Chapter 2　Performance of constructed wetland combined with biofilm-electrode reactor

2.1　Background

CW had been developed and applied worldwide since the 1990s,and were proved to be able to dispose a variety of wastewater during decades of study(Zhang et al.,2015).The CW had ideal treatment efficiency of ammonia nitrogen,especially in vertical flow constructed wetlands(Vymazal,2014).Among various physical and biochemical nitrogen removal processes in CW,microbial nitrification and heterotrophic denitrification were the two main processes carried out by nitrifying bacteria and heterotrophic denitrifying bacteria(Saeed and Sun,2012).The CW had the advantages of high efficient,low cost,widely applicable,and strong adaptability of load change.However,low C/N influent might obstruct the heterotrophic denitrification process in the posterior substrate of the CW and result in low nitrogen removal efficiency(Shen et al.,2015).Hence,external carbon source was usually added into the influent to enhance the denitrification effect(Saeed and Sun,2013).Whereas,there was a risk of overdosing or in exhaustive denitrification.Meanwhile,the addition of external carbon source increased the cost(Boley et al.,2000).

In Southern China,the municipal sewage was characterized as low COD level,and the carbon source was commonly insufficient.Thus,plenty of municipal sewage in Southern China was low C/N wastewater.At present,the denitrification crafts of low C/N wastewater were mainly biochemical process,physical process and the physical combined biochemical process.Nevertheless,these crafts in most cases had the shortage of immature technology,inefficiency,long start-up period,severe conditions,secondary pollution and high investment cost.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).

(2.1)

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).

(2.2)

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).

(2.3)

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.and Park et al.installed a new-type BER to dispose a high concentration of nitrate wastewater(Park et al.,2005; Szekeres et al.,2001).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 bioelectrochemical 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 HRT.

A plenty of investigations of BER or CW is separately carried out.But the studies of high effective denitrification by CW combined with BER were few.In this study,a constructed wetland combined with a biofilm-electrode reactor(CW-BER)generated a heterotrophic/autotrophic high effective denitrification system.On one hand,the nitrifying bacteria in the CW used carbon dioxide,carbonate,and bicarbonate as the carbon source to oxidate ammonia nitrogen to nitrate and compound new cells as well as gain energy at the aerobiotic condition.On the other hand,the heterotrophic/autotrophic denitrification(HAD)system at the anaerobic condition took advantage of the combined action of heterotrophic denitrification(HD)and autotrophic denitrification(AD)to promote the denitrification effect.Not only nitrate but also ammonia nitrogen had good removal effects in this system,thus,the removal efficiency of total nitrogen could be increased.This system made up the disadvantage that the CW lacked the capability of denitrification of low C/N wastewater as well as the BER could hardly dispose the ammonia nitrogen.The objective of this work was to investigate the effect of C/N ratio,electric current intensity(I),hydraulic retention time(HRT)and pH on denitrification rate and to optimize the operating parameters of the system.