The Hybrid Membrane Biofilm Process: A Novel System for Achieving Nitrogen Removal from Wastewater

This research examines the hybrid membrane-biofilm process (HMBP), a novel means to achieve total nitrogen removal from wastewater. The HMBP incorporates membrane aeration into an activated sludge basin. Nitrifying bacteria attach to the membranes, and heterotrophs are maintained as suspended growth. Oxygen transferred to the biofilm by the aeration membranes is consumed by the nitrifying bacteria, which oxidize ammonia to nitrate and nitrite. The nitrate and nitrite produced by the biofilm is reduced in the bulk liquid by suspended heterotrophs using influent BOD. This system allows for nitrification and denitrification in a single tank, reduces the required bulk liquid sludge retention time by maintaining the slow-growing nitrifiers in attachment, and utilizes almost all influent BOD for denitrification, minimizing the need for carbon augmentation for denitrification. Bench scale studies indicated that the HMBP was effective in removing total nitrogen. Nitrification in the HMBP was sensitive to the bulk liquid BOD concentration, with BOD concentration as low as 0.5 g m-3 impacting nitrification rates. However, if sufficient solids were maintained in the system to achieve a low bulk liquid BOD concentration, the HMBP was effective at BOD surface loadings of up to 17 gBOD m-2 day-1 at a BOD:N loading ratio of 12.5:1. This ability to treat high organic loading rates while maintaining high nitrification rates is a great improvement over past membrane aeration processes. Shortcut nitrogen removal (i.e. the reduction of nitrite rather than nitrate) was confirmed using microsensors and fluorescence in situ hybridization (FISH). Low oxygen concentrations in the were believed to cause the shortcut nitrogen removal. Shortcut nitrification was further explored in an isolated membrane aerated biofilm (MAB) reactor. Shortcut nitrification was quantified in a MAB as a function of membrane and bulk DO concentrations. The intra-membrane pressure was shown to effectively control the oxygen concentrations in a MAB. It appears that NOB are not able to effectively compete with AOB for oxygen in the biofilm when the DO was below 2 g m-3. A tradeoff was found between obtaining high ammonium oxidation fluxes at high membrane DO concentrations and obtaining high nitrite accumulation at low membrane DO concentrations. A combination of experimental and modeling results were used to assess the impact of bulk liquid BOD concentration on nitrification in a MAB. In the experimental MAB, bulk liquid BOD, when supplied as acetate, inhibited nitrification at concentrations as low as 1 g m-3. FISH results indicated heterotrophs had an increasing presence in the biofilm with increasing bulk liquid BOD concentrations, which would lead to the decreased nitrification rates. A distinct heterotrophic layer formed in the outer biofilm when the bulk liquid BOD concentration was above 3 g m-3. Modeling of varying BOD sources indicated that the type of BOD has a large impact on the concentration that impacts nitrification in a MAB. Nitrification was not impacted until the bulk liquid BOD concentration reached 3 g m-3 for a more slowly degradable BOD. Typical effluent BOD5 from nitrifying activated sludge plants are in the 3-5 g m-3 range. The majority of this BOD5 is more than likely recalcitrant BOD, therefore the nitrification rate in a system such as the HMBP treating a real wastewater would not be drastically impacted by these concentrations. The selection of membrane materials was shown to significantly impact the effectiveness of a membrane aerated treatment systems. Gas transfer rates and biofilm attachment efficiency both contributed to the nitrification rates in membrane aerated bioreactors (MABRs) constructed with polyethylene, cellulose tri-acetate, and polyester membrane materials. Biofilm adhesion studies indicated that the polyethylene membrane material produced the highest coverage after 28 days, and also the greatest biofilm adhesion rate. The polyethylene material also had the highest oxygen transfer rate, which was more than twice as high as the other membranes tested. The polyethylene MABR also exhibited the highest nitrification rate, which was more than 4 times as high as the MABRs constructed of cellulose tri-acetate and polyester. Pilot scale studies were used to assess the applicability of the HMBP to a real wastewater. The average nitrification rate at the pilot scale varied with nitrogen loading, but the maximum achieved rate was approximately 0.4 gN m-2 day-1. This is significantly lower than the 1 gN m-2 day-1 rate found at the bench-scale. Although nitrification rates were not as high as expected, denitrification and TN removal efficiencies were very favorable. Typically, greater than 75% of the nitrified ammonia was denitrified, and effluent TN concentrations below 6 gN m-3 were achieved with an average influent sCOD concentration of only 68 g m-3. The majority of sCOD consumed was shown to be utilized for denitrification. Although little nitrite was measured in the bulk liquid, nitrite may be preferentially reduced by denitrifying bacteria, and the high levels of denitrification with low influent sCOD would suggest that some nitrite reduction is occurring. Overall, the performance and function studies confirm that the HMBP is a promising technology. Nitrification and denitrification can be achieved in a single tank, with a short bSRT, limiting the process footprint. By incorporating nitrification and denitrification into a single tank, the use of influent BOD for denitrification is maximized, minimizing the need for carbon augmentation. The HMBP was also shown to achieve shortcut nitrogen removal through oxygen limitation of nitrite oxidation, decreasing both the oxygen demand for nitrification and the carbon requirement for denitrification. The energy efficient gas transfer associated with membrane aeration, combined with minimal oxygen supply for sCOD removal and decreased oxygen demand for shortcut nitrification, provide an excellent opportunity for increased energy efficiency in the aeration tank.