The research in this thesis was conducted at the Hampton Road Sanitation District's biological nutrient removal pilot, located at the Chesapeake-Elizabeth WWTP in Virginia Beach, VA. The pilot is operated in an A/B process with a high-rate, carbon-diverting A-stage, followed by a biological nitrogen removal B-stage containing four intermittently aerated CSTRs, followed by an anammox polishing MBBR. The goal of this research was to successfully combine short-cut nitrogen removal with sidestream enhanced biological nutrient removal (EBPR) in the most efficient way possible, specifically aiming to decrease cost and energy requirements, divert the most amount of carbon possible before B-stage, and to achieve low effluent nitrogen and phosphorus concentrations. A RAS fermenter (SBPR) and an A-stage WAS fermenter that feeds VFA into the SBPR (the supernatant of the fermenter is called fermentate) were implemented in order to enhance biological phosphorus removal. About 8 months after the RAS and WAS fermenter implementation, there was a 28 day consecutive period of low B-stage effluent OP <1 mg/L, with an average of 0.5 ± 0.1 mg/L OP. Following this low effluent OP period, bio-P became more unstable and there was high nitrite accumulation in the B-stage effluent for 106 days with concentrations ranging from 1.1-5.9 mg/L NO2. The nitrite accumulation was not due to NOB out-selection, confirmed by AOB and NOB maximum activity tests. It was determined that the nitrite accumulation was due to partial denitrification of nitrate to nitrite by bacteria using internally stored carbon, because profiles and activity tests showed anoxic nitrite accumulation at the end of the aerobic process. Post-anoxic denitrification using internally stored carbon compounds has been observed in other EBPR systems (Vocks, Adam, Lesjean, Gnirss, and Kraume, 2005). Fermentate addition was then halted, and nitrite accumulation and bio-P activity ceased all together, linking the fermentate addition to both bio-P activity and nitrite accumulation. Fermentate was then controlled to dose at 60% of the sCOD/OP (fermentate sCOD g/day / total OP- fermentate + influent - g/day) of the first low effluent OP period. During this fermentate dosing period where the average sCOD/OP was 15.6 ± 3.0 g/g, no nitrite accumulation was observed, but another consecutive low effluent OP period was observed with an average of 0.6 ± 0.2 mg/L OP. Linear correlation analysis shows that the highest r2 values relating the low effluent OP periods and the COD loads to the SBPR for both periods were between VFA g/day vs OP effluent mg/L, at r2=0.18 for the first low effluent OP period and r2=0.65 for the second. There were also high tCOD r2 values for the second low effluent OP period showing that COD hydrolysis in the SBPR could have impacted bio-P activity. However, the VFA r2 value was higher than any tCOD r2 value, concluding that the fermentate dosing mainly worked to enhance biological phosphorus removal by increasing the VFA load in g VFA as acetate/day. Since no nitrite was observed in a period with a lower VFA/OP dose, then the probable VFA load needed to provide enough internal storage to produce nitrite accumulation by partial denitrification is between 5-9 (g VFA as acetate/ g total OP). If sidestream EBPR systems could be studied further to promote nitrite accumulation and bio-P activity to produce low effluent OP, then short-cut nitrogen removal and EBPR could be successfully combined in an efficient way. / Master of Science / It is important to reduce nitrogen and phosphorus concentrations in wastewater treatment effluent in order to both protect the environment from eutrophication and to meet the increasingly stringent nutrient effluent discharge limits imposed by the EPA. Conventional biological nitrogen removal is achieved through nitrification and denitrification converting ammonia to nitrogen gas, where nitrogen gas is volatile and leaves the system naturally. Phosphorus removal can be achieved through either chemical addition or through biological phosphorus removal, where phosphorus is taken up in cells and removed from the system by the subsequent solids wasting of these cells. The combination of biological nitrogen and phosphorus removal can be improved to increase energy efficiency, reduce costs including aeration and chemical addition costs, increase system capacity and reduce tank sizes, and reduce biomass production, all while achieving low effluent N and P concentrations. Short-cut nitrogen removal can increase the efficiency of biological nitrogen removal. Deammonification, the combination of partial nitritation and anammox, has the potential to reduce wastewater treatment plant (WWTP) aeration costs by 63%, carbon requirements by 100%, and biomass production by 80% (Nifong, Nelson, Johnson, and B. Bott, 2013). Deammonification is the combination of partial nitritation and anammox. Anaerobic ammonia oxidation (anammox) is a useful class of bacteria that converts ammonia and nitrite straight to nitrogen gas in anaerobic conditions, which is a more direct pathway than the conventional nitrification-denitrification pathway. Anammox requires a nitrite supply, which can supplied by partial nitratation of ammonia to nitrite, performed by ammonia oxidizing bacteria (AOB) aerobically in the deammonification process. In order for partial nitratation to work, there needs to be nitrite oxidizing bacteria (NOB) out-selection so that the nitrite produced by AOB does not get oxidized to nitrate. Enhanced biological phosphorus removal (EBPR) is accomplished by the taking up and storing of orthophosphate (OP) by phosphorus accumulating organisms (PAOs). These organisms require an anaerobic carbon-storage phase followed by an aerobic growth phase where the internally stored carbon is used for growth. During the cell growth phase of PAOs in aerobic conditions, PAOs are able to take up more OP than they previously released in anaerobic conditions, creating a net OP removal from the system. There has been recent success in recycle activated