2. 广西科学院, 广西南宁 530007
2. Guangxi Academy of Sciences, Nanning, Guangxi, 530007, China
我国是世界上最大的肉类生产国,年产肉类占全球的30%以上[1]。肉制品的需求促进了现代禽畜养殖业迅速发展,使得禽畜养殖带来的环境污染问题日益严峻。据统计,禽畜养殖贡献了我国58.2%的农业面源污染来源和15%的温室气体排放[2, 3]。禽畜养殖业首要考虑且最重要的环境问题是如何管理禽畜粪便。据统计,我国主要禽畜(猪、牛、羊、家禽)粪便年产总量近20亿t (湿粪)[1]。禽畜粪便含有大量的有机污染物、抗生素、重金属,并携带大量病原菌及寄生虫,伴有NH3、H2S等恶臭气体产生,若露天堆放或受雨水淋溶,将对水体、土壤及大气环境等造成严重污染[4]。
禽畜粪便含有丰富的有机质和无机元素,是一种回收利用潜力巨大的生物质资源。然而,禽畜粪便是由多种复杂高分子有机化合物和少量矿物元素聚集而成的复合体,包含大量的氨氮、重金属和抗生素等成分以及病原体,导致其回收利用困难,特别是营养组分的生物回收[1]。目前,除少数禽畜粪便进行资源化处理(通常是堆肥处理)外,大部分未能得到有效利用,其中80%的粪便污水被直接排放至各类水体环境中,给生态环境造成严重的负担[5]。禽畜粪便的安全消纳或治理已是当前我国农业源废弃物污染防治必须要面对和解决的问题。
1 禽畜粪便的资源化潜力禽畜粪便主要来源于猪、牛、羊、鸡,可按其存栏数及不同月龄的日排粪量估算出实物量、可开发量以及标准煤的折算量[6]。禽畜粪便的资源量与禽畜养殖发展状况有关。近十年来,猪、牛、羊养殖量相对稳定,因此禽畜粪便的资源潜力相对稳定。2017年,我国禽畜存栏4.28亿头猪、0.89亿头牛、2.97亿只羊和60.4亿只家禽[7]。根据Wang等[1]提供的估算方法,我国主要禽畜粪便的年排放量为18.83亿t,干物质年总量为3.08亿t,其中猪粪、牛粪、羊粪、家禽粪便分别占40.6%、28.5%、27.1%、3.8%,折合化学需氧量(COD)约9 500万t,约为全国工业和生活污水排放量的4倍。从技术潜力分析,可产生沼气793亿m3(甲烷成分55%),折合甲烷产量436.2亿m3;相当于2.13亿t标准煤或者1.49亿t原油或者1 610亿m3天然气,可满足我国每年4%-5%的能源需求,或者满足我国居民日常生活中37%的能源消费[1]。若折算为有机肥,全国禽畜粪便养分总量为4 710万t,其中氮肥1 130万t、磷肥1 150万t、钾肥2 430万t[6]。但是,目前我国禽畜粪便能源化或肥料化的消费很少,只有少量被用作堆肥、沼气发酵原料或风干后燃烧。禽畜粪便富含有机质,是生产生物燃料的良好原料,同时禽畜粪便还含有大量的氮、磷、钾等营养成分,是很好的有机肥料来源[5]。对禽畜粪便进行加工处理,生产生物能源、有机肥或是营养成分回收,不但可以变废为宝,而且还可以减少环境污染,防止病虫害传播,促进当地经济发展。
2 禽畜粪便的资源化利用技术目前,禽畜粪便资源化利用技术主要包括焚烧产热、气化、水热液化、水热炭化、厌氧消化、好氧堆肥、蚯蚓堆肥等。
2.1 焚烧产热焚烧产热是利用空气作为助燃剂,燃烧禽畜粪便中的可燃性成分,以减少其体积和重量,并获得热能的过程[8]。焚烧可在短期内获得大量热能,以供发电或热源利用,这对寒冷的地区有较大吸引力。焚烧所获得的灰渣性质稳定,可回收磷、钾等无机养分,还能彻底灭活禽畜粪便中的病原体,并使有毒有害物质无害化,避免了禽畜粪便对周边环境的影响[9]。
