Scholarly article on topic 'Aquaculture sludge removal and stabilization within created wetlands'

Aquaculture sludge removal and stabilization within created wetlands Academic research paper on "Animal and dairy science"

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{Denitrification / "Horizontal flow" / Nitrification / "Overland flow" / "Reed beds" / "Sand drying beds" / "Sludge dewatering" / "Sludge treatment" / Vetiver / "Vertical flow"}

Abstract of research paper on Animal and dairy science, author of scientific article — Steven T. Summerfelt, Paul R. Adler, D.Michael Glenn, Ricarda N. Kretschmann

Abstract The objective of this research was to investigate treatment of the concentrated solids discharge produced during clarifier backwash within an aquaculture facility. Solids removal and stabilization were investigated within two types of created wetlands where water flowed either: (1) vertically, down through a porous substrate; or (2) horizontally, over soil and through plant hedges. Six 3.7×1.2×0.8-m (L×W×H) wetland cells were used to provide three replicates for both types of wetland. Approximately equal numbers of vetiver grass (Vetiveria zizanioides) tillers were planted on both wetlands types in November of 1994. Sludge (7500 mg l−1 solids) was loaded onto both wetland types six times day−1, with no scheduled drying cycle, from 12 May 1995 until 28 February 1996. Sludge was applied at a rate of about 1.35 cm day−1, or about 30 kg dry solids m−2 year−1. Results from this short study indicated that the vertical flow and horizontal flow wetlands, respectively, removed 98 and 96% TSS, 91 and 72% total COD, and 81 and 30% dissolved COD. Both types of wetland cells removed most (82–93%) of the total kjeldahl nitrogen, phosphorus, and dissolved phosphate. Measurements of sludge depths and TVS at the end of the study indicated considerable mineralization occurred in the wetlands; stored sludge at the end of the study had 50% less TVS than untreated sludge.

Academic research paper on topic "Aquaculture sludge removal and stabilization within created wetlands"

Aquacultural Engineering 19 (1999) 81-92

Aquaculture sludge removal and stabilization within

created wetlands

Steven T. Summerfelt a'*, Paul R. Adlerb, D. Michael Glenn b, Ricarda N. Kretschmann a

a The Conservation Fund's Freshwater Institute, P.O. Box 1746, Shepherdstown, WV 25443, USA b USDA-ARS, 45 Wiltshire Road, Kearneysville, WV 25430, USA

Received 31 July 1998; accepted 30 August 1998

Abstract

The objective of this research was to investigate treatment of the concentrated solids discharge produced during clarifier backwash within an aquaculture facility. Solids removal and stabilization were investigated within two types of created wetlands where water flowed either: (1) vertically, down through a porous substrate; or (2) horizontally, over soil and through plant hedges. Six 3.7 x 1.2 x 0.8-m (L x W x H) wetland cells were used to provide three replicates for both types of wetland. Approximately equal numbers of vetiver grass (Vetiveria zizanioides) tillers were planted on both wetlands types in November of 1994. Sludge (7500 mg l-1 solids) was loaded onto both wetland types six times day -1, with no scheduled drying cycle, from 12 May 1995 until 28 February 1996. Sludge was applied at a rate of about 1.35 cm day -1, or about 30 kg dry solids m - 2 year -1. Results from this short study indicated that the vertical flow and horizontal flow wetlands, respectively, removed 98 and 96% TSS, 91 and 72% total COD, and 81 and 30% dissolved COD. Both types of wetland cells removed most (82-93%) of the total kjeldahl nitrogen, phosphorus, and dissolved phosphate. Measurements of sludge depths and TVS at the end of the study indicated considerable mineralization occurred in the wetlands; stored sludge at the end of the study had 50% less TVS than untreated sludge. © 1999 Elsevier Science B.V. All rights reserved.

