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畜舍颗粒物减排技术研究现状

2020-12-03汪开英吴捷刚梅威达王校帅

农业工程学报 2020年18期
关键词:降尘除尘粉尘

汪开英,吴捷刚,梅威达,王校帅

畜舍颗粒物减排技术研究现状

汪开英,吴捷刚,梅威达,王校帅

(浙江大学农业生物环境工程研究所,杭州,310058)

随着大规模集约化畜牧业的发展,畜牧生产过程中产生的大量悬浮颗粒物(Particulate Matter,PM),已成为大气颗粒物PM10和PM2.5的重要来源。畜舍粉尘主要来源于饲料、粪便、皮屑、毛发等,其表面附着有细菌、真菌、病毒等致病微生物以及氨气、硫化氢等有害气体,不但严重威胁畜牧场工作人员和家畜的健康,还导致周边大气环境污染。科学适用的PM减排技术是保障畜舍及周边环境空气质量的重要手段。该研究概述了畜舍PM的排放源、特征及危害,从源头、过程、末端3个环节分别论述了国内外畜舍颗粒物减排技术的研究现状及存在问题。源头减排包括饲料、清粪工艺、饲养模式等方面的优化,经济且高效。过程减排包括喷雾降尘、通风除尘、静电除尘等技术,旨在降低舍内悬浮在空气中的颗粒物。喷雾技术相对成熟,但易滋生细菌且不适用于低温季节;通风技术对去除畜舍PM上应用最为广泛且高效,需要综合考虑满足畜舍通风换气与降尘的要求;静电除尘技术对人畜无干扰,但在除尘效率和二次扬尘方面有待进一步优化。末端减排包括洗涤降尘技术与过滤降尘技术,目的是减少PM对外界大气环境的污染。洗涤技术可以去除排气中多种污染物,但设备易腐蚀;过滤技术成本低,对大颗粒物的去除效率高,但易堵塞。该研究对畜舍PM减排技术研究现状进行综述,以期为未来开发高效、节能、经济、环保的畜舍PM减排技术提供参考。

颗粒物;环境调控;畜牧业;排放源;特征;危害;减排技术

0 引 言

大气颗粒物(Particulate Matter,PM)是在气溶胶体系中均匀分散的各种固态和液态颗粒状物质的总称,其大小通常用空气动力学直径(Aerodynamic Equivalent Diameter,AED)来描述,按粒径大小,PM可分为总悬浮颗粒物(Total Suspended Particulate,TSP)(AED≤100m)和可吸入颗粒物(Inhalable PM,IPM)(AED≤10m),其中IPM又可以分为粗颗粒物(2.5m

大气颗粒物主要来源于地表扬尘、煤等燃料能源的消耗、工业生产、机动车排放及生物质燃烧等[3]。然而,农业-畜牧养殖颗粒物源排放一直为大众所忽略,畜舍内的PM浓度比其他室内环境高10~100倍[4]。比较而言,畜舍所产生的PM危害更大,其携带有恶臭物质、挥发性有机物(Volatile Organic Chemicals,VOCs)以及其他有害气体,同时也是细菌、病毒等微生物传播的载体。畜舍内微生物以细菌为主,其中含量相对较高的是革兰氏阳性菌[5]。病毒虽然占比很小,若吸附于颗粒物表面自由扩散,将产生巨大的危害,如非洲猪瘟病毒可通过短距离气溶胶传播[6]。因此,畜舍降尘减排是保障舍内外人畜健康和保护周边环境空气质量的有效手段。本文重点介绍和分析畜舍PM源头、过程与末端3个环节的减排技术的研究现状及存在问题,以期为改善人畜生存环境和畜牧业可持续发展提供参考。

1 畜舍PM排放源、特征与危害

畜牧业是农业PM排放的重要来源,随着畜牧养殖业的高速发展,其对农业PM排放的贡献还将不断增加。不同畜舍内PM的来源与组成存在差异。鸡舍PM主要来自羽毛和粪便等[7]。其中细颗粒物(PM2.5)所占比例非常高,研究发现,此类PM2.5中有超过50%来源于二次颗粒物[8]。养殖场的氨气(NH3)与空气中酸性气体发生化学反应形成的无机盐粒子均属于二次颗粒物[9-12]。猪舍PM来源有饲料、皮屑、粪便与矿物灰等,舍内主要为粗颗粒物,其中饲料是最主要的颗粒物来源。许稳等[13]采用中流量颗粒物采样器(TH-150C Ⅲ型,100 L/min)对北京郊区一个集约化猪场舍内颗粒物的浓度进行了连续测定,发现舍内颗粒物的粒径主要在10~100m。

