The Conservation Fund Freshwater Institute, Shepherdstown, West Virginia 25443, USA PHILIP L. SIBRELL

North American Journal of Aquaculture 66:198 207, 2004 Copyright by the American Fisheries Society 2004 Application of Chemical Coagulation Aids for the Removal of Suspended Solids (TSS) and Phosphorus from the Microscreen Effluent Discharge of an Intensive Recirculating Aquaculture System JAMES M. EBELING* AND SARAH R. OGDEN The Conservation Fund Freshwater Institute, Shepherdstown, West Virginia 25443, USA PHILIP L. SIBRELL U.S. Geological Survey, Leetown Science Center, Kearneysville, West Virginia 25430, USA KATA L. RISHEL The Conservation Fund Freshwater Institute, Shepherdstown, West Virginia 25443, USA Abstract. An evaluation of two commonly used coagulation flocculation aids (alum and ferric chloride) was conducted to determine optimum conditions for treating the backwash effluent from microscreen filters in an intensive recirculating aquaculture system. Tests were carried out to evaluate the dosages and conditions (mixing and flocculation stirring speeds, durations, and settling times) required to achieve optimum waste capture. The orthophosphate removal efficiency for alum and ferric chloride were greater than 90% at a dosage of 60 mg/l. Optimum turbidity removal was achieved with a 60-mg/L dosage for both alum and ferric chloride. Both alum and ferric chloride demonstrated excellent removal of suspended solids from initial total suspended solid values of approximately 320 mg/l to approximately 10 mg/l at a dosage of 60 mg/l. Flocculation and mixing speed and duration played only a minor role in the removal efficiencies for both orthophosphates and suspended solids. Both coagulation flocculation aids also exhibited excellent settling characteristics, with the majority of the floc quickly settling out in the first 5 min. Because of its potential negative impact on freshwater systems, phosphorus is one of the most scrutinized nutrients discharged by aquaculture systems. Phosphorus is often the limiting nutrient in natural ecosystems, and excessive algae blooms can occur if discharge concentrations exceed the absorption capacity of the receiving body of water. Both state and federal regulatory agencies have imposed limitations on phosphorus discharge to high-quality receiving waters. Because of these limitations and the potential impact of phosphorus on the environment, numerous research projects have been conducted over the last few years on ways to reduce phosphorus in discharges from aquaculture systems. Much of the work in this area has focused on decreasing the total phosphorus in the feed or increasing dietary phosphorus availability. Recirculating aquaculture systems, however, provide excellent opportunities for phosphorus control. Because recirculating systems provide a small but concentrated waste stream, they pro- * Corresponding author: j.ebeling@freshwaterinstitute.org Received October 30, 2003; accepted February 14, 2004 vide opportunities for more efficient and economical phosphorus control. In contrast, in systems such as raceways, the equivalent treatment of the dilute effluent flow stream would be extremely difficult both from an engineering and an economic standpoint. Microscreen filters have become very popular for suspended solids removal because they require minimal labor and floor space and can treat large flow rates of water with very little head loss. Screen filters remove solids by virtue of physical restrictions (or straining) on a media when the mesh size of the screen is smaller than the particles in the wastewater. Microscreen filters, though, generate a separate solids waste stream that must be further processed before final discharge. The backwash flow will vary in volume, and the solids content will vary based on several factors (e.g., screen opening size, type of backwash control employed, frequency of backwash, and influent total suspended solid [TSS] load on the filter). Backwash flow is generally expressed as a percentage of the flow the filter treats, with reported backwash flows ranging from 0.2% to 1.5% of the treated flow (Ebeling and Summerfelt 2002). 198

