汪開英,吳捷剛,梅威達,王校帥
畜舍顆粒物減排技術研究現(xiàn)狀
汪開英,吳捷剛,梅威達,王校帥
(浙江大學農(nóng)業(yè)生物環(huán)境工程研究所,杭州,310058)
隨著大規(guī)模集約化畜牧業(yè)的發(fā)展,畜牧生產(chǎn)過程中產(chǎn)生的大量懸浮顆粒物(Particulate Matter,PM),已成為大氣顆粒物PM10和PM2.5的重要來源。畜舍粉塵主要來源于飼料、糞便、皮屑、毛發(fā)等,其表面附著有細菌、真菌、病毒等致病微生物以及氨氣、硫化氫等有害氣體,不但嚴重威脅畜牧場工作人員和家畜的健康,還導致周邊大氣環(huán)境污染??茖W適用的PM減排技術是保障畜舍及周邊環(huán)境空氣質(zhì)量的重要手段。該研究概述了畜舍PM的排放源、特征及危害,從源頭、過程、末端3個環(huán)節(jié)分別論述了國內(nèi)外畜舍顆粒物減排技術的研究現(xiàn)狀及存在問題。源頭減排包括飼料、清糞工藝、飼養(yǎng)模式等方面的優(yōu)化,經(jīng)濟且高效。過程減排包括噴霧降塵、通風除塵、靜電除塵等技術,旨在降低舍內(nèi)懸浮在空氣中的顆粒物。噴霧技術相對成熟,但易滋生細菌且不適用于低溫季節(jié);通風技術對去除畜舍PM上應用最為廣泛且高效,需要綜合考慮滿足畜舍通風換氣與降塵的要求;靜電除塵技術對人畜無干擾,但在除塵效率和二次揚塵方面有待進一步優(yōu)化。末端減排包括洗滌降塵技術與過濾降塵技術,目的是減少PM對外界大氣環(huán)境的污染。洗滌技術可以去除排氣中多種污染物,但設備易腐蝕;過濾技術成本低,對大顆粒物的去除效率高,但易堵塞。該研究對畜舍PM減排技術研究現(xiàn)狀進行綜述,以期為未來開發(fā)高效、節(jié)能、經(jīng)濟、環(huán)保的畜舍PM減排技術提供參考。
顆粒物;環(huán)境調(diào)控;畜牧業(yè);排放源;特征;危害;減排技術
大氣顆粒物(Particulate Matter,PM)是在氣溶膠體系中均勻分散的各種固態(tài)和液態(tài)顆粒狀物質(zhì)的總稱,其大小通常用空氣動力學直徑(Aerodynamic Equivalent Diameter,AED)來描述,按粒徑大小,PM可分為總懸浮顆粒物(Total Suspended Particulate,TSP)(AED≤100m)和可吸入顆粒物(Inhalable PM,IPM)(AED≤10m),其中IPM又可以分為粗顆粒物(2.5m 大氣顆粒物主要來源于地表揚塵、煤等燃料能源的消耗、工業(yè)生產(chǎn)、機動車排放及生物質(zhì)燃燒等[3]。然而,農(nóng)業(yè)-畜牧養(yǎng)殖顆粒物源排放一直為大眾所忽略,畜舍內(nèi)的PM濃度比其他室內(nèi)環(huán)境高10~100倍[4]。比較而言,畜舍所產(chǎn)生的PM危害更大,其攜帶有惡臭物質(zhì)、揮發(fā)性有機物(Volatile Organic Chemicals,VOCs)以及其他有害氣體,同時也是細菌、病毒等微生物傳播的載體。畜舍內(nèi)微生物以細菌為主,其中含量相對較高的是革蘭氏陽性菌[5]。病毒雖然占比很小,若吸附于顆粒物表面自由擴散,將產(chǎn)生巨大的危害,如非洲豬瘟病毒可通過短距離氣溶膠傳播[6]。因此,畜舍降塵減排是保障舍內(nèi)外人畜健康和保護周邊環(huán)境空氣質(zhì)量的有效手段。本文重點介紹和分析畜舍PM源頭、過程與末端3個環(huán)節(jié)的減排技術的研究現(xiàn)狀及存在問題,以期為改善人畜生存環(huán)境和畜牧業(yè)可持續(xù)發(fā)展提供參考。 畜牧業(yè)是農(nóng)業(yè)PM排放的重要來源,隨著畜牧養(yǎng)殖業(yè)的高速發(fā)展,其對農(nóng)業(yè)PM排放的貢獻還將不斷增加。不同畜舍內(nèi)PM的來源與組成存在差異。雞舍PM主要來自羽毛和糞便等[7]。其中細顆粒物(PM2.5)所占比例非常高,研究發(fā)現(xiàn),此類PM2.5中有超過50%來源于二次顆粒物[8]。養(yǎng)殖場的氨氣(NH3)與空氣中酸性氣體發(fā)生化學反應形成的無機鹽粒子均屬于二次顆粒物[9-12]。豬舍PM來源有飼料、皮屑、糞便與礦物灰等,舍內(nèi)主要為粗顆粒物,其中飼料是最主要的顆粒物來源。許穩(wěn)等[13]采用中流量顆粒物采樣器(TH-150C Ⅲ型,100 L/min)對北京郊區(qū)一個集約化豬場舍內(nèi)顆粒物的濃度進行了連續(xù)測定,發(fā)現(xiàn)舍內(nèi)顆粒物的粒徑主要在10~100m。 顆粒物的產(chǎn)生與動物活動有關,可以根據(jù)動物的排放速率評估。