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Tertiary treatment of municipal wastewater by cypress domes
Water Res. Vol. 17, No. 9, pp. 1027-1040, 1983 Printed in Great Britain. All rights reserved
0043-1354/83 $3.00+0.00 Copyright © 1983 Pergamon Press Ltd
TERTIARY TREATMENT OF MUNICIPAL WASTEWATER BY CYPRESS DOMES FORREST E. DIERBERG1 and PATRICK L. BREZONIK2 1Department of Environmental Science and Engineering, Florida Institute of Technology, Melbourne, FL 32901 and 2Department of Civil and Mineral Engineering, University of Minnesota, Minneapolis, MN 55455, U.S.A.
(Received July 1982)
Abstrae~The feasibility of using cypress domes as an alternative to physical-chemical methods for tertiary treatment of sewage effluent was demonstrated over a 5-year period. Surface water quality in domes receiving effluent was degraded (low dissolved oxygen, high nutrient levels) compared to control domes, and the degree of treatment within the dome surface waters was relatively small. However, water samples from shallow wells and ceramic soil moisture tubes indicated that the underlying organic soils and clay sands served as an effective barrier to the transport of biochemical oxygen demand, nitrogen, phosphorus, sulfate, fluoride, potassium, sodium, calcium, and magnesium to the shallow aquifer immediately below. High percentage removals were observed for all measured water quality parameters between the surface and groundwater stations. Standing water quality following the cessation of sewage pumping did not return to the same quality as found for the natural dome within 20 months.
Wetlands have long been recognized as nutrient sinks. In a recent review on various types of wetlands, van der Valk et al. (1979) found that phosphorus was removed in all 16 studies that measured phosphorus, and 12 of 14 studies involving nitrogen reported at least seasonal removal. Two studies found wetlands to act as nitrogen sources. Studies on palustrine wetlands (i.e. nontidal wetlands that are not confined by channels and are not marginal to lakes) have indicated removal of nitrogen and phosphorus (e.g. Boyt et al., 1977; Fetter et al., 1978; Richardson et al., 1978). Removal efficiencies in northern marshes decline in the spring and fall when high hydraulic loadings wash out some nutrients assimilated during the previous growing season and mineralized during fall (Lee et al., 1975; Spangler et al., 1977). Only one study (Steward & Ornes, 1975) has shown that the assimilative capacity of a palustrine wetland (the Everglades) was exceeded by the input of nutrient-laden sewage effluent. The capability of wetlands to trap nutrients has stimulated studies to evaluate their response to the addition of wastewater (Boyt et al., 1977; Sloey et al., 1978; Tilton & Kadlec, 1979; Whigham & Simpson, 1976; Dolan et al., 1981). Sloey et al. stated that palustrine wetlands are more amenable to management for wastewater treatment than other wetlands (tidal, riverine, and lacustrine) because they are hydraulically isolated from open surface water and their hydraulic residence times are high. Cypress domes are a common palustrine wetland throughout the pine-palmetto woodlands of the southern Atlantic and Gulf Coastal Plain. Cypress domes are topographic depressions that vary in size from about 1 ha to more than 10ha. Pond cypress
(Taxodium distichum var. nutans) is the most conspicuous canopy tree, and black gum (Nyssa sylvatica var. biflora) is the dominant understory tree. More detailed accounts of the floristic and soil characteristics of cypress domes are given by M o n k & Brown (1965), Coultas & Calhoun (1975), and Ewel (1983). Feasibility studies on the use of these ecosystems as alternatives to conventional tertiary treatment were initiated in 1974 when treated secondary sewage effluent was added to two cypress domes by O d u m and co-workers (see O d u m & Ewel, 1975, 1976; O d u m et al., 1975). This paper summarizes basic water quality data for the cypress dome feasibility study over the period March 1974 to May 1979. Its primary intent is to describe water quality in a manner that indicates the capability of cypress domes to treat domestic sewage effluent and reduce the pollutant levels. Quantitative descriptions (mass balances) of the nitrogen and phosphorus cycles in a cypress dome receiving treated effluent are presented elsewhere (Dierberg & Brezonik, 1982), and detailed results have been described in annual project reports (e.g. Dierberg & Brezonik, 1978).
METHODS
Description of study area The main study site consisted of three cypress domes on a large pine plantation about 5 km north of Gainesville, Florida (Fig. 1). A "package treatment plant" serving a 155-unit mobile home park adjacent to the site supplied secondary sewage effluent to the centers of sewage dome l (S-l: 0.51 hal and sewage dome 2 (S-2; 1.05 ha) (Fig. 2). Pumping was initiated to sewage dome 1 in March 1974 and to sewage dome 2 in December 1974. The sewage plant was an extended aeration waste treatment facility that
1027
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FORREST E. DIERBERGand PATRICK L. BREZONIK
olo . Fig. 1. Locations of the experimental (Owens-Illinois) and control (Austin Cary) sites in Alachua County, Florida. I
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Fig. 2. Site plan, groundwater monitoring wells and soil tube locations at the experimental (Owens Illinois) site.
Tertiary treatment of municipal wastewater by cypress domes treated about 95 m 3 day- i (25,000 gal day- l) or 83% of its design capacity of 114 m 3 day- ~. Treated effluent was discharged into an oxidation pond except during the period March 1974 to March 1975 when sewage was pumped to the cypress domes directly from the package plant. The plant was not operating efficiently during this period; hydraulic loadings ranged between 0 and 14 cm wk-~; and sludge occasionally was pumped into the domes. After March 1975, more uniform applications (2.5 cm wk- ~ with no sludge) were obtained using effluent that had been in the oxidation pond for about 10 days. The effluent pipe in the oxidation pond was relocated in March 1976 to a point closer to the treatment plant. Effluent additions to the domes were intermittent, with input pumps usually operating for 1 h in every 12 h. A combination of high application rates and heavy rainfall during summer of 1974 caused overflow from sewage dome 1. A weir was constructed on the west side of the dome to maintain standing water at a desired level and to allow overflow to be measured and sampled. A third dome (G-I; 0.70 ha) at the site received groundwater from a deep well (~50m) according to the same schedule and loading rate as sewage dome 1. It served as a hydrologic control, separating the response of acidic cypress domes to inputs of alkaline groundwater, from the effects of sewage pollutants (nutrients and organic matter). A larger dome (4.2 ha), about 17 km northeast of the main site in the Austin Cary Memorial Forest (Fig. 1), served as a "natural" control whose water levels fluctuated according to the normal hydroperiod. Two of the three domes (groundwater control and sewage dome 1) at the main site were burned extensively in an unintentional forest fire on 4 December, 1973; sewage dome 2 was burned only along the southwest edge.
Sampling procedures Standing water, sewage, and shallow groundwaters on the main site and standing water of the Austin Cary dome were monitored routinely for major ions, nutrients, and organic matter from March 1974 to May 1979. Samples were collected monthly from stations at the main site until December 1976, followed by quarterly sampling until May 1979. Austin Cary dome surface waters were sampled monthly from April 1978 to July 1979. The data from shallow wells (Fig. 2) were composited into three groups: (1) wells surrounding sewage domes 1 and 2 and wells located within sewage dome 1;(2) wells surrounding the groundwater control dome; and (3) wells distant from any of the experimental cypress domes and representative of the natural groundwater of the area.
Analytical Chemical analyses were performed according to accepted manuals (APHA, 1976; U.S. EPA, 1974). Analysis of major cations was performed by atomic absorption spectrophotometry (Varian Techtron Model 1200) with instrument settings recommended by the manufacturer. Automated colorimetric procedures were used for nitrate plus nitrite (cadmium reduction method), ammonium (indophenol method), Kjeldahl nitrogen (semi-micro digestion followed by determination of ammoniumt, and chloride (ferric thiocyanate method), Sulfate was analyzed turbidimetrically. Soluble reactive phosphorus and total phosphorus (persulfate digestion) were determined spectrophotometrically by the ascorbic acid-molybdenum blue method. Sulfide was analyzed by the colorimetric method of Strickland & Parsons (1972). Fluoride was measured by ion-selective electrode and pH by glass electrode. Dissolved organic and inorganic carbon were analyzed using a Beckman Model 915 carbon analyzer. Dissolved oxygen was determined by the azide modification of the Winkler method. Five-day biochemical oxygen demand (BOD-5) was determined by the amount of oxygen lost after incu-
1029
bating for 5 days in the dark at 20°C. Samples for anions (except chloride), cations, and nutrients were preserved with HgC12 and frozen from the time of collection until analysis within 1 week of collection. Sulfide and biochemical oxygen demand samples were kept refrigerated until analyzed within 24 h of collection.
