Abstract
To enhance the biodegradability of residual organic pollutants in secondary effluent of wastewater treatment plants, UV photolysis and ozonation were used in combination as pretreatment before a biological aerating filter (BAF). The results indicated that UV photolysis could not remove much COD (chemical oxygen demand), and the performance of ozonation was better than the former. With UV photolysis combined with ozonation (UV/O3), COD removal was much higher than the sum of that with UV photolysis and ozonation alone, which indicated that UV photolysis could efficiently promote COD removal during ozonation. This pretreatment also improved molecular weight distribution (MWD) and biodegradability greatly. Proportion of organic compounds with molecular weight (MW) <3 kDalton was increased from 51.9% to 85.9%. COD removal rates with BAF and O3/BAF were only about 25% and 38%, respectively. When UV/O3 oxidation was combined with BAF, the average COD removal rate reached above 61%, which was about 2.5 times of that with BAF alone. With influent COD ranging from 65 to 84 mg/L, the effluent COD was stably in the scope of 23–31 mg/L. The combination of UV/O3 oxidation with BAF was quite efficient in organic pollutants removal for tertiary wastewater treatment.
1. Introduction
In recent years, contamination of surface and subsurface water by organic pollutants has received much attention. Wastewater treatment plants (WWTPs) play important roles in such pollutants removal. In China, more and more WWTPs have been built not only in large cities, but also in small towns and counties. To meet the national discharge standard for municipal WWTPs, most of them have been built with secondary treatment processes, which usually comprise physical interception, precipitation, and biological treatment. Although the secondary treatment processes are efficient in bulk and soluble pollutants removal, there are still some residual organic compounds, nitrogen, phosphorus, and pathogenic bacteria in the effluent. Some researchers have found that there are more than 200 organic micropollutants in the secondary effluent of municipal WWTPs [1, 2]. Most of these organic pollutants are refractory and difficult to be further degraded, and therefore their discharge in water may cause long-term environmental effects to aquatic and terrestrial organisms [3, 4].
In late May, 2007, a drinking water crisis took place in Wuxi, Jiangsu Province, China, owing to the large scale growth of algae bloom in Wuxi’s drinking water source—Taihu Lake. This crisis has made nearly 170 WWTPs being required to be upgraded with deep treatment or tertiary treatment to relieve the pollution load to Taihu Lake. Coagulation and filtration have been usually applied in tertiary treatment for residual suspended solids and partial colloid substances removal. However, most refractory and volatile organic compounds cannot be eliminated by these processes [5]. Owing to the low biodegradability of residual organic compounds, it is difficult to treat the secondary effluent directly with further biological processes. Appropriate processes should be found for refractory pollutants removal in tertiary wastewater treatment.
Many advanced oxidation processes (AOPs) can be used for destruction of refractory and high molecular organic compounds in wastewater, such as ozonation, ultraviolet (UV) photolysis, hydrogen peroxide (H2O2) oxidation, and Fenton oxidation. These AOPs can be used alone or in combination. During these processes, hydroxyl radicals with redox potential of 2.8 V are usually generated as the basic oxidants for pollutants removal [6]. Hydroxyl radicals are highly reactive and consequently short-lived. In the application of these AOPs, it is vital to provide appropriate reaction conditions and produce enough hydroxyl radicals. Fenton oxidation is usually accomplished in acid conditions, resulting in large quantities of acid and base consumption [7]. It can be used in small scale industrial wastewater treatment or special pollutants removal. Ozone is a strong oxidant that decomposes in water to form hydroxyl radicals which are stronger oxidizing agents than ozone itself. The use of ozonation for refractory and complex organic pollutants oxidation is very popular in water and wastewater treatment [8, 9]. During ozonation, the high molecular pollutants are decomposed into small molecular compounds, which are readily to be biodegraded [10]. Ozonation can also reduce the toxic effects of some micropollutants, but does not decrease the viability of bacteria [11]. Therefore, it is feasible to apply ozonation with biological processes in wastewater treatment. UV light is often used in water and wastewater disinfection. UV photolysis can selectively reduce some organic compounds, but it alone is not efficient enough for pollutants removal. However, many researchers have found UV photolysis can enhance the oxidation potential of some oxidation processes. When UV is used with H2O2, a strong oxidant whose photolytic dissociation yields hydroxyl radicals [12, 13], many recalcitrant pollutants such as pharmaceuticals and pesticides can be oxidized [14]. When UV photolysis is combined with ozonation (UV/O3 oxidation), more hydroxyl radicals are produced via the photolysis of reaction intermediates such as H2O2 [15], and the organic pollutants are decomposed more completely. UV/O3 oxidation has been used in pharmaceuticals, drinking water and industrial wastewater treatment [16–18]. In WWTPs, UV/O3 oxidation can be used as pretreatment for biological process, in which complex pollutants are decomposed into more biodegradable substances. Therefore, more organic compounds can be removed in the subsequent biological treatment process.
