Simultaneous removal of nitrogen and phosphorus from swine wastewater in a sequencing batch biofilm reactor☆

Reti Hai,Yiqun He*,Xiaohui Wang,Yuan Li

Center of Environmental Science and Technology,Beijing University of Chemical Technology,Beijing 100029,China

Keywords:Sequencing batch biofilm reactor Swine wastewater Simultaneous nitrogen and phosphorus removal

ABSTRACT In this study,the performance of a sequencing batch biofilmreactor(SBBR)for removal of nitrogen and phosphorus from swine wastewater was evaluated.The replacement rate of wastewater was set at12.5%through out the experiment.The anaerobic and aerobic times were 3 h and 7 h,respectively,and the dissolved oxygen concentration of the aerobic phase was about 3.95 mg·L?1.The SBBR process demonstrated good performance in treating swine wastewater.The percentage removal of total chemical oxygen demand(COD),ammonia nitrogen(NH4+-N),total nitrogen(TN),and total phosphorus(TP)was 98.2%,95.7%,95.6%,and 96.2%at effluent concentrations of COD 85.6 mg·L?1,NH4+-N35.22 mg·L?1,TN 44.64 mg·L?1,and TP 1.13 mg·L?1,respectively.Simultaneous nitrification and denitrification phenomenon was observed.Further improvement in removal efficiency of NH4+-N and TN occurred at COD/TN ratio of 11:1,with effluent concentrations at NH4+-N 18.5 mg·L?1 and TN 34 mg·L?1,while no such improvement in COD and TP removal was found.Microbial electron microscopy analysis showed that the filler surface was covered with a thick biofilm,forming an anaerobic-aerobic micro environment and facilitating the removal of nitrogen,phosphorus and organic matters.A long-term experiment(15 weeks)showed that stable removal efficiency for N and P could be achieved in the SBBR system.

1.Introduction

Since the 1990s,the growth of large-scale industrial livestock enterprises has solved the livestock supply problem and promoted rural community economic development.However,this industrial growth is also responsible for severe environmental pollution.Generally,swine wastewater contains high levels of nutrient pollutants such as nitrogen and phosphorus,which contribute to eutrophication of water bodies[1,2].Therefore,swine wastewater treatment technologies are in demand for effective nutrient pollution control.Currently,swine wastewater is mainly treated by combined anaerobic/aerobic processes,which are effective for the removal of chemical oxygen demand(COD)and suspended solids(SS),confirmed by pilot study and engineering applications,but fail to remove nitrogen and phosphorus.More reliable and economic treatment processes are needed[3].

The sequencing batch biofilm reactor(SBBR)is a biological process based on the sequencing batch reactor(SBR)and utilizes microorganisms(attached to a plastic carrier)to treat wastewater.As a new biological wastewater treatment process,SBBR has been widely studied and applied because of its prominent advantages such as more biomass and less sludge,simple and flexible operation,and good sewage treatment effect[4,5].Fang and Chen conducted experiments to compare SBBR and SBR processes in treating slaughterhouse wastewater and showed that the outlet water quality with SBBR was better[6].Jiang and Zhai tested the ability of the SBBR process to remove nitrogen and phosphorus simultaneously from domestic sewage,with the removal efficiencies for TP and TNat84%and 80%,respectively,at dissolved oxygen(DO)concentration of 3.5 mg·L?1and sludge retention time of 25 days[7].Xiao et al.used the SBBR process to treat swine wastewater,with the removal of COD and NH4+-N at 92.8%-94.1%and 94.8%-96.1%,respectively,at a hydraulic retention time(HRT)of 10 days,temperature of 24-26°C,and DO concentration less than 2.0 mg·L?1[8],but they did not discuss the TP removal.The SBBR process combines the advantages of SBR process and biomass retention properties of attached biofilms and has been used for treating various types of wastewater,but few reports are on its application to remove nitrogen and phosphorus simultaneously from swine wastewater.The major objective of this study is to examine the factors influencing simultaneous removal of nitrogen and phosphorus from swine wastewater using SBBR and determine the optimal process parameters such as HRT,DO and COD/TN ratio.

