Plant Selection of Constructed Wetlands for Treatment of Piggery Wastewater. Ao Ziqiang1* ï¼Zhang Jie2 ï¼Peng Guiqun1 ï¼Fu Jiaqi1 ï¼Jiang Cheng1 ï¼Xiong ...
Meteorological and Environmental Ｒesearch 2017，8（ 2） ： 85 － 89
Editor： Yang Yingying
Proofreader： Yang Yingying
Plant Selection of Constructed Wetlands for Treatment of Piggery Wastewater Ao Ziqiang 1* ，Zhang Jie2 ，Peng Guiqun 1 ，Fu Jiaqi 1 ，Jiang Cheng 1 ，Xiong Jihai 1 1． Institute of Energy，Jiangxi Academy of Sciences，Nanchang 330029，China； 2． Poyang Lake Ｒesearch Center，Jiangxi Academy of Sciences， Nanchang 330096，China Abstract Piggery wastewater contains high concentrations of organic matter and ammonia nitrogen，and it is difficult to treat it and make the discharged wastewater to meet the standard． Since the treatment of piggery wastewater treated under anaerobic conditions by traditional aerobic methods requires large investment，high running expense and strict management，breeding enterprises could hardly accept the technique． According to the characteristics of piggery wastewater，Pennisetum sp． + Ｒumex acetosa Linn and Populus + Nerium oleander can be planted in constructed wetlands to treat piggery wastewater，which can improve the treatment effect of piggery wastewater by constructed wetlands，solve the difficulty of wastewater treatment in winter，and bring obvious economic benefit． Key words Constructed wetlands； Selection of plants； Woody plants； Piggery wastewater DOI 10． 19547 / j． issn2152 － 3940． 2017． 02． 017
Constructed wetland technology is a new type of ecological engineering technology created，supervised and controlled artificially for the purpose of treating wastewater based on the principle that natural wetlands can be used to degrade wastewater． Constructed wetlands have many advantages，such as low cost of building and operation，being easy to maintain and manage， stable quality of effluent，and buffering pollution load impact； organic wastewater containing low concentrations of nitrogen and phosphorus can be treated well by constructed wetlands， and nitrogen and phosphorus in the wastewater can be transformed into the components of plants，while some constructed wetlands can bring certain economic value［1］ ． Piggery wastewater contains high concentrations of organic matter and ammonia nitrogen． The quality of piggery wastewater treated by using traditional treatment methods can not reach the standard， and the treatment cost is high． Piggery wastewater can be treated by constructed wetlands after being treated by anaerobic methods，which has obvious advantages and can beautify environment around rural areas［2］ ． Plants in constructed wetlands are their important parts，studying the resistance to pollution， decontamination effect and growth conditions of plants in wastewater，analyzing their contribution to the removal of pollutants，discussing their purification mechanisms，and selecting a batch of specific plants with strong resistance to pollution and capacity to remove pollutants have importance significance to the building of constructed wetlands that are used to treat pig-
Ｒeceived： December 8，2016 Accepted： March 13，2017 Supported by National Natural Science Foundation of China （ 41263006， 2014BAC04B00，41661019，21567010） ； Program of Jiangxi Provincial Department of Science and Technology （ 20124ACB01200，20122BBG70086， 20142BCB24009） ； Program of Jiangxi Academy of Sciences （ JAS（ 2013） NO． 19-06，2016-YCXY-04，2013-XTPH1-14，2013H003） ． * Corresponding author． E-mail： aoziqiang628@163． com
gery wastewater［3］ ．
1 Present condition of treatment of piggery wastewater Piggery wastewater is mainly from pigs’excreta and rinse water from sites and contains high concentrations of nitrogen， phosphorus，organic matter，and suspended solids instead of toxic constituents that can harm plants． However，there are some problems of piggery wastewater presently． Firstly，most large-scale pig farms have been built into biomass pools at present，and the primary treatment of piggery waste has been realized． After piggery wastewater ferments in biomass pools， a part of biogas slurry is used to irrigate farmland，while another portion of biogas slurry is applied to fruit trees and vegetable． However，a part of biogas slurry has to be discharged outside due to the limits of planting area and seasonal fertilization． Secondly，the efficiency of nitrogen and phosphorus removal is low during anaerobic processes，so digestive juices of piggery wastewater contain large quantities of nitrogen and phosphorus，and the content of ammonia nitrogen will rise． Hence，the quality of piggery wastewater treated simply during anaerobic processes can not reach the standard． The discharged wastewater has brought great pressure to the environment，so measures to remove nitrogen and phosphorus from piggery wastewater deeply must be adopted． Thirdly，aerobic methods to treat anaerobic digester juices include activated sludge process，contact oxidation method，etc． ，and these methods have a large investment in projects，high cost of operation， high demand of management，but breeding enterprises could hardly realize them in respect of funds and management． Fourthly，very low temperature in winter can affect the running effect of current sewage treatment measures of large-scale pig farms greatly． Overground anaerobic reactors that have been