相對濃度作為畜舍內(nèi)PM的主要物理特征,直接影響動物生產(chǎn)性能及健康狀況,常被用來評價家畜生存環(huán)境及生長福利的優(yōu)劣。Takai等[14]與Winkel等[15]分別對不同種類畜舍內(nèi)PM10與PM2.5相對濃度及單個動物的平均排放率進行測量并分析,發(fā)現(xiàn)不同畜種間存在較大差異(表1)。 表1 不同種類畜舍內(nèi)PM10與PM2.5相對濃度及單個動物的平均排放率[14-15] 雖然家禽的PM平均排放率在雞、豬和牛3個畜種里偏低,但由于家禽個體的占地面積小,飼養(yǎng)密度遠大于生豬與奶牛。因此,一般情況下,雞舍中的PM濃度高于豬舍并顯著高于牛舍[16]。 畜舍PM表面附著有大量的重金屬離子、VOCs、NH3、內(nèi)毒素、細菌與真菌等,沉降在家畜體表會堵塞汗腺導致皮膚發(fā)炎,落在眼結膜上會引起眼結膜炎;PM2.5還會隨呼吸進入支氣管甚至肺泡,導致肺泡巨噬細胞吞噬能力下降進而影響機體的免疫功能,增加家畜患支氣管炎、肺炎的概率[17]。Michiels等[18]研究表明PM10對育肥豬的呼吸健康有顯著影響,易導致呼吸道疾病,降低豬的生產(chǎn)性能。相比于其他職業(yè),長時間處于畜舍高濃度PM環(huán)境下的工人更容易患呼吸系統(tǒng)疾病。 同時,畜舍PM通過排風系統(tǒng)進入大氣循環(huán)系統(tǒng),成為大氣污染物的重要組成部分。PM的粒徑越小,擴散的距離越遠[19]。大量持續(xù)的畜舍PM排放會加劇大氣質(zhì)量惡化,破壞大氣環(huán)境平衡,嚴重時可降低大氣能見度,引發(fā)霧霾[20]。畜舍PM擴散至居住區(qū)則會影響周圍居民的正常生活,誘發(fā)急慢性呼吸道疾病發(fā)生,嚴重時甚至致人死亡。Jerrett等[21]發(fā)現(xiàn)戶外空氣污染(主要為商業(yè)和住宅能源所貢獻的細顆粒物)在世界范圍內(nèi)造成每年超過320萬人死亡,農(nóng)業(yè)源作為第二大貢獻者,在許多歐洲國家,其造成的死亡率可以占到總空氣污染死亡率的40%。 畜舍PM源頭減排是指在借助PM減排設備減少畜舍內(nèi)環(huán)境中已產(chǎn)生的PM之前,通過改變一些日常的生產(chǎn)管理模式來切斷或減少舍內(nèi)PM源頭的產(chǎn)生量,從而降低舍內(nèi)PM濃度的過程。 畜舍的PM來源有飼料、糞便、皮屑、羽毛等,飼料是畜舍內(nèi)重要的PM來源,選擇合適的飼料類型可以顯著降低舍內(nèi)PM濃度。相比而言,粉狀飼料較顆粒狀飼料更易產(chǎn)生PM,使用液體飼料是減少粉塵源排放的有效途徑。另外,飼料添加劑或飼料涂層的使用也可以有效減少飼料對源頭粉塵的貢獻[22]。Cheng等[23]通過試驗發(fā)現(xiàn)在飼料中添加10%的新鮮發(fā)酵豆粕可降低了仔豬舍內(nèi)19.9%的PM10和11.6%的PM2.5濃度;張慶振等[24]用10%的菌肽蛋白替代保育豬日糧中的部分普通豆粕,試驗結果與對照組對比后發(fā)現(xiàn)能夠降低舍內(nèi)20.34%的PM10與8.09%的PM2.5濃度;吳勝等[25]在斷奶仔豬飼料中添加不同劑量的植物精油制劑并與對照組進行對比,結果表明添加植物精油可顯著降低舍內(nèi)微生物氣溶膠濃度,且大劑量的去除效果更優(yōu)。 研究發(fā)現(xiàn)雞舍中可吸入顆粒物主要來源于飼料羽毛和糞便等,通過改進清糞工藝、調(diào)整飼養(yǎng)模式等途徑可以減少PM源頭排放。程龍梅等[26]研究了刮糞板和傳送帶兩種清糞模式對蛋雞舍內(nèi)PM濃度的影響,發(fā)現(xiàn)傳送帶清糞的禽舍內(nèi)空氣質(zhì)量較好,更適合在蛋雞養(yǎng)殖中應用;Wathes等[27]發(fā)現(xiàn)不同飼養(yǎng)模式下舍內(nèi)粉塵濃度有顯著差異,地面飼養(yǎng)肉雞的粉塵水平高,棲架或分籠層飼養(yǎng)模式可有效降低舍內(nèi)粉塵濃度。 源頭控制是畜舍PM減排中相對經(jīng)濟而有效的環(huán)節(jié),且可有效降低畜舍PM的過程與末端減排壓力。如表2所示,改變飼料結構、在飼料中添加脂類物質(zhì)成本低且易實施,但需評估飼料結構及配方的改變對家畜營養(yǎng)攝入的影響。結合具體畜種選擇適宜的飼養(yǎng)模式,發(fā)展可有效降低PM等污染物排放的機械化、自動化的糞污清理模式是未來方向。 表2 畜舍PM源頭減排技術分析 畜舍PM過程減排是指利用多種除塵減排技術降低舍內(nèi)懸浮在空氣中的PM濃度,旨在提升畜舍內(nèi)環(huán)境質(zhì)量。其包括噴霧降塵、通風除塵、靜電除塵等技術。 噴霧降塵是一種發(fā)展較為成熟的技術,其原理是利用高速氣流的分裂作用將液體轉換成細霧。