定.

根據(jù)元素平衡法計(jì)算堆肥過程中C、N元素平衡,定量計(jì)算堆肥化處理過程中不同碳、氮化合物的百分含量.

堆肥過程中總碳、總氮損失計(jì)算公式:

式中:為堆肥總碳、總氮損失率,%;0、C1分別表示為堆肥初始和結(jié)束時(shí)的總碳、總氮質(zhì)量分?jǐn)?shù),g/kg;0、1分別表示堆肥初始和結(jié)束時(shí)物料干重,kg.

NH3、CH4、N2O和NH4+損失占總氮比例計(jì)算公式:

式中:M代表NH3、N2O、CH4的累積排放量,g/kg和冷凝水中NH4+的含量,mg/L;TC、TN代表總碳和總氮含量,g/kg.

1.4 數(shù)據(jù)分析

使用Origin 2021和Canoco 5作圖,SAS 8.2(SAS Institute,Cary,NC,USA)進(jìn)行顯著性差異分析.

2 結(jié)果與討論

2.1 溫度和氧氣

在好氧微生物的作用下,有機(jī)質(zhì)降解產(chǎn)生大量熱量,堆體快速升溫(圖2a).兩處理均在堆肥第8d達(dá)到溫度峰值(73.3℃),且高溫期(>55℃)持續(xù)時(shí)間超過20d,可有效殺滅堆體中病原菌,實(shí)現(xiàn)無害化.整個(gè)堆肥過程中,盡管WR處理隨著飽和水蒸汽排出會(huì)攜帶走部分熱量,但WR處理積溫(2279℃·h)仍高于W處理(2105℃·h),主要是由于水分的排出可使堆體處于更適宜的堆肥含水率條件,進(jìn)而促進(jìn)有機(jī)質(zhì)的降解.堆肥前4周,2個(gè)處理溫度無明顯差異,28d后隨著冷凝水回流量的增加,W處理溫度顯著低于WR處理(<0.01).

O2含量在堆肥升溫期急劇下降,隨后呈現(xiàn)上升趨勢(shì)(圖2b),溫度與O2含量呈顯著負(fù)相關(guān)關(guān)系(= -0.826,<0.01),堆肥前期好氧微生物分解有機(jī)質(zhì)消耗大量O2,這與之前的研究一致[9-10].值得注意的是,盡管堆肥前28d溫度無顯著差異,但WR處理O2含量顯著高于W處理(<0.01),主要是由于水分的及時(shí)排出增加了堆體的孔隙率[11],使堆肥結(jié)構(gòu)和自由孔隙率得以優(yōu)化,提高了微生物活性,使得WR處理在堆肥后期溫度有所升高.總體而言,堆肥過程冷凝水回流通過增加物料含水率,導(dǎo)致自由孔隙率降低,進(jìn)而影響O2擴(kuò)散和堆體發(fā)酵進(jìn)程.

2.2 含水率和冷凝水收集量

冷凝水回流處理堆肥過程含水率變化不顯著,至堆肥結(jié)束時(shí)含水率與初始值差異不大;增加水分去除措施的WR處理含水率呈逐漸下降趨勢(shì),含水率下降了12.3%(圖3a).堆肥過程水分去除規(guī)律與溫度變化趨勢(shì)一致,水分去除量也即冷凝水產(chǎn)量與溫度呈顯著正相關(guān)關(guān)系(=0.701,>0.05).水分去除主要發(fā)生在堆肥高溫期,占總水分去除量的95%以上(圖3b),這與之前的研究一致[5].整個(gè)堆肥過程,WR處理水分去除率達(dá)0.18kg/kg(基于濕重).

由表2可知,高溫氣流中飽和水蒸氣形成的冷凝水中主要含有溶解性碳、NH4+、K+、SO42-等離子,NH4+含量達(dá)到11.40g/L,造成較高的EC值(41.05)和pH值(9.22).冷凝水中NH4+含量達(dá)到《畜禽養(yǎng)殖業(yè)污染物排放標(biāo)準(zhǔn)》[12]中限定值(80mg/L)的142倍,不宜直接作為灌溉水利用,可進(jìn)一步回收其中的NH4+-N養(yǎng)分后進(jìn)行資源化利用.

