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Dissolved oxygen, COD, nitrogen and phosphorus profiles in a continuous sand filter used for WWTP effluent reclamation. Hongbin Xu, Sigrid M. Scherrenberg ...
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© IWA Publishing 2012 Water Science & Technology

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Dissolved oxygen, COD, nitrogen and phosphorus profiles in a continuous sand filter used for WWTP effluent reclamation Hongbin Xu, Sigrid M. Scherrenberg and Jules B. van Lier

ABSTRACT Continuous sand filtration (CSF) offers interesting potential for the extensive treatment of wastewater treatment plant (WWTP) effluents for water reclamation and/or restrictive discharge. Research on concentration profiles over the height of the CSF shows that most bacteriological conversions are restricted to the lower part of the filter bed. Dissolved oxygen (DO) rapidly decreases to below 1 mg/L in the first 0.4 m of the filter bed, applying hydraulic velocities of 12.9 ∼ 14.9 m/h and 10 ∼ 20 mm/min sand velocities, independent of the methanol dosage. The DO decrease agrees with the observed decrease in chemical oxygen demand (COD). At the given operational conditions, NOx-N and N-total removal is dedicated to the first 0.9 m of the filter bed. Results show that by optimising the CSF operational conditions the very restrictive effluent N and P values of 2.2 and 0.15 mg/L, respectively, as described in the European Water Framework Directive, can be met. Key words

| COD, continuous sand filter, denitrification, DO, NH4-N reduction, phosphorus removal,

Hongbin Xu Department of Environmental and Municipal Engineering, Zhengzhou University, 100 Science Road, 450001 Zhengzhou, China Sigrid M. Scherrenberg (corresponding author) Jules B. van Lier Section Sanitary Engineering, Department of Water Management, Faculty of Civil Engineering and Geosciences, Delft University of Technology, Stevinweg 1, 2628 CN Delft, The Netherlands E-mail: [email protected]

profiles

INTRODUCTION Increasing freshwater scarcity and growing environmental awareness increases the use of reclaimed wastewater especially in areas where the climate is (semi) arid and/or the population and economic growth are high (Yang & Abbaspour ). Municipal sewage is a promising alternative water resource and can satisfy non-potable water requirements such as landscaping and irrigation of agricultural fields (Foglar & Briški ; Hidaka et al. ). In order to produce high quality effluents, tertiary treatment of wastewater treatment plant (WWTP) effluent is expected to provide a feasible and cost-effective solution (Qin et al. ). Continuous sand filtration (CSF) has been successfully applied for tertiary denitrification in Belgium, Sweden and The Netherlands in both industrial and municipal WWTPs (Hultman et al. ; Hochstrat et al. ). A long-term stable denitrification efficiency exceeding 83% can be reached with appropriate influent dissolved oxygen (DO) concentrations, nitrate concentrations, methanol (MeOH) dosage and airlift velocity (Freed & Pauwels ). Results on nitrate removal with CSF are described by Hultman et al. () and show that nitrate (NO3-N) concentrations are removed from 7.3–20 mg/L in the doi: 10.2166/wst.2012.343

WWTP effluent to 0.5–2.2 mg/L in the filtrate, using hydraulic loading rates of 10 m/h during winter and 22 m/h during summer. In The Netherlands Scherrenberg et al. () investigated the influence of the hydraulic loading rate on the denitrification efficiency. Hydraulic loading rates of 17.5 and 22.5 m/h with a chemical oxygen demand (COD) dosage ratio of 7.5 kg COD/kg NOx-N and 1.2 kg COD/kg O2 were tested. The NOx-N concentration is the total inorganic oxidised nitrogen (sum of nitrate and nitrite). It was shown that removal rates for NOx-N of about 80% could be reached, resulting in filtrate concentrations below 2.2 mg/L for total nitrogen (T-N). For a hydraulic loading rate of 17.5 m/h the average denitrification rate was 1.9 kg NOx-N/m3d, for a hydraulic loading rate of 22.5 m/h the average denitrification rate was 1.8 kg NOx-N/m3. However, so far, the design, operation optimising and control of CSFs still relies on ‘in-house’ process engineering and empirical experimental knowledge (Yim et al. ; Sin et al. ). The European Water Framework Directive (WFD) needs to be fulfilled when discharging on surface water. To be able to reach the WFD criteria T-N concentrations need to be below 2.2 mg/L and total phosphorus (T-P)

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below 0.15 mg/L. Optimal operation of a CSF is required to reach this concentrations. The objective of this research is to investigate the denitrification profiles, in a CSF applied for tertiary treatment of WWTP effluent. These profiles are proven to be a useful tool to determine the removal processes within a filter bed and to investigate whether the filter bed is used optimally (Scherrenberg et al. ).

