İçme suyu şebekesinde bakteriyel yeniden çoğalmayı etkileyen faktörlerin belirlenmesi

Determination of environmental factors influencing bacterial regrowth in drinking water distribution system

Özet

İçme suyu arıtma tesisi çıkışında bakteriyel standartları sağlayan içme suyu, tüketiciye ulaşana kadar dağıtım sistemi içerisinde bir takım değişikliklere uğramakta ve heterotrofik bakteri sayısında artış meydana gelebilmektedir. Bursa İli içme suyu dağıtım sisteminde meydana gelen bakteriyel değişimlerin ve bunları etkileyen mikro-çevresel faktörlerin incelendiği bu çalışmada arıtma tesisi çıkışı itibari ile yaklaşık 10200 m 'lik bir hat üzerinde çalışılmıştır. Bu hat üzerinde seçilen 5 noktadan alınan örneklerde pH, bulanıklık, bakiye klor, nitrat, orto-fosfat, amonyum azotu, TOK (Toplam Organik Karbon), HBS (Heterotrofik Bakteri Sayısı), toplam koliform ve AOK (Asimile Edilebilir Organik Karbon) ölçümleri yapılmıştır. Elde edilen sonuçlar Bursa İli içme suyu dağıtım sisteminde arıtma tesisinden başlayarak şehrin doğusuna doğru uzanan hat üzerinde uzaklığa bağlı olarak bakteri sayısının arttığını göstermiştir. TOK, AOK ve amonyum azotu azalırken HBS değeri artmıştır. Bu parametreler arasındaki korelasyon katsayıları sırası ile -0.57, -0.74 ve -0.80 olarak belirlenmiştir. Bu değerler seçilen iletim hattı boyunca mikro-çevresel faktörlerin bakteriyel çoğalma üzerindeki etkisini ortaya koymaktadır. TOK, AOK ve amonyum-azotu ile uzaklık arasındaki korelasyon katsayıları ise -0.64, -0.89, -0,87 dir. Bazı noktalarda bakteri sayısının içme suyu standardı limit değerlerinin üzerinde olması, dağıtım sistemindeki bakiye klor konsantrasyonunun (

Abstract

In the present study, variations in microbial quality of drinking water and influential micro-environmental factors were examined in the distribution system of Bursa City. A 10200 m length of distribution line from the treatment plant was designated as the study area. pH, turbidity, residual chlorine, nitrate, ortho-phosphate, ammonium nitrogen, TOC (Total Organic Carbon), HPC (Heterotrophic Plate Count), total coliform and AOC (Assimilable Organic Carbon) measurements were carried out in water samples which were collected from 5 different points of the distribution system. The results of this study, indicated that the number of bacteria increased with distance between treatment plant and east side of the city. HPCs were determined to be 29, 62, 101, 133 and 176 CFU.$mg.l^{-1}$ for the sampling points of 1., 2., 3., 4. and 5., respectively. TOC concentrations for the same points were 3.45, 2.64, 2.13, 1.61 and 1.19 $mg.l^{-1}$. AOC concentrations measured in samples of point 1, 3 and 5 were 126.3, 99.1 and 92.1 $mu gC.l^{-1}$. It is known that AOC which is decomposed easily and utilized directly in cell synthesis forms 0.1-9% of TOC. In the present study, mean AOC concentration of water was measured to be 105.8 $mu gC.l^{-1}$ and formed 4-8% of TOC (2.2 $mg.l^{-1}$). Standard value for HPC is 100 CFU.$ml^{-1}$ according to WHO (World Health Organization) and AOC is required to be below 10 $mu gC.l^{-1}$ in order to accomplish this standard. However, bac¬terial regrowth appears to be inevitable in our distribution system since mean AOC concentration (105.8 $mu gC.l^{-1}$) was much above of this standard. Ammonium nitrogen concentrations were measured to be 0.26, 0.22, 0.17, 0.11 and 0.11 $mg.l^{-1}$ for the sampling points of 1., 2., 3., 4. and 5., respectively. Decreasing TOC, AOC and ammonium nitrogen concentrations were followed by increasing HPC. Correlation coefficients between HPC and TOC, AOC, and ammonium nitrogen were found to be -0.57, -0.74 and-0.80, respectively. These values signify the effect of micro-environmental factors on microbial growth. Concentrations of TOC, AOC and ammonium nitrogen decreased significantly with increasing distance from treatment plant. Correlation coefficients between distance and TOC, AOC and ammonium nitrogen were found to be -0.64, -0.89 and -0.87. Nitrate concentrations for the sampling points were measured to be 3.05, 3.13, 3.15, 3.50 and 4.15 $mg.l^{-1}$ and indicated an increasing trend line with distance. It is thought that potential nitrification process cause this increase in nitrate concentrations. A positive correlation (0.57) was determined between nitrate concentration and HPC whereas no correlation (0.37) existed between nitrate concentration and distance. Orhto-phosphate concentrations were measured to be 0.110, 0.087, 0.106, 0.082 and 0.078 $mg.l^{-1}$ for the same points. Although HPC increased with increasing ortho-phosphate at the end section of distribution system, no powerful correlation (-0.13) was determined between these parameters. Reduction of ortho-phosphate concentration in the distribution system indicated that ortho-phosphate was likely to be utilized by bacteria as a nutrient along with ammonium nitrogen and AOC. It is known that marginal variations in inorganic phosphate concentrations may stimulate bacterial growth in distribution systems. Mean pH values were measured to vary between 7.24 and 7.47 and this variation was found to be insignificant. However, this level of pH was thought to be appropriate for bacterial growth. Turbidity of water did not vary significantly in the distribution system. Mean turbidity levels were determined to be 2.49, 2.13, 2.36, 1.93 and 2.17 NTU for the sampling points of 1., 2., 3., 4. and 5., respectively. Mean turbidity level determined in the present study appears to comply with the drinking water standards. In general, turbidity is expected to increase with the increasing number of bacteria in water. However, turbidity doesn 't increase unless bacterial growth occurs at significant levels. Therefore, in the present study, turbidity of water remained at reasonable levels as a result of minor increases of HPC in water. Residual chlorine was found to be below 0.2 $mg.l^{-1}$ at all sampling points, indicating that residual chlorine in the distribution system was not maintained at a level to control bacterial growth. Accordingly, HPC in water increased with distance, which provided convincing evidence for inadequate regrowth control in the distribution system. Microbiological parameters measured in this study showed that microbial quality of water was at a level which would not show any adverse effects on human health. However, bacteriological quality and AOC level of the water must be monitored continuously since the bacterial regrowth potential exists.