Soil-solution studies Soil-solution sampling tubes (porous ceramic cups attached to 3.8 cm I.D. PVC pipes) were installed at three soil depths (60, 120, and 180cm) near the edges of the groundwater control dome and sewage domes 1 and 2 (Fig. 2). For the stations closest to the sewage domes, duplicate tubes were placed at each depth. The other stations had one tube at each depth. The ceramic cups were washed in 1 : 2 concentrated HNO3 and rinsed in demineralized water before installation in the field. Soil solutions were sampled by drawing a vacuum in the tubes and collecting the water 24 h later. Water that had accumulated in the tubes before a vacuum was applied was discarded. Samples were obtained in November and December 1976 and January, May, and August 1977. Nutrient concentrations in the soil water were determined by procedures cited above. EFFECTIVENESS OF THE TREATMENT PLANT/DOME SYSTEM IN POLLUTANT REMOVAL
The most significant change in appearance in the domes receiving sewage effluent was the growth of a rather dense blanket of duckweed (Lemna spp, Spirodela) that covered most of the surface water during most of the year. This cover inhibited reaeration and resulted in anoxic or near anoxic conditions and the formation of sulfide in the standing water. However, odors were not a problem at the sites, although the presence of reduced sulfur compounds could be detected when the water was artificially stirred. Similarly, insects such as mosquitoes were not found to be a problem, although there were some changes in diversity and numbers in the treated domes, compared to the controls. Details of insect studies done by others were presented in annual project reports (e.g. O d u m & Ewel, 1975, 1976). Bacterial contamination (fecal coliforms) was monitored on a routine basis during the project (e.g. Price, 1975, in O d u m & Ewel, 1975). In general, standing waters of sewage domes had elevated coliform levels (compared to controls), but water from the shallow wells showed no evidence of contamination by transport of bacteria in water percolating through the dome sediments. A few cases of elevated fecal coliform levels in shallow wells near sewage domes were attributed to overflow of surface waters from the domes during rainy periods and consequent contamination of the wells through broken casings. The effectiveness of cypress domes in reducing the levels of sewage pollutants in dome surface waters and associated groundwaters is illustrated in Figs 3-6. Stations were combined into nine subgroups within four major groups. Each subgroup is represented by a bar in the histograms of Figs 3-6; the legend of Fig. 3 defines the stations comprising each subgroup. The first category (three subgroups) follows the sewage
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Biochemical oxygen demand and total organic carbon BOD values more closely reflect the effect of sewage effluent on the highly colored surface waters of cypress domes than do total organic carbon (TOC) levels (Fig. 3). The package treatment plant removed 94% of the BOD of the raw sewage; the oxidation pond removed an additional 2%. Relatively small reductions in BOD were observed between the centers and edges of both sewage domes (35-38%), indicating the dome surface waters were not very efficient in oxidizing degradable organic matter. Groundwater from the four wells around sewage dome 2 had BOD levels below those in the natural dome surface water. The two sewage-enriched domes had BOD levels comparable to the two control domes when the influent pipe to the sewage domes was located at the far end of the oxidation pond (October 1975-February 1976) (Fig. 7). Except for that period, BOD levels were much higher in the sewage-enriched domes, probably from a combination of lower rainfall and higher levels of organic carbon in the sewage. Relocation of the influent pipe closer to the discharge from the treatment plant in March 1976 increased the input of organic carbon to the domes (Zoltek, 1976). BOD levels for both control domes normally were less than 5 mg 1- 1 whereas BOD levels in the sewage domes usually were above 5 mg 1- 1 (which is the standard of the State of Florida Department of Environmental Regulation (1979) for finished waters of domestic tertiary sewage treatment). It should be noted that all BOD samples were filtered through Whatman No. 4 qualitative filters to remove duckweed and sediments before incubation. This procedure gave a better
measurement of BOD levels attributable to the incoming sewage, but it probably resulted in an underestimate of the true BOD levels.
Nitrogen The package treatment plant substantially reduced influent total nitrogen (TN) levels (from 50 to 19mg1-1, or 62% removal; Fig. 4). The oxidation pond reduced the average concentration by another 19%. Higher mean concentrations of TN at the centers of the sewage domes than in the oxidation pond can be explained by the history of effluent discharge to the domes. Secondary effluent with higher TN concentrations was pumped to the domes directly from the treatment plant or from near the entrance to the oxidation pond during 47 of the 60 months of the study. Nitrogen concentrations in the wells surrounding and within the sewage domes maintained the same levels as the control wells. Except for a brief 3-month period (OctoberDecember 1975), TN concentrations in the surface waters of the sewage treated domes were higher than those in the control domes (Fig. 8). Mean TN concentrations for sewage domes 1 and 2, Austin Cary dome and the groundwater control dome over the study period were 8.6, 12.3, 1.6 and 1.5 mgl 1 respectively. The low TN levels during late 1975 and the increase beginning in February 1976 reflect changes in the quality of the incoming sewage effluent, as discussed previously. The center stations of the sewage domes had higher levels of TN than did edge stations. The average TN level was 33% lower at one edge station and 44~o lower at another compared to the average at the center of sewage dome 1. Similarly, average TN levels of edge stations in sewage dome 2 were 41 and 310/,i lower than the center station mean value. The lack of dissolved oxygen in the sewage domes resulted in a preponderance of ammonium in the inorganic N pool (see Dierberg, 1980}. Levels of TN in surface waters of the two sewage-
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FORREST E. DIERBERG and PATRICK L. BREZONIK
enriched domes generally were lower during the first 2 years of the project (Fig. 81, corresponding with a period of higher levels of nitrate and lower levels of TN in the effluent (Zoltek, 1976). Thus, it appears possible to maintain some control over the levels of TN in surface waters of cypress domes receiving sewage by maintaining a nitrified effluent.
Phosphorus The substantial reductions in BOD and nitrogen produced by the treatment plant did not occur for phosphorus (Fig. 4). Only 20~o of the total phosphorus (TP) was removed by the plant and another 7~/,, was removed by the oxidation pond. As a result, TP concentrations in the sewage domes were nearly the same as those for TN, even though TN levels were five times higher than TP levels in raw sewage. With one exception, concentrations of TP in the shallow wells were at background levels, implying complete removal of phosphorus by the domes. High TP concentrations in well 19 west of sewage dome 1 are attributed to naturally high levels; since this well was slightly deeper and penetrated the phosphorus-rich Hawthorn Formation (Dierberg, 1980). Concentrations of TP in the surface waters of sewage domes 1 and 2 ranged from 2 to 100 times higher than those of the two control domes (Fig. 9), which had mean values of 0.27 mg 1 1 (groundwater control) and 0.18 mgl-1 (Austin Cary control). Sewage dome 2 consistently had higher TP concentrations than did sewage dome 1 (Fig. 9), and concentrations generally were lower at the edges of both domes than at the center (Fig. 4). However, in most instances the decrease was less than 50~o, and concentrations almost always were higher than the lmg1-1 value recommended for finished waters of tertiary sewage treatment by the Florida Department of Environmental Regulation (1979). Over the entire study period, the average levels of TP at the edge of sewage dome 1 were 29-31~o lower than the average at the center (7.1 mg 1- 1). Average concentrations of TP at the edge stations in sewage dome 2 were 25-35~o lower than the average at the center (8.2 mg 1-1). Large temporal fluctuations in TP levels in sewage domes 1 and 2 (Fig. 9) at the beginning of the study reflect the variable nature of the sewage source. Relocation of the effluent pipe within the oxidation pond in March 1976 to a point closer to the treatment plant did not result in any observable changes of TP concentrations in the incoming sewage or the standing waters of the two sewage-enriched domes (Fig. 9). As mentioned previously, this was not the case for levels of BOD and TN. It is noteworthy that most of the TP in the sewage enriched domes was in the form of orthophosphate, which is readily used by plants.
Major cations The monovalent cations were similar to phosphorus in their concentration patterns (Fig. 5). Decreases of 77~o for sodium and 95~o for potassium
were found in comparing levels in wells around the domes to levels in surface water at the centers of the sewage domes. During 1978 and 1979 sodium concentrations in the wells surrounding sewage dome 2 increased to about half the levels in the surface water, pointing to downward movement of surface waters into the shallow aquifer. Concentrations of divalent cations were relatively constant through the treatment plant and oxidation pond, but they were lower (52Vo for Ca and 44~0 for Mgj at the edges of the sewage domes than at the centers (Fig. 5). In comparison, little change was noted in monovalent cations between the centers and edges. The decreased concentrations of divalent cations may reflect fixation onto exchange sites of the dome sediments. Concentrations of Mg in the groundwater under and around the two domes were equal to background levels in the control wells; but Ca levels were lower in the wells near the sewage domes than in the natural control wells. The proximity of the calcareous Hawthorn Formation to the control wells may explain the differences in Ca levels, since the latter wells were drilled deeper (av. depth = 4.3 m) than the wells surrounding the experimental domes (av. depth = 3.7 m/(Cutright, 1974).