As described above, although there are many studies on UV/O3 oxidation in organic pollutants removal from water and wastewater, there are few studies in the application of UV/O3 oxidation with biological processes in tertiary wastewater treatment. In this study, UV/O3 oxidation was combined with a biological aerating filter (BAF) to treat the secondary effluent from a WWTP. The application of UV/O3 oxidation in combination with BAF could achieve high organic pollutants removal with low cost [19]. The characteristics of the secondary effluent were evaluated. COD (chemical oxygen demand) removal performance and molecular weight distribution (MWD) changes during UV photolysis, ozonation, and their combination were investigated. Contrast experiments were also carried out to study the performances of COD removal with BAF, O3/BAF and UV/O3/BAF.
2. Materials and Methods
2.1. Reactors
A pilot system shown in Figure 1 was built in a WWTP in Nanjing, Jiangsu Province of China. UV/O3 oxidation was carried out in a cylinder glass reactor with diameter of 300 mm and effective volume of 4 L. A 10 W low-pressure mercury UV lamp with main wavelength of 254 nm was installed in the center of the reactor. Ozone was produced with an ozone generator (CF-G-3-2.5 g, Qingdao Guolin Industry Co., Ltd, China) with dry air. Ozone dosage was controlled with a flowmeter.
The wastewater after pretreatment with UV photolysis and ozonation was pumped into a BAF, an organic glass cylinder reactor with diameter of 100 mm. Clay ceramisites with diameter of 10 mm were used as filling material with depth of 1.5 m. This BAF was operated with hydraulic load of 1.5 m3/m2 h and gas/water ratio of 5 : 1.
2.2. Analytical Methods
The samples from the influent, oxidation reactor, and final effluent of BAF were taken regularly and the concentrations of COD, BOD5 (5-day biological oxygen demand) and -N (ammonia) were analyzed according to the standard methods (APHA, 1999). MWD was measured using ultrafiltration membrane filters with cutoff sizes of 1–100 kDalton. DOC (dissolved organic carbon) after membrane filtration with different cutoff sizes was measured using a Shimadzu TOC-V instrument. pH, dissolved oxygen, ozone concentration, and temperature were monitored regularly.
2.3. Influent Characteristics
The effluent from the secondary clarifiers in the WWTP was directly used as influent of the combined process. This WWTP was run with an A2/O (anaerobic/anoxic and oxic) process, which had removed most soluble organic compounds and nitrogen. The main indexes of influent during this study were as follows: COD 63.2–91.4 mg/L, BOD5 9–23 mg/L, -N 5.3–8.6 mg/L, pH 7.1–7.4, and water temperature 19–25°C.
3. Results and Discussion
3.1. Evaluation of Secondary Effluent
COD and BOD5 in the effluent from the secondary clarifiers of the WWTP in 2009 are shown in Figure 2. COD in the effluent ranged from 63.2 mg/L to 91.4 mg/L while BOD5 was in the scope of 9–23 mg/L. According to the national discharge standard for municipal wastewater treatment plant in China, when the effluent from a WWTP is discharged into an enclosed watershed, COD and BOD5 in the effluent are required below 50 mg/L and 10 mg/L, respectively. It is obvious that the effluent could not meet the national discharge standard. This WWTP was run with A2/O process, in which most biodegradable organic compounds had been degraded. The ratio of BOD5/COD (B/C) was low, ranging from 0.17 to 0.25, which indicated that most of the residual organic substances in the effluent were difficult to be biodegraded.
3.2. COD Removal with UV Photolysis, Ozonation, and UV/O3 Oxidation
Figure 3 shows COD variation in the oxidation reactor with time under UV photolysis, ozonation, and UV/O3 oxidation. It can be seen that COD removal with UV photolysis alone was low. In 30 min contacting time, COD only declined from 73.5 mg/L to 65.1 mg/L. The performance of ozonation was much better compared with UV photolysis. After 30 min ozonation, COD decreased from 72.2 mg/L to 53.7 mg/L. More than 25% of COD was removed during ozonation. The combination of UV photolysis and ozonation led to significant improvement of COD removal compared with UV photolysis and ozonation alone. In 30 min, COD reduced from 74.3 mg/L to 40.9 mg/L. The removal rate of COD attained about 45%, which was much higher than the sum of COD removal rates with UV photolysis and ozonation alone. The results indicated that UV photolysis could greatly improve COD removal during ozonation [20].
It can also be observed that COD concentration decreased with contact time rise. During UV/O3 oxidation, COD decreased from the original 74.3 mg/L to 59.7, 45.4, and 40.9 mg/L at contact time of 10, 20, and 30 min, respectively. The extension of contact time was beneficial for COD removal. Nevertheless, the reactor volume needed to be amplified with contact time extension. More construction investment and operation cost would be required. It can be seen that contact time influence on COD removal was relieved with contact time exceeding 20 min. Therefore the subsequent experiments were done with contact time of 20 min.
