Elsevier

Desalination

Volume 269, Issues 1–3, 15 March 2011, Pages 190-197
Desalination

Evaluation of biomass activity and wastewater characterization in a UCT-MBR pilot plant by means of respirometric techniques

https://doi.org/10.1016/j.desal.2010.10.061Get rights and content

Abstract

Over the last two decades, Membrane Bioreactors (MBRs) emerged even more for wastewater treatment, ensuring high removal efficiencies as well as very small footprint requirements. Indeed, in this kind of process, a modification in biomass activity and viability can exist compared to that of a CAS process. In this context, respirometric analysis represents a reliable tool in order to evaluate the actual biomass kinetic parameters, to insert in mathematical models in the design phase, as well as to monitor the biomass viability, especially when these processes are operated with high SRT values. The paper presents some results of respirometric techniques applied for the characterisation of wastewater and biomass activity in a pilot UCT-MBR plant for nutrient removal, operating with high SRT. In particular, the respirometric tests were specifically aimed at investigating heterotrophic and autotrophic bacterial activity. The pilot plant was built at Palermo WWTP and consisted of three reactors: anaerobic, anoxic and aerobic, followed by an aerobic compartment containing two submerged hollow fibre membrane modules with typical recycling lines.

The kinetic parameters for heterotrophic bacteria resulted lower respect to the CAS; regarding the nitrifying bacteria, the kinetic constants were in the range of CAS, suggesting a good nitrification activity.

Research Highlights

►Operational conditions and SRT strongly influence the biomass activity. ►Decrease of heterotrophic activity when operating with complete sludge retention. ►Good development of nitrifying bacteria to the detriment of heterotrophic one. ►Respirometry is a suitable tools for UCT-MBR biomass characterisation.

Introduction

Nowadays, due to the increasing awareness about environmental impact of pollutant discharges, the effluent standards are becoming more and more stringent, leading to increased requirements for Wastewater Treatment Plant (WWTP). In this context, the technical and scientific community in the last years showed a growing interest in developing innovative technologies that, together with very high removal efficiencies, can lead to a very low space and volume request. A possible solution to cope with such issues is represented by membrane bioreactors (MBRs), which are combined systems including a bioreactor and a filtration unit (usually an ultrafiltration or microfiltration membrane). More specifically, such systems compared to the traditional ones, like the conventional activated sludge (CAS) processes, which require large aeration and settling tanks, have shown higher efficiency in terms of effluent concentrations as well as smaller footprint and sludge production, due to higher biomass concentration in the bioreactor [1]. Further, MBRs enable high treatment levels in terms of effluent total suspended solid (TSS) concentrations, organic matter and total nitrogen (TN). More in detail, the very high biomass concentration, compared to the available food in the system, contributes to create an environment where bacteria are facing starvation condition so they are not in a physiological state for cell growth [2]. On the other hand, the high sludge retention times (SRTs), typical of MBR system, are highly advantageous for the growth of nitrifying bacteria [1]. Concerning nutrient removal, Ramphao et al. (2005) discussed about the influence of the MBR solid separation on the design of biological nutrient removal (BNR) systems which result smaller than the equivalent conventional systems [3]. On the other hand, Durante et al. and Parco et al. [4], [5] have focused the attention on the influence of the size of the MBR flocs on substrates diffusive transport. However, up to now, the knowledge concerning the influence that high solid concentrations in the mixed liquor as well as the nature of the selected biomass have on the BNR process is controversial and still limited. Indeed, in such systems, a modification in the biomass biokinetic behaviour can arise, compared to that of a CAS process. In this context, respirometric techniques [6] should represent a useful tool for the characterization of the biokinetic behaviour of bacteria in a MBR process, to insert in mathematical models in the design phase, as well as to monitor the biomass viability [7], [8]. Indeed, oxygen uptake rate (OUR), i.e. the oxygen consumption per unit volume per unit time, is widely recognized as an important parameter in order to monitor the biomass viability [9]. On the other hand, in the case of domestic wastewaters with contributions of industrial wastewater, and in a process operated with high SRTs, it might be of paramount interest the respirometric procedures in order to better characterise wastewater and biomass viability.

Bearing in mind such considerations, the paper presents some results of a study aimed at evaluating the heterotrophic and autotrophic kinetic parameters, in terms of organic and nitrogen removal, as well as wastewater chemical oxygen demand (COD) fractionation in a UCT-MBR pilot plant using respirometric techniques.

Section snippets

Pilot plant and operating conditions description

The experimental investigations were carried out on a MBR pilot plant, conceived for nutrient removal, built at the Acqua dei Corsari (Palermo) WWTP. In Fig. 1, the pilot plant layout is reported.

The pilot plant scheme was an adaptation of the UCT process [10] with a final membrane filtration unit instead of the conventional secondary clarifier. It consisted of three reactors in series, anaerobic (mean volume 71.6 l), anoxic (mean volume 164.9 l) and aerobic (mean volume 327 l) respectively,

Pilot plant performances

As previously discussed, during the investigated period the average MLSS concentrations for the pilot-scale MBR ranged from 3 to 6.5 g/l and the average percentage of volatile fraction was almost equal to 70%. The observed yield coefficient (Yobs) was near to 0.07 gVSSgCODox−1, after steady-state conditions were reached (day 76th), in good agreement with results obtained in previous experimental studies on MBR plants [16].

During the experimental period, the pilot plant reduced the average

Conclusions

An experimental gathering campaign on a UCT-MBR pilot plant, conceived for biological nutrient removal, was performed. One of the main aims of the study was the evaluation of the kinetic parameters of both heterotrophic and autotrophic biomass, as well as the COD fractionation, with the aid of respirometric batch tests. The experimental observations highlighted a decrease in the kinetic parameters for heterotrophic bacteria compared to that of a CAS system, even if they increased when the pilot

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