Keywords: Denitrification; Horizontal flow; Nitrification; Overland flow; Reed beds; Sand drying beds; Sludge dewatering; Sludge treatment; Vetiver; Vertical flow

* Corresponding author. Tel.: + 1 304 8762815; fax: + 1 304 8760739; e-mail: ssummerf@ ix.netcom.com

0144-8609/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S0144-8609(98)00042-9

1. Introduction

Removal of solids or nutrients from the effluents of fish farms is often required because of priority and regulations given to minimizing the effect of the discharge on the environment (Bastian, 1992; Ewart et al., 1995; Idaho DEQ, 1998). Aquaculture systems often have two separate discharges, and solids and/or nutrients in both, if left untreated, can have a negative affect upon receiving waters. When systems have two separate discharges, the effluent of largest volume usually contains comparatively low levels of solids and nutrients, particularly nitrogen and phosphorous. A second effluent, generated while trapped solids are washed from the solids treatment unit during backwash, is comparatively small but contains high levels of concentrated organic solids. The settleable fraction of solids in this clarifier-backwash effluent are often recaptured within a settling basins and are then removed as a thickened sludge containing 3-10% solids (Westers, 1991; Bergheim et al., 1993; Chen et al., 1997; Idaho DEQ, 1998). Disposal of this thickened sludge can be an issue.

Many states in the US classify and regulate aquaculture sludge as an industrial or municipal waste, because the sludge is a residual product of wastewater treatment (Bastian, 1992; Ewart et al., 1995). Other states, however, consider the aquaculture sludge to be an agricultural waste, because it is composed of manure and uneaten feed and is thus considered to be a non-toxic nutrient source. When classified as an agricultural waste, aquaculture sludge does not have to be considered a liability because it can be beneficially applied to land to fertilize agricultural crops. Using aquaculture effluents as inputs for production of other products can improve overall facility sustainability. Several technologies can be used to treat nutrients or biosolids in aquaculture effluents while producing other valuable products such as high-value fruits and vegetables (Adler et al., 1996a,b,c), grass turf (Adler et al., 1996d), and organic composts (Adler et al., 1996c). Although reuse of effluent streams is always worth considering, it is sometimes difficult to develop the technologies and markets required to support reuse as a form of effluent treatment. The two most common methods used to recycle solid wastes from aquaculture facilities are land application and composting (Ewart et al., 1995; Chen et al., 1997; Idaho DEQ, 1998). According to Ewart et al. (1995), land application of manure and other organic wastes (including wastewater) to fertilize agricultural crops is governed in most states by guidelines or regulations that limit the amount of pathogens, heavy metals, and other contaminants and the land application rates. In particular, application rates are based upon nutrient content, soil type, and plant nutrient uptake characteristics to prevent runoff or groundwater contamination (Ewart et al., 1995; Chen et al., 1997). Odor problems can also limit land application in populated areas. Sludge transport from the facility to another point of disposal or reuse is a major factor in the costs of sludge management, because the thickened sludge is greater than 90% water (Black and Veatch LLP, 1995; Reed et al., 1995).

Depending on an aquaculture facility's location and the local regulations, an aquaculture facility may have only limited and costly options available for sludge

disposal. If land application is not available adjacent to the facility, on-site treatment of the concentrated solids discharge with an uncomplicated, low-maintenance plant-based system could reduce solids disposal costs (Outwater, 1994).

Created horizontal flow wetland (HFW;, i.e. overland flow wetland) systems have been used with some success to treat high-strength aquacultural wastewaters (Pardue et al., 1994) and other agricultural, municipal, or industrial wastewaters (reviewed by Reed et al., 1995). HFW systems are usually operated with a hydroperiod to produce cycles of inundation and dewatering. However, HFW systems typically are not loaded with thickened sludges.

On the other hand, constructed vertical-flow wetland (VFW) systems have been used, over the past 20 years to treat thickened sludge (1 -7% solids) produced in the clarifier underflow at wastewater treatment plants (Hofmann, 1990; Lienard et al., 1990; Nielsen, 1990, 1993; Riggle, 1991; Outwater, 1994; Reed et al., 1995). VFW wetlands are generally referred to as 'reed beds' because they are often planted with reeds. When used for municipal treatment, these wetlands are loaded with 7-10 cm of 2% solids approximately once every 7-21 days (about 30-60 kg m- 2 year-1). During operation, a series of vegetated beds receives sequential batch applications of sludge. The sequential batch applications are such that the more recently flooded VFW cells are dewatering, while beds with older sludge applications are drying. Intervals between sludge addition allow for dewatering and drying. Plants facilitate dewatering by conducting water along their stem and root paths through previous sludge layers and by removing water through evapotranspiration (Outwater, 1994; Reed et al., 1995). The plants also increase biological stabilization of the solids by transporting oxygen to their root zones. Reed bed treatment system have been reported to have a useful lifetime of up to 10 years (Outwater, 1994; Reed et al., 1995).

