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Nitrification in porous media during rapid, unsaturated water flow
~
Pergamon
0043-1354(95)00206-5
Wat. Res. Vol. 30, No. 3, pp. 531-540, 1996 Elsevier ScienceLtd. Printed in Great Britain
NITRIFICATION IN POROUS MEDIA DURING RAPID, UNSATURATED WATER FLOW T. Y A M A G U C H I I*@, P. M O L D R U P : , D. E. R O L S T O N 3, S. ITO 4 and S. T E R A N I S H I 1 ~Department of Civil and Environmental Engineering, Faculty of Engineering, Hiroshima University, 1-4-I Kagamiyama, Higashi-Hiroshima 739, Japan, 2Environmental Engineering Laboratory, Department of Civil Engineering, Aalborg University, Sohngaardsholmsvej 57, DK-9000 Aalborg, Denmark, 3Soils and Biogeochemistry, Department of Land, Air and Water Resources, University of California, Davis, CA 95616, U.S.A. and 4Department of Applied Chemistry, Faculty of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima 739, Japan (First received October 1994; accepted in revised form August 1995) Abstraet--A substantial nitrification in rapid infiltration (RI) systems for wastewater treatment is a prerequisite for obtaining good N removal by denitrification. The purpose of this study is to investigate nitrification in porous media at conditions corresponding to RI treatment systems. Nitrification in six 50-cm porous media columns (98% weathered granite or sand and 2% field soil) during unsaturated leaching at constant flow rates of synthetic wastewater was investigated. Concentrations of NH4-N between 20 and 60 mg 1- l were applied and vertical concentration profiles of NO3-N, NO2-N and NH4-N were measured for 54 d at 30°C (three columns) and for 140 d at 10°C (three columns). A time lag in nitrification of 20 d was found at 10°C. Complete nitrification was obtained after 3-5 d at 30°C and after approximately 50 d at 10°C. Assuming first-order nitrification at steady-state, the corresponding first order reaction rate coefficients (kl) for NO 3 production in the columns were estimated to be between 0.4 and h -~ at 10°C and between 6 and 9 h -I at 30°C. Steady-state NO 3 profiles were obtained between 1.5 and up to 9 weeks after the experiments were started. At the actual soil-air contents (0.10 cm3 air phase cm -3 soil), oxygen limitations were not observed during the experiments. Nitrogen loadings (water flow times N concentration) above 100 mg N 1- t c m h -I (1 g N m -2 h - ' ) caused NH4 accumulation in the columns at 10°C and should probably be avoided during operation of RI system.
1
Key words--land treatment, wastewater, soil aquifer system, rapid infiltration, nitrogen removal, nitrification
INTRODUCTION Water reuse has become a central element in water resources planning in many areas of the world due to population growth, periodic droughts, and the increasing contamination of both surface and groundwaters (Asano, 1991). Soil-aquifer wastewater treatment, especially rapid infiltration (RI) of municipal wastewater into relatively permeable soils, is becoming an important treatment alternative as the need for water reuse increases and protection of groundwater resources becomes vital (Asano, 1985). Soil-aquifer treatment systems can offer many advantages compared to the conventional mechanical/chemical/biological wastewater treatment plants including lower cost and added storage capacity (Bouwer, 1991a), especially in countries where the economy and infrastructure make the use of high-cost conventional treatment non-feasible (e.g. Bennani et al., 1992).
*Author to whom all correspondence should be addressed. WR30:~--D
531
Most RI land treatment systems consist of coarsetextured porous media without plant cover and are operated at wastewater application rates between 15 and 350 mm d -1 (Iskandar, 1981). The most suitable soils are in the loamy sand, and fine sand range (USEPA/USAID, 1992). The RI plants normally contain a few percent finer-textured material (Bouwer, 1985) to promote transformation processes. The influent to RI plants is typically primary or secondary effluent with a total N concentration between 10 and 30 mg 1-~ but substantially larger values can occur (Iskandar, 1981; Bouwer, 1985). In the wastewater, a m m o n i u m is typically the principal form of nitrogen (Broadbent and Reisenauer, 1985). The quality improvement of the wastewater during RI treatment will mainly occur in the upper part of the vadose zone. For example, most of the nitrogen transformation in the Flushing Meadows RI plant (Phoenix, Arizona, U.S.A.) during 10 years of operation occurred in the upper 50 cm (Bouwer, 1991b). RI plants are normally operated analogously to a traditional municipal wastewater treatment plant, i.e. with alternating periods of wetting (application of
532
T. Yamaguchi et
wastewater) and drying (no application) to introduce sequential anaerobic and aerobic periods. During the wetting periods, however, both aerobic and anaerobic processes take place as the soil will never get completely water saturated, i.e. a certain soil-air phase will be present. Thus the nitrification process under rapid, unsaturated leaching needs to be understood to be able to optimize RI treatment plant design. A total nitrification of the ammonium during the aerobic periods is desirable to promote the denitrification process during subsequent anaerobic periods and for avoiding NH~ accumulation in the soil. If NH~ is accumulated in the soil during an aerobic period, it will reduce the amount of NH2 that can be adsorped during the subsequent anaerobic period, thus, increasing the NH4+ concentration in the renovated wastewater (Bouwer, 1985). Several studies on columns, lysimeters or full scale have been carried out to evaluate RI plant performance with respect to nitrogen removal (e.g. Lance and Whisler, 1976; Leach and Enfield, 1983; Crites, 1985; Bouwer, 1991b). However, these studies mainly dealt with temporal measurements for the influents and the effluents. To fully understand the influence of various soil and environmental parameters on RI system performance, it is necessary to follow both the temporal and spatial evolution of the inorganic N species under controlled circumstances. Yamaguchi et al. (1994) reviewed and discussed the results of 23 continuous leaching experiments at constant flow rates carried out by Yamaguchi and co-workers (1983, 1984, 1985, 1987, 1990, 1991, 1992) to investigate the temporal and spatial evolution of N removal in porous media columns at conditions corresponding to RI land treatment. They suggested the dispersive-convective nitrate flux (the hydrodynamic dispersion coefficient times the nitrate concentration) to be a potential key parameter in evaluating RI plant nitrogen removal efficiency. The studies all focused on the denitrification pro~ess during a near-water-saturated period (upward flow conditions). However, a substantial nitrification during a previous unsaturated period is a prerequisite for obtaining good N removal by denitrification. This study investigated nitrification in porous media columns at conditions corresponding to an unsaturated leaching period of a RI system where the inorganic nitrogen is present as NH4-N. The study focused on two objectives. The first was to measure the time until steady state was reached and determine possible time lags in nitrification by measuring evolution of NH4-N, NO2-N and NO3-N in time and depth at unsaturated leaching conditions. The second was to evaluate the influence of different parameters ( N H : N concentration in influent, oxygen, temperature and porous media composition) on estimated reaction rate coefficients using both the measured data and data reported in other studies. This nitrification study used the same porous media types and
al.
some of the same experimental conditions as were used in the denitrification study of Yamaguchi et al. (1990). THEORY
Nitrogen transport and transformation are traditionally described by the convection-dispersion equation (CDE) including a reaction term, equation
(1) Ri ~Ci
~2Ci
OCi
~t =D--~x2 - U ~ x +t~,, i= l, 2 . . . .
(1)
where C~ is the concentration of the N species (i) in the soil solution, Ri is a retardation factor (accounting for adsorption, anion exclusion etc.), D is the hydrodynamic dispersion coefficient (assumed constant for all i), u is the pore-water velocity (assumed constant for all i), ~i is the rate of transformation of the nitrogen species, x is distance within the soil, and t is time. The commonly accepted pathway for biological nitrification is NH4~NO2~NO3.
(2)
In this study, the concentration of NO2--N was insignificant during steady state. Hence, a single step reaction from NH4-N to NO3-N can be assumed for the nitrification process. Assuming first-order reaction for nitrification, the reaction terms of the simultaneous equation (1) with i = 1 and 2 are Ot = -k~'Cl and ¢P2=ki.C~, where - ~ is the amount of NH4-N oxidized per unit time, O2 is the amount of NO3-N produced per unit time, C1 is the NH4-N concentration, and kl is the first-order reaction rate coefficient for nitrification. The boundary conditions corresponding to the actual experimental conditions for the porous media columns are CI(X ) = C S CI(X) = 0
x = 0 x ----~oo
(3)
C2(x)=O x=O where Cs is the influent concentration of NH4-N, and C2 is the NO3-N concentration. At steady state, the retardation factor is assumed equal to R = 1, i.e. negligible adsorption of the influent NH4 is assumed after steady state is reached. The steady-state solution to the simultaneous equation (1) with R~ = R2 = 1, ~1 =-k~.C~ and ¢P2=k~.C~ under the boundary conditions of equation (3) is (Cho, 1971; Starr et al., 1974) Cl=exp Iux( 1Ca Cs
1
Cl Cs"
N~1 + 4Dkl'~l
(4)
Nitrification in porous media
If instead a zero-order reaction is assumed for nitrification, the corresponding reaction terms for N H 4 - N and NO3-N are of the form 0~ = - k 0 and 02 = k0, where k 0 is the zero-order reaction rate coefficient for nitrification. The steady-state solution to the simultaneous equation (1) with Rj = R 2 = 1, 0~ = - k o and • 2 = k 0 under the boundary conditions of equation (3) is C---!t= 1 k ° x uCs Cs - uCs' 0 <~x <<. k--o-
C2
Cs
1
(5)
Cj
Cs'
MATERIALS AND METHODS
Polyvinyl chloride pipes (20cm in diameter, 50cm in length) were used for the porous media columns (see Fig. 1). Sampling tubes were located at 5- to 10-cm intervals along the columns. Weathered granite passed through a 5-mm screen or sand was thoroughly mixed with cultivated soil passed through a 2.5-mm screen in the ratio of 98:2. The cultivated soil was taken from an agricultural field outside the city of Hiroshima and contained 41% clay and silt. The mixture was packed in six 50-cm long columns each with a porosity between 39.0 and 40.6% corresponding to between 25.5 and 26.2 kg porous media in each column. In Experiment 1, three concentrations of 20, 40, and 60 mg 1-~ of NH4-N were applied to three columns labeled N20, N40, and N60, respectively, at 30°C. Experiment 2 was conducted at 10°C and three concentrations of 20, 20, and 40mg 1-] of NH4-N were applied to three columns labeled NS20, NM20, and NM40, respectively. Column NS20 contained sand with cultivated soil while the remaining five columns contained weathered granite mixed with cultivated soil. The same porous media types as used by Yamaguchi et al. (1990) were used in the present study. The physical properties of the weathered granite and the sand are shown in Table 1. The chemical properties of the mixed porous media packed in the columns are shown in Table 2.
