A BENCH-SCALE ROCK-PLANT FILTER INVESTIGATION DONNA SKIPPER W.S. Neef & Associates, Inc., Lafayette, Louisiana, U.S.A. and MARTY TITTLEBAUM Department of Civil Engineering, Louisiana State UniversiO; Baton Rouge, Louisiana, U.S.A. (Received: March 1992; revised: December 1992)
Abstract. Three bench-scale rock filters containing 0.6 m of gravel were used in this investigation. Two of the filters were planted with Sagittaria Iancifolia and Scirpus validus, while the third filter was an unvegetated control filter. The wastewater directed through these systems was a synthetic mixture containing nutrient broth as the carbon source. An 80-day experiment was run on the filters using eight combinations of two flow rates and four influent biochemical oxygen demand (BODs) concentrations, each combination remaining constant for ten days. These combinations resulted in BOD5 surface loadings from 4.63-30.96 g/day/mz. From other studies of batch and continuous flow rock-plant filter systems, the first-order BOD5 reaction rate constant was tbund to decrease exponentially with detention time. In this study, however, little correlation was found between BOD5 loading rate and removal percentages, which averaged 69%, 57%, and 47% for the Scirpus, Sagittaria, and control systems, respectively. This is probably due to the relatively small change in detention times studied and the short time period of each loading rate. Oxidation/reduction potential (ORP) and dissolved oxygen (DO) measurements within these systems indicated that no free oxygen was available at any depth. Total Kjeldahl nitrogen (TKN) removal was higher in the plant systems relative to the control, with the Scirpus system achieving a higher overall removal than the Sagittaria system.
1.
Introduction
Due to their low operation and maintenance costs, rock-plant filter (RPF) systems are increasing in popularity, especially in the southeastern United States. Several studies have determined the effects o f various plants (Wolverton et al., 1983; Gersberg et al., 1986; Burgoon et al., 1991) or attempted to establish RPF design criteria (Trautmann et al., 1989; Lawson, 1985; Findlater et al., 1990; B a v o r et al., 1989). These studies have produced conflicting data, either because o f the different detention times used or the fact that some have been batch studies while the others were continuous flow studies. In addition, secondary or primary wastewater was used in these studies, meaning that the specific wastewater content variable between studies and, possibly, within a study. The research reported here was conducted in continuous flow systems using carefully controlled synthetic wastewater. T h e objectives o f this study were: (1) to compare the effect o f the emergent aquatic plants (Scirpus validus and Sagittaria lancifolia) on organic and nitrogen removal rates; (2) to determine whether there is a relationship between organic removal and oxidation reduction potential (ORP); and (3) to assess the o x y g e n content within vegetated and non-vegetated filters. Environmental Monitoring and Assessment 27: 69-80, 1993, (~) 1993 Kluwer Academic Publishers. Printed in the Netherlands.
70
DONNA SKIPPER AND MARTY TITTLEBAUM
2.
Related Research
Since the 1970s, research on rock-plant filters has been conducted using various types of aquatic plants, retention times, and wastewater sources. Removal rates observed have been varied and sometimes inconsistent, however, as discussed below. In one of the earlier studies, Spanler et al. (1976) planted bulrush (Scirpus validus) in flow-through trenches which were PVC lined and gravel filled to study the effect of retention time, nature of wastewater applied (primary or secondary treatment effluent), and frequency of harvesting on effluent quality. The retention times they used (5 hours, 16 hours, and 10 days) yielded equivalent biochemical oxygen demand (BODs) reductions for both control and bulrush basins. It was noted, however, that flow regulation was poor in the control basin and higher loadings were actually applied to the bulrush basins as compared to the control. For primary effluent, BOD5 reduction was nearly 10% greater when compared to secondary effluent. Furthermore, harvesting resulted in no observed changes in effluent quality. Contrary to Spangler et al. 