噴灑的液體一般為水、油、水與各種除臭菌劑的混合物或水油混合物等,液體在噴出過程中受空氣阻力作用迅速斷裂成細霧,粘附粉塵后受重力作用沉降,從而實現(xiàn)舍內(nèi)降塵。Aarnink等[28]在肉雞舍內(nèi)噴灑菜籽油,研究噴油量對降塵效率的影響,發(fā)現(xiàn)單位面積的日均噴油量越大,降塵效果越好;任景樂等[29]采用重量法探討噴霧前后蛋雞舍不同位置顆粒物的分布與變化,結果表明粉塵在雞舍的中部濃度最高,噴霧可有效降低PM濃度;Zheng等[30]在蛋雞舍內(nèi)使用中性電解水噴霧,與對照組比較后發(fā)現(xiàn),3 h即可降低舍內(nèi)34%的粉塵水平;Zhu等[31]在妊娠母豬舍內(nèi)噴霧并對顆粒物濃度變化進行了評估,發(fā)現(xiàn)在喂料期間開啟噴霧系統(tǒng),空氣中平均粉塵濃度降低了75%;Mostafa等[32]評價了在育肥豬舍內(nèi)噴淋水油混合物對顆粒物的降塵效果,結果表明PM平均去除效率為63%,使用小噴嘴的除塵效率高于大噴嘴;Chai等[33]在養(yǎng)雞場噴灑酸性電解水來降低舍內(nèi)PM水平,設置了3組噴施量并與無噴水組對照,結果發(fā)現(xiàn)電解水的噴施量越大,PM的排放率越低;Nonnenmann等[34]在養(yǎng)豬場每天噴5%的油水混合物并設置對照組來評估舍內(nèi)降塵效果,結果表明平均可減少52%的舍內(nèi)粉塵,應用大豆油與菜籽油的除塵效果無顯著差異。 噴霧降塵技術的成本低、除塵效率高,但其運行過程中存在耗水量大、噴頭易堵塞、容易滋生細菌、影響家畜健康等問題。因而舍內(nèi)噴霧降塵,要參照溫濕度、粉塵等多種環(huán)境參數(shù),在滿足家畜健康福利要求基礎上合理適度施用。 國外學者針對畜舍噴霧降塵技術研究了影響噴霧降塵效率的噴霧量與頻率、噴嘴選型與孔徑等關鍵因素對PM減排的影響。中國針對畜舍內(nèi)的噴霧降塵技術研究較少,通常噴霧的主要目的用于夏季降溫,緩解家畜的熱應激。未來噴霧技術應當著重對提高霧化效率、PM減排效率、廢水回收與凈化效率等方面展開研究。 通風是去除畜舍內(nèi)PM的有效方法,它能夠?qū)⑸醿?nèi)污濁、潮濕的氣體排出外界,同時帶走大量的PM、微生物及污染性氣體,補充進新鮮的空氣從而優(yōu)化家畜生長環(huán)境。 畜舍的通風方式主要有自然通風與機械通風。自然通風是指通過畜舍結構上的開口產(chǎn)生空氣流動,動力源于自然對流形成的風壓或熱壓差,主要有開窗式與敞開式兩種。郭霏等[35]研究了不同畜舍間距(1、2、3倍屋脊高)與不同污染物釋放位置(上、下風向)對自然通風畜舍氣流與污染物分布的影響,結果表明畜舍間距選擇2可有效減少舍間污染物擴散。由于自然通風受舍外天氣、風速、風向等多因素共同作用,可控性差,因此現(xiàn)代集約化畜牧場一般采用機械通風系統(tǒng)。機械通風根據(jù)驅(qū)動原理不同可分為負壓通風、正壓通風與等壓通風。不同的氣候或季節(jié)下畜舍通風量不同,畜舍內(nèi)氣流速率以及分布亦不同,進而影響舍內(nèi)的PM濃度。吳勝等[36]發(fā)現(xiàn)豬舍通風量是影響畜舍內(nèi)顆粒物分布的主要因素。夏季氣溫較高,需要加大通風量以帶走舍內(nèi)多余的熱量,通風率高于冬季,舍內(nèi)的顆粒物濃度低、排放率高。冬季畜舍需要保溫,舍內(nèi)通風率低、顆粒物濃度高、排放率低[37-38];黃藏宇[39]發(fā)現(xiàn)在封閉豬舍中安裝可調(diào)節(jié)風速和風量的新風系統(tǒng)可以顯著地降低舍內(nèi)PM濃度,最高降幅為65.8%;Kwon等[40]以通風率、室內(nèi)外溫度、豬數(shù)量、年齡與活動水平等作為參數(shù),利用CFD(Computational Fluid Dynamics)技術研究機械通風養(yǎng)豬場的粉塵排放規(guī)律,結果表明通風是影響TSP和PM10最主要的因素,調(diào)整通風速率和改善通風系統(tǒng)特性可以有效降塵;汪開英等[41]在豬舍內(nèi)應用CFD技術模擬PM濃度場,結果表明TSP、PM10、PM2.5均受通風率影響,通風速率越大、PM濃度越小,其中PM2.5受通風的影響最大。 機械通風畜舍內(nèi)PM分布、運動趨勢受氣流分布、通風率與通風量等多種通風系統(tǒng)特性的影響,如畜舍不同的通風機制會導致舍內(nèi)不同的氣流分布,進而影響舍內(nèi)PM等污染物分布。因此,在畜舍通風系統(tǒng)設計中應綜合考慮通風換氣與降塵的需要,可應用CFD技術深入研究經(jīng)濟高效的通風降塵策略。 靜電除塵技術的原理是靜電粒子電離(Electrostatic Particle Ionization,EPI),利用電極連接高壓電后產(chǎn)生電暈效果,放出自由電子和離子使顆粒物帶電,大氣中的帶電顆粒物受到周圍電場力的作用向集塵區(qū)聚集,從而實現(xiàn)顆粒物的收集減排[42]。