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533
Table 1. Physical properties of the weatheredgranite and the sand used as part of the mixed porous media in the columns Maximum grain size Specific wt. (g cm 3)
Porous media Granite (Exp. I) Granite (Exp. 2) Sand
2.64 2.67 2.69
100% 60% 30% (mm)
10%
5.00 5.00 5.00
0.29 0.27 0.35
2.10 1.80 1.35
0.80 0.75 0.66
Synthetic wastewater (solution of NH4C1, NaHCO3, and KH2PO4) was supplied by a pump from the top of the soil columns (sprinkler system) creating unsaturated downward flow for 54 d in Experiment 1 and 140 d in Experiment 2. NaHCO 3 was added to the synthetic wastewater as an inorganic carbon (IC) source at an IC/N ratio of 2.5. KH2PO 4 was added to obtain a P/N ratio of 0.1. The wastewater application rate was kept below 150-170mm d -] to avoid initial leaching of high-NO3 pulses due to preferential flow in macro pores (Leach and Enfield, 1983). The volumetric soil-air content of the columns was 0. I0 cm 3 air cm -3 soil. The additional experimental conditions for the six leaching experiments are shown in Table 3. The solution collected from the sampling tubes every 2-4d was centrifuged at 3500 r.p.m, and the supernatant analyzed for NO3-N , NO2-N , and NH4-N. The effluent from the columns was analyzed for each species of inorganic nitrogen, pH, total nitrogen (TN), total organic carbon (TOC), inorganic carbon (IC), and electrical conductivity (EC). The mixed porous media in the columns was analyzed at different depth for TN and number of nitrifiers both before and immediately after the experiment. In Experiment I (30°C), concentrations of NO3-N, NO2-N, and NH4-N were determined by u.v. spectrophotometry (220nm), Nesslerization and the sulfuric acid ee-naphthylethylenediamine procedure, respectively. In Experiment 2 (10°C), HPLC was used for the analysis of NO3-N and NO2-N while NH4-N was measured as in Experiment 1. The most probable number (MPN) method (Alexander, 1982) was applied to estimate the population of nitrifiers. Total N, TOC, and IC were analyzed by a TOC-TN analyzer. Gas in the soil pores was taken out through the gas sampling ports of the columns, and was analyzed for 02, N2, CO 2, and N20 using a gas chromatograph. Adsorption isotherm experiments were performed at I0, 20, and 30°C using 46 g of weathered granite (oven dry weight) and 200 ml of ammonium chloride solution. The concentrations were 10, 20, 40, and 80 mg N 1-~, respectively. After 24 h of shaking, the NH4-N concentration of the filtrated soil solution was analyzed. Total and micro-pore surface areas of the decomposed granite were determined by the Ethylene Glycol Monoethyl Ether (EGME) method and by Mercury Intrusion Porosimetry, respectively (Carter et al., 1986). The hydrodynamic dispersion coefficient, D, and the pore-water velocity, u, were determined by continuous application of NaCI solution (2000 mg CI 1- t as chloride) to the top of the columns. The change in chloride concentration in the effluent was measured continuously by an EC meter at the bottom of the columns. Using the measured breakthrough curves, D and u were simultaneously estimated according to the method of Yamaguchi et al. (1989).
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Table 2. Chemical properties of the mixed porous media in the columns Column
Fig. 1. Experimental set-up. Illustration of column set-up, wastewater application system, and sampling system used for the continuous leaching experiments.
N20, N40, N60 NS20 NM20, NM40
CEC (meq kg -I) 15 17 17
TN TP (mg kg -1)
TC
360 30 40
0.06 0.l 0. l
770 1000 560
(%)
TOC 0.05 <0.1 0. l
T. Yamaguchi et al.
534
Table 3. Experimental conditions during continuous unsaturated leaching of the porous media columns. The porous media of all columns were weathered granite mixed with cultivated soil except NS20 which was sand mixed with cultivated soil
Column N20 N40 N60 NS20 NM20 NM40
Influent NH4-N concentration (rag 1 i )
Application rate (ram d- t)
Pore-water velocity (cm h- t )
Dispersion coefficient (cm2 h t)
Temperature (°C)
20 40 60 20 20 40
150 150 150 150 150 150
2.9 2.8 2.9 3.2 3.8 3.3
7.0 6.3 7.2 3.1 7.2 7.2
30 30 30 10 10 10
RESULTS AND DISCUSSION
Evolution of the concentration profiles of N O 3 - N , NO2-N, and NH4-N in columns N60 (Exp. 1), NM20, and NM40 (both Exp. 2) is shown in Fig. 2.. The analogous profiles for columns N20 and N40 (both Exp. 1) are omitted, because the general trends in the profiles were similar to those shown for N60. Also, the profiles for column NS20 are omitted, because the general trends in the profiles were similar to those shown for column NM20.
Ammonia-nitrogen, nitrite-nitrogen, nitrate-nitrogen, and total-nitrogen In Experiment 1 (30°C) during the first 3 to 4 d, the relative concentrations of NH4-N decreased to between 0 and 0.08 within the upper 5- to 10-cm of column depth in all three columns (N20, N40, and N60). Since only small amounts of NO3-N or NO2-N were detected during the first 3 to 4 d, the decrease in NH4-N was caused by adsorption to soil. The NO3-N production started to occur from Day 3 and was promoted rapidly. The relative concentrations of NO3-N (relative to the influent NH4-N concentration, Cs) at Day 5 exceeded 1.0 (between 1.0 and 1.6) below 10-cm depth in all three columns (cf. Fig. 2, column N60). These high concentrations of NO~-N were likely the result of nitrification of the N H ~ N which had accumulated during the first 3-4 d. After Day 5 in Experiment 1, the relative concentration of NO~-N gradually decreased to 1.0, and steady-state nitrification was obtained between Day 10o15. At steady state, complete nitrification was obtained within the upper 5- to 10-cm depth. Significant concentrations of NO2-N were detected only between Day 3 and 5 as shown in Fig. 2 (column Nt0), and were scarcely detected thereafter (C/Cs < 0.01). In Experiment 2 (10°C) during the first 10020 d, the relative concentrations of NH4-N decreased to between 0 and 0.1 within the upper 20-cm depth in all three columns (NS20, NM20, NM40). Again, since only small amounts of NO3-N and NO2-N were detected within the first 10020d, the decrease in NH4-N was caused by adsorption to the soil. The large adsorption was partly due to the 2% finely textured, agricultural soil in the columns, but, as illustrated in Fig. 3, the weathered granite also had a
significant ammonium adsorption capacity. As expected, Fig. 3 shows a slight decrease in adsorption at the high temperature (30°C) compared to the lower temperatures (10020°C) because of higher molecular activity. In columns NM20 and NM40, the concentration of N O 3 - N started to increase slowly after Day 15-20. In column NS20 (sand), nitrification started to occur at Day 8-10, which is 5-10 d earlier compared to columns NM20 and NM40 (weathered granite). After approximately 4-7 weeks (wk) in Experiment 2 (4 wk for column NS20, 7 wk for columns NM20 and NM40), the decrease of NH4-N within the top part of the columns stopped temporarily and nitrification was strongly promoted as shown at Day 48 in Fig. 2 (columns NM20 and NM40). This phenomenon can be explained by a temporary saturation of the ammonium adsorption capacity of soil. This is in agreement with the observation by Broadbent and Reisenauer (1985) for slow infiltration systems that applications of wastewater substantially higher than 2.5-10cm wk -1 can result in saturation of the ammonium retention capacity of the soil. Relative concentrations of NO3-N of more than 1.0 (between 1.0 and 1.6) were detected below 20- to 30-cm depth between Day 52 and 68 in columns NM20 and NM40, and between Day 31 and 41 in column NS20. Compared to Experiment 1 (30°C) where this was observed at Day 5, a significant lag time was therefore introduced at 10°C. After Day 60 in Experiment 2, the relative concentration of N O 3 - N gradually decreased to 1.0, and steady-state nitrification was obtained after around 9wk. At steady state, complete nitrification was obtained within the upper 15- to 20-cm of column depth. Thus it took twice the column length to obtain complete nitrification at 10°C compared to 30°C. Figure 4 shows the amount of NO3-N produced as a function of time and depth for columns N20 (30°C) and NM40 (10°C), both representing the general trends found at 30 and 10°C, respectively. As can be seen from Fig. 4, a time lag in nitrification of approx. 20 d was found at 10°C, and furthermore, it took around 8 wk before nitrification had reached the maximum level. No time lag in nitrification was evident at 30°C. The relative concentration of N O : - N was very low (<0.06) throughout Experiment 2. Significant increases in TN in the columns were observed during
N60
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0.5
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Relative Concentration ( C / C s )
Fig. 2. Temporal and spatial evolution of NO3-N (*), NO2-N (A), and NH4-N ( 0 ) profiles in unsaturated columns of porous media at chosen days (t) during unsaturated leaching experiments. For experimental conditions, see Table 3. Cs is the influent NH4-N concentration. The temperature was 30°C (N60) and 10°C (NM20 and NM40), respectively. 535
536
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Fig. 3. Equilibrium adsorption isotherms for NH 4 on weathered granite and at three different temperatures.
through the water film of a second or less was calculated. Thus, even with a locally much thicker water film, it is unlikely that the nitrification performance in the columns were 02 diffusion limited. In Experiment 1 (30°C) a temporary decrease in 02 concentration was observed in all three columns during the first 4--6 d corresponding to the period with the largest nitrate production (cf. Fig. 4, column N20). The most significant decrease (to 15%) took place in the column with the highest nitrogen loading (column N60), see Fig. 5. In general the 02 concentration was close to 20% in all six columns. Hence both the theoretical considerations concerning film diffusion and the constantly high 02 concentrations measured in the soil air phase at various depths in the columns indicated that 02 was not the limiting parameter of the nitrification in the present experiments that were carried out at a volumetric soil-air content of approx. 0.10cm 3 air phase cm -3 porous media.
the period of the experiments confirming that NH~ adsorption was taking place. Column NM40 showed the most distinct increase in TN (7300 mg during 140d compared to 4200mg for column NM20). Hence, the low temperature (10°C) and the high NH4-N loading for column NM40 resulted in a non-optimal nitrification and a larger NH~ accumulation.
