Nutrient Removal from Wastewater Effluent Using an Ecological Water

settle from the water into a sediment trap. Both the fish and their feces can be harvested as nutrient sinks. To test this concept, effluent was pumpe...
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Environ. Sci. Technol. 2000, 34, 522-526

Nutrient Removal from Wastewater Effluent Using an Ecological Water Treatment System LAURA L. RECTENWALD† Department of Environmental Studies, Baylor University, Waco, Texas 76798 RAY W. DRENNER* Biology Department, Texas Christian University, Fort Worth, Texas 76129

This study investigated the use of a periphyton-fish system for removal of phosphorus and nitrogen from treated wastewater. In the periphyton-fish system, water flows through a series of vessels, and nutrients are taken up by periphyton growing on screens. Algal-grazing fish feed on the periphyton and either assimilate the nutrients into fish tissue or egest the nutrients in mucus-bound feces that settle from the water into a sediment trap. Both the fish and their feces can be harvested as nutrient sinks. To test this concept, effluent was pumped from a final clarifier at a wastewater treatment facility through a series of twelve 375-L mesocosms, each containing screens for periphyton growth and an average of 161 g of Tilapia mossambica, an algal-grazing cichlid. The system removed nutrients at an average rate of 27 mg of total phosphorus (TP) m-2 d-1 and 108 mg of total nitrogen (TN) m-2 d-1 and reduced wastewater TP and TN by 82% and 23%, respectively.

Introduction The water quality of lakes deteriorates with increased eutrophication (1). Eutrophic lakes experience nuisance algal blooms, which can result in fish kills, loss of water clarity, taste and odor problems, and increased cost associated with treatment of water for domestic uses. The primary causative agent of eutrophication is the excessive loading of nutrients such as phosphorus and nitrogen (2-4). Municipal wastewater can supply high amounts of nutrients to freshwater rivers and lakes and thus be an important cause of lake eutrophication (2). Although most municipal wastewater in the United States is treated with primary and secondary treatment, effluents from such wastewater plants still contain high concentrations of phosphorus and nitrogen, even in areas in which phosphorus bans have been instituted (5, 6). Tertiary treatment by chemical methods can remove phosphorus, but this process is expensive (5). Previous studies have indicated that algal periphyton treatment systems have potential as a method of tertiary treatment (7-10). Periphyton treatment systems use periphyton to take up phosphorus and nitrogen but require mechanical means such as scraping or vacuuming for removal of the periphyton from the system (9). As an alternative to * Corresponding author e-mail: [email protected]; phone: (817) 257-7165; fax: (817)257-6177. † Present address: Arthur Temple College of Forestry, Stephen F. Austin State University, Nacogdoches, TX 75961. 522

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FIGURE 1. Periphyton-fish system mesocosm for removal of nutrients from polluted water. mechanically harvesting periphyton, Drenner et al. (11, 12) suggested that herbivorous fish could be used as periphyton consumers. In the system proposed by Drenner et al. (11, 12) (Figure 1), water flows through a series of vessels, and nutrients are taken up by periphyton growing on screens and the sides of the vessels. Fish consume the periphyton, and nutrients are converted to fish tissue or mucus-bound feces that can be extracted from the system after they settle into a nutrient trap at the bottom. The experiment presented here examined the feasibility of using the periphyton-fish system for removal of nutrients from the effluent of a municipal wastewater treatment facility.

Materials and Methods The experiment was conducted at Waco Metropolitan Area Regional Sewerage System (WMARSS), which has many components typical of wastewater treatment facilities throughout the United States. Primary wastewater treatment begins as raw sewage enters the plant and passes through bar screens, which remove large objects. The raw sewage then flows to the primary clarifiers where rocks and sand are settled out. Secondary treatment begins in the aeration basins where activated sludge is used to release much of the organic carbon as CO2. Anaerobic zones in the basins create an environment in which nitrogen is reduced by bacteria, while phosphorus is removed in primary and waste-activated sludges (13). The water then moves into the final clarifiers where sludge and remaining solid matter are settled from the water. At WMARSS, water then passes to the sand filters, a tertiary treatment system that reduces total suspended solids. Finally, effluent from the sand filters is chlorinated to kill the remaining pathogenic organisms and is subsequently dechlorinated prior to outflow into the Brazos River. The experiment was conducted from July 10 to August 23, 1995, using twelve 375-L opaque fiberglass mesocosms positioned near a final clarifier. The mesocosms were interconnected by sections of rubber hose with an inside diameter of 2.54 cm and set on a slope so that water would gravity feed through the system from tank A to tank L (Figure 2A). Each conical-bottomed tank contained a plastic frame supporting 4 m2 of plastic mesh and a drain hose equipped with a shut-off valve (Figure 2B). A submersible pump was used to pump water from a WMARSS final clarifier into an elevated dechlorination tower. 10.1021/es9908422 CCC: $19.00

 2000 American Chemical Society Published on Web 12/31/1999

FIGURE 2. (A) Configuration of experimental tanks. (B) Periphyton-fish system mesocosm.

