Slow sand filtration for biofouling reduction in seawater desalination by reverse osmosis

• Published in: Water research (Oxford) Vol. 155; pp. 474 - 486
• Main Authors: Freitas de Oliveira, Fernando, Schneider, René Peter
• Format: Journal Article
• Language: English
• Published: England Elsevier Ltd 15.05.2019
• Subjects: Biofouling control; Microbial growth potential; Reverse osmosis; Seawater; Slow sand filtration; Slow sand filtration; Reverse osmosis; Biofouling control; Seawater; Microbial growth potential
• ISSN: 0043-1354; 1879-2448
• DOI: 10.1016/j.watres.2019.02.033

Control of the organic substrate pool that determines the microbial growth potential (MGP) of feedwater in seawater reverse osmosis (SWRO) is a challenge unresolved in conventional or advanced membrane pretreatment. Slow sand filtration (SSF) combines filtration with biodegradation, but its capability of reducing MGP, proteins and carbohydrates on seawater feeds is not known. Two SSF, one constructed with new media (newSSF) and one from a previous filtration run (oldSSF), reduced MGP as measured in a growth assay with the marine organism Pseudoalteromonas songiae by one order of magnitude after maturation periods of 76 and 61 days, respectively. The reduction of the amount of biopolymers deposited on the surfaces of SWRO membranes in laminar fluid flow cells was significant with filtrates from biologically non-acclimated SSF (proteins: 60% (oldSSF) and −66% (new SSF), carbohydrates: 75% (oldSSF) and −70% (newSSF)) and an even greater reduction was observed after filter maturation (proteins: 81% (oldSSF) and −76% (new SSF), carbohydrates: 88% (oldSSF) and −88% (newSSF). Turbidity was less than 0.3 nephelometric turbidity units (NTU) and silt density index (SDI) < 4 immediately after startup and during the 181 days operating period regardless of the oscillations of the raw sea water quality. Filtration and biological activity were restricted to the top 30 cm of the media column, with no significant further contribution of the deeper media layers to filtrate quality. [Display omitted] •Slow sand filtration of seawater produced filtrate with SDI<4 and turbidity <0.3NTU.•Microbial growth potential one order of magnitude lower in filtrate after maturation.•Filtration alone reduced proteins + carbohydrates on RO membranes by 60–70%.•Further reductions of proteins and carbohydrates after biological filter maturation.•Improvement of filtrate quality restricted to the first 30 cm of the sand column.