To Improve the Turbidity of Dam Reservoirs
 Dam reservoir muddied after heavy rain |
 Allophane used in the experiment |
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 Experimental cylinder |
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 Immediately after
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After heavy rain
When heavy rain falls in an area upstream of a dam, a large amount of water
flows into the dam reservoir but the dam prevents the rivers downstream
from flooding by storing the inflow water in the reservoir. In addition,
a dam gradually releases downstream the water stored during a heavy rain
after the rain has stopped, and we use the water for water supply and agriculture.
Meanwhile, the water stored in the dam reservoir during the heavy rain is more turbid than the water that usually flows in rivers and the turbidity of the water released from the dam may cause problems. In many dam reservoirs, intake facilities are provided with a way to release water at a level closer to the water surface where the water is less turbid. As a measure to take when an entire dam reservoir has become turbid, PWRI is conducting research to find a way of improving the turbidity of water in a dam reservoir.
Removal of turbidity using a coagulant
Methods of clarifying turbid water include filtration and use of coagulants.
A coagulant is an agent that removes the turbidity of water. When mixed
with turbid water, it characteristically attracts the particles causing
the turbidity and sinks to the bottom over time. As a result, the surface
layer becomes clear. While chemicals are usually used as coagulants, we
conducted a turbidity removal experiment with a natural coagulant to avoid
any adverse effect on the environment of the dam reservoir.
The coagulant used for the experiment is a material called allophane. Allophane
is a very fine particles contained in a high proportion in soil made from
volcanic ash that has fallen and accumulated. If allophane is dissolved
in water and acid or alkaline is added to it, its state changes according
to the pH condition and the particles distribute or stick together. The
intention was to take advantage of this property to attract the soil particles,
the cause of the turbidity, and remove that turbidity.
Continuing process of trial and error
For the turbidity removal experiment, we used a settling cylinder with
a diameter of about 40 cm and a depth of 2.2 m. Supersonic distributor
was provided at the top of the cylinder and a small pump at the bottom.
For the experiment, we used turbid water sampled from a dam reservoir in Tochigi Prefecture. The water was poured into the experimental cylinder, allophane was added, and then a small pump was used to mix the water thoroughly. Water-dissolved allophane can be distributed well by adding chemicals. For the experiment, however, we applied supersonic vibration rather than adding chemicals to distribute the allophane. The result may greatly vary depending on how much these mixing and distributing processes are carried out and the experiment was a repetition of trial and error.
Here we would like to present one case with an effect that we observed.
Immediately after the start of the experiment, the water was very muddy
due to the allophane that we had added to the turbid water. Subsequently,
the turbidity gradually decreased as shown in the photos taken 6 and 24
hours later, and eventually the turbidity almost disappeared and the soil
and allophane collected on the bottom of the cylinder. This experiment
has shown that allophane can be used to remove turbidity.
For practical use
While the turbidity was mostly removed in the experiment, many challenges
still remain before this technique can be put to practical use. In a dam
reservoir, soil particles, which cause turbidity, are distributed extensively
and very large-scale facilities are required to produce an effect. In addition,
completely removing the turbidity of a dam reservoir would require considerable
time and expense. In the future, we intend to conduct indoor experiments
to devise a method of efficiently removing turbidity, and later we will
conduct on-site experiments to advance research toward improving dam reservoir
turbidity.
(Contact: River and Dam Hydraulic Engineering Research Team)
Soil Amelioration by Application of Cow Waste Slurry as Fertilizer
- Clarification of grass yield holding effect and production environment amelioration effect
 Aerobic digestion facilities |
 Spraying of aerobically digested slurry |
 Grass yield investigation |
Cow waste slurry contains nutrition components including nitrogen, phosphoric acid and potassium and it can be used as a liquid fertilizer. In fact, in Hokkaido, slurrygation, in which cow waste slurry diluted with water and subjected to aerobic digestion process (feeding air into liquid for fermentation in aerobic conditions), or aerobically digested slurry, is applied as a fertilizer on pastures, has been practiced widely.
The nutrition component replenishment effect, achieved by applying aerobically digested slurry, can be identified in a relatively short-term investigation. For this reason, many studies have been conducted and the results used to reduce the amounts of chemical fertilizer applied on pastures. However, not many examples of research on the effect of applying aerobically digested slurry over the years on the change of pasture soil or grass yield have been conducted, and farmers engaged in slurrygation have made strong demands to verify the effect of long-term spraying of applied digested slurry.
CERI investigated a number of pastures on which aerobically digested slurry had not been applied, and others on which aerobically digested slurry had been applied for 4 to 18 years to verify the effect of slurrygation slurry on the change of pasture soil characteristics and grass yield.
As a result, the following results were obtained:
(1) No change in the soil characteristics was observed in pastures that had not been applied aerobically digested slurry. In pastures on which aerobically digested slurry had been applied, however, fertilizer retentivity was found to have increased while drainage and water retentivity of the soil surface improved as the application period got longer. This shows that slurrygation contributes to the development of soil suitable for growing grass.
(2) In pastures on which aerobically digested slurry has been applied,
no change in the yield was observed in case 2 to 23 years had passed after
grassland renovation (a process of re-preparation of a pasture in which
the grass yield fell due to a deterioration of the surface soil characteristics
and results in an increase in weeds) and high yield was maintained. This
indicates that slurrygation helps maintain grass yield over an extended
period.
In the future, we would like to publish a guide to the effect that slurrygation has on ameliorating the production environment, and have a plan to offer information to the engineers and the farmers practicing slurrygation.
(Contact: Rural Resources Conservation Research Team, CERI)