Denitrification characteristics of reject water in upflow biofiltration

Process Biochemistry 35 (2000) 1241 – 1245 www.elsevier.com/locate/procbio

Denitrification characteristics of reject water in upflow biofiltration Yongwoo Hwang a,*, Yutaka Yoneyama b, Hiroshi Noguchi c b

a Department of En6ironmental Engineering, Inha Uni6ersity, 253 Yonghyun-Dong, Nam-Gu, Inchon 402 -751, South Korea En6ironmental Research and De6elopment Center, Ebara Corporation, 4 -2 -1 Hon-Fujisawa, Fujisawa-shi, Kanagawa-ken 251, Japan c Bureau of Sewerage, Tokyo Metropolitan Go6ernment, 2 -8 -1 Nishishinjuku, Shinjuku-ku, Tokyo 163 -01, Japan

Received 7 January 2000; accepted 18 March 2000

Abstract Two types of bench-scale experiments using upflow biofilm reactors packed with granular floating polystyrene (GFP) or polyurethane foam cubes (PFC), were used to investigate the denitrification of reject water. A high denitrification rate was achieved in both upflow biofilm reactors since the highly concentrated volatile fatty acids in reject water served as effective hydrogen donors for denitrification. Of the two biofiltrations, the denitrification rate using GFP was 3.5 kg − 1 N m − 3 per day, and higher than that using PFC. The amount of total attached biomass and solid capture capacity were also greater in GFP than in PFC. Moreover, the backwashing of the GFP packed column was optimized with air and water agitation. Of the processes investigated in this study, upflow biofiltration using GFP was the most acceptable process for reject water treatment based on treatability and operation. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: Biofilm; Biofiltration; Denitrification; Reject water; Sludge treatment

1. Introduction Over the last few decades, the volume of municipal wastewater has increased drastically with rapid urbanization in the world [1]. In addition, the quantity of the by-products of wastewater treatment to be treated, sludge, has also significantly increased. In several densely populated cities in Japan, there are few available sites on which to construct new wastewater and sludge treatment facilities and few landfills for treated sludge. In order to upgrade treatment efficiency, especially on sludge treatment, local governments of large cities have planned and constructed central sludge treatment facilities (CSTFs) which treat pipe-collected sludge from several wastewater treatment plants (WWTPs) [2,3]. As a result, however, large volumes of reject water are produced from huge CSTFs, especially from the process of thickening and dewatering. Reject water contains highly concentrated nutrients such as ammonia or phosphorus as well as soluble organic * Corresponding author. Tel.: +82-328-607501; fax: + 82-328634267. E-mail address: [email protected] (Y. Hwang)

substances because bio-decomposition and phosphorus release occur during long pipe-transportation. The pollutant load of the integrated reject water is considered too high to return to a nearby WWTP for further treatment. Hence, the CSTFs require their own highrate on-site reject water treatment system. A number of studies on and applications of biological fixed film reactors for organic and nutrient removal, have been reported [4–7]. In all of these, however, attention was focused on municipal or industrial wastewater. There have been few studies dealing with reject water and its treatment characteristics, in a biofilm system. Teichgrer and Stein reported a successful nitrification/denitrification process for reject water from dewatering of digested or thermal conditioning sludge [8]. Wett et al. also suggested an effective pH control system in a sequencing batch reactor (SBR) for reject water treatment [9]. However, their investigation was limited to the activated sludge system. In this study, one of the primary goals in treating reject water, nitrogen removal performance was investigated with two biofiltration processes using different filter media, and their practical applicability was also determined.

0032-9592/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 2 - 9 5 9 2 ( 0 0 ) 0 0 1 7 1 - 0

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Fig. 1. Scheme of experimental reactors.

2. Materials and methods Two submerged denitrification columns were piloted at a municipal WWTP in Tokyo. A schematic diagram of the experimental reactors is shown in Fig. 1. In order to investigate the difference in the denitrification performance occurring with inert media, two different filter media, granular floating polystyrene (GFP, effective diameter 5 mm), polyurethane foam cube (PFC, size 20× 20× 12.5 mm), were packed in the columns, respectively. Coagulated supernatant following centrifugal thickening of an actual pipetransported mixed sludge (primary sludge+waste activated sludge) beyond 12 km was used as the influent. Sodium nitrate (NaNO3) was injected into the inflows of the experimental reactors as the oxidized nitrogen source for denitrification. The biofiltration columns equipped with four sampling ports consisted of a sealed vertical polyvinyl chloride pipe filled with 1.5 m of media, 150 mm in diameter and 2.5 m in height. The column was operated in an upflow mode. The top of the filter bed was sealed with a gasket cover to separate the filter media and the treated effluent. Backwashing was periodically conducted by employing gravity flow. The procedure for backwashing the filters began with air agitation followed by air and water scouring and finally water rinse. In both filters, the same hydraulic loading condition was applied and adjusted to ensure an equivalent nitrogen load. A flow-loading rate of 2.9 m3 per day, which corresponds to 166 m per day of linear filtration velocity (LV), was selected for design purpose. The actual retention time in the columns calculated by the flow rate and porosity of media, was about 8 min. Total organic carbon (TOC) and volatile fatty acids (VFAs) were analyzed instrumentally using a TOC analyzer (TOC-5000, Shimadzu) and high performance liquid chromatography (OA, Shodex), respectively. Routine analyses for determining suspended solids (SS),