86
Meteorological and Environmental Ｒesearch
built （ such as USＲ and UASB） are influenced by the decrease of temperature obviously，so the removal rate of pollutants in winter is low．
2
Ｒoles of plants in constructed wetlands
2． 1 Ｒemoving pollutants Nitrogen and phosphorus in piggery wastewater are necessary nutrient elements for the growth and reproduction of plants，and they are absorbed by plants and are changed into their components． Ｒoots of plants can secrete nutrients that are needed for the growth of microorganisms and a large amount of enzymes that can speed up degradation of macromolecular contaminants［4］ ． Therefore，enzyme activity is also an important indicator of selecting suitable plants for wetlands［5］ ，and the process of degrading pollutants by specific enzymes of plants is studied to analyze the potential of constructed wetlands to purify wastewater． 2． 2 Transporting oxygen Shortage of oxygen in wetland soil may make some reduced elements and organic matter toxic． Plants in wetlands transport oxygen produced by photosynthesis to roots to provide suitable environment for aerobic and anaerobic microorganisms and promote oxidation-reduction reaction in root zones［6 － 7］ ． The transportation of oxygen by wetland plants can influence the composition of organisms and diversity of microorganisms in wetlands and enzyme activity in root zones indirectly［8］ ． Fu Ｒongbing et al． ［9］ have suggested that the oxidation-reduction potential on the surface of roots of Phragmites australis is far higher than that of water，and it provide living environment for aerobic microorganisms and improve the oxidation capacity of root zones． Aerobic micro-environment in root zones can not change the internal anaerobic conditions of wetlands，but large quantities of roots can improve upper oxidation environment of wetlands． Aerobic microorganisms in aerobic zones of root zones of wetland plants can degrade organic matter that is retained by roots during metabolic process， and nitrifying bacteria can decrease nitrogen concentration in wetlands through nitrification． 2． 3 Increasing permeability of wetland matrices and transportation capacity of water Ｒoots of plants can penetrate through and expand wetland matrices to produce many small spaces，which can increase their porosity and permeability，thereby improving the transmission capability of water and avoiding hardening of wetland matrices［10］ ． When flowing through wetlands，wastewater can contact with roots of plants and wetland matrices completely to improve the removal rate of pollutants． 2． 4 Having ecological and economic value Plants in wetlands can release oxygen during the process of photosynthesis to adjust surrounding microclimate，provide habitats for birds and reptiles，block dust，purify air，reduce noise and beatify the environment［11］ ． P． communis，an important plant applied in constructed wetlands，can regulate local microclimate，prevent water and soil from losing，and maintain biodiversity，so it has very high ecological value； it plays a key role in industry， medicine and textile industry at the same time［12］ ． Alocasia macrorhiza has certain ornamental value and can purify live-
2017
stock wastewater effectively［1］ ． He Mingxiong et al． ［13］ have pointed out that 15 plants in constructed plants such as Eichhornia crassipes，P． communis，Typha latifolia，and Canna indica Linn． are good materials of biomass solid fuels and supplementary materials of biogas and alcoholic fermentation．