N20 1.6
1.4 1.2 1.0 0.8
0.6 Gas composition in soil pores
In Experiment 1 (30°C), the percentages of O2, N2, and CO2 in the soil pores in the columns were 15-21%, 74-84%, and 1-3%, respectively (16 measurements). The partial pressure of 02 in column N60 was a little lower than in columns N20 and N40. This is caused by the high concentration of NH4-N applied to column N60. No significant differences in the composition of these gases between the upper, middle, and lower layers of the soil column were observed. In Experiment 2 (10°C), the percentages of 02, N 2, and CO2 in the soil pores were 16--21%, 76-86%, and 0-5%, respectively (15 measurements). N 2 0 gas was not detected during Experiments 1 and 2. To evaluate possible O2 diffusion limitations, a mean water film thickness was estimated from the surface area measurements. The external surface area was estimated to approx. 10 m 2 g-i dry matter (DM) equal to the difference between the total surface area (13.3 m 2 g-I DM; measured by the EGME method) and the micro-pore surface area (3.1 m 2 g-~ DM; measured by mercury intrusion porosimetry). With a volumetric water content of 0.30cm 3 H20 cm -3 porous media and a bulk density of 1.6 g DM cm -3 the estimated, mean thickness of the water film equals 0.02/~m. Using a numerical solution to one-dimensional gas diffusion [solution of Moldrup et al. (1992) with u = 0 and the 02 diffusion coefficient in pure water at 10 or 30°C], an 02 diffusion time
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Fig. 4. Temporal and spatial evolution of NO3-N production in columns of porous media during unsaturated leaching experiments. C is the NO3-N concentration and Cs is the influent NH4-N concentration. For experimental conditions, see Table 3. The temperature was 30°C (N20) and 10°C (NM40), respectively.
537
Nitrification in porous media 20 19 18
16
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Time ( d a y s ) Depth,
cm
~
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Fig, 5. Temporal and spatial evolution of 0 2 (%) in the soil air phase in column N60 (30°C) during the first 2 weeks. For experimental conditions, see Table 3.
Number of nitrifiers and pH The distribution of nitrifiers in the columns is shown in Table 4 as number of ammonium- and nitrite-oxidizing bacteria (most probable number) for 1 g of dry porous media. After Experiment 1 (30°C), the numbers of both ammonium-oxidizing bacteria and nitrite-oxidizing bacteria were significantly higher at 0- to 5-cm depth, slightly higher at 5- to 15-cm depth, and almost the same at 35- to 45-cm depth compared to the initial values. This is because nitrification mainly occurred within the first 0- to 10-cm depth and, subsequently, there was almost no supply of NH4-N below 10-cm
depth (cf. Fig. 2, column N60) due to the extensive build-up of nitrifiers in the upper few cm. After Experiment 2 (10°C), the numbers of ammonium-oxidizing bacteria were significantly higher at 0- to 5-cm and 5- to 15-cm depth than the initial values, and slightly higher at 15- to 25-cm and 35- to 45-cm depth compared to the initial values. This is probably because NH4-N was able to leach deeply into the columns between Day 15 and 20 and between Day 40 and 50 in Experiment 2 (cf. Fig. 2, columns NM20 and NM40). After Experiment 2, the numbers of nitrite-oxidizing bacteria were almost the same throughout the columns compared with the initial values. This trend is different from Experiment 1 (30°C), since the growth of the nitrifying bacteria is likely inhibited at 10°C. In Experiment 1, the pH of the applied synthetic wastewater and the column effluents was between 8.2 and 8.6 and between 7.5 and 8.0, respectively. In Experiment 2, the pH of the wastewater and the effluents was between 8.3 and 8.6 and between 6.8 and 8.0, respectively. Hence, the pH of the effluents was generally lower than that of the applied wastewater because of the hydrogen ion production during the nitrification process.
Estimated reaction rate coefficients To be able to compare the results of the present experiments with earlier soil column studies, the reaction rate coefficients for nitrification were estimated from the measured steady-state concentration profiles of NH4-N and NO3-N, assuming first-order and zero-order reaction rates, respectively. The estimation was performed by curve-fitting the measured NH4-N and NO3-N profiles at steady state against the theoretical solutions [equations (4) and (5)]. The
Table 4. Number of nitrifiers (most probably number, MPN) in the porous media columns before (t = 0 d ) and after (t = 5 4 d and t ~ 140d, respectively) continuous unsaturated leaching. For experimental conditions, see Table 3 Number of nitrifying bacteria MPN/g dry porous media Soil column
Temperature (°C)
(A) Ammonium-oxidizing bacteria N20, N40, N60 30 N20 30 N40 30 N60 30 NS20 10 NS20 10 NM20, NM40 10 NM20 10 NM40 10 (B) Nitrite-oxidizing bacteria N20, N40, N60 30 N20 30 N40 30 N60 30 NS20 10 NS20 10 NM20, NM40 10 NM20 10 NM40 10 =Not measured.
Time (t = d)
0-5 cm
5-15 cm
15-25 cm
3 5 4 5 cm
0 54 54 54 0 140 0 140 140
I ' 103 8" 104 3' 105 3" l05 5" 103 9.105 5-102 6' 105 4. l0 s
1' 103 4- 103 3" 104 1. ]04 5' 103 5- 105 5.102 9' 104 2" 105
1- 103 NM a NM NM 3.102 5" 104 3.103 6" 104 4.104
1" 103 5" 102 1 • 103 1" 103 3" 102 4" 103 3.103 6" 103 6" 104
0 54 54 54 0 140 0 140 140
4.102 7.103 4.104 7.103 4.103 4.103 8.102 6" 103 9' 102
4.102 4- 102 3- 103 2.103 4.103 4.103 8.102 6' 102 9" 102
4.102 NM NM NM 6.102 4.103 1.103 6' 103 3' 10a
4.102 3.103 1.103 2.103 6.102 2.103 1.103 9- 102 9' 103
538
T. Yamaguchi et al.
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.~ .~, ~ .-~
~'.~ e~
estimation results for the 6 column experiments performed in the present study are shown in Table 5 (lower part). The rate coefficients for Experiment i (30°C) were each estimated from only one data point (5-cm depth) of the steady-state NH4-N profiles because of the rapid nitrification occurring at this high temperature. Hence, the calculation of the corresponding values of k 0 and k~ is sensitive to any experimental error associated with only one point. All three columns, however, showed the same trend during evolution of the inorganic N profiles• In Experiment 1 (30°C), the estimated values of k~ and k 0 were between 6.0 and 8.8 h -1, and between 11 and 34rag 1-~ h -~, respectively. An increasing k0 value corresponding to a higher net nitrification was obtained for an increasing influent concentration of NH4-N. The minimum kt value was estimated for an influent NH4-N concentration of 40 mg 1- ~ h - ~. The difference between the k~ values, however, is not significant recalling the estimation error associated with only one point as discussed above• For Experiment 2 (10°C), no significant difference in k0 values was observed for the three columns. The minimum value of kl (0.4h -~) was obtained for column NM40, the latter representing the largest concentration of NH4-N applied (40 mg N 1-'). Hence, the nitrification efficiency was not optimal at the highest NH4-N loading at 10°C. The effect of temperature (10°C vs 30°C) on the nitrification rate coefficients is apparent from Table 5. The values ofk~ at 30°C are 9 to 15 times larger than at 10°C, while the values of k0 at 30°C are 2 to 4 times larger than at 10°C (see Table 5). No significant effect of porous media composition was observed• The values of k 0 are at the same level when using sand (column NS20) compared to using weathered granite (NM20). In general, a zero-order nitrification rate seemed a slightly better assumption than a first-order rate for Experiment 2 (10°C) cf. the curve-fitted concentration profiles shown in Fig. 6. Comparison o f reported reaction rate coefficients
The estimated values of reaction rate coefficients and the experimental conditions for nitrification of previous studies by Starr et al. (1974), Misra et al• (1974), and Ardakani et al. (1974a, b) are shown in Table 5 (upper part). The values of rate coefficients for the experiment by Ardakani et al. (1974a, b) were estimated by the present authors assuming first- and zero-order nitrification, respectively. In the study of Ardakani et al. (1974a) and the present study, the pore-water velocities corresponded to RI systems, while the pore-water velocities of Starr et al. (1974) and Misra et al. (1974) corresponded to slow infiltration systems. The RI experiments gave nitrification rate coefficients that were two orders of magnitude higher than for the slow infiltration experiments (Table 5). In the field study of Ardakani et al. (1974b) the values of both u and kl showed to
539
Nitrification in porous media NM20
NM40
~. to 20 zo "?" 40 50 0
0.5
1.0 0
0.5
1.0
Relative Concentration (C/ICe)
NS20 0 ""
20 a
40
-"
•
1
50 ..
