Elsevier

Chemical Engineering Journal

Volume 311, 1 March 2017, Pages 63-71
Chemical Engineering Journal

Granular biomass floatation: A simple kinetic/stoichiometric explanation

https://doi.org/10.1016/j.cej.2016.11.075Get rights and content

Highlights

  • Reasons of anammox, denitrifying and anaerobic granules floatation are examined.

  • Substrates concentration and granules size are the main parameters to be controlled.

  • Data of literature fit into the zones with risk of floatation defined in this work.

  • Operating temperature could be used to control floatation episodes.

Abstract

Floatation events are commonly observed in anammox, denitrifying and anaerobic granular systems mostly subjected to overloading conditions. Although several operational strategies have been proposed to avoid floatation of granular biomass, until now, there is no consensus about the conditions responsible for this phenomenon. In the present study, a simple explanation based on kinetic and stoichiometric principles defining the aforementioned processes is provided. The operational zones corresponding to evaluated parameters where risk of floatation exists are defined as a function of substrate concentration in the bulk liquid and the radius of the granule. Moreover, the possible control of biomass floatation by changing the operating temperature was analyzed. Defined operational zones and profiles fit data reported in literature for granular biomass floatation events. From the study the most influencing parameter on floatation occurrence has been identified as the substrate concentration in the bulk media.

Introduction

Biomass concentration is known to be the main factor limiting the treating capacity of biological systems. Therefore, extensive effort has been made on developing technologies with good biomass retention capacity to accumulate large biomass amounts inside the reactors. Among these technologies, those based on granular biomass are widely used. Advantages come from the fact that the granulation process is relatively easily achieved by controlling the operational conditions inside the reactor and that the investment costs are very low compared to alternatives such as the use of membranes or carrier materials [1], [2], [3]. Granulation phenomenon was firstly observed in anaerobic digesters and it was further implemented through the Upflow Anaerobic Sludge Bed (UASB) reactor concept [1], which allowed spreading the anaerobic treatment of industrial effluents. In 1997, granular biomass obtained in aerobic conditions was reported for the first time and, nowadays, the developed technology is applied at full scale successfully [4]. Furthermore granular biomass is also used in several existing technologies based on the anammox process and applied nowadays at full scale [5]. Nevertheless, in the case of denitrifying systems, there are not reported applications at full scale although the formation of granular biomass is also feasible [6], [7].

Taking into account that reached biomass concentrations are larger in systems with granular than suspended sludge, the former are able to treat higher loading rates. In these systems if overloading conditions are imposed bubbles of produced gas (nitrogen and methane), which remain trapped inside the granules, can cause their floatation and the consequent biomass wash-out. This behavior is even more detrimental if the process taking place is inhibited by substrate which causes a loss of removal efficiency. Then, the accumulation of substrate increases causing a snowballing effect until the system totally loses its efficiency [8]. According to the observations of different authors, floatation happens when the volume of gas bubbles represents the 6–11% of the total granule volume [9], [10], [11]. Moreover, due to the low diameter of the pores inside the granules the compression pressure, caused by a biogas bubble, has been found to possibly deform and even break apart the granule [9], [12].

Floatation of granules was observed to occur in laboratory scale reactors where either anaerobic, denitrifying or anammox processes took place [13], [14], [15]. However, until now the key factors causing floatation of granular sludge remain unclear since most of the research works are focused on proposing methods to avoid floatation instead of defining those operational conditions which provoke it.

Several strategies have been proposed to avoid this phenomenon. Some of them are: a) to reduce the granules size [9], [16], [17]; b) to increase the shear stress conditions [11], [18]; c) to increase the effluent recycling ratio [19], [20]; d) to increase the granules density by promoting the formation of inner precipitates [16], [17], [21], [22] to a certain extent because an excess of precipitates formation could also decrease the biomass activity [9], [23]; e) to add nutrients like phosphate [13]; f) and to decrease biomass activity by decreasing operational temperature [17]. To sum up, there is not a well-defined strategy to mitigate floatation episodes.

Furthermore, extensive research has been focused on finding the causes for floatation. In the literature some hypotheses have been provided in order to explain granules flotation such as: a) change of bacteria populations distribution throughout the granule [19]; b) change of bacteria populations [20]; c) excess growth of filamentous bacteria [24]; d) or obstruction of gas channels inside the granules by exopolymeric substances (EPS) secreted by bacteria and accumulation of biogas [9].

Again a conclusive reason is not available and for this cause, in the present study, floatation of granular biomass is explained based on kinetic/stoichiometry of the involved processes. From this analysis appropriated ranges for the values of parameters like substrate concentration and granule diameter, which avoid floatation, together with the temperature effects are supplied.

Section snippets

Stoichiometric and kinetic calculations

To perform the study some initial assumptions have been made. The granules are considered spheres. The biological reactors, which contain the studied granular biomass, are considered that operate at complete mixture conditions. The external mass transfer resistance was considered negligible and, therefore, the substrate concentration on the granule surface corresponds to that present in the bulk liquid. From a physical point of view, gas pockets are formed inside granules when the concentration

Effect of substrate concentration in the bulk liquid and granule size on granules floatation

Initially, the minimum substrate concentration value exceeding the gas solubility at 30 °C was obtained from the profiles of gas production inside the granules (Fig. 1A, C and E). For this purpose, previously described calculations were performed for substrate concentrations in the following ranges: 5–25 mg NO2-N/L for anammox granules, 5–50 mg NO3-N/L for denitrifying granules and 25–200 mg COD/L for methanogenic granules. Results indicated that gas desorption will occur inside an anammox,

Conclusions

  • Substrates concentration and granules size are the main parameters to be controlled during the operation of anammox, denitrifying and anaerobic systems in order to avoid floatation of biomass and its concomitant washout.

  • To prevent floatation episodes, the loading rate applied to granular systems should not exceed their removal capacity in order to maintain the substrates concentrations lower than 16 mg NO2-N/L, 25 mg NO3-N/L and 130 mg COD/L (as acetate) for anammox, denitrifying and anaerobic

Acknowledgements

This work was funded by the Chilean Government through the project FONDECYT 1150285 and CONICYT/FONDAP/15130015 and by the Spanish Government through FISHPOL (CTQ2014-55021-R) and GRANDSEA (CTM2014-55397-JIN) projects. The authors from the USC belong to CRETUS (AGRUP2015/02) and the Galician Competitive Research Group (GRC 2013-032). All these programs are co-funded by FEDER. The authors also want to thank Professor Juan Lema, from Universidade de Santiago de Compostela, for his invaluable

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