Optimizing dewaterability of drinking water treatment sludge by ultrasound treatment: Correlations to sludge physicochemical properties

Highlights

• The relationship between dewaterability and solubilized organics and sludge flocs characteristics was established.

• Revealing the ultrasound mechanisms by analyzing cavitation field distribution.

• At higher energy densities of 3.0 and 5.0 W/mL, dewaterability deteriorated regardless of ultra-sonication time.

• The deterioration of the dewatering capacity was closely related to changes in floc size, proteins and polysaccharides, and Zeta potential of sonicated flocs.

Abstract

Sludge dewatering has proven to be an effective method to reduce the volume of sludge. In this study, drinking water treatment sludge (DWTS) was treated by ultra-sonication under variable conditions comparing two sonoreactor types (bath and probe), four frequencies (25, 40, 68, 160 kHz) and four energy density levels (0.03, 1, 3, 5 W/mL). The effects of these conditions were studied using specific resistance to filtration and capillary suction time as measures of dewaterability, and floc size, the Brunauer, Emmett and Teller (BET) specific surface area and Zeta potential to determine treated sludge characteristics. The results indicated that the dewaterability of sonicated sludge improved at relatively low energy densities of 0.03 and 1.0 W/mL, while an optimum for sonication duration (within 10 min) was also identified. Higher frequencies (tested up to 160 kHz) with acoustic energy density of 0.03 W/mL also reduced the dewatering property. At higher energy densities of 3.0 and 5.0 W/mL, dewaterability of sludge deteriorated regardless of ultra-sonication time, with an increase of solubilized organic matter content and severely changed floc characteristics. The deterioration of the dewatering capacity was closely related to the considerably reduced floc sizes, dissolution of proteins and polysaccharides, and to the Zeta potential of sonicated sludge flocs. The dewaterability was not correlated with BET specific surface area. Mechanistic explanations for the observations were discussed by analyzing corrosion patterns of aluminum foil as a measure for cavitation field distribution.

Introduction

A typical byproduct resulting from the production of drinking water by use of aluminum or iron-based salts as coagulants is sludge that collects in the primary settling tanks and backwash drainage of filters [1]. Disposal of this drinking water treatment sludge (DWTS) depends on local conditions and customs, and may include reuse as building materials, burial into soil, application to farmland, sanitary landfills or ocean disposal [2], [3]. The most common disposal strategy in China is burying DWTS in deserted soils and sanitary landfills. However, prior to disposal, efficient dewatering of the sludge is of paramount importance as a reduced volume and weight results in lower management, transport and disposal costs. In addition to reducing waste disposal, the reuse of DWTS could contribute to improving coagulation and absorption process in terms of the removal of particulates [4], phosphorus [5], arsenate [6], boron [7], and heavy metals, i.e., copper (II) and lead (II) [8]. However, the enhanced capacity for pollutants removal associated with DWTS recycling process is undesirable due to the organic solubilization [9]. An effective pretreatment should be considered, aiming to degrading or inactivating the constituents in sludge, and to facilitating the solid-liquid separation of sonicated sludge flocs.

Specific treatments of waste-activated sludge (WAS) that facilitate the dewatering process include the addition of polymers and oxidants, thermal conditioning, and ultra-sonication individually or the combinations of thermo-chemo-sonic methods [10], [11], [12], [13], [14], [15], [16], [17], [18]. Of these, we consider ultra-sonication as an environmental friendly and economically viable method. Ultra-sonication performs a series of compression and rarefaction cycles, leading to the generation of millions of cavitation bubbles. Spontaneous implosion of these bubbles results in extreme micro-localized conditions, with temperatures reaching up to 5000˚C, pressures of up to 100 MPa, and the generation of free radicals such as OH, HO₂ and O [19]. Low frequency is widely used for WAS disintegration, whereas high frequency is applied for the decontamination of water and wastewater through sonochemical reactions [19], [20], [21].

Most information on ultra-sonication was available from treatment of WAS, with both positive and negative effects on dewatering efficiency being reported. In a number of studies ultra-sonication enhanced the dewaterability of WAS as evidenced by a decrease in capillary suction time (CST) and in the specific resistance to filtration (SRF) [22], [23]. In contrast, other studies described that WAS dewaterability was negatively affected by ultra-sonication, whereby CST and SRF increased in a time-dependent manner [24], [25]. WAS filterability was further affected by factors such as the concentration and properties of extracellular polymeric substance (EPS), floc size, the surface charge of flocs, and viscosity [18], [26], [27]. A high concentration of EPS generally leads to low WAS filterability, but the effect depends on the nature of the EPS and stratification structure of sludge (soluble, loosely bound or tightly bound EPS, polysaccharide vs. protein, etc.); soluble EPS most strongly decreases the filterability of WAS [12], [26], [27]. The abundance of floc size as supra- and true colloidal particles (smaller than 10 μm) correlates with a decreased WAS filterability. An increased negative surface charge of flocs also decreases the filterability, by enhancing the dispersivity of WAS suspension [28]. Thus, the increased stability of colloidal particles results in less aggregation and subsequently increases the packing density of flocs.

Knowledge on the effect of ultra-sonication on the dewaterability of DWTS is limited. There is large quantities of inorganic compounds in DWTS including sediment mainly hydroxides due to use of aluminum or iron-based coagulants, and other mineralizer origins i.e., wollastonite and silicon dioxide. The organic compounds in DWTS are attributed to the presence of algae and bacteria, of which the endogenous organics in the DWTS are humus-like derived from the decay of plants, and a small quantity of EPS are mainly attributed to the presence of pathogenic microorganisms (Escherichia coli, Enterococcus faecalis, etc) greatly differing from that in WAS predominated by nitrifying bacteria, denitrifying bacteria and polyphosphate-accumulating bacteria, etc. Our preliminary study demonstrated that the content of soluble chemical oxygen demand (sCOD), polysaccharide and protein in DWTS is very low, and that the number of microorganisms present in raw DWTS samples was four orders of magnitude lower than what is typically found in WAS [29]. Given the differences in physicochemical and biological properties of DWTS and WAS, ultrasound treatment will have different effects, ultimately affecting the sludge dewaterability in unexpected ways. Furthermore, DWTS can largely vary in terms of physical, chemical and biological characteristics, leading to a relatively unpredictable dewatering behavior.

To the best of our knowledge, there are few literatures throughout the world on the dewaterability of DWTS using ultra-sonication individually, and the relationship of solubilized organics from DWTS and/or sludge surface characteristics with dewaterability has not been well constructed. Therefore, the objectives of the present study were (i) to assess the dewatering performance of DWTS, as expressed as SRF and CST following different sonication conditions; for this we compared two sonoreactor types and varied frequency and energy density; (ii) to clarify the cause of dewaterability variation by establishing the relationship between dewaterability and solubilized organics and DWTS flocs surface characteristics; and (iii) to reveal the involved ultrasound mechanisms by analyzing corrosion patterns of aluminum foil as a measure for cavitation field distribution.

The results presented here can provide a theoretical basis for the practical application of ultra-sonication for DWTS dewatering, and can support implications for an effective dewatering strategies of DWTS when using ultra-sonication alone or combinations of ultra-sonication with polymers, acids, alkalis or oxidants.