颗粒物的产生与动物活动有关,可以根据动物的排放速率评估。相对浓度作为畜舍内PM的主要物理特征,直接影响动物生产性能及健康状况,常被用来评价家畜生存环境及生长福利的优劣。Takai等[14]与Winkel等[15]分别对不同种类畜舍内PM10与PM2.5相对浓度及单个动物的平均排放率进行测量并分析,发现不同畜种间存在较大差异(表1)。

表1 不同种类畜舍内PM10与PM2.5相对浓度及单个动物的平均排放率[14-15]

虽然家禽的PM平均排放率在鸡、猪和牛3个畜种里偏低,但由于家禽个体的占地面积小,饲养密度远大于生猪与奶牛。因此,一般情况下,鸡舍中的PM浓度高于猪舍并显著高于牛舍[16]。

畜舍PM表面附着有大量的重金属离子、VOCs、NH3、内毒素、细菌与真菌等,沉降在家畜体表会堵塞汗腺导致皮肤发炎,落在眼结膜上会引起眼结膜炎;PM2.5还会随呼吸进入支气管甚至肺泡,导致肺泡巨噬细胞吞噬能力下降进而影响机体的免疫功能,增加家畜患支气管炎、肺炎的概率[17]。Michiels等[18]研究表明PM10对育肥猪的呼吸健康有显著影响,易导致呼吸道疾病,降低猪的生产性能。相比于其他职业,长时间处于畜舍高浓度PM环境下的工人更容易患呼吸系统疾病。

同时,畜舍PM通过排风系统进入大气循环系统,成为大气污染物的重要组成部分。PM的粒径越小,扩散的距离越远[19]。大量持续的畜舍PM排放会加剧大气质量恶化,破坏大气环境平衡,严重时可降低大气能见度,引发雾霾[20]。畜舍PM扩散至居住区则会影响周围居民的正常生活,诱发急慢性呼吸道疾病发生,严重时甚至致人死亡。Jerrett等[21]发现户外空气污染(主要为商业和住宅能源所贡献的细颗粒物)在世界范围内造成每年超过320万人死亡,农业源作为第二大贡献者,在许多欧洲国家,其造成的死亡率可以占到总空气污染死亡率的40%。

2 畜舍PM源头减排

畜舍PM源头减排是指在借助PM减排设备减少畜舍内环境中已产生的PM之前,通过改变一些日常的生产管理模式来切断或减少舍内PM源头的产生量,从而降低舍内PM浓度的过程。

畜舍的PM来源有饲料、粪便、皮屑、羽毛等,饲料是畜舍内重要的PM来源,选择合适的饲料类型可以显著降低舍内PM浓度。相比而言,粉状饲料较颗粒状饲料更易产生PM,使用液体饲料是减少粉尘源排放的有效途径。另外,饲料添加剂或饲料涂层的使用也可以有效减少饲料对源头粉尘的贡献[22]。Cheng等[23]通过试验发现在饲料中添加10%的新鲜发酵豆粕可降低了仔猪舍内19.9%的PM10和11.6%的PM2.5浓度;张庆振等[24]用10%的菌肽蛋白替代保育猪日粮中的部分普通豆粕,试验结果与对照组对比后发现能够降低舍内20.34%的PM10与8.09%的PM2.5浓度;吴胜等[25]在断奶仔猪饲料中添加不同剂量的植物精油制剂并与对照组进行对比,结果表明添加植物精油可显著降低舍内微生物气溶胶浓度,且大剂量的去除效果更优。