sludge (McIlroy et al.) fermentation to enhance biological phosphorus removal, which works to promote hydrolysis, fermentation, and EBPR enhancement (Houweling, Dold, and Barnard, 2010). A portion of the RAS is introduced to an anaerobic zone before returning to the main process, allowing for extra VFA production and adsorption by PAOs. RAS fermentation solves the issue of carbon needed for EBPR in VFA/carbon limited systems without having to add too much additional carbon, creating a carbon efficient EBPR system. The research outlined in this study was done at the Hampton Road Sanitation District's (HRSD) pilot plant located within HRSD's Chesapeake-Elizabeth WWTP in Virginia Beach VA. The pilot is run in an A/B process that works in two separate steps: the A-stage is the first step that works to remove carbon by oxidation, and by adsorption so it can potentially be diverted, and the B-stage is the second step where biological nitrogen removal (BNR) is done. The BNR phase consists of an anaerobic selector followed by four completely stirred tank reactors (CSTRs) that are intermittently aerated to provide aerobic and anoxic phases. The pilot also has an anammox polishing step following B-stage. The nitrogen removal goal for this research was short-cut nitrogen removal via deammonification, by producing partial nitritation in B-stage and polishing with anammox. A B-stage RAS fermenter, along with an A-stage waste activated sludge (WAS) fermenter that feeds VFA into the RAS fermenter, was implemented to the existing pilot to enhance biological phosphorus removal. The overall goal of this study was to successfully combine short-cut nitrogen removal with sidestream EBPR to achieve low effluent N and P concentrations in the most energy and carbon efficient way possible. EBPR was achieved about eight months after the implementation of the RAS and WAS fermenter to the pilot. A period of B-stage effluent OP that was consistently below 1 mg/L OP was observed right before an unexpected period of high nitrite in the B-stage effluent. The high effluent nitrite lasted for 106 days and ranged from 1.1-5.9 mg/L of effluent nitrite during this time. The nitrite accumulation was unexpected because weekly maximum activity tests for AOB and NOB showed that NOB out-selection was not occurring. The first phase of this research investigates the cause of the nitrite accumulation. Based on profiles taken in the reactors in the aerobic and anoxic phases, and based on denitrification activity tests, it was determined that the nitrite accumulation was due to partial denitrification of nitrate to nitrite. Because this partial denitrification was happening in the reactor anoxic times where external should have been used up, it was determined that the source of the partial denitrification was from a bacteria using internally stored carbon during anoxic periods as the electron supply for partial denitrification. Research has showed that EBPR systems promote bacteria that are capable of storing carbon internally and keeping that carbon stored through an aerobic phase and then using that stored carbon for denitrification following an aerobic phase (Vocks et al., 2005), like observed in this research. The second phase of this research sought to link the nitrite accumulation and bio-P activity to the VFA added to the RAS fermenter. The VFA addition was decreased in phases, and with that a decrease in nitrite in the effluent was observed. The bio-P activity became more unstable after the nitrite accumulation occurred, but all bio-P activity ceased after VFA addition to the RAS fermenter ceased. It was concluded, unsurprisingly, that the VFA added to the RAS fermenter was the source of the internally stored carbon that caused the nitrite accumulation, and necessary for bio-P enhancement. The third phase of this research sought to recreate the low effluent OP period and the nitrite accumulation by controlling the VFA dose to the RAS fermenter. The average soluble chemical oxygen demand (sCOD) per OP (fermenter sCOD g/day / total OP-fermenter + influent- g/day) of the period of low effluent OP was calculated, and the dose from the WAS fermenter was controlled to meet 60% of the calculated value. The calculated dose was 13.6 gC/gP, but the actual average dose from controlling the load during this period was 15.6 ± 3.0 gC/gP. The average VFA/OP (g VFA as acetate/ g total OP) dose for the first low effluent OP period was 9.4 ± 3.6 g/g, and the average dose for the third phase of research was 5.5 ± 1.3 g/g. No nitrite accumulation occurred in this phase, but another consistent low effluent OP period did occur. From linear correlation analysis, the highest r2 values relating the low effluent OP periods and the COD loads to the RAS fermenter for both periods were between VFA g/day vs OP mg/L, at r2=0.18 for the first period and r2=0.65 for the second. This shows that effluent OP < 1 mg/L can be achieve at 5.5 or 9.4 (g VFA as acetate/ g total OP). Since no nitrite was observed in phase 3, than the probable VFA load needed to provide enough internal storage to produce nitrite accumulation by partial denitrification is probably between 5.5-9.4 (g VFA as acetate/ g total OP). This research was significant because the link between nitrite accumulation and bio-P enhancement with sidestream RAS and WAS fermentation was confirmed. Partial denitrification of nitrate to nitrite could be used as an alternative source of nitrite for anammox, instead of NOB out-selection and partial nitritation of ammonia to nitrite by AOB, in combined EBPR and short-cut nitrogen removal systems. If sidestream EBPR systems could be used to promote nitrite accumulation and bio-P activity to produce low effluent OP and nitrogen removal efficiently than short-cut nitrogen removal and EBPR could be successfully combined in an efficient way. Future work needs to be done on the organism that is capable of nitrite accumulation and if that organism can be enhanced in conjunction with EBPR organisms to promote both nitrite accumulation and low effluent OP simultaneously.