焚烧产热是最简单、最直接的禽畜粪便处理方法,受到人们的广泛推崇。例如,2014年开始,欧盟鼓励在农场利用焚烧产热处理家禽垃圾,以实现家禽养殖场的热量自给自足[10]。焚烧使粪便体积和质量均减少80%以上,显著节约了粪便处理的土地使用量[5]。禽畜粪便的燃烧性能与助燃剂的成分有关,氧含量越高,粪便燃烧的最低活化能越低。例如使用富氧燃料(CO2/O2值低)作为助燃剂时,牛粪燃烧具有最低的活化能(180.6 kJ/mol),优于空气(N2/O2)[11]。通过禽畜粪便造粒等致密化操作也可以提高热量回收效率[12]。焚烧剩余的灰渣含有丰富的磷元素,羟基磷灰石(CaHPO4·2H2O)是从灰渣回收磷的主要形式,回收率达92%[13]。此外,最近也有禽畜粪便与其他原料共燃烧的研究,以提高能量和无机元素的回收效率,如牛粪和污水污泥[14]、粪便与城市固废物[15]、猪粪与煤[16]等的共燃烧处理。
焚烧法投资成本低、操作程序简便,适合于经济欠发达的地区。但是,燃烧过程是大量温室气体(GHG)排放的潜在来源,其中N2O和CH4的排放量分别为0.019-0.06 g N2O-N/g干粪和0.000 046-0.002 6 g CH4/g干粪[8]。另一方面,焚烧法只适合于干湿粪便分离的养殖场(草场地区),对大部分含水量较高的粪便不适用。新鲜粪便的干物质通常仅有10%-20%,其燃烧热值只有2.9-4.2 MJ/kg,但经干燥后可达11.9-19.4 MJ/kg,约为标准煤的一半(29.3 MJ/kg)[17, 18]。Da Lio等[19]认为高水分含量是粪便燃烧效率的唯一限制因素,当水分含量>60%时,燃烧效率显著降低,但可以添加木质纤维生物质(如木屑)以提高产热性能。然而,对于大部分集约型养殖场,对粪便进行干燥较困难,这使得焚烧产热难以运行。因此,如何利用禽畜粪便直接燃烧产能,是一项有待深入研究的技术。
2.2 气化气化是利用空气中的氧气或含氧物作气化剂,在高温(300-1 400℃)条件下使得有机高分子碳氢化合物发生链裂解,并与气化剂发生热化学反应,生成可燃性合成气的过程[20]。合成气热值和能量密度高,经提质净化后可替代化石燃料,也可以通过费托工艺进一步加工成甲醇、二甲醚等化学产品[21]。气化是一种清洁、环境友好的热化学转化技术,它可以在几秒钟或几分钟内完成,对能源生产具有较大吸引力。
气化过程包括预热、干燥、热解、炭气化和氧化等步骤。热解是气化的基本步骤之一,通常在200-500℃下进行[20],该过程生成的水汽和CO2可作为气化剂参与气化反应。但生物质的热解过程十分复杂,确切的基本原理尚不清楚[22]。气化的产物(合成气)主要由H2、CO和CH4等可燃性气体组成,各成分比例主要取决于温度、气化剂、原料化学性质等因素[23]。在高温(>800℃)或低O2浓度(10%)条件下更容易实现更高的CO流速和产率[23]。Kirubakaran等[24]报道鸡粪中的碳和氧含量几乎相等,理论上可以使其成为自给自足的原料,不需要额外的气化剂,这种方法被称为“自动气化”。但气化剂的使用对气化效率和合成气的组分发挥着决定作用。当使用空气气化时,合成气的热值为4-7 MJ/m3;而使用纯O2气化时,热值可高达12-28 MJ/m3[20]。如果使用Ni-Al2O3为催化剂,则可获得高产率的富H2合成气[25]。利用CO2为气化剂有助于重新利用热废气,但气化温度要求较高(>700℃),同时还增加了气化设备的投资成本[26]。
