1.3　Bio-electrode film technique

1.3.1　Theory of Bio-electrode process

The aim of electrode biofilm method for nitrogen removal is to reduce oxynitride to N2.The denitrifying bacteria is attached on the cathode surface by the way of membrane-hanging domestication to form a microbial film.Among these microorganisms,the autotrophic denitrifying bacteria on the film took the hydrogen produced by microelectrolysis as electron donor.Ultimately,the oxynitride is reduced to N2.It is a treatment of sewage which combines electrochemical interaction and denitrification.Therefore,it is an efficient and clean treatment method for sewage with low C/N content,which unites with electrolysis theory and biological theory.Besides,both the properties of autotrophic denitrifying bacteria and a high utilization ratio of the produced nitrogen are used in this system.

1.3.2　Electrolysis theory

Electrolysis reaction is a process of converting electrical energy into chemical energy.In the process,the reaction of hydrogen autotrophication is caused by the addition of micro-current.In micro-electrolytic reactor,cathode and anode are respectively connected with negative pole and positive pole by power supply.Therefore,the oxidizing compounds around cathode are converted to reduction state by getting the electrons from the negative pole.Meanwhile,the electrons of reducing compounds around anode are transferred to anode because of the electrons accumulation around anode.And then,the reducing compounds are converted to oxidation state by the function of electrons.

In general,there are basic ions such as so on in domestic sewage.To deal with this kind of sewage contains many kinds of ions,the first precipitation ions need to be obtained by comparing the electric potential of the equilibrium electrode and the actual precipitation electrode potential.On the cathode,the higher precipitation potential ions are firstly precipitated out.On the contrary,the ions with lower precipitation potential are precipitated out on the anode.The hydrogen is firstly obtained by the reaction of electrolytic water on the cathode.On the anode(carbon-based),carbon dioxide is firstly produced after a comprehensive analysis of the precipitation potential of various basics ions.

The first reaction on the cathode surface:

(1.8)

(1.9)

The first reaction on the anode(carbon-based):

(1.10)

(1.11)

In the electrode biofilm reactor,both cathode and anode play an important role.If the anode is made of carbon,it will produce carbon dioxide and other compounds that neutralize the base of denitrification to buffer the pH of the system.Meanwhile,it could also be used as inorganic carbon sources by the autotrophic denitrifying bacteria.If the anode is metal material,oxygen may be produced from the reaction,which form suitable nitration conditions around the anode.It will also create favorable conditions for the simultaneous nitrification and denitrification reaction in the reactor.The cathode is not only the conductor connected circuit but also played a role as a biological carrier.In addition,the hydrogen which is obtained by the reaction of electrolytic water flowed out of the introversion through the biofilm on the cathode.It is beneficial for enhancing an anaerobic environment around the cathodic biofilm and providing a good condition for the smooth progress of the reaction of denitrification.

1.3.3　Biological theory

The denitrification method of traditional biological denitrification is used for electrode biofilm.Ultimately,oxynitride is reduced to nitrogen after a series of enzymatic reactions as follows:

Denitrifying bacteria are classified as heterotrophic bacteria and autotrophic bacteria including the carbon source type that denitrifying bacteria need for growth.

Heterotrophic bacteria take organics as carbon source.For this kind of bacteria,the carbon source is the energy power supply for its denitrification.The traditionally used organic carbon sources contain acetic acid,glucose,methanol and so on.The process of the heterotrophic denitrification which takes acetic acid as the electron donor is shown as follows:

(1.12)

A multitude of denitrifying bacteria in nature are heterotrophic such as Micrococcus,Pseudomonas and so on.

Autotrophic bacteria take inorganics like carbon dioxide,carbon ion,hydrogen carbonate ion for carbon source.This kind of bacteria which normally takes elemental sulfur,sulfide and hydrogen as electron donor reduced nitrate to nitrogen.This type of denitrifying bacteria is a minority in nature.Hydrogen bacteria are one of them and their growth rate is fast.The reaction process which takes hydrogen as its electron donor is shown as follows:

(1.13)

Hydrogen bacteria are mainly cultured on the cathode of the electrode in biofilm reactor.There are many strains of the bacteria such as Pseudomonas,Paracoccus,Alcaligenes,Xanthomonas.Most hydrogen bacteria are aerobic bacteria,and a few of them are anaerobic or facultative anaerobes.Most of them are suitable for growing at room temperature and an unalkaline or neutral environment.