表2 冷凝水的理化性質(zhì)

2.3 腐熟度指標(biāo)

如圖4(a)所示,2個(gè)處理的初始pH值為6.4,在堆肥過程中呈現(xiàn)相似的變化趨勢(shì),升溫期和高溫期pH值快速升高,主要是由于有機(jī)質(zhì)分解礦化產(chǎn)生大量的NH4+[13]以及高溫作用促進(jìn)有機(jī)酸的揮發(fā)[14],第14d達(dá)到最高值8.5后保持穩(wěn)定,這與Cui等[15]的研究結(jié)果一致.EC值反映堆肥可溶性離子的濃度,EC值過高會(huì)對(duì)植物產(chǎn)生毒性作用[9].一般認(rèn)為EC值低于4mS/cm,即為植物安全生長(zhǎng)的界限[16].如圖4(b)所示,堆肥過程EC值始終低于該值,且隨著堆肥的進(jìn)行EC值逐漸降低.堆肥前3周,兩處理EC值均呈現(xiàn)下降趨勢(shì),一方面是由于氨氣的大量損失,另一方面與堆肥腐殖化過程有關(guān)[17].堆肥后期WR處理EC值顯著高于W處理(<0.05),主要是由于堆肥后期微生物活性仍較高,有機(jī)質(zhì)降解產(chǎn)生小分子物質(zhì),這與高溫的變化趨勢(shì)一致.

堆肥C/N可一定程度表征堆肥腐熟程度[18].以往研究表明,初始C/N為18時(shí)堆肥達(dá)到腐熟,且微生物多樣性較高[19].本研究原料的初始C/N為18.6(圖4c),隨著堆肥的進(jìn)行C/N呈現(xiàn)下降趨勢(shì),W和WR處理的C/N至堆肥結(jié)束時(shí)分別為11.51和10.01,主要是因?yàn)橛袡C(jī)質(zhì)礦化速率高于氮素?fù)p失速率[14].至堆肥結(jié)束,W處理的C/N高于WR處理,表明冷凝水回流不利于有機(jī)質(zhì)降解[20].

GI是評(píng)價(jià)堆肥腐熟度的最直接有效的指標(biāo)[9,19],堆肥初期GI值極低,種子幾乎不發(fā)芽,主要是由于堆肥原料中較高含量的小分子有機(jī)酸,銨態(tài)氮等植物毒性物質(zhì),抑制種子發(fā)芽[21].隨著堆肥溫度的升高及植物毒性物質(zhì)的降解轉(zhuǎn)化,GI值逐漸升高.堆肥結(jié)束時(shí)W和WR處理的GI值分別為71.4%和95.8%,達(dá)到《NY/T 525-2021有機(jī)肥料》中對(duì)GI的要求(370%)[22].堆肥過程WR處理的GI值始終高于W處理.主要原因:一是,冷凝水回流使堆體中NH4+-N含量較高(圖5a),對(duì)種子發(fā)芽產(chǎn)生負(fù)面作用[23],相關(guān)性分析也發(fā)現(xiàn)NH4+-N與GI呈顯著負(fù)相關(guān)(=-0.954,<0.01).二是,堆肥過程水分去除為微生物提供了更加適宜的生存環(huán)境,進(jìn)一步促進(jìn)有機(jī)質(zhì)降解和物料腐熟,這與Li等[24]的研究結(jié)果一致.

2.4 氨態(tài)氮和硝態(tài)氮

原料的初始NH4+-N含量為3.5g/kg DM(圖5a),堆肥1周后達(dá)到最大值4.8g/kg DM,主要是由于有機(jī)氮的氨化作用產(chǎn)生了NH4+離子[25].隨后NH4+-N含量快速下降,與初始含量相比,W和WR分別降低了64.2%和98.1%,WR處理NH4+-N含量堆肥結(jié)束時(shí)顯著低于W處理,主要是由于大量的NH4+離子進(jìn)入冷凝水中(表2).堆肥原料NO3--N含量為0.23g/kg DM,WR在第14d NO3--N含量降低至最低,是反硝化作用對(duì)NO3--N的轉(zhuǎn)化以及硝化作用被高溫抑制的結(jié)果[26],W處理NO3--N含量變化趨勢(shì)與WR一致,但降低程度較慢,在第21d降低至最低,這主要是由于冷凝水回流抑制了反硝化細(xì)菌活性[27],這與N2O排放結(jié)果一致,冷凝水回流降低了N2O排放.