's observations, a study by Wolverton (1982) indicated a significant difference in BOD5 removal between an unvegetated filter and a filter containing reed (Phragmites communis). BOD5 removal rates for the control filter were 62% and 83% after 6 and 24 hours retention time, respectively; the rockreed filters achieved 87% and 96% BOD5 removal for the same retention times. In addition, nutrient removals were an order of magnitude higher in Wolverton's rock-reed filter relative to his control. Relatively equal levels of BOD5 removals in vegetated and unvegetated filters were, however, observed by Wolverton et al. (1983) in a later study. This batch experiment began by first setting raw sewage anaerobically for 24 hours, then delivering the effluent to various rock filters, each containing a different aquatic emergent plant plus an unvegetated control. Plants used include reeds (Phragmites communis), cattails (Typha latifolia), rush (Juncus effusus), and bamboo (Bambusa multiplex). The filter influent BOD5 was adjusted to approximately 60 mg/1 and 300 mg/1 on separate occasions by adding different amounts of water hyacinth juice to the raw sewage. The results of this study indicated that retention times of 6 and 29 hours were required to achieve the 30 mg/1 BOD5 secondary treatment standard for the 60 mg/1 and 300 mg/l influent BOD5 loading, respectively. Although the BOD5 removal in the vegetated filters was comparable to the control, the system containing reeds performed slightly better than the other systems. In a later study, Gersberg et al. (1986) treated primary wastewater in four 0.76 m deep gravel ditches containing bulrush (Scirpus validus), reed (Phragmites communis), and cattail (Typha latifolia), plus an unvegetated control. Parameters monitored during this study included ammonia, BODs, and total suspended solids (TSS). Ammonia removals were found to be 94%, 78%, and 28% for bulrush, reed, and cattail, respectively. BOD5 removals for bulrush, reed, cattail, and the control
A BENCH-SCALE ROCK-PLANT FILTER INVESTIGATION
71
were 96%, 81%, 74%, and 69%, respectively. Essentially equal TSS removal was observed in all ditches and was, therefore, assumed to be an exclusively physical process. In addition, the depths of root zones were recorded for each species. Cattail was found to have the shallowest root zone: approximately 12" (30 cm). The bulrush root zone extended to a depth of over 24" (60 cm); while the root zone of reed averaged around 30" (76 cm). In a more recent study, Burgoon et al. (1991) found equal BOD5 removal in nonvegetated, batch-fed systems as compared to batch-fed systems containing Sagittaria latifolia, Scirpus pungens, Phragmites austral&, or Typha latifolia. Nitrogen removal, however, was found to be enhanced in the vegetated systems versus the unvegetated systems. The wastewater used in this study originated from secondary domestic wastewater and ranged between 2 and 24 g BODs/(m 2 day) and between 0.6 to 4.4 g nitrogen/(m 2 day). One reported difference in the above studies is detention time. Influent and effluent BOD5 concentrations are theoretically tied to the detention time through the first-order reaction rate constant for BOD5 removal, K, as follows:
K = {ln(Co/Ce)}/t where: Co = influent BOD5 concentration, mg/1; Ce = effluent BOD5 concentration, mg/1; and t = detention time, days. A tabulation of the K values versus detention times for the projects described above are listed in Table I. These data reveal an exponential decay relationship between K and detention time. Therefore the reaction rate for BOD5 degradation in an RPF system depends on the detention time used, according to the studies discussed above. In other words, most of the reaction apparently takes place in a short period of time (or within the first few feet of the filter). This explains the discrepancy reported by Burgoon et al. (1991) between continuous and batch flow systems. 3.
Methods and Procedures
Three rectangular, welded-aluminum tanks (1.5 m long x 0.45 m wide) housed the filter systems. Each tank had an interior polyurethane coating and was constructed as shown in Figure 1. The influent flow entered near the bottom of one end of each tank through a perforated PVC pipe spanning the width of the tank while the effluent spilled into a standpipe at the opposite end of the tank. Oxidation/reduction potential (ORP) probes were placed near the mid-tank access ports at mid-depth. These tanks were installed in a greenhouse which supplied an equal amount of sunlight to all tanks. The water temperature of all three tanks remained within I°C of each other and varied from 21°C to 31°C during this study. On November 19, one tank was planted with ten Sagittaria lancifolia (duck potato) seedlings and another tank with ten Scirpus validus (bulrush) seedlings. These plants grew for five months prior to the commencement of the analyses.
72
DONNA SKIPPERAND MARTYTITTLEBAUM TABLE I First-order BaD5 rate constants vs. detention time. Time (h)
Control
Bulrush
5 5 6 12 24 24 48 144 240
5.96 5.51 3.87 4.14 2.29 1.77 1.51 0.2 0.16
9.41 8.19 8.16 4.35 2.49 3.22 1.57 0.54 0.19
/ /•lnfluent Air Release
6" Pea Grovel 216 It
- - ~
°
Reed
Reference Spangler et el. (1976) Burgoon e t a L (1991) Wolverton (1982) Burgoon et aL (1991) Burgoon et el. (1991) Wolverton (1982) Burgoon et el. (1991) Gersberg et el. (1986) Spangler et el. (1976)
~,..~-Mid-Tank Access Ports j
,..//
/
/
/-Effluent
/
StandPipe
° Sampling Port
I' - 6" I"x 3" Gravel
6,if// Fig. 1. Rock-aquatic plant filter systems, inner tank details. (Not to Scale.)