含重芳香烃的焦油是气化过程中的副产品,主要是在气化温度较低时产生,它可进一步热裂、催化裂化或重整,从而提高合成气的产率,但会增加气化的成本[26]。另外,与焚烧类似,气化通常用于干粪(水分含量<25%)而不是湿粪,避免因蒸发水分而造成的能量损失[20]。近年来,生物质的超临界水气化(SCWG)已应用于湿生物质,不需要预干燥是其与其他传统气化技术相比的主要优势[20]。例如,Cao等[27]研究鸡粪的SCWG过程,在不添加催化剂的情况下仍获得25.2 mol H2/kg湿粪和99.2%的碳气化效率。但是,SCWG具有高吸热性和长停留时间的缺点,而且这种技术设备投资大,运营成本高[26]。
2.3 水热液化水热液化(HTL)是有机物在亚/超临界(240-380℃,5-30 MPa)条件下分解转化成生物油的过程[28-30]。禽畜粪便HTL的优点包括:①粪便可以直接转化,无须高耗能的干燥过程;②可对粪便进行消毒杀菌, 以达到无害化处理的目的;③生物油具有更高的能量含量(21-35 MJ/kg)[31],可用作液体燃料或用于化学品制造[32]。HTL是一种环境友好型技术,生物质和氧气在水热条件下会迅速氧化或矿化形成CO2或H2O,无废气的排放[33, 34]。此外,与气化技术相比,HTL所需的能源消耗较低[35]。
HTL通常是在配备磁力搅拌器的间歇式不锈钢高压釜反应器中进行[36]。目前生物质HTL的生物油产量一般低于60%,大部分在20%至40%之间[37]。HTL过程受多种因素影响,如温度、粪便种类和催化剂等,其中温度和粪便种类是影响HTL的重要参数。不同粪便种类HTL生产生物油的最佳温度不同,例如牛粪在300℃至310℃之间[38],而家禽在310℃至340℃之间[33]。适当提高温度有利于生物油的生产,如Xiu等[32]报道将温度从260℃提高到340℃,猪粪HTL的生物油产量从14.9%提高到24.2%。但当生物油产量达到最大值后,进一步升高温度会抑制液化,导致气体和炭的生成[33]。此外,HTL生物油产率也与催化剂有关:以CO为催化剂,310℃时牛粪HTL的生物油产率达到48.8%[38];而以Na2CO3为催化剂,350℃时牛粪HTL获得67.6%的生物油产率[36]。针对猪粪的HTL,添加粗甘油能使生物油产量从24.2%提高到68%[32]。然而,增加粗甘油添加量对生物油的碳含量和热值都有不利影响[39]。
尽管HTL具有显著的优点,如可直接使用湿粪、能耗低和环境友好,但是目前禽畜粪便HTL的数据几乎都来自小批量小型高压灭菌反应器的实验,缺乏更接近实际应用的规模化实验研究,且禽畜粪便HTL的连续模式仍然缺乏研究[33]。另一方面,禽畜粪便富含氮素,在HTL过程中对生物油的质量有显著影响[37]。富氮的生物油在燃烧或进行高级应用时,可能会导致大量的氮氧化物排放,而且HTL中氮的转化行为尚未完全了解,因此脱氮仍是禽畜粪便生物油生产和纯化过程中的关键问题[37]。
2.4 水热炭化水热炭化(HTC)是以水为反应介质,在130-250℃和自生压力下,生物质经水热反应得到固体富碳材料(称为“水炭”)和增值液体化学品的过程[40]。HTC是一种简便、环保、经济高效的禽畜粪便资源化技术,不需要对粪便进行预干燥,且能在短时间内有效降解或钝化粪便中的抗生素和重金属[41, 42]。HTC主要的转化产品——水炭具有多种应用潜力,包括制作固体燃料[43]、吸附剂[44]和土壤改良剂[45]等。除了形成水炭,HTC过程还产生一种含有可溶性无机盐和有机化合物的悬浮水溶液,这些工艺液体可能需要额外的处理,如厌氧消化、湿空气氧化等,也可以进行氮、磷、钾等养分的回收[46, 47]。