研究发现鸡舍中可吸入颗粒物主要来源于饲料羽毛和粪便等,通过改进清粪工艺、调整饲养模式等途径可以减少PM源头排放。程龙梅等[26]研究了刮粪板和传送带两种清粪模式对蛋鸡舍内PM浓度的影响,发现传送带清粪的禽舍内空气质量较好,更适合在蛋鸡养殖中应用;Wathes等[27]发现不同饲养模式下舍内粉尘浓度有显著差异,地面饲养肉鸡的粉尘水平高,栖架或分笼层饲养模式可有效降低舍内粉尘浓度。

源头控制是畜舍PM减排中相对经济而有效的环节,且可有效降低畜舍PM的过程与末端减排压力。如表2所示,改变饲料结构、在饲料中添加脂类物质成本低且易实施,但需评估饲料结构及配方的改变对家畜营养摄入的影响。结合具体畜种选择适宜的饲养模式,发展可有效降低PM等污染物排放的机械化、自动化的粪污清理模式是未来方向。

表2 畜舍PM源头减排技术分析

3 畜舍PM过程减排

畜舍PM过程减排是指利用多种除尘减排技术降低舍内悬浮在空气中的PM浓度,旨在提升畜舍内环境质量。其包括喷雾降尘、通风除尘、静电除尘等技术。

3.1 喷雾降尘

喷雾降尘是一种发展较为成熟的技术,其原理是利用高速气流的分裂作用将液体转换成细雾。喷洒的液体一般为水、油、水与各种除臭菌剂的混合物或水油混合物等,液体在喷出过程中受空气阻力作用迅速断裂成细雾,粘附粉尘后受重力作用沉降,从而实现舍内降尘。Aarnink等[28]在肉鸡舍内喷洒菜籽油,研究喷油量对降尘效率的影响,发现单位面积的日均喷油量越大,降尘效果越好;任景乐等[29]采用重量法探讨喷雾前后蛋鸡舍不同位置颗粒物的分布与变化,结果表明粉尘在鸡舍的中部浓度最高,喷雾可有效降低PM浓度;Zheng等[30]在蛋鸡舍内使用中性电解水喷雾,与对照组比较后发现,3 h即可降低舍内34%的粉尘水平;Zhu等[31]在妊娠母猪舍内喷雾并对颗粒物浓度变化进行了评估,发现在喂料期间开启喷雾系统,空气中平均粉尘浓度降低了75%;Mostafa等[32]评价了在育肥猪舍内喷淋水油混合物对颗粒物的降尘效果,结果表明PM平均去除效率为63%,使用小喷嘴的除尘效率高于大喷嘴;Chai等[33]在养鸡场喷洒酸性电解水来降低舍内PM水平,设置了3组喷施量并与无喷水组对照,结果发现电解水的喷施量越大,PM的排放率越低;Nonnenmann等[34]在养猪场每天喷5%的油水混合物并设置对照组来评估舍内降尘效果,结果表明平均可减少52%的舍内粉尘,应用大豆油与菜籽油的除尘效果无显著差异。

喷雾降尘技术的成本低、除尘效率高,但其运行过程中存在耗水量大、喷头易堵塞、容易滋生细菌、影响家畜健康等问题。因而舍内喷雾降尘,要参照温湿度、粉尘等多种环境参数,在满足家畜健康福利要求基础上合理适度施用。

国外学者针对畜舍喷雾降尘技术研究了影响喷雾降尘效率的喷雾量与频率、喷嘴选型与孔径等关键因素对PM减排的影响。中国针对畜舍内的喷雾降尘技术研究较少,通常喷雾的主要目的用于夏季降温,缓解家畜的热应激。未来喷雾技术应当着重对提高雾化效率、PM减排效率、废水回收与净化效率等方面展开研究。