During this five months, a solution containing Hydrosol (a hydroponic fertilizer by Peters), calcium nitrate (CaNO3), and dextrose was recirculated through each tank at a flow o f 500 to 1000 ml/min. This solution was replenished twice per week, with daily dextrose additions. On January 25, and then again on March 8, the tanks were seeded with activated sludge mixed liquor. After the five month start-up period, the flow through the tanks was changed to once-through flow using a synthetic wastewater containing constituents listed in Table 2. Although the use of a synthetic wastewater creates a somewhat unre-
A BENCH-SCALE ROCK-PLANT FILTER INVESTIGATION
73
TABLE II Synthetic wastewater recipe. Constituent MgSO4 • 7H20 FeCI3 • 6H20 MnSO4. H20 K2HPO4 KHzPO4 CaCO3 BOD5
Concentration (rng/1) 300 14 5 120 30 14 28, 78, 107, 51
alistic situation, it allows the effluent to vary while the influent is controlled. The wastewater recipe shown in Table II is the result of several trial mixtures. Initial mixtures contained glucose and ammonium sulfate as carbon and nitrogen sources, respectively. This was changed when most of the BOD5 was consumed by residual microorganisms in the reservoir before entering the filters. Nutrient broth, a more complex and less readily degraded carbon source, corrected this situation satisfactorily; however, the nitrogen source in nutrient broth is solely organic, which is found in primary wastewater but not usually in secondary wastewater, where ammonia is the typical nitrogen form. This restricted the comparability of nitrogen removal rates between this study and other studies. To feed each filter with synthetic wastewater, separate chemical feed pumps (Cole Parmer Model N-07141-28) distributed wastewater from a common reservoir to each filter. This wastewater was freshly mixed daily. Initially, the goal was to arrive at a steady percentage of organic removal before stepping to a different hydraulic/organic loading scheme. No steady removal rate was found, however, after over a month at a flow of 80 ml/min and an organic loading of 4.96 g BODs/day/m 2 surface area (30 mg/l BOD5 influent). When steady-state operation was not obtained within a feasible time-frame, an alternative monitoring program was contrived. This program involved randomly changing the influent BOD5 concentration in consecutive ten-day periods at two flow rates. The four influent BOD5 concentrations (28, 78, 107, and 51 mg/l) were first applied under a flow of 80 ml/min (24-hour theoretical retention time), which resulted in BOD5 surface loadings of 4.63, 12.90, 17.69, and 8.43 g/day/m2. The influent BOD5 concentrations listed above were then repeated under a 140 ml/min flow (42-hour theoretical retention times), giving BOD5 surface loadings of 8.10, 22.57, 30.96, and 14/76 g/day/m2. During each ten-day period, the effluent from each filter and an influent composite were analyzed on days 4, 8, 9, and 10 for BODs, total Kjeldahl nitrogen (TKN), and ammonia; and on days 2, 4, 6, 8, 9, and 10 for chemical oxygen demand (COD). Only random nitrate analyses were
74
DONNA SKIPPER AND MARTY TITTLEBAUM
performed since negligible amounts of this compound were measured during the preliminary portion of this study. Daily measurements of water temperature, pH, and ORP were also obtained in each tank. The influent pH was adjusted daily to between 7.0 and 7.5 by addition of sulfuric acid. Ambient air temperature and influent and effluent flow measurements were also recorded daily. Above-grade dry plant matter weight and TKN plant mass were analyzed at the end of every 10-day period for each vegetated filter. Many of the BOD5 removal measurements were calculated through COD values due to the greater ease, speed, and accuracy of the COD analysis in comparison with the BOD5 analysis. Ordinarily this relationship is not reliably consistent; however, with a controlled wastewater, such as the one used in this study, the ratio of these parameters is relatively constant. Measurements of both were performed here to verify this relationship. COD measurements were facilitated through the use of standard range COD twist-tubes supplied by O.I. Corporation. For BODs, TKN, and ammonia, and nitrate analyses, methods specified in the 16th edition to Standard Methods for the Examination of Water and Wastewater (APHA, 1984) were employed. Flow rates were obtained manually by measuring the volume accumulated in one minute. Platinum probes installed at two points in each tank yielded ORP measurements. Data on plant growth and nitrogen content were recorded by randomly selecting 6 or 12 plants from each tank and drying them at 70°C to a constant weight. The above-grade dry weight of plant matter for each system was estimated by counting the number of plants in each tank at the time of sampling. A small portion of dried plant matter from each tank was ground in a Wiley mill equipped with a #40 sieve for TKN analysis to obtain a plant TKN mass estimate for each tank. 4.
