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Control of Clay Turbidity in Ponds


									                                                                                                SRAC Publication No. 460

                                       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
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-
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.

                                                                                                                       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                (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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