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SRAC Publication No. 460 VI PR May 1999 Control of Clay Turbidity in Ponds John A. Hargreaves* What is turbidity? objectionable to pond owners bottom soils to be resuspended. In from an aesthetic standpoint. such cases, oxygen may decline to Turbidity is a very general term critically low levels and make it that describes the “cloudiness” or Some sources of clay turbidity are runoff from clear-cut or over- necessary to aerate the pond. “muddiness” of water. Turbidity can be caused by many sub- grazed watersheds, road or build- stances, including microscopic ing construction, the activities of The effect of turbidity on algae (phytoplankton), bacteria, cattle watering in farm ponds, off-flavor in fish dissolved organic substances that pond bank erosion from wave action, excessive aeration, or the Not much algae can grow in stain water, suspended clay parti- muddy water because clay parti- cles, and colloidal solids. feeding activities of certain bot- tom-dwelling fish such as com- cles limit the penetration of light Although turbidity can be a prob- into water. Blue-green algae are lem in many different types of mon carp or buffalo. This fact sheet will discuss the control of adapted to the dimly lit waters of water, turbidity caused by sus- moderately turbid ponds. pended clay tends to occur most undesirable forms of turbidity, specifically that caused by sus- Unfortunately, some of these algae often in soft, poorly-buffered (low can cause off-flavor in fish, which alkalinity) waters. pended clay particles. could be reason enough to clear Some of the substances that cause water of clay turbidity. Interest- turbidity are more desirable in The effect of clay turbidity ingly, extremely muddy ponds fish culture or recreational farm on dissolved oxygen have few, if any, algae in the water ponds than others. In moderate The dissolved oxygen in sportfish and often less problem with off- amounts, phytoplankton is a or farm ponds normally fluctuates flavor than moderately muddy desirable form of turbidity widely during the summer. ponds. because it provides food for During the day, plant photosyn- microscopic animals (zooplank- thesis increases the oxygen con- The chemistry of colloidal ton) and filter-feeding fish, and centration; during the night, plant clay suspensions improves water quality by pro- and fish respiration reduces the ducing dissolved oxygen and oxygen concentration in the water. The chemistry of colloidal clay removing potentially toxic com- Clay turbidity reduces the magni- suspensions is not completely pounds such as ammonia. On the tude of daily fluctuations in dis- understood, primarily because other hand, turbidity caused by solved oxygen concentration, so fairly complex physical and chem- clay particles is generally undesir- that it gets neither very high nor ical processes are involved. Clay able because it keeps light from very low. However, muddy water particles are extremely small; penetrating the water, and light is tends to have a lower average some are even smaller than bacte- required for algal growth. At very concentration of dissolved oxygen ria. Therefore, they will not settle high concentrations, clay particles than water with a green phyto- readily, even in still water. The can also clog fish gills or smother plankton bloom. Clay turbidity small size of these particles means fish eggs. Turbidity also may be can sometimes develop quite sud- that they have an extremely high denly, as when heavy storm surface area relative to the volume runoff enters the pond or high of the particle. A clay particle can *Mississippi State University. winds churn the water and cause be envisioned as a flat plate cov- ered with a negative electrical charge that attracts the positive ions in water. Positive ions that are immediately adjacent to the clay particle are said to be “adsorbed,” while others that are farther away are less strongly attracted. In water, negatively charged clay particles are sur- rounded by clouds of positively charged ions. When these parti- cles, surrounded by their ion clouds, come close to each other they are repulsed, much the same way similar poles of two magnets will repel each other (Fig. 1). The cumulative effect of the repulsion of a huge number of small parti- cles prevents their aggregation into larger, heavier particles that would settle more readily. Taken together then, the extremely small Figure 1. Small clay particles remain in suspension because they have the same size of clay particles and the sur- surface charge and repel each other when they get too close. face electrical charge explain how particles remain in suspension. Flocculation and coagulation Flocculation is a way of control- ling