Advanced Treatment of Polluted Source Water by Pre-ozonation

Jinsong ZHANG, Xin SUN
Shenzhen Water (Group)Co.Ltd.
Water Building,1019 Shennan Rd.Cent.Shenzhen 518031,P.R.CHINA

Abstract:The source water in Shenzhen is relatively serious polluted by organic matter and algae. The Ames Test results of the tap water are positive and the odor of the tap water is objectionable. The pilot-study results on pre-ozonation showed, compared with pre-chlorination, pre-ozonation reduced the mutagenic activities of finished water significantly , with its far less MR value, the total chlorinated organic matter and chloroform in finished water is lower by 61.4 and 30.6 percent respectively, the total removal of COD_Mn, TON was higher by 3.1 and 6.3 percent respectively, the ammonia, nitrite and color can also be removed more effectively. The proper ozone dose and contact time is about 0.5 mg O₃ /mg TOC and 6~8 min respectively. Based on the pilot-study results, Donghu water plant(350,000m³/d) was upgraded with pre-ozonation. The systematic examination of the process performance showed the upgradation had achieved the targets, the pre-ozonation can replace pre-chlorination to produce better and safer drinking water.

Introduction ：Increasing improvement in human living conditions and unceasing deterioration in aqueous environment lead to more and more attention paid to safe drinking water. Advanced water treatment by ozonation biological activated carbon(O₃/BAC)process after conventional treatment has been among the urgent demands from the inhabitants in economically developed areas in China(1). However, for those water plants confined by limited lands or restrained with the existing processes, enhanced conventional treatment by pre-ozonation instead of pre-chlorinaton is one of the potential alternatives to improve drinking water quality ,especially for coastal areas in southern China, where rapid economic growh is accompanied with serious pollution.

The source water in Shenzhen is polluted by receiving untreated domestic and industrial wastewaters after a long-distance transportation by open channel.Total nitrogen,total phosphorus,mineral oil,ammonia,nitrite,BOD5,and manganese violate the national water quality standards for secondary source water frequently. The high concentrations of algae indicate the eutrophication in Shenzhen Reservoir, from which the source water is taken.Under the existing conventional treatment, it is difficult to meet the water quality goals for the local water company. The positive results of Ames Test show the existence of organic pollutants in the tap water. In some cases, high concentration of nitrite in filtered water consumes most part of chlorine. The insufficient residue chlorine in finished water cannot guarantee an effective disinfection and cannot avoid bacteria regrowth in pipe system. The odor caused by algae and excess residue chlorine brings unceasingly complaints.

To study the feasibility of applying pre-ozonation to improve the drinking wter quality in Shenzhen, Pilot experiments on pre-ozonatin were conducted in 1998~2000 and the conclusion pre-ozonation is superior to pre-chlorination was drawn. Based on this conclusion, Donghu Water Plant(350,000 m³/d) was first upgraded with pre-ozonation in May,2001.The systematic examination of the process performance showed the upgradation had achieved the targets.

Materials and Methods

Source water.In 1998 and 2001, the source water was taken from Shenzhen Reservoir, it‘ s a typical polluted source water with high content of ammonia, nitrite and algae. In 2000, the source water was taken from Xili Reservoir, the water quality was better than that of Shenzhen Reservoir( Table Ⅰ).

Table Ⅰ Source water qualities

Pilot experiment system.The process flow was pre-ozonation ― conventional treatment, the experiment equipments( Figure 1) are installed in Dayong Water Plant. Most experiments was performed with a 3-m.-high,320-mm.-diameter stainless steel ozone contactors. Ozone was produced on site from liquid oxygen(1998) and air(2000) respectively(Model CFS-1A Ozone Generator, Ozonia).The off-gas was exhausted out directly.

Pre-ozonation and pre-chlorination was runned alternately. Ozone and sodium hypochlorite was added by injector, they were both contacted with water for about 8 min, the dose of ozone and sodium hypochlorite was 0.4~1.6mg/l and 2~4mg/l respectively. The water velocity in pre-oxidization contactor was about 40m/h. The conventional treatment unites was runned at the rate of 3 m³/hr.The liquid polyaluminum chloride(PAC) was used as coagulant and its dose was 2.5mg/l or so.The post-chlorine dose was changed with the source water quality.

