Study On The Mechanism Of Microbial Regrowth And Resitriction Technique In Shanghai Drinking Water Supply System

Water is a crutial natural resource to human beings. In water supplysystems, drinking water is usually purified at the waterworks throughsedimentation, filtration, disinfection process to satisfy the sensory andhealthy requirements before drainaging into the secondary water supplylayer via pipeline distribution network, and finally arrives at the user’send: the faucet. In the entire distribution process, the quality of drinkingwater will be decreased because of the growth of microorganisms as wellas the related biochemical reactions. Therefore it is of obvious practicalsignificance to study the principle of microorganisms regrown in watersupply systems and water microbial regeneration control manners toguarantee the feedwater safety water.In this paper, the Nanshi and Changqiao waterworks which belong tothe Southern City Company in Shanghai and their water supply systemare chosen to be the research objects. Each month microbiological relatedindexes of water samples have been detected since2009to2011. Thenthe results are analysed to draw the relevant conclusion of Shanghai city water supply system in microbial regeneration mechanism as below:1. The advanced treatment technology can greatly reduce thedissolved organic matter and ammonia nitrogen’s concentration, whichhelps to control the water biological stability. In2010the conventionaltreatment process was taken placed by advanced treatment process atNanshi waterworks, when the source water quality has not changed muchthe water pollution index decreased significantly except watermicroorganism demonstrated by HPC and total bacteria. With the help ofozone and O3-BAC technology, the CODMnremoval efficiency isincreased from43.5%to52.6%, and the effluent AOC also decreasesfrom178μg/L to91μg/L. In the meantime, the ammonia of outlet isreduced from1.45mg/L to0.36mg/L.2. In2011, Nanshi and Changqiao waterworks were experiencedsource water switch from Huangpu River water to Yangtze River water.The outlet water quality was improved after this process. The effluentHPC has decreased from181CFU/mL to154CFU/mL. CODMndecreasesfrom2.12mg/L to1.38mg/L, and AOC decreases from91μ g/L to73μg/L. Changqiao waterworks has similar changes, whose HPC in pipewater has decreased from286CFU/mL to60CFU/mL, and CODMndecreases from3.34mg/L to1.44mg/L, and AOC droppes from181μ g/Lto81μg/L.3. Water quality index including microorganism indicators of total bacteria and HPC showes a rising trend in the ditribution network excepttotal chlorine. Either advanced treatment or water source switching doesnot affect these patterns in the netwwork. During the survey, significantmicrobial regrowth in network had emerged conpaired with the outlet ofwaterworks. When the microbial in outlet was hardly detected, the meansof total bacteria in the network of Nanshi waterworks in2009,2010and2011were73CFU/mL,43CFU/mL and34CFU/mL.As a matter of factor,conbined with the introduction of advanced treatment process and watersource switch we may find out in which way the microbial regenerationrate has been reduced in the distribution system. It is clear that treatmenttechnology and source water alternate can effets outlet water quality andmight improve drinking water biological stability to restrain networkmicrobial reproduce.4. It is showed that treatment process changes and water sourceswitch both successfully makes microbial regeneration reduced inscondary water supply system. During the conventional treatment processstage, total bacteria of Nanshi waterworks’ sampling point is993CFU/mLin average; after introducing the advanced treatment process, the meansof total bacteria is269CFU/mL; when the souce water was switched tothe Yangtze River water, total bacteria declined to69CFU/mL.5. It can be found that the the bioavailability organics carbonnutrient represented by AOC reflects the same trend as microbial regeneration patterns in the distribution system while analyzing therelationship between water quality index and microbial regrowth. Thisresult inplis that the mechanism which affects microbial regrowth is todecrease the concentration bioavailable organic matters such as AOC andenhances the biological stability, thereby inhibiting the regrowth ofmicrobial in the water distribution system.6. The influence of three main nutrient elements to microbialregrowth which might affect biological stability was tested viaexperiment in vitro. Microbial regrowth shall be restrained, and HPCreproduced rate in3days was declined when dissolved AOC in the formof Acetic acid carbon, compared to ammonia nitrogen in the presence ofnitrogen, with phosphate in the form of phosphorus are below100μg/L,20μg/L and2μg/L. Based on the fact that the concentration of ammoniaand phosphate in networks usually exceed those ammounts determined bythis study, AOC is the only reliable biological stability index which maysatisfy the requirements of regular and control in waterworks anddrinking water supply system.7. A proportions of chlorine resstance bacteria are present in thewaterworks outlet and distribution system via determination thedisinfection curves of microoganisms in waterworks outlet and supplynetwork. These bacteria can resist fairly high concentrations ofdisinfectant of the outlet (total chlorine1.5–2.2mg/L), and it is a potential threat to the distribution system using only chlorine disinfectant toprevent the microbial regrowth. This result in another aspect reflects theimportance of the studies of biological stability to restrict microbialregeneration.8. By analyzing the outlet AOC and secondary water supplysampling microbial plant counting results by linear regression, we mayget the conclusion of the restriction to waterworks outlet AOC. Take100CFU/mL of total bacteria and500CFU/mL of HPC as microbialregeneration limit value, AOC should be restricted under34μg/L at thespot of waterworks outlet in the worst situations. In the case that residuelchlorine is satisfied which could be kept above0.5mg/L, and theresidence time is not too long during secondary water supply stage, thepromission of the effulent AOC should be under93μ g/L to ensure thatthe microbial regeneration would not exceed the limits. Actually in thesurvey, when Nanshi waterworks applied advanced treatment process andusing Yanzi river water as source water, the sample results of thesecondary water supply system showed that both total bacteria and HPCwere below the limit value of100CFU/mL and500CFU/mL. In themeantime the outlet AOC was71μg/L, the other water quality indicatorswere in a fairly nice situation.