Seabed gallery intakes: Investigation of the water pretreatment effectiveness of the active layer using a long-term column experiment

Abstract

Seabed gallery intake systems used for seawater reverse osmosis facilities employ the same principle of water treatment as slow sand filtration in freshwater systems. An investigation concerning the effectiveness of the active layer (top layer) in improving raw water quality was conducted by using a long-term bench-scale columns experiment. Two different media types, silica and carbonate sand, were tested in 1 m columns to evaluate the effectiveness of media type in terms of algae, bacteria, Natural Organic Matter (NOM) and Transparent Exopolymer Particles (TEP) removal over a period of 620 days. Nearly all algae in the silica sand column, 87% (σ = 0.04) of the bacteria, 59% (σ = 0.11) of the biopolymer fraction of NOM, 59% (σ = 0.16) of particulate and 32% (σ = 0.25) of colloidal TEP were removed during the last 330 days of the experiment. Total removal was observed in the carbonate sand column for algal concentration, while the bacterial removal was lower at 74% (σ = 0.08). Removal of biopolymers, particulate and colloidal TEP were higher in the carbonate column during the last 330 days with 72% (σ = 0.15), 66% (σ = 0.08) and 36% (σ = 0.12) removed for these organics respectively. Removal of these key organics through the 1 m thick column, representing the active layer, will likely reduce the rate of biofouling, reduce chemical usage and minimize operating cost in SWRO systems. The data show that the media will require several months at the beginning of operation to reach equilibrium so that high organic removal rates can be achieved. No development of a "schmutzdecke" layer occurred. The experimental results suggest that unlike freshwater slow sand filtration wherein most water treatment occurs in the upper 10 cm, in seawater systems treatment occurs throughout the full active layer depth of 1 m. The results of this study will help in designing and operating seabed gallery intake systems in varied geological conditions.

Green method of stemming the tide of invasive marine and freshwater organisms by natural filtration of shipping ballast water

“…In recent years, considerable research has been conducted on the use of subsurface intakes to remove organic macroscopic debris, algae, and bacteria. These natural filtration-based intakes also remove significant parts of the smaller-sized organic matter, including transparent exopolymer particles and the biopolymer fraction of natural organic matter (Missimer 2009;Missimer et al 2013;Rachman et al 2014;Dehwah et al 2015;Dehwah and Missimer 2017). This technology uses either shallow wells located adjacent to the shoreline or some type of gallery intake and has been used successfully to remove organic materials from the raw seawater that allows seawater reverse osmosis desalination plants to operate more economically with addition of chemicals (e.g., chlorine) and less cleaning of the membranes (Missimer et al 2015).…”

“…Measurements of the organic matter transport in well intake systems have been conducted at several locations around the world (Missimer 2009;Missimer et al 2013;Rachman et al 2014;Dehwah et al 2015;Dehwah and Missimer 2017). Select data on the removal of combined algae and cyanobacteria and marine bacteria from these investigations are summarized in Table 1, and additional data are provided in the reference papers.…”

“…These installed capacities can be met on the low side by a well system based on existing systems used by SWRO desalination intakes (Sur, Oman well system, installed capacity of 160,000 m 3 /d) and on the high side by a seabed gallery system which has essentially unlimited capacity (Dehwah and Missimer 2017).…”

“…In these areas, a seabed gallery could be developed to obtain seawater free of algae and a high percentage of marine bacteria. Since the flow pathway through a seabed gallery system is generally less than a well system, the removal of bacteria is not as effective in terms of overall percentage (Dehwah and Missimer 2017). Experimental work conducted by Dehwah and Missimer (2017) verified that 100% of the algae are removed in the filter and up to 84% of the marine bacteria are removed.…”

Impacts of natural pore-water and offshore aquifer chemistry on the operation and economics of some subsurface intakes types for SWRO plants

“…While it is unlikely that membrane biofouling can ever be eliminated, the rate of biofouling can be controlled by using extensive pretreatment processes [4,5] designed to remove inorganic and organic particulates including algae, fineorganic solids, and bacteria; and semi-dissolved or dissolved organic matter, such as particulate and colloidal transparent exopolymer particles (TEP) and other sticky polysaccharides that occur mostly within the biopolymer fraction of natural organic matter (NOM) [6]. Many of the precursors to biofouling are also known to be effectively removed by using subsurface intakes, such as conventional vertical wells [7][8][9][10][11] and seabed galleries [12,13].…”