Aquaculture sludges are good candidates for use in both crop or created wetland. However, if transportation costs make sludge disposal on crop land uneconomical, disposing of the sludge on-site within created wetlands might be the next best alternative. The objectives of the work reported in this paper were to investigate disposal and treatment within created wetlands of an aquaculture sludge produced during clarifier backwash. This research focused on the variables controlling capture and stabilization of solids within created wetland systems. Solids removal and stabilization were investigated within two types of created wetlands where water flowed either: (1) vertically, down through a porous substrate; or (2) horizontally, over soil and through hedges. These two wetland types differed in both physical characteristics and in hydraulic distribution and collection.

Both created wetlands types were planted with vetiver grass (Vetiveria zizan-ioides ). Vetiver grass was selected because it is tolerant of a wide range of environmental conditions, and has been proven to control soil erosion throughout the world (Becker, 1992). When planted as narrow hedges, the dense vetiver shoots act as a filter, allowing water to pass through while holding soil back to settle by gravity, thereby preventing erosion. Vetiver also has an extensive and deeply growing root system that would help maintains the bed's hydraulic conductivity and contribute to oxygen transport into the bed.

2. Methods

Sludge used in these studies was collected from the recirculating trout-production system at the Freshwater Institute (Heinen et al., 1996). Sludge originated from the clarifier backwash and was collected and thickened to about 5% solids in a septic tank before it was pumped to the greenhouse where the wetland cells were located. However, the manner in which the sludge was collected and pumped from the septic tank to the equalization tank within the greenhouse diluted the sludge to about 0.75% dry solids by weight. Sludge pumped from the equalization tank was thoroughly mixed before it was applied to the wetland cells. Solids loading onto both horizontal and vertical wetland types was about 30 kg m - 2 year -1. About 60 l day -1 of sludge was loaded onto each wetland cell (1.35 cm water applied to each cell day-1), in six equally spaced batch applications of 10 l, approximately every day from 12 May 1995, until 18 February 1996. No drain and dry period was provided for either type of wetland. However, three times flow to the HFW cells had to be discontinued for several days to prevent water levels from over-flowing the vessels. Flow rates to each wetland were checked three times week -1. Occasionally, a plugged distribution pipe kept sludge from being applied to a given wetland cell.

Six 3.7 x 1.2 x 0.8-m (L x W x H) wetland cells were used to provide three replicates for both types of vetiver beds.

The VFW cells (Fig. 1) are sand drying beds planted with vegetation. The VFW cells consisted of a 10-cm layer of sand and three layers of increasingly larger gravel to support the sand over a flow collection pipe (Fig. 1), based on criteria provided by Cooper (1993). Sludge was distributed across the top of each VFW cell through

sludge distribution pipe

Fig. 1. Vertical flow wetland cell, 3.7 x 1.2 x 0.8 m (I x W x H), planted with vetiver grass, and sloping 2% to drain.

sludge influent pipe

Fig. 2. Horizontal flow wetland cell, 3.7 x 1.2 x 0.8 m (i x W x H), planted with vetiver grass, and sloping 2% to drain.

a 2.5-cm inside diameter pipe (Fig. 1). Solids were trapped on and within the sand as the flow passes vertically through the bed. A 7.5-cm inside diameter drainage pipe at the bottom of the bed collected and carried the flow from each VFW cell. Each VFW cell sloped 2% down to the point where the drain pipe exited the tank. Vetiver tillers were planted at about 15-cm intervals across the entire top of each VFW cell.

The HFW cells (Fig. 2) were designed to have the flow travel overland, passing horizontally along the tank's long axis, from one narrow end of the cell to the other. The HFW cells were loaded to a depth of 51 cm with a local topsoil. Rooted vetiver shoots were planted close against each other in three 35-cm wide rows; each row was oriented perpendicular to the long axis of the vessel, and each row was about 61 cm apart (Fig. 2). About the same number of vetiver tillers were planted in a HFW cell as in a VFW cell. Sludge was distributed at the upper end of the tank onto a brick to disperse the energy of the flow. The flow passed through the vetiver hedges in the process of traveling from one end of the wetland to the other (Fig. 2). The dense shoots of mature vetiver hedges were expected to enhance solids removal by straining and settling. After passing horizontally through the wetland cell, the flow was collected in a perforated drain pipe placed at the end of the cell's long axis and buried under sand and three supporting layers of gravel. Each HFW cell sloped 2% down to the point where the drain pipe exits the tank.

