The dredging industry generates a large quantum of dredged material each year, which needs to be properly handled, treated and disposed of. This material can be disposed of in open waters, depending on its pollutant levels, or on land. Land disposal is usually preceded by dewatering techniques, and barge/pipeline systems are employed to transport the dredged material to the disposal site. However, the dredged material needs to be treated both during and after dredging, in order to be fit for disposal. The processes that can be used for this treatment include aeration, which leads to the elimination of displeasing odours; and chemical treatment to improve spoil quality before dredging works are commenced.
Several treatment techniques are also deployed post-dredging, including flocculation, which is used to clarify the resultant effluent. This technique depends on quiescent water, the quantum of settling prior to the addition of chemicals and the efficient mixing of chemicals. Another prominent treatment technique is that of hydro-cyclones, which is used to separate solids from liquids, and can thus dewater fill material for reclamation works. In addition, vacuum filters can also be deployed for dewatering and sludge removal, with the resultant liquid being treated through other processes. Besides this, aeration can be used to stabilise highly organic dredge spoils and the incineration technique can be deployed to handle solid waste.
However, while several techniques exist for the treatment of dredged material, most of them are equipped to handle single elements in the waste generated. Therefore, a combination of techniques needs to be adopted for effective treatment of dredged materials.
Once treated, several techniques can be deployed to dispose of this material.
- Pipeline dredges: These hydraulic dredges discharge the waste either to the side of a channel (open water disposal) or into a confined/unconfined disposal area.
- Hopper dredgers and scows: Hopper dredgers and scows can be used by dredging companies to discharge waste by using various methods – overflow, bottom dump or pump out. Overflow is a method designed to increase the economic solids load. The bottom dump method involves disposing materials in areas which are mostly located close to the shore so as to minimise the turnaround time of the process. Meanwhile, the direct pump-out method disposes of the waste to confined disposal areas. A frequently used combination of the three methods involves bottom dump into a rehandling basin and hydraulic pipeline removal to a confined area.
- Beach nourishment: Dredged material can be used for beach nourishment. While pipeline dredgers pumping material from offshore areas may be subject to wind and wave limitations, submerged and flexible pipelines can also be employed for this purpose.
- Silt curtains: These are essentially vertical barriers consisting of polyvinyl chloride-type floating screens which prevent the spread of dredge turbidity. However, they may not be feasible for use in currents. Thus, they can be used when current flows are controlled, including use within confined disposal areas, to ensure adequate retention time by preventing short-circuiting of flows; and at open-water disposal sites, possibly in conjunction with flocculant or aeration techniques.
- Bubble barriers: Pneumatic bubble screens involve the creation of barriers to stop floating of suspended materials. However, for its effective use, current velocity must be minimal and high power requirements for adequate air compression should be met.
- Long-distance pipeline transport: Long-distance pipeline disposal can be used to pump dredged material over land to rehabilitate strip mine areas, or to pump it far out into the sea. But this technique is heavily dependent on the location, type of material, and existing facilities and equipment.
- Road and rail transport: If the dredged material is sufficiently dewatered, it can be handled by dragline and bucket loaders, and be hauled by rail or road from temporary storage areas to permanent inland locations.
Regardless of the method deployed for disposing of dredged material, some considerations need to be kept in mind while selecting a dumpsite. For instance, land sites need to be selected factoring in the potential environmental impact. Identifying water dumpsites comes with caveats such as the physical, chemical and biological characteristics of the water-column/ seabed; other uses of the area under consideration; the assessment of the constituent fluxes associated with dumping in relation to the existing fluxes of substances already present in the marine environment; and economic and operational feasibility of the dumpsite.
The way forward
The efficient handling, treatment and disposal of dredged material is essential to dredging activity. While the aforementioned techniques are being deployed at present, several new techniques are also gaining prominence. For instance, friction-reducing chemicals are being added to the dredged material to increase solid concentration, and cutter head and suction dredging is being eliminated to reduce turbidity in the material. Going forward, an increased focus on undertaking precise surveys, as well as relying on historical data and predictive techniques can go a long way in ensuring accurate dredging. This in turn can lead to a reduction in the quantity of material required to be dredged.
Based on a presentation by V. K. Shukla, scientist, Central Water and Power Research Station, at a recent India Infrastructure conference