Behavioral Analysis of Suspended Particles in the Mesh Tube Media Filter and the Applicability of the Pretreatment for the SWRO Desalination Plant

Abstract

In 2010, a new water treatment process using a roughing filter medium composed of low-density polyethylene (LDPE) was developed to remove suspended solids and dissolved phosphorus during wastewater treatment. This process, known as mesh tube media filtration (MTF), is expected to be used in the primary pretreatment of seawater desalination, where there are variations in the amount of suspended solids and organic matter in the raw water. For stable operation of a seawater desalination plant, a pretreatment process that efficiently removes suspended particles in seawater is required, and for stable water supply, this pretreatment process must consume less or similar energy than other conventional pretreatment processes. The fate of SPs injected into a mesh tube media column is divided into discharge, deposition, and precipitation. After the kaolin and SiO2 particle filtration experiments were completed, the mass balance was analyzed by measuring the concentration of SPs in the treated water, the mesh tube filter media, and the media column tank. However, the Tufenkji model is challenging to use for predicting the precipitation of SPs, because the Tufenkji model is designed using a sand media filter in which it is difficult for SPs to precipitate. Mesh tube filter media are much coarser than sand media, so the transport of SPs passing through coarse media is affected by precipitation, and this tendency is more prominent when the fluid flow velocity is slow. By the three main mechanisms of single collector contact presented by Tufenkji and Elimelech, the gravitational phenomena (ηG) are strongly influenced by the particle size. Therefore, it is necessary to analyze the particle size under each condition. The particle size of the suspended solids in the influent and the treated water under each condition was measured. In the chapter 4, we present an improved MTF design, a porous filter bed (PFB), which exhibits superior SP removal performance to conventional MTF media. We then compare the applicability of MTF and PBF to both the primary pretreatment process for seawater desalination and the water reuse process. In bench-scale SP removal experiments, PFB shows removal rates of 46.7%, 68.0%, 67.6%, and 68.4% at hydraulic retention times of 15, 20, 30, and 60 min, respectively, which are better than those of MTF. The specific energy consumption of batch dissolved air flotation was known to range from 0.035 to 0.047 kWh/m3 , whereas the SEC calculated for pilot-scale MTF and PFB is 0.027 kWh/m3 and minimum energy for influent supply, respectively. This suggests that PFB can compete with DAF as a primary pretreatment process. MTF predominantly removes SP by sedimentation, whereas SP removal in PFB typically occurs via deposition of SP on the mesh tube media. Laboratory-scale experiments have limited influent capacity, making it difficult to increase the operating time. Therefore, field experiments were conducted at Yeosu wastewater treatment plant (Yeosu-si, Jeollanam-do, Republic of Korea) to analyze the SP removal characteristics and organic pollutants during more than 72 h of MTF operation. MTF maintained an SP removal rate of approximately 47% for approximately 40 h of operation before it rapidly decreased. Backwashing (clean-in-place) using air scouring was performed 72 h after the start of operation, which showed stable performance throughout operation. Low-molecular-weight (LMW) acids were hardly detected in the secondary treated wastewater, and biopolymers were detected at low concentrations (55 μg/L, 6.2%) in Yeosu seawater. LMW acids have a particle size of 350 Dalton (Da) or less and the lowest percentage of LC-OCD fractions in natural seawater. Due to the very small particle size of LMW acids, the removal rate was approximately 10% in the dual media filtration (DMF) process, approximately 5% in the ultra-filtration (UF) process, and 55% in the reverse osmosis (RO) process. Biopolymers have a particle size of 20,000 Da or more and are the largest particulate substances in the LC- OCD fraction, including polysaccharides and proteins. Polysaccharides and proteins contained in surface water are known to contribute to UF membrane fouling, as well as irreversible fouling of coarse membranes with a pore size of 750 kDa. Humic acids have a particle size of less than 1,000 Da and are hard to remove during pretreatment processes such as DMF due to their small molecular weight. In chapter 5 & 6, the water quality of the influent and treated water of each stage was analyzed, and fouling of the mesh tube media of MTF bed and the surface of the FO membranes of the PAFO process after the 76 days operation of the plant as analyzed to examine the potential of membrane fouling occurring in the PAFO-RO plant. This plant consisted of a wastewater lane and a seawater lane, two fouling aspects which is fouling by wastewater and seawater in the MTF bed and PAFO process were compared, and the correlations between microbial adenosine triphosphate (ATP) which is an indicator of microbial activity and modified fouling index were also determined. The purpose of this study is to provide strategies for mitigating membrane fouling of PAFO process. In the SEM photographic images of the FO membranes on the surface contacting with seawater, noticeable fouling was not found. Membrane perforation by sample drying, salt crystals, and a few marine microalgae such as diatoms on the membranes. However, very large amounts of bacterial cells were found on the side contacting the wastewater. Because of the hydraulic pressure at the level of 2 bar, bacterial cells present in the wastewater were deposited on the surface of the FO membrane, and the cells on the surface appeared to proliferate and form biofilms.