2.5 溫室氣體排放規(guī)律

堆肥過程N(yùn)H3排放主要發(fā)生在高溫期(圖6a),堆肥溫度與NH3排放呈顯著正相關(guān)關(guān)系(=0.729,<0.05).W和WR處理NH3排放速率分別在第10d和第11d達(dá)到峰值1.72和0.94g/kg DM.堆肥過程水分的排出顯著降低了高溫期NH3排放速率(<0.05),主要原因是NH3以NH4+的形式溶解在冷凝水中,并隨冷凝水排出(表2),從而減少了NH3向大氣中排放. NH3的累積排放量在W-A和WR-A中分別為7.81和4.92g/kg DM.研究表明,有機(jī)物料快速生物干化去除水分過程會(huì)減少好氧發(fā)酵過程中NH3的排放[5],與本研究結(jié)論一致.因此,冷凝水回流(W-A)增加了NH3排放,較WR-A增加了55.2%.

兩個(gè)處理堆肥過程的N2O排放存在顯著差異(<0.05).N2O主要在堆肥初期和腐熟期排放,這與以往研究的排放規(guī)律一致[10,28].研究顯示,較低的O2和C/N均會(huì)導(dǎo)致N2O的排放[29].堆肥初期較低的O2濃度和堆肥高溫抑制硝化作用,反硝化菌將亞硝酸鹽和硝酸鹽轉(zhuǎn)化為N2O[30].堆肥后期有機(jī)碳含量下降,微生物進(jìn)入內(nèi)源代謝階段導(dǎo)致N2O積累排放.與W-A相比,WR-A在堆肥后期產(chǎn)生了大量的N2O,是水分的去除增強(qiáng)微生物活性,有機(jī)質(zhì)降解導(dǎo)致的低C/N和低O2含量的共同結(jié)果.與此同時(shí),堆肥后期好氧條件下,NH4+-N通過硝化和反硝化作用產(chǎn)生了N2O[30],進(jìn)一步表明密閉條件下冷凝水回流會(huì)抑制硝化作用,進(jìn)而減少N2O排放.

堆肥過程CH4排放主要集中在堆肥初期和高溫期(圖6c).堆肥初期,由于初始階段易降解小分子有機(jī)物分解礦化快速消耗氧氣以及高含水率形成厭氧環(huán)境,從而導(dǎo)致CH4的排放[31].研究顯示,CH4排放主要集中在堆肥前期[32-33].而本研究中CH4的產(chǎn)生主要是在堆肥高溫期和降溫期(圖6c),主要為堆體中較高的水分含量及堆體壓實(shí)度影響了通風(fēng)擴(kuò)散.相關(guān)性分析也表明含水率與CH4排放速率呈現(xiàn)顯著負(fù)相關(guān)(=-0.903,<0.01).W處理CH4排放速率在第20d達(dá)到最高值0.17g/(kg DM·d),而WR處理的排放峰值僅為W處理的1/3,主要是由于水分的去除減少了堆體的壓實(shí)度,增加了堆肥的孔隙率[33].堆肥過程WR-A的CH4累積排放量顯著低于W-A處理(< 0.05),與W-A處理相比,降低了60.2%.因此,堆肥過程中冷凝水回流會(huì)增加CH4排放.

堆肥過程CO2排放規(guī)律與溫度變化曲線相似,這兩個(gè)參數(shù)都反應(yīng)了微生物的活性[34].高溫期作為有機(jī)質(zhì)大量降解的階段,堆肥過程水分的排出使得堆體含水率適度降低至合適范圍,微生物活性增加.水分的排出促進(jìn)了有機(jī)質(zhì)的礦化產(chǎn)生CO2,WR排放速率在第10d達(dá)到峰值19.75g/(kg DM·d),較W處理(13.03g/(kg DM·d))增加了34.1%.相關(guān)性分析表明,含水率與CO2排放呈現(xiàn)顯著負(fù)相關(guān)(=-0.839,<0.01),WR-A的CO2累積排放量(299.08g/kg DM)顯著高于W-A處理(248.94g/kg DM).因此,水分的排出促進(jìn)了有機(jī)質(zhì)的降解,增加了CO2的排放.