Results
During the preliminary period of this study, oscillation of the COD effluent concentration at a constant influent BOD5 of approximately 30 mg/1 and a flow of 80 ml/min, even after a month, indicated that a steady-state situation had not been achieved. Overall average BOD5 removal percentages (by mass) were 75%, 60%, and 44% for the Scirpus, Sagittaria, and control systems, respectively. The removal performance of the filter systems, in terms of COD removal, was substantiated by the pH measurements for each system. A larger drop in pH throughout the system indicates microbial activity due to the organic acids or carbon dioxide (CO2) produced by microbial metabolism. For the systems containing plants, the pH of the effluent lay consistently between 7.2 and 7.7, while the feed water and control effluent pH consistently ranged between 7.6 and 8.7, with the control having a higher pH than the feed water on several days. Higher pH readings than the feed water were probably due to the presence of algae on the surface of the control filter. Similarly, mass percentage TKN removals were enhanced by the systems con-
A BENCH-SCALE ROCK-PLANT FILTER INVESTIGATION
75
taining plants (50%-75 % TKN removal), when compared to the control (10%-20% TKN removal). Within the dried plant tissue, Sagittaria contained essentially the same percentage of TKN (25 mg N/g dry plant weight, average) as the Scirpus (22 mg N/g dry plant weight, average). Due to the higher moisture content of the Sagittaria (91%) over the Scirpus (84%), however, the total mass of Kjeldahl nitrogen in the system was about 40% higher for the Scirpus system as compared to the Sagittaria system. ORP measurements taken near the influent and effluent one-third of the three filter systems indicated that each of the systems, with negative ORP values, were well into the anaerobic range. Dissolved oxygen (DO) profiles within the systems also revealed either an absence or negligible (
Abstract. Three bench-scale rock filters containing 0.6 m of gravel were used in this investigation. Two of the filters were planted with Sagittaria Iancifolia and Scirpus validus, while the third filter was an unvegetated control filter. The wastewater directed through these systems was a synthetic mixture containing nutrient broth as the carbon source. An 80-day experiment was run on the filters using eight combinations of two flow rates and four influent biochemical oxygen demand (BODs) concentrations, each combination remaining constant for ten days. These combinations resulted in BOD5 surface loadings from 4.63-30.96 g/day/mz. From other studies of batch and continuous flow rock-plant filter systems, the first-order BOD5 reaction rate constant was tbund to decrease exponentially with detention time. In this study, however, little correlation was found between BOD5 loading rate and removal percentages, which averaged 69%, 57%, and 47% for the Scirpus, Sagittaria, and control systems, respectively. This is probably due to the relatively small change in detention times studied and the short time period of each loading rate. Oxidation/reduction potential (ORP) and dissolved oxygen (DO) measurements within these systems indicated that no free oxygen was available at any depth. Total Kjeldahl nitrogen (TKN) removal was higher in the plant systems relative to the control, with the Scirpus system achieving a higher overall removal than the Sagittaria system.
1.
Introduction
Due to their low operation and maintenance costs, rock-plant filter (RPF) systems are increasing in popularity, especially in the southeastern United States. Several studies have determined the effects o f various plants (Wolverton et al., 1983; Gersberg et al., 1986; Burgoon et al., 1991) or attempted to establish RPF design criteria (Trautmann et al., 1989; Lawson, 1985; Findlater et al., 1990; B a v o r et al., 1989). These studies have produced conflicting data, either because o f the different detention times used or the fact that some have been batch studies while the others were continuous flow studies. In addition, secondary or primary wastewater was used in these studies, meaning that the specific wastewater content variable between studies and, possibly, within a study. The research reported here was conducted in continuous flow systems using carefully controlled synthetic wastewater. T h e objectives o f this study were: (1) to compare the effect o f the emergent aquatic plants (Scirpus validus and Sagittaria lancifolia) on organic and nitrogen removal rates; (2) to determine whether there is a relationship between organic removal and oxidation reduction potential (ORP); and (3) to assess the o x y g e n content within vegetated and non-vegetated filters. Environmental Monitoring and Assessment 27: 69-80, 1993, (~) 1993 Kluwer Academic Publishers. Printed in the Netherlands.
