per capita water resources of china are only a quarter of average value of the world .
china was belonging to one of 13 countries with an extreme deficiency of water resources in the world . at the same time
problems caused by water shortage have already become the bottleneck of economic and social sustainable development in china .
so municipal reclaimed water rather than surface water was replenished into recycling cooling water system in power plants in some severe water shortage cities , such as beijing , tianjin , taiyuan , qingdao , and xi'an . compared with surface water
, municipal reclaimed water contains more microorganisms and higher content of inorganics , organics , nitrogen , and phosphorus .
recycling cooling water system would face more microbial and corrosive problems with the system makeup by municipal reclaimed water instead of surface water .
in fact , biofilm growth , pipe corrosion , and water quality deterioration have been found in the real system [ 3 , 4 ] . using surface water to replenish recycling cooling water system has been studied systematically .
many works are concentrated in the chemical stability , corrosion , and scaling mechanism , corresponding to control technology [ 58 ] .
the existing studies provide important theoretical and technical support for the system security . however , water quality of municipal reclaimed water is more complicated than that of surface water .
so more biochemical , physical , and chemical reactions will happen in the recycling cooling water system when made up by municipal reclaimed water instead of surface water , which will result in more foulants and severe metal corrosion on the interface of pipe . at present ,
fewer reports about foulant , metal corrosion , and microorganism in the recycling cooling water system makeup by municipal reclaimed water are found . two kinds of foulants on the carbon steel coupons in the system makeup by either municipal reclaimed water or surface water were studied , respectively , by static experiment in the paper .
firstly , 23 water quality indexes of municipal reclaimed water and surface water were comprehensively analyzed by chemical methods .
then foulant characteristics of both systems , mainly including the total number of bacteria , sulfate reducing bacteria , iron bacteria , extracellular polymeric substance ( eps ) , protein ( pn ) , polysaccharide ( ps ) , and weight of foulant and its morphology , were analyzed by microbiological methods and scanning electron microscopy ( sem ) technology . based on experimental results ,
the effects of different makeup water resources on foulants in recycling cooling water system were discussed .
after 20 carbon steel coupons were hanged , 10  l municipal reclaimed water and 10  l surface water were added into the reactors , respectively .
then temperatures , flow velocities , and concentration ratios of the reactors were set to 35c , 0.7  m / s , and 1.0 times .
size of the coupon was 5  cm  2.5  cm  0.2  cm .
municipal reclaimed water was taken from municipal reclaimed water pipeline of one thermal power plant in beijing .
the national standard analytical methods for water quality were adopted in the experiment , as shown in table 1 .
rapid microbial fluorescence detector pi-102 produced by american hygiene company was employed for total number of bacteria analysis .
principle of the instrument is that atp was released from the microbial cells through the cell lysis reagent extractant .
then luciferase reaction agent , bioluminescence reagent , was added and fluorescence was generated due to the reaction between bioluminescence reagent and atp . at last ,
sulfate reducing bacteria ( srb ) were analyzed by mpn method according to  examination of bacteria and algae in industrial circulating cooling water 
part 5 : examination of sulfate - reducing bacteria  ( gb / t14643.5 2009 ) .
iron bacteria were analyzed by mpn method according to  examination of bacteria and algae in industrial circulating cooling water 
( gb / t 14643.6 2009 ) . peel off foulant from coupons and put it into a beaker .
add 50  ml buffer solution ( 2  mmol / l na3po4 , 4  mmol / l nah2po4 , 9  mmol / l nacl , and 1  mmol / l kcl , ph = 10 ) into the beaker .
then dip 2 to 5 cotton swabs into the buffer solution and use them to scrub coupons back and forth . put the swabs into the beaker also .
add 0.3  ml formaldehyde into the beaker and stir the beaker 0.5  h at 300  r / min .
after adding 50  ml 0.04  mol / l naoh solution into the beaker , stir it for 1  h and centrifuge it for 10~20  min .
filtrate supernatant fluid by 0.45  m microporous membrane and take clear liquid for experiment .
comparison of water quality of municipal reclaimed water and surface water was shown in table 2 . according to table 2 , except aluminum and copper , the concentrations of turbidity , ph , alkalinity , zinc , manganese , total iron , and bicarbonate in municipal reclaimed water were lower than those in surface water , but the concentrations of conductivity , tds , chloride ion , sulfate , calcium hardness , codmn , total hardness , total phosphorus , soluble phosphorus , orthophosphate , silicon , surfactant , and total bacteria were higher .