clay turbidity by adding sub- stances to water that facilitate the formation of bridges between par- ticles (Fig. 2), allowing them to combine into groups of small par- ticles called “flocs” (Fig. 3). Metal salts make good flocculants, depending on pH. These Figure 2. Coagulants (CG) such as alum form bridges between particles. hydrolyzed metal compounds destabilize colloids by shrinking the layer of positively charged ions surrounding clay particles, which increases the attraction of one particle to another (coagula- tion). Hydrolyzed metals also can be adsorbed onto the surfaces of clay particles and create bridges to other particles (flocculation). As these particles begin to settle, they ensnare other particles, become progressively heavier, and settle much more readily from suspen- sion. In general, the effectiveness of coagulants increases with the charge on the metal ion. The sodi- um (Na+) in sodium chloride (NaCl) is not a very effective coag- ulant. The calcium (Ca2+) in gyp- sum (CaSO4) is more effective because it carries a +2 charge. The aluminum (Al3+) in alum and Figure 3. Adding coagulants to turbid water causes particles to aggregate into the ferric-iron (Fe3+) in ferric sul- fate are more effective yet because “flocs,” which settle out more readily than individual particles. they carry a +3 charge. Some companies now manufacture vari- Severe turbidity (25 mg/L alum) ous synthetic “polyelectrolytes,” which are large, long-chained Alum application rate (lbs/acre) molecules with even more charge than the metal salt coagulants list- ed here. Alum Moderate turbidity One of the most effective coagu- (15 mg/L alum) lants is alum, or aluminum sul- fate, which has been used to clari- fy muddy waters since the time of the early Egyptians (2000 B.C.). Although alum is not always available from farm supply busi- Average pond depth (ft) nesses, many companies selling industrial chemicals will carry it. Severe turbidity A dose of 15 to 25 mg/L (150 to (300 mg/L gypsum) 250 pounds per acre) should be Gypsum application rate (lbs/acre) sufficient to remove the turbidity from most waters (Fig. 4). Use the lower concentration for moderate- ly turbid (less than 12-inch visibil- ity) waters and the higher concen- tration for highly turbid (less than 6-inch visibility) waters. Alum Moderate turbidity makes water more acidic. In (100 mg/L gypsum) ponds with low alkalinity (less than 20 mg/L as CaCO3) it can reduce water pH to levels that may affect fish growth and sur- vival. In low alkalinity ponds, add 1/2 part hydrated lime for every Average pond depth (ft) part of alum applied in order to Figure 4. Guidelines for alum and gypsum application rates are a function of maintain proper pH. pond depth and the severity of the turbidity problem. Apply alum in calm weather because excessive turbulence will coagulate, so fertilizing to start a Other coagulants slow the settling of the flocs. The phytoplankton bloom may also key to success with alum is to Although not nearly as effective clear water of suspended clay par- thoroughly and quickly mix the as alum, gypsum also can be used ticles. coagulant with the water. This can to control turbidity but without be accomplished by releasing a the loss of alkalinity. Gypsum Organic matter such as chopped mixture of 10 parts water to 1 part must be added to achieve a con- hay or cottonseed meal can reduce alum into the prop wash of a boat centration of 100 to 300 mg/L for clay turbidity in farm ponds. as it is driven back and forth effective turbidity control. For However, large amounts of mater- around the pond. Or, a slurry of most ponds, gypsum application ial must be added to the pond, alum and water can be spread rates will range from about 1,000 which may deplete the dissolved over the pond surface. In ponds to 2,000 pounds per acre (Fig. 4). oxygen as the organic matter equipped with aerators, releasing In hard-water ponds (calcium decomposes. It may also be diffi- a slurry of alum and water in hardness greater than 50 mg/L), cult and costly to transport and front of the aerator will distribute the water is nearly saturated with uniformly distribute large it quickly. Wear a particle (dust) calcium and gypsum may be inef- amounts of organic matter. mask when mixing the dry chemi- fective. In that situation, alum will cal with water. If the dose is suffi- be the only effective coagulant. The bucket test cient, water should be noticeably All the coagulants mentioned can Although the application rates clearer within hours, although the remove phosphorus from water. recommended here for coagulants full effect may not be apparent for As phosphorus is an essential are applicable for most situations, several days. plant nutrient, it may be necessary there are