A Pilot Study On Reject Water Treatment And Nitrification Enhancement In Wastewater Treatment Process Through Bioaugmentation

Reject water discharged from sludge treatment processes in Wastewater TreatmentPlant （WWTP） includes supernatant of sludge anaerobic digester, supernatant of sludgeconcentrator and sludge dewatering filtrate. It is characterized by small amount, highammonium concentration and low C/N ratio. The reject water, representing typicallyonly1%³%of the influent wastewater stream, can contribute up to15%²5%of the Nload of the influent to the activated sludge process. It is normally recycled and to betreated with the influent, which definitely overloads the plant. In this study, nitrifiercultivated by reject water in a side-stream was bioaugmented to a main-stream to treatwastewater in the pilot plant which was built in Xi’an. The activated sludge process hasbeen started-up quickly and operated at a better volume ratio of anaerobic zone, anoxiczone and aerobic zone. Cultivating nitrifiers by reject water in side-stream, andbioaugmentation to enhance nitrification in main-stream have been studied. The mainresults and conclusions are as following.（1） The concentration of suspended sludge in influent is a key factor for the startingup of the wastewater treatment plant by self-cultivation. There was about91.2%of thetotal bacterial count which was detected by DAPI staining in the primary sludge onaverage. The main-stream was started up by self-cultivation. After11d, the COD ineffluent was lower than50mg/L and the MLSS was more than2000mg/L. The ammonianitrogen and nitrite nitrogen both came to0mg/L, and the concentration of nitrate wasmore than25mg/L in effluent after15d.（2） When the primary sludge was inoculated to start up the activated sludge system, the COD was lower than50mg/L and the ammonia nitrogen was lower than1mg/L after9d. The total bacterial count in primary sludge can reach as high as1.70×10¹⁰cell/mL.The primary sludge was good seeding sludge and can be used to compensatethe activated sludge losing in the daily operating.（3） The side-stream was a pseudo-continuous O/A process. When the SRT was15dand the HRT was32h, nitrifying bacteria has been cultivated by the reject water. After64d, the side-stream came to stabilization, the average ammonia volume load was0.484kgNH₄⁺-N/（m³d）, the concentration of ammonia nitrogen and nitrite nitrogen wasboth lower than5mg/L in effluent. Nitrosospira preponderated in the population ofAOB before inoculating, and Nitrsomonas europaea and nitrosococcus mobilispreponderate in the population of AOB after inoculating. The population of NOB has nomarked change. The total AOB population （counted by Nitrsomonaseuropaea/Nitrosococcus mobilis and Nitrosospira） was108.82-108.86cell/mL, and thetotal NOB population （counted by Nitrobacter and Nitrospira） was10^8.58-10^8.63cell/mLafter cultivation. And there was35.2mg nitrifier was produced when1L reject waterwas treated. The maximum AUR （ammonia utilization rate） was12.43mgNH₄⁺-N/（gMLSS·h） and the maximum NUR （nitrite utilization rate） was13.18mgNO₂^--N/（gMLSS·h）.（4） The volume ratio of anaerobic zone, anoxic zone and aerobic zone was animportant factor for the removal of nitrogen and phosphorus in the A²/O process （mainstream）. When the HRT was10.1h, and the volume ratio of anaerobic zone, anoxic zoneand aerobic zone was1:2:7.1, the concentration of COD, ammonia nitrogen, totalnitrogen and phosphorus in effluent were16.98mg/L,0.81mg/L,24.75mg/L and2.45mg/L respectively, and the removal rate of COD, ammonia nitrogen, total nitrogenand phosphorus in effluent were91.4%,97.8%,54.4%and27%respectively. When thevolume ratio was changed to1:1.7:2.4, the concentration of COD, ammonia nitrogen,total nitrogen and phosphorus in effluent were28.92mg/L,5.95mg/L,21.97mg/L and0.41mg/L respectively, and the removal rate of COD, ammonia nitrogen, total nitrogenand phosphorus in effluent were88.4%,84.4%,59.8%and85%respectively. When theaerobic zone was reduced, the ammonia nitrogen was oxidized less, nitrate was less, andthe nitrate taken back to the anaerobic zone was less. So the less organic was needed by denitrifier. At the same time, the anaerobic zone was enlarged, the polyphosphateaccumulating organisms （PAOs） release phosphate more completely, and the removalrate of phosphate was improved. When the anoxic zone was enlarged, there was longertime for denitrification, and the nitrogen removal was enhanced.（5）The AUR and NUR of main-stream rose from2.61mgNH₄⁺-N/（gMLSS h）and2.38mgNO₂^--N/（gMLSS h） to5.32mgNH₄⁺-N/（gMLSS h） and3.81mgNO₂^--N/（gMLSS h） respectively by bioaugmentation in the main stream. The removal rateof ammonia were improved more than30%.（6） Phosphorus removal efficiency in the main stream could be improved throughnitrifier bioaugmentation from63.44%to89.15%.The experiment results confirmed that nitrification in wastewater treatment plantcould be enhanced and microbial population in the process has been optimized bybioaugmenting the nitrifier cultivated by reject water in side-stream.