Dissolved organic matter (DOM) wildly exists in aquatic system and affects water quality such as color, taste, odor and produce disinfection by-products (DBP). The most important is that the presence of DOM would promote bacterial regrowth that lead to biofouling on membrane surface and distribution pipes. According to the published, the biofouling potential is represented by the dissolved organic matter (DOM), microorganisms and the inorganic nutrient concentration in feed water. Among nutrient, phosphorus is an important component of bacterial cell. In addition, low phosphate concentrations can restrict biomass growth even at high organic carbon concentrations have been published. Therefore, minimizing DOM and nutrient (phosphate) availability by effective pretreatment is an effective biofouling reduction strategy.

Ferrihydrite (Fh) is a poorly crystalline iron oxide, has a large adsorption capacity for the phosphate, arsenate and organic matter present in natural systems since it presents an extremely high surface area and reactivity. The objective of this study was to investigate the effects of Fh on removal of both DOM and phosphate to control biofouling, and hybrid Fh with membrane system to separate Fh for application on practice.

The surface area and pore size of Fh was analyzed by nitrogen adsorption method. The synthesized Fh exhibited about 300 m2/g and comprised primarily mesopores. The average particle size of Fh was range 5 to 10 μm. The isotherm and kinetic adsorptions of DOM by Fh were tested with surrogate organic matters e.g. serine, glutanic acid, resorcinol and tannic acid as well as Suwannee river NOM (SRNOM) and Suwannee river fulvic acid (SRFA). Sorption increased in the order of tannic acid > glutamic acid > resorcinol ≒ serine. The results show tannic acids have high adsorption capacity, especially tannic acid was removed 81% by Fh at 10 mg Fe/L of Fh. Phenolic -OH groups of the tannin molecules were bound to Fh by hydrogen bonding, the adsorbed surfactant can become a partitioning phase on Fh which is more effective for DOM adsorption. The adsorption of glutamic acid increased at lower pH, FT-IR spectra of treated Fh show the peak of COO- appeared, suggested γ-carboxylate group was deprotonated and reacted with Fh by ligand exchange mechanism. Both hydrophobic and hydrophilic neutral DOM were difficult deprotonated and have low active functional groups on the surface to react with Fh. Fh more effectively remove iron DOM which contain carboxyl or hydroxyl function groups.

Adsorption of SRNOM onto Fh at different pH, Both DOC and UV254 removal by Fh increased as pH decreased from 11 to 6. It was constant as pH further decreased from 6 to 3. Adsorption of SRNOM by Fh was attributable to ligand exchange-surface complexation between carboxylic acids (20% of total C) of SRNOM and metal oxide surface, as well as hydrophobic interaction between aromatic moieties (23% of total C) and the oxide surfaces by FT-IR analysis. EEM spectra show that SRNOM mainly contains fulvic acid and humic acid, it was almost completely removed at pH 6 by Fh, and only very small residual of fulvic acid was observed by EEM at pH 9.

The effects of Fh adsorption were compared with powdered activated carbon (PAC) by Sanshiro Pond, coagulation, ozonation and biological activated carbon (BAC) processes by Arakawa River water. Assimilable organic carbon (AOC) and bacterial regrowth potential (BRP) were firstly used to evaluate bacterial regrowth potential after removal of both DOM and phosphate by different water treatment processes. The surface area of PAC was 956 m2/g with a highly porous when Fh was 300 m2/g. Based on the pseudo second order equation, the adsorption rate of DOM by Fh was faster than that of PAC. Fh could be a better adsorbent for removing NOM within limited hydraulic residence times. Fh shows higher adsorption efficiency for DOM with MW larger than 1000Da. PAC adsorbed more DOM in a range of 800< MW <1000Da. That is, PAC preferentially removed smaller MW organics than Fh. About 80% of phosphate was removed by Fh at the concentration of 50 mg/L as Fe, while PAC removed 15% of phosphate. Fh removed both DOM and phosphate in natural water.