Anions Primarily because of dietary use of salt, chloride concentrations are higher in domestic sewage than in most freshwaters. Since chloride is a conservative ion (i.e. it is not adsorbed by soil components or assimilated by plants) it can be used to trace the effluent. Levels of chloride in wells thought to be influenced by sewage percolation were more than half the values found in the sewage dome centers (Fig. 6). Average chloride concentrations were the same from the raw sewage to the dome edges, pointing to the conservative nature of chloride. The higher chloride concentrations in wells surrounding the groundwater control dome (middle bar of well group in Fig. 6) are unexplainable since the groundwater pumped into this dome had low chloride levels ( ~ 7 m g l - 1 ) . During 1978 and 1979, chloride levels in the wells surrounding sewage dome 2 increased to concentrations comparable to those in the surface water. A considerable decrease in the concentration of sulfate (39%) occurred between the center and edge stations of the sewage domes, probably because of reduction of sulfate to sulfide (Fig. 6). Sulfate levels in wells around the sewage domes were similar to those in control wells. A decrease in the concentrations of fluoride (37~o) between the center and edge stations of the sewage domes suggests that chemical precipitation may have occurred. Given the low solubility of fluorapatite (Kso ~ 10 -55) and the high levels of phosphate and calcium, the observed concentrations of fluoride are considerably above the solubility limits for fluorapatite, even at the dome edges. Fluoride levels in wells surrounding the groundwater control dome were
*Median pH. t N u m b e r of samples. :~Based on two samples.
Sewage D o m e 2 4 m northwest of edge (replicate soil tubes)
Sewage D o m e 2 13 m southeast of edge
Sewage Dome I 7 m west of edge (replicate soil tubes)
Sewage D o m e 1 36 m west o1 edge
Sewage D o m e I 15 m east of edge
Groundwater Control D o m e 3 m west of edge
Station 60 120 180 60 120 180 60 120 180 60 60 120 120 180 180 60 120 180 60 60 120 120 180 180
Depth (cm) 0.01 0.03 0.02 0.01 0.01 0.02 0.02 0.02 0.03 0.03 0.03 0.03 0.03 0.02 0.03 0.03 0.04 0.06 0.09 0,02 0.02 0.02 0.02 0.03
( N O 3 and NO2-) N ( m g l -~} 0.35 0.14 0.25 1.1 0.05 0.02 0.07 0.13 0.04 0.37 0.12 0.05 0.05 0.07 0.09 0.28 0.11 0.06 0.06 0.05 0.19 0,16 0.12 0.09
NH4 N {mgl ~) 0.6 0.6 0.7 0.6 0.5 0.3 0.5 0.8 0.7 0.7 0.4 0.7 1.2 0.5 0.5 0.9 0.5 0.6 1.1 0.5 0.8 0.3 0.5 0.8
Organic N ( m g l -~) 0.02 0.02 0.01 0.03 0.01 0.02 0.04 0.02 0.02 0.01 0.01 0.01 0.01 0.02 0.01 0.01 0.01 0.02 + 0.005 0.01 0.02 0.02 0.01 0.005
Total P ( m g l ~) 54 49 84 67 38 37 91 88 132 144 156 166 157 169 171 1415 200 95 179 179 156 150 150 149
Specific conductance (/Lmhocm ~) 6 10 10 11 7 7 20 21 34 39 40 45 42 42 44 > 100 55 26 46 48 43 39 37 39
CI (mgl 4.2 4.4 4.5 5.0 4.6 4.8 4.0 4.5 4.8 4.3 4.2 4.4 4.6 4.5 4.4 4.7 4.4 4.4 4.4 4.3 4.2 4.4 4.1 4.2
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Table 1. Mean levels for selected chemical parameters of the interstitial water taken at three depths from soil tubes surrounding the cypress d o m e s at the Owens Illinois site in November, December, January, M a y and August (1976 1977)
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FORREST E. DIERBERG and PATRICK L. BREZONIK
similar to those in wells around the sewage domes. However, fluoride levels were slightly higher (Fig. 6) in the deeper control wells that penetrated the Hawthorn Formation.
Hydrogen sulfide Concentrations of H2S greater than 1 mg 1-1 were found regularly at several stations in the experimental domes. The sewage effluent had high concentrations of sulfate (~40 mgl-1) compared to the natural surface waters and shallow groundwaters of the area; apparently some sulfate was reduced in the anoxic waters and sediments of the experimental domes. Because pH values for the surface waters of the sewage domes (~6.0) are below pK1 for H2S (7.0), the undissociated volatile species (HzS) predominated, comprising 65-92°/~, of the total dissolved sulfide in the water. SOIL S O L U T I O N STUDIES
Chloride concentrations indicate that infiltration from sewage dome 1 occurred in a westerly direction, and the extent of infiltration increased with depth at increasing distances from the dome (Table 1). On the other hand, infiltration from sewage dome 2 occurred about equally to the southeast and northwest. Unusually high chloride levels (up to 1000mg1-1) at 60 cm depth southeast of sewage dome 2 are an artifact from the disposal of filtrate high in MgC12 during virus sampling. Concentrations of chloride in standing water at the edges of the sewage domes ranged from 12 to 105mg1-1 for the study period (.,Y = 46 mg 1-1 for sewage dome 1 and 51 mg 1-1 for sewage dome 2). It is evident from the chloride data that the soil tubes located 7 m west of sewage dome 1 (all depths), 13 m southeast of sewage dome 2 (all depths except 180 cm), and 4 m northwest of sewage dome 2 (all depths) were in zones receiving a large amount of infiltration from the sewage domes. However, nutrient concentrations in these samples were consistently low, indicating a high degree of removal
of nutrients from infiltrating dome waters. Nutrient concentrations in samples with large fractions of sewage infiltrate (as indicated by chloride levels) did not differ significantly from the nutrient concentrations in samples with low or negligible fractions of sewage in the soil solution (Table 1). Nitrate contamination in groundwater is not a danger at present loading rates, nor does enrichment of adjacent waterways by other nitrogen and phosphorus species seem a matter of concern. Apparently, the spodic horizon in these soils is important in the removal of phosphorus which can move freely only to the depth at which this horizon occurs (Hortenstine, 1976). The latter author reported that phosphorus from secondary sewage effluent sprayed onto a spodosol near Orlando, Florida, moved freely in the soil solution above the spodic horizon, but efficient removal by this horizon was indicated by the absence of detectable phosphorus in soil solution below this layer. According to Coultas & Calhoun (1975), the spodic horizon in soil adjacent to the cypress domes occurs at depths of 2 ~ 3 0 cm. This fact explains the low levels of phosphate even in the shallow (60cm) soil tubes. Field data from the soil tubes corroborate results from the monitoring wells. Efficient removal (>95~0) of TP, nitrate, ammonium and TN by the sediments and soils surrounding the cypress domes occurred throughout the 5-year period of sewage disposal into the domes. WATER QUALITY F O L L O W I N G CESSATION OF SEWAGE I N P U T S
Pumping of treated sewage influent into sewage dome 1 was discontinued in September 1977. Sampiing was continued on an irregular schedule (depending on the presence of standing watert for the following 20 months to determine the extent of recovery in surface water quality. Wide variations were observed for most water quality parameters during the period following termination of sewage loading (Table 2). These variations reflected two major factors: (1)
Table 2. Changes in water quality at the center station of sewage dome 1 after
Date 2 Aug. 1977t 16 Dec. 1977¶ 1 Feb. 1978 7 Mar. 1978 6 May 1978¶ 4 Aug. 1978 22 Feb. 1979¶ 26 May 1979
Conductivity (•mhocm 1)
pH
Ca
Mg
Na
K
SO24-
HCO3*
C1-
F-
520 547 151 73 269 35 119 100
6.80 5.10 5.00 3.30 5.25 4.21 4.60 5.43
18.3~ 41.9 3.5 2.3 8.6 2.6 4.8 4.2
11.6~ 23.2 3.5 1.5 5.8 1.0 2.3 2.0
39.0~ 75.4 18.2 7.3 24.5 2.6 13.0 9.0
8.0~ 12.8 2.0 3.9 6.7 2.5 1.3 1.2
28.9§ 237.0 9.0 43.0 60.0 < 1.0 18.0 10.0
125.6 4.9 7.3 0.0 8.5 0.0 0.0 --
63.0 65.0 26.0 8.5 22.5 2.5 14.2 17.0
0.65 0.13 0.08 0.06 0.11 0.04 0.08
*Calculated from H +-HCO~- equilibrium using alkalinity. tLast date of sampling before cessation of treated sewage loading. :~Mean of 17-18 surface water samples taken at dome center from April 1974 to August 1976 during sewage loading. §Mean of 5 surface water samples taken at dome center from August 1975 to March 1977 during sewage loading. ¶Dry period with no standing water preceded sampling date.