FIGURE 3. TP and TN concentrations (µg L-1) in water column samples from tanks A-L on weekly sampling dates. VOL. 34, NO. 3, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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This tower served a dual purpose: to control the rate of flow into the series of mesocosms and to prevent fish mortality from the chlorine occasionally used to control algae in the final clarifiers. Water was gravity-fed from the dechlorination tower into tank A and down the series of mesocosms. Water flowing out of tank L passed into a chlorinated tub to eliminate fish that might have been present prior to return to the final clarifier. Mesocosms were filled with water from the final clarifier on June 15. On June 30 and July 3, 104 adult cichlids (Tilapia mossambica) were distributed among the 12 experimental tanks with an average of 161 g of fish per tank. T. mossambica is a mouth-brooder, and mouth-fry were present when the fish were distributed among the tanks. This cichlid feeds on periphytic detrital aggregate composed of detritus, algae, and bacteria (14). From July 10 to August 21, a daily aliquot of 975 L of water was introduced to the system. Daily inflow and outflow samples were collected from the dechlorination tower and tank L, respectively. Daily inflow and outflow samples (20 mL) were composited weekly and frozen in Nalgene bottles. From July 17 to August 21, water column and drain samples were collected weekly. Water column samples were taken by submerging a 125-mL Nalgene bottle below the water surface of each tank. Drain samples were taken by flushing sediment through each drain hose into a bucket in 5-10-L increments until the water appeared clear, stirring the contents, collecting a subsample, and freezing it in a Nalgene bottle. All samples were thawed, and drain samples were homogenized in an Oster blender prior to analysis for total phosphorus (TP) and total nitrogen (TN). Samples for TP analysis were digested with potassium persulfate (15) and analyzed using a modification of the malachite green method (16) in which 1 mL of color reagent was added to subsamples of the total digestions, and absorbance was measured at 610 nm. Samples for TN were digested with alkaline potassium persulfate (17), and absorbance was measured at 220 nm (18). The TP and TN contents of the sediment were calculated by subtracting the amounts of TP and TN in 5-10 L of the water column from the TP and TN present in the 5-10-L drain sample on the same date. TP and TN removal rates were calculated by dividing the TP and TN content of the sediment by tank surface area (0.6 m2) and the number of days in the sample period. Tanks were drained, and the fish were removed and weighed on August 24. Periphyton were scraped in a single vertical path from the side of each tank using a 3.3 cm wide razor blade and were frozen for analysis of TP and TN. Periphyton samples were thawed, standardized to 100-mL samples by adding deionized water, homogenized with the blender, and analyzed for TP and TN using the techniques given above. Concentrations of TP and TN in the periphyton scrape were calculated by dividing the amount of TP and TN in the scrape by the surface area of the scrape.

Results Water column samples indicated a decline in nutrients as the water from the clarifier passed through the tanks with TP declining more than TN (Figure 3). Comparison of inflow and outflow samples also revealed reduction of TP and TN by the system (Figure 4A,B). The system reduced TP levels by an average of 82% and reduced TN levels by an average of 23%. The efficiency of the system the final week of the experiment was improved over the first week. The TN:TP ratio (by mass) of the water was altered as it passed through the system (Figure 4C). While the TN:TP ratios of the inflow water ranged from 8 to 46 and remained fairly consistent throughout the experiment, the TN:TP ratio of the outflow water increased during the experiment. At the 524

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FIGURE 4. (A) TP and (B) TN (µg L-1) present in composited inflow and outflow water samples. (C) TN:TP ratios of composited weekly inflow and outflow samples. Samples reflect week ending on the dates listed. end of the experiment, the outflow water had a TN:TP ratio of 219, which was approximately 18 times greater than the inflow. Phosphorus removal rates in the sediments ranged from 10 to 121 mg of TP m-2 d-1 (mean ) 27 mg m-2 d-1), and nitrogen removal rates ranged from 15 to 385 mg of TN m-2 d-1 (mean ) 108 mg m-2 d-1) (Figure 5). The first six tanks had higher TP and TN removal rates than the last six tanks. The decreased removal rates for phosphorus and nitrogen during the week of August 21 may have been caused by low TP in inflow water for the August 12 sampling date. We observed minimal fish mortality and an increase in the biomass of the fish during the experiment. Three adult fish (each approximately 20 g) died in tanks A, G, and L during the experiment and were removed from the mesocosms. At the conclusion of the experiment, we collected a total of 101 adults (1964 g) and 336 fry (321 g) for an increase in total fish biomass of 355 g. The greatest increase in fish biomass occurred in tanks at the front of the system (Figure 6A).