70
DONNA SKIPPER AND MARTY TITTLEBAUM
2.
Related Research
Since the 1970s, research on rock-plant filters has been conducted using various types of aquatic plants, retention times, and wastewater sources. Removal rates observed have been varied and sometimes inconsistent, however, as discussed below. In one of the earlier studies, Spanler et al. (1976) planted bulrush (Scirpus validus) in flow-through trenches which were PVC lined and gravel filled to study the effect of retention time, nature of wastewater applied (primary or secondary treatment effluent), and frequency of harvesting on effluent quality. The retention times they used (5 hours, 16 hours, and 10 days) yielded equivalent biochemical oxygen demand (BODs) reductions for both control and bulrush basins. It was noted, however, that flow regulation was poor in the control basin and higher loadings were actually applied to the bulrush basins as compared to the control. For primary effluent, BOD5 reduction was nearly 10% greater when compared to secondary effluent. Furthermore, harvesting resulted in no observed changes in effluent quality. Contrary to Spangler et al. 's observations, a study by Wolverton (1982) indicated a significant difference in BOD5 removal between an unvegetated filter and a filter containing reed (Phragmites communis). BOD5 removal rates for the control filter were 62% and 83% after 6 and 24 hours retention time, respectively; the rockreed filters achieved 87% and 96% BOD5 removal for the same retention times. In addition, nutrient removals were an order of magnitude higher in Wolverton's rock-reed filter relative to his control. Relatively equal levels of BOD5 removals in vegetated and unvegetated filters were, however, observed by Wolverton et al. (1983) in a later study. This batch experiment began by first setting raw sewage anaerobically for 24 hours, then delivering the effluent to various rock filters, each containing a different aquatic emergent plant plus an unvegetated control. Plants used include reeds (Phragmites communis), cattails (Typha latifolia), rush (Juncus effusus), and bamboo (Bambusa multiplex). The filter influent BOD5 was adjusted to approximately 60 mg/1 and 300 mg/1 on separate occasions by adding different amounts of water hyacinth juice to the raw sewage. The results of this study indicated that retention times of 6 and 29 hours were required to achieve the 30 mg/1 BOD5 secondary treatment standard for the 60 mg/1 and 300 mg/l influent BOD5 loading, respectively. Although the BOD5 removal in the vegetated filters was comparable to the control, the system containing reeds performed slightly better than the other systems. In a later study, Gersberg et al. (1986) treated primary wastewater in four 0.76 m deep gravel ditches containing bulrush (Scirpus validus), reed (Phragmites communis), and cattail (Typha latifolia), plus an unvegetated control. Parameters monitored during this study included ammonia, BODs, and total suspended solids (TSS). Ammonia removals were found to be 94%, 78%, and 28% for bulrush, reed, and cattail, respectively. BOD5 removals for bulrush, reed, cattail, and the control
A BENCH-SCALE ROCK-PLANT FILTER INVESTIGATION
71
were 96%, 81%, 74%, and 69%, respectively. Essentially equal TSS removal was observed in all ditches and was, therefore, assumed to be an exclusively physical process. In addition, the depths of root zones were recorded for each species. Cattail was found to have the shallowest root zone: approximately 12" (30 cm). The bulrush root zone extended to a depth of over 24" (60 cm); while the root zone of reed averaged around 30" (76 cm). In a more recent study, Burgoon et al. (1991) found equal BOD5 removal in nonvegetated, batch-fed systems as compared to batch-fed systems containing Sagittaria latifolia, Scirpus pungens, Phragmites austral&, or Typha latifolia. Nitrogen removal, however, was found to be enhanced in the vegetated systems versus the unvegetated systems. The wastewater used in this study originated from secondary domestic wastewater and ranged between 2 and 24 g BODs/(m 2 day) and between 0.6 to 4.4 g nitrogen/(m 2 day). One reported difference in the above studies is detention time. Influent and effluent BOD5 concentrations are theoretically tied to the detention time through the first-order reaction rate constant for BOD5 removal, K, as follows:
K = {ln(Co/Ce)}/t where: Co = influent BOD5 concentration, mg/1; Ce = effluent BOD5 concentration, mg/1; and t = detention time, days. A tabulation of the K values versus detention times for the projects described above are listed in Table I. These data reveal an exponential decay relationship between K and detention time. Therefore the reaction rate for BOD5 degradation in an RPF system depends on the detention time used, according to the studies discussed above. In other words, most of the reaction apparently takes place in a short period of time (or within the first few feet of the filter). This explains the discrepancy reported by Burgoon et al. (1991) between continuous and batch flow systems. 3.
