Effectiveness of Mechanical Aeration in Floating Aquatic Macrophyte-Based Wastewater Treatment Systems

Outdoor tanks containing the floating aquatic macrophyte water hyacinth [Eichhornia crassipes (Mart) Solms] were provided with diffuse aeration to evaluate its effect on wastewater treatment efficiency and plant growth. Light aeration (0.003 and 0.021 L nr min ') had no effect on the treatment of primary domestic effluent in the batch-fed water hyacinth tanks. Heavy aeration (1.03 and 3.53 L nr min-) raised wastewater dissolved oxygen (DO) concentrations, but did not improve biochemical oxygen demand (BOD,) removal efficiency or increase plant growth rates during 21-d experiments. Heavy aeration slightly increased wastewater N removal, but the effectiveness of aeration for nutrient removal was not consistent among experiments. In continuous-flow raceways fed primary domestic effluent, vigorous aeration (6.1 L nr min') improved effluent quality, with contaminant removal rates averaging 77% (BODS) and 76% (suspended solids [SS]) in nonaerated raceways, and 94% (BOD5) and 89% (SS) in aerated raceways. Results of this study suggest that the high aeration requirement for enhanced contaminant removal in floating aquatic macrophyte systems (FAMS) is due not only to poor O2 transfer efficiencies (1.6-4.0%), but also to the inefficient utilization of O2 for BOD, removal. The primary benefit of moderate aeration (0.1-1.0 L nr min-) in FAMS appears to be the elevation of wastewater DO concentrations (from 0.5 to 2.5 mg Ir), which may be useful for controlling odors and mosquito proliferation. I in the use of shallow ponds containing floating aquatic macrophytes for treating domestic wastewaters has increased in the past decade. Floating aquatic macrpphyte-based treatment systems (FAMS) have low capital and operating costs, but are somewhat land intensive (Crites and Mingee, 1987; Reddy and DeBusk, 1987). Pilot-scale studies conducted in Florida have shown that annual mean biochemical oxygen T.A. DeBusk and K.S. Clough, Reedy Creek Energy Services, Inc., P.O. Box 10000, Lake Buena Vista, FL 32830; and K.R. Reddy, Univ. of Florida, Inst. of Food and Agric. Sci., Soil Science Dep., Gainesville, FL 32611. Florida Agric. Exp. Stn. Journal Series no. 9666. Received 31 Aug. 1988. 'Corresponding author. Published in J. Environ. Qual. 18:349-354 (1989). demand (BOD5) and suspended solids (SS) removal rates of 200 and 65 kg ha" d~', respectively, can be achieved in floating aquatic macrophyte systems (FAMS), which receive primary domestic effluent. In FAMS that receive secondary domestic effluent, average N and P removal rates of 11 and 3 kg ha" d" typically are attained (DeBusk and Reddy, 1987). Many of the contaminant-removal processes functioning in FAMS require oxygen, which is used as an electron acceptor by bacteria during the oxidation of reduced substrates such as organic C or NH4 (Reddy, 1983; Good and Patrick, 1987). Oxygen is supplied to the water column of FAMS via atmospheric diffusion, or by transport directly through the plant's aerenchyma tissues (Moorhead and Reddy, 1988). In conventional activated sludge systems utilized for secondary domestic wastewater treatment, dissolved oxygen (DO) concentrations of from 1 to 3i mg L" are maintained (Metcalf and Eddy, 1979). In contrast, oxygen concentrations in FAMS that receive a high organic matter loading (e.g., primary domestic effluent) frequently are less than 0.5 mg L" (G. Tchobanoglous et al., 1987, Univ. of California-Davis, unpublished data). Oxygen availability therefore may be a factor limiting BOD5 removal in FAMS. Recently, investigators have suggested that FAMS be equipped with aeration devices to improve effluent quality and to reduce potential aesthetic problems (e.g., odors and mosquito production) (Reed et al., 1988). To date, however, no studies have been conducted to test the effectiveness of aeration in FAMS. The present study was conducted to evaluate the effects of aeration on contaminant removal in a FAM (water hyacinth-based) system that received primary sewage effluent. MATERIALS AND METHODS Three experiments were conducted in central Florida during July and August of 1986 and 1987 to determine the effects of aeration on wastewater quality in FAMS. 350 J. ENVIRON. QUAL., VOL. 18, JULY-SEPTEMBER 1989 Experiment 1. Effects of Aeration Level on Wastewater Quality in Batch-Fed Tanks Twelve 800-L tanks, 1.7 ms in area and 0.5 m deep, were stocked with the floating macrophyte water hyacinth [Eichhornia crassipes (Mart.) Solms] at a standing crop of kg wet wt. m-L Daytime (12 h) aeration was provided duplicate tanks at rates of 0, 0.003, 0.021, 0.074, 1.03, and 3.53 L m-2 min-1. These aeration levels (from lowest to highest) were designated 0 through 5, respectively. The tanks were aerated using glass air stones (one stone per tank) suspended from floating distribution manifolds. Air was provided to the manifolds with a 0.7 kW, low pressure air blower. Each tank was filled with primary domestic effluent obtained from the Reedy Creek Improvement District wastewater treatment plant. The DO concentration of the wastewater in each tank was measured periodically during the 21d incubation (YSI model 54A 02 meter). Biochemical oxygen demand of tank wastewater samples was determined on Days 0, 1, 4, 7, and 15 (APHA, 1985). Water