HTC技术可实现禽畜粪便除臭、除菌、固碳和营养元素高效回收的目的。猪粪和牛粪等粪便经HTC处理后,产生的水炭具有与高端次烟煤相当的燃烧能量(19.3-26.8 MJ/kg),磷回收率为80%-90%[48]。一般认为,在低温或碱性条件下进行HTC更有利于水炭燃烧能量的提高[49]。此外,添加CaO[50]、H2SO4[51]、柠檬酸[52]等催化剂不但可以显著改善HTC水炭的产量和热值,而且还可以提高无机养分的溶出率。例如,Qaramaleki等[52]利用HTC从牛粪中回收的养分,发现相对低温(170℃)和柠檬酸添加使粪便中磷、氮的溶出率分别达到98%和60%。另外,经HTC处理后,粪便中Cu和Zn的去除率分别达到55%和59%[49]。
与其他热化学转化技术(气化、HTL)相比,HTC的温度和压力都较低,反应条件相对较温和,因此HTC在禽畜粪便处理方面可能更具竞争力[53]。然而,目前HTC仍处于发展阶段,确切的反应机制尚不清楚[54]。虽然水炭具有富碳、低氧氮硫和高热值的优点[55],但水炭的灰分含量高,导致燃烧效率低,且结渣和结垢倾向高[56]。同时禽畜粪便HTC的水炭呈弱酸性,并可能仍然有较高的重金属含量,这些都限制了水炭在土壤方面的应用[46, 57]。
2.5 厌氧消化厌氧消化是多种功能微生物在厌氧条件下分解代谢有机物产生沼气的过程[1]。沼气主要由CH4(50%-75%)和CO2(25%-50%)组成,它是一种清洁的环境友好型燃料,平均热值为20 MJ/m3,可替代化石燃料用于供热和发电,或用作汽车燃料等[58]。厌氧消化技术具有技术成熟、操作简便等特点,它能使禽畜粪便减量化、稳定化、能源化,在推动禽畜产业发展中发挥着重要作用[59]。
厌氧消化过程包括水解、酸生成、产氢产乙酸、甲烷生成4个阶段,多种功能微生物参与了这4个阶段,并通过协同和拮抗作用共同维系着厌氧系统的稳定性[60]。影响厌氧消化效率的因素有底物特性、反应器类型、反应温度、有机负荷(OLR)、水力停留时间(HRT)等[60]。例如,牛粪厌氧消化的OLR为1.50-6.75 kg VS/(m3·d),HRT为15-30 d,COD降解率为36%-85%,甲烷产量为0.16-0.39 m3/(kg VS);而猪粪厌氧消化的OLR为0.8-4.0 kg VS/(m3·d),HRT为15-60 d,COD降解率为44%-77%,甲烷产量为0.16-0.32 m3/(kg VS)[58, 61]。反应器类型主要根据禽畜粪便的类型来选择:对于干物质含量高的粪便(如羊粪),通常使用连续搅拌罐式反应器(CSTR);而对于干物质较低的禽畜粪便(如猪粪),则通常使用上流式厌氧污泥床(UASB)或膨胀颗粒污泥床(EGSB)[60]。粪便的厌氧消化通常在中温(30-40℃)和高温(50-60℃)条件下运行,高温消化运行的OLR大约是中温发酵的2.5-3倍[62],并且对病原体灭活能力高,但高温消化需要更高的能耗成本,系统稳定性也比中温发酵差。目前,也有中温和高温发酵组合的方案,即利用中温消化稳定、能耗低的优点,再结合高温消化以灭活病原体,达到沼渣和沼液无害化还田的目的[63]。
禽畜粪便的沼气/甲烷潜力可根据Buswell公式(基于C、H、O、N含量)进行估算[64],但相比于其他原料(如秸秆、餐厨垃圾),禽畜粪便的厌氧消化受到高浓度有机污染物、氨氮、重金属的干扰,导致其能量回收效率较低[61, 65]。强化粪便厌氧消化的策略有以下3点。①预处理。热预处理和机械预处理粪便是最常用的预处理方法,它们可以促进可溶性物质释放,增加微生物/酶对底物的可及性[66]。预处理成本可占到沼气厂运营成本的20%-50%,但它能使沼气产量提高15%-80%[66]。②厌氧共消化。农业秸秆(如稻草、玉米秆等)是粪便厌氧共消化最常见的共底物,该策略可有效改善禽畜粪便的厌氧消化性能[67]。通过添加秸秆(C/N>30),使厌氧消化的C/N在最优值(20-30)范围内,降低氨氮浓度和稳定pH值(6.8-7.5),稀释有毒物,达到营养平衡,使沼气产量提高25%-400%[68, 69]。③添加碳基/铁基材料。添加碳基(如生物碳)/铁基(例如Fe、Fe3O4)材料可改善微生物对微量元素的吸收和酶活性,从而提高甲烷生产率[70, 71]。这些材料促进了微生物产甲烷过程的直接种间电子传递(DIET)[71],但是细菌和古细菌之间的DIET增强机制仍在探索中。