the concentrations of conductivity , tds , chloride ion , silicon , and surfactant in municipal reclaimed water were , respectively , 1.5~2 times , 1.6~1.9 times , 1.9~4.5 times , 2.9 times , and 2.8 times higher than those in surface water .
the concentrations of codmn , total phosphorus , soluble phosphorus , orthophosphate , and total nitrogen were about 1.5~2 times higher than those in surface water .
the total number of bacteria in municipal reclaimed water was one order of magnitude higher than that in surface water . compared with surface water , more sulfate , calcium , phosphate , and silica in municipal reclaimed water would increase scale risks in the recycling cooling water system . however
, higher anionic surfactant would play an opposite role . by comprehensive analysis according to table 2 , the concentration of surfactant in municipal reclaimed water was 2.8 times higher than that in surface water . because the difference of surfactants in municipal reclaimed water and surface water was bigger than that of sulfate , phosphate , and silica , scaling risk would reduce with the system makeup by municipal reclaimed water instead of surface water .
chloride ion , sulfate , orthophosphate , and ph were the key indices for corrosion .
radius of chlorine ion is small and therefore that leads to its strong penetrability or adsorption ability .
chlorine ion could destroy the structure of oxide film on the metal and result in metal into active corrosion state from passive corrosion state .
although radius of sulfate ion was of larger and lower penetrability through passivation film than chloride ion , sulfate would be reduced to hydrogen sulfide by sulfate reducing bacteria and result in low ph nearby .
so the pitting corrosion of metals happened . on the contrary , phosphate was beneficial to passivation of the metal surface .
main ingredient of the passivation membrane was -fe2o3 , which could delay the metal corrosion .
due to high chlorine ion and sulfate and low ph in municipal reclaimed water , risk of metal corrosion would increase in recycling cooling water system made up by municipal reclaimed water . there were sunshine , high dissolved oxygen , and suitable temperature in recycling cooling water system , which were beneficial to microorganism growth .
moreover , high codmn , total phosphorus , orthophosphate total nitrogen , and the total number of bacteria in municipal reclaimed water would also increase microbial risk in recycling cooling water system .
weight of foulant in recycling cooling water system makeup by municipal reclaimed water and surface water was shown in figure 4 . according to figure 4 ,
weight of foulant on the coupon in the system makeup by municipal reclaimed water was 46.8  mg / cm , which was 0.56 times higher than that by surface water .
sem morphology in figure 5 also clearly showed the differences of the foulants . according to figure 5 , it was clear that the surface of carbon steel coupon in the system makeup by surface water was almost entirely covered by foulant , whereas that by municipal reclaimed water was only covered partly .
foulant in recycling cooling water system in power plant mainly formed by crystallization , particle , corrosion , and microorganism .
the weight of foulant is mainly composed of the first three kinds of foulant . according to results of section 3.1 , crystallization foulant
would be relieved in recycling cooling water system under the role of anionic surfactant in municipal reclaimed water , which was the main reason for the foulant weight decrease .
meanwhile , chloride ion in municipal reclaimed water was 2.5~4.5 times higher than that in surface water .
images by sem confirmed the judgments above . according to figure 6 , carbon steel 's corrosion in the system makeup by municipal reclaimed water belonged to pitting corrosion , which produced fewer foulants but more and deeper corrosion pits .
by contrast , for carbon steel coupon in that by surface water , its corrosion surface was relatively uniform .
the weight of corrosion foulant was more , but the depth of corrosion pit was relatively lighter .
in addition , compared with surface water , low turbidity in municipal reclaimed water would result in fewer particle foulants in recycling cooling water system .
based on the above results , when municipal reclaimed water took the place of surface water to replenish into recycling cooling water system , the amount of foulant would be lower due to reduction of crystallization , corrosion , and particle deposition , which would lead to decrease of heat transfer resistance and energy losses of the heating equipment .
comparison of eps , ps , and pn in foulants in recycling cooling water system makeup by municipal reclaimed water and surface water was shown in figure 7 . according to figure 7 , eps in unit mass of foulant in the system makeup by municipal reclaimed water was 1.70  mg / g .
the value in that by surface water was 6.54  mg / g , which was 4 times higher compared with that by municipal reclaimed water . in the system makeup by municipal reclaimed water , contents of pn and ps in unit mass of foulant were , respectively , 0.47  mg / g and 0.89  mg / g , in which the ratios in eps were corresponding to 27.6% and 52.4% .
the proportion of ps was an absolute advantage . on the contrary , in the system makeup by surface water , the proportion of pn in eps was up to 51.7% and the ratio of ps was relatively lower .