Data were collected on influent and effluent concentrations of total suspended solids (TSS), total volatile solids (TVS), total and dissolved chemical oxygen demand (COD), nitrate, dissolved phosphate, total nitrogen, and total phosphorus. Data were collected on 11 separate weeks from June through February. TSS and TVS were measured using standard methods (APHA, 1989). Total and dissolved

Table 1

Mean ( + standard error) concentrations of TSS, TVS, percent volatile solids, total COD, and dissolved COD fed to and within the effluent of two wetland cell types

Wetland type TSS (mg l-1) TVS (mg I"1) Volatile solids (%) Total COD (mg l"1) Dissolved COD (mg l"1)

Influent 7860 +1849 6204 + 1362 82.8 + 1.6 6855 + 1251 2173 + 110

Effluent

Vertical flow 156 + 29 93.3 + 14.4 57.2 + 2.3 539 + 134 419 + 68

Horizontal flow 229 + 30 147.5 + 17.9 65.1 + 1.5 1761 +289 1486 + 136

Table 2

Mean (+ standard error) percent TSS, TVS, percent volatile solids, total COD, and dissolved COD removed across two types of wetland cells

Wetland type TSS TVS Volatile solids Total COD Dissolved COD

(mgl-1) (mgl-1) (%) (mg l-1) (mg l-1)

Vertical flow 97.2 + 0.8 98.0 + 0.4 30.4 + 3.6 91.3 + 1.9 81.0 + 3.0

Horizontal flow 95.8 + 0.9 96.8 + 0.6 21.1 + 2.7 71.9 + 4.2 29.7 + 7.6

COD were measured using a Hach spectrophotometer test kit (Loveland, CO). In water samples, nitrate and phosphate were quantified by ion chromatography (APHA, 1989) as described by Adler et al. (1996d). After chemical digestion, total kjeldahl nitrogen (TKN) and total phosphorus were determined by ion chromatog-raphy as described by Adler et al. (1996d).

Just before the end of the study, single samples from the inlet and outlet of each wetland cell were collected and tested for total ammonia nitrogen (TAN) using the Nessler method and a Hach DR/2000 spectrophotometer (Hach).

Sludge depths and sludge samples were also taken from each wetland at the end of the 1-year study and were analyzed for percent volatile solids.

3. Results and discussion

Results indicated that sludge removal and stabilization occurred within both wetland types (Tables 1 and 2). The VFW and HFW cells, respectively, removed 98 and 96% TSS, 91 and 72% total COD, and 81 and 30% dissolved COD (Table 2). Because little dissolved COD was expected to be removed by physical mechanisms, the increased removal of dissolved COD within the VFW cells was likely due to better microbiological treatment that occurred as the water percolated down through the sand and gravel layers of the VFW cells.

Both wetland types removed most, 82-93%, of the dissolved phosphate, total kjeldahl nitrogen, and total phosphorus (Tables 3 and 4). Particulate phosphorus was the major form of phosphorus in the treated effluent from both wetland types (Table 3).

Table 3

Mean ( + standard error) concentrations of nitrate, phosphate, total kjeldahl nitrogen, and total phosphorus fed to and within the effluent of two wetland cell types

Wetland type Nitrate Phosphate Total kjeldahl nitrogen Total phosphorus

(mg l-1 as N) (mg l-1 as P) (mg l-1 as N) (mg l-1 as P)

Influent 0.057 + 0.009 106 + 7 234 + 20 238 + 19

Effluent

Vertical flow 45.4 + 8.7 7.07 + 1.38 26.9 + 3.5 30.9 + 3.2

Horizontal flow 0.38 + 0.14 8.96 + 1.72 32.5 + 2.8 42.2 + 3.4

Table 4

Percent increase of dissolved nitrate and percent removal of dissolved phosphate, total kjeldahl nitrogen, and total phosphorus across two types of wetland cells

Wetland type Net nitrate Phosphate Total kjeldahl nitrogen Total phosphorus

production removal removal removal

Vertical flow 80 000 93 89 90

Horizontal flow 570 92 86 82

The data collected on TAN was limited; however, it indicates a net production of ammonia across the wetlands cells, i.e. 10-20 and 100-150 mg l-1 at inlets and outlets, respectively. The increase in TAN is attributed to the breakdown of organic kjeldahl nitrogen in the sludge (Table 3). Kjeldahl nitrogen was the major form of nitrogen entering the wetland cells, however, TAN and nitrate were the major forms of nitrogen leaving the wetlands.

