WO2003093180A1

WO2003093180A1 - Biological denitrification apparatus and method using fluidized-bed reactor filled with elemental sulfur

Info

Publication number: WO2003093180A1
Application number: PCT/KR2002/000822
Authority: WO
Inventors: Jae-Ho Bae; Kyu-Hong Ahn; Seock-Heon Lee
Original assignee: Korea Institute Of Science And Technology
Priority date: 2002-05-02
Filing date: 2002-05-02
Publication date: 2003-11-13
Also published as: AU2002306099A1

Abstract

The present invention relates to a biological denitrification apparatus and a denitrification method using a fluidized-bed reactor filled with elemental sulfur. The apparatus comprises: a reactor filled with elemental sulfur and limestone therein; an inlet port at a lower portion of the reactor for introducing the wastewater to flow from the lower portion to an upper portion of the reactor; a treated water sedimentation basin over the reactor for collecting the introduced water after passing through the reactor; a treated water recycle line between the treated water sedimentation basin and the inlet port of the reactor for recycling the treated water from the treated water sedimentation basin to the inlet port of the reactor; a pump mounted to an end of the treated water recycle line for increasing the flow rate of the recycled water; and a T-shaped precipitator at one end of the recycle line for precipitating suspended solids such as microbes contained in the treated water recycled to the reactor, in which the treated water sedimentation basin has a diameter at least twice that of the reactor in order to function as a sedimentation basin.

Description

BIOLOGICAL DENITRIFICATION APPARATUS AND METHOD USING FLUIDIZED-BED REACTOR FILLED WITH ELEMENTAL SULFUR

Technical Field The present invention relates to a biological denitrification apparatus and a method using a fluidized-bed reactor filled with elemental sulfur.

Background Art

As well known to those skilled in the art, biological denitrification for removing nitrate nitrogen or nitrite nitrogen contained in the wastewater has widely utilized heterotrophic denitrification which uses organic substances as electron donors. For heterotrophic denitrification, reduced nitrogen such as ammonia contained in the wastewater first be oxidized to nitrate or nitrite by nitrifying bacteria, and then the oxidized nitrogen are reduced into nitrogen gas by heterotrophic denitrifying bacteria. A biological denitrification apparatus based upon the above principle is so constructed that the introduced wastewater generally passes through a nitrification reactor after passing through a denitrification reactor by the following reason: Nitrification is inhibited by the presence of the organic substances, whereas heterotrophic denitrification require organic substances as the electron donors. Therefore, the denitrification reactor is arranged upstream of the nitrification reactor in order to effectively use the organic substances contained in the wastewater and to prevent the organic substances from inhibiting nitrification. A recycle line is added to return a part of the nitrification reactor effluent into the denitrification reactor so as to construct a biological nitrogen removal process. A typical biological denitrification process is anoxic/aerobic (A/O) process, and various alternative methods have been developed based upon the A O process. Total nitrogen removal efficiency of the above nitrification/denitrification processes depends on the recycle ratio (the ratio of recycle flow rate to influent flow rate). Therefore, to achieve high total nitrogen removal efficiency, the recycle ratio should be kept high, which requires a high pumping cost. Furthermore, it is impossible to achieve complete total nitrogen removal no matter how high the recycle ratio is increased. When influent total nitrogen concentration is very high, it become impossible to meet the discharge standard for total nitrogen with nitrification/denitrification processes. In that case, an additional post-denitriflcation process is required to remove total nitrogen to a desired level. The heterotrophic denitrification adopted as the post-denitrifϊcation process produces a larger amount of sludge than the autotrophic denitrification thereby increasing the treatment cost. Further, it is necessary to add external carbon sources such as methanol since the nitrified wastewater does not contain any organic substances. The addition of external carbon sources to heterotrophic post-denitrifϊcation process would yield following problems as the concentration of nitrate nitrogen changes during the treatment. When the influent nitrate nitrogen concentration is low compared to the carbon sources added, some of the added external carbon sources reside thereby requiring an additional process for removing the same. On the opposite case, the external carbon sources became insufficient thereby degrading the denitrification efficiency. Various processes have been developed in order to solve the problems associated with the heterotrophic post-denitrification, and examples thereof include autotrophic denitrification using sulfur (sulfur-utilizing denitrification). In sulfur-utilizing denitrification, sulfur granules are filled in the reactor, which is used as electron donors, and provide surface area for bacterial growth. As autotrophic denitrification, sulfur-utilizing denitrification produce less sludge than heterotrophic denitrification. Also, no excess electron donor remains in the effluent as the dissolution of sulfur is limited to provide the required amount of sulfur to achieve 100% denitrification efficiency regardless of the variations in nitrate concentration of the wastewater because of very low solubility of sulfur. Further, sulfur is cost effective electron donor than organic substances used for heterotrophic denitrification.

