11 months ago


Water treatment


Longer “initial-ripening” periods for turbidity than for indicator bacteria could simplify monitoring and operational decisions about connecting new SSF units to main treated water effluents, particularly in small WS systems with limitations for microbiological analyses. However, this possibility requires further observations before making any practical recommendation. During ripening periods for indicator bacteria SSF effluents should be wasted, especially when other parameters which interfere with chemical disinfection, like turbidity, are also high and unstable. Exceptionally, chemical disinfection could be applied to the effluent of SSF units being ripened for indicator bacteria. In this case chemical disinfection should be an integral part of the treatment system and not only a final safety barrier, thus also requiring stricter measures for water quality control, operation and maintenance. Water quality changes in CGF lines and SSF units The filterability test was carried out with samples taken from the effluents of the CGF lines and their respective SSF units. Descriptive statistics of mean volumes of filtered water are summarised in table 3.16. The volumes of filtered water are in the range 346 to 491 ml.l -1 h - for samples taken from the CGF effluents and 513 to 550 ml.l -1 h -1 for samples taken from the SSF effluents. This suggests that SSF units are adapting their solids removal efficiencies to smooth out filterability differences found in effluents of CGF lines. This should be reflected in higher O&M requirements in those SSF units treating influent water with low filterability or higher solid concentrations. 1 Table 3.16 Descriptive statistics for volumes (ml.3min -1 ) of water obtained during the application of filterability tests to the effluents of CGF lines and SSF units. Test period I, March-July 1991. Descriptive Int. Line 1 Line 2 Line 3 Line 4 Line 5 statistics water UGFS SSF1 UGFL SSF2 MHGF SSF3 HGF SSF4 DGFS SSF5 Mean 62 383 513 346 535 453 539 456 513 491 550 SD 26 97 58 95 30 88 26 79 42 66 0 Minimum 33 241 426 205 474 325 486 357 457 421 550 Maximum 99 490 550 475 550 550 550 550 550 550 550 Data (N) 6 6 6 6 6 6 6 6 6 6 6 The pH, total alkalinity and total hardness were fairly consistent in the 2 nd and 3 rd filtration stages. Therefore the buffer capacity of the water was not significantly altered and precipitation did not take place in the filter beds. The mean dissolved oxygen (DO) levels in the effluents of the vertical flow CGF lines were in the range of 3.8 to 7.1 mgl -1 of O 2 . Higher values were observed in vertical flow CGF options with aeration taken place after the inlet flow weirs of each filtration step. The lowest mean value (1.9 mgl -1 of O 2 ) was observed in HGF line during the IV test period having the highest tested filtration rate (0.75 mh -1 ). Long “ conventional” HGF units may have problems caused by anaerobic environments when treating polluted waters at high filtration rates. SSF units processing effluents from UGFS, HGF, and DGFS produced water consistently within the guidelines for total iron (table 2.3) during all test periods. SSF 2 (after UGFL) had effluents >0.3 mgl -1 in 25 and 33% of samples taken during the 1 st and 2 nd test periods 105

espectively. SSF 3 (after MHGF) produced effluents with total iron levels above the guideline value during test period II in 17% of all analysed samples. All CGF+SSF lines showed mean removal efficiencies of total iron in the range 98 to 99% during the 1 st testing period, when the mean influent value was 11 mgl -1 . The efficiencies were around 90% in all other periods, when the mean influent values were in the range of 1 to 2.1 mgl -1 . The great majority of total iron concentrations in SSF influents were 0.1 mgl -1 in 20% of samples taken during test period IV. Integrated water had mean volatile solids concentrations in the range 48 to 52 mgl -1 during test periods I, II, and IV. CGF+SSF mean removal values for volatile solids were in the range 21 to 42% during these three periods, with CGF contributing in the range 18 to 30%. During all test periods mean integrated water chemical oxygen demand (COD) was in the range

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