3 Shortcomings of treatment of piggery wastewater by plants in constructed wetlands Firstly，the growth of plants has certain seasonal features， and the treatment effect of wastewater is good in the growing season． In winter，most plants in wetlands wither at low temperatures，so the capacity of wetlands to treat wastewater will decrease greatly． Therefore，many plants that can not overwinter can not be applied widely even though they have strong capacity to remove pollutants，which has restricted the popularization of treatment of piggery wastewater by plants in constructed wetlands． Secondly，roots of plants are distributed in limited zones， and most roots are distributed underground at a depth of about 30 cm； the treatment effect is good in regions where lots of roots are distributed，but the treatment ability is limited in regions where there are no roots［14］ ． Thirdly，the resistance of plants to different concentrations of pollutants is limited，and most wastewater treated by constructed wetlands contain low and middle concentrations of organic matter，so constructed wetlands should be combined with anaerobic treatment and other treatment methods to treat wastewater． Within a certain range of nitrogen and phosphorus concentration，the removal rate of nitrogen and phosphorus in wastewater by plants rises with the increase of their concentration． When their concentration exceeds the load concentration of plants，the removal rate will decrease． Besides，pH and conductivity of water can affect the growth of plants to a certain degree． Fourthly，most plants in constructed wetlands are herbaceous plants，and it is troublesome to maintain and manage them． Enough manpower is needed to cut and remove them out of a sewage treatment system． Meanwhile，they compete with native species and exotic species． Fifthly，some plants in constructed wetlands wither at low temperatures in winter，and the residue of some plants that are not removed in time will be decomposed naturally，so that the concentration of total nitrogen and organic matter will rise，and the concentration of dissolved oxygen will reduce to result in secondary pollution． Meanwhile，plant residue that is decomposed difficultly will accumulate in constructed wetlands to decrease the capacity of constructed wetlands to treat wastewater．
4 Principles of selecting plants for constructed wetlands Wetland plants are important parts of constructed wetlands，and selection of wetland plants can influence the running and removal rate of constructed wetlands，so selection of wet-
Ao Ziqiang et al． Plant Selection of Constructed Wetlands for Treatment of Piggery Wastewater
land plants must follow certain principles according to characteristics of piggery wastewater． 4． 1 Selecting plants for constructed wetlands according to local conditions Differences between various regions in the environment should be considered during the process of selecting plants for constructed wetlands，and local dominant species should be chosen． The normal growth of selected plants in a region should be ensured firstly． Secondly，they can adapt the special adversity of piggery wastewater，otherwise it is difficult to treat pollutants effectively． Constructed wetlands are parts of surrounding landscapes，so the selecting plants should be consistent with surrounding landscapes． To increase the treatment effect of pollutants by constructed wetlands and their beauty，dominant plants should be chosen to match other plants． According to the seasonal growth of plants，the selected plants can make constructed wetlands run effectively all the year round． In addition，the interaction between plants should be considered to reduce antagonism and increase symbiotic action． 4． 2 Selecting plants with strong stress resistance Piggery wastewater is an adverse environment for the growth of plants，so plants in constructed wetlands should have strong vitality，resistance to pollution，aquatic and wet environment， plant diseases and insect pests，and perennial plants should be chosen． Ｒoots of plants in constructed wetlands are submerged by water and contact with pollutants，so the plants have certain adaptability and resistance to pollutants，but their resistance to pollution and adversity are different． Plants’resistance to pollution is one of an important indicator to choose plants for constructed wetlands． Choosing plants with strong resistance to pollution to build constructed wetlands can ensure the normal growth of plants and is conducive to the removal of plants by plants． When the concentration of ammonia nitrogen is higher than 50，20 and 30 mg / L respectively in wastewater， the wastewater is not suitable for the growth of Iris pseudacorus， Pontederia cordata L． and Arundo donax var． ； as the concentration of ammonia nitrogen is 50 mg / L，Cyperus alternifolius does not wither，and the resistance of Thalia dealbata and Typha orientalis Presl． to ammonia nitrogen is next only to that of C． alternifolius［15］ ． Hence，when livestock wastewater containing high concentrations of ammonia nitrogen is treated by constructed wetlands，suitable plants should be selected according to their resistance to ammonia nitrogen． Plants with strong resistance to pollutants should be used to treat various types of wastewater to improve the treatment effect． 4． 3 Choosing plants with strong capacity to remove pollutants To improve the capacity of constructed wetlands to remove pollutants，the purification capacity of plants should be considered comprehensively． Different plants has various capacities to remove pollutants in wastewater，and the stronger the capacity of plants to purify wastewater is，the higher the removal rate of pollutants per unit area is． Among four typical plants in constructed wetlands （ P． australis，Thalia dealbata， Miscanthus sacchariflorus and Canna indica） ，T． dealbata has high annual production of biomass，moderate depth of roots，