靜電除塵按照不同的除塵方式可分為靜電空間電荷系統(tǒng)(Electrostatic Space Charge System,ESCS)、干式靜電除塵器(Dry Electrostatic Precipitator,DESP)和濕式靜電除塵器(Wet Electrostatic Precipitator,WESP)[43]等。 ESCS系統(tǒng)一般安裝在天花板等不易接觸的位置,對人畜無干擾、維護要求低,但除塵效率低。Cambra-Lopez等[44]在肉雞舍內(nèi)采用靜電除塵技術并評估了其對顆粒物分布的影響,表明電離系統(tǒng)能有效減少畜舍PM的排放。劉濱疆等[45]采用靜電空間電荷系統(tǒng)(ESCS)對仔豬保育舍與籠養(yǎng)蛋雞舍進行影響評估,結果表明ESCS能去除70%~94%的粉塵與50%~93%的微生物;孫利[46]研究了靜電空間電荷系統(tǒng)(ESCS)對雞舍內(nèi)環(huán)境改善作用,并比較了功率高低對除塵效率的影響,結果表明,與對照雞舍相比,安裝ESCS能顯著降低雞舍內(nèi)粉塵(<0.05),在高功率下空氣凈化效率高;李永明等[47]在保育豬舍內(nèi)安裝靜電空間電荷系統(tǒng)(ESCS),結果表明能夠顯著增加空氣中負離子的濃度(<0.01),同時顯著降低空氣中各種粒徑的粉塵含量(<0.01);Ritz等[48]設計了一種靜電空間電荷系統(tǒng)(ESCS)用于減少商品肉雞舍的粉塵排放,設置對照組對比后發(fā)現(xiàn)ESCS平均降低了空氣中43%的粉塵;閆懷峰等[49]在豬舍天花板安裝3DDF—450型號靜電空間電荷系統(tǒng)(ESCS),與對照豬舍相比,減少了空氣中67.6%的TSP;王樹華等[50]在保育豬舍內(nèi)使用3DDF—450型靜電空間電荷系統(tǒng)(ESCS),與對照豬舍相比后發(fā)現(xiàn)該系統(tǒng)最大可降低72.59%的舍內(nèi)TSP濃度(<0.01);徐鑫等[51]在試驗蛋雞舍內(nèi)安裝了10套300型靜電空間電荷系統(tǒng)(ESCS),與對照蛋雞舍比較后發(fā)現(xiàn)舍內(nèi)粉塵的平均質(zhì)量濃度降低了35.9%;Nicolai等[52]比較了裝有靜電空間電荷系統(tǒng)(ESCS)與未安裝的豬舍內(nèi)粉塵含量,發(fā)現(xiàn)ESCS系統(tǒng)平均減少了63%的舍內(nèi)顆粒物;張開臣等[53]利用八套3DDF—300型靜電空間電荷系統(tǒng)(ESCS)對1個平養(yǎng)肉雞舍的粉塵和微生物進行控制,結果表明其對粉塵與微生物的去除率分別為97.8%與90%;Dolejs等[54]使用靜電空間電荷系統(tǒng)(ESCS)對奶牛舍進行降塵,結果表明奶牛舍內(nèi)粉塵濃度降低了12.7%~26.2%;吳新[55]利用空氣電凈化技術構建了一套靜電空間電荷系統(tǒng)(ESCS),在封閉式保育豬舍內(nèi)進行試驗,結果表明該系統(tǒng)能夠去除70%的粉塵;焦洪超等[56]利用靜電空間電荷系統(tǒng)(ESCS)在雞舍施加人工負離子,設置對照組并研究了其對粉塵的清除效率,結果表明空氣負離子可以顯著降低舍內(nèi)粉塵含量,且其對粒徑大于1m的粉塵清除效率較高。 DESP系統(tǒng)利用分布板將氣流均勻分布,通過高壓電場使氣流中的顆粒物帶電,并受電場作用向陽極板運動,吸附于陽極后通過振打的方式使PM震落于灰斗中。該系統(tǒng)除塵效率優(yōu)于ESCS系統(tǒng),功耗低且易于集成,但其除塵效果較依賴于系統(tǒng)的通風特性,其對細顆粒物的去除效果差且震動過程易造成二次揚塵。Winkel等[57]比較了商用干式過濾器和干式靜電除塵器(DESP)對禽舍粉塵的去除效率,結果表明DESP對粉塵的去除效果更好,能平均減少57%的PM10與45.3%的PM2.5;Manuzon等[58]運用CFD技術優(yōu)化干式靜電除塵器(DESP),并在試驗室以及現(xiàn)場條件下評估優(yōu)化后的除塵器對禽舍顆粒物收集效率,發(fā)現(xiàn)優(yōu)化后除塵器的總收集效率可達89%(實驗室)與82%(現(xiàn)場);Chai等[59]使用優(yōu)化后的干式靜電除塵器(Improved Dry Electrostatic Precipitator,IDESP)對禽舍除塵,總降塵效率最高可達79%。 WESP系統(tǒng)早前主要應用于冶金、化工等領域,近年來在燃煤電廠迅速推廣,靜電場空間的水霧在一定程度上提高了電場特性,大大提高了對顆粒物的收集效率[60]。