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Biology

Making potable water safe for the seafloor

Del-Pilar-Ruso, Y., E. Martinez-Garcia, F. Gimenez-Casalduero, A. Loya-Fernandez, L.M. Ferrero-Vicente, C. Marco-Mendez, J. A. de-la-Ossa-Carrertero, J.L. Sanchez-Lizaso, (2015) Benthic community recovery from brine impact after the implementation of mitigation measures, Water Research (70) pp. 325-336 http://dx.doi.org/10.1016/j.watres.2014.11.036

Background

Basic steps in sea water reverse osmosis processes

Figure 1: Basic steps in sea water reverse osmosis processes

Desalination through seawater reverse osmosis (SWRO) is an effective way to make potable water. The basic process of reverse osmosis desalination involves three steps (figure 1): 1) a pretreatment to remove particulates that could interfere with the reverse osmosis process, 2) a reverse osmosis processes which separates dissolved constituents (including salts) from the water, and 3) a treatment to make the ‘clean’ water meet potable standards. After the reverse osmosis step, brine concentrated in the dissolved constituents is discharged back into the ocean. The density of the discharge is greater than the surrounding seawater so it settles to the seabed. The introduction of high salinity brine may impact the benthic community. It is important to understand what the impacts are and how to control them so that the community is minimally affected by the discharge.

Diffuser in laboratory, dyed blue water represents dense brine.

Figure 2: Diffuser in laboratory, dyed blue water represents dense brine.

One solution to mitigate the disruption of brine on benthic communities near discharge sites is to install a diffuser. Diffusers enhance mixing by forcing the dense discharge up; the dense brine is diluted by ambient seawater (figure 2).

In 2008 the global production potential of SWRO desalinization processes was 24.5 million cubic meters per day, 17% of which occurred in the Mediterranean Sea. Two of the desalination plants in the Mediterranean are in southeast Spain in San Pedro; each with a production capacity of 65,000 cubic meters per day. The discharge from these two plants has a salinity of around 70, about double the salinity of ambient seawater. The brine is pipelined 5 kilometers off shore and discharged into 33 meters of water. In May of 2010 a diffuser was installed at the end of the pipeline to enhance mixing. The study reviewed here was conducted to determine the effectiveness of mitigation efforts at the pipeline discharge point.

 Methods

Polychaetes

Figure 3: Polychaetes

Station location

Figure 4: station location

Researchers used polychaetes (figure 3) as a bioindicator of the community response to salinity impacts from a SWRO desalination plant in the Mediterranean Sea.   They monitored salinity, polychaete abundance, family richness (the number of families found in a sample), and diversity at 12 stations between 29-38 meters depth (figure 4) in autumn of 2005-2012.  The first year of the study, 2005, was conducted before there was discharge. Between 2006 and 2010 there was discharge without a diffuser. In May of 2010 a diffuser at a 60-degree angle to the horizontal was installed; observations continued through 2012.

Polychaetes were chosen as the bioindicator because they are known to adapt to a range of environmental properties. Six polychaete families were focused on in the study: Magelonidae, Paraonidae, Capitellidae, Syllidae, Cirratulidae, Sabellidae.

The extent of the brine plume was monitored by a conductivity temperature depth sensor (CTD).  A CTD sensor was also used to measured bottom salinity at the stations. Polychaete communities were sampled by a grab sampler in replicates of four: three for biological analysis and one for environmental analysis.

Various statistical analyses were preformed to determine the spatial changes in the community over the study period.

Results

Salinity:
Salinity increased noticeably at the three stations (B1, B2, B3) within 250 meters of the discharge location (figure 5). None of the sites further than 2 kilometers from the discharge area were impacted, suggesting that the initial brine impact on the benthic community, even without the diffuser, is constrained in area. After the diffuser was installed there was clear dilution of the plume(figure 6).

OCT salinty vs time

Figure 5: Salinity at each station over time

Oct salinity pre and post

Figure 6: Change in brine plume before (left) and after (right) diffuser installation

Total Abundance

OCT abundance at station vs time

Figure 7: Abundance change over time at each station

Eleven on the twelve stations shared similar patterns in polychaete abundance: an initial high before the discharge in 2005, then a decrease after the discharge began followed by an abundance recovery that peaked in 2008. In 2010, after the diffuser was installed the abundance was low but rebounded through the conclusion of the study in 2012.   The only site that saw a decline in the abundance throughout the discharge period was B2, the one in closest proximity to the discharge; after 2010 the abundance began to recover (figure 7).

Diversity and Richness:

In general, with the exception of station B4, the species richness and diversity decreased with the start of discharge and then began to rebound after the installation of the diffuser (figure 8)

OCT richness at stations vs. time

OCT diversity at station vs. time Figure 8: Richness (top) and diversity (bottom)  change over time at each station

Information on how different families recover from ecological disturbances is limited; further research on family specific response and recovery may be required to understand results for the specific families.

Discussion

The study found that mitigation of the adverse affects of SWRO is simple, and the results come quickly.   The recovery is considered rapid, occurring in a period of months, relative to the recovery from other anthropogenic processes, like fish farming, which can take greater than 5 years to recover.   This may be related to the impacts each process has. SWRO is simple because the discharge introduces high saline low nutrient water, where as more complicated process may introduce additional pollutants into the sediment.

Importance

This study is important because it is related to satisfying anthropogenic needs while minimizing the impact on the environment. As the population continues to grow and resources change it is vital that we put our best effort into understanding how our engineering will impact the environment. Once we have that understanding, it is critical that we apply it so that unnatural impacts are minimized.

 

 

 

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