The sulfur-utilizing denitrification has been applied using a sulfur packed-bed reactor. The sulfur packed-bed reactor is filled with sulfur granules, in which the wastewater is introduced to flow upward or downward through an elemental sulfur layer. The elemental sulfur layer is stationary since the specific gravity of sulfur filled into the reactor is greater. Since the sulfur layer in the packed-bed reactor has a low porosity, growth of microbial films on the surface of the sulfur granules and residual nitrogen gas, which is a product of the denitrification, in pores create clogging problem as the denitrification progresses. Such clogging not only restricts the maximum flow rate passing through the reactor but also decreases the transfer of nitrate nitrogen from the bulk liquid to the microbes thereby to decrease denitrification efficiency. In order to avoid operational problems and the deterioration of treatment efficiency due to such clogging, it is necessary to practice regular backwashing with water or air so as to remove excess microbial films and the entrapped nitrogen gas in the pores. However, the backwashing not only reduces the reactor operating time decreasing treatment capacity but also requires supplements such as a water storage bath, a backwashing pump, rejected water treatment facility. Also, if the intensity of water or air for backwashing is not properly adjusted, excess washout of microbes attached on the sulfur granules may result, which greatly decrease the denitrification efficiency and take long time to recover a steady-state. Furthermore, the decrease in mass transfer efficiency of nitrate nitrogen from bulk liquid to microbes is inevitable with sulfur packed-bed reactor as the accumulation of nitrogen gas begins just after the backwashing.

Disclosure of the Invention Accordingly the present invention has been made to solve the above problems and it is an object to provide a biological denitrification apparatus using an elemental sulfur, which does not need backwashing, has a high denitrification efficiency at higher loading rate by increasing the mass transfer rate of nitrate nitrogen from the bulk liquid to microbes, removes the necessity of additional facilities associated with backwashing, and permits easy way of providing alkalinity.

To accomplish the above object and other advantages, a biological denitrification apparatus using elemental sulfur is provided. The apparatus comprises: a reactor filled with the elemental sulfur granules (elemental sulfur) and limestones therein; an inlet port at a lower portion of the reactor for introducing the wastewater to flow from the lower portion to an upper portion of the reactor; a treated water sedimentation basin over the reactor for collecting the introduced water after passing through the reactor; a treated water recycle line between the treated water sedimentation basin and the inlet port of the reactor for returning the treated water from the treated water sedimentation basin to the inlet port of the reactor; a pump mounted to an end of the treated water recycle line for increasing the flow rate of the recycled water; and a T-shaped precipitator at one end of the recycle line for precipitating suspended solids such as microbes contained in the treated water recycled to the reactor, in which the treated water sedimentation basin has a diameter at least twice that of the reactor in order to function as a sedimentation basin. In the above construction, the inlet port is installed at the lower portion of the reactor to fluidize elemental sulfur filled within the reactor with high upward velocity of recirculated fluid to prevent clogging as microbe films are excessively formed on the surfaces of sulfur granules or nitrogen gas stacks in pores of the sulfur granules, thereby enhancing denitrification efficiency and applicable loading rate. The treated water sedimentation basin over the reactor is provided with a relatively larger diameter compared to that of the reactor to decelerate the upward flow velocity of the wastewater moving from the lower portion of the reactor to the upper portion thereof thereby functioning for sedimentation. The treated water sedimentation basin functions to remove suspended solids together with the small-sized T-shaped precipitator provided at one end of the recycle line so that an additional sedimentation basin is not necessary. Further, limestones are filled with elemental sulfur within the reactor to function as an alkalinity-providing substance to provide alkalinity necessary for sulfur-utilizing denitrification. Generally, limestones sink to the lower portion of the reactor due to greater specific gravity compared to that of elemental sulfur. As the invention employs high recycle ratio, alkalinity supply from limestones are stable and maintaining the desired sulfur to limestone ratio is easy as those two granular materials are separated by the difference in specific gravity.