2.Materials and Methods

2.1.Laboratory scale SBBR

Fig.1.A schematic diagram of SBBR.

Fig.1 shows a lab-scale SBBR reactor in this study,with an internal diameter of 15 cm,a height of 55 cm,a total volume of 9.7 L,and a working volume of 8.0 L.It was fabricated with Plexiglas material for easy observation of biofilm carriers and sludge in the reactor.The feed was introduced at the bottom of the reactor to achieve better mixing in the reaction.An activated carbon fiber filler was installed in the reactor,which is 40 cm in length and contains about 12000 carbon fiber threads.Tens of thousands of filaments were dispersed to form a huge surface area,and flexible carbon fiber yarns swung with the water in the reactor under the aeration condition,so that the mass transfer could be improved in the biofilm.

The reaction time for each stage in SBBR was controlled by a timer.The concentration of DO in the reactor was maintained by microporous aerators during the aerobic phase,and the anaerobic cycle was achieved through the circulation pump during the anaerobic phase.The controller was programmed to operate on a repeating 24 h cycle with a sub-program,and the output went to the operation of each controllable element.Temperature,which has a remarkable effect on the respiratory rate of microorganisms,was maintained between 25-30°C in the experimental process to maintain high activity of microorganisms.

2.2.Wastewater

In this study,the swine wastewater was taken from a swine breeding farm of Tongzhou district in the suburbs of Beijing.It was allowed to settle for 6-12 h to remove some suspended solids.The collected swine wastewater was stored at 4°C.

2.3.Reactor start-up and operation

The swine wastewater was instantly charged into the SBBR and drawn through the outlet after the completion of an operation cycle(20%water exchanged,1.6 L).Table 1 shows the major characteristicsof the swine wastewater.In the first stage of microbial culture,aerobic condition(DO ＞2.0 mg·L?1)was maintained,and the original swine wastewater was diluted(with COD 470 mg·L?1,NH4+-N 85 mg·L?1,and TP 3.2 mg·L?1).Seed activated sludge taken from a sewage plant in Beijing was used for the SBBR setup.The activated sludge had mixed liquor suspended solids(MLSS)of 7568 mg·L?1and mixed liquor volatile suspended solids(MLVSS)of 4762 mg·L?1,with the value of MLVSS/MLSS at 0.63.The SBBR was subsequently operated at an HRT of 24 h to obtain steady performance,and then the influent wastewater concentration was gradually increased until it equaled the original concentration.After 8 days of cultivation,significant biofilm formed on the carbon fiber threads in SBBR,and the percentage removal values of COD and NH4+-N were 73%and 62%,respectively.With microscopic examination,the biofilms were found to be mature.

Table 1 Characteristics of the swine wastewater used in this study

The second stage was to nurture the phosphorus accumulating capacity of the system.The operation strategy was: fill(5 min)-anaerobic phase(1.5 h)-aeration phase(6 h)-settle(20 min)-draw(5 min).As shown in Fig.2,after 65 days of cultivation,effluent concentrations of COD and TP decreased to 99.2 and 3.11 mg·L?1,respectively,indicating that the phosphorus accumulating bacteria domestication was completed.

Fig.2.COD and TP data of phosphate accumulating bacteria domestication stage.

2.4.Scanning electron microscopy

After 45 days of cultivation,a sponge cuboid was extracted from the carbon fiber thread in SBBR,and scanning electron microscopy(SEM)was used to visualize the large amount of microorganisms in the biofilm.The biofilm sample was suspended in a 0.1 mol·L?1phosphate buffer solution(pH 7.4)for 24 h at room temperature.Then,the sample was dehydrated by six consecutive extractions in ethanol/water solutions at increasing ratios(from 10 to 100).Finally,the sample was freeze dried,gold-coated by a sputter,and examined by SEM(S-4700,Japan)operated at 20 kV.