alkalinity, nitrate and biochemical oxygen demand (BOD) were performed according to Wastewater Examination Methods [10].

3. Results and discussion The average quality of influent water during 2 months of operation is shown in Table 1. Since moderate septic conditions for sludge were maintained during underground transportation, the concentrations of pollutants in the received reject water were very high even though a pretreatment was performed with inorganic/ organic flocculants. Changes in water temperature and denitrification rate in both columns during a filtration cycle after steady state are shown in Fig. 2. A steady state was assumed when the concentration of nitrogen compounds in the effluent was relatively constant. During the period of investigation, the water temperature ranged between 10°C and 14°C, and the average concentration of NO3 – N in the prepared influent was 44 mg l − 1. For GFP, a very high volumetric denitrification rate was achieved and stabilized at 3.5 kg N m − 3 per day in spite of backwashing, which was higher than that for PFC. With the PFC column, an average denitrification rate of 2.0 kg N m − 3 per day was achieved with a nitrogen Table 1 Influent water quality Water temperature (°C) pH

7.0

Alkalinity (CaCO3 mg l−1) SS (mg l−1) NH4 N (mg l−1) S-BODa (mg l−1) S-TOCa (mg l−1) a

15.0

Soluble values filtered by 0.8 mm membrane filter.

240 180 65 350 200

Y. Hwang et al. / Process Biochemistry 35 (2000) 1241–1245

Fig. 2. Changes in water temperature and denitrification rate during a filtration cycle.

Fig. 3. PFC filter depth change before and after backwashing. Table 2 VFAs of reject water measured in winter VFAs

Concentration (mg l−1)

TOC equivalent (mg l−1)

Acetic acid Propionic acid n-Butyric acid n-Valeric acid i-Valeric acid Total Soluble TOC in reject water (mg l−1) VFAs TOC/soluble TOC (%)

132.1 86.6 24.8 18.6 25.7 287.8 195.8

52.9 42.1 13.5 10.9 15.0 134.4

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of VFAs to total soluble organic matter was very high, approximately 70% as carbon equivalent. As mentioned previously, the VFA concentration, even in winter, was due to the hydrolysis-acidification of organic sludge in the buried pipeline. Measurements of VFA throughout a year showed few seasonal variations in concentration. It is well known that VFA are very effective external hydrogen donors for biological denitrification [11]. The high denitrification performance in GFP filtration was, therefore, probably due to media selection and also the contribution of the concentrated VFA of reject water. The SS removal performance and head loss change in each column during the same filtration cycle mentioned above are shown in Fig. 4. The concentrations of SS in the influent and effluent were determined for every composite samples stored for 2 h until the columns reached a head loss of approximately 2000 mmAq. For GFP, head loss increased almost linearly with time elapse and reached 2100 mmAq after 26 h. The SS concentrations in the influent and effluent were 120 and 100 mg − 1, respectively, and 17% of removal was obtained on average. In the case of PFC, the head loss was increased at the final stage of filtration although 42 h of an extended treatment was possible before head loss reached to 2050 mmAq. This phenomenon was probably due to the clogging caused by PFC compression. As shown in Fig. 3, PFC buffered the in-pressure of the column by the compression of itself to some extent. However, the head loss increased drastically over the breakpoint. Accordingly, when the upflow filtration equipment using PFC is actually designed for reject water denitrification treatment, it would be very difficult to determine the effective filtration period and the backwashing cycle.

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load of 4.9 kg N m − 3 per day. The denitrification rate decreased gradually during filtration (Fig. 3). This can be explained by the reason that, as the head loss increases, PFCs are compressed, causing a shorter hydraulic retention time (HRT) in the biofilter and a smaller effective contact area between liquid and biofilm. The measured concentrations of VFAs present in the used reject water are shown in Table 2. The proportion

Fig. 4. Changes in head loss and SS concentration in each biofiltration.