Tertiary treatment of municipal wastewater by cypress domes elapsed time since cessation of sewage influent; and (2) the frequency, extent and intensity of rainfall prior to sampling. Because of an extended dry period (OctoberNovember 1977), the first sampling of the surface water after termination of sewage input was in December 1977, and the drought resulted in higher levels of conductance, minerals, nutrients, total carbon, BOD and turbidity than occurred in any other subsequent period (Table 2). Levels of most major ions were higher than the levels found in the dome during sewage loading. Oxidation of reduced sulfur that had been immobilized as organic or metal sulfides during sewage pumping may have accounted for the high sulfate level (237 mg 1-1) after the dry period. Other sampling dates on which high concentrations of water quality constituents were found (May 1978, and February and May 1979) also were preceded by dry periods that resulted in the disappearance of standing water from the dome, thus allowing a build-up of the products of aerobic decomposition and oxidation. Sampling dates on which the lowest concentrations were found (February, March and August 1978) were preceded by heavy rainfall that diluted the pollutants remaining from the period of sewage loading. On one occasion (August 1978), all water quality parameters except potassium and TP were within the ranges measured for the natural dome. The effects of sewage could still be discerned 20 months after sewage inputs had ceased. Specific conductance, major ions, fluoride, ammonium and total phosphorus were still higher than the mean values for the natural dome. Evidently, sediments and vegetation on the swamp floor released these substances to the standing water long after the end of sewage pumping. However, many important water quality parameters, including pH, nitrate, BOD total carbon, H2S, dissolved oxygen, turbidity, color and silica, returned to natural background levels within 8 months after sewage inputs were discontinued.
1039
CONCLUSIONS Compared to the effluent concentrations, rather small reductions (<33%) occurred in the concentrations of nitrogen, phosphorus, BOD, sodium, potassium, chloride and fluoride in the surface waters of domes receiving treated sewage effluent. Larger reductions (~50%) occurred for calcium, magnesium and sulfate. The conventional treatment plant/oxidation pond itself was effective in reducing levels of BOD and TN, but was ineffective in reducing levels of TP and other minerals. The effectiveness of cypress domes in removing high percentages (> 90%) of organic matter, nutrients and minerals is more obvious when concentrations of these substances in the shallow watertable aquifer below the sewage-enriched domes are examined. Concentrations of these parameters in shallow wells in and around the sewage domes were at background levels throughout the study. High chloride values (compared to control wells) indicated that treated sewage was percolating into the shallow aquifer. Cypress domes and their associated sediments and soils thus can reduce the levels of major water quality parameters to levels comparable to those of conventional tertiary treatment processes. Sediments and vegetation continued to release inorganic ions to the standing water for more than 20 months after the cessation of sewage inputs. On the other hand, parameters associated with organic matter (BOD) and with the reducing environment of the sewage-enriched domes (H2S, NH,~) displayed a rapid return to background levels.
Acknowledgements--The work reported here was performed at the Center for Wetlands and Department of Environmental Engineering Sciences, where the authors formerly were graduate student and professor, respectively. Research was supported by the National Science Foundation (Grant AEN 73-07823 A01) and the Rockefeller Foundation (Grant 73029), H. T. Odum and K. C. Ewel, principal investigators, and by the Center for Wetlands and En-
cessation of sewage loading on September 16, 1977. All values in mg I ~ unless otherwise noted NO3 N and NO~-N 4.10 <0.01 0.02 0.04 <0.01 0.04 0.01 0.03
NH2-N
Org. N
TN
TP
TC
8.30 4.50 1.50 1.85 0.92 0.22 0.08 1.18
1.2 3.3 0.4 0.4 2.1 2.8 1.2 2.0
13.6 7.8 1.9 2.3 3.0 3.0 1.3 3.2
8.10 38.5 2.72 98.0 1.44 72.0 0.69 100.0 2.40 -0.40 57.5 0.50 -1.20 82.0
SiO2
BOD 5
H2S
DO
Turbidity (NTU)
Color (CPU)
-15.2 4.3 0.5 3.9 1.3 -1.0
26.8 18.8 3.3 2.8 10.0 2.6 3.5 2.9
6.2 <0.01 <0.01 -<0.01 <0.01 <0.01 <0.01
1.05 0.00 4.25 4.10 0.35 0.00 6.10 0.30
18.0 13.0 4.0 0.0 0.0 ----
148 380 620 479 522 800 683 1200
1040
FORREST E. DIERBERG and PATRICK L. BREZONIK
gineering and Industrial Experiment Station, University of Florida, Gainesville. REFERENCES
APHA (1976) Standard Methods for Analysis of Water and Wastewater, 14th Edition. American Public Health Association, New York. Boyt F. L., Bayley S. E. & Zoltek J. Jr (1977) Removal of nutrients from treated municipal wastewater by wetland vegetation. J. Wat. Pollut. Control Fed. 49, 789-799. Coultas C. L. & Calhoun F. G. (1975) A toposequence of soils in and adjoining a cypress dome in north Florida. Soil Crop Sci. Soe. Proc. 35, 186-191. Cutright B. L. (1974) Hydrogeology of a cypress swamp in northcentral Alachua County, Florida. M.S. Thesis, University of Florida, Gainesville. Dierberg F. E. (1980) The effects of secondary sewage effluent on the water quality, nutrient cycles and mass balances, and accumulation of soil organic matter in cypress domes. Ph.D. Thesis, University of Florida, Gainesville. Dierberg F. E. & Brezonik P. L. (1978) The effect of secondary sewage effluent on the surface water and groundwater quality of cypress domes. In Cypress Wetlands for Water Management, Recycling and Conservation (Edited by Odum H. T. & Ewel K. C.), pp. 178 270. Fourth Annual Report to the National Science Foundation and The Rockefeller Foundation. Center for Wetlands, University of Florida, Gainesville. Dierberg F. E. & Brezonik P. L. (1983) Nitrogen and phosphorus mass balances for sewage treated cypress domes. In Cypress Swamps (Edited by Ewel K. C. & Odum H. T.). University Presses of Florida, Gainesville. Dolan T. J., Bayley S. E., Zoltek J. Jr & Herman A. J. (1981) Phosphorus dynamics of a Florida freshwater marsh receiving treated wastewater. J. appl. Ecol. 18, 205 219. Ewel K. C. (1983) Effects of fire and sewage on understory vegetation in cypress domes. In Cypress Swamps (Edited by Ewel K. C. & Odum H. T.). University Presses of Florida, Gainesville. Fetter C. W. Jr, Sloey W. E. & Spangler F. L. (1978) Use of a natural marsh for wastewater polishing. J. War. Pollut. Control Fed. 50, 290-307. Florida Department of Environmental Regulation (1979) Domestic and Industrial Waste Treatment Requirements, Chap. 17 6, Part 1. Florida Administrative Code, Tallahassee. Hortenstine C. C. (1976) Chemical changes in the soil solution from a spodosol irrigated with secondary-treated sewage effluent. J. envir. Qual. 5, 335-338. Lee G. F., Bentley E. & Amundson R. (1975) Effects of marshes on water quality. In Coupling of Land and Water Systems (Edited by Hasler A. D.), pp. 105 127. Springer-Verlag, New York. Monk C. D. & Brown T. W. (1965) Ecological consideration of cypress heads in northcentral Florida. Am. Mid. Nat, 74, 126-140.