3.3. Influence of O3 Dosages on UV/O3 Oxidation
A series of experiments of UV/O3 oxidation was done at different ozone dosages in the range of 0–12 mg/L. Results of COD variation with ozone dosages are shown in Figure 4. It can be seen that COD removal rose with ozone dosage increase. COD removal increased from 8.3% to 41.6% with dosage ranging from 0 to 12 mg/L, while residual COD decreased from 64.2 mg/L to 44 mg/L. It can also be noticed that when ozone dosage was under 2 mg/L, COD removal was low. When ozone dosage was increased from 2 to 8 mg/L, COD removal was enhanced greatly. These results indicated that ozone dosage was an important factor of UV/O3 oxidation. However, COD changed little when ozone dosage was above 8 mg/L. This indicated that there were still some organic compounds which were difficult to be degraded with UV/O3 oxidation, and excessive ozone dosage could not make further COD removal.
3.4. Molecular Weight Distributions under UV Photolysis, Ozonation, and UV/O3 Oxidation
MWD under different operations is shown in Figure 5. In the effluent from the secondary clarifiers, organics with molecular weight (MW) <1 kDalton were 42.4%. After UV photolysis, there was a minor increase of organics with MW <3 kDalton. After ozonation, the proportions of organics with MW <1 kDalton and 1–3 kDalton were increased obviously. Organics with MW <3 kDalton attained about 74%, which was much higher than 51.9% in the original influent. The combination of UV photolysis and ozonation changed MWD more greatly. Organics with MW <1 kDalton and 1–3 kDalton reached 64.7% and 21.2%, respectively. The proportion of organics >10 kDalton decreased from the original 21.5% to 4.7%. UV/O3 oxidation improved MWD of organics greatly and produced more biodegradable substances [21]. Consequently, the wastewater after UV/O3 oxidation was more appropriate for further biological treatment.
3.5. Performance of UV/O3 Oxidation Combined with BAF
Figure 6 shows COD removal with BAF alone (1–26 d), O3/BAF (27–56 d), and UV/O3/BAF (57–90 d). At the beginning, the BAF was started with the activated sludge of this WWTP as inoculum. COD removal increased gradually with time as more biofilm grew on the ceramisites. At the end of Day 16, COD removal rate attained about 24%. Subsequently, COD removal rate was maintained around 25%. Owing to the low biodegradability of organics in the influent, BAF alone could not remove more COD. COD in the effluent from the BAF was still above 50 mg/L, exceeding the standard limit for WWTPs in China.
In the second phase (27–56 d), ozonation was combined with BAF, and there was an obvious increase in COD removal. The combination of ozonation and BAF could remove 33%–44% COD. The average removal rate reached 38%. Ozonation had decomposed partial organic compounds into biodegradable substances, which could be readily biodegraded in subsequent BAF.
When UV photolysis and ozonation were combined with BAF (UV/O3/BAF, from Day 57 to Day 90), there was further improvement of COD removal. The average COD removal rate was above 61%, which was 2.5 and 1.6 times of that in BAF and O3/BAF, respectively. With influent COD ranging from 65 mg/L to 84 mg/L, the effluent COD was stably maintained in the scope of 23–31 mg/L. These results indicated that the combination of UV/O3 oxidation with BAF was an appropriate process for low biodegradable wastewater treatment. UV/O3 oxidation could not only remove partial organic pollutants, but also efficiently enhance wastewater biodegradability [22]. With UV/O3 oxidation combined with BAF, most organics were removed from the wastewater with the comprehensive activities of physicochemical decomposition and biodegradation.
4. Conclusion
The effluent from the secondary clarifiers of the WWTP comprises many refractory organic pollutants, most of which are difficult to be biodegraded. Direct treatment with further biological processes cannot make satisfactory performance. AOPs are usually efficient in refractory pollutants removal, and can be combined with biological processes in low biodegradable wastewater treatment. In this study, UV/O3 oxidation was combined with BAF in tertiary treatment. The results indicated that although UV photolysis alone was not quite efficient for COD removal, it could improve the performance of ozonation. When UV photolysis was combined with ozonation, 45% of COD in the wastewater from the secondary effluent was removed, and more biodegradable organic compounds were produced. It was really feasible to combine UV/O3 oxidation with BAF to improve COD removal in tertiary wastewater treatment. The combination of UV/O3 oxidation with BAF could remove more than 61% of COD in the wastewater from the second clarifiers, which was nearly 2.5 and 1.6 times of that in BAF and O3/BAF, respectively.
Acknowledgments
This research was supported by the project of the priority academic program development of Jiangsu Higher Education Institutions (2011-6), and science and technology project of China Housing and Urban-Rural Development Ministry (2010-K7-10).