Ru等[61]在模擬實驗室的條件下評價了濕式靜電除塵器(WESP)對家禽養(yǎng)殖顆粒物的減排效果,建立顆粒物去除效率預測經(jīng)驗模型,結果表明在特定電壓、風速等條件下最多能減少94%的PM10與88%的PM2.5排放。與DESP系統(tǒng)不同之處在于WESP系統(tǒng)采用噴淋的方式在集塵板上形成流動水膜,利用水膜將集塵板捕獲的粉塵沖刷清洗至灰斗中,避免了二次揚塵等問題。其對PM的凈化效果優(yōu)于ESCS與DESP,但安裝較為困難、耗水量大且易形成污水等二次污染物,因而無法大面積推廣與使用。 畜舍內(nèi)靜電除塵技術的使用效果與其自身結構(極板間距、電暈線間距、密封性)、供電裝置(絕緣性、接地性)、風速等密切相關,對于夏季大通風量下的畜舍,靜電除塵效果有限。目前,畜牧養(yǎng)殖業(yè)主要應用ESCS對畜舍降塵,雖然可以將粉塵收集至集電極、天花板、地面或其他金屬表面上,降低空氣中的粉塵濃度,但并沒有減少粉塵的總量,除塵效率、二次揚塵等問題亟待解決。DESP與WESP系統(tǒng)的降塵效率優(yōu)于ESCS系統(tǒng),但存在二次揚塵、投入與維護成本高等關鍵性問題。如何解決經(jīng)濟性與普適性使DESP與WESP系統(tǒng)適用于畜舍環(huán)境降塵是未來發(fā)展的關鍵。中國主要開展了ESCS的降塵研究,針對DESP與WESP系統(tǒng)在畜牧業(yè)應用的相關研究鮮有。 畜舍PM過程減排是畜牧養(yǎng)殖管理過程中非常重要的一個環(huán)節(jié)。通過降低舍內(nèi)PM濃度可以提高動物福利,促進動物生長、生產(chǎn),進而提高經(jīng)濟效益。如表3所示,現(xiàn)階段針對畜舍PM過程減排技術有一定的優(yōu)勢,但仍存在影響動物生長、除塵效率低、成本高、二次污染等問題。如何對現(xiàn)有除塵設備在結構、性能上進行優(yōu)化,制定適宜的除塵策略是提高畜舍內(nèi)除塵效率的關鍵。 表3 畜舍PM過程減排技術分析 畜舍PM末端減排是指通過在畜舍排風機外安裝特定的除塵裝置對畜舍排出的廢氣凈化,從而減少對外界大氣環(huán)境污染的過程。其包括洗滌除塵、過濾除塵等末端減排技術。 洗滌除塵技術是指利用洗滌器的慣性、截留和擴散等原理,在相對密閉的空間通過液體過濾沉降廢氣中的PM,從而實現(xiàn)減少排放的過程,該技術已被證明能夠有效去除畜舍內(nèi)排出的顆粒物[62]。Zhao等[63]比較了豬舍外的3種空氣洗滌器對顆粒物的去除效果,結果發(fā)現(xiàn)三級洗滌器的降塵效果優(yōu)于二級洗滌器,去除效率與洗滌級數(shù)成正比;Melse等[64]利用空氣洗滌器去除畜舍內(nèi)顆粒物,研究發(fā)現(xiàn)多級洗滌器具有較高的平均空氣停留時間,去除效率更佳,最高可去除79%的PM10;Aarnink等[65]利用三級空氣洗滌器來去除育肥豬舍內(nèi)粉塵,發(fā)現(xiàn)可以去除93%的PM10與90%的PM2.5;李世才等[66]設計了一種空氣洗滌器以提高畜舍空氣質(zhì)量,該洗滌器利用水溶解有害氣體以及吸附粉塵和病原微生物,同時結合植物栽培,經(jīng)測試發(fā)現(xiàn)其對空氣中的PM有良好的凈化效果。 研究發(fā)現(xiàn)洗滌除塵的效率主要與洗滌器級數(shù)有關,增加洗滌器級數(shù)能夠大幅提高對洗滌器對顆粒物的除塵效率,但提高降塵效率的同時增加了能耗與成本。且洗滌處理過程中產(chǎn)生大量的污水等二次污染物,因此洗滌除塵技術尚未在畜牧領域推廣使用。中國對畜舍洗滌除塵技術的研究與應用較少,目前該技術多應用于工業(yè)領域除塵。因此,洗滌降塵技術在畜牧領域的應用有待進一步研究和改進,基于大廢氣排放量的經(jīng)濟高效畜舍洗滌降塵技術是未來研究重點。 過濾除塵是通過在畜舍排風口外一定距離建造具有一定透過率的多孔介質(zhì)如防風林和擋風墻,迫使氣流突然改變方向,通過離心力的原理,部分顆粒物在氣流過后會被阻擋而沉淀,從而實現(xiàn)有害氣體與顆粒物的分離減排,其對粗顆粒物的阻擋效果優(yōu)于細顆粒物。過濾系統(tǒng)應距離風機口2~4個風機直徑以確保風機性能,高度至少3 m以確保攔截。研究表明不同種類植物的除塵效率不同[67-68]。范舒欣等[69]評估了8種常綠闊葉樹種對PM的捕獲能力,發(fā)現(xiàn)衛(wèi)矛和早園竹的擋塵效率高;Adrizal等[70]通過在5個商品雞場風機外分別種植云杉、雜交楊樹、雜交柳樹等來評估防風林對粉塵的捕獲潛力,試驗證明云杉與雜交柳樹對顆粒物具有更好地捕獲效率;張家洋等[71]選擇了9種植物并分析它們的滯塵量,結果表明國槐、鳳尾蘭、紫葉李這3種植被的降塵率較高;馬淑麗[72]通過計算流體力學模擬與實地驗證發(fā)現(xiàn)防風林能夠使下風向的顆粒物濃度顯著降低,在試驗過程中比較了5種植被的滯塵能力,發(fā)現(xiàn)紫葉稠李形成的防風林具有最高的滯塵效率,植被葉片的表面粗糙度與攔截效率呈正相關。 