Odum H. T. & Ewel K. C. (Eds) (1975) In Cypress Wetlands for Water Management, Recycling and Conservation. Second Annual Report to the National Science Foundation and The Rockefeller Foundation. Center for Wetlands, University of Florida, Gainesville. Odum H. T. & Ewel K. C. (Eds) (1976) In Cypress Wetlands for Water Management, Recycling and Conservation. Third annual report to the National Science Foundation and The Rockefeller Foundation. Center for Wetlands, University of Florida, Gainesville. Odum H. T., Ewel K. C., Mitsch W. & Ordway J. (1975) Recycling treated sewage through cypress wetlands in Florida. Occasional Publication No. 1, Center for Wetlands, University of Florida, Gainesville. Richardson C. J., Tilton D. L., Kadlec J. A., Chamie J. P. & Wentz W. A. (1978) Nutrient dynamics of northern wetland ecosystems. In Freshwater Wetlands (Edited by Good R. E., Whigham D. F. & Simpson R. L.), pp. 217-241. Academic Press, New York. Sloey W. E., Spangler F. L. & Fetter C. W. Jr (1978) Management of freshwater wetlands for nutrient assimilation. In Freshwater Wetlands (Edited by Good R. E., Whigham R. E. & Simpson R. L.), pp. 321 340. Academic Press, New York. Spangler F. L., Fetter C. W. Jr & Sloey W. E. (1977) Phosphorus accumulation-discharge cycles in marshes. Water Res. Bull. 13, 1191-1202. Steward K. K. & Ornes W. H. (1975) Assessing a marsh environment for wastewater renovation. J. War. Pollut. Control Fed. 47, 1880-1891. Strickland J. E. H. & Parson T. R. (1972) A Practical Handbook of Seawater Analysis, Bull. 167. Fisheries Research Board of Canada, Ottawa. Tilton D. L. & Kadlec R. H. (1979) The utilization of a freshwater wetland for nutrient removal from secondarily treated waste water effluent. J. envir. Qual. 8, 328 334. U.S. Environmental Protection Agency (1974) Methods for Chemical Analysis of Water and Wastes. U.S. Government Printing Office, Washington, DC. van der Valk A. G., Davis C. B., Baker J. L. & Beer C. E. (1979) Natural freshwater wetlands as nitrogen and phosphorus traps. In Wetland Functions and Values: The State of Our Understanding (Edited by Greeson P. E., Clark J. R. & Clark J. E.), pp. 457 467. American Water Resources Association, Minneapolis, MN. Whigham D. F. & Simpson R. L. (1976) The potential use of tidal marshes in the management of water quality in the Delaware River. In Biological Control of Water Pollution (Edited by Tourbier J. & Pierson R. R. Jr), pp. 173-186. University of Pennsylvania Press, Philadelphia, PA. Zoltek J. Jr (1976) Monitoring of sewage treatment plant influent and effluent and cypress dome influent. In Cypress Wetlands for Water Management, Recycling, and Conservation (Edited by Odum H. T. & Ewel K. C.), pp. 44-52. Third Annual Report to the National Science Foundation and The Rockefeller Foundation, Center for Wetlands, Univ. Florida, Gainesville.
0043-1354/83 $3.00+0.00 Copyright © 1983 Pergamon Press Ltd
TERTIARY TREATMENT OF MUNICIPAL WASTEWATER BY CYPRESS DOMES FORREST E. DIERBERG1 and PATRICK L. BREZONIK2 1Department of Environmental Science and Engineering, Florida Institute of Technology, Melbourne, FL 32901 and 2Department of Civil and Mineral Engineering, University of Minnesota, Minneapolis, MN 55455, U.S.A.
(Received July 1982)
Abstrae~The feasibility of using cypress domes as an alternative to physical-chemical methods for tertiary treatment of sewage effluent was demonstrated over a 5-year period. Surface water quality in domes receiving effluent was degraded (low dissolved oxygen, high nutrient levels) compared to control domes, and the degree of treatment within the dome surface waters was relatively small. However, water samples from shallow wells and ceramic soil moisture tubes indicated that the underlying organic soils and clay sands served as an effective barrier to the transport of biochemical oxygen demand, nitrogen, phosphorus, sulfate, fluoride, potassium, sodium, calcium, and magnesium to the shallow aquifer immediately below. High percentage removals were observed for all measured water quality parameters between the surface and groundwater stations. Standing water quality following the cessation of sewage pumping did not return to the same quality as found for the natural dome within 20 months.
Wetlands have long been recognized as nutrient sinks. In a recent review on various types of wetlands, van der Valk et al. (1979) found that phosphorus was removed in all 16 studies that measured phosphorus, and 12 of 14 studies involving nitrogen reported at least seasonal removal. Two studies found wetlands to act as nitrogen sources. Studies on palustrine wetlands (i.e. nontidal wetlands that are not confined by channels and are not marginal to lakes) have indicated removal of nitrogen and phosphorus (e.g. Boyt et al., 1977; Fetter et al., 1978; Richardson et al., 1978). Removal efficiencies in northern marshes decline in the spring and fall when high hydraulic loadings wash out some nutrients assimilated during the previous growing season and mineralized during fall (Lee et al., 1975; Spangler et al., 1977). Only one study (Steward & Ornes, 1975) has shown that the assimilative capacity of a palustrine wetland (the Everglades) was exceeded by the input of nutrient-laden sewage effluent. The capability of wetlands to trap nutrients has stimulated studies to evaluate their response to the addition of wastewater (Boyt et al., 1977; Sloey et al., 1978; Tilton & Kadlec, 1979; Whigham & Simpson, 1976; Dolan et al., 1981). Sloey et al. stated that palustrine wetlands are more amenable to management for wastewater treatment than other wetlands (tidal, riverine, and lacustrine) because they are hydraulically isolated from open surface water and their hydraulic residence times are high. Cypress domes are a common palustrine wetland throughout the pine-palmetto woodlands of the southern Atlantic and Gulf Coastal Plain. Cypress domes are topographic depressions that vary in size from about 1 ha to more than 10ha. Pond cypress
(Taxodium distichum var. nutans) is the most conspicuous canopy tree, and black gum (Nyssa sylvatica var. biflora) is the dominant understory tree. More detailed accounts of the floristic and soil characteristics of cypress domes are given by M o n k & Brown (1965), Coultas & Calhoun (1975), and Ewel (1983). Feasibility studies on the use of these ecosystems as alternatives to conventional tertiary treatment were initiated in 1974 when treated secondary sewage effluent was added to two cypress domes by O d u m and co-workers (see O d u m & Ewel, 1975, 1976; O d u m et al., 1975). This paper summarizes basic water quality data for the cypress dome feasibility study over the period March 1974 to May 1979. Its primary intent is to describe water quality in a manner that indicates the capability of cypress domes to treat domestic sewage effluent and reduce the pollutant levels. Quantitative descriptions (mass balances) of the nitrogen and phosphorus cycles in a cypress dome receiving treated effluent are presented elsewhere (Dierberg & Brezonik, 1982), and detailed results have been described in annual project reports (e.g. Dierberg & Brezonik, 1978).
METHODS
Description of study area The main study site consisted of three cypress domes on a large pine plantation about 5 km north of Gainesville, Florida (Fig. 1). A "package treatment plant" serving a 155-unit mobile home park adjacent to the site supplied secondary sewage effluent to the centers of sewage dome l (S-l: 0.51 hal and sewage dome 2 (S-2; 1.05 ha) (Fig. 2). Pumping was initiated to sewage dome 1 in March 1974 and to sewage dome 2 in December 1974. The sewage plant was an extended aeration waste treatment facility that
1027
1028
FORREST E. DIERBERGand PATRICK L. BREZONIK
olo . Fig. 1. Locations of the experimental (Owens-Illinois) and control (Austin Cary) sites in Alachua County, Florida. I
[ OWENS- ILLINOIS SITE Nj.._
38e
~
41
~BURNED
AND CLEARED AREAS
Fig. 2. Site plan, groundwater monitoring wells and soil tube locations at the experimental (Owens Illinois) site.
Tertiary treatment of municipal wastewater by cypress domes treated about 95 m 3 day- i (25,000 gal day- l) or 83% of its design capacity of 114 m 3 day- ~. Treated effluent was discharged into an oxidation pond except during the period March 1974 to March 1975 when sewage was pumped to the cypress domes directly from the package plant. The plant was not operating efficiently during this period; hydraulic loadings ranged between 0 and 14 cm wk-~; and sludge occasionally was pumped into the domes. After March 1975, more uniform applications (2.5 cm wk- ~ with no sludge) were obtained using effluent that had been in the oxidation pond for about 10 days. The effluent pipe in the oxidation pond was relocated in March 1976 to a point closer to the treatment plant. Effluent additions to the domes were intermittent, with input pumps usually operating for 1 h in every 12 h. A combination of high application rates and heavy rainfall during summer of 1974 caused overflow from sewage dome 1. A weir was constructed on the west side of the dome to maintain standing water at a desired level and to allow overflow to be measured and sampled. A third dome (G-I; 0.70 ha) at the site received groundwater from a deep well (~50m) according to the same schedule and loading rate as sewage dome 1. It served as a hydrologic control, separating the response of acidic cypress domes to inputs of alkaline groundwater, from the effects of sewage pollutants (nutrients and organic matter). A larger dome (4.2 ha), about 17 km northeast of the main site in the Austin Cary Memorial Forest (Fig. 1), served as a "natural" control whose water levels fluctuated according to the normal hydroperiod. Two of the three domes (groundwater control and sewage dome 1) at the main site were burned extensively in an unintentional forest fire on 4 December, 1973; sewage dome 2 was burned only along the southwest edge.