3.2 通风降尘

通风是去除畜舍内PM的有效方法,它能够将舍内污浊、潮湿的气体排出外界,同时带走大量的PM、微生物及污染性气体,补充进新鲜的空气从而优化家畜生长环境。

畜舍的通风方式主要有自然通风与机械通风。自然通风是指通过畜舍结构上的开口产生空气流动,动力源于自然对流形成的风压或热压差,主要有开窗式与敞开式两种。郭霏等[35]研究了不同畜舍间距(1、2、3倍屋脊高)与不同污染物释放位置(上、下风向)对自然通风畜舍气流与污染物分布的影响,结果表明畜舍间距选择2可有效减少舍间污染物扩散。由于自然通风受舍外天气、风速、风向等多因素共同作用,可控性差,因此现代集约化畜牧场一般采用机械通风系统。机械通风根据驱动原理不同可分为负压通风、正压通风与等压通风。不同的气候或季节下畜舍通风量不同,畜舍内气流速率以及分布亦不同,进而影响舍内的PM浓度。吴胜等[36]发现猪舍通风量是影响畜舍内颗粒物分布的主要因素。夏季气温较高,需要加大通风量以带走舍内多余的热量,通风率高于冬季,舍内的颗粒物浓度低、排放率高。冬季畜舍需要保温,舍内通风率低、颗粒物浓度高、排放率低[37-38];黄藏宇[39]发现在封闭猪舍中安装可调节风速和风量的新风系统可以显著地降低舍内PM浓度,最高降幅为65.8%;Kwon等[40]以通风率、室内外温度、猪数量、年龄与活动水平等作为参数,利用CFD(Computational Fluid Dynamics)技术研究机械通风养猪场的粉尘排放规律,结果表明通风是影响TSP和PM10最主要的因素,调整通风速率和改善通风系统特性可以有效降尘;汪开英等[41]在猪舍内应用CFD技术模拟PM浓度场,结果表明TSP、PM10、PM2.5均受通风率影响,通风速率越大、PM浓度越小,其中PM2.5受通风的影响最大。

机械通风畜舍内PM分布、运动趋势受气流分布、通风率与通风量等多种通风系统特性的影响,如畜舍不同的通风机制会导致舍内不同的气流分布,进而影响舍内PM等污染物分布。因此,在畜舍通风系统设计中应综合考虑通风换气与降尘的需要,可应用CFD技术深入研究经济高效的通风降尘策略。

3.3 静电除尘

静电除尘技术的原理是静电粒子电离(Electrostatic Particle Ionization,EPI),利用电极连接高压电后产生电晕效果,放出自由电子和离子使颗粒物带电,大气中的带电颗粒物受到周围电场力的作用向集尘区聚集,从而实现颗粒物的收集减排[42]。静电除尘按照不同的除尘方式可分为静电空间电荷系统(Electrostatic Space Charge System,ESCS)、干式静电除尘器(Dry Electrostatic Precipitator,DESP)和湿式静电除尘器(Wet Electrostatic Precipitator,WESP)[43]等。

ESCS系统一般安装在天花板等不易接触的位置,对人畜无干扰、维护要求低,但除尘效率低。Cambra-Lopez等[44]在肉鸡舍内采用静电除尘技术并评估了其对颗粒物分布的影响,表明电离系统能有效减少畜舍PM的排放。刘滨疆等[45]采用静电空间电荷系统(ESCS)对仔猪保育舍与笼养蛋鸡舍进行影响评估,结果表明ESCS能去除70%~94%的粉尘与50%~93%的微生物;孙利[46]研究了静电空间电荷系统(ESCS)对鸡舍内环境改善作用,并比较了功率高低对除尘效率的影响,结果表明,与对照鸡舍相比,安装ESCS能显著降低鸡舍内粉尘(<0.05),在高功率下空气净化效率高;李永明等[47]在保育猪舍内安装静电空间电荷系统(ESCS),结果表明能够显著增加空气中负离子的浓度(<0.01),同时显著降低空气中各种粒径的粉尘含量(<0.01);Ritz等[48]设计了一种静电空间电荷系统(ESCS)用于减少商品肉鸡舍的粉尘排放,设置对照组对比后发现ESCS平均降低了空气中43%的粉尘;闫怀峰等[49]在猪舍天花板安装3DDF—450型号静电空间电荷系统(ESCS),与对照猪舍相比,减少了空气中67.6%的TSP;王树华等[50]在保育猪舍内使用3DDF—450型静电空间电荷系统(ESCS),与对照猪舍相比后发现该系统最大可降低72.59%的舍内TSP浓度(<0.01);徐鑫等[51]在试验蛋鸡舍内安装了10套300型静电空间电荷系统(ESCS),与对照蛋鸡舍比较后发现舍内粉尘的平均质量浓度降低了35.9%;Nicolai等[52]比较了装有静电空间电荷系统(ESCS)与未安装的猪舍内粉尘含量,发现ESCS系统平均减少了63%的舍内颗粒物;张开臣等[53]利用八套3DDF—300型静电空间电荷系统(ESCS)对1个平养肉鸡舍的粉尘和微生物进行控制,结果表明其对粉尘与微生物的去除率分别为97.8%与90%;Dolejs等[54]使用静电空间电荷系统(ESCS)对奶牛舍进行降尘,结果表明奶牛舍内粉尘浓度降低了12.7%~26.2%;吴新[55]利用空气电净化技术构建了一套静电空间电荷系统(ESCS),在封闭式保育猪舍内进行试验,结果表明该系统能够去除70%的粉尘;焦洪超等[56]利用静电空间电荷系统(ESCS)在鸡舍施加人工负离子,设置对照组并研究了其对粉尘的清除效率,结果表明空气负离子可以显著降低舍内粉尘含量,且其对粒径大于1m的粉尘清除效率较高。