MATERIAL AND METHODS This research was conducted at the Harnaschpolder WWTP located in the Netherlands. This WWTP is conventional; the treatment process includes biological nutrient removal (BNR) treatment i.e. nitrogen and phosphorus. The quality of the WWTP effluent in 2010 is presented in Table 1. The values indicate that the COD, ammonia (NH4-N), NO3-N, T-N, orthophosphorus (PO4-P) and T-P fluctuate significantly. Figure 1

Pilot scale sand filter

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Schematic illustration of the CSF. The numbers represent: 1. feed pipe, 2. supply pipe, 3. distributions arms, 4. sand bed, 5. effluent outlet, 6. inlet air-lift pipe, 7. air-lift pipe, 8. sand washer, 9. drain pipe.

The applied CSF is schematically illustrated in Figure 1. WWTP effluent is pumped into the CSF through the feed pipe (1), directed downwards through a supply pipe (2) and is subsequently distributed by distribution arms (3) in the lower section. The WWTP effluent flows upwards through the downwards moving sand bed (4) reaching the effluent outlet (5) at the top. The intensive contact between water and sand leads to biomass growth on the sand grains. From the bottom part (6) sand with biomass is lifted by compressed air via an air-lift pipe (7) towards the sand washer (8), in which the superfluous biomass is separated from the sand. At the top part of the sand washer, the wash water and detached biomass are discharged through a drain pipe (9). The cleaned sand falls down on top of the sand bed. In this way, the sand is re-circulated continuously and filtration remains in service without interruption. The operational parameters are described in Table 2. The CSF is used for denitrification only. The sand velocity

Table 1

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was measured every week at the four quadrants of the filter bed. This is done by measuring the downward velocity of a disk, attached to a pipe, which is dragged down together with the sand bed. Sand velocities of 10–20 mm/min result in bed turnover rates of 7–14 times per day. Lower turnover rates were tested but resulted in filter bed clogging even in combination with lower MeOH dosage ratios. The MeOH dosage was controlled based on online measured NOx-N concentrations in the WWTP effluent. The MeOH dosage compensated for the O2 concentration fixed for 6 mg O2/L. The applied MeOH dosage ratio was 1.0 g COD/g O2. The filtrate of the CSF is stored in a buffer tank. Sampling system for profile measurements The sampling system is illustrated in Figure 2. The key equipment of the sampling system is a 5-channel pump, which

The quality of the WWTP effluent in 2010

COD

NH4-N

NO3-N

T-N

PO4-P

T-P

pH

Temp.

mg/L

mg/L

mg/L

mg/L

mg/L

mg/L



W

Minimum

29.2

0.06

0.7

0.04

0.37

6.7

11.0

Maximum

51.7

3.34

11.4

1.38

1.48

7.8

22.2

Average

35.3

0.60

0.43

0.83

7.1

15.9

4.27

2.39 13.7 5.69

C

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Table 2

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Denitrification profiles in a continuous sand filter

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Operational parameters of the pilot CSF

Flow rate m3/h

Cross-sectional area m²

Hydraulic loading rate m/h

Sand velocity mm/min

Bed height m

Grain size mm

Wash water %

MeOH kg COD/kg NO x -N

9–10

0.7

13–15

10–20

2

1.4–2.0

5–10

6.0–7.5

RESULTS AND DISCUSSION Overall filter performance From February 2010 to January 2011, hydraulic loading rate, sand velocity and MeOH dosage ratio were adjusted between 9 and 10.4 m3/h, 10 and 20 mm/min and 6.0 and 7.5 kg COD/kg NOx-N, respectively. COD, NH4-N, NO3-N, PO4-P, T-P concentrations in the filtrate and the removal efficiencies are presented in Table 3. The denitrification efficiency in this period was 77%. In addition to denitrification the results show significant NH4-N and phosphorus removal. Profile measurements

Figure 2

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Illustration of sampling system.

W

pumps samples from 5 different depths of the filter bed. The vertical distance between the two sampling points is 0.5 m. In this way samples from the different heights could be pumped to the sample bottles. The lowest point is just above the distribution arms, the highest point in the top of the sand bed. The samples were analysed for COD, NH4-N, NO3-N, nitrite (NO2-N), T-N, PO4-P and T-P using Hach Lange cuvette tests. DO, pH and temperature (T) are measured by handheld meters (Oxi 323, pH 315i, WTW Company).