Figure 1　 The layout of the pilot plant for pre-ozonation advanced treatment

Full-scale experiment system.The experiment was conducted in Donghu Water Plant from August to December in 2001. The process flow is shown as figure 2.Ozone was produced on site from liquid oxygen(Model ZF06/60　 Ozone Generator,Ozonia).The off-gas was destroyed under high temperature. The designed ozone dose and contact time was 1.6 mg/l and 6min respectively. In contrast with different pre-oxidation technology, the dose of ozone and sodium hypochlorite was 1.5mg/l and 3.5mg/l respectively, the contact time was both 6min.Pre-ozonation and pre-chlorination was runned alternately.

Sampling methods.Routine analysis of source water, pre-oxidized water and finished water were 5 times per week. During the pilot experiment, the total chlorinated organic matter was analyzed and Ames Test were performed for source water and finished water, the dose of ozone and sodium hypochlorite was 1.6 and 3mg/l respectively. During the full-scale experiment, Ames Test were performed for source water, pre-oxidized water and finished water.

Analytical methods.Chemical analysis on COD_Mn, NH₃- N, NO₂ ^— - N, CHCl₃ and CCl₄ were carried out according to the standard methods(2). TOC was determined on a carbon analyzer by persulfate-ultraviolent oxiding method, UV₂₅₄ was measured as absorbance at 254 nm( without adjusting pH), TON was determined by dilution method,details of them were shown in American standard methods(3).Ames Test was carried out by the preincubation method with Salmonella typhimurium TA98 and TA100 strains(4),negative control(DMSO) and positive controls(SA,2-AF and 2,7-AF) were submitted to the assay.Algae numbers was counted by cells(5). Apparent molecular weight distribution(AMWD) of dissolved organic matter was measured using Ultrafiltration Technique(6). Aldehyde was measured by chromotrope　 spectrophotometric method(7).Turbidity and residual ozone was measured with HACH turbidity analyzer and HACH ozone analyzer on site respectively.

Results and Measurements

Results of pilot experiments

analysis of water qualities

When the dose of ozone and sodium hypochlorite was 1.6mg/l and 3mg/l respectively,the finished water quality of pre-ozonation flow was superior to that of pre-chlorination flow, especially on ammonia,nitrite,odor and organic matter(Table Ⅱ).The total removal of ammonia of pre-ozonation flow was higher by 42.3 percent than that of pre-chlorination flow,the difference of total removal of nitrite between these two pre-oxidation flow was more obvious. Compared with pre-chlorination,the total removal of color,TON and COD_Mn of pre-ozonation flow was higher by 12.4,6.3 and 3.1 percent, and capability to remove turbidity and algae was similar.

Table Ⅱ　 　process performance of pilot experiments

Note：removal value was average in TableⅡ.

As for chlorinated organic matter in finished water, pre-ozonation flow was superior to pre-chlorination flow.With the same source water,the total chlorinated organic matter and chloroform in finished water of pre-ozonation flow was lower by 61.4 and 30.6 percent than those of pre-chlorination flow.

Table Ⅲ　 chlorinated organic matter in finished water（μg/L）

Pre-ozonation reduced the mutagenic activities of finished water significantly ,especially for mutagens inducing frameshift mutations(Figure 3). When test dose was 2L/P,without S9 mix,MR value for TA98 of the source water of pre-ozonation and pre-chlorination was 1.95 and 1.73,that of the finished water was higher by 6.2 and 166.5 percent respectively. With S9 mix, MR value for TA98 of the source water of pre-ozonation and pre-chlorination was 1.36 and 1.48,that of the finished water was higher by 94.1 and 143.9 percent respectively.The negative results of Ames Test for TA100 showed there was few mutagens inducing base-pair substitution mutations.

Figure 3　 Mutagenic activities of different water sample

Ozone dose and contact time

When the ozone dose was varied between 0.47~1.6mg/l,the content of　 residual ozone in pre-ozonated water was very low(TableⅣ),and it was independent of contact time.This result showed most of the ozone reacted with water.According to table Ⅴ,it was beneficial to remove more organic matter.

　Table Ⅳ　 residual ozone of different pre-ozonation conditions

Table Ⅴ　 pre-ozonated water qualities of different contact time

Results of full-scale experiment

When ozone dose was 1.5mg/l and 1.0mg/l respectively, the total removal of TON, algae, organic matter, ammonia and turbidity of the former was both higher that that of the latter respectively, the CHCl₃ content in finished water of the former was lower by 22.3 percent than that of the latter, and the content of CCl₄ and aldehyde in finished water could not exceed 0.5μg/l and 80 μg/l respectively. When the dose of ozone and sodium hypochlorite was 1.5mg/l and 3.5 mg/l respectively, the total removal of TON of the former was higher by 13.1 percent than that of the latter, the CHCl₃ content in finished water of the former was lower by 98.0 percent than that of the latter, the total removal of algae was similar, the turbidity, color of finished water was both 0.25NTU and 5unit or so, and the content of CCl₄ and NO₂^—-N could not exceed 0.5μg/l and 0.001mg/l respectively.