除比較不同植物間的滯塵能力外,多位國內(nèi)外學者評估了過濾系統(tǒng)在畜舍外的除塵效率。Willis等[73]利用激光雷達技術來估計隧道通風的肉雞舍外防風林對顆粒物的捕獲效率,發(fā)現(xiàn)在夜間低風速與湍流強度的條件下最多可以捕獲74%的顆粒物;Hernandez等[74]通過風洞試驗和現(xiàn)場監(jiān)測,確定了防風林對養(yǎng)豬場內(nèi)氣流的影響,并監(jiān)測了防風林前后的顆粒物濃度,結果表明防風林可以減少40%以上的顆粒物;Winkel等[75]對養(yǎng)雞場糞肥干燥隧道(Manure Drying Tunnels,MDTs)的除塵效果進行評價,發(fā)現(xiàn)MDTs能有效沉降廢氣中的顆粒物并將其回收至糞肥中,肥料層厚度增加也能提高對PM10的去除效率;Mostafa等[76]創(chuàng)新設計了一款干式過濾系統(tǒng)并評估了對雞舍PM去除效率,試驗結果表明其可以降低55%的舍內(nèi)PM濃度,減少72%的PM排放率。使用生物質(zhì)材料的擋風墻對排出的廢氣過濾,同樣取得了較好的降塵效果。董紅敏等[77]將秸稈與雜草作為擋風墻的材料來凈化蛋雞舍排出的廢氣,結果表明,可降低77%的TSP濃度與55%的IPM濃度。相比生物質(zhì)材料,使用活性炭纖維過濾材料在去除雞舍廢氣中PM的同時,對細菌和大腸桿菌的吸附效率更優(yōu)[78]。若將過濾除塵系統(tǒng)與靜電除塵技術相結合可以進一步優(yōu)化除塵效果,Jerez等[79]在肉雞舍排風口末端生物質(zhì)擋風墻的基礎上增加了EPI系統(tǒng)并比較了增加前后對粉塵的減排效率,結果表明增加EPI后可以進一步減少39%的TSP排放。 過濾系統(tǒng)對畜舍內(nèi)排出的顆粒物具有一定的攔截效率,其攔截效果依賴于植物種類、系統(tǒng)的組成、材料、結構、孔隙率與位置等多種因素。該技術的成本低,對大顆粒物的降塵效果好,但擋風墻長期使用易堵塞,導致系統(tǒng)前后壓降增大,影響風機通風性能,需要頻繁清理。目前國內(nèi)外現(xiàn)有的過濾除塵技術在降塵效率上仍有很大的提升空間,結合多種除塵技術、篩選除塵介質(zhì)、優(yōu)化除塵系統(tǒng)結構等是未來過濾系統(tǒng)除塵技術的研究方向。 畜舍PM末端減排對保護外界大氣環(huán)境、防止污染物自由擴散及疫病傳播等有至關重要的作用。如表4所示,洗滌技術在去除PM的同時還可以去除空氣中氨氣等污染性氣體,對細顆粒物降塵效果好。過濾技術建造的成本低,對大顆粒物降塵效果好。但其均存在維護成本高、空氣阻力大等問題。因此,畜舍洗滌技術的經(jīng)濟性有待提高,擋塵墻的自清潔系統(tǒng)有待研發(fā),防風林滯塵能力的相關研究有待進一步開展。 表4 畜舍PM末端減排技術分析 畜舍PM的來源、特征十分復雜,而各種PM減排技術都有其適用場合、優(yōu)勢與不足,且畜舍PM排放特征因畜種及生長階段不同而差異顯著,因而單一的技術難以滿足復雜、多樣的畜舍環(huán)境PM減排需求。在實際生產(chǎn)中應針對不同畜舍PM排放特征與排放規(guī)律科學選用PM減排技術。 畜舍PM減排是一個系統(tǒng)工程,從源頭、過程至末端的PM全程減排是未來的發(fā)展方向。源頭控制是畜舍內(nèi)PM減排中經(jīng)濟而有效的環(huán)節(jié),有效控制源頭排放,可降低過程與末端的PM減排壓力;過程減排旨在通過減少畜舍內(nèi)漂浮在空氣中的PM,可顯著改善畜舍內(nèi)環(huán)境空氣質(zhì)量;末端減排可降低對外界大氣環(huán)境的污染及PM中攜帶的病毒傳播與擴散風險。雖然現(xiàn)有的畜舍PM源頭、過程和末端的減排技術在一定程度上可去除畜舍PM,但在降塵效率、適應性、穩(wěn)定性、二次污染風險、對人畜健康和畜舍通風性能的影響等方面存在的問題仍有待進一步研究解決。 中國作為世界家畜養(yǎng)殖大國,在畜舍PM減排技術的研究與應用方面仍處于起步階段,不能滿足畜舍內(nèi)外環(huán)境友好型畜牧業(yè)發(fā)展的需要。因此,為實現(xiàn)精準、高效的畜舍PM減排,改善與保障畜舍內(nèi)外環(huán)境空氣質(zhì)量,亟待結合我國畜牧養(yǎng)殖實際情況,開展畜舍PM源解析和排放特征等基礎研究,研發(fā)高效、節(jié)能、經(jīng)濟、環(huán)保的畜舍PM減排技術,建立畜舍PM全程減排技術體系。 [1] Berlo D, Hullmann M, Schins R P. 