2.5.Analytical methods

The concentrations of COD,NH4+-N,NO3?-N,NO2?-N,TN,TP,MLSS and SS were measured according to the Water and Wastewater Monitoring Analysis Method[9].The pH,oxidation reduction potential(ORP),and temperature were tested by using a HANA HI8424 pH/ORP/T tester.The DO was measured continuously online by using a DO detector(HACH sc100,USA).The microorganisms were observed through an Olympus CX31 type biological microscope.

3.Results and Discussion

3.1.The effect of reaction time on nitrogen and phosphorus removal

3.1.1.Effect of anaerobic reaction time on phosphorus release and denitrification

The residence time of the anaerobic stage is one of the key factors that affect the nitrogen and phosphorus removal in SBBR.Phosphorus release at the anaerobic stage is a prerequisite of excess phosphorus uptake at the aerobic stage.During this period,denitrifying bacteria utilize anaerobic fermentation products as carbon source and oxygen of nitric acid in swine wastewater under anaerobic conditions and reduce nitrates to N2[10].The anaerobic time was extended to 4 h in this experiment in order to determine appropriate anaerobic time.

As shown in Fig.3,TP concentration slowly increased in the first hour,and then increased rapidly in the next2 h.The phosphorus release mainly occurred in the first 3 h,and the TP concentration in SBBR reached 11.29 mg·L?1from 6.51 mg·L?1with a release quantity of 4.78 mg·L?1.The phosphorus release process continued even after 3.5 h,but the release rate was very slow[11].The TP concentration was 11.42 mg·L?1after4 h.NO3?-N concentration sharply declined during the first hour,indicating that denitrifying bacteria reduced nitrate to N2under hypoxic conditions.After 1.5 h of anaerobic denitrification,NO3?-N concentration was 0.44 mg·L?1.In the early anaerobic stage,higher NO3?-N concentrations inhibited the activity of phosphorus release bacteria,slowing phosphorous release in the first hour.As the nitrate concentration reduced,phosphorus release rate increased rapidly.

Fig.3.Influence of anaerobic reaction time on phosphorus release and denitrification.

3.1.2.Effect of aerobic reaction time on removal of nitrogen and phosphorus

In order to determine a more appropriate aerobic time,concentrations of COD,NH4+-N,TP,and TN in the effluent were determined for 1-8 h in the experiments[12].The DO concentration was kept around 2.8-4.0 mg·L?1[13].As shown in Fig.4,in the aerobic phase,COD concentration decreased rapidly in the first 2 h and reached 303.2 mg·L?1.The COD concentration reached a stable state 5 h later.The NH4+-N concentration reduced gradually and became stable after 6 h(at 41.28 mg·L?1),indicating that the nitrifying bacteria had a good activity.TN concentration had no significant change at the beginning since NH4+-N was converted into nitrate and nitrite.After 4 h,TN concentration reduced significantly,and further reduced to 63.3 mg·L?1after 7 h.NH4+-N concentration gradually reduced at the same time,meaning simultaneous nitrification and denitrification at this stage.The TP concentration decreases rapidly in the early aerobic phase,showing that polyhydroxybutyrate(PHB)stored inside the body was fully oxidized.The TP concentration in the SBBR became stable at 2.49 mg·L?1after 5 h,so the phosphorus removal process was completed.Long time of aeration makes more PHB in the bacteria,which in turn hampers their phosphorus removal ability.Considering various times for different reaction processes in SBBR,the aerobic time of 7 h is recommended.

Fig.4.Influence of aerobic reaction time on nitrogen and phosphorus removal.

3.2.Influence of aeration on the nitrogen and phosphorus removal

In the microbial experiments,aeration has a major effect on the removal of NH4+-N and TN,and a very large or a very small amount of aeration does not favor the removal of TN.In this experiment,the amount of aeration was controlled so that DO concentration in the SBBR gradually increased from1.98 mg·L?1to 7.02 mg·L?1.The quality of raw water at this stage is given in Table 2.

Table 2 Quality of the raw water and water in the reactor influent(mg·L?1)

Fig.5.Influence of different DO concentration on nitrogen and phosphorus removal.