Brief Description of the Drawings The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

Fig. 1 illustrates a biological denitrification apparatus of the wastewater according to one embodiment of the invention; Fig. 2 is a graph comparatively illustrating the relationship between the loading rate and the denitrification efficiency of a sulfur fluidized-bed reactor of the invention and a conventional sulfur packed-bed reactors having no fluidization of sulfur in treatment of the wastewater containing high-concentration nitrate nitrogen (700 mg NO₃ ^"-N/L); Fig. 3 is a graph comparatively illustrating the relationship between the empty bed contact time (EBCT) of the reactor and the denitrification efficiency of a sulfur fluidized-bed reactor of the invention and conventional sulfur packed-bed reactors for the treatment of the wastewater containing high-concentration nitrate nitrogen (700 mg NO₃ ^"-N/L); Fig. 4 is a graph comparatively illustrating the relationship between the loading rate of nitrate nitrogen and the denitrification efficiency of a sulfur fluidized-bed reactor of the invention and conventional sulfur packed-bed reactors for the treatment of the wastewater containing low-concentration nitrate nitrogen (20 mg NO₃ ^"-N/L);

Fig. 5 is a graph comparatively illustrating the relationship between the EBCTs of the wastewater and the denitrification efficiency of a sulfur fluidized-bed reactor of the invention and a conventional sulfur packed-bed reactor for the treatment of the wastewater containing low-concentration nitrate nitrogen (20 mg NO₃ ^"-N/L); and

Fig. 6 is a graph comparatively illustrating the N₂O composition of the produced gases at various loading rates of nitrate nitrogen in sulfur fluidized-bed reactors of the invention and a conventional sulfur packed-bed reactor.

Best Mode for Carrying Out the Invention

The following detailed description will present a preferred embodiment of the invention in reference to the accompanying drawings, but it shall be understood that the invention is not restricted to the embodiment thereof. Fig. 1 illustrates a biological denitrification apparatus for denitrification of nitrate nitrogen and nitrite nitrogen in the wastewater into nitrogen gas. The biological denitrification apparatus comprises a biological denitrification reactor 10 having a wastewater inlet port 13 containing nitrate nitrogen at the bottom and filled with an elemental sulfur column 11 and limestone column 12 functioning as an agent for providing alkalinity therein, a treated water sedimentation basin 20 placed over the reactor 10 for separating suspended solids from the wastewater which passed through the reactor 10, a recycle line 30 and a treated water outlet 14, recycle line 30 for recycling the treated water collected in the sedimentation basin 20 to the inlet port 13, a pump 40 installed at one end of the recycle line 30 for increasing the flow rate of the recycled water flowing through the recycle line 30 to fluidize the elemental sulfur column 11 and the limestone column 12 functioning as the alkalinity-providing agent within the reactor 10, a T-shaped precipitator 50 mounted at one end of the recycle line 30 for precipitating the suspended solids including microbes in the recycled water and a discharge valve 130 for discharging the suspended solids stacked in the T-shaped precipitator 50.

Describing in more detail, the wastewater introduced through a wastewater-introducing pipe 100 flows into the inlet port 13 of the reactor 10, joined with the recycled water which is carried to an entrance of the reactor 10 via the recycle line 30.