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Fig. 5. Material balances of two types of denitrification columns during a filtration cycle. Table 3 Yield coefficients acquired from the denitrification of reject water Produced biomass (VSS)/reduced NO3 N (kg kg−1) Consumed S-BOD/reduced NO3 N (kg kg−1) Produced biomass (VSS)/consumed S-BOD (kg kg−1)

1.04 2.26 0.46

Fig. 6. Proportion of attached and produced biomass in denitrification columns.

The material balance of nitrogen, SS, and soluble BOD (S-BOD) for the two columns obtained from the experiments described above is shown in Fig. 5. All values in the figure were calculated on the basis of unit packed volume (m3) of the reactors. The total SS capture capacities determined from backwashed drainage were 5.0 kg m − 3 in PFC and 6.5 kg m − 3 in

GFP. Of these, net amounts of filtered SS which were calculated from the experimental data shown in Fig. 4 were only 1.5 kg m − 3 in PFC and 2.3 kg m − 3 in GFP. Therefore, most of the captured SS in the columns was produced by microbial growth due to the oxidation of organic substances during denitrification. The net yield coefficients obtained from the above denitrification experiment are shown in Table 3. When the organic materials contained in reject water were used as the hydrogen donors, the biomass production by nitrate removal was 1 kg-VSS kg − 1 N. The S-BOD consumption during denitrification was approximately 2.3-fold of the amount of removed nitrate, and the conversion ratio of S-BOD to VSS was approximately 0.5 kgVSS kg − 1 S BOD. From the additional activated sludge experiment with the same reject water, a denitrification rate of 11.7 mg N g − 1 SS per h was obtained at 20°C. On the assumption that the existing microorganisms in the biofiltration columns have denitrification rates equal to activated sludge, the following can be calculated. The amount of biomass attached on the media in filtration columns calculated from the denitrification rate (PFC, 2.2 kg N m − 3 per day; GFP, 3.5 kg N m − 3 per day) after backwashing, and the amount of produced VSS by the denitrification reaction are shown in Fig. 6. In both cases, the amount of biomass attached and present in the filtration columns after backwashing (PFC, 7.9 kg m − 3; GFP, 12.5 kg m − 3) are more than twice as much as the amount of produced VSS by denitrification (PFC, 3.5 kg m − 3; GFP, 4.1 kg m − 3). This indicates that a high rate of denitrification could be maintained in the columns even after backwashing. Moreover, it seems that the VSS produced by the denitrification reaction contributes to the enhancement of the rate of denitrification. Actually, however, the rate of denitrification is being stabilized as shown in Fig. 2. Presumably, the contribution was diminished by the nitrogen gas trapped between the medium, or by shortcircuiting resulted from partial clogging. Based on these observations, comparing the two media, GFP is a more effective filtration media than PFC for reject water denitrification, when considering backwashing control as well as denitrification rate. The relationship between nitrogen load and denitrification rate in these two reactors investigated under the same water temperature is shown in Fig. 7. The biofilm reactor using GFP as a filtration media showed a higher volumetric denitrification rate than PFC. In the case of applying PFC in an actual treatment plant, it is difficult to set up an operation cycle because the head loss increases quite suddenly due to the compression of PFC. Moreover, the denitrification rate also decreases due to bed depth changes in the PFC. Thus, PFC cannot be applied as a suitable material for fixedbed denitrification if there exists no agitation device

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temperature. The amount of total attached biomass and solid capture capacity which decide the biotreatment rate were also greater in the GFP than in the PFC. In addition, the backwashing of the GFP packed column was very easily conducted with air and water agitation. In conclusion, of the processes investigated in this study, the upflow biofilm reactor using GFP was the most acceptable process for reject water denitrification treatment based on treatability and operation.

References Fig. 7. Nitrogen load and denitrification rate in two types of denitrification reactors.

inside the reactor. In view of the above, GFP filtration was considered the most acceptable denitrification process for reject water based on treatability and operation.

4. Conclusions In this study, two types of upflow biofilm reactors packed with PFC and GFP, were analyzed to investigate the denitrification performance for reject water treatment. The results showed that a very high denitrification rate was achieved in both types of upflow biofilm reactors because the high concentration of VFAs, mainly acetic acid and propionic acid contained in the reject water provided the effective hydrogen donors for denitrification. Of the two types of biofiltration, a high denitrification rate of 3.5 kg N m − 3 per day was achieved by using GFP even below 15°C of water

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