Degradation Of Polycyclic Aromatic Hydrocarbon (PAH) in Wastewater by Combination of Solar Photocatalytic and Polyvinylidene Flouride (PVDF) Submerged Membrane Bio Reactor (sMBR)

Agung Sri Hendarsa, Department of Chemical Engineering, University of Indonesia

Background

Polycyclic aromatic hydrocarbons (PAHs), also known as poly-aromatic hydrocarbons or polynuclear aromatic hydrocarbons, are potent atmospheric pollutants that consist of fused aromatic rings and do not contain heteroatoms or carry substituents. Naphthalene is the simplest example of a PAH. PAHs occur in oil, coal, and tar deposits, and are produced as byproducts of fuel burning (whether fossil fuel or biomass). As a pollutant, they are of concern because some compounds have been identified as carcinogenic, mutagenic, and teratogenic. PAHs are also found in water. Studies have shown that high levels of PAHs are found, for example, in industrially polluted rivers.

Apart from highly industrially polluted rivers, the concentrations of individual PAHs in surface and coastal waters are generally =50 ng/litre (WHO, 1997). Concentrations above this level (sometimes into the 10 μg/litre range) indicate contamination by PAHs mainly through industrial point sources and shipyards, atmospheric deposition, and urban runoff. Ships for inland navigation are periodically treated with coal tar to prevent corrosive damage. The leaching/abrasion of this coating is a source of PAHs (Berbee, 1992). In addition, wood preserved with creosote can leach PAHs into the environment, especially into waters where wood is used for bank protection or harbours and in the disposal of creosote-impregnated railway ties (Berbee, 1992; Sandell & Tuominen, 1996).

PAH levels in uncontaminated groundwater are usually in the range of 0–5 ng/litre. Leaching of PAHs from soils into groundwater is negligible, as the compounds tend to adsorb strongly to the soil organic matter (Woidich et al., 1976; Stuermer et al., 1982). Only at heavily contaminated sites do the PAHs reach the groundwater, giving concentrations above 10 μg/litre (Environment Canada, 1994).

The typical concentration range for the sum of the selected PAHs in drinking-water is from about 1 ng/litre to worst cases of 11 μg/litre (see Table 1). Many individual PAHs are at concentrations below the detection limit. As an example, in 1988–1989, the sum of the six Borneff PAHs was below the detection limit of 5 ng/litre in 88% (5287 of 5975) of drinking-water samples from waterworks in Germany; the concentrations were below 40 ng/litre in 10% (588 samples); and concentrations above 200 ng/litre were detected in 0.08% (5 samples) (Dieter, 1994).

The main source of PAH contamination in drinking-water is usually not the raw water sources but the coating of the drinking-water distribution pipes. At least in the past, coal tar was a common coating material for water pipes, used to give effective protection against corrosion. After the passage of drinking-water through those pipes or after repair work, significantly increased PAH levels have been detected in the water (Vu Duc & Huynh, 1981; Basu et al., 1987; Davi et al., 1994); for example, a concentration of 2.7 μg of Borneff PAHs per litre was detected in one sample of such water (State Chemical Analysis Institute, 1995). Although WHO has called for a cessation of this practice (WHO, 1996), many countries still have a large amount of pipes lined with coal tar coating. If BaP is present at elevated concentrations in drinking-water, this is indicative of the presence of particulate matter (e.g. from the deterioration of the coal tar coating).

In Canada, significantly increased levels of PAHs in drinking-water were reported for which the reason is not known (Environment Canada, 1994). Also, the PAH concentrations in spa waters from 10 different spas in the Sudetes region (Poland) are surprisingly high (Babelek &

Ciezkowski, 1989). In most of the PAH-contaminated spas, groundwater, presumably polluted, also contributes to the spa water.

In the majority of drinking-water samples taken in England and Wales, PAHs are not detected above the standard (EEC, 1980; CEC, 1995) for PAHs of 0.2 μg/litre. Only 5% of the reported samples fail to meet the standard. In practically every case where the PAH standard has been exceeded, the only PAH detected to any significant extent is FA. This is indicative of a coal tar pitch lining in good condition where the hard groundwater very slowly dissolves the lining. There are very few cases where other PAHs have been detected in significant 6 concentrations, and these occur mainly where soft corrosive water is derived from surface water sources. This is probably indicative of physical deterioration of the lining, releasing particulate containing PAHs into the water supply (Drinking Water Inspectorate, personal communication, 1997).

Methodology

Wastewater containing polycyclic aromatic hydrocarbons (PAHs) contaminants is proposed to be treated by a coupled system which consists of a photocatalytic pretreatment followed by a biological oxidation process Membrane Bio Reactor.

In chemistry, photocatalysis is the acceleration of a photoreaction in the presence of a catalyst. In catalysed photolysis, light is absorbed by an adsorbed substrate. In photogenerated catalysis, the photocatalytic activity (PCA) depends on the ability of the catalyst to create electron–hole pairs, which generate free radicals (hydroxyl radicals: ·OH) able to undergo secondary reactions. Its comprehension has been made possible ever since the discovery of water electrolysis by means of the titanium dioxide. Commercial application of the process is called advanced oxidation process (AOP). There are several methods of achieving AOP's, that can but do not necessarily involve TiO₂ or even the use of UV light. Generally the defining factor is the production and use of the hydroxyl radical.

When TiO₂ is subjected to radiation exceeding the material's band gap, electron-hole pairs, known as excitons, are generated so that additional electrons enter the conduction band, while holes remain in the valence band. These photo-generated electron-hole pairs facilitate redox reactions through the formation of adsorbed radicals on TiO₂ surfaces. The photocatalytic activity of TiO₂ depends on the relative rates of generation and recombination of electron-hole pairs as well as the levels of adsorbed radical-forming species on TiO₂ surfaces.

The two most commonly used phases of TiO₂ are anatase and rutile. While rutile exhibits a lower band gap (~3.0 eV) in comparison to anatase (~3.2 eV) and can thus be excited by irradiation at longer wavelengths, anatase is generally exhibits superior photocatalytic activity to rutile as a result of a significantly higher surface area and thus higher levels of adsorbed radicals. It is likely that mixed phase anatase-rutile materials exhibit enhanced photocatalytic activity through an improvement in electron-hole separation, as conduction band elections become trapped in the rutile phase.

Membrane bioreactor technology (MBR) is particularly suitable for advanced biological treatment of wastewater. Membrane bioreactor (MBR) is the combination of a membrane process like microfiltration or ultrafiltration with a suspended growth bioreactor, and is now widely used for municipal and industrial wastewater treatment with plant sizes up to 80,000 population equivalent (i.e. 48 MLD).

When used with domestic wastewater, MBR processes could produce effluent of high quality enough to be discharged to coastal, surface or brackish waterways or to be reclaimed for urban irrigation. Other advantages of MBRs over conventional processes include small footprint, easy retrofit and upgrade of old wastewater treatment plants.

It is possible to operate MBR processes at higher mixed liquor suspended solids (MLSS) concentrations compared to conventional settlement separation systems, thus reducing the reactor volume to achieve the same loading rate.

In this research area, there is a lack of research about the integration of the solar photocatalytic oxidation process with biodegradation in polyvinylidene flouride-Membrane Bio Reactor (PVDF MBR). The aim of this work is to demonstrate the viability of the coupled system to treat toxic wastewater containing polycyclic aromatic hydrocarbons (PAHs).