2.6 影響堆肥腐熟度和溫室氣體排放的關(guān)鍵因子分析

圖7 堆肥過程中理化性質(zhì)和腐熟度、溫室氣體的相關(guān)性熱圖和冗余分析

Fig.7 Correlation heatmanps and redundancy analysis among physicochemical properties, maturity indexes and polluting gases during composting

熱圖中*和**分別代表顯著性水平<0.05和<0.01

通過相關(guān)性分析探究理化性質(zhì)對(duì)堆肥腐熟度和溫室氣體排放影響.結(jié)果表明,堆肥NH3排放與溫度具有顯著正相關(guān)關(guān)系(=0.777,<0.05).冷凝水回流(W)處理中供氧效率的降低直接影響NH3的排放.因此,W處理中NH3排放與O2含量呈現(xiàn)極顯著負(fù)相關(guān)(=-0.862,<0.01).2個(gè)處理N2O排放主要與pH值、EC、NO3--N呈現(xiàn)顯著相關(guān)性,而CH4排放與理化指標(biāo)沒有顯著相關(guān)性.堆肥C/N與CO2排放相關(guān)性在W(=-0.948,<0.01)和WR(=-0.966,<0.01)處理中不一致,主要是由于W處理中高含水率和低C/N條件限制了有機(jī)質(zhì)的降解[18],而WR處理中堆肥后期有機(jī)質(zhì)的降解促進(jìn)了CO2的排放.W處理中EC值(=-0.895,<0.01)和WR處理中含水率(= -0.895,<0.01)與CO2排放呈現(xiàn)負(fù)相關(guān)關(guān)系,主要通過影響微生物活性影響有機(jī)質(zhì)降解[24,35].EC作為代表堆肥腐熟度的重要指標(biāo)之一,W處理中的EC與GI值呈現(xiàn)顯著的負(fù)相關(guān)關(guān)系(=0.895,<0.01),這與之前的研究結(jié)果一致[23].WR處理中含水率的降低提升微生物活性,促進(jìn)有毒物質(zhì)的降解GI值上升.因此,含水率與GI值呈顯著負(fù)相關(guān)(=-0.895,<0.01).同時(shí),NH4+-N、NO3--N、C/N、pH值均與GI值呈現(xiàn)顯著相關(guān)性.

冗余分析結(jié)果顯示,理化指標(biāo)在W(C/N、溫度、pH值)和WR(C/N、NO3--N、EC)處理堆肥過程中對(duì)于溫室氣體(NH3、N2O、CH4、CO2)排放和GI值的變化解釋率達(dá)到90.0%和89.6%.其中,最主要的影響因素為C/N,解釋率分別為54.0%和51.5%.C/N與NH3、N2O和CO2排放正相關(guān),與CH4排放、GI值負(fù)相關(guān).堆肥初期C/N較高會(huì)增加溫室氣體的排放[36],而堆肥過程中隨著C/N的降低GI值逐漸增加[37].W處理中,溫度與NH3、CO2、CH4排放正相關(guān),這與之前的研究結(jié)論一致[5,34,38].WR處理中, NO3--N和EC值與N2O、CO2排放正相關(guān),與CH4排放、GI值負(fù)相關(guān),NO3--N在反硝化細(xì)菌作用下對(duì)N2O在堆肥后期排放貢獻(xiàn)較大[39].

2.7 碳、氮元素平衡分析

堆肥過程中的物料碳氮元素平衡分析和總溫室效應(yīng)分析如表3所示.W和WR處理的總碳損失率分別為67.4%和68.2%,總氮損失率分別為43.3%和43.9%.碳素主要以有機(jī)質(zhì)分解礦化生成CO2-C形式損失,損失量占初始TC的57.4%～64.1%;CH4-C的排放占初始TC比例較低,僅為0.07%～0.17%.氮素?fù)p失主要為NH3-N形式,在W和WR處理中,NH3-N損失量占初始總氮的33.3%和22.4%,產(chǎn)生差異的主要原因是WR處理中NH3-N以NH4+-N形態(tài)通過冷凝水損失,占初始總氮的17.4%;兩個(gè)處理的N2O損失占初始總氮比例較小,分別為1.3%和1.8%,但因其具有較大的溫室效應(yīng)系數(shù),對(duì)溫室效應(yīng)貢獻(xiàn)較大.

堆肥過程中排放的溫室氣體主要包括CO2、CH4和N2O.CO2作為有機(jī)質(zhì)分解礦化的產(chǎn)物,是堆肥過程中產(chǎn)生的最主要的溫室氣體,W和WR處理堆肥過程CO2溫室效應(yīng)占總溫室效應(yīng)的86.9%和85.8%,WR處理CO2當(dāng)量溫室效應(yīng)(1277.42kg/t)較W處理(1050.48kg/t)高18.0%,主要是由于堆肥后期有機(jī)質(zhì)的充分降解和N2O的大量排放.N2O對(duì)總溫室效應(yīng)貢獻(xiàn)高于CH4,有研究表明堆肥過程CO2的產(chǎn)生量可不計(jì)算入總溫室效應(yīng),若不考慮CO2的貢獻(xiàn),在W處理中,總溫室效應(yīng)為137.7kg CO2-eq/t,其中N2O占80.6%;在WR處理中,總溫室效應(yīng)為180.8kg CO2-eq/t,其中N2O占94.0%.

