A three step approach to the degradation of natural organic matter (NOM) from water sources

Abstract

Natural Organic Matter (NOM) is a complex blend of organic compounds that forms naturally via the degradation of plant and animal materials into water sources. NOM in water negatively affects water quality (by causing odor, taste and color problems), negatively affects consumers health (through the disinfection by-products formation which are carcinogenic), increases costs in plant operations (by causing membrane fouling and high coagulant dosage demand) and negatively impacts the ecosystem (through bacterial regrowth and deterioration of surface water sources). In addition, the complexity and the size of NOM hinders most of the available water treatment processes that are in place in South Africa and worldwide from effectively and efficiently removing NOM from water sources. The varying character of NOM in various sources makes it difficult to remove NOM as its composition is not uniform; it depends on the climate, topology, industrial and agricultural activities around a particular area. Hence there is a need for methods that can effectively characterize and degrade NOM (such as photodegradation using TiO2) into smaller pieces for easy removal during water treatment processes. The characterization of NOM in water was done by collecting samples from different water treatment plants located in various South African geographic locations. The purpose was to get a better understanding regarding the type and the composition of NOM occurring in water. The treatment plants of interest were Magalies Water (MP1, MP2 and MP3); Rietvlei Water (RV); Umgeni Water (HL, UM, MT and AM); Lepelle Water (LE, LO and LF); Midvaal Water (MV); Veolia Water (VP and VH) and Plettenberg Bay Water (P). The sampling was done during the period of September 2015 to September 2016 in order to account for seasonal variations. Samples were collected after each treatment stage for each treatment plant in order to study the treatability of NOM by various treatment processes. Conventional characterization methods such as dissolved organic carbon (DOC), ultra-violet at 254 nm (UV254) and specific UV-absorbance. Natural Organic Matter (NOM) is a complex blend of organic compounds that forms naturally via the degradation of plant and animal materials into water sources. NOM in water negatively affects water quality (by causing odor, taste and color problems), negatively affects consumers health (through the disinfection by-products formation which are carcinogenic), increases costs in plant operations (by causing membrane fouling and high coagulant dosage demand) and negatively impacts the ecosystem (through bacterial regrowth and deterioration. Furthermore, N, Pd co-doped TiO2 (NPT) and MWCNTs/N, Pd co-doped TiO2 (CT) were successfully synthesized via sol-gel method and characterized using FTIR (to confirm for the available functional groups), UV-Vis (to study the effect of doping TiO2 with N and Pd and the effect of the presence of MWCNTs on the absorption edge of TiO2), XRD (to verify the presence of the crystalline phases), Raman (to determine the nature of TiO2 and to verify the presence of MWCNTs), SEM (for morphology), EDS (for elemental composition) and TGA (for thermal stability and to evaluate the amount of MWCNTs present on the nanocomposite). NPT and CT were then tested for their photodegradation efficiency on various NOM containing samples collected from selected treatment plants.Conventional NOM characterization methods include both the on-site characterization (pH, turbidity and conductivity); and bulk characterization (DOC, UV254 and SUVA). The pH was used to determine the alkalinity or the acidity of the water; and it was found to be in a range of 2.50-9.13 with Midvaal (MV) raw water being the most alkaline and Preekstoel (VP) being the most acidic water. The turbidity (a measure of the amount of all the clay particles and colloids in water) of all the water samples at their final stages of the treatment process was found to be in the range of 0.00-3.00 NTU, with the Flag Boshielo Water (LF) having the highest turbidity value and the Magalies Water (MP1) having the lowest turbidity. Lastly, the water conductivity was found to be in the range of 135.3–781.3 mS/cm, with the Olifantspoort plant (LO) having the highest conductivity and Plettenberg Bay plant (P) plant having the lowest conductivity. Bulk characterization results showed that the VP raw water had the highest SUVA value (i.e. 7.24 ℓ·mg-1 m) thus high content of high molecular weight and hydrophobic NOM compared to other raw water sources. Regardless of the observed high SUVA in VP raw water; the P plant showed the highest DOC removal efficiency of 90.03% and Hazelmere (HL) plant showed the highest UV254 removal of 88.07%. DOC and UV254 were also used to study the effect of seasonal variations on NOM quantity, quality and treatability. It was shown that the DOC and UV254 was high in autumn (R2) compared to other seasons due to the aromatic nature of the soluble compounds found in leaves, which end up deposited into water sources during the autumn season. Advanced NOM characterization technique, FEEM, gave more and deeper understanding about the composition of NOM in water. FEEM showed that all the raw water samples contain, amongst others, the aromatic protein fraction. NOM fractions (humic and fulvic) were also observed albeit in different quantities in raw waters of VP, HL and P treatment plants. FEEM also proved that the observed high UV254 removal efficiency for VP, HL and P treatment plants was because of the presence of high content of humic substances in the raw waters of these treatment plants. FEEM was also used to link the treatability of NOM to various treatment processes (i.e coagulation and filtration) of P treatment plant. Water after the coagulation showed little traces of humic and fluvic components compared to the raw water samples. Whereas, water after filtration showed very little or no traces of humic fractions. The N, Pd co-doped TiO2 (0.0-1.0%) was evaluated for its photodegradation efficiency towards NOM containing water samples under visible-light irradiation. The highest photodegradation of 58.8% was achieved with NPT (0.5% Pd) on MV raw water samples. The results were in close approximation to those of conventional processes applied at MV treatment plant (60.0%). NPT (0.5% Pd) was also used to conduct the treatability studies with NOM containing samples obtained from various raw water samples. The results showed different UV254 (aromatic content of NOM) removal efficiencies thus proving the varying character of NOM from various water sources. On the other hand, MWCNTs/N, Pd co-doped TiO2 (CT) (0.5 - 5.0%) nanocomposites were evaluated for their photocatalytic efficiency towards P raw water samples. It was observed that the highest photocatalytic activity was with 1.0% MWCNTs. About 91.2% (UV254) reduction was achieved with CT (1.0% MWCNTs), which is much higher compared to 68.2% achieved with NPT (1.0% Pd). The observed enhanced UV254 reduction is attributable to the large surface area of TiO2 which allows bigger amounts of NOM to be adsorbed onto the surface of the TiO2. Adsorption of high amounts of NOM on the surface of the TiO2 permits the photogenerated radicals to have enough time to interact with NOM.