COAGULATION AIDS REMOVAL OF TSS AND PHOSPHORUS 199 Several chemical and biological processes have been investigated for the removal of phosphorus from aquaculture effluent water. Adler and Sibrell (2003) investigated the use of neutralized acid mine drainage to reduce the loss of soluble P from agricultural fields and animal wastewater. A biological means of phosphate removal by Barak and van Rijn (2000) demonstrated that some denitrifiers were capable of phosphate uptake in excess of their metabolic requirements. Kioussis et al. (1999) developed a polymeric hydrogel, which decreased phosphorus in aquaculture wastewater effluents by more than 99% to less than 0.01 ppm. However, it has been determined that the majority of the phosphorus discharged from intensive aquaculture systems (50 85%) is contained in the filterable or settleable solids fraction (Bergheim et al. 1993; Heinen et al. 1996). Thus, any mechanism that could enhance solids removal would also contribute to a reduction in the overall level of phosphorus discharge. Coagulation and flocculation processes, with aids such as alum or ferric chloride, are standard techniques in the wastewater and drinking water industry for the removal of suspended solids. These aids have not been extensively applied in the aquaculture industry primarily because of the dilute nature of most aquaculture waste streams. However, the concentrated waste stream from recirculating systems, especially the backwash from microscreen filters, makes this option feasible from both an engineering and an economic standpoint. Over the past several years, the Conservation Fund s Freshwater Institute in Shepherdstown, West Virginia, has demonstrated several technologies and strategies to manage and reduce the wastes generated during aquaculture production, including improved feed and feeding strategies (Tsukuda et al. 2000), technologies to minimize water use and concentrate waste streams (Timmons and Summerfelt 1997; Summerfelt et al. 2000), and overall waste management and treatment reviews (Summerfelt 1998; Summerfelt et al. 1999; Ebeling and Summerfelt 2002). In related research, several coagulation aids were evaluated for the physical chemical treatment of the supernatant discharge from manure-thickening tanks (Ebeling et al. 2003). Future research at the Freshwater Institute will include the application of microfiltration technology using semipermeable membranes and the construction of a demonstration pilot-scale compost facility. The objectives of this research were to evaluate the physical chemical treatment of a recirculating system s microscreen backwash effluent using several common coagulation flocculation aids employed in the drinking and wastewater treatment industry (i.e., alum and ferric chloride). In addition to determining their effectiveness in removing both suspended solids and phosphorus, a systematic evaluation of the variables encountered in the coagulation flocculation process (mixing and flocculation stirring speeds and durations, and settling times) was conducted. Background One of the most commonly used methods for the removal of suspended solids in drinking water is the addition of coagulant and flocculation aids, such as alum, ferric chloride, and long-chain polymers (AWWA 1997). Coagulation is the process of decreasing or neutralizing the electric charge on suspended particles or zeta potential. Similar electric charges on small particles in water cause the particles to naturally repel each other and hold the small, colloidal particles apart and keep them in suspension. The coagulation flocculation process neutralizes or reduces the negative charge on the particles. This allows the van der Waals force of attraction to encourage the initial aggregation of colloidal and fine suspended materials to form microfloc. Flocculation is the process of bringing together the microfloc particles to form large agglomerations by physically mixing them or through the binding action of flocculants (such as long-chain polymers). A classical coagulation flocculation unit process (Metcalf and Eddy, Inc. 1991) consists of three separate steps: (1) Rapid or flash mixing the suitable chemicals (coagulants, flocculants, and, if required, ph adjusters) are added to the wastewater stream, which is stirred and intensively mixed at high speed; (2) Slow mixing (coagulation and flocculation) the wastewater is only moderately stirred in order to form large flocs, which are easily settled out; and (3) Sedimentation the floc formed during flocculation is allowed to settle out and is separated from the effluent stream. Numerous substances have been used as coagulant and flocculation aids, including alum (Al 2 [SO 4 ] 3 18H 2 O), ferric chloride (FeCl 3 6H 2 O), ferric sulfate (Fe 2 [SO 4 ] 3 ), ferrous sulfate (FeSO 4 7H 2 O), and lime (Ca[OH] 2 ) (Metcalf and Eddy, Inc. 1991). Aluminum sulfate (alum) is the most commonly