The influent and recycled water introduced into the inlet port as above to fluidize sulfur granules and limestones filled in the reactor 10. This maintains pore volume of sulfur layer large and remove biofilms excessively grown on the sulfur granules with the shearing force of fluid thereby to prevent clogging caused by nitrogen gas accumulation in the pores, excessive microbial growth on sulfur granules, and the diameter reduction of the sulfur granules as the denitrification progresses. Further, as the clogging is prevented, the mass transfer rate of nitrate nitrogen from bulk liquid to the microbes can be significantly enhanced so as to increase the removal rate per unit volume of the reactor. The upward flow rate for fluidization in the reactor is mainly determined by the flow rate of the recycled water, and sufficient if the expansion ratio of a sulfur layer (i.e. the height ratio of the sulfur layer before and after fluidization) is at 30 % or less. The recycle flow rate can be adjusted to the minimum flow rate capable of preventing the clogging. As presented in the invention, the fluidization of sulfur granules can eliminate regular backwashing and supplements accompanying to the same, and maximize the treatment capacity. Nitrate nitrogen contained in the wastewater is removed as denitrified in the form of nitrogen gas by the microbes growing on the surface of sulfur granules according to Equation 1 : NO₃ + 1.1S + 0.76H₂O + 0.4CO₂ + 0.08NH₃ = 0.08C₅H₇O₂N + 0.5N₂ + 1.1 SO₄ ^2" + 1.2H⁺ ... Equation 1. The microbes (hereinafter will be referred to sulfur-utilizing bacteria) mediating in Equation 1 are autotrophic bacteria using sulfur as electron donor and nitrate nitrogen as an electron acceptor, and widely exist in the natural habitats, such as tidal flats. Therefore, the sulfur-utilizing bacteria can be easily obtained since they are collected in the tidal flats and are cultured with reduced sulfur. Since the sulfur-utilizing bacteria use sulfur as a substrate, the size of sulfur granules gradually decrease as denitrification proceeds. Nitrate nitrogen functioning as the electron acceptor is released into the atmosphere as nitrogen gas while oxidating sulfur into sulfate.

In order to attach the sulfur-utilizing bacteria to sulfur granules, the sulfur-utilizing bacteria are simply mixed with elemental sulfur in a fluidized bed, and then the bacteria are cultured with nitrate nitrogen contained in wastewater. For the purpose of rapid enrichment of sulfur-utilizing bacteria, sodium thiosulfate instead of sulfur granules can be used.

Typical sulfur-utilizing bacteria includes Thiobacillus denitrificans and Thiomicrospira denitrificans. The sulfur-utilizing bacteria are referred in various documents on autotrophic denitrification such as "Sulfur: Limestones Autotrophic Denitrification Processes for Treatment of Nitratecontaminated Water: Batch Experiments", published from Pergamon Press Wat. Res. Vol. 33, pp. 599 to 609, 1999. This document shows that the sulfur-utilizing bacteria widely exist in natural soils, sediments, tidal flats, and the like. Therefore, as described above, the sulfur-utilizing bacteria can be collected in the tidal flats, easily obtained with sulfur, and cultured.

As shown in Equation 1, since the sulfur-utilizing denitrification is a reaction which consumes alkalinity unlike the heterotrophic denitrification, it is necessary to economically provide the alkalinity if the alkalinity of the wastewater is insufficient. As one of methods for solving this, sulfur and limestones can be mixed in the reactor. Limestones dissolve in water to produce Ca²⁺ and CO₃ ^2" (carbonate), in which carbonate produced functions as the alkalinity. Since limestone has a specific gravity greater than sulfur, sulfur granules are fluidized in the upper layer and limestones are placed at the bottom layer, in which limestone are not necessarily fluidized. The upward flow velocity in the reactor for fluidization is sufficient when the expansion ratio of the sulfur layer is about 30 %.

The alkalinity-providing agent or limestone column 12 and the elemental sulfur column 11 are primarily filled into the reactor 10 by mixedly filling sulfur granules and limestones into the same. When limestones are applied as the alkalinity-providing agent, as shown in Fig. 1, limestones having the greater specific gravity moves to the bottom and sulfur granules having smaller specific gravity compared to limestones moves to the top of the limestone layer thereby automatically making their own layers as time lapses. When limestones are applied as the alkalinity-providing agent, sulfur and limestones should be refilled into the reactor after they are consumed by a predetermined amount. In regard of refill, the sulfur fluidized-bed reactor is more advantageous than a sulfur packed-bed reactor. The ratio of sulfur to limestone consumption in the sulfur packed-bed reactor depends on the ratio of alkalinity to nitrate nitrogen of the influent wastewater. Therefore, the ratio of sulfur to limestone changes from the original filling condition as denitrification proceeds, leaving sulfur or limestone as major component of the filling. In the sulfur packed-bed reactor, mixing of sulfur granules and limestones is practically hard to achieve. As a result, denitrification efficiency decreases with the following reasons: The alkalinity is insufficient if sulfur dominantly resides. Otherwise, the microbes attached to sulfur granules become insufficient if limestones dominantly resides.