many factors that can to fertilize the pond after treating affect the effectiveness of the treat- it for turbidity. On occasion, phy- ment process. These include the toplankton and clay can mutually amount and kind of turbidity, chemical characteristics of the rate from Table 1 by the number pond edges to minimize scouring coagulant, mineral composition of of surface acres of the pond. of shallow edges by wave action. the water, pH, temperature, and If gypsum is the coagulant select- Windward levees in ponds with a the amount of mixing during and ed, the bucket test and Table 1 can long fetch (maximum length) ori- after application. So, it is best to be modified slightly to determine ented to the prevailing wind are take an experimental approach to application rates. Simply multiply subject to erosion by waves. turbidity control. This can be done the amount of coagulant added to Protect windward banks with rip- with a bucket test. each bucket by 10, adding 2, 3, 4 rap consisting of large boulders Obtain a small sample of a select- or 5 g gypsum to each bucket. placed at the shoreline or log ed coagulant (alum or gypsum). Multiply the rates in Table 1 by 10 booms (logs linked with chain) Collect four 5-gallon buckets of to determine the gypsum applica- placed along the base of the turbid pond water. Carefully tion rate. For example, if the mini- levee. Shallow sediments of old weigh four separate, small quanti- mum gypsum dose that cleared ponds may be periodically resus- ties of alum: 0.2, 0.3, 0.4 and 0.5 g. water was added to bucket 3 ( 4 g pended by wind-driven waves. Add each weighed amount of gypsum), and average pond Renovate old ponds after about coagulant to one bucket of water depth is 3 feet, then the gypsum 10 to 15 years by removing sedi- and stir vigorously for 1 to 2 min- application rate is 1,810 pounds ments that have accumulated. utes. Then, stir briefly every 5 per acre. Spread and compact the excavat- minutes for up to 30 minutes. ed material on the pond levee. Observe the clarity of the water. Finally, if practical, limit livestock Prevention is the best access to a small section of the Select the minimum dose of coag- ulant that clears the water. For control method pond, preferably at the shallow example, suppose the water Coagulants should be applied end. cleared in buckets 3 and 4, but did after the cause of the turbidity not clear in buckets 1 and 2. The problem is corrected. Watershed References dose of alum added to bucket 3 protection and soil conservation Avnimelech, Y. and R. G. Menzel. (0.4 g) would be the proper one. practices should receive the high- 1984. Algal clay flocculation Next, estimate average pond est priority for attention. If a as a means to clarify turbid depth by measuring depth with a watershed is to be clear-cut, leave impoundments. Journal of Soil weighted line at 10 to 20 locations buffer strips (stream-side manage- and Water Conservation 39:200- around the pond. Average depth ment zones) about 50 to 100 feet 203. also can be estimated by multiply- wide along each side of feeder streams. These strips can trap a Boyd, C.E. 1979. Aluminum sul- ing the maximum depth by 0.4. fate (alum) for precipitating Select the application rate in Table large quantity of sediment run- ning off cleared slopes. If pond clay turbidity from fish ponds. 1 by first reading across the line Transactions of the American for the minimum alum dose (0.4 g layout permits, divert turbid feed- er streams around the pond or Fisheries Society 108:307-313. in the example) and then reading down the table to the average direct them through a sedimenta- Wu, R. and C. E. Boyd. 1990. pond depth. The table entry tion basin upstream from the Evaluation of calcium sulfate where the two lines cross is the pond. If a watershed is in pasture, for use in aquaculture ponds. coagulant application rate in balance livestock stocking rates Progressive Fish-Culturist 52:26- pounds per acre. To determine the with the availability of forage to 31. total amount of coagulant minimize overgrazing. Within the required, multiply the application pond, maintain grass cover along levees and pond margins. Deepen Table 1. Alum application rates (pounds per acre) determined by a bucket test. Alum addition to 5-gallon Average pond depth (feet) bucket Bucket (g) 2 2.5 3 3.5 4 4.5 5 1 0.2 60 75 91 106 121 136 151 2 0.3 91 113 136 159 181 204 226 3 0.4 121 151 181 211 242 272 302 4 0.5 151 189 226 267 302 340 377 The work reported in this publication was supported in part by the Southern Regional Aquaculture Center through Grant No. 94-38500-0045 from the United States Department of Agriculture, Cooperative States Research, Education, and Extension Service.
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