eps was one kind of high - molecular compound which was secreted by microorganism and surrounded outside of the cell wall under certain environmental conditions .
eps mainly came from microbial metabolism , cell autolysis , deciduous of cell material , and influent water matrix . by studying eps of 27 kinds of heterotrophic bacteria , tsuneda et al . found that ps and pn account for 75%~89% of eps
pn mainly came from intracellular secretion or substance release by cell digestion , which concluded many hydrophobic functional groups with a positive charge , such as alanine , cysteine , valine , and amino acid . according to the principle of thermal dynamics ,
the increase of hydrophobicity was helpful to reduce gibbs energy on the surface of foulant .
hydrophobicity played a key role in the initial stages of fouling formation [ 10 , 1217 ] .
its chemical structure was highly branched and included many hydrophilic functional groups with negative charge , such as hydroxyl , carboxyl , and phosphate .
thus , an amorphous network structure formed , which played a role of adsorption and flocculation by adsorption bridging .
some scholars had put forward that acidic ps plays a role of bridge support for microbial aggregation and its strong three - dimensional structure by ionic bonds , neutral polysaccharide , and hydrogen bonding [ 18 , 19 ] .
the content and composition of eps were related to microbial species , growth status , and the types of substrate . in low organic matter and nutrient environment , available substrate by bacteria was less and its proliferation rate was low .
in high organic matter and nutrient conditions , microbial cell can not put the whole carbon source for cell synthesis ; excess carbon source was converted into eps and extracellular polymeric substances were accumulated in eps .
eps mainly came from the process of synthetic cell metabolism [ 21 , 22 ] .
microbial death and cell autolysis were the main source of eps in the foulant . according to table 2 , the concentrations of codmn , total phosphorus , soluble total phosphorus , orthophosphate , and total nitrogen in municipal reclaimed water were 1.5~2 times higher than those in surface water .
results of section 3.2 showed that the contents of crystallization , corrosion , and particulate depositions in the system makeup by municipal reclaimed water were less than those by surface water , which reduced the mass transfer resistance of organic matter , nutrients , and dissolved oxygen in foulant . at the same time , microbial assimilation was enhanced . on the whole , there were stronger microbial anabolic activity and lower death and cell autolysis rate in the system makeup by municipal reclaimed water than those in the surface water , which was the major cause of results of eps decrease in foulant .
polyvalent metal ion , suspended organic matter , and inorganic matter in water were adsorbed by electrostatic interactions of protein in eps .
higher proportion of pn in eps would cause stronger hydrophobicity , higher cross linking degree , and more compact structure of foulant [ 2327 ] .
ps would promote the flocculation by reducing the effective critical potential of the bacterial cells due to cell walls wrapped by it .
lower pn and higher ps in the system makeup by municipal reclaimed water than that by surface water meant more microbial foulant and low compactness of foulant in the system .
comparisons of the total number of bacteria , iron bacteria , and srb in the foulant in recycling cooling water system makeup by municipal reclaimed water and surface water were shown in figure 8 . according to figure 8 ,
total numbers of bacteria and iron bacteria in the system makeup by municipal reclaimed water were , respectively , 8.64 times and 8.64 times higher than those by surface water , but srb was 80.8% lower . in poor nutrition environment ,
the higher the nutrient concentration , the greater the microbial growth rate . according to table 2 , the contents of codmn , total phosphorus , soluble total phosphorus , orthophosphate , and total nitrogen in municipal reclaimed water were 1.5~2 times higher than those in surface water , which was the main reason for the higher total number of bacteria and iron bacteria in the system .
moreover , based on results in sections 3.2 and 3.3 , in recycling cooling water system , less foulant weight and looser foulant structure were found if reclaimed water was used as makeup water instead of surface water .
so transfer resistance decreased and dissolved oxygen increased in the foulant when the system is made up by reclaimed water instead of surface water .
more microorganisms in municipal reclaimed water would increase biofilm and microbial foulant formation in the recycling cooling water system .
iron bacteria belonged to aerobic bacteria , which could oxidize fe to fe and produce iron oxide precipitation on the metal surface .
moreover , precipitation on the surface also led to the formation of oxygen concentration cell and thus resulted in metal corrosion accelerated .
it used lactic acid or pyruvic acid as electron donor and sulfate as electron acceptor . by redox reaction
in addition , role of cathodes depolarization was also the main reason of srb corrosion .
high iron bacteria and low srb in municipal reclaimed water meant that metal corrosion would mainly be influenced by iron bacteria when recycling cooling water system replenished by reclaimed water .