o
a higher degree of oxygen transport in the gaseous phase during the initial periods of wastewater infiltration. This is probably best done by adjusting the wastewater infiltration rate so a minimum, volumetric soil-air content of 0.1 cm 3 cm -3 is present, especially in the topsoil to avoid putting a "lid" on the oxygen diffusion into the soil. Also, a wastewater recycling system where secondary effluent wastewater with high ammonia content is infiltrated at a relative low flow velocity to obtain rapid nitrification after which recycled wastewater with high nitrate content is infiltrated at a higher flow velocity to obtain anaerobic conditions (denitrification) may be a solution for obtaining a larger degree of nitrogen removal in RI treatment plants. CONCLUSIONS
o25
Relative Concentration ( C / C z )
Fig. 6. Curvefitted steady-state NO3-N (*) and NH4-N ( 0 ) profiles for unsaturated columns of porous media (see Table 3 for experimental conditions) using equations (4) and (5) assuming first-order ( ..... ) and zero-order ( - - - ) nitrification, respectively. The estimated values for the corresponding reaction rate coefficients are given in Table 5. Cs is the influent NH4-N concentration. The temperature was 10°C for all three columns.
be in between those obtained for slow and rapid infiltration. The extremely high NH4-N loading in the RI experiment of Ardakani et al. (1974a) equal to 5.2 cm h-~ of water velocity times an influent NH4-N concentration of 75 mg N 1-j resulted in a higher N application than the microorganisms were able to use optimally, and subsequently, the k I value was significantly smaller compared to the present RI experiments. The nitrification studies in Table 5 were carried out with artificial ammonia-nitrogen wastewater and at a volumetric soil-air content/>0.1 cm 3 cm -3, Based on the results of the present study, oxygen limitations are not likely to have occurred at these high soil-air contents. Thus the nitrification rates in Table 5 should probably be considered potential rates i.e. based on optimal oxygen supply in the porous media. In the real treatment situation with both biodegradable organic matter and ammonia in the wastewater and smaller or larger degree of heterogeneity in the soil system, there may be competition between heterotrophs and autotrophs (nitrifiers) in the biofilm surrounding the soil particles leading to local oxygen limitations. Also, clogging problems and channeling may occur resulting in a non-homogeneous distribution of oxygen to the soil matrix. To obtain a high nitrification rate, it therefore seems vital to maintain
In the rapid infiltration (RI) column studies, steady-state nitrification was obtained from 9 wk at 10°C and from 1.5 wk at 30°C, i.e. a relatively long time period compared with the normally used drying sequences in RI treatment systems (5-16 d for most of the presently operating plants cf. Smith, 1991). Hence, a steady-state approach can in some cases provide a rough estimate of the net nitrification (from the k0 value) and the nitrification efficiency (from the kl value), especially at high temperature, but may not be adequate to describe the inorganic N transformations in RI treatment systems. This is the case for both nitrification during unsaturated periods (this study) and for denitrification during subsequent water-saturated periods (Yamaguchi et al., 1990). A low temperature (10°C) in combination with a high N loading (u. Cs > 100 mg N 1-l cm h -j = 1 g N m -2 h -~) resulted in a non-optimal nitrification and should probably be avoided during RI plant operation. Oxygen limitations or effects of the coarse porous media composition were not observed in this study. If the water application rate in RI systems can be adjusted so that a sufficiently large soil-air phase /> 0.10 cm 3 air cm -3 soil is present, the present experiments suggest that O: diffusion in the gaseous phase will not become the limiting factor for nitrification. This study may suggest a potential for using RI porous media reactors for nitrification of NH4 in for example refinery wastewater with typical NH4 concentrations between 20 and 80 mg N 1- i. The present RI results, i.e. full conversion of NH4 to NO3 within 3-5 d at 30°C, compare favorably with the achievements of activated sludge or rotating biological contact systems (cf. Fang et al., 1993). Also, the present data together with the data from Yamaguchi et al. (1994) suggest that even shallow RI porous media systems with alternating downward (nitrification) and upward (denitrification) flow could provide complete nitrogen removal from wastewater if operated at the correct flow velocities and nitrogen loadings. This needs to be tested at both laboratory and field scale.
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Acknowledgements--Mr Y. Harada from AMCO, Japan, is gratefully acknowledged for the mercury intrusion porosimetry measurements. A travel grant from the Japanese Ministry of Education (Monbusho International Scientific Research Program: Joint Research, No. 06044158) has greatly enhanced the work and cooperation of the authors and is hereby gratefully acknowledged. This study was supported by the Danish Center for Root Zone Processes under the Danish Environmental Research Programme.
REFERENCES Alexander M. (1982) Most probable number method for microbial populations. In Methods of Soil Analysis (Edited by Page A. L. et aL), Part 2, 2nd edn, pp. 815-820. Agron. Mongr. 9. ASA and SSSA, Madison, Wisc. Ardakani M. S., Rehbock J. T. and Mclaren A. D. (1974a) Oxidation of ammonium to nitrate in a soil column. Soil Sci. Soc. Am. Proc. 38, 96-99. Ardakani M. S., Schulz R. K. and McLaren A. D. (1974b) A kinetic study of ammonium and nitrate oxidation in a soil field plot. Soil Sci. Soc. Am. Proc. 38, 273-277. Asano T. (1985) Overview: Artificial Recharge of Groundwater. In Artificial Recharge of Groundwater (Edited by Asano T.), pp. 3-19. Butterworth, Boston, Mass. Asano T. (1991) Wastewater reclamation and reuse. In Wastewater Engineering: Treatment, Disposal and Reuse (Edited by Tchobanoglous G. and Burton F. L.) 3rd edn, Chap. 16, pp. 1137-1193. McGraw-Hill, New York. Bennani A. C., Lary J., Nrhira A., Razouki L., Bize J. and Nivault N. (1992) Wastewater treatment of Greater Agadir (Morocco): an original solution for protecting the Bay of Agadir by using the dune sands. Wat. Sci. Technol. 25, 239-245. Bouwer H. (1985) Renovation of wastewater with rapid infiltration land treatment systems. In Artificial Recharge of Groundwater (Edited by Asano T.), pp. 249-282. Butterworth, Boston, Mass. Bouwer H. (1991a) Role of soil-aquifer treatment in water reuse. Proc. Spec. Conf. Environ. Eng., Am. Soc. Cir. Eng., p. 738. Bouwer H. (1991b) Groundwater recharge with sewage effluent. Wat. Sci. Technol. 23, 2099-2108. Broadbent F. E. and Reisenauer H. M. (1985) Fate of wastewater constituents in soil and groundwater: nitrogen and phosphorus. In Irrigation with Reclaimed Municipal Wastewater--A Guidance Manual (Edited by Pettygrove G. S. and Asano T.), Chap. 12, pp. 12-1 to 12-16. Lewis, Chelsea, Mich. Carter D. L., Mortland M. M. and Kemper W. D. (1986) Specific surface area. In Methods o f Soil Analysis, Part 1, Physical and Mineralogical Methods (Edited by Klute A.), pp. 413-423. Agron. Mongr. No. 9 (2nd edn). ASA, SSSA, Madison, Wisc. Cho C. M. (1971) Convective transport of ammonium with nitrification in soil, Can. J. Soil Sci. 51, 339-350. Crites R. W. (1985) Nitrogen removal in rapid infiltration systems. J. environ. Engng 111, 865-873. Fang H.-Y., Chou M.-S. and Huang C.-W. (1993) Nitrification of ammonia-nitrogen in refinery wastewater. Wat, Res. 27, 1761-1765.
Iskandar I. K. (1981) Introduction. In Modeling Wastewater Renovation: Land Treatment (Edited by Iskandar I. K.), pp. 3-19. Wiley, New York. Lance J. C. and Whisler F. D. (1976) Stimulation of denitrification in soil columns by adding organic carbon to wastewater. J. Wat. Pollut. Control Fed. 48, 346-356. Leach L. E. and Enfield C. G. (1983) Nitrogen control in domestic wastewater rapid infltration systems. J. Wat. Pollut. Control Fed. 55, 1150-1157. Misra C., Nielsen D. R. and Biggar J. W. (1974) Nitrogen transformation in soil during leaching: II. Steady-state nitrification and nitrate reduction. Soil Sci. Soc. Am. Proc. 38, 294-299. Moldrup P., Yamaguchi T., Hansen J. Aa. and Rolston D. E. (1992) An accurate and numerically stable model for one-dimensional solute transport in soils. Soil Sci. 153, 261-273. Smith R. G. (1991) Natural treatment systems. In Wastewater Engineering: Treatment, Disposal and Reuse (Edited by Tchobanoglous G. and Burton F. L.) 3rd edn, Chap. 13, pp. 927-1016. McGraw-Hill, New York. Starr J. L., Broadbent F. E. and Nielsen D. R. (1974) Nitrogen transformation during continuous leaching. Soil Sci. Soc. Am. Proc. 38, 283-289. U.S. Environmental Protection Agency and U.S. Agency for International Development (1992) Guidelines for Water Reuse. EPA/625/R-92/004. Yamaguchi T. and Teranishi S. (1983) Denitrification of wastewater in land treatment. (in Japanese with English abstract.) J. Jap. Sewage Wks Assoc. 20, 28-35. Yamaguchi T. and Teranishi S. (1984) Denitrification in soil under continuous leaching (in Japanese with English abstract, figures, and tables). Proc. environ. Sanit. Engng Res. 20, 223-229. Yamaguchi T. and Teranishi S. (1985) Denitrification in soil columns under saturated flow (in Japanese with English abstract) J. Jap. Sewage Wks Assoc. 22, 55-59. Yamaguchi T. and Teranishi S. (1987) Denitrification during unsaturated filtration in soil columns (in Japanese with English abstract, figures, and tables). Proc. environ. Sanit. Engng Res. 23, 211-217. Yamaguchi T., Moldrup P. and Yokosi S. (1989) Using breakthrough curves for parameter estimation in the convection-dispersion model of solute transport. Soil Sci. Soc. Am. J. 53, 1635-1641. Yamaguchi T., Ito S., Masumoto M. and Teranishi S. (1991) Nitrogen removal from secondary effluent in soil columns (in Japanese with English abstract, figures, and tables). Jap. J. Wat. Pol. Res. 14, 747-754. Yamaguchi T., Moldrup P., Rolston D. E. and Hansen J. A. (1992) A simple, inverse model for estimating nitrogen reaction rates from soil-column leaching experiments at steady water flow. Soil Sci. 154, 490-496. Yamaguchi T., Moldrup P., Teranishi S. and Rolston D. E. (1990) Denitrification in porous media during rapid, continuous leaching of synthetic wastewater at saturated water flow. J. environ. Qual. 19, 676-683. Yamaguchi T., Moldrup P., Ito S., Rolston D. E. and Teranishi S. (1994) Nitrogen removal from wastewater by rapid infiltration land treatment--an evaluation based on soil column studies. In Nutrient Removal from Wastewater (Edited by Horan N. J.). Technomic, Lancaster, Pa.