200 EBELING ET AL. used coagulant, is easy to handle and apply, and produces less sludge than lime. Its primary disadvantage is that it is most effective over a limited ph range of 6.5 7.5. Ferric chloride is also a commonly used coagulant and is effective over a wider ph range of 4 11. The ferric hydroxide floc is also heavier than the alum floc, improving its settling characteristics and reducing the size of the clarifier. Neither ferric sulfate nor ferrous sulfate is commonly used today, but ferric sulfate is slowly replacing ferric chloride because it is easier to store and handle. Lime is commonly used and is effective, but is quite ph-dependent and produces a large quantity of sludge requiring disposal. When alum is added to a wastewater, the following reaction takes place: A1 (SO ) 18 H O 3 Ca(HCO ) 3 CaSO 2 4 3 2 3 2 4 2 Al(OH) 3 6CO2 18HO. 2 The insoluble aluminum hydroxide, Al(OH) 3,isa gelatinous floc that settles slowly through the wastewater, sweeping out the suspended material. Alkalinity is required for the reaction and, if not available, must be added at the rate of 0.45 mg/l as CaCO 3 for every 1 mg/l alum. Similarly, for ferric chloride: 2 FeCl 3 6 H2O 3 Ca(HCO 3) 2 3 CaCl2 2 Fe(OH) 3 6CO2 12HO. 2 The insoluble ferric hydroxide, Fe(OH) 3, is also a gelatinous floc that settles through the wastewater, sweeping out the suspended material. Alkalinity is required for the reaction and, if not available, must be added at the rate of 0.55 mg/l CaCO 3 for every 1 mg/l ferric chloride. In addition, both aluminum and iron salts can also be used for the chemical precipitation of phosphorus. The basic reactions involved are Al 3 PO 3 4 AlPO4 and Fi 3 PO 3 4 FePO 4. The above equations are the simplest forms of the reaction (Metcalf and Eddy, Inc. 1991; Lee and Lin 2000). Due to the many other competing reactions, the effects of alkalinity, ph, trace elements, and other compounds in the wastewater, the actual chemical dosage required to remove a given quantity of phosphorus is usually established on the basis of a bench-scale test or sometimes pilotscale tests. Mixing disperses precipitating agents, coagulants, and coagulant aids throughout the wastewater to ensure the most rapid precipitation reactions and subsequent settling of precipitates possible. For engineering design, the degree of mixing is dependent, in turn, upon the amount of energy supplied, the mixing residence time, and the related turbulence effect (which depends on the size and shape of the mixing tank). Mixing can be subdivided into two types: (1) flash or rapid mixing of chemicals and (2) continuous mixing in reactors or holding tanks. Both are employed during the coagulation flocculation process. In flash or rapid mixing, the principal objective is to provide complete and uniform dispersion of a chemical added to the water (i.e., alum in wastewater). In continuous mixing, the principal objective is to maintain the contents of a reactor in a completely mixed state that promotes the aggregation of particles (e.g., flocculation). Types of mixing include propeller, turbine, paddle, pneumatic, and hydraulic mixers. The power input per unit volume of liquid can be used as a rough measure of mixing effectiveness (Metcalf and Eddy, Inc. 1991). The following equation is normally used to estimate the velocity gradients in coagulation tanks and is used for the design and operation of mixing systems (Metcalf and Eddy, Inc. 1991): where G (P/ V) 1/2, G mean velocity gradient (s 1), P power applied (W or ft lb/s), dynamic viscosity (N s/m2or lb s/ft 2), and V tank volume (m3or ft 3). The actual values for G are usually provided by the equipment manufacturers, although they can be estimated from the area of the paddle, fluid density, and paddle velocity in the fluid. Rapid or flash mixing residence times typically range from 30 s to 2 min, with 1 min being the most common. The intensity and duration of the mixing of the coagulants must be controlled to prevent the breakup the microfloc or the uneven dosing of coagulant. The typical range of velocity gradient values for rapid mixing is 250 1,500/s (Metcalf and Eddy, Inc. 1991). Flocculation mixing is much slower to allow the maximum interaction of the floc to aggregate together. Typical values of the velocity gradient for flocculation range from 20 to 80/s, with a retention time of 20 30 min (Lee and Lin 2000). In our previous coagulation/flocculation testing on the supernatant discharge from fish ma-