2.6 好氧堆肥好氧堆肥是在通气条件下,利用微生物将有机物降解为水蒸气、CO2、热量和其他气体并产生热量的过程[72]。堆肥的最终产品为富含腐殖质和营养物质的有机肥料,可替代无机肥料使用。堆肥可使有机废弃物的体积减少约50%,能有效节约禽畜粪便处理的土地使用量[5]。堆肥还可以有效钝化重金属,降低抗生素含量,并最大限度地减少杂草种子和病原体的传播风险,基本上使禽畜粪便达到无害化[72]。堆肥因其易于操作、投资成本低、不需要复杂的高科技基础设施,目前被许多国家视为禽畜粪便管理的首选技术。
影响堆肥的因素包括温度、水分含量、氧气含量和碳氮比C/N等[4]。生物质在堆沤过程能产生约70℃的高温,能杀死绝大多数病菌和虫卵。根据Li等[72]、Kim等[73]、Zang等[74]的研究,堆肥的水分含量一般设定为50%-70%,以达到最佳堆肥质量,过高会影响通风,过低则会延缓微生物的生长和作用。堆肥堆的孔隙率通常控制在35%-50%,以便空气可以自由流过基质[74]。堆肥适宜的C/N为20-30,过高会延长腐熟时间; 过低意味着有效氮过量,氮可能会以NH3的形式流失,产生臭气[75]。禽畜粪便通常具有较低的C/N(<15),可以通过添加秸秆、锯末、稻壳、木屑、生物炭等基质调整。这些基质不仅可以增加粪便堆肥的C/N和孔隙率,通过提供更多的曝气通道来调节含水量,而且还可以降低病原体、重金属和抗生素污染的风险,提高微生物活性,减少温室气体排放[75-77]。然而,目前堆肥过程中有关重金属、抗生素的结果大多源于实验室阶段的研究,关于大规模商业堆肥过程中这些污染物变化的具体信息较少, 仍待深入研究。
堆肥作为一种潜在的化肥替代产品,在世界各国备受青睐。因为与堆肥相比,其他禽畜粪便处理技术相对昂贵。在欧洲,过去的20年里堆肥的使用量增加了近50%;到2024年,全球堆肥市场预计将达到92亿美元[78]。但是,如果施用未完全腐熟的堆肥,会存在烧根烧苗、滋生病虫、产生毒气并缺氧、盐化土壤、游离重金属和激素超标等风险[78]。另外,在粪便堆肥过程中,初始粪便中大约46%的C和67%的N以气体形式损失掉,造成大量的温室气体排放[75]。根据联合国政府间气候变化专门委员会(IPCC)统计,堆肥过程中温室气体排放为0.44-2.41 g CH4/kg干粪和6-100 g N2O/kg总氮[75]。因此,如何减少C、N养分的损失仍是禽畜粪便堆肥技术需要考虑的问题。
2.7 蚯蚓堆肥蚯蚓堆肥是通过蚯蚓消化道将有机废弃物彻底分解并加速堆肥腐熟的过程[79]。蚯蚓堆肥技术生态友好、成本低、附加值高,在有机废物管理和减轻环境负担方面受到世界各国的广泛关注[80]。蚯蚓堆肥主要有3个优势[80, 81]:一是通过提高微生物和酶活性来加速去除有毒物质(重金属、抗生素等),二是通过增强微生物多样性和功能来抑制微生物病原体,三是获得富含腐殖酸和益生菌的优质生物肥料(蚯蚓粪)。蚯蚓堆肥是推进种养结合产业中的一种突出技术,能有效降低禽畜粪便带来的环境污染问题,满足养殖业绿色健康发展需求。