In the sulfur fluidized-bed reactor, however, the newly refilled limestones move to the bottom of the reactor due to the difference in specific gravity between sulfur granules and limestones, making it easy to maintain the desired ratio of sulfur to limestone. In addition, when alkalinity is insufficient, the dissolution rate of limestone in the fluidized-bed reactor will become much higher that that in the packed-bed reactor as bulk liquid efficiently contact with limestone in the fluidized-bed reactor. Therefore, alkalinity can be easily maintained at the optimum conditions in the fluidized-bed reactor than in the packed-bed reactor.

Further, for the purpose of stable supply of alkalinity, an additional column (not shown) filled with the alkalinity-providing agent can be installed in any positions of the recycle line. The water introduced and treated as above passes through the alkalinity-providing agent column 12 and the elemental sulfur column 11 filled within the reactor 10, and is collected in the treated water sedimentation basin 20 provided over the reactor 10 and functioning as a collector. The sedimentation basin 20 placed over the reactor 10 has a diameter enlarged compared to a reactor portion for fluidizing sulfur filled therein to function as the sedimentation basin. Since the flow velocity decreases as the diameter is enlarged, the microbes washed out from the sulfur granules precipitate in the sedimentation basin over the reactor and thus the precipitated microbes are preferably removed. The wastewater collected in the treated water sedimentation basin 20 is carried to the inlet port 13 of the reactor 10 via the inside of the recycle line 30 connecting between the sedimentation basin 20 and the inlet port 13 of the reactor 10. A portion of the treated water is flown out of the reactor instead of returning to the inlet port 13, in which the amount of the discharged water is identical with the amount of the influent wastewater. The pump 40 is mounted in the middle of the recycle line 30 to supply adequate recycle flow rate returning to the inlet port 13 of the reactor 10 so as to allow fluidization of the elemental sulfur column 11 filled within the reactor 10. Further, the T-shaped precipitator 50 is mounted at the middle of the recycle line 30 to precipitate residual organic substances or sludge contained in the recycled water returning to the inlet 13 of the reactor 10. As set forth above, an additional alkalinity-providing agent column (not shown) may be attached to a middle portion of the recycle line to promote stable supply of the alkalinity.

In order to express the effect of the invention, treatment results are compared according to the following operation conditions between the sulfur fluidized-bed reactors presented in the invention and conventional sulfur packed-bed reactors with no fluidization of sulfur.

The expansion ratio of the sulfur layer was maintained for 25-30%) in the fluidized-bed, and the upward flow velocity was maintained for 2.97 cm/sec in the reactor by using the return to obtain the expansion ratio. The experiments were performed with two different influent concentrations of nitrate nitrogen of 700 and 20 mg NO₃ ^"-N/L (referred to as high-concentration and low concentration, respectively). The high-concentration influent was leachate from a landfill pretreated with nitrification and denitrification process, in which nitrate nitrogen was added to the leachate to maintain the concentration of nitrate nitrogen the same. The low-concentration influent was made by mixing the sewage with the artificial wastewater.

