Research On Control Strategy Of Low Dissolved Oxygen Limited Filamentous Sludge Bulking

Activated sludge process is the most widely applied municipal wastewatertreatment technology. It owns the characteristics of simple configuration andconvenient management, but has the disadvantage of big aeration energyconsumption. What is more, it is suffered by sludge bulking seriously. Sludgebulking has the characteristics of many influence factors, huge damage, difficulttreatment, high occurrence frequency and so on. Our laboratory team had beentreating municipal wastewater by activated sludge process many years, in order totreat sludge bulking more scientifically,"The theory and technique of saving energyachieved by limited filamentous bulking under low dissolved oxygen （DO）" wasproposed firstly by our laboratory team through summarizing practical experiences.Its core content is adjusting the activated sludge system operate under low DOcondition through decreasing aeration rate artificialy. Filamentous bacterium will beproliferated by low DO, meanwhile, sludge bulking extent is prevented fromdamaging normal operation. On the basis of saving energy, effluent quality can beimproved through using the filamentous bacterial physiological characteristics suchas large specific surface area, strong low DO enduring ability and strongdecomposing low substrate ability.In order to apply the low DO limited filamentous sludge bulking in practicebetter, lab-scale sequencing batch reactor （SBR） was used through adjustingdifferent operation patterns and conditions. The approaches to induce limitedfilamentous bulking, inhibit excessive filamentous bulking and maintain low DOlimited filamentous bulking were all investigated. Under low DO condition, theinfluences of various normal environmental factors and process parameters onactivated sludge process operation effects, especially the sludge settleability, werealso investigated. Through optimizing the experimental datas, the characteristics ofactivated sludge process had been understood further, and the effective approachesto induce and inhibit filamentous bacterium proliferation were established. Finally,the established approaches to control sludge settleability had been verified in theSBR treating synthetic wastewater and real municipal wastewater, respectively.Meanwhile, the pollutants removal characteristics under low DO limited bulkingwere investigated.The approaches to induce limited filamentous bulking were established. It wasfound that sludge settleability and organic loading rate （OLR） had close relationship under low DO condition. When there was no pre-anoxic phase, filamentous bacteriacan not proliferate excessively by adjusting low DO alone when OLR was in therange of0.30⁰.35kgCOD·（kgMLSS·d）^-1, while it will proliferate excessivelywhen OLR was smaller than0.25or larger than0.40kgCOD·（kgMLSS·d）^-1. Whenthere was pre-anoxic phase, filamentous bacteria can not proliferate excessively byadjusting low DO alone when OLR was smaller than0.30kgCOD·（kgMLSS·d）^-1,while it will also proliferate excessively when OLR was larger than0.40kgCOD·（kgMLSS·d）^-1. After optimization, it was proposed the most effectiveapproach to induce filamentous bacteria proliferation was decreasing OLR under0.25kgCOD·（kgMLSS·d）^-1by prolonging aeration time without pre-anoxic phase.The approaches to inhibit excessive filamentous bulking was established. Itwas found increase DO or OLR alone was difficult to inhibit filamentous bacteria,while operate as anoxic（anaerobic）/aerobic was effective to improve sludgesettleability. For anoxic selector, excellent phosphorus removal ability is necessaryto inhibit filamentous bacteria effectively. When phosphorus removal performancewas well and the anoxic time was sufficient, filamentous bacteria can be inhibitedsuccessfully when influent RBCOD: NO₃^--N ratio was in the range of33.00⁶6.00,while it will lost when the ratio was smaller than33.00or larger than66.00. Foranaerobic selector, excellent phosphorus removal ability was also important toinhibit filamentous bacteria. When PO₄^3-–P was excessive, the influentCOD:PO₄^3-–P ratio had no impact on sludge settleability improvement rate. Duringsludge settleability deterioration or improvement processes, sludge volume index（SVI） existed a threshold effect around200mL·g^-1, where it will change sharply ina short time.It was found the environmental factors like temperature, pH, DO and theprocess parameters like sludge retention time （SRT）, mixed liquid suspended solids（MLSS）, feeding pattern had different impacts on activated sludge processoperation effect, especially the sludge settleability. Temperature variation in a shortterm had no impact on filamentous bacterial content. Sludge settleabilitydeteriorated by the excessive excreted extracellular polymeric substances （EPS）when temperature was decreased, while it will deteriorated by the looser flocsconfiguration when temperature was increased. Nitrification and phosphorusremoval processes fluctuated slightly when temperature varied smal. Theydeteriorated seriously when temperature decreased sharply （22¹2°C）. Whentemperature incresed sharply （12²2°C）, nitrification will deteriorated seriously,while phosphorus removal was not affected. Nitrification, denitrification and many other biochemical reactions can consume or produce alkalinity, with a lot of ionpairs existed ionization equation in the water solution, the activated sludge mixturehad excellent pH adjusting ability. When influent pH was in the range of6⁹, pHhad little influence on sludge settleabiliy. The supernatant was turbid under aciditycondition, while it was clear under alkaline condition. DO was not an importantfactor to influence filamentous bacterial proliferation. Sludge settleability improvedwith SRT deduction, nitrification rate, the amounts of PO₄^3-–P released and uptakendecreased with SRT deduction, while the specific nitrification rate, specific PO₄^3-–Preleasing rate and uptaken rate increased with SRT deduction. No matter non-bulking sludge or bulking sludge, when MLSS was too low the activated sludgewould have the characteristics like dispersed or semi-dispersed configuration, rapidsedimentation rate, fuzzy sludge-water interface, tuibid supernatant. When MLSSwas bigger, the sedimentation rate would decrease, and the sludge-water interfacebecome clear. It was verified the storage selection effect caused by different feedingpatterns can not be acceptted as a general law to inhibit filamentous bacterialproliferation.Through combined utilizing the approaches to induce or inhibit filamentousbacterial proliferation estabilished above, the low DO limited filamentous bulkinghad been firstly maintained steadily in SBR treating synthetic municipal wastewaterand real municipal wastewater, respectively. When SVI was close to the thresholdvalue, although sludge settleability could be changed easily, the activated sludgesystem was very unsteady. In order to maintain the limited filamentous bulkingsteadily, it was recommended operation patterns should be altered when SVI was inthe range of250³00mL·g^-1. Operating under low DO in a long term is benificalfor short-cut nitrification and simultaneous nitrification denitrification （SND） totake place. It was found when each cycle duration and influent quality wereconstant, the value of anoxic （anaerobic） time to aerobic time ratio was a determinefactor for SND ratio.