Table Ⅵprocess performance of full-scale experiment

Figure 4　 Mutagenic activities of different water sample

Note:Values before and after“/”was for source water and finished water，values in”（）”was average.

As shown in figure 4，the results of Ames Test for TA98 of the source water was positive,and mutagens inducing frameshift mutations were contributed to the positive results.Pre-oxidation increased the mutagenic activities of treated water ,the results for TA98±S9 of the finished water in both pre-oxidation flow were positive,while those for TA100±S9 of the finished water in pre-chlorination flow were possibly positive.Compared with source water,the extent to increase mutagenic activities of pre-chlorination flow was more than that of pre-ozonation flow obviously. When the dose was 2L/P，without S9，the MR value for TA98 and TA100 of the finished water in pre-ozonation flow increased 41.6 and -11.6 percent respectively,that of the finished water in pre-chlorination flow increased 69.3 and 49.6 percent respectively.

Discussion

Pre-ozonation can remove odor more effectively than pre-chlorination. It is due to the strong oxidizing power of ozone and OH radical, their redox potential was higher by 0.71 and 1.44V than that of chlorine.In addition,new odorous matter may be produced by chlorination and chlorine is odorous itself.

The experiment results showed the TOC removal was very low(below 5 percent),but the UV₂₅₄ removal was increased as the ozone dose was added.This results confirmed that the main purpose of pre-ozonation is to transform complex organic matter to simple ones,and ozone recat with unsaturated　 functional groups.And the measurements of AMWD of dissolved organic matter in source water may explain the low TOC removal,the percent of organic matter whose molecular weight was below 4,000 in source water and pre-ozonated water was 72.6 and 95.8 percent respectively.

When the ozone dose is low,pre-ozonation is mainly applied to decompose algae ,but not to oxidize algae thoroughly.full-scale experiment results showed, the algae numbers did not decrease until the ozone dose was added up to 2.0mg/l.The experiment results on algae removal in Wiggins Water Plant were the same(8).The pH value may play an important role in algae removal by pre-ozonation.When the alkaline source water was pre-ozonated,though the ozone dose was as low as 1.5mg/l,the unit removal for algae could exceed 40 percent.This may be due to more OH radical under alkaline condition.

The precursors of mutagens inducing frameshift mutations could be converted to mutagens by pre-oxidation,other study results also showed only if the ozone dose is high can the mutagenic activities of treated water be reduced(9) .Mutagens in pre-ozonated water would not be added during the following treatment,but that was contrary for pre-chlorination.This may due to the poorer oxidizing power of sodium hypochlorite.Pre-ozonation did not add mutagens inducing base-pair substitution mutations,while pre-chlorination did.

When the ammonia content in source water exceed 2mg/l，the nitrite content in iltered water may increase dramatically, no matter what pre-oxidation method was applied. The major possible reason was the imcomplete biological nitration, which was due to deficient dissolved oxygen. During the full-scale experiment, the total ammonia removal in pre-ozonation flow was lower than that of pre-chlorination, the possible reason was chlorination prior to filtration.

Conclusion

The extent to improve water quality by pre-ozonation was dependent of the source water qualities and ozonation conditions. In accordance with the source water in Shenzhen, the proper ozone dose and contact time was about 0.5mg O₃/mg TOC and 6~8min.Under this optimal pre-ozonation conditions, the total removal of TON, algae, COD_Mn and NH₃-N could be up to 87,95,43 and 48 percent，the content of NO₂^—-N、CHCl₃、CCl₄、aldehyde was stringently controlled within the limit respectively. Compared with the source water, the MR value for TA98-S9 of finished water was higher by 3 percent, the MR value for TA100-S9 of finished water was lower by 12 percent.

The pre-ozonation can replace pre-chlorination to enhance conventional treatment and　 produce better and safer drinking water.When the ozone dose was half of sodium hypochlorite dose, odor, chlorinated disinfection by-product and mutagenic activities of finished water in pre-ozonation flow was both　 superior to that of pre-chlorination flow, while the other was similar.

When the ammonia content in source water is above 2mg/l，alternative methods to remove ammonia will be developed.The influence of pre-ozonation on the biological stability and the optimal running will be studied further.

References

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