The effluent COD concentration was higher than 200 mg·L?1and the water quality was poor when DO concentration was less than 1.98 mg·L?1.Fig.5 shows that the COD effluent concentrations significantly reduced(to about 100 mg·L?1or so)when DO concentration reached 3.02 mg·L?1.The lack of proper DO concentration reduced the microbial activity,so that the microorganisms could not adequately decompose organic matters in the absence of oxygen.As DO concentration increased,NH4+-N and TN removal efficiencies were improved to certain degrees,and the TN effluent concentration decreased from 88.21 mg·L?1to 44.63 mg·L?1.However,when DO concentration reached 5.06 mg·L?1,the NH4+-N concentration in the effluent remained high(e.g.,effluent concentration of 37.24 mg·L?1at DO concentration of 7.02 mg·L?1),while the TN concentration was low(e.g.,effluent concentration of 51.25 mg·L?1).A thick biological membrane formed in the SBBR where DO presented a certain gradient due to the thickness of the membrane,and a transfer process for DO was needed from the liquid to biological membrane phase.At the beginning of the experiment,DO concentration in the biological membranes is low(＜1.98 mg·L?1),decreasing the activity of aerobic nitrifying bacteria(i.e.,complete nitrification does not take place).Therefore,NH4+-N and TN concentrations in the effluent were high.However,at high DO concentration(e.g.,＞5.06 mg·L?1),the rate of transfer of oxygen to the biofilm is accelerated.A high DO concentration enlarges the aerobic layer and compresses the anoxic layer,so that the degradation speed of organic matters is accelerated.Therefore,both denitrification activities and carbon source are reduced,indicating that the activity of denitrifying bacteria is inhibited and the nitrate nitrogen could not transform to N2[14-16].This results in a gradual increase in the effluent TN concentration.According to the results in this study,efficient removal of TN occurs at DO concentration of about 3.95 mg·L?1.

In this phase of the trial,the effluent TP concentration reduced from 1.06 mg·L?1to 0.88 mg·L?1as DO concentration increased from 1.98 mg·L?1to 7.02 mg·L?1.The effluent TP concentration in the initial stage was expected to increase with DO concentration,but no such change was observed.This is because at that time the phosphorus absorption rate is reduced.It can be concluded that at the same phosphorous release content,an increase in the DO concentration results in greater aerobic phosphorus uptake,and therefore shortens the time for desired aeration.This result is consistent with that of previous research[17].Considering the efficiency of phosphorus removal,phosphorus uptake rate,aeration time,and power consumption,the DO concentration was ensured to be about 3.95 mg·L?1in this experiment while the aerobic reaction time was 7 h.In this case,the effluent concentrations of COD,NH4+-N,TN,and TP were 85.6,35.22,44.64,and 0.92 mg·L?1with the percentage removal rates of 98.2%,95.7%,95.6%,and 96.2%,respectively.

It is found that the removal rates are higher than those in Xiao et al.'s research and the HRT is reduced to 4 days.Larger amount of aeration was carried out in this experiment to keep the DO concentration at 3.95 mg·L?1.This makes simultaneous removal of nitrogen and phosphorus possible.The SBBR technology has a good effect on swine wastewater treatment.

3.3.Effect of COD/TN ratio on the performance

The COD/TN ratio of swine wastewater was 4.4:1 and gradually showed a lack of carbon source as the experiment proceeded.By supplying an external carbon source,the COD/TN ratio could be controlled(varied from 5:1 to 18:1).As shown in Fig.6,the overall COD removal efficiency at different COD loading rates is good,and the effluent is stable below 200 mg·L?1,indicating that the system possesses a good ability to remove organic pollutants and a good antiorganic shock loading capability[organic loading rate(OLR)changed from 1.88 to 6.77 kg·m?3·d?1].This has also been demonstrated in a previous study[18].

Fig.6.Influence of different COD/TN ratio on the nitrogen and phosphorus removal.

