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Role of external carbon and metal salt dosing in membrane bioreactor system to achieve limits of technology nutrient removal from municipal wastewater

Authors
Publisher
University of British Columbia
Publication Date
Disciplines
  • Biology
  • Chemistry

Abstract

Membrane bioreactor (MBR) technology in conjunction with conventional biological nutrient removal has been demonstrated to be successful in recent years. However, the limits of technology (LoT) effluent goal, ≤ 3 mg TN/L (total nitrogen) and ≤ 0.1 mg TP/L (total phosphorus), could potentially push a system to the limits of its capability. The broad objective of the long-term PhD study was to investigate role of external dosing of alum in a membrane biological nutrient removal (MBNR) system targeting LoT effluent nutrient levels. Two parallel MBNR systems, modified Bardenpho configuration, were operated under similar process conditions with metal salt addition being the only difference. The continuous flow MBNR system performance data signified the importance of external methanol and alum dosing in accomplishing the LoT nutrient removal goal. The stoichiometric methanol ratio, i.e. mg methanol required / mg NO₃-N removed, was calculated to be 6.1 in reducing average permeate NO₃-N concentration to 1.4 mg/L. Similarly, an average molar Al/TP ratio of 1.9 was required to reduce PO₄-P concentration to 0.07 mg/L in the permeate. Chemical phosphorus removal did not have any influence on COD removal, nitrification (except for a brief period) and denitrification. The relationship between chemical P removal and enhanced biological phosphorus removal (EBPR) was dynamic and was dependent on alum dosage concentration. At high dosage levels (i.e. 80 mg/L), alum supplementation competed with and finally, inhibited EBPR until the MBNR system was converted to a chemical P removal system. Activated sludge modeling was undertaken to analyze its suitability in predicting the performance of an MBNR system targeting LoT goals. The model was successful in predicting nitrogen removal, while parameter calibration was required for fitting of the measured suspended solids and EBPR data. Moreover, the model could not predict the relationship between the simultaneous biological and chemical P removal accurately. A direct batch DON measurement method, batch anion exchange resin adsorption followed by persulfate digestion, was developed and validated successfully. Using the method, the DON contribution to permeate total nitrogen was observed to vary from 7 percent to 96 percent in the parallel MBNR systems, when permeate TN concentrations were less than 3 mg/L.

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