COAGULATION AIDS REMOVAL OF TSS AND PHOSPHORUS 201 TABLE 1. Water quality characteristics of the microscreen backwash effluent in nine samples. Abbreviations are as follows: NTU nephelometric turbidity units; TP total phosphorus; SRP soluble reactive phosphorus; TSS total suspended solids; TVS total volatile solids; TN total nitrogen; TAN total ammonia nitrogen; and cbod 5, carbonaceous biochemical oxygen demand. Parameter Mean SD Range ph Temperature ( C) Alkalinity (mg/l) Turbidity (NTU) TP (mg/l) SRP (mg/l) TSS (mg/l) TVS (mg/l) TN (mg/l) TAN (mg/l) NO 2 (mg N/L) NO 3 (mg N/L) cbod 5 (mg/l) 7.43 19.4 292 12.5 4.0 1015 753 77.8 14.8 0.43 38.8 548 Over range 0.26 1.4 21 6.3 1.9 401 206 89.6 24.5 0.34 9.2 190 6.97 7.78 18 21 260 324 5.8 21 2 7.2 517 1540 489 1100 8 236 3.4 92 0.23 1.36 25.5 48.6 281 947 nure thickening tanks, optimum conditions were a mixing speed of 165/s for 1 min, a 14/s flocculation speed for 10 min, and a settling time of 10 20 min. Methods The coagulation flocculation tests were carried out following the standard practice for coagulation flocculation testing of wastewater to evaluate the chemicals, dosages, and conditions required to achieve optimum results (ASTM 1995). Jar tests provide insight into the overall process effectiveness, particularly with regard to mixing intensity and duration as it affects floc size and density (Lee and Lin 1999). Samples for jar tests were taken directly from the holding tanks receiving the backwash water from several rotating microscreen filters used for suspended solids removal in two, commercial-size recirculating production systems growing Arctic char Salvelinus alpinus. The first of these is a pilot-scale, partial-reuse system consisting of three 3.66-m-diameter 1.1-m-deep circular Cornell-type dual-drain culture tanks with a maximum feed loading rate of 45 50 kg feed/d (Summerfelt et al. 2002). The second system is a fully recirculating system consisting of a 150-m 3 circular production tank with a maximum daily feed rate of 200 kg feed/d (Summerfelt et al. 2003). Water quality characteristics of the microscreen backwash effluent are summarized in Table 1. Because of the excess alkalinity of the water at this location (i.e., approximately 260 mg/l as CaCO 3 ), no alkalinity additions were required in conjunction with alum and ferric chloride treatments. Jar tests. For over 50 years, the jar test has been the standard technique used to optimize the addition of coagulants and flocculants used in the wastewater and drinking water treatment industry. The Standard Practice for Coagulation Flocculation Jar Test of Water, ASTM-D-2035, was first approved in 1980 and reapproved in 1999 (ASTM 1995). The scope of this practice covers a general procedure for the evaluation of a treatment to reduce dissolved, suspended, colloidal, and nonsettleable matter from water by chemical coagulation flocculation, followed by gravity settling. This standard was utilized to provide a technique to systematically evaluate the variables normally encountered in the coagulation flocculation process. Since coagulant interactions are very complex, laboratory studies are used to determine the optimal dosage, duration, and intensity of mixing and flocculation. The coagulation flocculation process consists of three distinct steps. First, the coagulant is added to the effluent water and a rapid and high intensity mixing is initiated. The objective is to obtain a complete mixing of the coagulant with the wastewater to maximize the effectiveness of destabilization of colloidal particles and initiate coagulation. Critical parameters for this step are the duration and the paddle speed or mixing intensity (velocity gradient, G). Second, the suspension is slowly stirred to increase contact between coagulating particles and to facilitate the development of large flocs. Again, the flocculation duration and intensity are critical parameters (e.g., too high an intensity can break up the aggregate floc). Third, mixing is terminated and the floc is allowed to settle. The velocity gradient is a measure of the mixing energy and allows an engineer to scale the test results to proportionally larger system sizes. A standard jar test apparatus, the Phipps and Bird six-paddle stirrer with illuminated base (Figure 1), was employed for the tests, with six 2-L square B-Ker 2 Plexiglas jars, sometimes called gator jars. The jars are provided with a sampling port, 10 cm below the water line, which allows for repetitive sampling with minimal impact on the test. This type of jar has several advantages over the more traditional 1-L circular jars, including a larger volume for reduced errors in mixing and a larger volume of supernatant for analysis. In addition, the square walls reduce water rotation, making baffles or stators unnecessary. Finally, the thick Plexiglas walls offer sufficient thermal insulation to minimize temperature changes during the testing

