Sludge disintegration using a hydrocyclone to improve biological nutrient removal and reduce excess sludge

Highlights

• A novel sludge disintegration method using a hydrocyclone is proposed.

• Hydrocyclone disintegration leads to the release of carbon source.

• Achieved disintegration led by hydrocyclone shedding, rotation and centrifugation.

• Lower energy consumption by hydrocyclone than the other disintegration methods.

Abstract

Sludge disintegration can simultaneously improve biological nutrient removal and reduce excess sludge. A novel sludge disintegration method is proposed wherein a hydrocyclone is used for internal release of carbon source. The effect of hydrocyclone disintegration on mixed liquor recirculation was studied in a side-stream device of an anoxic/aerobic (A/O) wastewater treatment plant (WWTP). The mechanisms of hydrocyclone disintegration were comprehensively investigated, and its energy consumption was compared with those of other disintegration methods. The sludge disintegration degree (DD) reached 6.66–12.25% in the hydrocyclone processed mixed liquor recirculation, leading to a significant increase in the concentration of soluble chemical oxygen demand (SCOD), protein and polysaccharide. However, the sludge size distribution and the stable structure for microorganism aggregation and cell attachment changes only slightly. The sludge disintegration process using the hydrocyclone involves sheared shedding, rotation desorption and centrifugation lysis which are mainly induced by disordered turbulence. The biological degradation experiments with continuous modes revealed significant improvements in microbial activity in disintegrated mixed liquor recirculation. Similarly, the averaged denitrification rate of disintegrated mixed liquor recirculation increased by 14.43% in batch experiments. The continuous operation indicated that the observed biomass yield and efficiency of excess sludge reduction of hydrocyclone-improved A/O process were 0.34 kg VSS/kg COD and 30.61%, respectively. Compared with other disintegration methods, hydrocyclone disintegration exhibited less intensity but lower energy consumption, which indicated that this method shows promises in advanced nitrogen removal and excess sludge reduction.

Introduction

For the past 100 years, the activated sludge process has been the most commonly used biological process in wastewater treatment plant (WWTP) because of its simplicity, high efficiency and low cost [1]. With the increasingly strict legislation on environmental protection and persistent pursuit of more economical operation, the long-standing problems of insufficient removal of organic matter and nutrients and the production of large amounts of excess sludge are clearly exposed [2], [3]. The potential avenues for improvement mainly focus on the utilization of novel environmental technology, such as the symbiotic interaction between tubificidae and microorganisms [4] and the investigation of filamentous bacteria for nutrient removal [5], the optimization of operational parameters such as DO concentration [6], and the improvement of conventional biological treatment technology [7], [8]. The typical factors that require improvement to facilitate the efficient release of the carbon source from sludge, include excess sludge and mixed liquor recirculation. An environmentally begin and economical disintegration method needed to yield total solid solubilization. Such a method would simultaneously benefit the denitrification process for total nitrogen removal and sludge digestion for reducing excess sludge [9], [10], [11], [12].

To date, several methods have been applied for sludge disintegration, including thermal treatment [13], chemical treatment using acids or alkali [14], mechanical disintegration [10], [15], [16], biological hydrolysis [17] and a combination of the above methods [18]. Organic substances can be release from the sludge due to the desorption of loosely bound extracellular polymeric substances (LB-EPS) and/or the breaking of the cellular plasma membrane [18]. Extracellular polymeric substances (EPS), which represent up to 50–90% of the total sludge [19], originate from the metabolism or cell lysis of microorganisms and are more valuable for disintegration [20]. Specifically, EPS are the construction material of bacterial settlements. They either remain attached to the cell’s or sludge floc’s outer surface or are secreted into its growth medium. The thermal, chemical or biological disintegration treatment mainly break the protein-lipid bilayer and releases the intracellular substances [21], whereas mechanical methods comprehensively disintegrate the sludge, including EPS solubilization [22]. The development of a more economical and feasible disintegration method, such as hydrocyclone disintegration, can be significant for its ability to fully release available carbon.

As conventional separation device, the hydrocyclone consists of one or more tangential inlets, a cylinder and a conical shape. These components cause not only the known centrifugal motion of mixture [23] but also the revolution of dispersed particles on its axis [24]. Therefore, particles with a porous structure, such as sludge floc, will be improved for ad/desorption in the hydrocyclone [25]. The sludge interface will be continually updated, and the mass transfer efficiency of the cell, which is embedded in sludge, will be enhanced due to the shearing force in the hydrocyclone induced by the velocity difference between flow layers. All of these factors demonstrate the advantages of hydrocyclone disintegration over other conventional mechanical disintegration methods [26].

Based on the above characteristics, a side-stream device of an anoxic/aerobic (A/O) WWTP was setup for sludge disintegration by a hydrocyclone. Such a device is typically designed for separation. Because of the close correlation between sludge microbial activity and DD, as reported by Li et al. and Schlafer et al. [27], [28], the sludge in the mixed liquor recirculation was chosen as the disintegrate target. Our new attempt of disintegrating the mixed liquor recirculation is also more convenient to the improvement of WWTP rather than excess sludge. The main objective of this study was to investigate the effects of hydrocyclone disintegration of sludge in the mixed liquor recirculation, conducted with different ΔP values on organic compound release, DD, the particle size distribution from sludge and microorganism activity. The mechanisms of hydrocyclone disintegration were explored based on the measurement and simulation of the hydrocyclone flow field. In our experiments, we also monitored the effect of hydrocyclone disintegration on the biological process and quantitatively compared the economic efficiency of operation. This latter analysis helps instruct application.