Table 3

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Concentration profiles were assessed for DO, COD, NH4-N, NO3-N, T-N and T-P. The feed water DO concentrations, pH and T fluctuated between 5.5 and 6.5 mg O2/L, 7.04 and 7.46 and 11.0 and 16.7 C, respectively. The flow rate was 9 m3/h in combination with a sand velocity of 10 mm/ min. The applied MeOH dosage ratio was 7.5 kg COD/kg NOx-N. The average profiles from 8 samples are shown in Figures 3 and 4. DO profiles indicate that DO was consumed from 6.0 to 0.7 mg O2/L within the first 0.4 m sand bed and to 0.5 mg DO/L within 0.9 m sand bed. The DO decrease in combination with the reduction of NH4-N from 0.4 mg/L to almost zero within the first 0.4 m sand bed removal might indicate that the bottom part of sand bed was aerobic, the middle and top part were anoxic. Stoichiometric 0.093 mol MeOH/mol NH4-N is required for cell growth, the observed

COD, NH4-N, NO 3 -N, PO4-P, T-P concentrations in CSF filtrate and CSF removal efficiencies between September 2010 and January 2011 COD mg/L

NH4-N mg/L

NO3-N mg/L

T-N mg/L

PO4-P mg/L

T-P mg/L

WWTP effluent

Average

34.9

0.067

3.0

6.66

0.59

0.80

CSF filtrate

Min–Max Average

29.8–50.6 39.1

0.003–0.05 0.025

0.09–1.40 0.70

1.08–3.03 2.53

0.04–1.18 0.42

0.11–0.92 0.54

–12%

63%

77%

62%

29%

33%

Removal efficiency

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Figure 3

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DO, COD and NH4-N profiles over the sand bed. Average of eight samples.

Figure 4

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NO3-N, T-N and T-P profiles over the sand bed. Average of eight samples.

NH4-N reduction was below the stoichiometric requirements for cell growth, therefore it is concluded that the NH4-N reduction is not due to nitrification, but due to uptake for cell growth. The exact bed height required for the DO and NH4-N reduction is not known since no intermediate sample points were available. COD, NO3-N, T-N and T-P profiles show a rapid decline within 0.9 m of the sand bed, even with aerobic or anoxic circumstances. This is in agreement with the NO3-N profiles made by Scherrenberg et al. () for a dual media filter. Due to the MeOH dosage, the COD concentration at the distribution arms was higher compared with the WWTP effluent. The results indicate that the biofilm is mainly present in the bottom part of the sand bed and that the nutrient loads likely can be increased since only half of the sand bed is used.

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Influence of methanol dosage on the profiles In order to investigate if the MeOH dosage could be lowered, the dosage ratio was adjusted from 7.5 to 6 kg COD/kg NOx-N from September to October 2010. The WWTP effluent temperature was 19.0 ± 1.0 C. The flow rate and sand velocity were 9 m3/h and 10 mm/min, respectively. The average profiles of 8 samples are shown in Figure 5 and 6. The DO profiles indicate that the DO reaches 0.5 and 0.9 mg/L within 0.9 m of sand bed for dosages of 6 and 7.5 kg COD/kg NOx-N. This resulted in a maximum oxygen reduction of 8.7 kg O2/m3 filter bed. The DO for a dosage of 7.5 kg COD/kg NOx-N was lower in every measured height compared with 6 kg COD/kg NOx-N. NH4-N decreased rapidly (below 0.05 mg/L) for both dosage ratios. W

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Figure 5

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DO, COD and NHþ 4 -N profiles in the sand bed for a MeOH dosage of 6-7.5 kg COD/kg NOx-N. Average of five samples.

It was shown for NO3-N and T-N removal that a MeOH dosage of 7.5 kg COD/kg NOx-N was more efficient compared with 6 kg COD/kg NOx-N. The first 0.9 m of 4.8 kg NO3-N/m3 filter bed and 38.6 kg COD/m3 filter bed were removed with a dosage of 7.5 kg COD/kg NOx-N. Theoretically 8.7 kg O2/m3 filter bed will require 5.8 kg COD, this leaves a COD consumption of 5.2 kg COD/kg NO3-N within the first 0.9 m of filter bed. This is slightly higher compared with the theoretical value of 4.8 kg COD/kg NO3-N (Koch & Siegrist ). T-N profiles also indicated that nitrogen removal in CSF mainly occurs in the bottom, independent of the MeOH dosage ratio. This is underlined by the NO3-N graph in Figure 5 and the COD graph in Figure 6. These

Figure 6

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graphs indicate that NO3-N as well as COD are available in the top 1 m of the filter bed, but no removal takes place. The T-P profiles indicate that T-P was removed quickly in the bottom. PO4-P shows a total decrease of 0.1 mg/L, therefore the T-P removal is probably due to the removal of particulate phosphorus. The T-P removal efficiency increased with a higher MeOH dosage ratio. A clear explanation is not found. The NH4-N uptake, denitrification rate, T-N and T-P loads within 0.9 m of sand bed are calculated for both MeOH dosages in Table 4. The calculated denitrification rates are lower compared with the results described by Scherrenberg et al. (). During this research a mean denitrification rate of 1.9 kg NOx-N/(m3 d) was found concerning

NO3-N, T-N and T-P profiles in the sand bed for a MeOH dosage of 6 and 7.5 kg COD/kg NOx-N. Average of five samples.