87
and roots with a large specific surface area，can create a better environment for biological denitrogenation and has the highest denitrification efficiency［16］ ． 4． 4 Selecting plants with large biomass and developed roots According to the principles and actual situation of wastewater treatment by constructed wetlands， the chosen plants for constructed wetlands should have developed roots and large overground biomass． Developed roots of plants can make plants secrete more secretions to provide good living and growth conditions for micro-organisms in root zones and then promote the degradation of pollutants in rhizosphere． The secreted enzymes can take part in many biochemical processes to improve the purification effect of constructed wetlands． Developed roots of plants are conducive to the fixation of surface of the bed body and soil and have great significance to the stability of wetland ecosystem． 4． 5 Choosing plants with high economic value and diversified functions Besides having high removal rate of pollutants，plants in constructed wetlands should have high economic value and good landscape effect to realize diversified functions of constructed wetlands and obtain good eco-environmental and economic benefit，so constructed wetlands can be applied more widely in the treatment of livestock wastewater． T． latifolia is a kind of perennial herbaceous plants that have developed roots and can be used to treat wastewater in constructed wetlands． It has green leaves and strange ears，so it has certain ornamental value； it can be used to make artificial cotton and paper after being harvested［17］ ． Scirpus validus Vahl has poor capacity to purify wastewater but has certain landscape value and can be used as a landscape plant； its overground part can be used as medicine，and its stems can be used to make paper［18］ ． P． australis，E． crassipes，etc． can be used by using biomass solid fuel molding technology，biogas technology，and fuel ethanol technology to establish the coupling utilization mode of biomass resources in constructed wetlands．
5 Selection of plants for constructed wetlands used to treat piggery wastewater There are more than 6 700 species of higher plants in the world，of which only dozens of species of higher plants have been used to treat wastewater effectively，and most plants are aquatic herbaceous plants． To use constructed wetlands to treat piggery wastewater，more potential plants in constructed wetlands need to be studied further． According to the characteristics of piggery wastewater as well as advantages and disadvantages of plants in constructed wetlands，it is found that the follow-up plant combinations can be used to build constructed wetlands to treat piggery wastewater． 5． 1 Pennisetum sp． + Ｒumex acetosa Linn P． sp． ，an upright tufted perennial gramineous plant，is 4 － 5 m in height， has large biomass （ its fresh weight reaches 250 － 400 t / hm2 ） and developed roots，grows fast，and has high calorific value， so it is a good energy plant． It can be used extensively as materials of cultivation of good edible and medicinal mushrooms，
88
Meteorological and Environmental Ｒesearch
as fodder in breeding industry，and as an energy plant to produce methane through anaerobic fermentation，so it has high economic value［19 － 21］ ． It has strong resistance to pollution and can grow well in piggery wastewater that has been treated under anaerobic conditions （ COD，NH4+ -N and TP concentration are 1 075 －1 800，643 － 745，and 63 － 70 mg / L） ． Ｒ． acetosa Linn，a perennial herbaceous plant，does not need to be planted repeatedly，so it is easy to manege it． It can grow in winter，which can make up the defect of poor treatment effect of wastewater by traditional constructed wetlands in winter and ensure the treatment effect of wastewater by constructed wetlands． Meanwhile，it has developed roots and large biomass， and it can be used as fodder and has medicinal value． In addition，it has strong resistance to pollution，so it can also grow well in piggery wastewater that has been treated under anaerobic conditions （ COD，NH4+ -N and TP concentration are 1 075 － 1 800，643 － 745，and 63 － 70 mg / L） ． Constructed wetlands where P． sp． + Ｒ． acetosa Linn are planted are used to treat piggery wastewater，because the two plants have strong resistance to pollution and can remove nitrogen and phosphorus． Moreover，they have obvious economic value and can make up the defect of poor treatment effect of wastewater by constructed wetlands in winter． The quality of piggery wastewater treated by the constructed wetlands can meet the demands of Discharge Standard of Pollutants for Livestock and Poultry Breeding （ GB 18596 －2001） （ COD 400 mg / L，NH4+ -N 80 mg / L，and TP 8 mg / L） ． 