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The dust of the livestock houses mainly originates from the feed, manure, dander, hair, etc., and carries various types of microorganisms, including bacteria, fungi, viruses, and harmful gases such as ammonia and hydrogen sulfide. No doubt that livestock related PM can seriously threaten not only the health of livestock farm workers and animals, but also the surrounding atmospheric environment. Applicable technologies of PM emission reduction are the important methods to maintain the comfortable indoor air quality as well as the ambient environment. This review firstly overviewed the emission sources, characteristics, and hazards of PM in livestock houses. Then, major focuses were put into the research status and problems of state-of-the-art reduction technologies of PM in livestock houses. In this part, the reduction technologies were classified into three groups: source reduction methods, process reduction methods, and terminal reduction methods. According to literature review, the source control of PM in livestock houses was generally done by changing either the form of feed or adding feed additives (e.g., animal fat, tallow). Source emission reduction included optimization of feed, manure removal technology, and breeding mode, etc. However, reports showed that both modification of feed form and usage of feed additives might negative affect the growth of animals, and affect animal welfare to a certain extent. Process control includes spray technology, optimization of ventilation system, and electrostatic precipitator technology, which were designed to reduce the particulate matter suspended in the barn air. The spray technology was relatively mature, low in cost, and high in efficiency. But it was easy to breed bacteria, which affected the health of animals, and it was also not suitable for low temperature season. Dust removal via ventilation was the most widely used and efficient way for removing PM in livestock houses, and requirements of ventilation and dust reduction in livestock houses should be taken into