According to the results in previous studies,theremoval is mainly concentrated in the aerobic segment.As shown in Fig.6,at firstremoval efficiency is improved,and then decreases as COD/TN ratio increases while the TN concentration overall shows a downward trend.The reason for this is as follows:additional carbon source and high consumption of oxygen due to the decomposition of organic matters inhibit nitrification and result in higher concentration of effluentconcentration[19,20].Therefore,it is necessary to strictly control the influent concentration of organic matters and ensure that nitrobacteria complete the nitrification process.According to Elisabeth,the best COD/TN ratio is 11.2:1[21],and the results in this study confirm the best COD/TN ratio to be 11:1.Theand TN concentrations were 18.5 and 34 mg·L?1,respectively,at this COD/TN ratio.

The results also show that an increase in OLR does not affect TP removal,because the anaerobic stage has sufficient carbon source throughout the experiment and the whole anaerobic process releases phosphorus completely,resulting in higher TP removal.At the COD/TN ratio of 11:1,the TP concentration of water achieved the best value of 1.225 mg·L?1.

3.4.Scanning electron microscopy

Bio film is significantly heterogeneous in structure,metabolism,and activity[22].Its structure includes cell clusters,pores and channels,and extracellular polymeric substances(EPS)[23].Fig.7(B)shows large numbers of cells in the form of biofilms, flocs,and sludge.Fig.7(C)and(D)shows that the surface of the filler has a complex structure resembling the molecules of EPS,which is responsible for that in Fig.7(B).Fig.7(D)shows a large number of filamentous bacteria,cocci,multi-streptococci,and Bacillus inserted in the framework of EPS,forming a dense biological network.

The SEM images of the biofilm also indicate that the adhesion strength of the network is large with a certain elastic force.Thus the activated carbon fiber filler demonstrates good performance in forming biofilm and immobilizing microorganisms.This is helpful in forming an anaerobic-aerobic micro-environment,providing the possibility for removal of nitrogen,phosphorus as well as organic matters.

3.5.Long-term operation experiment

The SBBR was run for 15 consecutive weeks under the following conditions:anaerobic and aerobic residence times of3 h and 7 h,respectively,aerobic DO concentration of 3.95 mg·L?1,and COD/TN ratio of 11:1.The COD,TN,and TP data from this experiment are shown in Fig.8.The effluent COD concentrations were always stable below 200 mg·L?1and effluent TN and TP concentrations maintained stable as well.Stable P removal was achieved,which was absorbed by the biological membrane and excluded from the reactor along with the sludge once a month.Effluent TP concentrations were below 1.5 mg·L?1at the end of aerated period and TN was below 40 mg·L?1.Thus the SBBR could achieve a long-term stability for removal of nitrogen,phosphorus as well as organic matters.

Fig.7.SEM images of activated carbon fiber filler(A)and biofilm formed on the filler(B,C,and D).

Fig.8.Data of long-term operation experiment.

4.Conclusions

In this study,simultaneous removal of nitrogen and phosphorus nutrient pollutants from swine wastewater was studied by using a sequencing batch biofilm reactor(SBBR)in order to explore its industrial applications.The following conclusions are obtained from the experimental data.

In the SBBR for examining factors influencing the process,the replacement rate of wastewater was set at 12.5%,the anaerobic and aerobic residence times were 3 h and 7 h,respectively,and the aerobic DO concentration was 3.95 mg·L?1.The SBBR process demonstrated efficient treatment ability for swine wastewater.The effluent concentrations of COD,,TN,and TP were 85.6,35.22,44.64,and 1.13 mg·L?1with the percentage removal rates of 98.2%,95.7%,95.6%,and 96.2%,respectively.

With an external carbon source added into the SBBR,the removal efficiency forand TN was further improved,and the effluent concentrations achieved the best results at 18.5 and 34 mg·L?1with a COD/TN ratio of 11:1.However,the COD and TP effluent concentrations had no significant change.

On the surface of carbon fiber materials used in SBBR,dense biological membranes form and aid the treatment of swine wastewater.For 15 weeks of operation,the SBBR presents long-term stability for removal of nitrogen,phosphorus and organic matters.Thus this technology is worth popularizing.