Direct filtration of Cryptosporidium surrogates in drinking water treatment-a multiscale approach

• Author / Creator

  Liu, Lu

• Cryptosporidium, a waterborne protozoan pathogen that can cause gastrointestinal illness, is often found in surface waters that are used to supply drinking water. Filtration is a major barrier against Cryptosporidium in drinking water treatment processes. However, interactions between oocysts and filter media are still unclear and no satisfactory surrogates have been identified for quantifying their filtration removal in porous media. In the phase I of this study, polystyrene microsphere with a size, density, and shape similar to Cryptosporidium was modified with glycoprotein or synthesized biomolecules to mimic the surface properties of live Cryptosporidium oocyst. Interaction kinetics between live Cryptosporidium/modified microspheres and filter media were studied at the molecular-scale using a quartz crystal microbalance with dissipation monitoring (QCM-D) and at the laboratory-scale using sand-packed columns. Both QCM-D and column experiments underlined the importance of Cryptosporidium surface charge and hydrophobicity on their fate and transport in porous media. As compared to live Cryptosporidium, glycopolymer and zwitterionic polymer co-modified polystyrene microspheres (later called copolymers-modified microspheres) represent comparable surface properties, adsorption kinetics on filter surfaces, and transport and deposition behaviors in filter columns; hence were selected as appropriate Cryptosporidium surrogates for the lab-scale and pilot-scale filtration investigation. Factors contributing to the solution chemistry were investigated using Cryptosporidium surrogates (copolymers-modified microspheres) established previously in the laboratory-scale filtration columns; they are ionic strength, pH, and DOC (dissolved organic matter) of aqueous solution. Single-collector contact efficiency (η0) and DLVO (Derjaguin-Landau-Verwey-Overbeek) interaction energy were calculated to facilitate the explanation of transport and retention of Cryptosporidium oocysts surrogates in the porous media. The value of single-collector contact efficiency (η0) was determined using Tufenkji and Elimelech model (T-E model). In general, the results of surrogate transport experiments demonstrate an increase in surrogate removal with increasing solution ionic strength or decreasing solution pH. This observed dependence of surrogate attachment with changes in solution salt concentration and pH confirms that the significant role of physicochemical filtration in surrogate removal in the filtration columns when porous media are saturated with water. On the other hand, the natural organic matter (NOM) had negative effect on the retention of surrogates in the packed-bed column, which emphasizes the importance of optimal coagulation to remove NOM for the better filtration performance. This phase II study also validates the using of copolymers-modified microspheres as representative surrogates for Cryptosporidium oocysts in the condition that relevant to drinking water. In the phase III pilot-scale study, the removal of Cryptosporidium oocyst surrogates was determined in a pilot-scale granular media filtration system operated in direct filtration mode at cold water temperature condition. The surface characteristics of these modified microspheres, including surface charge and hydrophobicity, resembled those of viable oocysts in drinking water conditions. Pilot-scale direct filtration challenge experiments were conducted to determine the impact of chemical pretreatments and filter design on the removal of Cryptosporidium surrogates dosed into the influent water. The operational parameters examined in the direct filtration mode included coagulant type (alum versus PACl), filter aid polymer type (polyamine Magnafloc® LT-7981 versus polyDADMAC Magnafloc® LT-7995) and dose (0.5 versus and 2.0 mg/L), and filter configuration (i.e. regular versus and deep bed filters). The results indicated a higher Cryptosporidium surrogate removal was associated with higher polymer dose (2 mg/L), polyDADMAC polymer and the deep bed filter configuration. The chemical pretreatment conditions played important roles in the transport and removal of Cryptosporidium surrogates in the filter bed. Specifically, optimized doses of coagulants alum (0.454 mg/L as Al) determined from previous full-scale and pilot-scale investigation and filter aid polymer polyDADMAC (Magnafloc® LT-7995) together with a deep bed configuration achieved the highest log removal of Cryptosporidium surrogates. This study improves our understanding on how surface characteristics impact Cryptosporidium transport behaviors in porous media and contributes to our capacity to evaluate the fate of Cryptosporidium in natural water purification systems and engineered aquatic environments. It demonstrates the effect of solution chemistry on the transport and deposition of surrogates. It also emphasizes the importance of optimizing chemical pretreatment and filter configuration for removing oocysts in cold-water conditions in granular media direct filtration.

• Subjects / Keywords

  • Cryptosporidium
  • Pilot direct filtration
  • Packed-bed column
  • Surrogates

• Graduation date

  Spring 2019

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/r3-w5dz-0997

• License

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