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None of the treatment lagoons warn that they are silted up.
Sediment accumulation is gradual and invisible—it happens underwater, centimeter by centimeter, over years. When the problem becomes evident, it is already advanced: treatment efficiency has decreased, the effluent discharge is substandard, and environmental penalties have become a real risk.
This article explains how to identify the need for intervention before it gets to that point.
What is dredging?
Desilting is the removal of sediment from the bottom of ponds, reservoirs, canals, and wastewater treatment systems.
This material — sludge, sand, organic matter, and various solids — naturally settles during operation. As it accumulates, it occupies the volume that should be available for treatment, compromising the system's capacity and efficiency.
This phenomenon is common in:
- Stabilization ponds
- Wastewater Treatment Plants (WWTPs)
- Retention basins
- Industrial reservoirs
- Drainage systems
- Settling ponds in mining
5 signs that dredging is necessary
1. Decrease in treatment effectiveness
This is the most important sign—and the most frequently ignored. When the usable volume of the lagoon decreases, the hydraulic retention time also decreases. The effluent passes through the system more quickly and does not receive adequate treatment.
In practice, this shows up in the analyses: BOD, COD, and suspended solids start to rise at the outlet, even without a change in the input load.
“When a client tells me that the wastewater treatment plant has 'stopped working properly' for no apparent reason, the first thing I ask for is a bathymetric survey. In many cases, the lagoon has lost 30%, 40% of usable volume to sediment. The system hasn't broken down—it has simply become too small for the load it receives.”
— Luiz Varandas, Project Coordinator at TSL
2. Bad odor
Excessive sludge buildup creates anaerobic zones at the bottom of the lagoon. Decomposition without oxygen generates hydrogen sulfide gas and other compounds—the characteristic rotten egg odor.
Besides being a nuisance, it's a technical indicator: it means the lagoon has lost its ability to maintain the aerobic conditions required for treatment.
3. Visible reduction in depth
A bathymetric survey compares the current depth with the design depth. But even without bathymetry, visual signs appear: emerging sediment banks, evident shallow areas, vegetation growth in places that should be deep.
4. Risk of spillage
A lagoon with less usable volume has less capacity to absorb peak flows. In events of intense rainfall or a sudden increase in load, overflow becomes possible — with serious environmental and regulatory consequences.
5. Increase in solids at the outlet
With less volume available for settling, particles that should settle end up being carried to the system's outlet. It's the lagoon "returning" the sediment it can no longer retain.
How is dredging carried out?
The operation can be performed with the lagoon in operation — without the need to empty it or stop the treatment. This is possible because the removal is done by pumping, not by excavation.
Dredging by suction and pumping
A dredger positioned over the lagoon — usually mounted on dredging barge — It sucks sediment from the bottom and pumps it to the treatment area. The system operates below the water surface, without interfering with the treatment operation.
Dehydration of the removed material
The extracted sediment has a very high water content — often above 90%. Transporting it in this state would be economically unfeasible: you would be paying freight to transport water.
Therefore, dehydration is an integral part of the process, carried out by geobags or decanter centrifugesdepending on the available area and the project timeline.
Final destination
The dehydrated material is classified and disposed of according to its composition, with all traceability documentation required by Law 12.305/10 (National Solid Waste Policy). The separated liquid phase returns to the treatment system.
“Many people think of dredging as simply 'removing the silt.' But removal is only the first step. The real challenge is what to do with the material afterward: dewatering, sorting, proper disposal, and documenting everything. That's where most projects stall.”
— Luiz Varandas, Project Coordinator at TSL
The benefits
- Restoration of the usable volume and design capacity of the lagoon.
- Restoration of treatment efficiency — output parameters return to standard.
- Elimination of bad odors and anaerobic zones.
- Reducing the risk of flooding and environmental penalties.
- Increased lifespan of the installation — avoids the need to build a new pond.
- Compliance with the environmental agency's release parameters.
The calculation is worthwhile: the cost of dredging is a fraction of the cost of building a new lagoon — which is, in practice, the alternative when the existing one irreversibly loses its capacity.
How often to
There is no standard range. The rate of sedimentation depends on the influent solids load, the type of effluent, and the operating regime.
What works in practice is monitoring: a periodic bathymetric survey — every two or three years — allows us to track the evolution of the sediment and plan intervention before it becomes an emergency.
The difference between scheduled and emergency dredging is significant in terms of cost and risk. In scheduled dredging, you choose the window of opportunity, negotiate the timeline, and plan the disposal of the sediment. In emergency dredging, you do what you can—under pressure from the environmental agency.
Frequently Asked Questions
What is the difference between dredging and desilting?
Dredging is the method—the technique of removing sediment by pumping or excavation. Desilting is the objective: to recover the lost volume of a lagoon or reservoir. In practice, desilting is carried out through dredging.
Is it necessary to drain the lagoon to carry out the dredging?
No. Suction and pump dredging operates with the lagoon full and the treatment plant running. The dredger works below the water surface, without interrupting operations.
How long does dredging take?
It depends on the volume of sediment, access to the lagoon, and the installed dewatering capacity. Medium-sized projects can take from a few weeks to a few months. A preliminary bathymetric survey allows for an estimation of the timeframe.
How often should I perform dredging?
There is no fixed interval — it depends on the solids load of the effluent. It is recommended to conduct bathymetric surveys every two or three years and schedule intervention when the usable volume drops significantly.
What happens to the removed material?
It undergoes dehydration (geobags or centrifuges), is classified according to its composition, and sent to a licensed location with traceability documentation. The separated water returns to the treatment system.
What are the risks of not carrying out the dredging?
Progressive loss of treatment efficiency, effluent that does not meet discharge standards, risk of environmental fines, foul odor, risk of overflow and, ultimately, the need to build a new lagoon.
TSL Engenharia e Serviços executes projects of dredging of lagoons with its own dredging barges, dewatering using geobags or centrifuges, and documented final disposal — with the lagoon operation in full swing.
