A year-long, bench-scale treatability study was performed to assess whether an activated sludge sequencing batch reactor could be used to treat an influent stream of 50/50 (v/v) municipal wastewater and landfill leachate to discharge standards established by the Oregon Department of Environmental Quality. One primary obstacle that was exposed during this study was incomplete nitrification that resulted in high effluent nitrite (NO₂-N) for many months of the study. The data presented in this thesis was collected as a response to this issue.
In an effort to remedy the partial nitrification issue, as well as to address other contaminants that were not being effectively treated by the SBR (including recalcitrant COD, metals, and color), abiotic pretreatments were analyzed. These pretreatments included sorption by powder activated carbon (PAC), coagulation and flocculation by ferric chloride (FeCl₃), and precipitation by sodium hydroxide (NaOH). Biological batch experiments using pretreated influent were conducted to determine each pretreatment’s effect on nitrification activity. While the pretreatments influenced removal of the contaminants named above to varying degrees, there was little indication that these pretreatments affected nitrification rates.
It was found that nitrification was greatly influenced by ammonia loading and pH. Both are factors that contribute greatly to the formation of inhibitory free ammonia (FA). Temperature and aeration intensity also play a significant role in ensuring complete nitrification. Using experimental nitrification rate data, modeled analyses of limitation by FA in addition to low dissolved oxygen (DO) were generated. These modeled analyses indicated that inhibition of ammonia-oxidizing bacteria (AOBs) were primarily affected by low DO while inhibition of nitrite-oxidizing bacteria (NOBs) were affected by both FA and low DO.
Over time, our system was able to sustain complete nitrification at temperatures as low as 16 °C with an average DO concentration that remained far below 0.5 mg/L DO during active aerobic treatment. This is likely due to the effects of a long solids retention time (SRT) that aided the enrichment of nitrifying bacteria with enhanced oxygen affinity as well as seasonally decreased ammonia loading to the system.
The 16S rRNA genome of reactor sludge treating the 50/50 (v/v) leachate and wastewater influent was compared to reactor sludge treating wastewater alone in order to clarify what microbial populations become enriched in the presence of leachate. Overall, it was found that, with the addition of leachate, a small number of taxa came to represent a greater fraction of the microbial population given analysis at the phylum, class, and genus level. At the genus level, a number of genera of carbon degraders were enriched with the addition of leachate as were AOBs. Alternatively, NOBs were greatly reduced.
Additionally, Droplet Digital PCR analyses of nitrifying populations over time revealed that β-proteobacterial AOBs became enriched following the addition of leachate due to increased ammonia loading. It is likely that their populations decreased with decreasing temperature and increased with increasing aeration. Looking temporally at NOB populations, it became clear that leachate addition was inhibitory to Nitrospira-like NOBs that thrive under low substrate and low FA conditions and thus their populations significantly decreased in leachate-treating SBRs. In time, a population of Nitobacter-like NOBs, which dominate under high-substrate conditions, came to exist and contributed to sustained complete nitrification in the system.
The results of this work demonstrate that the issue of incomplete nitrification and nitrite buildup in the SBR during the first six months of operation with a 50/50 (v/v) wastewater and leachate influent was most likely a result of low DO concentrations in the reactor, FA at levels inhibitory to NOBs, and a resultant ineffectual NOB population.