Pergamon
0043-1354(95)00206-5
Wat. Res. Vol. 30, No. 3, pp. 531-540, 1996 Elsevier ScienceLtd. Printed in Great Britain
NITRIFICATION IN POROUS MEDIA DURING RAPID, UNSATURATED WATER FLOW T. Y A M A G U C H I I*@, P. M O L D R U P : , D. E. R O L S T O N 3, S. ITO 4 and S. T E R A N I S H I 1 ~Department of Civil and Environmental Engineering, Faculty of Engineering, Hiroshima University, 1-4-I Kagamiyama, Higashi-Hiroshima 739, Japan, 2Environmental Engineering Laboratory, Department of Civil Engineering, Aalborg University, Sohngaardsholmsvej 57, DK-9000 Aalborg, Denmark, 3Soils and Biogeochemistry, Department of Land, Air and Water Resources, University of California, Davis, CA 95616, U.S.A. and 4Department of Applied Chemistry, Faculty of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima 739, Japan (First received October 1994; accepted in revised form August 1995) Abstraet--A substantial nitrification in rapid infiltration (RI) systems for wastewater treatment is a prerequisite for obtaining good N removal by denitrification. The purpose of this study is to investigate nitrification in porous media at conditions corresponding to RI treatment systems. Nitrification in six 50-cm porous media columns (98% weathered granite or sand and 2% field soil) during unsaturated leaching at constant flow rates of synthetic wastewater was investigated. Concentrations of NH4-N between 20 and 60 mg 1- l were applied and vertical concentration profiles of NO3-N, NO2-N and NH4-N were measured for 54 d at 30°C (three columns) and for 140 d at 10°C (three columns). A time lag in nitrification of 20 d was found at 10°C. Complete nitrification was obtained after 3-5 d at 30°C and after approximately 50 d at 10°C. Assuming first-order nitrification at steady-state, the corresponding first order reaction rate coefficients (kl) for NO 3 production in the columns were estimated to be between 0.4 and h -~ at 10°C and between 6 and 9 h -I at 30°C. Steady-state NO 3 profiles were obtained between 1.5 and up to 9 weeks after the experiments were started. At the actual soil-air contents (0.10 cm3 air phase cm -3 soil), oxygen limitations were not observed during the experiments. Nitrogen loadings (water flow times N concentration) above 100 mg N 1- t c m h -I (1 g N m -2 h - ' ) caused NH4 accumulation in the columns at 10°C and should probably be avoided during operation of RI system.
1
Key words--land treatment, wastewater, soil aquifer system, rapid infiltration, nitrogen removal, nitrification
INTRODUCTION Water reuse has become a central element in water resources planning in many areas of the world due to population growth, periodic droughts, and the increasing contamination of both surface and groundwaters (Asano, 1991). Soil-aquifer wastewater treatment, especially rapid infiltration (RI) of municipal wastewater into relatively permeable soils, is becoming an important treatment alternative as the need for water reuse increases and protection of groundwater resources becomes vital (Asano, 1985). Soil-aquifer treatment systems can offer many advantages compared to the conventional mechanical/chemical/biological wastewater treatment plants including lower cost and added storage capacity (Bouwer, 1991a), especially in countries where the economy and infrastructure make the use of high-cost conventional treatment non-feasible (e.g. Bennani et al., 1992).
*Author to whom all correspondence should be addressed. WR30:~--D
531
Most RI land treatment systems consist of coarsetextured porous media without plant cover and are operated at wastewater application rates between 15 and 350 mm d -1 (Iskandar, 1981). The most suitable soils are in the loamy sand, and fine sand range (USEPA/USAID, 1992). The RI plants normally contain a few percent finer-textured material (Bouwer, 1985) to promote transformation processes. The influent to RI plants is typically primary or secondary effluent with a total N concentration between 10 and 30 mg 1-~ but substantially larger values can occur (Iskandar, 1981; Bouwer, 1985). In the wastewater, a m m o n i u m is typically the principal form of nitrogen (Broadbent and Reisenauer, 1985). The quality improvement of the wastewater during RI treatment will mainly occur in the upper part of the vadose zone. For example, most of the nitrogen transformation in the Flushing Meadows RI plant (Phoenix, Arizona, U.S.A.) during 10 years of operation occurred in the upper 50 cm (Bouwer, 1991b). RI plants are normally operated analogously to a traditional municipal wastewater treatment plant, i.e. with alternating periods of wetting (application of
532
T. Yamaguchi et
wastewater) and drying (no application) to introduce sequential anaerobic and aerobic periods. During the wetting periods, however, both aerobic and anaerobic processes take place as the soil will never get completely water saturated, i.e. a certain soil-air phase will be present. Thus the nitrification process under rapid, unsaturated leaching needs to be understood to be able to optimize RI treatment plant design. A total nitrification of the ammonium during the aerobic periods is desirable to promote the denitrification process during subsequent anaerobic periods and for avoiding NH~ accumulation in the soil. If NH~ is accumulated in the soil during an aerobic period, it will reduce the amount of NH2 that can be adsorped during the subsequent anaerobic period, thus, increasing the NH4+ concentration in the renovated wastewater (Bouwer, 1985). Several studies on columns, lysimeters or full scale have been carried out to evaluate RI plant performance with respect to nitrogen removal (e.g. Lance and Whisler, 1976; Leach and Enfield, 1983; Crites, 1985; Bouwer, 1991b). However, these studies mainly dealt with temporal measurements for the influents and the effluents. To fully understand the influence of various soil and environmental parameters on RI system performance, it is necessary to follow both the temporal and spatial evolution of the inorganic N species under controlled circumstances. Yamaguchi et al. (1994) reviewed and discussed the results of 23 continuous leaching experiments at constant flow rates carried out by Yamaguchi and co-workers (1983, 1984, 1985, 1987, 1990, 1991, 1992) to investigate the temporal and spatial evolution of N removal in porous media columns at conditions corresponding to RI land treatment. They suggested the dispersive-convective nitrate flux (the hydrodynamic dispersion coefficient times the nitrate concentration) to be a potential key parameter in evaluating RI plant nitrogen removal efficiency. The studies all focused on the denitrification pro~ess during a near-water-saturated period (upward flow conditions). However, a substantial nitrification during a previous unsaturated period is a prerequisite for obtaining good N removal by denitrification. This study investigated nitrification in porous media columns at conditions corresponding to an unsaturated leaching period of a RI system where the inorganic nitrogen is present as NH4-N. The study focused on two objectives. The first was to measure the time until steady state was reached and determine possible time lags in nitrification by measuring evolution of NH4-N, NO2-N and NO3-N in time and depth at unsaturated leaching conditions. The second was to evaluate the influence of different parameters ( N H : N concentration in influent, oxygen, temperature and porous media composition) on estimated reaction rate coefficients using both the measured data and data reported in other studies. This nitrification study used the same porous media types and
al.
some of the same experimental conditions as were used in the denitrification study of Yamaguchi et al. (1990). THEORY
Nitrogen transport and transformation are traditionally described by the convection-dispersion equation (CDE) including a reaction term, equation
(1) Ri ~Ci
~2Ci
OCi
~t =D--~x2 - U ~ x +t~,, i= l, 2 . . . .
(1)
where C~ is the concentration of the N species (i) in the soil solution, Ri is a retardation factor (accounting for adsorption, anion exclusion etc.), D is the hydrodynamic dispersion coefficient (assumed constant for all i), u is the pore-water velocity (assumed constant for all i), ~i is the rate of transformation of the nitrogen species, x is distance within the soil, and t is time. The commonly accepted pathway for biological nitrification is NH4~NO2~NO3.
