ABSTRACT
Media filtration is the most common used pre-treatment process in full-scale SWRO desalination plants. The removal of particulate fouling in seawater media filtration has been extensively addressed. However, the removal of organic/biological fouling potential in seawater media filtration is less extensively covered in the literature, mainly due to a lack of standard methods for these types of fouling. Recently, several biological/organic fouling indicators have been developed such as assimilable organic carbon and bacterial growth potential (BGP). This work aims (i) to investigate the removal of particulate, organic and biological fouling potential during two stage dual media filtration (DMF) pre-treatment in a full-scale SWRO desalination plant and (ii) to compare the removal of fouling potential in two stage DMF (DMF pre-treatment) with the removal in two stage DMF installed after dissolved air floatation (DAF-DMF pre-treatment). For this purpose, the silt density index (SDI), modified fouling index (MFI), BGP, organic fractions and microbial ATP were monitored through the pre-treatment processes of two full-scale SWRO desalination plants.
140Particulate fouling potential was well controlled through the two stages of DMF with significant removal of SDI15 (80 %), MFI-0.45 (94 %), and microbial ATP (> 95 %). However, lower removal of biological and organic fouling potential (24 – 41 %) was observed compared to the observed removal of particulate fouling, which was attributed to low biological activity in the DMFs due to frequent chlorination (weekly), which probably destroyed the biofilm layer on the filter media.
Comparing overall removal of DAF-DMF pre-treatment to DMF pre-treatment showed that DAF significantly improved the removal of biological/organic fouling potential, in which the removal of BGP and biopolymer concentration increased by 40 % and 15 %, respectively. However, the removal of biological/organic fouling potential in the two stage DMFs in both plants were in the same range in terms of BGP (90 – 95 µg-C/L), CDOC (320 – 460 µg-C/L), and biopolymers (100 – 160 µg-C/L). The low removal of biological/organic fouling potential in both stages of DMF (in DAF-DMF) was attributed to the periodic use of SWRO brine for backwashing, which probably resulted in osmotic shock of the biofilm on the filter media, and reduced removal of biodegradable organic matter.
Overall, relatively low removal of biological/organic fouling potential was observed in DMFs in two full-scale SWRO plants due to operational practices such as chlorination of the intake and brine backwashing. Monitoring ATP and BGP along the pre-treatment processes, particularly in DMF, would be beneficial to enhance biological degradation and lower the BGP of SWRO feed water.