Table 1 and Figs. 2 and 3 comparatively illustrate experimental results obtained from a sulfur fluidized-reactor of the invention and conventional sulfur packed-bed reactors with high-concentration influent. As shown in Fig. 2, the denitrification efficiency of the sulfur fluidized-bed reactor was greater than 98 %> up to the loading rate of 2.68 kg NO₃ ^"-N/m -day per unit volume of the reactor, in which the empty bed contact time (EBCT) was 4.06 hr. The denitrification efficiency at the loading rate of 4.06 kg NO₃ ^"-N/m -day was 83%> suggesting denitrification efficiency did not decreased significantly at the high loading rate. In the meantime, denitrification efficiency of the sulfur packed-bed reactor 1 decreased significantly at the loading rate of 2.68 kg N0₃ ^"-N/m³-day or less. The sulfur packed-bed reactor 2 showed a similar result; denitrification efficiency decreased lower than 80 % at the the loading rate of 2 kg NO₃ ^"-N/m -day. The maximum removal rate per unit volume of the sulfur packed-bed reactor was about 2 kg NO₃ ^"-N/m³-day, whereas that of the sulfur fluidized-bed reactor was greater than 3 kg NO₃ ^"-N/m³-day. Further, denitrification efficiency of the sulfur packed-bed reactor decreased more significantly than that of the sulfur fluidized-bed reactor at high loading rates. As shown in Fig. 3, the sulfur fluidized-bed reactor requires shorter EBCT than the sulfur packed-bed reactors to achieve greater than 95%> of denitrification efficiency.

Table 1. Operational Conditions and Results of a Sulfur Fluidized-Bed Reactor Fed with High-Concentration Nitrate Nitrogen.

Table 2 summarizes the operational conditions and results of the fluidized-bed reactor fed with low-concentration influent (20 mg/L nitrate nitrogen).

Table 2. Operational Conditions and Results of a Sulfur Fluidized-Bed Reactor Fed with Low-Concentration Nitrate Nitrogen.

Fig. 4 illustrates the relationship between the nitrate nitrogen loading rate and the denitrification rate of sulfur packed-bed reactors and a sulfur fluidized-bed reactor fed with wastewater containing low-concentration nitrate nitrogen. With sulfur fluidized-bed reactor, denitrification efficiency was greater than 97% at the influent flow rate up to 192 L/day, or loading rate of 2.5 kg NO₃ ^"-N/m -day. The corresponding EBCT was 0.19 hr. On the other hand, denitrification efficiency of sulfur packed-bed reactors decreased significantly when the loading rate increased higher than 0.5 kg NO₃ ^~-N/m³-day.

For low-concentration influent, the minimum EBCT necessary for greater than 95%) denitrification efficiency for the sulfur packed-bed reactors was generally 1.1 to 1.9 hr as shown in Fig. 5. However, the minimum EBCT can be reduced to 0.19 hr with the sulfur fluidized-bed reactor. This result indicates that the sulfur fluidized-bed reactor can treat wastewater with 1/10 of EBCT for the sulfur packed-bed reactors. Such reduction in the EBCT means that the same treatment efficiency can be obtained even if the volume of the reactor is reduced to 1/10. Therefore, the sulfur fluidized-bed reactor requires a small-sized reactor thereby minimizing the area required for the treatment equipment. Therefore, the sulfur fluidized-bed reactor can be advantageously installed in an existing treatment plant as a supplementary process.

Fig. 6 comparatively illustrates N₂O composition of the produced gases at various nitrate nitrogen loading rates in sulfur fluidized-bed reactors and a sulfur packed-bed reactor. It is known that N₂O is produced when the loading rate of nitrate nitrogen is high, and the discharge thereof is restricted as far as possible since N₂O is one of strong greenhouse gases. At the same loading rate, the sulfur fluidized-bed reactor produced less N₂O than the sulfur packed-bed reactor. For example, N₂O composition of the sulfur packed-bed reactor was 21 to 25%>, whereas that of the sulfur fluidized-bed reactor was 0.01 to 6% N₂O even though it is operated at higher loading rates. Therefore, the sulfur fluidized-bed reactor has a low potential to produce N₂O compared to the sulfur packed-bed reactor, which is related to the higher denitrification efficiency of the sulfur fluidized-bed reactor.