Methods and Procedures
Three rectangular, welded-aluminum tanks (1.5 m long x 0.45 m wide) housed the filter systems. Each tank had an interior polyurethane coating and was constructed as shown in Figure 1. The influent flow entered near the bottom of one end of each tank through a perforated PVC pipe spanning the width of the tank while the effluent spilled into a standpipe at the opposite end of the tank. Oxidation/reduction potential (ORP) probes were placed near the mid-tank access ports at mid-depth. These tanks were installed in a greenhouse which supplied an equal amount of sunlight to all tanks. The water temperature of all three tanks remained within I°C of each other and varied from 21°C to 31°C during this study. On November 19, one tank was planted with ten Sagittaria lancifolia (duck potato) seedlings and another tank with ten Scirpus validus (bulrush) seedlings. These plants grew for five months prior to the commencement of the analyses.
72
DONNA SKIPPERAND MARTYTITTLEBAUM TABLE I First-order BaD5 rate constants vs. detention time. Time (h)
Control
Bulrush
5 5 6 12 24 24 48 144 240
5.96 5.51 3.87 4.14 2.29 1.77 1.51 0.2 0.16
9.41 8.19 8.16 4.35 2.49 3.22 1.57 0.54 0.19
/ /•lnfluent Air Release
6" Pea Grovel 216 It
- - ~
°
Reed
Reference Spangler et el. (1976) Burgoon e t a L (1991) Wolverton (1982) Burgoon et aL (1991) Burgoon et el. (1991) Wolverton (1982) Burgoon et el. (1991) Gersberg et el. (1986) Spangler et el. (1976)
~,..~-Mid-Tank Access Ports j
,..//
/
/
/-Effluent
/
StandPipe
° Sampling Port
I' - 6" I"x 3" Gravel
6,if// Fig. 1. Rock-aquatic plant filter systems, inner tank details. (Not to Scale.)
During this five months, a solution containing Hydrosol (a hydroponic fertilizer by Peters), calcium nitrate (CaNO3), and dextrose was recirculated through each tank at a flow o f 500 to 1000 ml/min. This solution was replenished twice per week, with daily dextrose additions. On January 25, and then again on March 8, the tanks were seeded with activated sludge mixed liquor. After the five month start-up period, the flow through the tanks was changed to once-through flow using a synthetic wastewater containing constituents listed in Table 2. Although the use of a synthetic wastewater creates a somewhat unre-
A BENCH-SCALE ROCK-PLANT FILTER INVESTIGATION
73
TABLE II Synthetic wastewater recipe. Constituent MgSO4 • 7H20 FeCI3 • 6H20 MnSO4. H20 K2HPO4 KHzPO4 CaCO3 BOD5
Concentration (rng/1) 300 14 5 120 30 14 28, 78, 107, 51
alistic situation, it allows the effluent to vary while the influent is controlled. The wastewater recipe shown in Table II is the result of several trial mixtures. Initial mixtures contained glucose and ammonium sulfate as carbon and nitrogen sources, respectively. This was changed when most of the BOD5 was consumed by residual microorganisms in the reservoir before entering the filters. Nutrient broth, a more complex and less readily degraded carbon source, corrected this situation satisfactorily; however, the nitrogen source in nutrient broth is solely organic, which is found in primary wastewater but not usually in secondary wastewater, where ammonia is the typical nitrogen form. This restricted the comparability of nitrogen removal rates between this study and other studies. To feed each filter with synthetic wastewater, separate chemical feed pumps (Cole Parmer Model N-07141-28) distributed wastewater from a common reservoir to each filter. This wastewater was freshly mixed daily. Initially, the goal was to arrive at a steady percentage of organic removal before stepping to a different hydraulic/organic loading scheme. No steady removal rate was found, however, after over a month at a flow of 80 ml/min and an organic loading of 4.96 g BODs/day/m 2 surface area (30 mg/l BOD5 influent). When steady-state operation was not obtained within a feasible time-frame, an alternative monitoring program was contrived. This program involved randomly changing the influent BOD5 concentration in consecutive ten-day periods at two flow rates. The four influent BOD5 concentrations (28, 78, 107, and 51 mg/l) were first applied under a flow of 80 ml/min (24-hour theoretical retention time), which resulted in BOD5 surface loadings of 4.63, 12.90, 17.69, and 8.43 g/day/m2. The influent BOD5 concentrations listed above were then repeated under a 140 ml/min flow (42-hour theoretical retention times), giving BOD5 surface loadings of 8.10, 22.57, 30.96, and 14/76 g/day/m2. During each ten-day period, the effluent from each filter and an influent composite were analyzed on days 4, 8, 9, and 10 for BODs, total Kjeldahl nitrogen (TKN), and ammonia; and on days 2, 4, 6, 8, 9, and 10 for chemical oxygen demand (COD). Only random nitrate analyses were