DESP系统利用分布板将气流均匀分布,通过高压电场使气流中的颗粒物带电,并受电场作用向阳极板运动,吸附于阳极后通过振打的方式使PM震落于灰斗中。该系统除尘效率优于ESCS系统,功耗低且易于集成,但其除尘效果较依赖于系统的通风特性,其对细颗粒物的去除效果差且震动过程易造成二次扬尘。Winkel等[57]比较了商用干式过滤器和干式静电除尘器(DESP)对禽舍粉尘的去除效率,结果表明DESP对粉尘的去除效果更好,能平均减少57%的PM10与45.3%的PM2.5;Manuzon等[58]运用CFD技术优化干式静电除尘器(DESP),并在试验室以及现场条件下评估优化后的除尘器对禽舍颗粒物收集效率,发现优化后除尘器的总收集效率可达89%(实验室)与82%(现场);Chai等[59]使用优化后的干式静电除尘器(Improved Dry Electrostatic Precipitator,IDESP)对禽舍除尘,总降尘效率最高可达79%。

WESP系统早前主要应用于冶金、化工等领域,近年来在燃煤电厂迅速推广,静电场空间的水雾在一定程度上提高了电场特性,大大提高了对颗粒物的收集效率[60]。Ru等[61]在模拟实验室的条件下评价了湿式静电除尘器(WESP)对家禽养殖颗粒物的减排效果,建立颗粒物去除效率预测经验模型,结果表明在特定电压、风速等条件下最多能减少94%的PM10与88%的PM2.5排放。与DESP系统不同之处在于WESP系统采用喷淋的方式在集尘板上形成流动水膜,利用水膜将集尘板捕获的粉尘冲刷清洗至灰斗中,避免了二次扬尘等问题。其对PM的净化效果优于ESCS与DESP,但安装较为困难、耗水量大且易形成污水等二次污染物,因而无法大面积推广与使用。

畜舍内静电除尘技术的使用效果与其自身结构(极板间距、电晕线间距、密封性)、供电装置(绝缘性、接地性)、风速等密切相关,对于夏季大通风量下的畜舍,静电除尘效果有限。目前,畜牧养殖业主要应用ESCS对畜舍降尘,虽然可以将粉尘收集至集电极、天花板、地面或其他金属表面上,降低空气中的粉尘浓度,但并没有减少粉尘的总量,除尘效率、二次扬尘等问题亟待解决。DESP与WESP系统的降尘效率优于ESCS系统,但存在二次扬尘、投入与维护成本高等关键性问题。如何解决经济性与普适性使DESP与WESP系统适用于畜舍环境降尘是未来发展的关键。中国主要开展了ESCS的降尘研究,针对DESP与WESP系统在畜牧业应用的相关研究鲜有。

畜舍PM过程减排是畜牧养殖管理过程中非常重要的一个环节。通过降低舍内PM浓度可以提高动物福利,促进动物生长、生产,进而提高经济效益。如表3所示,现阶段针对畜舍PM过程减排技术有一定的优势,但仍存在影响动物生长、除尘效率低、成本高、二次污染等问题。如何对现有除尘设备在结构、性能上进行优化,制定适宜的除尘策略是提高畜舍内除尘效率的关键。