蚯蚓粪是蚯蚓堆肥最重要的转化产物,它是一种新型有机肥,具有良好的通气性和保水性,而且腐殖质含量和脲酶活性高,有助于改善土壤结构、提高农产品的品质与产量[82, 83]。影响蚯蚓堆肥的因素有温度、pH值、水分含量、C/N、通风等。为保证微生物和酶的高活性,蚯蚓堆肥需在10-32℃的中温条件下运行,以更快地将有机物转化为蚯蚓粪[84]。然而,正是蚯蚓堆肥的温度要求,使其缺乏杀死病原体的能力,这被认为是蚯蚓堆肥过程的主要缺点。高质量的蚯蚓堆肥要求基质pH值为5.5-5.8;与初始基质相比,蚯蚓堆肥处理过程的pH值降低5%-9%[85],这是由于有机物的分解、有机酸产生导致的CO2排放以及NH4+、腐殖酸的存在[86]。蚯蚓堆肥的水分含量通常在50%至90%之间,水分过低不利于蚯蚓生长,过高则会阻碍通气[87]。此外,蚯蚓通过皮肤呼吸,曝气有助于蚯蚓生长,加快对有机物的分解,缩减腐熟时间,但这同时也增加了温室气体的排放量[88, 89]。
与传统堆肥相比,蚯蚓堆肥耗能更少,经济上可行且具有成本效益。Gong等[90]评估认为每公顷蚯蚓堆肥能生产蚯蚓粪2 172 kg,农民收入增加4 008美元,作物产量和地上生物量分别增加18.3%和7.7%。同时,蚯蚓堆肥具有更大的养分回收潜力,蚯蚓堆肥过程中氮损失比传统堆肥减少10%-20%,氮回收率可达76%,从而减少堆肥过程中的N2O排放[84, 91, 92]。与厌氧消化相比,蚯蚓堆肥避免了沼液、沼渣的管理问题。但是,在某些情况下需要对禽畜粪便进行预处理,因为它们可能含有对蚯蚓有毒的物质,如酸性化合物[93]。而且,由于蚯蚓的生长温度较低,蚯蚓堆肥对粪便中病原菌的灭活作用有限。
3 禽畜粪便养分的回收技术近年来,生物质精炼技术受到了人们的广泛关注[94]。禽畜粪便的生物质精炼旨在有针对性地回收粪便中的营养养分,并进一步增值为高附加值的生物材料和生物产品,同时达到处理禽畜粪便的目的。禽畜粪便,特别是其液体部分(粪水)以及粪便处理产生的工艺液体(如沼液、水热处理的悬浮液),含有大量的氮、磷、钾等营养元素,非常适合有针对性的资源回收[65, 95]。氨汽提、化学沉淀、膜分离和离子交换(电渗析)是常见的禽畜粪便养分回收技术。
氨汽提工艺是指在一定温度和pH条件下,粪便中的氮以NH3排放的形式从液相转移到气相,进而通过酸吸收回收氨[96]。NH4+/NH3是决定工艺效率的关键因素,但NH4+/NH3受pH值控制[97]。较高的pH值有利于NH3的形成,较低的pH值有利于NH4+的形成,因此在氨汽提之前提高粪便的pH值有利于氮回收。氨汽提已在实际中得到证实,氮回收率为80%-90%,但是该技术的能耗相对较高,需1.54-12 kWh/m3 NH3,运行成本估计为4.5-8.6欧元/m3 NH3[98]。为此,把厌氧消化和氨汽提结合是理想的禽畜粪便处理途径。在厌氧消化中,几乎70%的总氮被矿化为NH4+/NH3,而且沼液的NH4+/NH3较低,使沼液成为通过氨汽提回收氮的合适来源[99]。
磷酸铵镁[Mg(NH4)PO4·6H2O]俗称鸟粪石(MAP),为白色粉末状无机晶体矿物,其P2O5含量约为58%,是一种极好的缓释肥(有机肥料),在自然界中的储量极少[100]。禽畜粪便中含有丰富的氨氮和矿物质,它们是鸟粪石形成的基本原料。从禽畜粪便中制备鸟粪石的方式有两种,一是通过微生物的长期作用,使氨氮和矿物质转化为鸟粪石;二是通过人工添加化学试剂(如Mg2+)加快鸟粪石沉淀的形成速度[100]。影响鸟粪石沉淀形成的影响因素有pH值(8.5-9.5)、离子浓度(N/P>3和Mg/P>1)、温度(25℃)和悬浮固体浓度(<1 g/L)等[101]。Mg2+浓度是影响从禽畜粪便回收鸟粪石的关键因素,而禽畜粪便的Mg2+浓度通常较低,因此可以通过添加Mg(OH)2、MgCl2或MgSO4来解决[102]。鸟粪石沉淀法实现了禽畜粪便的氨氮和磷养分的同步回收,氮和磷回收率分别达到10%-40%、80%-90%[103]。鸟粪石沉淀法能耗低,运营成本为520-2 800欧元/t鸟粪石[98]。Song等[104]开发了一种无须添加化学试剂的鸟粪石形成工艺,显著降低了从禽畜粪便中生产鸟粪石的成本。
