Study recommends alternatives to submarine outfall

A study performed at the USP Polytechnic School indicates that changes to the treatment of sewage from the São Paulo coast’s oldest outfall will allow the system to maintain its current level of discharge.

By Elton Alisson

Agência FAPESP – Some Brazilian coastal cities have sewage waste treatment systems that use submarine outfalls to release household or industrial effluents in deep ocean waters because of the ocean’s high capacity for the dispersion of contaminants.

However, Brazil still has no legislation establishing the type of treatment to be used by submarine outflow systems or which compounds in the sewage must be removed before they reach the sea to minimize environmental impacts.

A doctoral study performed by Eduardo Lucas Subtil in the Hydraulic and Environmental Engineering Department at the University of São Paulo’s Polytechnic School, with FAPESP funding, provides information regarding the best option for pre-treatment and the main compound to be removed from the sewage released by high-discharge submarine outflows such as the one in Santos on the São Paulo coast.

The study concluded that the application of specific chemical products in a type of treatment called Advanced Primary Treatment would be able to remove the suspended solids and elements, such as phosphorus, present in the sewage that is released by the submarine outfall.

The results of this study have already begun to raise interest and may serve as foundation for environmental control organizations to create specific legislation regarding submarine outfalls.

“Most countries already have environmental laws for this,” Subtil told Agência FAPESP. “In Brazil, the first legislation that mentions submarine outfalls was released by Conama [the National Environmental Council] in 2011. It established minimum treatment as removal of 20% of suspended solids. But this amount is still insufficient for heavy flow submarine outfalls and those located in restricted marine circulation areas.”

According to Subtil, adjustments to the treatment system for sewage released by the Santos submarine outfall would  allow maintenance of the current discharge levels.

Administered by Sabesp, the Santos submarine outfall, which is approximately 4,500 meters long and was installed in 1978, is the oldest along the São Paulo coast and releases the most sewage.

Currently authorized to release up to 5.3 cubic meters of sewage per second, this outfall is unable to adequately disperse the load of contaminants that it releases into the ocean using the current treatment system.

As preliminary treatment, screens are used to remove the larger solids from the water. This preliminary system is used at the Santos submarine outfall and other outflows in Brazil. The material that passes through the screens is released into the ocean through a diffusion system located at the end of the outfall that helps to dilute and mix the effluent with the ocean water.

However, as the amount of discharge from the submarine outfall continues to increase, and with the adoption of the preliminary sewage treatment process, this outfall system has become unable to control and reduce phosphorous levels. At excessive levels in the water, phosphorus can cause the growth of toxic algae, the contamination and death of fish and plants and unpleasant odors.

“In the case of the Santos submarine outfall, preliminary sewage treatment has become unsustainable. Either the discharge from the outfall needs to be reduced, or the treatment system has to be improved if the authorized discharge levels are to remain. This would be the most feasible solution, because there is already a sewage collection system installed,” said Subtil.

Best treatment option

According to Subtil, the advanced primary sewage treatment process for the Santos submarine outfall recommended by the study is the best solution in economic and environmental terms.

Used in the United States, China and other countries, this system is able to remove phosphorus from sewage in a more efficient manner than secondary or tertiary treatments.

Secondary treatment, for example, removes larger amounts of organic material but has a lower capacity for the elimination of phosphorus and nitrogen.

Tertiary systems, even though they can eliminate organic material as well as phosphorus, require nearly twice as much space as that required by advanced primary treatment systems.

Subtil noted that the advance primary system  is  “easier and cheaper to implement than the secondary and tertiary treatment systems, which can cost three times more.”

According to Subtil, in addition to improvements to the sewage treatment system, another solution that would help reduce the environmental impacts caused by the Santos submarine outfall would be to increase its length so that the sewage would not be released in the middle of the bay as it is today. The release of the wastewater in this location makes the dispersal of contaminants more difficult.

This solution, however, is made difficult by the low declivity of the ocean floor along the São Paulo coastline. The declivity here is less than that along the north coast of Spain, where Subtil did part of his research. “The release depth of submarine outfalls in Spain can be up to 40 meters, while the Santos and Praia Grande outfalls aren’t any deeper than 15 meters,” he said.

“This means that, in the case of the Santos submarine outfall, there isn’t sufficient dilution to reduce the concentration of pollutants released to safe levels before the plume reaches the surface and increases risk of algae growth,” he explained.
 
According to Subtil, some studies performed at outfalls with low discharge levels and that release the effluent at great depths—which is not the case for the Santos outflow—showed that the effects of these outfalls on the environment are negligible. These studies also showed that, because of the negligible environmental effect for some marine outfalls, it is difficult to establish legal restrictions regarding these systems.

Subtil pointed out that the creation of environmental legislation for this purpose would allow the increased use of this type of sewage treatment system. “The existence of specific environmental legislation for this, including the concept of a legal mixing zone, would offer security not only to environmental agencies but also to companies that intend to adopt this system as a sewage treatment solution in coastal zones,” he commented.