FIGURE 5. TP and TN removal rates (mg m-2 d-1) in sediments from tanks A-L each week of the experiment. Phosphorus taken up by fish tissue was estimated using Tan’s (19) approximation that TP represents 2.4% of the dry weight of fish tissue and by assuming the dry weight of the fish to be 23.9% of wet weight. Fish growth removed an average of 6.4 mg of TP m-2 d-1 over the course of the experiment. Fish growth in the first six tanks removed more phosphorus than fish growth in the last six tanks (Figure 6B). Periphyton on the sides of the tanks contained an average of 0.54 mg of TP cm-2. Because fish grazed periphyton and reduced it to a thin biofilm, periphyton on tank walls only accounted for 1.3% of the TP in tanks on the last sampling date, but this estimate of the TP content of the periphyton does not include periphyton on the screens, which was not sampled. We found an average of 9 mg of TP in the periphyton, 418 mg of TP in the water column, 169 mg of TP in the fish, and 95 mg of TP in the sediments of each tank.

Discussion Only a few studies have examined phosphorus removal rates using periphyton-based systems. Adey et al. (9) examined nutrient removal from agricultural runoff by natural algal populations in raceways. Using hand and vacuum harvesting of the periphyton, Adey et al. (9) achieved phosphorus removal rates ranging from 104 to 139 mg of TP m-2 d-1.

Drenner et al. (12) examined phosphorus removal from test solutions using periphyton-fish systems and during the latter part of the experiment achieved sediment removal rates of 21-31 mg of TP m-2 d-1. Nutrient removal rates in the study presented here may be lower than those achieved by Adey et al. (9) because the periphyton-fish system uses fish to harvest periphyton while Adey et al. (9) vacuumed the periphyton. Fish excrete phosphorus into the water and thus may decrease the resultant removal rate of the periphytonfish system (20). The results of this study demonstrate that a periphytonfish system can reduce phosphorus and nitrogen in wastewater previously treated to secondary standards. The system reduced TP by an average of 82% and TN by an average of 23%. The TN:TP ratio of the effluent increased each week of the experiment, reaching 219:1 in the final week, an adjustment that would probably reduce the possibility that the effluent would stimulate blue-green algae. Smith (21) found that blue-green algae were usually not abundant in lakes with N:P ratios greater than 29:1, but other ecologists have questioned the effects of N:P ratio on the abundance of bluegreen algae (22). In the future, the periphyton-fish system could be used as an alternative form of tertiary treatment to remove VOL. 34, NO. 3, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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Lind and W. M. Alexander for helpful suggestions. We also thank Tom Conry and the Brazos River Authority for allowing us to conduct the research at the Waco Metropolitan Regional Sewerage System. Research was supported by a grant from the Texas Christian University Research Fund. The periphyton-fish system is covered by U.S. Patent No. 5254252.

Literature Cited

FIGURE 6. (A) Fish biomass (g) stocked and recovered. (B) TP (mg m-2 d-1) removed by fish growth. phosphorus at wastewater treatment facilities, but a large surface area would be required to treat large volumes of effluent water. For example, to remove 1 mg of P L-1 from 3 785 306 L d-1 would require 3.8 ha of surface area assuming an optimal removal rate of 100 mg of P m-2 d-1. In the case of the study presented here, 98 ha of surface area would be required to reduce phosphorus by 1 mg of P L-1 in the 265 000 000 L of WMARSS effluent at the mean removal rate (27 mg of TP m-2 d-1) achieved in our study. A smaller surface area could be used if we can achieve more efficient nutrient removal rates after additional study of the relationship between nutrient removal rates, retention time, nutrient load, mesocosm surface area and depth, and algal growth substrate. Because the efficiency of the periphyton-fish system declines with cool temperatures, larger surface areas would be required in cooler seasons (11). In addition, a periphytonfish system installed in latitudes with cold winter temperatures would require provisions for winterization because temperatures of 18 °C and below may be lethal to tilapia (23). The periphyton-fish system may have greatest applicability in tropical countries where temperatures will permit year-round operation of the system and the fish produced might be a valuable food resource.

Acknowledgments We thank Jan Fox, Kristina Stewart, Donald Day, Chris Conley, Misty Ethridge, and Scott Rectenwald for assistance and Owen

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Received for review July 26, 1999. Revised manuscript received November 11, 1999. Accepted November 15, 1999. ES9908422