5． 2 Populus L． + Nerium oleander In winter，overground parts of most herbaceous plants in constructed wetlands will wither，so functions of constructed wetlands will be affected． In recent years，the introduction of woody plants into constructed wetlands expands the range of selection of plants for constructed wetlands，relieves the problem of single structure of plants in constructed wetlands，and provides a new idea for the solution of seasonal problems constructed wetlands． Populus L． ，a deciduous species，belongs to Populus，Salicaceae and has advantages of growing fast，deep roots，large biomass，high transpiration rate，and resistance to low temperature． Its leaves can be used as fodder of cows and sheep，and its branches， inflorescences and barks can be used as medicine． It has strong resistance to external toxic substances and can enrich and transport heavy metals to overground parts［22］ ． N． oleander，a dicotyledon，belongs to Apocynaceae． It has high survival rate and increases of plant height and root length，and its average increment reaches 663． 46 g / plant，while the content of nitrogen and phosphorus is up to 14 098． 53 and 902． 31 mg / plant respectively［23］ ． Besides，its leaves and barks has medicinal value，and they are good ornamental plants and can purify the atmosphere［24］ ． P． L． + N． oleander can be planted in constructed wetlands at the same time，because fast-growing poplars grow fast，have strong resistance to pollution （ COD，NH4+ N and TP concentration in piggery wastewater that has been treated under anaerobic conditions are 1 075 － 1 800，643 － 745，and 63 －70 mg / L respectively） and can remove nitrogen and phosphorus effectively； N． oleander is an evergreen spe-
2017
cies，which can make up the defect of poor treatment effect of wastewater by constructed wetlands in winter and ensure the treatment effect of wastewater by constructed wetlands in all the year round． The quality of piggery wastewater treated by the constructed wetlands can meet the demands of Discharge Standard of Pollutants for Livestock and Poultry Breeding （ GB 18596 －2001） （ COD 400 mg / L，NH4+ -N 80 mg / L，and TP 8 mg / L） ．
6
Conclusions
Piggery wastewater has become one of main sources of non-point pollution in rural areas． The application of constructed wetlands in treatment of piggery wastewater can obtain stable effluent，decrease running cost，and beautify the environment． According to the features of piggery wastewater containing high concentrations of organic matter and ammonia nitrogen and the principles of selecting plants for constructed wetlands，P． sp． + Ｒ． acetosa Linn and P． L． + N． oleander can be planted in constructed wetlands to treat piggery wastewater，which can improve the treatment effect of piggery wastewater by constructed wetlands，solve the difficulty of wastewater treatment in winter，and bring obvious economic benefit．
Ｒeferences ［1］ ZHU QH，XIA HX． Study on selection of new artificial wetland plant species［J］． Journal of Hydroecology，2010，31 （ 6） ： 30 － 35． （ in Chinese） ． ［2］ JIANG YP，GE Y，YUE CL，et al． Nutrient removal role of plants in constructed wetlands on sightseeing water［J］． Acta Ecologica Sinica，2004，24（ 8） ： 1718 － 1723． （ in Chinese） ． ［3］ YANG K． Selection of plants for constructed wetlands and experimental research［D］． Nanning： Guangxi University，2007． （ in Chinese） ． ［4］ WANG SＲ，NIAN YG，HOU WH，et al． Selection of plants for constructed wetlands［J］． Journal of Lake Sciences，2004，16 （ 1） ： 91 － 96． （ in Chinese） ． ［5］ WU ZB，LIANG W，CHENG SP，et al． Studies on correlation between the enzymatic activities in the rhizosphere and purification of wastewater in constructed wetlands［J］． Acta Scientiae Circumstantiae，2001，21（ 5） ： 622 － 624． （ in Chinese） ． ［6］ BＲIX H． Treatment of wastewater in the rhizosphere of wetland plants———the root-zone method［J］． Water Science ＆ Technology，1987，19： 107 － 118． ［7］ GAGNON V，CHAZAＲENC F，COMEAU Y，et al． Influence of macrophyte species on microbial density and activity in