Sampling procedures Standing water, sewage, and shallow groundwaters on the main site and standing water of the Austin Cary dome were monitored routinely for major ions, nutrients, and organic matter from March 1974 to May 1979. Samples were collected monthly from stations at the main site until December 1976, followed by quarterly sampling until May 1979. Austin Cary dome surface waters were sampled monthly from April 1978 to July 1979. The data from shallow wells (Fig. 2) were composited into three groups: (1) wells surrounding sewage domes 1 and 2 and wells located within sewage dome 1;(2) wells surrounding the groundwater control dome; and (3) wells distant from any of the experimental cypress domes and representative of the natural groundwater of the area.
Analytical Chemical analyses were performed according to accepted manuals (APHA, 1976; U.S. EPA, 1974). Analysis of major cations was performed by atomic absorption spectrophotometry (Varian Techtron Model 1200) with instrument settings recommended by the manufacturer. Automated colorimetric procedures were used for nitrate plus nitrite (cadmium reduction method), ammonium (indophenol method), Kjeldahl nitrogen (semi-micro digestion followed by determination of ammoniumt, and chloride (ferric thiocyanate method), Sulfate was analyzed turbidimetrically. Soluble reactive phosphorus and total phosphorus (persulfate digestion) were determined spectrophotometrically by the ascorbic acid-molybdenum blue method. Sulfide was analyzed by the colorimetric method of Strickland & Parsons (1972). Fluoride was measured by ion-selective electrode and pH by glass electrode. Dissolved organic and inorganic carbon were analyzed using a Beckman Model 915 carbon analyzer. Dissolved oxygen was determined by the azide modification of the Winkler method. Five-day biochemical oxygen demand (BOD-5) was determined by the amount of oxygen lost after incu-
1029
bating for 5 days in the dark at 20°C. Samples for anions (except chloride), cations, and nutrients were preserved with HgC12 and frozen from the time of collection until analysis within 1 week of collection. Sulfide and biochemical oxygen demand samples were kept refrigerated until analyzed within 24 h of collection.
Soil-solution studies Soil-solution sampling tubes (porous ceramic cups attached to 3.8 cm I.D. PVC pipes) were installed at three soil depths (60, 120, and 180cm) near the edges of the groundwater control dome and sewage domes 1 and 2 (Fig. 2). For the stations closest to the sewage domes, duplicate tubes were placed at each depth. The other stations had one tube at each depth. The ceramic cups were washed in 1 : 2 concentrated HNO3 and rinsed in demineralized water before installation in the field. Soil solutions were sampled by drawing a vacuum in the tubes and collecting the water 24 h later. Water that had accumulated in the tubes before a vacuum was applied was discarded. Samples were obtained in November and December 1976 and January, May, and August 1977. Nutrient concentrations in the soil water were determined by procedures cited above. EFFECTIVENESS OF THE TREATMENT PLANT/DOME SYSTEM IN POLLUTANT REMOVAL
The most significant change in appearance in the domes receiving sewage effluent was the growth of a rather dense blanket of duckweed (Lemna spp, Spirodela) that covered most of the surface water during most of the year. This cover inhibited reaeration and resulted in anoxic or near anoxic conditions and the formation of sulfide in the standing water. However, odors were not a problem at the sites, although the presence of reduced sulfur compounds could be detected when the water was artificially stirred. Similarly, insects such as mosquitoes were not found to be a problem, although there were some changes in diversity and numbers in the treated domes, compared to the controls. Details of insect studies done by others were presented in annual project reports (e.g. O d u m & Ewel, 1975, 1976). Bacterial contamination (fecal coliforms) was monitored on a routine basis during the project (e.g. Price, 1975, in O d u m & Ewel, 1975). In general, standing waters of sewage domes had elevated coliform levels (compared to controls), but water from the shallow wells showed no evidence of contamination by transport of bacteria in water percolating through the dome sediments. A few cases of elevated fecal coliform levels in shallow wells near sewage domes were attributed to overflow of surface waters from the domes during rainy periods and consequent contamination of the wells through broken casings. The effectiveness of cypress domes in reducing the levels of sewage pollutants in dome surface waters and associated groundwaters is illustrated in Figs 3-6. Stations were combined into nine subgroups within four major groups. Each subgroup is represented by a bar in the histograms of Figs 3-6; the legend of Fig. 3 defines the stations comprising each subgroup. The first category (three subgroups) follows the sewage
FORREST E. DIERBERG and PATRICK L. BREZONIK
1030
3.; o
o} E o
bJ
(170)
~"
Organic
C
BOD 5
•,
(288)
80-
o
70-
~8 ® ~-
i
3 ~ ,-
60-
:,: o .E "~ ,~ m c = E =
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o
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ackage Sewage! Wells ant R, dome' fox pond surface water
Natural dome
Package Sewage Wells plant ~ dome[
surface
as pond
water
surfaCe
Natural
dome
surface water
water I
Fig. 3. Average concentrations (___l SE) of organic carbon and biochemical oxygen demand at sites within the treatment plant/dome system and in natural (control) sites over the study period (March 1974-May 1979). Bars represent (left to right): raw sewage; secondary sewage effluent; oxidation pond; mean of center stations for S-1 and S-2; mean of edge stations in S-1 and S-2; mean of seven wells in and around S-1 and S-2; mean of two wells near G - l ; mean of four control wells (distant from all experimental domes); and natural control dome (Austin Cary surface water).
50
-
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-
~
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Natural dome
ox p o n d
surf=ml wa~r I
surface water
=ackoge ~WCge Wells Natural ;)lont a Idome J dome =x pond Isur~ I surface water iwmlr
Fig. 4. Average concentrations -t- 1 SE) of total nitrogen and total phosphorus at sites within the treatment plant/dome system and in natural (control) sites over the study period. See Fig. 3 for key to sites.
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1032
FORREST E. DIERBERG a n d PATRICK L. BREZONIK
0.7' 160
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Fig. 6. Average concentrations (+ 1 SE) of chloride, sulfate and fluoride at sites within the treatment plant/dome system and in natural (control) sites over the study period. See Fig. 3 for key to sites. through the treatment plant and oxidation pond. The second category (two subgroups) follows sewage through dome surface water, and the third category (three subgroups) compares contaminant levels in shallow groundwaters of wells near sewage domes and at control sites. The fourth category represents the surface water control (Austin Cary dome).
Biochemical oxygen demand and total organic carbon BOD values more closely reflect the effect of sewage effluent on the highly colored surface waters of cypress domes than do total organic carbon (TOC) levels (Fig. 3). The package treatment plant removed 94% of the BOD of the raw sewage; the oxidation pond removed an additional 2%. Relatively small reductions in BOD were observed between the centers and edges of both sewage domes (35-38%), indicating the dome surface waters were not very efficient in oxidizing degradable organic matter. Groundwater from the four wells around sewage dome 2 had BOD levels below those in the natural dome surface water. The two sewage-enriched domes had BOD levels comparable to the two control domes when the influent pipe to the sewage domes was located at the far end of the oxidation pond (October 1975-February 1976) (Fig. 7). Except for that period, BOD levels were much higher in the sewage-enriched domes, probably from a combination of lower rainfall and higher levels of organic carbon in the sewage. Relocation of the influent pipe closer to the discharge from the treatment plant in March 1976 increased the input of organic carbon to the domes (Zoltek, 1976). BOD levels for both control domes normally were less than 5 mg 1- 1 whereas BOD levels in the sewage domes usually were above 5 mg 1- 1 (which is the standard of the State of Florida Department of Environmental Regulation (1979) for finished waters of domestic tertiary sewage treatment). It should be noted that all BOD samples were filtered through Whatman No. 4 qualitative filters to remove duckweed and sediments before incubation. This procedure gave a better
measurement of BOD levels attributable to the incoming sewage, but it probably resulted in an underestimate of the true BOD levels.
Nitrogen The package treatment plant substantially reduced influent total nitrogen (TN) levels (from 50 to 19mg1-1, or 62% removal; Fig. 4). The oxidation pond reduced the average concentration by another 19%. Higher mean concentrations of TN at the centers of the sewage domes than in the oxidation pond can be explained by the history of effluent discharge to the domes. Secondary effluent with higher TN concentrations was pumped to the domes directly from the treatment plant or from near the entrance to the oxidation pond during 47 of the 60 months of the study. Nitrogen concentrations in the wells surrounding and within the sewage domes maintained the same levels as the control wells. Except for a brief 3-month period (OctoberDecember 1975), TN concentrations in the surface waters of the sewage treated domes were higher than those in the control domes (Fig. 8). Mean TN concentrations for sewage domes 1 and 2, Austin Cary dome and the groundwater control dome over the study period were 8.6, 12.3, 1.6 and 1.5 mgl 1 respectively. The low TN levels during late 1975 and the increase beginning in February 1976 reflect changes in the quality of the incoming sewage effluent, as discussed previously. The center stations of the sewage domes had higher levels of TN than did edge stations. The average TN level was 33% lower at one edge station and 44~o lower at another compared to the average at the center of sewage dome 1. Similarly, average TN levels of edge stations in sewage dome 2 were 41 and 310/,i lower than the center station mean value. The lack of dissolved oxygen in the sewage domes resulted in a preponderance of ammonium in the inorganic N pool (see Dierberg, 1980}. Levels of TN in surface waters of the two sewage-
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1036
FORREST E. DIERBERG and PATRICK L. BREZONIK
enriched domes generally were lower during the first 2 years of the project (Fig. 81, corresponding with a period of higher levels of nitrate and lower levels of TN in the effluent (Zoltek, 1976). Thus, it appears possible to maintain some control over the levels of TN in surface waters of cypress domes receiving sewage by maintaining a nitrified effluent.