The sulfur-utilizing bacteria require the alkalinity for denitrification. Limestone is cheap and thus functions as an economical alkalinity-providing source. In the invention, limestones and sulfur granules are filled with the volume ratio of 1 :6 into the sulfur fluidized-bed reactor to provide the required alkalinity. With low concentration influent (20 mg NO₃ ^"-N/L), the alkalinity was efficiently provided from limestone through the entire reactor operation. Even if the conventional sulfur packed-bed reactor also discloses a method of providing the alkalinity from limestone, the ratio of sulfur and limestone consumption during the treatment change according to the variation of the ratio of alkalinity to nitrate nitrogen of the introduced water. Therefore, if the ratio of limestone to sulfur is not maintained correctly, either sulfur or limestone excessively reside in conventional packed-bed reactor layers. For the former case, alkalinity cannot be supplied efficiently, and for the latter case, the amount of microbes in the reactor is decreased. As both cases result in the decrease in denitrification rate or denitrification efficiency, it is desirable to maintain the optimum sulfur to limestone ratio. However, to maintain the optimum sulfur to limestone ratio in the sulfur packed-bed is not easy as the mixing of those two substances is not easy to implement. On the contrary, as limestones are always placed at the bottom of the sulfur fluidized-bed reactor due to the difference in specific gravities between sulfur and limestones, it is easy to maintain the optimum sulfur to limestone ratio. Further, the alkalinity-providing ability in the fluidized-bed reactor is enhanced as efficient dissolution of limestone can be achieved because of clogging-free operation and high recycle ratio. Therefore, the use of limestone as the alkalinity-providing source is more efficient and stable in the sulfur fluidized-bed reactor than in the sulfur packed-bed reactor. The invention provides a method of operating a fluidized-bed reactor filled with elemental sulfur to remove nitrate nitrogen contained in the wastewater. The sulfur fluidized-bed reactor of the invention is adopted to prevent the clogging which frequently takes place in the sulfur packed-bed reactor and to increase the mass transfer rate between the microbes growing on the surfaces of the sulfur and nitrate nitrogen contained in the bulk liquid thereby increasing the treatment capacity significantly. This reduces the size of the sulfur fluidized-bed reactor compared the sulfur packed-bed reactor thereby greatly reducing the installation cost thereof and the area for the reactor. When limestone is used in order to provide the alkalinity necessary for sulfur-utilizing denitrification, sulfur fluidized-bed reactor provides more efficient and stable alkalinity supply than the sulfur packed-bed reactor. Further, the production of N₂O, a greenhouse gas, can be greatly reduced with sulfur fluidized-bed reactor than with the sulfur packed-bed reactor. The sulfur fluidized-bed reactor can also reduce the treatment cost by utilizing sulfur as electron donor, instead of relatively expensive external carbon source, reducing sludge production and cost related to the treatment of the produced sludge. Although the preferred embodiment of the present invention has been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions can be made without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

Claims

Claims:

1. A biological denitrification apparatus with a fluidized-bed reactor filled with elemental sulfur, comprising: a biological denitrification reactor filled with elemental sulfur granules and having a wastewater inlet port at a lower portion; a treated water sedimentation basin provided over said reactor; a treated water recycle line connecting said treated water sedimentation basin and said inlet port of the reactor; and a pump provided at one end of said treated water recycle line.

2. The apparatus as claimed in claim 1, wherein said treated water sedimentation basin has a diameter at least twice that of said reactor.

3. The apparatus as claimed in claim 1, further comprising a T-shaped precipitator provided at one end of said recycle line.

4. The apparatus as claimed in claim 1 , wherein an alkalinity-providing agent is mixedly filled within said reactor together with the elemental sulfur.

5. The apparatus as claimed in claim 1, further comprising an additional column provided at one end of said recycle line and filled with an alkalinity-providing agent.

6. The apparatus as claimed in claim 4, further comprising an additional column provided at one end of said recycle line and filled with an alkalinity-providing agent.

7. A biological denitrification method using a fluidized-bed reactor filled with elemental sulfur, wherein the fluidized-bed reactor is operated to apply elemental sulfur granules as media for microbial growth and a main electron donor substrate to reduce nitrate nitrogen with autotrophic denitrifying bacteria attached to sulfur surface, whereby nitrate nitrogen contained in the wastewater is removed.

8. The method as claimed in claim 7, wherein an alkalinity-providing agent is filled at the same time into the fluidized-bed reactor to provide alkalinity necessary for the denitrifying bacteria using sulfur.

9. The method as claimed in claim 7 or 8, wherein an alkalinity-providing agent is filled into an additional alkalinity-providing agent column to provide the alkalinity.

10. The method as claimed in claim 9, wherein the alkalinity-providing agent is limestone to provide the alkalinity from dissolution of limestone.