Impact of Recirculating Nitrified Effluent on the Performance of Passive Onsite Hybrid Adsorption and Biological Treatment Systems

Approximately 25% of households in the U.S. treat their wastewater onsite using conventional onsite wastewater treatment systems (OWTS). These systems typically include a septic tank or a series of septic tanks followed by a soil absorption system. They effectively remove biochemical oxygen demand (BOD), total suspended solids (TSS), fats and grease but are not designed to remove significant amounts of nitrogen. High nitrogen loading to coastal and ground waters can be dangerous to aquatic life and public health. Hence, there is a need for advanced onsite wastewater treatment systems that can effectively remove nitrogen. Making enhanced nitrogen removal for OWTS as our primary goal, a laboratory scale Hybrid Adsorption and Biological Treatment Systems (HABiTS) was developed and upon observation of its effective nitrogen removal capacity, a pilot demonstration study with two side-by-side HABiTS, one with recirculation and one without recirculation (only forward flow) were constructed and tested at the Northwest Regional Water Reclamation Facility in Hillsborough County (Florida). HABiTS employ biological nitrogen removal and ion exchange for effective nitrogen removal. HABiTS is a two-stage process which uses nitrification for the oxidation of ammonium to nitrate and ion exchange for ammonium adsorption that helps buffer transient loading and also acts as a biofilm carrier in its stage 1 biofilter and it uses tire-sulfur hybrid adsorption denitrification (T-SHAD) in its stage 2 biofilter. These sulfur pellets help promote sulfur oxidation denitrification (SOD) and tire chips are used for nitrate adsorption during transient loading conditions, as biofilm carriers for denitrifying bacteria, and can also be used as organic carbon source to promote heterotrophic denitrification because they leach organic carbon. For this research, HABiTS without recirculation is considered as the control system and the performance of HABiTS with recirculation was tested for its ability to further enhance nitrogen removal from HABiTS. Nitrified effluent recirculation is a common strategy employed in wastewater treatment for enhanced nitrogen removal. It is the reintroduction of semi-treated wastewater to pass through an anoxic pre-treatment chamber to achieve better quality effluent. Recirculation is said to improve and consistently remove nitrogen at any hydraulic loading rate and/or nitrogen concentration. This is because of the dilution of high BOD septic tank effluent with nitrified effluent which lowers COD:TKN ratio and also improves mass transfer of substrates in the stage 1 biofilter. Recirculation also provides some pre-denitrification in the pre-treatment chamber, thereby reducing nitrogen load on the system. The HABiTS with recirculation (R) was run at 1:1 ratio of nitrified effluent recirculation rate to the influent flow rate for 50 days, and at 3:1 ratio for the remaining period of this research (200 days). The forward flow system (FF) was run under constant conditions throughout the research and comparisons between the two systems were made for different water quality parameters (pH, DO, conductivity, alkalinity, TSS, chemical oxygen demand (COD), total nitrogen (TN), total phosphorus (TP) and various nitrogen species). The final effluent ammonium results showed that the system with recirculation removed consistently \u3e 80% NH4+-N during 1:1 and 3:1 recirculation ratios whereas the forward flow system achieved 57% removal. Further, an average of 81% total inorganic nitrogen (TIN) removal from the system influent was seen in the recirculation system’s final effluent when compared to an average of 55% in forward flow system’s final effluent. This research explains in detail, the impact of nitrified effluent recirculation on enhanced nitrogen removal in onsite systems and the results presented in this thesis proved that nitrified effluent recirculation provides promising enhanced nitrogen removal in an onsite wastewater treatment system