膜技术也可以从禽畜粪便中回收养分,其最重要的优势是对养分进行浓缩。正/反渗透、纳滤、膜蒸馏等是一类压力驱动膜技术,能耗为4-6 kWh/m3,运营成本为4-13欧元/m3[99]。禽畜粪便经过纳滤或反渗透后,NH4-N浓度高达10 g/L[105]。但是,由于禽畜粪便富含有机物并且具有高总固体含量,不能直接利用这些技术进行养分回收,需经预处理后获得液体部分才可以使用。电渗析是一种无压膜技术,其目的是将阴离子转移到阳极室,并在阴极室收集阳离子。据报道,电渗析浓缩的NH4-N浓度高达16-21 g/L,明显高于压力驱动工艺[106]。电渗析的氮回收率为80%以上,能源需求为3.25-3.6 kWh/kg NH4-N[98]。值得注意的是,在膜技术的实际应用中应尽可能避免膜污染风险[107]。
4 资源化技术的可持续性每种禽畜粪便资源化处理技术对自然环境的影响不同,构成了其不同的可持续性。通过生命周期评估(LCA)或环境影响评估(EIA)可以从环境角度比较不同的禽畜粪便处理技术,以获得最环保的解决方案[59]。以养牛场为例,小型农场(<100只动物)的温室气体(GHG)排放量为45.5 kg CO2 eq./t粪便,而大型农场(200-999只动物)为104.9 kg CO2 eq./t粪便,大型农场具有更高的GHG排放主要是由于其缺乏大容量储存、加工场所和土地应用[108]。堆肥是许多发展中国家首选的禽畜粪便处理技术,其全球变暖潜能值(GWP)为226-236 kg CO2 eq./t堆肥[109]。一般认为,当其他处理技术在经济或环境层面不可行时,堆肥是一个不错的禽畜粪便管理方案[99]。当使用气化技术处理禽畜粪便时,GWP为-643 kg CO2 eq./t干粪,大约是传统粪便堆放处置(GWP=119 kg CO2 eq./t干粪)的6倍[110]。如果所有的禽畜粪便都用于沼气发电,大约可以减少4%的一次能源电力(如煤电)使用所造成的温室气体排放[111]。另外,HTL不需要干燥或任何其他预处理,这对于禽畜粪便的管理是巨大的优势。Connelly等[112]指出基于粪便的HTL技术生产生物油可以减少50%的温室气体排放,但这取决于生物油的进一步利用方式。这种情况同样会出现在其他禽畜粪便管理技术中,例如厌氧消化技术,生产的沼气是用于热电联产还是纯化为生物甲烷将会导致不同的环境影响[113, 114]。
厌氧消化是无害化处理禽畜粪便的高效途径,它能够获得生物能源和生物肥料[115, 116]。但与热解、气化等其他能源化技术相比,厌氧消化的净能源回收率较低[5]。此外,沼气厂配置涉及粪便运输、粪便预处理、厌氧消化、沼气利用、沼肥利用等步骤,每一个步骤都会对厌氧消化的整体环境效益产生影响。例如,禽畜粪便运输到沼气厂的过程中,需要考虑温室气体和污染物排放,包括CO2、颗粒物(PM)、NH3、NOx、SO2和非甲烷挥发性有机化合物(NMVOC)等[59]。同时,沼气生产阶段效率低,会造成CO2、CH4和SO2的大量排放[117]。另一方面,当沼肥应用于农田过程中,沼肥储存、运输、分配及其相关排放是一个严重的问题,如果不认真对待,可能会对生态系统造成负面影响[118, 119]。据统计,关于沼肥应用的研究论文,50%的文章显示沼肥对土壤有积极影响,7%的文章显示有负面影响[120];与其他有机肥料相比,25%的文章显示沼肥对土壤微生物刺激较大,而17%的文章显示对土壤微生物质量的刺激较小[120, 121]。