Taking comprehensive analysis of principle,the heterotrophic denitrifying bacteria taking organics as carbon source.Meanwhile,autotrophic denitrifying bacteria which take the hydrogen produced by micro-electrolysis as electron donor and used inorganic carbon compounds like anodized carbon dioxide as an inorganic carbon source for denitrification.Both the heterotrophic denitrification and the autotrophic denitrification take place simultaneously.Then,both two reactions convert the nitrate nitrogen into nitrogen and take it out of the system.Finally,the nitrogen is removed by denitrification.The general equation can be expressed as follows:

(1.14)

(1.15)

1.3.4　The development of biofilm-electrode reactor

Recently,biofilm-electrode reactor(BER)had been widely used to study the nitrogen removal of low C/N wastewater.And it had become a favorite way of nitrogen removal of low C/N wastewater due to its advantage of high-efficiency as well as free of external carbon source supply.

BER was one of the most effective nitrate removal devices which combines biological and electrochemical methods(Tong et al.,2013).In such a reactor,autotrophic denitrifying microorganisms immobilized on the surface of the cathode and the internal production of H2 which was generated by the electrolysis of water was utilized as an electron donor for denitrification(Park et al.,2006).

(1.16)

The anode which was made by carbon materials provides the inorganic carbon source and served as a pH buffer for denitrification(Hao et al.,2013).

(1.17)

The denitrification in BER used hydrogen which was generated in the reactor instead of organic carbon source.What’s more,cathode generated H2 and could not be overdosed because of its low solubility in water,which could convert nitrate completely into harmless nitrogen gas in the following way(Kurt et al.,1987).

(1.18)

Which made the process no further steps to remove excess substrates.On this account,BER was considered as a high nitrogen removal efficiency of low C/N wastewater,cost saving and security reactor(Feleke et al.,1998; Sakakibara et al.,1997; Szekeres et al.,2002).

Since the first report on the biofilm-electrode reactor,substantial studies had been carried out(Mellor et al.,1992).Flora et al.made a cylindrical BER and found that the quantity of nitrogen increased with the increase of the electric current intensity.This result indicated that electric current intensity was able to promote and control the denitrification process.In addition,if the value of electric current intensity exceeds the limiting value,the phenomenon of “hydrogen inhibition” occurred and led to the reduction of nitrate removal efficiency(Flora et al.,1994).Feng et al.,Bao et al.and Kuroda et al.added a part of organic carbon source(solid or liquid)in BER to study the removal of nitrate with different C/N ratios(Bao and Hao,2006; Feng et al.,2013; Kuroda et al.,1996).The result showed that in the case of high C/N ratio,the autotrophic denitrification still took place.Moreover,the coexistence of heterotrophic bacteria and autotrophic bacteria had a better nitrogen removal effect.Szekeres et al.(2001)and Park et al.(2005)installed a new-type BER to dispose a high concentration of nitrate wastewater.The new reactor could reach high nitrate removal efficiency and the maximum nitrate removal efficiency was 98% at an applied current of 200mA,corresponding to a nitrate removal of 0.17mg NO3-N/(cm2 of biofilm surface area day).Ghafari used palm shell granular activated carbon(GAC)(Ghafari et al.,2009)and Zhao et al.immobilized denitrifying bacteria fiber threads as the biocarrier to install an upflow bio-electrochemical reactor(UBER)and an intensified biofilm-electrode reactor(IBER)(Zhao et al.,2011),respectively.The nitrate could be highly reduced within a widely operational range of electric current and hydraulic retention time(HRT).

In the electrode biofilm reactor,both cathode and anode play an extremely important role.If the anode is made of carbon,it will produce carbon dioxide and other compounds that neutralize the base of denitrification to buffer the pH of the system.Meanwhile,it could also be used by the autotrophic denitrifying bacteria as inorganic carbon sources.If the anode is metal material,oxygen may be produced from the reaction,which form suitable nitration conditions around the anode.It will also create favorable conditions for the simultaneous nitrification and denitrification reaction in the same reactor.The cathode is not only the conductor that connect circuit but also played a role as a biological carrier.In addition,the hydrogen which is obtained by the reaction of electrolytic water flowed out of the introversion through the biofilm on the cathode.It is beneficial for enhancing an anaerobic environment around the cathodic biofilm and providing a good condition for the smooth progress of the reaction of denitrification.