Nitrate production (Tables 3 and 4) indicates that there was some aerobic bacterial activity (e.g., nitrification) in both types of wetland cells. These localized aerobic conditions may have been created within wetlands through either root transport of oxygen or by aeration of the flow as it trickled through the gravel-support layers within the VFW cells; the lower gravel layers were not saturated with water due to the large void spaces between the aggregate material. However, much more nitrate was produced in the VFW cells than in the HFW cells (Tables 3 and 4), probably because oxygen could be transferred from the atmosphere as the flow trickled through the aerated gravel-support layers. Denitrification probably accounted for the removal of some nitrate from both wetland types, but the low level of nitrate in the effluent from the HFW cells may have been due to both insufficient oxygen transfer for nitrification, and to anoxic conditions that allowed rapid denitrification of nitrate when it was produced. Gas bubbles observed in the upper saturated regions of both VFW and HFW cells, in combination with the high levels of organic solids present, appears to indicate the presence of anoxic and probably anaerobic conditions.

At the end of the study, depths of accumulated sludge in each wetland averaged 11 and 8.1 cm in the VFW and HFW cells, respectively. Although the density of the accumulated sludge was not measured directly, the sludge that accumulated within the VFW cells was less dense than the sludge that accumulated within the HFW cells due to the presence of large voids (air pockets) within the sludge from the VFW cells. Additionally, these sludges contained an average of 43 and 37% volatile solids, respectively. In comparison, the fraction of volatile solids in the sludge that was treated was about 83% volatile (Table 1), and it was 57-65% volatile in the treated wetland effluents (Table 2). Therefore, considerable mineralization occurred in the accumulated sludge.

Resistance to water flow through the wetland cells was greater within the HFW cells than in the VFW cells, as indicated by deeper water ponded above the HFW surface (on average 12-18 cm deep) than above the VFW surface (on average 5-12

cm deep). Additionally, we suspect that most of the water flowed horizontally above the soil and across the HFW cells and then filtered through the sand layer covering the collection pipe at the end of the cell. The distance the sludge had to flow horizontally and the thickness and number of hedges in the HFW cells were probably inadequate to physically remove most of the solids. These conclusions were supported by observations of the vetiver hedges and the sludge distribution across the top of the HFW cells at the end of the experiment, which indicated that the three hedges planted across each HFW cell did not develop stem and root masses thick enough to trap most of the solids. Performance may have been enhanced by allowing hedges to thicken more before application of sludge began. Therefore, we think that the similar and favorable particulate removal found in both the HFW and VFW cells were largely due to the sand layers that cover the effluent collection pipes within each wetland cell.

In this research, solids were loaded onto both horizontal- and vertical-flow wetland cells semi-continuously at a rate of 30 kg m-2 year -1 (dry weight). Sludge was loaded on the wetland cells at about the same rate as others have recommended for wetland drying beds (Hofmann, 1990; Lienard et al., 1990; Nielsen, 1990, 1993; Riggle, 1991; Outwater, 1994; Reed et al., 1995); however, sludge used in this experiment was relatively dilute (0.75% dry solids) when compared to the thickened sludges (1-7%) these same others reported. Additionally, the semi-continuous application of sludge in this experiment meant that only a small volume of sludge was distributed at any given application. Over a 2-week period, the more dilute sludge concentrations applied (i.e. higher water content) resulted in a higher hydraulic loading rate (1.35 cm water day -1) than others generally applied to VFW cells (Outwater, 1994; Reed et al., 1995). After the first few weeks of operation, the hydraulic loading used in this experiment always maintained a flooded condition. Maintaining surface flooded conditions was our original intent when we selected semi-continuous applications. We expected that, when flooded, the sand layer of the VFW and the soil within the HFW would make effective anaerobic filters, which proved true. This hydraulic loading strategy was contrary to conventional wisdom, as others have recommended alternating flooding and drying intervals to enhance plant growth and sludge stabilization by air- and photo-oxidation (Hofmann, 1990; Lienard et al., 1990; Nielsen, 1990, 1993; Riggle, 1991; Outwater, 1994; Reed et al., 1995). It is generally held that an aerobic environment helps to minimize odors, breaks down organic matter more rapidly, and makes phosphorus less susceptible to leaching than would anaerobic conditions. However, it is also generally believed that an anaerobic environment stabilizes sludge to its minimum solids mass and requires less energy (e.g. trickling filter height, blower/aerator power) than an aerobic environment. Additionally, this study showed that the anaerobic sand filter proved effective at removing dissolved organic matter.