(2)
In this study, the concentration of NO2--N was insignificant during steady state. Hence, a single step reaction from NH4-N to NO3-N can be assumed for the nitrification process. Assuming first-order reaction for nitrification, the reaction terms of the simultaneous equation (1) with i = 1 and 2 are Ot = -k~'Cl and ¢P2=ki.C~, where - ~ is the amount of NH4-N oxidized per unit time, O2 is the amount of NO3-N produced per unit time, C1 is the NH4-N concentration, and kl is the first-order reaction rate coefficient for nitrification. The boundary conditions corresponding to the actual experimental conditions for the porous media columns are CI(X ) = C S CI(X) = 0
x = 0 x ----~oo
(3)
C2(x)=O x=O where Cs is the influent concentration of NH4-N, and C2 is the NO3-N concentration. At steady state, the retardation factor is assumed equal to R = 1, i.e. negligible adsorption of the influent NH4 is assumed after steady state is reached. The steady-state solution to the simultaneous equation (1) with R~ = R2 = 1, ~1 =-k~.C~ and ¢P2=k~.C~ under the boundary conditions of equation (3) is (Cho, 1971; Starr et al., 1974) Cl=exp Iux( 1Ca Cs
1
Cl Cs"
N~1 + 4Dkl'~l
(4)
Nitrification in porous media
If instead a zero-order reaction is assumed for nitrification, the corresponding reaction terms for N H 4 - N and NO3-N are of the form 0~ = - k 0 and 02 = k0, where k 0 is the zero-order reaction rate coefficient for nitrification. The steady-state solution to the simultaneous equation (1) with Rj = R 2 = 1, 0~ = - k o and • 2 = k 0 under the boundary conditions of equation (3) is C---!t= 1 k ° x uCs Cs - uCs' 0 <~x <<. k--o-
C2
Cs
1
(5)
Cj
Cs'
MATERIALS AND METHODS
Polyvinyl chloride pipes (20cm in diameter, 50cm in length) were used for the porous media columns (see Fig. 1). Sampling tubes were located at 5- to 10-cm intervals along the columns. Weathered granite passed through a 5-mm screen or sand was thoroughly mixed with cultivated soil passed through a 2.5-mm screen in the ratio of 98:2. The cultivated soil was taken from an agricultural field outside the city of Hiroshima and contained 41% clay and silt. The mixture was packed in six 50-cm long columns each with a porosity between 39.0 and 40.6% corresponding to between 25.5 and 26.2 kg porous media in each column. In Experiment 1, three concentrations of 20, 40, and 60 mg 1-~ of NH4-N were applied to three columns labeled N20, N40, and N60, respectively, at 30°C. Experiment 2 was conducted at 10°C and three concentrations of 20, 20, and 40mg 1-] of NH4-N were applied to three columns labeled NS20, NM20, and NM40, respectively. Column NS20 contained sand with cultivated soil while the remaining five columns contained weathered granite mixed with cultivated soil. The same porous media types as used by Yamaguchi et al. (1990) were used in the present study. The physical properties of the weathered granite and the sand are shown in Table 1. The chemical properties of the mixed porous media packed in the columns are shown in Table 2.
__Yl_ ~.~
:: 203 mm =:
f,0
T O
sol, so,,,,io° ~ : I L " : - I - -
oo, Po~
533
Table 1. Physical properties of the weatheredgranite and the sand used as part of the mixed porous media in the columns Maximum grain size Specific wt. (g cm 3)
Porous media Granite (Exp. I) Granite (Exp. 2) Sand
2.64 2.67 2.69
100% 60% 30% (mm)
10%
5.00 5.00 5.00
0.29 0.27 0.35
2.10 1.80 1.35
0.80 0.75 0.66
Synthetic wastewater (solution of NH4C1, NaHCO3, and KH2PO4) was supplied by a pump from the top of the soil columns (sprinkler system) creating unsaturated downward flow for 54 d in Experiment 1 and 140 d in Experiment 2. NaHCO 3 was added to the synthetic wastewater as an inorganic carbon (IC) source at an IC/N ratio of 2.5. KH2PO 4 was added to obtain a P/N ratio of 0.1. The wastewater application rate was kept below 150-170mm d -] to avoid initial leaching of high-NO3 pulses due to preferential flow in macro pores (Leach and Enfield, 1983). The volumetric soil-air content of the columns was 0. I0 cm 3 air cm -3 soil. The additional experimental conditions for the six leaching experiments are shown in Table 3. The solution collected from the sampling tubes every 2-4d was centrifuged at 3500 r.p.m, and the supernatant analyzed for NO3-N , NO2-N , and NH4-N. The effluent from the columns was analyzed for each species of inorganic nitrogen, pH, total nitrogen (TN), total organic carbon (TOC), inorganic carbon (IC), and electrical conductivity (EC). The mixed porous media in the columns was analyzed at different depth for TN and number of nitrifiers both before and immediately after the experiment. In Experiment I (30°C), concentrations of NO3-N, NO2-N, and NH4-N were determined by u.v. spectrophotometry (220nm), Nesslerization and the sulfuric acid ee-naphthylethylenediamine procedure, respectively. In Experiment 2 (10°C), HPLC was used for the analysis of NO3-N and NO2-N while NH4-N was measured as in Experiment 1. The most probable number (MPN) method (Alexander, 1982) was applied to estimate the population of nitrifiers. Total N, TOC, and IC were analyzed by a TOC-TN analyzer. Gas in the soil pores was taken out through the gas sampling ports of the columns, and was analyzed for 02, N2, CO 2, and N20 using a gas chromatograph. Adsorption isotherm experiments were performed at I0, 20, and 30°C using 46 g of weathered granite (oven dry weight) and 200 ml of ammonium chloride solution. The concentrations were 10, 20, 40, and 80 mg N 1-~, respectively. After 24 h of shaking, the NH4-N concentration of the filtrated soil solution was analyzed. Total and micro-pore surface areas of the decomposed granite were determined by the Ethylene Glycol Monoethyl Ether (EGME) method and by Mercury Intrusion Porosimetry, respectively (Carter et al., 1986). The hydrodynamic dispersion coefficient, D, and the pore-water velocity, u, were determined by continuous application of NaCI solution (2000 mg CI 1- t as chloride) to the top of the columns. The change in chloride concentration in the effluent was measured continuously by an EC meter at the bottom of the columns. Using the measured breakthrough curves, D and u were simultaneously estimated according to the method of Yamaguchi et al. (1989).
I~~.~,,~,O.!]icr~).-....?::. '-~.. :.~"-
r.: :,. -.:
E
.i g
o
t
20
I
30
~, E f f l u e n f
Table 2. Chemical properties of the mixed porous media in the columns Column
Fig. 1. Experimental set-up. Illustration of column set-up, wastewater application system, and sampling system used for the continuous leaching experiments.
N20, N40, N60 NS20 NM20, NM40
CEC (meq kg -I) 15 17 17
TN TP (mg kg -1)
TC
360 30 40
0.06 0.l 0. l
770 1000 560
(%)
TOC 0.05 <0.1 0. l
T. Yamaguchi et al.
534
Table 3. Experimental conditions during continuous unsaturated leaching of the porous media columns. The porous media of all columns were weathered granite mixed with cultivated soil except NS20 which was sand mixed with cultivated soil
Column N20 N40 N60 NS20 NM20 NM40
Influent NH4-N concentration (rag 1 i )
Application rate (ram d- t)
Pore-water velocity (cm h- t )
Dispersion coefficient (cm2 h t)
Temperature (°C)
20 40 60 20 20 40
150 150 150 150 150 150
2.9 2.8 2.9 3.2 3.8 3.3
7.0 6.3 7.2 3.1 7.2 7.2
30 30 30 10 10 10
RESULTS AND DISCUSSION
Evolution of the concentration profiles of N O 3 - N , NO2-N, and NH4-N in columns N60 (Exp. 1), NM20, and NM40 (both Exp. 2) is shown in Fig. 2.. The analogous profiles for columns N20 and N40 (both Exp. 1) are omitted, because the general trends in the profiles were similar to those shown for N60. Also, the profiles for column NS20 are omitted, because the general trends in the profiles were similar to those shown for column NM20.
Ammonia-nitrogen, nitrite-nitrogen, nitrate-nitrogen, and total-nitrogen In Experiment 1 (30°C) during the first 3 to 4 d, the relative concentrations of NH4-N decreased to between 0 and 0.08 within the upper 5- to 10-cm of column depth in all three columns (N20, N40, and N60). Since only small amounts of NO3-N or NO2-N were detected during the first 3 to 4 d, the decrease in NH4-N was caused by adsorption to soil. The NO3-N production started to occur from Day 3 and was promoted rapidly. The relative concentrations of NO3-N (relative to the influent NH4-N concentration, Cs) at Day 5 exceeded 1.0 (between 1.0 and 1.6) below 10-cm depth in all three columns (cf. Fig. 2, column N60). These high concentrations of NO~-N were likely the result of nitrification of the N H ~ N which had accumulated during the first 3-4 d. After Day 5 in Experiment 1, the relative concentration of NO~-N gradually decreased to 1.0, and steady-state nitrification was obtained between Day 10o15. At steady state, complete nitrification was obtained within the upper 5- to 10-cm depth. Significant concentrations of NO2-N were detected only between Day 3 and 5 as shown in Fig. 2 (column Nt0), and were scarcely detected thereafter (C/Cs < 0.01). In Experiment 2 (10°C) during the first 10020 d, the relative concentrations of NH4-N decreased to between 0 and 0.1 within the upper 20-cm depth in all three columns (NS20, NM20, NM40). Again, since only small amounts of NO3-N and NO2-N were detected within the first 10020d, the decrease in NH4-N was caused by adsorption to the soil. The large adsorption was partly due to the 2% finely textured, agricultural soil in the columns, but, as illustrated in Fig. 3, the weathered granite also had a
significant ammonium adsorption capacity. As expected, Fig. 3 shows a slight decrease in adsorption at the high temperature (30°C) compared to the lower temperatures (10020°C) because of higher molecular activity. In columns NM20 and NM40, the concentration of N O 3 - N started to increase slowly after Day 15-20. In column NS20 (sand), nitrification started to occur at Day 8-10, which is 5-10 d earlier compared to columns NM20 and NM40 (weathered granite). After approximately 4-7 weeks (wk) in Experiment 2 (4 wk for column NS20, 7 wk for columns NM20 and NM40), the decrease of NH4-N within the top part of the columns stopped temporarily and nitrification was strongly promoted as shown at Day 48 in Fig. 2 (columns NM20 and NM40). This phenomenon can be explained by a temporary saturation of the ammonium adsorption capacity of soil. This is in agreement with the observation by Broadbent and Reisenauer (1985) for slow infiltration systems that applications of wastewater substantially higher than 2.5-10cm wk -1 can result in saturation of the ammonium retention capacity of the soil. Relative concentrations of NO3-N of more than 1.0 (between 1.0 and 1.6) were detected below 20- to 30-cm depth between Day 52 and 68 in columns NM20 and NM40, and between Day 31 and 41 in column NS20. Compared to Experiment 1 (30°C) where this was observed at Day 5, a significant lag time was therefore introduced at 10°C. After Day 60 in Experiment 2, the relative concentration of N O 3 - N gradually decreased to 1.0, and steady-state nitrification was obtained after around 9wk. At steady state, complete nitrification was obtained within the upper 15- to 20-cm of column depth. Thus it took twice the column length to obtain complete nitrification at 10°C compared to 30°C. Figure 4 shows the amount of NO3-N produced as a function of time and depth for columns N20 (30°C) and NM40 (10°C), both representing the general trends found at 30 and 10°C, respectively. As can be seen from Fig. 4, a time lag in nitrification of approx. 20 d was found at 10°C, and furthermore, it took around 8 wk before nitrification had reached the maximum level. No time lag in nitrification was evident at 30°C. The relative concentration of N O : - N was very low (<0.06) throughout Experiment 2. Significant increases in TN in the columns were observed during
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Fig. 2. Temporal and spatial evolution of NO3-N (*), NO2-N (A), and NH4-N ( 0 ) profiles in unsaturated columns of porous media at chosen days (t) during unsaturated leaching experiments. For experimental conditions, see Table 3. Cs is the influent NH4-N concentration. The temperature was 30°C (N60) and 10°C (NM20 and NM40), respectively. 535
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through the water film of a second or less was calculated. Thus, even with a locally much thicker water film, it is unlikely that the nitrification performance in the columns were 02 diffusion limited. In Experiment 1 (30°C) a temporary decrease in 02 concentration was observed in all three columns during the first 4--6 d corresponding to the period with the largest nitrate production (cf. Fig. 4, column N20). The most significant decrease (to 15%) took place in the column with the highest nitrogen loading (column N60), see Fig. 5. In general the 02 concentration was close to 20% in all six columns. Hence both the theoretical considerations concerning film diffusion and the constantly high 02 concentrations measured in the soil air phase at various depths in the columns indicated that 02 was not the limiting parameter of the nitrification in the present experiments that were carried out at a volumetric soil-air content of approx. 0.10cm 3 air phase cm -3 porous media.