表3 畜舍PM过程减排技术分析

4 畜舍PM末端减排

畜舍PM末端减排是指通过在畜舍排风机外安装特定的除尘装置对畜舍排出的废气净化,从而减少对外界大气环境污染的过程。其包括洗涤除尘、过滤除尘等末端减排技术。

4.1 洗涤除尘

洗涤除尘技术是指利用洗涤器的惯性、截留和扩散等原理,在相对密闭的空间通过液体过滤沉降废气中的PM,从而实现减少排放的过程,该技术已被证明能够有效去除畜舍内排出的颗粒物[62]。Zhao等[63]比较了猪舍外的3种空气洗涤器对颗粒物的去除效果,结果发现三级洗涤器的降尘效果优于二级洗涤器,去除效率与洗涤级数成正比;Melse等[64]利用空气洗涤器去除畜舍内颗粒物,研究发现多级洗涤器具有较高的平均空气停留时间,去除效率更佳,最高可去除79%的PM10;Aarnink等[65]利用三级空气洗涤器来去除育肥猪舍内粉尘,发现可以去除93%的PM10与90%的PM2.5;李世才等[66]设计了一种空气洗涤器以提高畜舍空气质量,该洗涤器利用水溶解有害气体以及吸附粉尘和病原微生物,同时结合植物栽培,经测试发现其对空气中的PM有良好的净化效果。

研究发现洗涤除尘的效率主要与洗涤器级数有关,增加洗涤器级数能够大幅提高对洗涤器对颗粒物的除尘效率,但提高降尘效率的同时增加了能耗与成本。且洗涤处理过程中产生大量的污水等二次污染物,因此洗涤除尘技术尚未在畜牧领域推广使用。中国对畜舍洗涤除尘技术的研究与应用较少,目前该技术多应用于工业领域除尘。因此,洗涤降尘技术在畜牧领域的应用有待进一步研究和改进,基于大废气排放量的经济高效畜舍洗涤降尘技术是未来研究重点。

4.2 过滤除尘

过滤除尘是通过在畜舍排风口外一定距离建造具有一定透过率的多孔介质如防风林和挡风墙,迫使气流突然改变方向,通过离心力的原理,部分颗粒物在气流过后会被阻挡而沉淀,从而实现有害气体与颗粒物的分离减排,其对粗颗粒物的阻挡效果优于细颗粒物。过滤系统应距离风机口2~4个风机直径以确保风机性能,高度至少3 m以确保拦截。研究表明不同种类植物的除尘效率不同[67-68]。范舒欣等[69]评估了8种常绿阔叶树种对PM的捕获能力,发现卫矛和早园竹的挡尘效率高;Adrizal等[70]通过在5个商品鸡场风机外分别种植云杉、杂交杨树、杂交柳树等来评估防风林对粉尘的捕获潜力,试验证明云杉与杂交柳树对颗粒物具有更好地捕获效率;张家洋等[71]选择了9种植物并分析它们的滞尘量,结果表明国槐、凤尾兰、紫叶李这3种植被的降尘率较高;马淑丽[72]通过计算流体力学模拟与实地验证发现防风林能够使下风向的颗粒物浓度显著降低,在试验过程中比较了5种植被的滞尘能力,发现紫叶稠李形成的防风林具有最高的滞尘效率,植被叶片的表面粗糙度与拦截效率呈正相关。

除比较不同植物间的滞尘能力外,多位国内外学者评估了过滤系统在畜舍外的除尘效率。Willis等[73]利用激光雷达技术来估计隧道通风的肉鸡舍外防风林对颗粒物的捕获效率,发现在夜间低风速与湍流强度的条件下最多可以捕获74%的颗粒物;Hernandez等[74]通过风洞试验和现场监测,确定了防风林对养猪场内气流的影响,并监测了防风林前后的颗粒物浓度,结果表明防风林可以减少40%以上的颗粒物;Winkel等[75]对养鸡场粪肥干燥隧道(Manure Drying Tunnels,MDTs)的除尘效果进行评价,发现MDTs能有效沉降废气中的颗粒物并将其回收至粪肥中,肥料层厚度增加也能提高对PM10的去除效率;Mostafa等[76]创新设计了一款干式过滤系统并评估了对鸡舍PM去除效率,试验结果表明其可以降低55%的舍内PM浓度,减少72%的PM排放率。使用生物质材料的挡风墙对排出的废气过滤,同样取得了较好的降尘效果。董红敏等[77]将秸秆与杂草作为挡风墙的材料来净化蛋鸡舍排出的废气,结果表明,可降低77%的TSP浓度与55%的IPM浓度。相比生物质材料,使用活性炭纤维过滤材料在去除鸡舍废气中PM的同时,对细菌和大肠杆菌的吸附效率更优[78]。若将过滤除尘系统与静电除尘技术相结合可以进一步优化除尘效果,Jerez等[79]在肉鸡舍排风口末端生物质挡风墙的基础上增加了EPI系统并比较了增加前后对粉尘的减排效率,结果表明增加EPI后可以进一步减少39%的TSP排放。

