3081
A System of dehydration, purification, and reduction for dredged soil
–
Release
inhibition of nutrient salts from bed mud using natural zeolite
–
Un système de déshydratation, d'épuration et de réduction de sols dragués - Prévention du
relâchement de sels nutritifs des lits de boue à l'aide d'une zéolithe naturelle -
Umezaki T., Kawamura T.
Department of Civil Engineering, Shinshu University, Japan
ABSTRACT: In a closed water body such as a lake, many nutrient salts such as nitrogen and phosphorus are contained in bed mud
deposited on the bottom. Their release is one of the causes of eutrophication. We propose a zero-emission system to preserve the
ecosystem in a closed water body. Applying the system obviates landfill sites for dredged soil and reduces countermeasure costs. In
the system, the bed mud is dredged and then reduced to the bottom after dehydration and purification treatments that reduce their
volume and absorb nutrient salts. To realize a suitable bottom environment, reclaimed soil requires moderate flexibility. Consolidation
tests were conducted on the bed mud with natural zeolite powder as absorbent, and column tests were also conducted for release
inhibition of nutrient salts on the treated soil and lake water. Based on test results, the effectiveness of dehydration and purification
was investigated. Main conclusions are as follows: Applying about 30 kPa of low consolidation pressure, water contents approach the
liquid limit and the volume decreases to about two-thirds. For simulating the proposed system, the release of nutrient salts is inhibited
and development of algae is prevented.
RÉSUMÉ : Dans un plan d'eau fermé tel qu'un lac, le lit de boue déposé au fond contient des sels nutritifs tels que l'azote et le
phosphore. Leur libération est l'une des causes de l'eutrophisation. Nous proposons un dispositif sans émission pour préserver ce type
d'écosystème. Le système permet d'éviter la mise en décharge de ces sols dragués et réduit les coûts économiques. Dans ce principe, le
lit de boue est dragué puis redéposé au fond après déshydratation et purification pour en réduire son volume et absorber les sels
nutritifs. Pour réaliser un environnement
convenable au fond du plan d’eau
, le sol récupéré exige une souplesse modérée. Des tests de
consolidation sur de la boue avec de la poudre de zéolithe naturelle comme absorbant ont été effectués, ainsi que des essais en colonne
pour inhiber le relâchement de sels nutritifs dans le sol traité et l
’
eau du lac. L'efficacité de la déshydratation et de la purification ont
été étudiées sur la base de ces résultats. Les principales conclusions sont que l'application d'une pression de consolidation faible
d'environ 30 kPa permet d'approcher la limite de liquidité des sols dont le volume diminue d'environ deux tiers.
L’avantage du
système proposé est de prévenir le relâchement de sels nutritifs et le développement d
’
algues.
KEYWORDS: closed water body, eutrophication, nutrient salts, bed mud, natural zeolite, consolidation, protection of ecosystem
1 INTRODUCTION
In a closed water body such as a lake, eutrophication is
accelerated by nutrient salts such as nitrogen and phosphorus
that inflow from rivers and which are released from bed mud
deposited on the bottom. Many algae develop in the water,
causing problems such as water pollution and offensive odors.
Dredging of bed mud has been used as a countermeasure.
However, dredging work is rarely conducted because of high
transportation costs of dredged soil with high water content.
Moreover, landfill sites are scarce.
As described herein, we propose a zero-emission system for
ecosystem preservation in a closed water body. The system, as
shown in Figure 1, dredges, dehydrates, purifies, and reduces
bed mud. First, the system outline is discussed. Next,
consolidation tests and column tests of bed mud simulating this
system were conducted to verify the system effectiveness.
Based on test results, dehydration, volume reduction, and
release inhibition of nutrient salts from the treated soil were
discussed.
2 A ZERO-EMISSION SYSTEM FOR ECOSYSTEM
PROTECTION
Figure 1 shows an outline of the proposed system, which has
three processes: (a) dredging, (b) dehydration and purification,
and (c) reduction. The system concept is the following.
(1) To remove nutrient salts, heavy metals, etc. from the bed
mud, the bed mud is dredged.
(2) By adding an environment-friendly adsorbent such as
natural material to the bed mud, nutrient salts and heavy
metals are adsorbed and immobilized.
(3) The volume of bed mud is reduced by dehydration, and
nutrient salts and heavy metals are removed with drained
water. After water quality analysis and suitable treatment,
the drainage water is discharged.
(4) The treated soil is reclaimed to the bottom of the water and a
zero-emission system is realized. The soil should be
deformed flexibly with self-weight to avoid increasing the
volume of reclaimed soil and release of nutrient salts from
Figure 1. Proposed zero-emission system.