202 EBELING ET AL. FIGURE 1. Phipps and Bird six-paddle stirrer with illuminated base. period. The six flat paddles are all driven by a single variable speed motor from 0 to 300 revolutions per minute (rpm). Velocity gradient curves (G; s 1 ) versus agitator paddle speed (rpm) are provided by the manufacturer. An illuminated base helps in the observation of the floc formation and settling characteristics. Stock solutions of the coagulants and flocculants were used to improve the ease of handling and measuring and to ensure good mixing in the jars. Simple dilutions of alum and ferric chloride with distilled water to a 0.2% solution by weight were employed. Normally, the actual test procedures are representative of an existing treatment system (for example, a wastewater treatment plant s mixing, flocculation, and settling tanks) in terms of the duration of mixing and flocculation and the velocity gradients, as well as the settling time employed. In contrast, in this engineering design study (and in future work), a wide range of chemicals, dosages, and conditions (mixing and flocculation stirring speeds and durations, and settling times) were examined to achieve optimum removal of suspended solids and phosphorus. Based on these tests, recommendations can then be made as to the engineering design and operation of both a pilot-scale study and large-scale treatment systems. For each jar test, the following procedure was followed (ASTM 1995). Each jar was filled with 2 L of sample measured with a graduated cylinder, and the initial temperature was recorded. The coagulant or flocculant dose destined for each jar was carefully measured into 150-mL beakers, and distilled water was added to yield equal volumes in all the beakers. The multiple stirrer speed was set to the flash mix values of 50, 75, 150, and 225 rpm (corresponding to G values of 42, 70, 165, and 272/s, respectively), and the test solutions were added. After the predetermined flash mix duration, the mixing speed was reduced to the flocculation or slow mix values of 10, 20, 30, or 40 rpm (corresponding to G values of 6, 14, 22, and 32/s, respectively) for a specified duration. After this time period, the paddles were withdrawn and the floc allowed to settle for specified time (0, 5, 10, 15, 20, 30, or 45 min). Samples were then withdrawn from the sampling ports located 10 cm below the water level for analysis. A baseline set of mixing and flocculation speeds and durations were used for comparison purposes (150 rpm for 1 min mixing, 20 rpm for 20 min flocculation, and a settling time of 30 min). Analysis. For all of the jar tests, ph, turbidity, and soluble reactive phosphorus (SRP, orthophosphate) were measured. For the purpose of comparing the effect of various operating parameters such as mixing and flocculation speed, turbidity was used as an indicator of suspended solids and orthophosphate for phosphorus content. Table 2

COAGULATION AIDS REMOVAL OF TSS AND PHOSPHORUS 203 TABLE 2. Laboratory methods used for analysis via a Hach DR/2010 colorimeter. Parameter Alkalinity a Reactive phosphorus b Total suspended solids a Turbidity a Method; range Standard Methods 2320B Hach Method 8048 (orthophosphate; 0 2.50 mg PO 4 3 /L expressed as P Standard Methods 2540D Hach Model 2100P turbidimeter; nephelometric turbidity units a Adapted from Standard Methods for the Examination of Water and Wastewater (APHA 1989). b Approved for reporting by the U.S. Environmental Protection Agency. shows the methods used for each analysis. When appropriate, reagent standards and blanks were analyzed along with the samples to ensure quality control. The comparison of alum or ferric chloride treatments were statistically analyzed using a single-factor analysis of variance (ANOVA) test (with triplicate samples at each dosage and at 0.01). For tests on the effect of mixing and flocculation variables using alum, the results at each dosage level were lumped, averaged, and then tested using ANOVA for significant differences. Results and Discussion Comparison of Alum and Ferric Chloride The two coagulant aids, alum and ferric chloride, were tested. Figure 2 shows a comparison of the effectiveness of alum and ferric chloride in removing orthophosphate from the microscreen backwash effluent as compared with the control of settling only. The turbidity data showed a similar relationship (Figure 3). For orthophosphate percent removal and normalized turbidity remaining, no significant difference was seen between the alum and ferric chloride (P 0.01) using a singlefactor ANOVA test of triplicate