粪便资源化回收的成功取决于所采用工艺的经济可行性。热化学转化技术虽然有较高的能量回收,但它们对设备要求较高,投资成本大。厌氧消化的投资成本相对较低,其盈利能力依赖于沼气的销售。因此,建立更好的沼渣市场可以提高沼气厂的收入[122]。将沼气升级为生物甲烷的消费税较低,进一步提高了厌氧消化技术的经济性[123]。但与堆肥相比,厌氧消化的投资成本较高,而堆肥的盈利能力依赖于堆肥的销售。针对性回收禽畜粪便的营养物质是生物精炼的重要内容。尽管氨汽提效率很高,但它对NH4+/NH3的高要求和硫酸等的消耗增加了生产成本。鸟粪石沉淀是经济效益更高的工艺,但它依赖于NaOH、H2SO4、MgCl2等化学品的大量使用[118]。膜技术也可以有效地用于养分回收,但是其运行成本较高,而且膜污染的风险仍然未得到有效解决。
当前农业绿色发展相关政策对禽畜粪便排放量进行了严格控制,而养殖户作为面源污染政策的集中执行对象,其经营成本在一定程度上有所增加。禽畜规模化程度越来越高,传统的粪便堆放或自然发酵方式已经无法满足现代化的养殖需求以及环保要求,因此,建造以及购置禽畜粪便处理设施是大多养殖场处理禽畜粪便的必选之路。但是,设备设施投入大、后期运营成本高成为困扰养殖户的一大难题。使用粪肥回收有机肥料可减少无机肥料的使用,并通过改善土壤质量和生产力为农民增加收入[124]。此外,粪便循环利用的一些管理实践及其经济分析表明,依照政府的激励措施和优惠政策,加上养殖户的积极参与,禽畜粪便回收利用在经济上是可行的[5, 125]。粪便循环利用的主要受益者是农业系统,建立种养之间的循环经济势在必行。
5 展望做好禽畜粪便处理与利用工作,既可以实现零污染、零排放,促进农业全产业链清洁生产,又可以实现废弃物的资源化,促进可再生能源对化石燃料、有机肥对化肥的有效替代,真正做到“变污为净”“变废为宝”。未来的研究应着眼于以下4点。
(1) 我国的禽畜养殖业朝着更加集中化、规模化的方向发展,禽畜粪便管理技术的应用也面临着更严峻的挑战。热化学转化技术(焚烧产热、气化、水热液化、水热炭化)能有效节约禽畜粪便处理的土地使用量,处理时间短,并能彻底灭活病原体,但能耗大、设备投资成本高。生物转化技术(堆肥、厌氧消化)能获得优质的有机肥,但禽畜粪肥的液体部分仍然是潜在的环境威胁。因此,将生物精炼的概念用于禽畜粪便管理,以最大程度利用粪便的营养成分,对现代养殖业向循环生物经济加速迈进有重要意义。
(2) 禽畜粪便的排放会对全球的生态环境产生负面影响,然而,限制禽畜业的发展和禁止在禽畜养殖业中使用抗生素和重金属等方法是不切实际的,如何更好地处置这些污染物关乎环境、生态与人类的健康发展。因此,不仅要提升现有技术,而且还应继续加大禽畜粪便资源化实用技术的创新,例如发展可持续循环的源头分离策略和耦合技术[126]。
(3) 各种禽畜粪便处理技术及其转化产品具有不同的特性,如何为特定区域选择最佳的禽畜粪便管理技术是决策者最关心的问题。但这一问题十分复杂,涉及当地废弃物供应链、区域参数、气候条件、技术可能性、环境政策、人类健康、生态、产品的市场价格、最终需求产品等因素。因此,关于各种禽畜粪便管理技术的综合评估仍然任重而道远。
(4) 政府和养殖户是现代养殖业管理的主要责任群体。政府可以进一步加大对禽畜粪便资源化技术相关研究的科研投入,建立相关的政策法规,通过奖金、免税、购买设备、提供场地等激励措施,进一步引导企业和农户参与禽畜粪便资源化利用,推动清洁型和经济型养殖,建立种养有机结合的循环经济体系,从而构建禽畜粪便能源化、肥料化、产业化、市场化新格局。
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