the period of the experiments confirming that NH~ adsorption was taking place. Column NM40 showed the most distinct increase in TN (7300 mg during 140d compared to 4200mg for column NM20). Hence, the low temperature (10°C) and the high NH4-N loading for column NM40 resulted in a non-optimal nitrification and a larger NH~ accumulation.
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In Experiment 1 (30°C), the percentages of O2, N2, and CO2 in the soil pores in the columns were 15-21%, 74-84%, and 1-3%, respectively (16 measurements). The partial pressure of 02 in column N60 was a little lower than in columns N20 and N40. This is caused by the high concentration of NH4-N applied to column N60. No significant differences in the composition of these gases between the upper, middle, and lower layers of the soil column were observed. In Experiment 2 (10°C), the percentages of 02, N 2, and CO2 in the soil pores were 16--21%, 76-86%, and 0-5%, respectively (15 measurements). N 2 0 gas was not detected during Experiments 1 and 2. To evaluate possible O2 diffusion limitations, a mean water film thickness was estimated from the surface area measurements. The external surface area was estimated to approx. 10 m 2 g-i dry matter (DM) equal to the difference between the total surface area (13.3 m 2 g-I DM; measured by the EGME method) and the micro-pore surface area (3.1 m 2 g-~ DM; measured by mercury intrusion porosimetry). With a volumetric water content of 0.30cm 3 H20 cm -3 porous media and a bulk density of 1.6 g DM cm -3 the estimated, mean thickness of the water film equals 0.02/~m. Using a numerical solution to one-dimensional gas diffusion [solution of Moldrup et al. (1992) with u = 0 and the 02 diffusion coefficient in pure water at 10 or 30°C], an 02 diffusion time
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Number of nitrifiers and pH The distribution of nitrifiers in the columns is shown in Table 4 as number of ammonium- and nitrite-oxidizing bacteria (most probable number) for 1 g of dry porous media. After Experiment 1 (30°C), the numbers of both ammonium-oxidizing bacteria and nitrite-oxidizing bacteria were significantly higher at 0- to 5-cm depth, slightly higher at 5- to 15-cm depth, and almost the same at 35- to 45-cm depth compared to the initial values. This is because nitrification mainly occurred within the first 0- to 10-cm depth and, subsequently, there was almost no supply of NH4-N below 10-cm
depth (cf. Fig. 2, column N60) due to the extensive build-up of nitrifiers in the upper few cm. After Experiment 2 (10°C), the numbers of ammonium-oxidizing bacteria were significantly higher at 0- to 5-cm and 5- to 15-cm depth than the initial values, and slightly higher at 15- to 25-cm and 35- to 45-cm depth compared to the initial values. This is probably because NH4-N was able to leach deeply into the columns between Day 15 and 20 and between Day 40 and 50 in Experiment 2 (cf. Fig. 2, columns NM20 and NM40). After Experiment 2, the numbers of nitrite-oxidizing bacteria were almost the same throughout the columns compared with the initial values. This trend is different from Experiment 1 (30°C), since the growth of the nitrifying bacteria is likely inhibited at 10°C. In Experiment 1, the pH of the applied synthetic wastewater and the column effluents was between 8.2 and 8.6 and between 7.5 and 8.0, respectively. In Experiment 2, the pH of the wastewater and the effluents was between 8.3 and 8.6 and between 6.8 and 8.0, respectively. Hence, the pH of the effluents was generally lower than that of the applied wastewater because of the hydrogen ion production during the nitrification process.
Estimated reaction rate coefficients To be able to compare the results of the present experiments with earlier soil column studies, the reaction rate coefficients for nitrification were estimated from the measured steady-state concentration profiles of NH4-N and NO3-N, assuming first-order and zero-order reaction rates, respectively. The estimation was performed by curve-fitting the measured NH4-N and NO3-N profiles at steady state against the theoretical solutions [equations (4) and (5)]. The
Table 4. Number of nitrifiers (most probably number, MPN) in the porous media columns before (t = 0 d ) and after (t = 5 4 d and t ~ 140d, respectively) continuous unsaturated leaching. For experimental conditions, see Table 3 Number of nitrifying bacteria MPN/g dry porous media Soil column
Temperature (°C)
(A) Ammonium-oxidizing bacteria N20, N40, N60 30 N20 30 N40 30 N60 30 NS20 10 NS20 10 NM20, NM40 10 NM20 10 NM40 10 (B) Nitrite-oxidizing bacteria N20, N40, N60 30 N20 30 N40 30 N60 30 NS20 10 NS20 10 NM20, NM40 10 NM20 10 NM40 10 =Not measured.
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estimation results for the 6 column experiments performed in the present study are shown in Table 5 (lower part). The rate coefficients for Experiment i (30°C) were each estimated from only one data point (5-cm depth) of the steady-state NH4-N profiles because of the rapid nitrification occurring at this high temperature. Hence, the calculation of the corresponding values of k 0 and k~ is sensitive to any experimental error associated with only one point. All three columns, however, showed the same trend during evolution of the inorganic N profiles• In Experiment 1 (30°C), the estimated values of k~ and k 0 were between 6.0 and 8.8 h -1, and between 11 and 34rag 1-~ h -~, respectively. An increasing k0 value corresponding to a higher net nitrification was obtained for an increasing influent concentration of NH4-N. The minimum kt value was estimated for an influent NH4-N concentration of 40 mg 1- ~ h - ~. The difference between the k~ values, however, is not significant recalling the estimation error associated with only one point as discussed above• For Experiment 2 (10°C), no significant difference in k0 values was observed for the three columns. The minimum value of kl (0.4h -~) was obtained for column NM40, the latter representing the largest concentration of NH4-N applied (40 mg N 1-'). Hence, the nitrification efficiency was not optimal at the highest NH4-N loading at 10°C. The effect of temperature (10°C vs 30°C) on the nitrification rate coefficients is apparent from Table 5. The values ofk~ at 30°C are 9 to 15 times larger than at 10°C, while the values of k0 at 30°C are 2 to 4 times larger than at 10°C (see Table 5). No significant effect of porous media composition was observed• The values of k 0 are at the same level when using sand (column NS20) compared to using weathered granite (NM20). In general, a zero-order nitrification rate seemed a slightly better assumption than a first-order rate for Experiment 2 (10°C) cf. the curve-fitted concentration profiles shown in Fig. 6. Comparison o f reported reaction rate coefficients
The estimated values of reaction rate coefficients and the experimental conditions for nitrification of previous studies by Starr et al. (1974), Misra et al• (1974), and Ardakani et al. (1974a, b) are shown in Table 5 (upper part). The values of rate coefficients for the experiment by Ardakani et al. (1974a, b) were estimated by the present authors assuming first- and zero-order nitrification, respectively. In the study of Ardakani et al. (1974a) and the present study, the pore-water velocities corresponded to RI systems, while the pore-water velocities of Starr et al. (1974) and Misra et al. (1974) corresponded to slow infiltration systems. The RI experiments gave nitrification rate coefficients that were two orders of magnitude higher than for the slow infiltration experiments (Table 5). In the field study of Ardakani et al. (1974b) the values of both u and kl showed to
539
Nitrification in porous media NM20
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a higher degree of oxygen transport in the gaseous phase during the initial periods of wastewater infiltration. This is probably best done by adjusting the wastewater infiltration rate so a minimum, volumetric soil-air content of 0.1 cm 3 cm -3 is present, especially in the topsoil to avoid putting a "lid" on the oxygen diffusion into the soil. Also, a wastewater recycling system where secondary effluent wastewater with high ammonia content is infiltrated at a relative low flow velocity to obtain rapid nitrification after which recycled wastewater with high nitrate content is infiltrated at a higher flow velocity to obtain anaerobic conditions (denitrification) may be a solution for obtaining a larger degree of nitrogen removal in RI treatment plants. CONCLUSIONS
o25
Relative Concentration ( C / C z )
Fig. 6. Curvefitted steady-state NO3-N (*) and NH4-N ( 0 ) profiles for unsaturated columns of porous media (see Table 3 for experimental conditions) using equations (4) and (5) assuming first-order ( ..... ) and zero-order ( - - - ) nitrification, respectively. The estimated values for the corresponding reaction rate coefficients are given in Table 5. Cs is the influent NH4-N concentration. The temperature was 10°C for all three columns.