samples at each dosage. Also no significant difference (P 0.01) was seen in percent phosphorus removal for both coagulant aids above a concentration of 60 mg/l. At concentrations above 90 mg/l, orthophosphate concentrations were reduced to about 0.12 mg P/ L for both alum and ferric chloride. At concentrations of 60 mg/l, the turbidity approached its lowest value. The results show that both coagulants had similar effects despite the different chemicals involved. A comparison of the molecular weights of these compounds indicates that the ferric chloride should be slightly more effective on a weight basis given its lower formula weight. However, this difference was not observed in these tests. Effect of Mixing Intensity Since there was no practical difference between the two coagulation agents, a series of tests was conducted with alum to examine the effect of the initial mixing intensity (paddle speed or velocity gradient) and the flocculation intensity on the removal efficiency of suspended solids and orthophosphate. The results of these jar tests are shown in Figures 4 and 5 over a range of alum dosages from 0 to 120 mg/l and for mixing speeds of 50, 75, 150, and 225 rpm. As can be seen in Table 1, the effluent varied slightly from day to day due to changes in, for example, feed addition, biomass, and makeup water addition. Therefore, the results in Figure 4 are expressed in terms of percent re- FIGURE 2. Comparison of alum and ferric chloride in terms of the percent removal of orthophosphate under standard conditions (mixing at 150 rpm for 1 min, flocculation at 20 rpm for 20 min, and settling for 30 min). FIGURE 3. Comparison of alum and ferric chloride in terms of normalized turbidity (nephelometric turbidity units [NTU]) under standard conditions (mixing at 150 rpm for 1 min, flocculation at 20 rpm for 20 min, and settling for 30 min).

204 EBELING ET AL. FIGURE 4. Effect of mixing intensity (rpm) and alum dosage on the percent removal of orthophosphate using the standard jar test with mixing for 1 min, flocculation at 20 rpm for 20 min, and settling for 30 min. moved, and turbidity in Figure 5 has been normalized to minimize the effect of different initial starting concentrations of orthophosphate and turbidities (although normally very small). For the removal of orthophosphorus, a significant difference was seen between the dosage levels (P 0.01) between each treatment level except for the 90 and 120 mg/l dosage. For the normalized turbidity data, no significant difference was seen between the treatment levels for dosages of alum above 40 mg/l (P 0.01). As can be seen in Figure 5, for alum dosages above 60 mg/l, there is a practical effect of mixing speed on turbidity. The lower mixing speed may improve the removal of turbidity at high concentrations due to reduced shearing of the floc during initial formation. FIGURE 6. Effect of flocculation intensity (rpm) and alum dosage on the percent removal of orthophosphate using the standard jar test with mixing at 150 rpm for 1 min, flocculation for 20 min, and settling for 30 min. Effect of Flocculation Intensity Figures 6 and 7 shows the effect of flocculation speed on the removal of orthophosphorus and normalized turbidity as a function of alum dosage. Similar to previous results, no significant difference was seen between dosage levels in the removal of phosphorus (P 0.01) above a 90-mg/ L alum dosage. Although statistically significant differences were found at the lower dosages, these were both small in magnitude and outside the recommended operational range of dosages. For the normalized turbidity data, no significant difference was seen between any of the dosage levels. The lower mixing speed may improve the removal of turbidity at lower concentrations of alum due to reduced shearing during the flocculation phase. Effect of Flocculation Time Figures 8 and 9 show the effect of flocculation mixing time on the orthophosphate concentration and turbidity and total suspended solids concentration. Based on the previous results, a minimum alum dosage of 60 mg/l was chosen as a good operating point. Flocculation mixing times of 0, FIGURE 5. Effect of mixing intensity (rpm) and alum dosage on normalized turbidity (nephelometric turbidity units [NTU]) using the standard jar test with mixing for 1 min, flocculation at 20 rpm for 20 min, and settling for 30 min. FIGURE 7. Effect of flocculation intensity (rpm) and alum dosage on normalized turbidity (nephelometric turbidity units [NTU]) removal using the standard jar test with mixing at 150 rpm for 1 min, flocculation for 20 min, and settling for 30 min.