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Table 4

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Denitrification profiles in a continuous sand filter

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NH4-N uptake, denitrification rates, T-N and T-P loads for a sand bed of 0.9 m height

MeOH dosage kg COD/kg NOx-N

NH4-N uptake kg NH4-N/(m3d)

Denitrification rate kg NO3-N/(m3d)

T-N load kg T-N/(m3d)

T-P load kg T-P/(m3d)

6.0

0.10

1.00

1.05

0.07

7.5

0.12

1.20

1.50

0.14

the entire filter bed of 2 m height at a hydraulic loading rate of 17.5 m/h and MeOH dosage ratios of 3.5–5 kg MeOH/kg NOx-N. Influence of the inflow loads on the profiles To investigate the influence of the NOx-N loading rate on the profiles the hydraulic loading rate was increased. During this experiment the WWTP effluent temperature was 17.0±1.0 C. The sand velocity and MeOH dosage ratio were 10 mm/min and 7.5 kg COD/kg NOx-N, respectively. Figures 7 and 8 present the results for the experiments conducted with a flow rate of 9 m3/h, i.e. hydraulic loading rate 12.8 m/h, and 10.4 m3/h, i.e. hydraulic loading rate 14.8 m/h. The DO profiles made for the flow rates indicated that the DO quickly decreases within 0.4 m of the sand bed, but the DO was higher for 10.4 m3/h. After 1.5 m of sand bed the DO profiles were comparable. The COD and T-P profiles for the different hydraulic loading rates were similar. 3 The NO 3 -N profile for 10.4 m /h indicates that the middle part of the sand bed and also the top part, were used for denitrification. NO2-N concentrations were 1.6 mg/L in the middle of the filter bed and 0.7 mg/L in the filtrate, W

Figure 7

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DO, COD and NHþ 4 -N profiles in the sand bed at different hydraulic loading rates.

indicating unstable denitrification, but the WFD target limit of 2.2 T-N/L is met. The denitrification rates in the entire sand bed at 10.4 and 9.0 m3/h were calculated as 0.51 kg 3 3  NO 3 -N /(m d) and 0.54 kg NO3 -N /(m d), respectively. Considering that for an inflow of 9 m3/h, only 0.9 m of filter bed is 3 needed, a denitrification rate of 1.2 kg NO 3 -N/(m d) can be 3 calculated. The T-N removal rate at 10.4 m /h was calculated as 0.83 kg T-N /(m3d) and 0.67 kg T-N /(m3d) at 9.0 m3/h. For T-N removal, the filtrate of CSF at both hydraulic loading rates could satisfy the required quality for Harnaschpolder WWTP secondary effluent reuse. For an increased hydraulic load the NH4-N uptake and phosphate removal still mainly took place in the bottom part of the sand bed, but denitrification took place in the whole sand bed. Therefore, hydraulic loading rates exceeding 10.4 m3/h are likely not accommodated by the researched CSF without compromising on effluent quality. This hydraulic loading rate is rather low compared with Hultman et al. (), who reached a hydraulic loading rate of 22 m/h with 90% NOx-N removal, and Scherrenberg et al. (), describe stable NOx-N removal efficiency of about 80% for hydraulic loading rates between 17.5 and 25 m/h. The retention time in the filter bed was 9.4 min for a flow of 9.0 m3/h. This resulted in a removal

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Figure 8

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Denitrification profiles in a continuous sand filter

Water Science & Technology

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NO3-N, T-N and T-P profiles in the sand bed at different hydraulic loading rates.

rate of >95%. For a flow of 10.4 m3/h, i.e. retention time of 8.1 min, a removal rate of 75% was found. Schauer et al. () describe the effect of the retention time on the removal rate. This research indicated that varying retention times between 22.6 and 11.4 min had a slight impact on the removal rates. Schauer et al. () found removal rates between 60 and 95% for retention times between 11 and 14 min.

DISCUSSION AND CONCLUSIONS 1. T-N concentrations in the CSF filtrate of 2.2 mg/L can be reached for a maximum T-N concentration of 7.0 mg/L in the WWTP effluent, a dosage of 7.5 mg COD/mg NOx-N, a sand velocity of 10 mm/min and an upward velocity of 14.8 m/h, agreeing with a hydraulic loading rate of 10.4 m3/h. Although the results show that the required T-N values can be met, due to the presence of an organic nitrogen fraction that varies between 1.1 and 2.8 mg/L in the Harnaschpolder WWTP effluent, an effluent quality guarantee