表3 堆肥過程中碳、氮元素平衡分析及溫室效應(yīng)分析

注:碳、氮元素平衡分別為碳、氮損失占初始總碳、總氮的百分比;溫室氣體排放當(dāng)量值以物料的干基計(jì)算;N2O和CH4對(duì)溫室效應(yīng)的貢獻(xiàn)率分別為CO2的256倍和28倍.

3 結(jié)論

3.1 封閉式堆肥體系高溫期產(chǎn)生的飽和水蒸汽形成的冷凝水產(chǎn)量占初始總重的18.0%.其主要成分為NH4+-N(11.40g/L)、溶解性有機(jī)碳(6.3g/L)以及無機(jī)鹽離子,具有較高的EC值(41.05mS/cm)和pH值(9.22).

3.2 反應(yīng)器堆肥過程中冷凝水回流不會(huì)對(duì)堆肥升溫造成影響,但是會(huì)影響堆肥腐熟度.堆肥產(chǎn)品種子發(fā)芽指數(shù)(GI)達(dá)到腐熟(71.4%),與不回流相比GI下降了25.5%.

3.3 冷凝水回流增加了55.2%的NH3排放和60.2%的CH4排放,降低了34.8%的N2O排放,且改變了氮素?fù)p失途徑,部分NH3-N以冷凝水銨態(tài)氮形式損失,損失量占初始TN的17.4%.

3.4 對(duì)于堆肥過程總溫室氣體排放,冷凝水回流處理總溫室效應(yīng)為137.7kg CO2-eq/t,N2O貢獻(xiàn)率為80.6%;冷凝水排出處理總溫室效應(yīng)為180.8kg CO2-eq/t,N2O貢獻(xiàn)率為94.0%.

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Effect of condensed water reflux on maturity and greenhouse gas emissions during composting.

WANG Jia-ni1, MA Ruo-nan1, TANG Ruo-lan1, LI Li-qiong1, PENG Li-juan1, LI Guo-xue1, LIN Jia-cong2, WANG Ding-mei2, LI Qin-fen2, YUAN Jing1*

(1.Beijing Key Laboratory of Farmland Soil Pollution Control and Remediation, College of Resources and Environment, China Agricultural University, Beijing 100193, China；2.Environment and Plant Protection Institute, Chinese Academy of Tropical Agricultural Sciences, Haikou 571101, China)., 2023,43(1)：234～243

In order to investigate the influence of condensed water reflux on maturity and greenhouse gas emissions during composting, the trial was conducted with pig manure and kitchen waste in a 60L reactor for 48 days. The results showed that the saturated steam produced during thermophilic composting could not be emitted due to the closed system of reactor and transformed into condensed water, accounting for 18% of the wet weight of raw metarials. In addition, the main components of the condensed water were ammonium nitrogen (NH4+-N, 11.4g/L), dissolved total organic carbon (TOC, 6.3g/L) and inorganic salt ions, resulting a higher EC (41.05mS/cm) and pH (9.22). However, due to its higher NH4+-N concentration of the condensed water, the seed germination index (GI) in the condensed water reflux treatment had a lower maturity (71.4%), which decreased by 25.47% comparing with non-reflux. Furthermore, condensate reflux increased NH3and CH4emissions by 55.17% and 60.24%, but CO2and N2O emissions decreased by 16.77% and 34.76%, respectively. Overall, condensate reflux reduced the total greenhouse gas emission by 17.8%. On the whole, we suggested the saturated steam should be condensed and discharged during composting, which could improve maturity and reduce moisture content of final compost, and decreasing the NH3emissions. Further, if the condensed water can be collected, the nitrogen element will be recycled and reused. This paper provids theoretical basis and data support for the water removal and maturity in closed aerobic composting system.

composting；condensate reflux；maturity；nitrogen loss；greenhouse gas

X705

A

1000-6923(2023)01-0234-10

王佳妮(1998-),女,山西長(zhǎng)治人,碩士研究生,主要研究方向?yàn)楣腆w廢棄物處理與資源化.

2022-06-07

海南省重大科技計(jì)劃項(xiàng)目(ZDKJ2021009)

* 責(zé)任作者, 副教授, jingyuan@cau.edu.cn