过滤系统对畜舍内排出的颗粒物具有一定的拦截效率,其拦截效果依赖于植物种类、系统的组成、材料、结构、孔隙率与位置等多种因素。该技术的成本低,对大颗粒物的降尘效果好,但挡风墙长期使用易堵塞,导致系统前后压降增大,影响风机通风性能,需要频繁清理。目前国内外现有的过滤除尘技术在降尘效率上仍有很大的提升空间,结合多种除尘技术、筛选除尘介质、优化除尘系统结构等是未来过滤系统除尘技术的研究方向。

畜舍PM末端减排对保护外界大气环境、防止污染物自由扩散及疫病传播等有至关重要的作用。如表4所示,洗涤技术在去除PM的同时还可以去除空气中氨气等污染性气体,对细颗粒物降尘效果好。过滤技术建造的成本低,对大颗粒物降尘效果好。但其均存在维护成本高、空气阻力大等问题。因此,畜舍洗涤技术的经济性有待提高,挡尘墙的自清洁系统有待研发,防风林滞尘能力的相关研究有待进一步开展。

表4 畜舍PM末端减排技术分析

5 结论与展望

畜舍PM的来源、特征十分复杂,而各种PM减排技术都有其适用场合、优势与不足,且畜舍PM排放特征因畜种及生长阶段不同而差异显著,因而单一的技术难以满足复杂、多样的畜舍环境PM减排需求。在实际生产中应针对不同畜舍PM排放特征与排放规律科学选用PM减排技术。

畜舍PM减排是一个系统工程,从源头、过程至末端的PM全程减排是未来的发展方向。源头控制是畜舍内PM减排中经济而有效的环节,有效控制源头排放,可降低过程与末端的PM减排压力;过程减排旨在通过减少畜舍内漂浮在空气中的PM,可显著改善畜舍内环境空气质量;末端减排可降低对外界大气环境的污染及PM中携带的病毒传播与扩散风险。虽然现有的畜舍PM源头、过程和末端的减排技术在一定程度上可去除畜舍PM,但在降尘效率、适应性、稳定性、二次污染风险、对人畜健康和畜舍通风性能的影响等方面存在的问题仍有待进一步研究解决。

中国作为世界家畜养殖大国,在畜舍PM减排技术的研究与应用方面仍处于起步阶段,不能满足畜舍内外环境友好型畜牧业发展的需要。因此,为实现精准、高效的畜舍PM减排,改善与保障畜舍内外环境空气质量,亟待结合我国畜牧养殖实际情况,开展畜舍PM源解析和排放特征等基础研究,研发高效、节能、经济、环保的畜舍PM减排技术,建立畜舍PM全程减排技术体系。

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Research status on particulate reduction technology in livestock houses

Wang Kaiying, Wu Jiegang, Mei Weida, Wang Xiaoshuai

(,,310058,)

With the development of large-scale intensive animal husbandry, a large amount of Particulate Matter (PM) originated from livestock houses has become an important source of atmospheric particles PM10and PM2.5. 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

汪开英,吴捷刚,梅威达,等. 畜舍颗粒物减排技术研究现状[J]. 农业工程学报,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

浙江省科技厅重点研发计划项目(2018C02035)

汪开英,博士,副教授,主要从事农业空气质量及动物福利方面研究。Email:zjuwky@zju.edu.cn

10.11975/j.issn.1002-6819.2020.18.025

X513

A

1002-6819(2020)-18-0204-09

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