comprehensive consideration. Electrostatic precipitator technology was divided into three categories: Electrostatic Space Charge System (ESCS), Dry Electrostatic Precipitator (DESP), and Wet Electrostatic Precipitator (WESP) according to different structures, which had no interference to humans and animals, but their efficiency were needed to be further optimized. Terminal control included scrubber technology and filter technology, aiming to reduce the harm to the ambient environment. Scrubber technology could effectively remove multiple pollutants in the air by means of chemical and biological methods, but such equipments were more susceptible to corrosion. Filter technology was a cheap method and had a good dust-reducing performance on large particles. However, it was easy to clog, resulting in an increased air resistance and decreased efficiency of dust reduction. By comparing and analyzing the research status of various emission reduction technologies, hopefully, it could serve as a reference for the future development of high-efficiency, energy-saving, economical, and environment-friendly reduction technologies of particulate matter in livestock houses. particulate matter; environmental regulation;animal husbandry; emission source; characteristic; harm; emission reduction technology 汪開英,吳捷剛,梅威達,等. 畜舍顆粒物減排技術研究現(xiàn)狀[J]. 農(nóng)業(yè)工程學報,2020,36(18):204-212.doi:10.11975/j.issn.1002-6819.2020.18.025 http://www.tcsae.org Wang Kaiying, Wu Jiegang, Mei Weida, et al. Research status on particulate reduction technology in livestock houses[J]. Transactions of the Chinese Society of Agricultural Engineering (Transactions of the CSAE), 2020, 36(18): 204-212. (in Chinese with English abstract) doi:10.11975/j.issn.1002-6819.2020.18.025 http://www.tcsae.org 2020-06-20 2020-08-19 浙江省科技廳重點研發(fā)計劃項目(2018C02035) 汪開英,博士,副教授,主要從事農(nóng)業(yè)空氣質(zhì)量及動物福利方面研究。Email:zjuwky@zju.edu.cn 10.11975/j.issn.1002-6819.2020.18.025 X513 A 1002-6819(2020)-18-0204-091 畜舍PM排放源、特征與危害
2 畜舍PM源頭減排
3 畜舍PM過程減排
3.1 噴霧降塵
3.2 通風降塵
3.3 靜電除塵
4 畜舍PM末端減排
4.1 洗滌除塵
4.2 過濾除塵
5 結論與展望