74
DONNA SKIPPER AND MARTY TITTLEBAUM
performed since negligible amounts of this compound were measured during the preliminary portion of this study. Daily measurements of water temperature, pH, and ORP were also obtained in each tank. The influent pH was adjusted daily to between 7.0 and 7.5 by addition of sulfuric acid. Ambient air temperature and influent and effluent flow measurements were also recorded daily. Above-grade dry plant matter weight and TKN plant mass were analyzed at the end of every 10-day period for each vegetated filter. Many of the BOD5 removal measurements were calculated through COD values due to the greater ease, speed, and accuracy of the COD analysis in comparison with the BOD5 analysis. Ordinarily this relationship is not reliably consistent; however, with a controlled wastewater, such as the one used in this study, the ratio of these parameters is relatively constant. Measurements of both were performed here to verify this relationship. COD measurements were facilitated through the use of standard range COD twist-tubes supplied by O.I. Corporation. For BODs, TKN, and ammonia, and nitrate analyses, methods specified in the 16th edition to Standard Methods for the Examination of Water and Wastewater (APHA, 1984) were employed. Flow rates were obtained manually by measuring the volume accumulated in one minute. Platinum probes installed at two points in each tank yielded ORP measurements. Data on plant growth and nitrogen content were recorded by randomly selecting 6 or 12 plants from each tank and drying them at 70°C to a constant weight. The above-grade dry weight of plant matter for each system was estimated by counting the number of plants in each tank at the time of sampling. A small portion of dried plant matter from each tank was ground in a Wiley mill equipped with a #40 sieve for TKN analysis to obtain a plant TKN mass estimate for each tank. 4.
Results
During the preliminary period of this study, oscillation of the COD effluent concentration at a constant influent BOD5 of approximately 30 mg/1 and a flow of 80 ml/min, even after a month, indicated that a steady-state situation had not been achieved. Overall average BOD5 removal percentages (by mass) were 75%, 60%, and 44% for the Scirpus, Sagittaria, and control systems, respectively. The removal performance of the filter systems, in terms of COD removal, was substantiated by the pH measurements for each system. A larger drop in pH throughout the system indicates microbial activity due to the organic acids or carbon dioxide (CO2) produced by microbial metabolism. For the systems containing plants, the pH of the effluent lay consistently between 7.2 and 7.7, while the feed water and control effluent pH consistently ranged between 7.6 and 8.7, with the control having a higher pH than the feed water on several days. Higher pH readings than the feed water were probably due to the presence of algae on the surface of the control filter. Similarly, mass percentage TKN removals were enhanced by the systems con-
A BENCH-SCALE ROCK-PLANT FILTER INVESTIGATION
75
taining plants (50%-75 % TKN removal), when compared to the control (10%-20% TKN removal). Within the dried plant tissue, Sagittaria contained essentially the same percentage of TKN (25 mg N/g dry plant weight, average) as the Scirpus (22 mg N/g dry plant weight, average). Due to the higher moisture content of the Sagittaria (91%) over the Scirpus (84%), however, the total mass of Kjeldahl nitrogen in the system was about 40% higher for the Scirpus system as compared to the Sagittaria system. ORP measurements taken near the influent and effluent one-third of the three filter systems indicated that each of the systems, with negative ORP values, were well into the anaerobic range. Dissolved oxygen (DO) profiles within the systems also revealed either an absence or negligible (
