Feasibility of hydraulic separation in a novel anaerobic-anoxic upflow reactor for biological nutrient removal.

Rubén Díez-Montero,Marta González-Viar,Iñaki Tejero,Loredana De Florio,Eveline I P Volcke

doi:10.1007/s00449-014-1247-9

Rubén Díez-Montero, Marta González-Viar + Show 3 more

Open Access

https://doi.org/10.1007/s00449-014-1247-9

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Abstract

This contribution deals with a novel anaerobic-anoxic reactor for biological nutrient removal (BNR) from wastewater, termed AnoxAn. In the AnoxAn reactor, the anaerobic and anoxic zones for phosphate removal and denitrification are integrated in a single continuous upflow sludge blanket reactor, aiming at high compactness and efficiency. Its application is envisaged in those cases where retrofitting of existing wastewater treatment plants for BNR, or the construction of new ones, is limited by the available surface area. The environmental conditions are vertically divided up inside the reactor with the anaerobic zone at the bottom and the anoxic zone above. The capability of the AnoxAn configuration to establish two hydraulically separated zones inside the single reactor was assessed by means of hydraulic characterization experiments and model simulations. Residence time distribution (RTD) experiments in clean water were performed in a bench-scale (48.4 L) AnoxAn prototype. The required hydraulic separation between the anaerobic and anoxic zones, as well as adequate mixing in the individual zones, were obtained through selected mixing devices. The observed behaviour was described by a hydraulic model consisting of continuous stirred tank reactors and plug-flow reactors. The impact of the denitrification process in the anoxic zone on the hydraulic separation was subsequently evaluated through model simulations. The desired hydraulic behaviour proved feasible, involving little mixing between the anaerobic and anoxic zones (mixing flowrate 40.2 % of influent flowrate) and negligible nitrate concentration in the anaerobic zone (less than 0.1 mgN L(-1)) when denitrification was considered.

Highlights

The presence of the nutrient elements nitrogen and phosphorus in wastewater discharged into water bodies is the major cause of eutrophication
RTD1 shows a significant delay in the peak, which is attributed to slow mixing
The delay of approximately 4 minutes in the sharp peak of RTD2 compared to the theoretical continuous stirred tank reactors (CSTR) profile can be explained by the fact that the internal recycle is pumped from the bottom to the top of the anaerobic zone, producing a countercurrent downflow and in this way slightly delaying the arrival of the tracer in the outlet

Summary

Introduction

The presence of the nutrient elements nitrogen and phosphorus in wastewater discharged into water bodies is the major cause of eutrophication. Conventional configurations for biological nutrient removal (BNR) require anaerobic and anoxic compartments, besides aerobic ones which are sufficiently large to establish nitrification, which results in a significant volume increase compared to the one needed for organic matter removal only. The larger footprint needed for the retrofitting of existing wastewater treatment plants (WWTP) to achieve BNR is often not available. For BNR, separate anoxic and anaerobic conditions are required. Phosphate is released through the phosphate accumulating organisms (PAO) metabolism, which can only take place under strict nitrate absence. Nitrate serves as an electron acceptor allowing organic matter consumption for denitrification. The accumulation of phosphate by PAO takes place in excess of metabolic requirements, under aerobic conditions. Phosphate uptake is feasible using nitrate as sole electron acceptor, instead of oxygen [1], which leads to energy savings for aeration, less sludge production and maximal influent organic substrate exploitation [2]

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