Advanced Treatment Of Secondary Effluent By Combined Pre-Ozonation And Biofiltration

Both the pollution of water environment and the shortage of water resources are driving the researches in applications of advanced treatment and in the reuse of wastewater. The secondary effluent of municipal wastewater can be used as a good potential water source due to its stable quality and large quantity. Generally, the secondary effluent of chembiological treatment has lower particle size and concentration of suspended solid (SS). Evenhough, biological processes (i.g. biofilm processes) are often adoptted as a combined unit for advanced treatment of wastewater due to its economical characteristics. Based on the systematic analysis of the secondary effluent, the combined process of ozonation and biofiltration (O3/BAF) was established for the advanced treatment of secondary effluent aiming at water reuse. Except for the general index, some chemical and biological methods including the molecular weight distribution (MWD), GC/MS, biodegradable analysis was employed to assess the quality fo the water in the research of operating characteristics of O3/BAF process and impact factors. Further more, the enhancement of the subsequent BAF by ozonation, the mechamisms of removal in BAF processes as well as some biofilm characteristic analysis were discussed.Commonly, the secondary effluent is characterized with the following index, such as BOD,COD,SS,NH3-N,TN and TP. However, they are too general to depict the characteristics of the secondary effluent with the upgrading of the standard of reused water and broadening of the water reuse field. Therefore, the significant characteristics need more research such as molecular weight distribution of organics, type of organics characterized by their functional groups, as well as the classes and structures of the organics in colloidal and suspended forms. Thus, the secondary effluent was fractionated into three parts: dissovled, colloidal and suspended and characterized individually. The analysis results indicated that the dissolved organics were dominating since the dissolved COD accounted for 78.2-86.5% in the secondary effluent. The secondary effluent is poorly biodegradable since the BDOC accounted for 15.5-27% of DOC. Furthermore, the organics with molecular weight <1k Dalton accounted for 56.3- 62.8%. The organic components dominantly accounted for 75.54-89.93% of the total mixture of suspended and colloidal pollutants. The paticle size distribution analysis showed that 80% of the particles ranged from 2 to 6.84μm. GC/MS analysis of the organics in dissolved form and in suspended solid indicated that 58% and 35% of them had cyclic structures, respectively. From this point, most of the organics were refractory. In summary, the secondary effluent is poorly biodegradable, which makes it necessary to adopt chemical oxidation as a pretreatment unit for improvement of the removal efficiency of the organic matter. The ozonation of the secondary effluent indicated that the removals of COD, TOC, UV254 and color were 25.7%, 16.5%, 69.3% and 79.2%, respectively, at the ozone dose of 10 mg/L and the contact time of 4 min. At the same time, the ozonation changed the percentage of organics with MW<1k Dalton from original 52% to final 72.6%. More important, the ozonation enhanced the biodegradability of secondary effluent by inproving BDOC from 1.08 mg/L to 2.6 mg/L, which accelerated the growth of biofilm and improved the biological activity of the biofilm. As a result, the removal function of the sequent BAF is thus efficiently enhanced.The long-term running data showed that the removal of organic matter by O3/BAF was higher that the sum of those by ozonation alone and BAF alone.At the same time, the removal of TOC, UV254 and color reached 20-27%, 65-75% and 85-95%, respectively. The percentage of organics with MW<1k Dalton decreased obviously.Furthermore, BAF was adaptable for the changes of hydraulic loading and pH for the organics removal. The removal of NH3-N and TN by O3/BAF is 87-92% and 13-17%, respectively. It is considered that the TN removal arised mainly from some anaerobic ammonia oxidation. Temperature, NH3-N loading and pH significiantly affected the NH3-N removal. The NH3-N removal increased with the increasing of temperature and decreased with the increasing of NH3-N loading. Besides, the optimal pH for Nitrification is from 7.27-7.89. The N element balance analysis indicated that NH₃-N was oxidized to NO₂-N by aerobic ammonia oxidizer firstly and then NO₂-N oxidized to dominant N₂ and a little of NO₃-N at the same time, by anaerobic ammonium oxidizing bacteria. The anaerobic ammonium oxidization was the main cause of nitrogen loss. The combined water-air backwashing could effectively remove the suspended pollutants and the unwanted non-active biofilm in BAF and backwashing only had a little negative impact on the removal of COD and nitrification by controlling proper condition such as backwashing intensity and backwash time. The removal function could be recoveried within shorter time after backwashing.The kinetic analysis of the reaction between ozone and organics indicated that the reactions were all one orders in 0-1.6, 1.6-16, 16-30 min and the kinetic constants were 0.0241, 0.00296, 0.000732 (L·mg-1·min-1), respectivly. Based on the exprimental and thoredical analysis, it was summarized that the removal of organic matter was attributed to biological oxidation, filtration, biological flocculation and the consumption in the food chain in BAF. It was found that the biofilm in the forepart of BAF is much thicker through microscope and the color is deeper, the hole between media particles is smaller.The diversities of microbial polulation and special characteristics of community sucession of BAF were observed. There were a lot of communities bacillus, filamentous, vorticella, algae, coccus and so on. The configuration of the biofilm and the composition of micorbial populaiton differed from different height of meida layer. On the whole, the distribution of heterotrophic bacteria and nitrobacter is related to the organics degradation and nitrification characteristics.An experiential model was established, which is only based on the organics concentration and height of the media layer. The model constants were calculated based on the experimental data. At last, the model was validated and it can be used as a reference for actual engineering design and operating management.In conclusion, the concentration of COD and NH3-N is lower than 20mg/L and 2 mg/L, respectively. O3/BAF is an effective as well as promising combined process for advanced treatment of the secondary effluent for reuse purpose.