UV-advanced oxidation process for taste and odor removal – comparing low pressure and medium pressure UV for a full-scale installation in Korea

The described UV-advanced oxidation process (UV AOP) project is part of an infrastructure project in South Korea to support high tech industry with high quality water. The exisiting treatment train at the WTP consists of the following steps:

• (1) flocculation & coagulation;

• (2) sedimentation;

• (3) sand filtration;

• (4) chlorination.

The design capacity is 101,000 m³/d (27 MGD) which should be extended to 129,000 m³/d (34 MGD) in 2017. Due to the seasonal occurence of 2-methylisoborneol (2-MIB) which is a taste and odor influencing compound produced by blue algae, the need for an additional treatment step was identified. As UV AOPs are well known to be an economical and reliable technology for the removal of micro-pollutants like 2-MIB, as well as assuring disinfection of the water, this technology was pre-selected by K-Water and tested in pilot trials.

During this pilot phase two different UV AOP technologies were compared by conducting a side by side test of a six lamp low pressure reactor and a four lamp medium pressure reactor using flow rates of 12–171 m³/h.

To determine the necessary electrical energy demand (EED) for each UV AOP to achieve a 0.5 LOG removal, 2-MIB was spiked to the water at a level of up to 100 ng/L. The dose of hydrogen peroxide was varied from 5 to 15 ppm to determine the most economic setting. The results of the pilot trials were used to size and design the full-scale treatment system capable of treating up to 4,419 m³/h of water. During the taste & Odor season the system has to assure a 0.5 Log reduction of 2-MIB and out of the season a 3.0 LOG removal of Crypto has to be guaranteed. To determine which UV technology is the best under economic aspects, captital and operational expenditure (CAPEX & OPEX) evaluations were carried out.

Two different UV technologies were tested which were supplied by two different suppliers. One supplier delivered a UV low pressure reactor whereas the other supplied a medium pressure reactor. The low pressure reactor was equipped with six low pressure lamps in parallel to the flow each one with a rated power of 0.36 kW incl. the ballast (Figure 1).

The medium pressure system was equipped with four lamps perpendicular to the flow each one with a rated power of 3 kW incl. ballast (Figure 2). Both reactors were installed at the WTP and fed with different flow rates of water from the sedimentation stage. The water quality is characterized by the following parameters (Table 1).

The water was artificially enriched with 2-MIB levels of up to 100 μg/L to evaluate the degradation rates achievable by the different UV systems. Hydrogen peroxide was dosed upstream at levels of 5–10 ppm. This 2-MIB was analyzed by Gaschromatography-Mass Spectra (GC-MS) equipped with a purge and trap device. In order to detect low levels (ppt) of 2-MIB, solid phase microextraction method was applied for each sample. The stock solution concentration of hydrogen peroxide (35%) was checked by using a spectroscopic method (UV spectrophotometer). The molar extinction coefficient (19 at 254 nm) was used for concentration calculation. The hydrogen peroxide in the influent was measured by 2,9-dimethyl-1,10-phenanthroline method.

Goal of the tests was to demonstrate at least a 0.5 LOG (70%) removal of 2-MIB. Therefore different flowrates and peroxide levels were evaluated to identify the setting capable of reaching the treatment goal. The values given are based on the EED. This value expresses the energy needed to treat 1 m³ of water (Table 2).

Table 3 shows an excerpt of the pilot tests carried out at the Siheung WTP. Three different hydrogen peroxide dosages were applied to each settled flow rate. The results of these tests are visualized in Figure 3.

At a hydrogen peroxide dose of 5 mg/L the low pressure UV systems needs to apply about 60 W/m³ of energy to reach the target reduction of 0.5 LOG. The medium pressure system needs up to 180 W/m³ to achieve this goal safely (Figure 4).

With a peroxide dose of 10 mg/L all applied EEDs of the low pressure system are resulting in a 2-MIB reduction of at least 0.8 LOG. At a maximum energy input of 180 W/m³ a 0.95 Log reduction is achieved. The medium pressure system achieves the treatment goal with an energy input of 144 W/m³ and could reduce up to 0.85 LOG of 2-MIB with a corresponding EED of 280 W/m³ (Figure 5).

The addition of up to 15 mg/L of hydrogen peroxide does not result in a significant better Log reductioin compared to 10 mg/L. This could be explained by the possibility of excess peroxide quenching hydroxyl radicals (Rosenfeld 2011).

The pilot tests showed that comparing the necessary energy input a low pressure UV AOP is significantly more efficient to remove 2-MIB than a medium pressure system. But the taste and odor event only occurs for about 2–3 months a year, whereas for the rest of the year only disinfection takes place. A low pressure system usually consists of more lamps and reactors than a medium pressure system cause the total rated power of medium pressure lamps is up to 20 times higher than for low pressure lamps. In the following OPEX evaluation these circumstances were considered to see which system offers the best life cycle costs. Therefore the following assumptions were made (Table 4).

The assumptions of the reactor design are based on the pilot test results and the possible improvements due to optimized design of a full-scale system. From Product Service System (PSS) modeling it was concluded that the EED of the medium pressure system will be two factors higher than the LP system. The detailed price list of the spares (lamps, ballasts, wiper rings and sleeves) as well as the total capital expenditures cannot be given in this paper due to secrecy clauses.

Figure 6 shows the result of the OPEX evaluation and the costs for the operation of the low pressure system are significantly lower than for the medium pressure system. This is mainly caused by the higher energy consumption of the MP system during the operation in AOP mode but also in the disinfection mode. The savings of the MP system due to lower lamp replacement costs are equalized by higher energy costs.

To see which system offers the best life cycle costs the capital expenditures have to be included in the evaluation. In this specific project the low pressure system also offered the lowest capital costs which finally resulted in awarding manufacturer A, who was supplying a low pressure UV AOP system to the client.

The results from this study show that low pressure UV AOP systems should be considered also for seasonal operations as the advantage of a significant lower EED and a longer lifetime of the lamps can cover the necessary costs of a higher amount of lamps to be replaced. Low pressure UV AOP's can provide higher reduction rates with lower energy input compared to medium pressure systems.