samples collected on Days 0, 11, and 21 also were analyzed for NH4, NO3, total Kjeldahl N (TKN), soluble reactive P (SRP) total P (TP) (APHA, 1985). Mosquito larvae counts (numbers per 250-mL dip) were conducted in each tank on Days 11 and 21. The water hyacinth in all tanks were harvested and weighed at the end of the experiment to determine the net plant productivity. Experiment 2. Effect of Continuous Aeration on Wastewater Quality Another experiment was conducted using similar procedures to determine the effects of continuous (24 h -t) aeration on effluent quality. Eight 800-L tanks were stocked with water hyacinth, and four tanks (designated NM) were not stocked with macrophytes. All tanks were filled with primary sewage effluent, and continuous aeration (24 h -~) was provided at four levels (0, 0.074, 1.03, and 3.53 L -2 min t) to the water hyacinth tanks, and at two levels (0 and 1.03 m-2 min-~) to the tanks with no macrophytes. The BOD5 concentrations were measured on wastewater samples collected on Days 0, 1, 4, 16, and 21. Wastewater samples collected on Days 0, 7, 14, and 21 were analyzed for mosquito larvae, DO, N, and P forms as described above. The water hyacinths were weighed at the end of the experiment. Experiment 3. Effects of Aeration on Wastewater Quality in Continuous-Flow Raceways Four rectangular (5:1, length/width) fiberglass tanks (3000 L, 5.9 m2 in area, 0.5 m deep) were fed primary domestic effluent on a continuous basis, providing a theoretical hydraulic retention time (HRT) of 6 d. The tanks were stocked with water hyacinth to a standing crop of 15 kg wet wt m -2. Aeration was provided to two of the tanks through two airstones positioned near the effluent region of each tank. Air was provided continuously (24 h -l) at a rate of 6.1L m-2 rain-t (designated aeration level 6). Influent and effluent wastewater grab samples were collected twice weekly from each tank and analyzed for BOD5 (APHA, 1985). Suspended solids concentrations (nonfilterable residue) (APHA, 1985) of wastewater samples were measured once per week. The water hyacinths were harvested and weighed at the end of the experiment. Oxygen Transfer Measurements Oxygen was purged from well water held in a small outdoor tank (130 L, 0.5 m deep) by adding sodium sulfite and cobalt chloride (46.2 and 0.3 mg -~, respectively). Airstones (calibrated to deliver a desired airflow) immediately were placed in the tank, and the increase in DO was measured over time. The oxygen transfer coefficient (KL~) for each airflow rate was calculated from the expression: (KLa)T -(In ODlo -In OD70)/(t70 where (KLa)T = oxygen transfer Coefficient for the existing water temperature (h -t) ODto = oxygen deficit at 10% of saturation (mg -t) ODTo = oxygen deficit at 70% of saturation (mg -t) tto = time when DO reaches 10% of saturation (h) t70 = time when DO reaches 70% of saturation (h) The "standard" oxygen transfer rate (SOTR) [kg 02 -~] in well water was calculated from the following equation (Ahmad and Boyd, 1988): SOTR = (gLa)2O X (9.07) × V X -3 where (KLa)2o = measured KLa values corrected to standard atmosphere pressure and temperature 9.07 -DO concentration at 20 °C and standard atmospheric pressure V -volume of the test container No correction for the influence of well water constituents on oxygen transfer (relative to deionized water) was made. Oxygen transfer efficiency was calculated as SOTR per g O5 pumped per hour. RESULTS AND DISCUSSION Aeration level had little effect on BOD5 removal from the primary sewage effluent in the batch-fed treatment systems (experiment 1). The nonaerated tanks performed as well or better than the aerated systems on four sampling dates (Fig. 1). Wastewater BOD5 declined from an initial concentration of 124 mg L-l to ca. 18 mg L-1 on Day 7 with all aeration rates, representing a mass-removal rate of 71 kg ha-1 d-1. Biochemical oxygen demand removal rates of from 50 to 250 kg ha-l d-1 have been reported in previous studies of nonaerated, water hyacinth-based treatment systems (Stowell et al., 1981). The aeration regime had little effect on wastewater nutrient removal from the water hyacinth tanks (Table 1). On Day 11, highest Nand P-removal rates (% basis) were observed in the nonaerated tanks. On Day 21, no relationship between aeration and N removal was observed, although P-removal rates declined with increasing aeration on this sampling date. Mean removal rates of 0.57 g N m-2 d-I and 0.11 g P m-2 d-1 were provided by the 0 level aeration treatment during the 21-d study. Some NO3 accumulation was noted in the level 5 aeration tanks on Day 11 (Table 1). NitrateN subsequently was lost from the system via plant uptake or denitrification, since wastewater NO3 concentrations were low on Day 21. Ammonium-N concentrations dropped sharply between Days 0, 11, and 21, but concentrations of this constituent were similar at all aeration levels on each sampling date (Table 1). While none of the aeration levels utilized in this experiment enhanced contaminant removal, level 4 and 5 aeration did increase wastewater DO concertDEBUSK ET AL.: AQUATIC MACROPHYTE-BASED WASTEWATER TREATMENT SYSTEMS Table 1. Effect of daytime (12 h) aeration on wastewater (primary domestic effluent) quality in water hyacinth tanks.t 351 Aeration level Air flow TN TP NO~ NH4 DO Mosq.$ L m-2 min -~ -% removal ~ mg L -m no.