Adsorption kinetics of DOC and phosphate were investigated in Arakawa River water. Adsorption was fast and instantly reached equilibrium in less than 5 min, the k2 value for phosphate adsorption was higher than that for DOC. Phosphate forms a very strong surface complex with hydroxyl groups and has a higher affinity with Fh than DOC. The adsorption of DOM and UV254 decreased as pH was lowered from 11 to 3 in river water and consistent with synthetic water. Fh present negative charge at pH was larger than 8, which electrostatic repulsion with negative charged DOM. Comparison of different water treatments, removal of DOC and UV254 by Fh were higher than those after coagulation-sand filtration, which indicated that Fh effectively removed both DOM and aromatic organic matter. Ozonation was more effective than Fh adsorption in reduction of UV254 and SUVA, which indicated that ozonation decomposed aromatic organic matter and produced more hydrophilic smaller molecules.

The AOC of Arakawa River water were ranged from 100 to 218 μg・acetate-C/L during one year analysis. The value was slightly decreased after coagulation and increased again after ozonation treatment. Coagulation-sedimentation-sand filtration process removed 30% of DOC and 50%-60% of phosphate from raw water, resulting in higher reduction of BRP than AOC. This result showed that higher phosphate removal may effectively decrease BRP. In all the results of Fh treated water, the AOC level was nearly same with coagulated water. The concentration of phosphate was removed by Fh as low as 0.3 mg/L. It can effectively control the bacteria regeneration and provide low BRP value.

For application of Fh, a bench-scale unit was used for microfiltration (MF) and ultrafiltration (UF) to separate Fh and some parts of DOM in the feed water. On SRNOM solution, the flux decline of RC membrane not very different after addition of Fh in the existence of DOM and phosphate. After Fh reacted with DOM and phosphate, the particle size and characteristics of Fh were changed and formed loose cake larer on membrane surface which increased the permeability. During all processes, Fh hybrid membrane system shows the 90% removal efficiency for phosphate when membrane alone almost does not remove phosphate. Different with RC hybrid system, the flux of Fh-PES system reduced 50%. Since, the PES membrane existence of large negative charge which has strong attraction with positive charged Fh. Then, the Fh coated and deposited on the membrane surface, reduce the permeability of PES membrane. MF membrane alone practically did not reject DOM from water sample due to the large pore size. However, more than 90% of UV254 and 70% of DOC were removed after addition of Fh. Therefore, the Fh combined RC and MF membrane system show the high permeability and removal efficiency for DOM and phosphate.

Arakawa River water and Hanoi ground water were used to evaluate the effects of Fh-RC and Fh-MF systems. The flux sharply declined in all processes for filtration of Arakawa River water. Even though, the DOC and UV removal rates by Fh-membrane system also reached to 30% and 50%, respectively. The phosphate removal rate was higher than 60%. In order to determine the membrane fouling mechanisms on these two kinds of water, fouling models were used to distinguish the fouling type. The fouling of RC and MF membrane were pore blocking and pore constriction mechanism, respectively. However, very little fouling took place in filtering Hanoi water. The intramolecular binding of Ca2+ of Mg2+ decreased the DOM molecular volume, and then the DOM size distribution became smaller to pass through the membrane. The DOM rejection and flux decline were lower than filtration of Arakawa River water. After addition of Fh, the flux was very stable and the removal rates of DOC, UV254, and arsenic reached 35%, 56% and 90%, respectively. Therefore, Fh hybrid membrane system exhibited higher removal rate for DOM, phosphate and arsenate in natural water. More importantly, the Fh not reduce membrane flux in combination system. Fh may combined with membrane system for application of raw water which contain DOM, phosphate or arsenate to control biofouling.

楊禹氏は、「微生物再増殖制御を目的としたフェリハイドライトによる溶存有機物とリンの除去」と題する論文において、水道プロセスにおける微生物の再増殖制御のため、鉄系の吸着剤であるフェリハイドライト(Fh)を利用し、溶存有機物とリンの同時除去について検討した。その結果、フェリハイドライトは溶存有機物の除去とリンの除去ともに有効な吸着剤であり、既存の高度浄水処理プロセスに代えて、新しい高度浄水処理を実現可能とする方法の一つであることを示した。具体的には、以下の点のついて研究を行った。