be in between those obtained for slow and rapid infiltration. The extremely high NH4-N loading in the RI experiment of Ardakani et al. (1974a) equal to 5.2 cm h-~ of water velocity times an influent NH4-N concentration of 75 mg N 1-j resulted in a higher N application than the microorganisms were able to use optimally, and subsequently, the k I value was significantly smaller compared to the present RI experiments. The nitrification studies in Table 5 were carried out with artificial ammonia-nitrogen wastewater and at a volumetric soil-air content/>0.1 cm 3 cm -3, Based on the results of the present study, oxygen limitations are not likely to have occurred at these high soil-air contents. Thus the nitrification rates in Table 5 should probably be considered potential rates i.e. based on optimal oxygen supply in the porous media. In the real treatment situation with both biodegradable organic matter and ammonia in the wastewater and smaller or larger degree of heterogeneity in the soil system, there may be competition between heterotrophs and autotrophs (nitrifiers) in the biofilm surrounding the soil particles leading to local oxygen limitations. Also, clogging problems and channeling may occur resulting in a non-homogeneous distribution of oxygen to the soil matrix. To obtain a high nitrification rate, it therefore seems vital to maintain
In the rapid infiltration (RI) column studies, steady-state nitrification was obtained from 9 wk at 10°C and from 1.5 wk at 30°C, i.e. a relatively long time period compared with the normally used drying sequences in RI treatment systems (5-16 d for most of the presently operating plants cf. Smith, 1991). Hence, a steady-state approach can in some cases provide a rough estimate of the net nitrification (from the k0 value) and the nitrification efficiency (from the kl value), especially at high temperature, but may not be adequate to describe the inorganic N transformations in RI treatment systems. This is the case for both nitrification during unsaturated periods (this study) and for denitrification during subsequent water-saturated periods (Yamaguchi et al., 1990). A low temperature (10°C) in combination with a high N loading (u. Cs > 100 mg N 1-l cm h -j = 1 g N m -2 h -~) resulted in a non-optimal nitrification and should probably be avoided during RI plant operation. Oxygen limitations or effects of the coarse porous media composition were not observed in this study. If the water application rate in RI systems can be adjusted so that a sufficiently large soil-air phase /> 0.10 cm 3 air cm -3 soil is present, the present experiments suggest that O: diffusion in the gaseous phase will not become the limiting factor for nitrification. This study may suggest a potential for using RI porous media reactors for nitrification of NH4 in for example refinery wastewater with typical NH4 concentrations between 20 and 80 mg N 1- i. The present RI results, i.e. full conversion of NH4 to NO3 within 3-5 d at 30°C, compare favorably with the achievements of activated sludge or rotating biological contact systems (cf. Fang et al., 1993). Also, the present data together with the data from Yamaguchi et al. (1994) suggest that even shallow RI porous media systems with alternating downward (nitrification) and upward (denitrification) flow could provide complete nitrogen removal from wastewater if operated at the correct flow velocities and nitrogen loadings. This needs to be tested at both laboratory and field scale.
T. Yamaguchi et al.
540
Acknowledgements--Mr Y. Harada from AMCO, Japan, is gratefully acknowledged for the mercury intrusion porosimetry measurements. A travel grant from the Japanese Ministry of Education (Monbusho International Scientific Research Program: Joint Research, No. 06044158) has greatly enhanced the work and cooperation of the authors and is hereby gratefully acknowledged. This study was supported by the Danish Center for Root Zone Processes under the Danish Environmental Research Programme.
REFERENCES Alexander M. (1982) Most probable number method for microbial populations. In Methods of Soil Analysis (Edited by Page A. L. et aL), Part 2, 2nd edn, pp. 815-820. Agron. Mongr. 9. ASA and SSSA, Madison, Wisc. Ardakani M. S., Rehbock J. T. and Mclaren A. D. (1974a) Oxidation of ammonium to nitrate in a soil column. Soil Sci. Soc. Am. Proc. 38, 96-99. Ardakani M. S., Schulz R. K. and McLaren A. D. (1974b) A kinetic study of ammonium and nitrate oxidation in a soil field plot. Soil Sci. Soc. Am. Proc. 38, 273-277. Asano T. (1985) Overview: Artificial Recharge of Groundwater. In Artificial Recharge of Groundwater (Edited by Asano T.), pp. 3-19. Butterworth, Boston, Mass. Asano T. (1991) Wastewater reclamation and reuse. In Wastewater Engineering: Treatment, Disposal and Reuse (Edited by Tchobanoglous G. and Burton F. L.) 3rd edn, Chap. 16, pp. 1137-1193. McGraw-Hill, New York. Bennani A. C., Lary J., Nrhira A., Razouki L., Bize J. and Nivault N. (1992) Wastewater treatment of Greater Agadir (Morocco): an original solution for protecting the Bay of Agadir by using the dune sands. Wat. Sci. Technol. 25, 239-245. Bouwer H. (1985) Renovation of wastewater with rapid infiltration land treatment systems. In Artificial Recharge of Groundwater (Edited by Asano T.), pp. 249-282. Butterworth, Boston, Mass. Bouwer H. (1991a) Role of soil-aquifer treatment in water reuse. Proc. Spec. Conf. Environ. Eng., Am. Soc. Cir. Eng., p. 738. Bouwer H. (1991b) Groundwater recharge with sewage effluent. Wat. Sci. Technol. 23, 2099-2108. Broadbent F. E. and Reisenauer H. M. (1985) Fate of wastewater constituents in soil and groundwater: nitrogen and phosphorus. In Irrigation with Reclaimed Municipal Wastewater--A Guidance Manual (Edited by Pettygrove G. S. and Asano T.), Chap. 12, pp. 12-1 to 12-16. Lewis, Chelsea, Mich. Carter D. L., Mortland M. M. and Kemper W. D. (1986) Specific surface area. In Methods o f Soil Analysis, Part 1, Physical and Mineralogical Methods (Edited by Klute A.), pp. 413-423. Agron. Mongr. No. 9 (2nd edn). ASA, SSSA, Madison, Wisc. Cho C. M. (1971) Convective transport of ammonium with nitrification in soil, Can. J. Soil Sci. 51, 339-350. Crites R. W. (1985) Nitrogen removal in rapid infiltration systems. J. environ. Engng 111, 865-873. Fang H.-Y., Chou M.-S. and Huang C.-W. (1993) Nitrification of ammonia-nitrogen in refinery wastewater. Wat, Res. 27, 1761-1765.
Iskandar I. K. (1981) Introduction. In Modeling Wastewater Renovation: Land Treatment (Edited by Iskandar I. K.), pp. 3-19. Wiley, New York. Lance J. C. and Whisler F. D. (1976) Stimulation of denitrification in soil columns by adding organic carbon to wastewater. J. Wat. Pollut. Control Fed. 48, 346-356. Leach L. E. and Enfield C. G. (1983) Nitrogen control in domestic wastewater rapid infltration systems. J. Wat. Pollut. Control Fed. 55, 1150-1157. Misra C., Nielsen D. R. and Biggar J. W. (1974) Nitrogen transformation in soil during leaching: II. Steady-state nitrification and nitrate reduction. Soil Sci. Soc. Am. Proc. 38, 294-299. Moldrup P., Yamaguchi T., Hansen J. Aa. and Rolston D. E. (1992) An accurate and numerically stable model for one-dimensional solute transport in soils. Soil Sci. 153, 261-273. Smith R. G. (1991) Natural treatment systems. In Wastewater Engineering: Treatment, Disposal and Reuse (Edited by Tchobanoglous G. and Burton F. L.) 3rd edn, Chap. 13, pp. 927-1016. McGraw-Hill, New York. Starr J. L., Broadbent F. E. and Nielsen D. R. (1974) Nitrogen transformation during continuous leaching. Soil Sci. Soc. Am. Proc. 38, 283-289. U.S. Environmental Protection Agency and U.S. Agency for International Development (1992) Guidelines for Water Reuse. EPA/625/R-92/004. Yamaguchi T. and Teranishi S. (1983) Denitrification of wastewater in land treatment. (in Japanese with English abstract.) J. Jap. Sewage Wks Assoc. 20, 28-35. Yamaguchi T. and Teranishi S. (1984) Denitrification in soil under continuous leaching (in Japanese with English abstract, figures, and tables). Proc. environ. Sanit. Engng Res. 20, 223-229. Yamaguchi T. and Teranishi S. (1985) Denitrification in soil columns under saturated flow (in Japanese with English abstract) J. Jap. Sewage Wks Assoc. 22, 55-59. Yamaguchi T. and Teranishi S. (1987) Denitrification during unsaturated filtration in soil columns (in Japanese with English abstract, figures, and tables). Proc. environ. Sanit. Engng Res. 23, 211-217. Yamaguchi T., Moldrup P. and Yokosi S. (1989) Using breakthrough curves for parameter estimation in the convection-dispersion model of solute transport. Soil Sci. Soc. Am. J. 53, 1635-1641. Yamaguchi T., Ito S., Masumoto M. and Teranishi S. (1991) Nitrogen removal from secondary effluent in soil columns (in Japanese with English abstract, figures, and tables). Jap. J. Wat. Pol. Res. 14, 747-754. Yamaguchi T., Moldrup P., Rolston D. E. and Hansen J. A. (1992) A simple, inverse model for estimating nitrogen reaction rates from soil-column leaching experiments at steady water flow. Soil Sci. 154, 490-496. Yamaguchi T., Moldrup P., Teranishi S. and Rolston D. E. (1990) Denitrification in porous media during rapid, continuous leaching of synthetic wastewater at saturated water flow. J. environ. Qual. 19, 676-683. Yamaguchi T., Moldrup P., Ito S., Rolston D. E. and Teranishi S. (1994) Nitrogen removal from wastewater by rapid infiltration land treatment--an evaluation based on soil column studies. In Nutrient Removal from Wastewater (Edited by Horan N. J.). Technomic, Lancaster, Pa.