Phosphorus The substantial reductions in BOD and nitrogen produced by the treatment plant did not occur for phosphorus (Fig. 4). Only 20~o of the total phosphorus (TP) was removed by the plant and another 7~/,, was removed by the oxidation pond. As a result, TP concentrations in the sewage domes were nearly the same as those for TN, even though TN levels were five times higher than TP levels in raw sewage. With one exception, concentrations of TP in the shallow wells were at background levels, implying complete removal of phosphorus by the domes. High TP concentrations in well 19 west of sewage dome 1 are attributed to naturally high levels; since this well was slightly deeper and penetrated the phosphorus-rich Hawthorn Formation (Dierberg, 1980). Concentrations of TP in the surface waters of sewage domes 1 and 2 ranged from 2 to 100 times higher than those of the two control domes (Fig. 9), which had mean values of 0.27 mg 1 1 (groundwater control) and 0.18 mgl-1 (Austin Cary control). Sewage dome 2 consistently had higher TP concentrations than did sewage dome 1 (Fig. 9), and concentrations generally were lower at the edges of both domes than at the center (Fig. 4). However, in most instances the decrease was less than 50~o, and concentrations almost always were higher than the lmg1-1 value recommended for finished waters of tertiary sewage treatment by the Florida Department of Environmental Regulation (1979). Over the entire study period, the average levels of TP at the edge of sewage dome 1 were 29-31~o lower than the average at the center (7.1 mg 1- 1). Average concentrations of TP at the edge stations in sewage dome 2 were 25-35~o lower than the average at the center (8.2 mg 1-1). Large temporal fluctuations in TP levels in sewage domes 1 and 2 (Fig. 9) at the beginning of the study reflect the variable nature of the sewage source. Relocation of the effluent pipe within the oxidation pond in March 1976 to a point closer to the treatment plant did not result in any observable changes of TP concentrations in the incoming sewage or the standing waters of the two sewage-enriched domes (Fig. 9). As mentioned previously, this was not the case for levels of BOD and TN. It is noteworthy that most of the TP in the sewage enriched domes was in the form of orthophosphate, which is readily used by plants.
Major cations The monovalent cations were similar to phosphorus in their concentration patterns (Fig. 5). Decreases of 77~o for sodium and 95~o for potassium
were found in comparing levels in wells around the domes to levels in surface water at the centers of the sewage domes. During 1978 and 1979 sodium concentrations in the wells surrounding sewage dome 2 increased to about half the levels in the surface water, pointing to downward movement of surface waters into the shallow aquifer. Concentrations of divalent cations were relatively constant through the treatment plant and oxidation pond, but they were lower (52Vo for Ca and 44~0 for Mgj at the edges of the sewage domes than at the centers (Fig. 5). In comparison, little change was noted in monovalent cations between the centers and edges. The decreased concentrations of divalent cations may reflect fixation onto exchange sites of the dome sediments. Concentrations of Mg in the groundwater under and around the two domes were equal to background levels in the control wells; but Ca levels were lower in the wells near the sewage domes than in the natural control wells. The proximity of the calcareous Hawthorn Formation to the control wells may explain the differences in Ca levels, since the latter wells were drilled deeper (av. depth = 4.3 m) than the wells surrounding the experimental domes (av. depth = 3.7 m/(Cutright, 1974).
Anions Primarily because of dietary use of salt, chloride concentrations are higher in domestic sewage than in most freshwaters. Since chloride is a conservative ion (i.e. it is not adsorbed by soil components or assimilated by plants) it can be used to trace the effluent. Levels of chloride in wells thought to be influenced by sewage percolation were more than half the values found in the sewage dome centers (Fig. 6). Average chloride concentrations were the same from the raw sewage to the dome edges, pointing to the conservative nature of chloride. The higher chloride concentrations in wells surrounding the groundwater control dome (middle bar of well group in Fig. 6) are unexplainable since the groundwater pumped into this dome had low chloride levels ( ~ 7 m g l - 1 ) . During 1978 and 1979, chloride levels in the wells surrounding sewage dome 2 increased to concentrations comparable to those in the surface water. A considerable decrease in the concentration of sulfate (39%) occurred between the center and edge stations of the sewage domes, probably because of reduction of sulfate to sulfide (Fig. 6). Sulfate levels in wells around the sewage domes were similar to those in control wells. A decrease in the concentrations of fluoride (37~o) between the center and edge stations of the sewage domes suggests that chemical precipitation may have occurred. Given the low solubility of fluorapatite (Kso ~ 10 -55) and the high levels of phosphate and calcium, the observed concentrations of fluoride are considerably above the solubility limits for fluorapatite, even at the dome edges. Fluoride levels in wells surrounding the groundwater control dome were
*Median pH. t N u m b e r of samples. :~Based on two samples.
Sewage D o m e 2 4 m northwest of edge (replicate soil tubes)
Sewage D o m e 2 13 m southeast of edge
Sewage Dome I 7 m west of edge (replicate soil tubes)
Sewage D o m e 1 36 m west o1 edge
Sewage D o m e I 15 m east of edge
Groundwater Control D o m e 3 m west of edge
Station 60 120 180 60 120 180 60 120 180 60 60 120 120 180 180 60 120 180 60 60 120 120 180 180
Depth (cm) 0.01 0.03 0.02 0.01 0.01 0.02 0.02 0.02 0.03 0.03 0.03 0.03 0.03 0.02 0.03 0.03 0.04 0.06 0.09 0,02 0.02 0.02 0.02 0.03
( N O 3 and NO2-) N ( m g l -~} 0.35 0.14 0.25 1.1 0.05 0.02 0.07 0.13 0.04 0.37 0.12 0.05 0.05 0.07 0.09 0.28 0.11 0.06 0.06 0.05 0.19 0,16 0.12 0.09
NH4 N {mgl ~) 0.6 0.6 0.7 0.6 0.5 0.3 0.5 0.8 0.7 0.7 0.4 0.7 1.2 0.5 0.5 0.9 0.5 0.6 1.1 0.5 0.8 0.3 0.5 0.8
Organic N ( m g l -~) 0.02 0.02 0.01 0.03 0.01 0.02 0.04 0.02 0.02 0.01 0.01 0.01 0.01 0.02 0.01 0.01 0.01 0.02 + 0.005 0.01 0.02 0.02 0.01 0.005
Total P ( m g l ~) 54 49 84 67 38 37 91 88 132 144 156 166 157 169 171 1415 200 95 179 179 156 150 150 149
Specific conductance (/Lmhocm ~) 6 10 10 11 7 7 20 21 34 39 40 45 42 42 44 > 100 55 26 46 48 43 39 37 39
CI (mgl 4.2 4.4 4.5 5.0 4.6 4.8 4.0 4.5 4.8 4.3 4.2 4.4 4.6 4.5 4.4 4.7 4.4 4.4 4.4 4.3 4.2 4.4 4.1 4.2
~) pH*
5 5 2 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5
N+
Table 1. Mean levels for selected chemical parameters of the interstitial water taken at three depths from soil tubes surrounding the cypress d o m e s at the Owens Illinois site in November, December, January, M a y and August (1976 1977)
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1038
FORREST E. DIERBERG and PATRICK L. BREZONIK
similar to those in wells around the sewage domes. However, fluoride levels were slightly higher (Fig. 6) in the deeper control wells that penetrated the Hawthorn Formation.