はじめに、実験室において合成したFhを測定したところ、比表面積は300m2/gと大変大きく、メソポアを有し、粒径は5-10μmであった。Fhの吸着特性について検討するため、天然有機物を模擬した4種類の有機物、即ちタンニン酸、グルタミン酸、セリン、レソシノールを用いて、吸着実験を行った。その結果、タンニン酸は最も吸着しやすく、次いで、グルタミン酸、レソシノール、セリンの順であった。Fhの濃度が10mg/L のとき、81%のタンニン酸が吸着除去された。タンニン酸はこれら4種類の有機物の中で最も分子量が大きく、分子量の大きな有機物ほど吸着除去されやすいことが明らかとなった。グルタミン酸の荷電はpH に依存し、Fhによる吸着量もpHに大きく依存していた。即ち、pHが低いほどグルタミン酸の吸着量は多かった。FT-IR分析によると、グルタミン酸の吸着は、カルボキシル基のリガンド交換によるものと考えられた。疎水性及び親水性の中性有機物(レソシノール、セリン)は、荷電していないためにFhによる吸着除去が難しかった。

天然有機物として標準的にもちいられるスワニー川天然有機物(SRNOM)は、DOCとして想定した場合も紫外線吸光度UV254として測定した場合も、pH6まではpHが低いほどFhへの吸着量が多かった。EEM分析によると、SRNOMは主にフミン酸とフルボ酸からなり、たんぱく質などの成分は検出されなかった。SRNOMはpH6でFhに完全に吸着し、pH 9ではわずかにフルボサンの一部が吸着せずに残された。SRNOMのFhへの吸着には、リガンド交換や疎水性相互作用が関与していることが示された。

溶存有機物のFhへの吸着を粉末活性炭(PAC)への吸着と比較したところ、溶存有機物のFhへの吸着は、PACへの吸着よりも早かった。溶存有機物のFhおよびPACへの吸着は、有機物の分子量によって異なり、分子量1,000Da以上の有機物はFhへの吸着量がPACへの吸着量よりも多く、反対に分子量1,000Da以下の有機物は、PACへの吸着量がFhへの吸着量よりも多かった。また、リン酸イオンは、Fh添加量50mg/L で80%が除去されたのに対して、PACは15%除去にとどまった。

荒川河川水を用いて溶存有機物とリン酸の除去実験を行ったところ、リン酸の吸着は溶存有機物の吸着よりも早く、リン酸はFhの表面に強く結びついていることが確認できた。Fhとオゾン処理を比較したところ、Fhは親水性及び疎水性の有機物をともに除去する物の、UV254の除去率は、オゾン処理の方が高かった。荒川河川水の資化性有機炭素(AOC)を測定したところ、100-218μg/L であった。FhによるAOCの除去率は、凝集・沈殿プロセスと同程度であった。また、FhによるAOCの除去率を生物再増殖能(BRP)の除去率と比較したところ、BRPの除去率の方が高かった。これは、Fhによるリンの除去が影響しているものと推察された。

吸着後のFhを効率的に固液分離するため、精密ろ過(MF)と限外ろ過(UF)とFh吸着を組み合わせた実験を行った。溶存有機物としてSRNOMを用いて、FhとMF/UFの組み合わせの実験を行ったところ、Fhが溶存有機物とリン酸を吸着することで、Fhの粒径が変化し、膜表面に透過性の高いケーキ層を形成した。膜単独ではリン酸の除去が不可能であったが、Fhと膜との組み合わせにより、リン酸の90%が除去された。また、RCとPESとの二種類のUF膜とFhとのハイブリッド処理を比較したとこと、膜表面のマイナス荷電が大きいPES膜では、膜表面にプラスに帯電したFhを吸着しやすく、Fhのケーキ層を形成することで、フラックスが大きく低下した。MF膜は膜の孔径が大きいため、単独ではほとんど溶存有機物を除去することができない。しかし、Fhを添加することで、DOCとして90%の有機物を除去し、UV254として70%除去した。

荒川の河川水とハノイの地下水を供給水として、Fhと膜ろ過処理との組み合わせによる処理を行った。その結果、荒川河川水ではDOC除去率30%、UV254除去率50%をしましたが、フラックスの低下が著しかった。しかし、ハノイの地下水を用いた場合、ほとんどフラックスの低下が起こらなかった。これらの違いは、溶存有機物の性質や、それいがいの水の成分によるものと考えられた。特に水中のCaやMgなどのイオンは、有機物の構造に影響を与えるため、膜表面でのケーキ抵抗に影響を及ぼした可能性が示された。

これらの実験結果から、本研究において検討したFhと膜との組み合わせは、有機物除去の新しいプロセスとして、有効性が高いことが示された。また、膜ろ過におけるフラックスの低下には、溶存有機物のほかに、溶存イオンなどが影響することが示唆された。

よって本論文は博士(工学)の学位請求論文として合格と認められる。