At the conclusion of the experiment, root growth was observed when all material was removed from the wetland vessels. Root growth was thick below the vetiver all the way to the base of the 51-cm sand and gravel or soil media. Roots had even grown into the bottom drain pipes and had enmeshed the bottom layers of large gravel sufficiently to make manual gravel removal much more difficult.

Vegetation played an important role in dewatering the sludge, as evapotranspiration accounted for 12-20% of the water balance across both types of wetland cells during the summer months as others have also reported (Outwater, 1994; Reed et al., 1995). Plant growth was vigorous from spring until fall, when all but the base 20-25 cm of plant stems were cut and removed from all wetland cells. Much of the vegetation senesced through the winter, but shoot growth was occurring in portions of the wetlands by the end of February 1996, when the experiment was terminated. It was apparent from the pattern of uneven shoot growth that occurred in all three VFW cells, however, that some factor had limited plant revegetation within the lower third of each VFW cell. There was a total lack of vegetation in these regions. It is uncertain why revegetation did not occur in the lower regions of the VFW cells. However, because both the sand surface and the vessel base of each wetland cell had been sloped 2% down to the drain, an additional 5 cm of ponded sludge (when flooded) had accumulated at the lower end of each cell. It is possible that the additional sludge, along with the continuous anaerobic digestion, ammonia production, and flooded conditions were critical factors that limited revegetation in the lower regions of the VFW cells. Therefore, in future studies we hope to investigate the impact of hydroperiod on revegetation and solids removal and stabilization within created wetlands.

4. Conclusions

This 9.5-month study showed that created wetlands can be used to dewater and stabilize aquaculture sludge applied at an annualized rate of about 30 kg dry solids m- 2. The continuously flooded wetlands functioned reasonably well, removing: 72% total COD; 82% total kjeldahl nitrogen, phosphorus, and dissolved phosphate; and 96% TSS. As well, sludge stored in the wetlands mineralized, which reduced its TVS by > 50% compared to the untreated sludge. The VFW technology has also been used successfully over the past 20 years within the municipal wastewater treatment industry to dewater and stabilize solids (Hofmann, 1990; Lienard et al., 1990; Nielsen, 1990, 1993; Riggle, 1991; Outwater, 1994; Reed et al., 1995). The VFW bed design and solids application rates that we used were based on criteria reported by Outwater (1994) and Reed et al. (1995). However, they also recommended applying solids in a series of sequential batches, approximately one solids application per wetland bed every 7-21 days. The 1 -3-week interval between solids application is used to provide sufficient time for sludge dewatering and drying. In hindsight, we think we could have improved the long-term operation of our created wetlands if we had applied sludge every 1-3 weeks. Even though our study was of short duration, it did show that applying sludge every 4 h, without providing a lengthy period for dewatering and drying, produced problems with water ponding. As the sand beds plugged under continuous loading conditions, depths of ponded water increased and environmental factors eventually became detrimental to the vegetation within the ponded sludge.

Other than our decision to use a continuous sludge-loading cycle, we think that the created wetland designs evaluated, especially the VFW design, could be used to effectively dispose of aquaculture waste solids. We conclude that on site treatment of waste solids with this uncomplicated, low-maintenance VFW technology can offer an alternative to landfill disposal when land application of waste solids is not a viable option.

Acknowledgements

This work was supported by the US Department of Agriculture, Agricultural Research Service under grant agreement numbers 0500-00022-003-00D and 591931-3-012. We thank Dan Bullock, Kevin Webb, Joseph Morton, Susan Glenn, C. Frederick Ford, and Robert Crouse for their invaluable assistance.

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