COAGULATION AIDS REMOVAL OF TSS AND PHOSPHORUS 205 FIGURE 8. Effect of flocculation time on phosphorus concentration using the standard jar test with mixing at 150 rpm for 1 min, flocculation at 20 rpm, and settling for 30 min at an alum concentration of 60 mg/l. 5, 10, 15, 20, and 25 min were used. As can be seen from the bar charts in Figures 8 and 9, after the first 20 min, there was very little change in the concentration of orthophosphate, and after 20 min both turbidity and TSS concentration are at their minimum values. Total suspended solids concentration data were included here to demonstrate the close relationship between turbidity and TSS for this particular effluent at these relatively low concentrations. Effect of Settling Time The effect of settling time on removal efficiencies was also examined. A standardized set of mixing and flocculation speeds and durations were used for comparison purposes: 150 rpm for 1 min mixing; 20 rpm for 20 min flocculation; and a settling time of 5, 10, 15, 20, and 30 min at an alum dosage of 60 mg/l. Figures 10 and 11 show the FIGURE 10. Effect of settling time on the orthophosphate concentration using the standard jar test with mixing at 150 rpm for 1 min and flocculation at 20 rpm for 20 min and settling at an alum concentration of 60 mg/l. effect of settling time on the removal of soluble reactive phosphorus and on turbidity and TSS with alum. As both graphs show, the floc quickly settles out within the first 5 min, with little change in final values after 10 min. Conclusions An evaluation of two commonly used coagulation flocculation aids (alum and ferric chloride) was conducted to determine the optimum conditions for treating the backwash effluent from microscreen filters in an intensive recirculating aquaculture system using standard jar test procedures. The backwash effluent contains very high concentrations of total suspended solids (ranging from 517 to 1,540 mg/l) due to the nature of the microscreen operation. The orthophosphate removal efficiencies for alum and ferric chloride were greater than 90% at a dosage of 60 mg/l, with final concentrations of SRP approaching 0.15 mg FIGURE 9. Effect of flocculation time on the turbidity (nephelometric turbidity units [NTU]) and total suspended solid (TSS) concentration using the standard jar test with mixing at 150 rpm for 1 min, flocculation at 20 rpm, and settling for 30 min at an alum concentration of 60 mg/l. FIGURE 11. Effect of settling time on turbidity (nephelometric turbidity units [NTU]) and total suspended solid (TSS) concentration using the standard jar test with mixing at 150 rpm for 1 min, flocculation at 20 rpm for 20 min, and settling at an alum concentration of 60 mg/l.

206 EBELING ET AL. P/L. Optimal turbidity removal was achieved with a 60 mg/l dosage for both alum and ferric chloride. Both alum and ferric chloride demonstrated an excellent removal of suspended solids from initial TSS values of approximately 320 mg/l to approximately 10 mg/l at a dosage of 60 mg/l. Flocculation and mixing intensity and duration played only a minor role in the removal efficiencies for both orthophosphates and suspended solids. Both coagulation flocculation aids also exhibit excellent settling characteristics, with the majority of the floc quickly settling out in the first 5 min. It is important that mixing and flocculation speeds and times be determined by jar test for each effluent, but good results were obtained for this aquaculture effluent with a mixing speed of 150 rpm (G 165/s) for 1 min, 20 rpm (G 14/s) flocculation speed for 20 min, and a settling time of 10 20 min. Based on these initial studies, for maximum orthophosphate removal the dosages of both alum and ferric chloride applied should be greater than 60 mg/l. For maximum turbidity removal, a dosage of 60 mg/l or more of both alum and ferric chloride is required. Future studies will include additional tests with varying mixing, flocculation, and settling times at various dosages for other potential coagulation flocculation aids, including a screening of potential polymer additions. Acknowledgments This work was supported by the U.S. Department of Agriculture, Agricultural Research Service, under Cooperative Agreement number 59-1930-1-130. The experimental protocol and methods used in this study were in compliance with Animal Welfare Act requirements and were approved by the Freshwater Institute Institutional Animal Care and Use Committee. References Adler, P. R., and P. L. Sibrell. 2003. Sequestration of phosphorus by acid mine drainage floc. Journal of Environmental Quality 32:1122 1129. 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