Discharge of Polyphosphonate-Based Antiscalants via Desalination Brine: Impact on Seabed Nutrient Flux and Microbial Activity

Abstract

Desalination brine is a hypersaline byproduct that contains various operational chemicals such as polyphosphonate-based antiscalants. Brine often sinks and flows over the seabed by density currents; therefore, it may affect sediment-water nutrient fluxes and thus microbial activity. We quantified these parameters in brine plumes around two large-scale desalination facilities located in the P-limited Southeastern Mediterranean Sea. The benthic nutrient fluxes and microbial activity were determined using ex-situ core benthocosms, to which we added brine from the dispersion area in excess salinities of ∼3% and 5% above natural levels. A higher influx of dissolved organic phosphorus (∼6-fold) and an efflux of dissolved organic carbon (∼1.7-fold) were measured in the brine-amended cores relative to the controls. This was accompanied by increased oxygen consumption (15%) and increased microbial activity (∼1.5–6.5-fold). Field observations support the results from experimental manipulations, yielding ∼4.5-fold higher microbial activity rates around the brine plume compared to uninfluenced locations. Our results imply that desalination brine can alter sedimentary processes affecting benthic nutrients inventories. Moreover, we show that brine acts as a vector of anthropogenic P, stimulating microbial activity in the sediment–water interface.

Impacts of Desalination Brine Discharge on Benthic Ecosystems

“…It should be noted that the density differences between the diluted brine and the ambient seawater required to support the formation and propagation of gravimetric density currents are quite small . The above-mentioned study argued that the far-field propagation (<13 km 2 ) of desalination brines will form a higher density layer above the seabed that can impede the vertical mass transfer of nutrients from the sea bottom to the overlying water, , similarly to reports from the mixing zone …”

“…Polyphosphonate-based antiscalants are not considered to be a form of phosphorus used by organisms. However, a recent study that sampled sediment and porewater from the discharge area of a large-scale desalination facility indicated that these antiscalants can be degraded, leading to the release (∼40x higher than the intake water) of dissolved organic phosphate, DOP . This additional load of organic phosphate has been shown to enhance bacterial metabolism at the diffusive boundary layer .…”

“…93 The above-mentioned study argued that the far-field propagation (<13 km 2 ) of desalination brines will form a higher density layer above the seabed that can impede the vertical mass transfer of nutrients from the sea bottom to the overlying water, 22,34 similarly to reports from the mixing zone. 54 Previous studies have shown that the recycling of organic matter and remineralization of nutrients in coastal-shelf sediments are vital processes that support up to 40% of primary production in coastal waters. 94,95 Considering the observed and modeled far-field dynamic effects of seawater desalination brine discharge discussed above, it is likely that the propagating density currents may transport recycled nutrients that are emitted from the sea bottom downslope and away from the shore.…”

Efficient Treatment of Phosphonate-Laden Wastewater by CaO2/UV: Partial Oxidation, Complexation, and Coprecipitation

“…17,18 Hence, phosphonates could be a potential source of bioavailable P. Indeed, Sisma-Ventura et al found that the discharge of polyphosphonate-containing desalination brine to coast areas stimulated microbial activity in the seabed. 19 Hasanin et al also reported that phosphonates significantly promote bacterial growth under phosphorus-limiting conditions, thus leading to membrane fouling when used as antiscalants. 20 Moreover, phosphonate compounds also exhibit toxicity to aquatic organisms.…”

“…However, it is important to note that phosphonates cannot be mineralized in WWTPs . Along with their widespread application yet poor degradation in WWTPs, phosphonates are frequently detected in sewage sludge, effluents of WWTPs, lakes, rivers, and oceans. − In water bodies, phosphonates can be slowly converted to phosphate through enzymatic or photochemical reactions. , Hence, phosphonates could be a potential source of bioavailable P. Indeed, Sisma-Ventura et al found that the discharge of polyphosphonate-containing desalination brine to coast areas stimulated microbial activity in the seabed . Hasanin et al also reported that phosphonates significantly promote bacterial growth under phosphorus-limiting conditions, thus leading to membrane fouling when used as antiscalants .…”