above all , in recycling cooling water system makeup by municipal reclaimed water instead of surface water , there were more numbers of microorganisms , high foulant viscosity , and more serious aerobic microbial corrosion .
based on comparison of municipal reclaimed water and surface water , amount of foulant , the total number of bacteria , iron bacteria , srb , eps , pn , and ps in foulant on the carbon steel in recycling cooling water system makeup by municipal reclaimed water and surface water were studied , respectively , by microbiological methods and sem technology .
main conclusions were shown as follows.according to analysis results of 23 water quality indexes , compared with that in surface water , concentrations of conductivity , tds , chloride ion , and surfactant in municipal reclaimed water were 2~4 times higher , concentrations of codmn , total phosphorus , soluble total phosphorus , orthophosphate , and total nitrogen were 1.5~2 times higher , and the total number of bacteria was one order of magnitude higher .
the results showed that lower scaling , higher corrosion risk , and microbial risk would happen in the recycling cooling water system when it was replenished by municipal reclaimed water instead of surface water.the weight of foulant in the recycling cooling water system makeup by municipal reclaimed water was 44% lower than that by surface water , but the total numbers of bacteria and iron bacteria in unit mass foulant were , respectively , 8.6 times and 2.7 times higher .
the results indicated that heat transfer resistance and energy losses of the heating equipment would decrease but microbial corrosion would increase in the recycling cooling water system when it was replenished by municipal reclaimed water instead of surface water.eps in foulant in the recycling cooling water system makeup by municipal reclaimed water was 74% lower than that by surface water .
proportions of pn and ps in eps were , respectively , 27.6% and 52.4% in the system makeup by municipal reclaimed water .
the proportion of ps was an absolute advantage . on the contrary , in the system makeup by surface water , pn ratio in eps was up to 51.7% ; content of ps was relatively lower .
the results revealed more microbial foulant and looser foulant structure in the recycling cooling water system when it was replenished by municipal reclaimed water instead of surface water.results of sem showed that thicker foulant and uniform corrosion appeared in the recycling cooling water system makeup by surface water . when the system was replenished by municipal reclaimed water instead of surface water ,
the amount of foulant became less and metal pitting corrosion was more serious . according to analysis results of 23 water quality indexes , compared with that in surface water , concentrations of conductivity , tds , chloride ion , and surfactant in municipal reclaimed water were 2~4 times higher , concentrations of codmn , total phosphorus , soluble total phosphorus , orthophosphate , and total nitrogen were 1.5~2 times higher , and the total number of bacteria was one order of magnitude higher .
the results showed that lower scaling , higher corrosion risk , and microbial risk would happen in the recycling cooling water system when it was replenished by municipal reclaimed water instead of surface water .
the weight of foulant in the recycling cooling water system makeup by municipal reclaimed water was 44% lower than that by surface water , but the total numbers of bacteria and iron bacteria in unit mass foulant were , respectively , 8.6 times and 2.7 times higher .
the results indicated that heat transfer resistance and energy losses of the heating equipment would decrease but microbial corrosion would increase in the recycling cooling water system when it was replenished by municipal reclaimed water instead of surface water .
eps in foulant in the recycling cooling water system makeup by municipal reclaimed water was 74% lower than that by surface water .
proportions of pn and ps in eps were , respectively , 27.6% and 52.4% in the system makeup by municipal reclaimed water .
on the contrary , in the system makeup by surface water , pn ratio in eps was up to 51.7% ; content of ps was relatively lower .
the results revealed more microbial foulant and looser foulant structure in the recycling cooling water system when it was replenished by municipal reclaimed water instead of surface water .
results of sem showed that thicker foulant and uniform corrosion appeared in the recycling cooling water system makeup by surface water . when the system was replenished by municipal reclaimed water instead of surface water ,
 due to water shortage , municipal reclaimed water rather than surface water was replenished into recycling cooling water system in power plants in some cities in china . in order to understand the effects of the measure on carbon steel corrosion , characteristics of two kinds of foulant produced in different systems were studied in the paper . 
 differences between municipal reclaimed water and surface water were analyzed firstly . 
 then , the weight and the morphology of two kinds of foulant were compared . 
 moreover , other characteristics including the total number of bacteria , sulfate reducing bacteria , iron bacteria , extracellular polymeric substance ( eps ) , protein ( pn ) , and polysaccharide ( ps ) in foulant were analyzed . 
 based on results , it could be concluded that microbial and corrosive risk would be increased when the system replenished by municipal reclaimed water instead of surface water . 
1. Introduction
2. Materials and Methods
3. Results and Discussion
4. Conclusion