Hydrogen sulfide Concentrations of H2S greater than 1 mg 1-1 were found regularly at several stations in the experimental domes. The sewage effluent had high concentrations of sulfate (~40 mgl-1) compared to the natural surface waters and shallow groundwaters of the area; apparently some sulfate was reduced in the anoxic waters and sediments of the experimental domes. Because pH values for the surface waters of the sewage domes (~6.0) are below pK1 for H2S (7.0), the undissociated volatile species (HzS) predominated, comprising 65-92°/~, of the total dissolved sulfide in the water. SOIL S O L U T I O N STUDIES
Chloride concentrations indicate that infiltration from sewage dome 1 occurred in a westerly direction, and the extent of infiltration increased with depth at increasing distances from the dome (Table 1). On the other hand, infiltration from sewage dome 2 occurred about equally to the southeast and northwest. Unusually high chloride levels (up to 1000mg1-1) at 60 cm depth southeast of sewage dome 2 are an artifact from the disposal of filtrate high in MgC12 during virus sampling. Concentrations of chloride in standing water at the edges of the sewage domes ranged from 12 to 105mg1-1 for the study period (.,Y = 46 mg 1-1 for sewage dome 1 and 51 mg 1-1 for sewage dome 2). It is evident from the chloride data that the soil tubes located 7 m west of sewage dome 1 (all depths), 13 m southeast of sewage dome 2 (all depths except 180 cm), and 4 m northwest of sewage dome 2 (all depths) were in zones receiving a large amount of infiltration from the sewage domes. However, nutrient concentrations in these samples were consistently low, indicating a high degree of removal
of nutrients from infiltrating dome waters. Nutrient concentrations in samples with large fractions of sewage infiltrate (as indicated by chloride levels) did not differ significantly from the nutrient concentrations in samples with low or negligible fractions of sewage in the soil solution (Table 1). Nitrate contamination in groundwater is not a danger at present loading rates, nor does enrichment of adjacent waterways by other nitrogen and phosphorus species seem a matter of concern. Apparently, the spodic horizon in these soils is important in the removal of phosphorus which can move freely only to the depth at which this horizon occurs (Hortenstine, 1976). The latter author reported that phosphorus from secondary sewage effluent sprayed onto a spodosol near Orlando, Florida, moved freely in the soil solution above the spodic horizon, but efficient removal by this horizon was indicated by the absence of detectable phosphorus in soil solution below this layer. According to Coultas & Calhoun (1975), the spodic horizon in soil adjacent to the cypress domes occurs at depths of 2 ~ 3 0 cm. This fact explains the low levels of phosphate even in the shallow (60cm) soil tubes. Field data from the soil tubes corroborate results from the monitoring wells. Efficient removal (>95~0) of TP, nitrate, ammonium and TN by the sediments and soils surrounding the cypress domes occurred throughout the 5-year period of sewage disposal into the domes. WATER QUALITY F O L L O W I N G CESSATION OF SEWAGE I N P U T S
Pumping of treated sewage influent into sewage dome 1 was discontinued in September 1977. Sampiing was continued on an irregular schedule (depending on the presence of standing watert for the following 20 months to determine the extent of recovery in surface water quality. Wide variations were observed for most water quality parameters during the period following termination of sewage loading (Table 2). These variations reflected two major factors: (1)
Table 2. Changes in water quality at the center station of sewage dome 1 after
Date 2 Aug. 1977t 16 Dec. 1977¶ 1 Feb. 1978 7 Mar. 1978 6 May 1978¶ 4 Aug. 1978 22 Feb. 1979¶ 26 May 1979
Conductivity (•mhocm 1)
pH
Ca
Mg
Na
K
SO24-
HCO3*
C1-
F-
520 547 151 73 269 35 119 100
6.80 5.10 5.00 3.30 5.25 4.21 4.60 5.43
18.3~ 41.9 3.5 2.3 8.6 2.6 4.8 4.2
11.6~ 23.2 3.5 1.5 5.8 1.0 2.3 2.0
39.0~ 75.4 18.2 7.3 24.5 2.6 13.0 9.0
8.0~ 12.8 2.0 3.9 6.7 2.5 1.3 1.2
28.9§ 237.0 9.0 43.0 60.0 < 1.0 18.0 10.0
125.6 4.9 7.3 0.0 8.5 0.0 0.0 --
63.0 65.0 26.0 8.5 22.5 2.5 14.2 17.0
0.65 0.13 0.08 0.06 0.11 0.04 0.08
*Calculated from H +-HCO~- equilibrium using alkalinity. tLast date of sampling before cessation of treated sewage loading. :~Mean of 17-18 surface water samples taken at dome center from April 1974 to August 1976 during sewage loading. §Mean of 5 surface water samples taken at dome center from August 1975 to March 1977 during sewage loading. ¶Dry period with no standing water preceded sampling date.
Tertiary treatment of municipal wastewater by cypress domes elapsed time since cessation of sewage influent; and (2) the frequency, extent and intensity of rainfall prior to sampling. Because of an extended dry period (OctoberNovember 1977), the first sampling of the surface water after termination of sewage input was in December 1977, and the drought resulted in higher levels of conductance, minerals, nutrients, total carbon, BOD and turbidity than occurred in any other subsequent period (Table 2). Levels of most major ions were higher than the levels found in the dome during sewage loading. Oxidation of reduced sulfur that had been immobilized as organic or metal sulfides during sewage pumping may have accounted for the high sulfate level (237 mg 1-1) after the dry period. Other sampling dates on which high concentrations of water quality constituents were found (May 1978, and February and May 1979) also were preceded by dry periods that resulted in the disappearance of standing water from the dome, thus allowing a build-up of the products of aerobic decomposition and oxidation. Sampling dates on which the lowest concentrations were found (February, March and August 1978) were preceded by heavy rainfall that diluted the pollutants remaining from the period of sewage loading. On one occasion (August 1978), all water quality parameters except potassium and TP were within the ranges measured for the natural dome. The effects of sewage could still be discerned 20 months after sewage inputs had ceased. Specific conductance, major ions, fluoride, ammonium and total phosphorus were still higher than the mean values for the natural dome. Evidently, sediments and vegetation on the swamp floor released these substances to the standing water long after the end of sewage pumping. However, many important water quality parameters, including pH, nitrate, BOD total carbon, H2S, dissolved oxygen, turbidity, color and silica, returned to natural background levels within 8 months after sewage inputs were discontinued.
1039
CONCLUSIONS Compared to the effluent concentrations, rather small reductions (<33%) occurred in the concentrations of nitrogen, phosphorus, BOD, sodium, potassium, chloride and fluoride in the surface waters of domes receiving treated sewage effluent. Larger reductions (~50%) occurred for calcium, magnesium and sulfate. The conventional treatment plant/oxidation pond itself was effective in reducing levels of BOD and TN, but was ineffective in reducing levels of TP and other minerals. The effectiveness of cypress domes in removing high percentages (> 90%) of organic matter, nutrients and minerals is more obvious when concentrations of these substances in the shallow watertable aquifer below the sewage-enriched domes are examined. Concentrations of these parameters in shallow wells in and around the sewage domes were at background levels throughout the study. High chloride values (compared to control wells) indicated that treated sewage was percolating into the shallow aquifer. Cypress domes and their associated sediments and soils thus can reduce the levels of major water quality parameters to levels comparable to those of conventional tertiary treatment processes. Sediments and vegetation continued to release inorganic ions to the standing water for more than 20 months after the cessation of sewage inputs. On the other hand, parameters associated with organic matter (BOD) and with the reducing environment of the sewage-enriched domes (H2S, NH,~) displayed a rapid return to background levels.
Acknowledgements--The work reported here was performed at the Center for Wetlands and Department of Environmental Engineering Sciences, where the authors formerly were graduate student and professor, respectively. Research was supported by the National Science Foundation (Grant AEN 73-07823 A01) and the Rockefeller Foundation (Grant 73029), H. T. Odum and K. C. Ewel, principal investigators, and by the Center for Wetlands and En-
cessation of sewage loading on September 16, 1977. All values in mg I ~ unless otherwise noted NO3 N and NO~-N 4.10 <0.01 0.02 0.04 <0.01 0.04 0.01 0.03
NH2-N
Org. N
TN
TP
TC
8.30 4.50 1.50 1.85 0.92 0.22 0.08 1.18
1.2 3.3 0.4 0.4 2.1 2.8 1.2 2.0
13.6 7.8 1.9 2.3 3.0 3.0 1.3 3.2
8.10 38.5 2.72 98.0 1.44 72.0 0.69 100.0 2.40 -0.40 57.5 0.50 -1.20 82.0
SiO2
BOD 5
H2S
DO
Turbidity (NTU)
Color (CPU)
-15.2 4.3 0.5 3.9 1.3 -1.0
26.8 18.8 3.3 2.8 10.0 2.6 3.5 2.9
6.2 <0.01 <0.01 -<0.01 <0.01 <0.01 <0.01
1.05 0.00 4.25 4.10 0.35 0.00 6.10 0.30
18.0 13.0 4.0 0.0 0.0 ----
148 380 620 479 522 800 683 1200
1040
FORREST E. DIERBERG and PATRICK L. BREZONIK
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