constructed wetlands［J］． Water Science ＆ Technology，2007，56 （ 3 ） ： 249 － 254． ［8］ KONG L，WANG Y，ZHAO L，et al． Enzyme and root activities in surface-flow constructed wetlands［J］． Chemosphere，2009，76： 601 － 608． ［9］ FU ＲB，ZHU YP，YANG HZ，et al． DO and OＲP conditions and their correlation with plant root distribution in a continuous-flow constructed wetland treating eutrophic water［J］． Acta Scientiae Circumstantiae， 2008， 28（ 10） ： 2036 － 2041． （ in Chinese） ． ［10］ FU WJ，TANG Y． Ｒoles of plants in constructed wetlands and 2005， 24（ 6） ： 45 － species selection［J］． Sichuan Environment， 49． （ in Chinese） ． ［11］ DONG JK，HE F，WU ZB． Valuation of ecosystem service for constructed wetlands［J］． Environmental Science ＆ Technology， 2009， 32（ 8） ： 187 － 193． （ in Chinese） ． ［12］ WANG M，WNAG YB，CHEN ZH． Germplasm resources of
Ao Ziqiang et al． Plant Selection of Constructed Wetlands for Treatment of Piggery Wastewater Phragmites adans and its application in constructed wetlands［J］． Chinese Journal of Applied and Environmental Biology，2010，16 （ 4） ： 590 － 595． （ in Chinese） ． ［13］ HE MX，HU QC，LUO AJ，et al． Assessment of constructed wetland plant biomass for energy utilization［J］． Chinese Journal of Applied and Environmental Biology， 2011， 17（ 4） ： 527 － 531． （ in Chinese） ． ［14］ ZHANG W． Selection of plants for restoration of eutrophic water and comparison between constructed wetland substrates in purification［D］． Changchun： Northeast Normal University，2010． （ in Chinese） ． ［15］GUO X，ZHANG LY，XI BD，et al． Denitrification and selection of wetland plants under high ammonia-nitrogen concentration environment［J］． Journal of Agro-Environment Science，2011，30 （ 5） ： 993 － 1000． （ in Chinese） ． ［16］ SHI L，YANG X． Phytomass and its impact on the pollutant removal ability of constructed wetland［J］． Ecology and Environ2010， 19（ 1） ： 28 － 33． （ in Chinese） ． ment Sciences， ［17］ DONG N，DENG F． Brief discussion on the functions and values of the economic plants in the constructed wetlands［J］． Industrial 39（ 8） ： 22 － 25． Safety and Environmental Protection，2013， ［18］ XIONG Y，SU ZG，GAO JM． Ｒesearch on growth and purifica-
89
tion capacity of various emerging plants［J］． Environmental Study and Monitoring，2010， 23（ 3） ： 10 － 13． （ in Chinese） ． ［19］ LIN XS，LIN ZX，LIN DM，et al． Effects of different years of planting Pennisetum sp． on the plant-and insect diversity in Pennisetum sp． communities［J］． Chinese Journal of Applied Ecology， 2012， 23（ 10） ： 2849 － 2854． （ in Chinese） ． ［20］ XIAO Z，ZHAO C，LIU B． Ｒesearch on thermophicic anaerobic fermentation of Pennisetum sp． ［J］． Anhui Agricultural Science Bulletin， 2012， 18（ 17） ： 18 － 21． （ in Chinese） ． ［21］ ZHENG JY，CHEN LF，LIN ZX． Analysis of Juncao industrial growth and its versatility［J］． Chinese Agricultural Science Bulle2011， 27（ 1） ： 304 － 308． （ in Chinese） ． tin， ［22］LIU YL，WU FX，XU Y，et al． Ｒesearch progress in the remediation of heavy metal contaminated soils with populus［J］． Scientia 2012， 48（ 9） ： 139 － 144． （ in Chinese） ． Silvae Sinicae， ［23］ CHEN YH，WU XF，HAO J，et al． Selection and purification potential evaluation of woody plant in vertical flow constructed wetlands in the subtropical area［J］． Environmental Science，2014， 35（ 2） ： 585 － 591． （ in Chinese） ． ［24］ ＲUI H，WANG Y，WANG Z． Environmental and economic value of Nerium indicum Mill． ［J］． Northern Horticulture，2011，（ 20） ： 119 － 120． （ in Chinese） ．
About the Database of M eteorological and Environm ental Ｒesearch The journal of Meteorological and Environmental Ｒesearch ［ISSN： 2152 － 3940］has been included and stored by the following famous databases： UPD，CAS，CABI，CSA，EBSCO，Chinese Science and Technology Periodical Database，and CNKI，as well as Library of Congress，United States． CAS is a division of the American Chemical Society established in 1907，and is the most authoritative and comprehensive source for chemical information． Centre for Agriculture and Bioscience International（ CABI） is a not-for-profit international agricultural information institute with headquarters in Britain． ProQuest CSA belongs to Cambridge Information Group （ CIG） ，and it provides access to more than 100 databases published by CSA and its publishing partners． EBSCO is a large document service company with a history of more than 60 years，providing subscription and publication services of journals and documents． CNKI （ China National Knowledge Infrastructure） ，universally acclaimed as the most valuable Chinese website，boasts the greatest information content，covering natural science，humanities and social science，engineering，periodical，doctor / master’s dissertations，newspapers，books，meeting papers and other miscellaneous public information resources in China．
