Influence of physical, chemical and dual sewage sludge conditioning methods on the dewatering efficiency

Highlights

• The assessment of physical, chemical and dual conditioning of sewage sludge

• A change of sewage sludge parameters after conditioning was observed.

• The improvement of sludge dewatering was noted.

• The dual conditioning influenced best the effectiveness of sludge dewatering.

Abstract

This paper presents the influence of different methods of sewage sludge conditioning on the effectiveness of dewatering. In the laboratory research, three conditioning methods were tested: (1) with the use of biomass ash, (2) chemical conditioning by means of polyelectrolyte and (3) dual conditioning with the application of biomass ash in conjunction with polyelectrolyte. The specific resistance to filtration, the moisture content and characteristics of filtrates were measured. The influence of sludge conditioning on capillary suction time and the initial pH was also examined.

The obtained results showed that MC and SRF decreased after sludge conditioning. Among all tested methods, the dual conditioning influenced the sludge dewaterability to the highest extent. Due to their effectiveness, physical and dual conditioning methods might be a promising alternative for the application of polyelectrolytes. The reduction of polyelectrolyte consumption has also economic benefits, which was confirmed in a previous economic analysis.

Introduction

Sewage sludge is a residue of wastewater treatment, which consists of water and solid particles [2]. Due to the development of sewerage system, the amount of generated sewage sludge is steadily growing every year [11]. In Europe, the annual production of sewage sludge is approximately 11 million tons of sewage sludge dry mass (DM) [3]. Among all European countries, most of sewage sludge is produced in Germany (approximately 1.9 million tons per year) [17]. On the other hand, Cyprus and Malta are characterized by the lowest production of wastewater sludge [25]. Excessive production of sludge is also a significant problem outside Europe. According to Qiao et al. [21], approximately 14 million tons of dewatered sewage sludge was produced in China in 2007. In Unites States, the annual production of biosolids from stabilized sewage sludge is at the level of 8 million tons of dry mass [20].

Due to specific characteristics, especially the content of heavy metals, organic compounds and microorganisms, sewage sludge should be properly processed. The inappropriate sludge treatment might result in secondary pollution [9]. The typical sewage sludge treatment in wastewater treatment plants usually includes: thickening, stabilization, conditioning, dewatering and utilization [10]. Because sewage sludge is not ‘typical’ waste, the selection of efficient treatment requires individual approach [19].

Among all the processes of sewage sludge treatment, the most important, as well as the most expensive one, is dewatering. According to Wang and Wang [30], the annual cost of sludge dewatering in the USA is approximately 5 billion dollars. Sludge dewatering is a key process which results in the reduction of its moisture content (MC) and its volume. Depending on the susceptibility of sewage sludge to dewatering, the sludge moisture might change from 95 to 99% to 65–85% [5,21]. As reported by Schaum and Lux [24], the volume of sewage sludge with initial solids content of approximately 5% might decrease by even 90%. It contributes to reduction of costs associated with transport and utilization of sewage sludge.

The effectiveness of sewage sludge dewatering depends on different factors, including: the wastewater treatment technology, the type of sewage sludge, the composition of sludge and the method of dewatering [26]. Generally, sewage sludge is characterized by the low dewaterability, which is connected with strong hydrophylicity of sludge particles [40]. According to Chen et al. [4] and Wójcik et al. [34], raw sewage sludge is characterized by the negative charge and it creates a stable system with a low sedimentation and low dewatering capacity. In order to improve the dewaterability of sewage sludge, a change in its structure is necessary.

The dewatering process might be enhanced by sludge conditioning. In general, there are three main sewage sludge conditioning methods: chemical, biological and physical. The sewage sludge dewatering might be also improved by the application of microwaves, ultrasounds or thermal methods [12,23]. In laboratory tests, unconventional methods, such as Fenton's process are also examined [7].

Chemical conditioning with the use of polyelectrolytes is commonly applied in wastewater treatment plants. The addition of cationic polyelectrolyte results in neutralization of the sludge charge and bridging of particles. Chemical conditioning eliminates repulsion of particles across the created of short-term forces and enables the connection of fine particles into larger agglomerates [6,22]. The effectiveness of chemical conditioning was proved by many researchers. Jin et al. [13] used cationic polyacrylamide in order to improve sludge dewatering. Kuglarz et al. [14] proved that the application of Praestol 610BC cationic polyelectrolyte in the dosage of 2.5 g/kg DM resulted in the decrease of moisture and the specific resistance to filtration (SRF), respectively, by approximately 4 and 81%. However, chemical conditioning indicates some limitations. According to Wu et al. [38], conditioned sewage sludge might be more compact during filtration. It can influence the limitation of further dewatering. Other disadvantages are related with high dosages of reagents and the relatively high cost of polyelectrolytes. According to Stachowicz et al. [27], the monthly cost of the acquisition of polyelectrolyte is approximately EURO 90 for a treatment plant which produces 560 tons of sewage sludge DM per year.

Apart from chemical conditioning, physical conditioners called ‘skeleton builders’ are applied. Recently, the addition of different fractions of waste into sludge has been tested. In a laboratory research, the usefulness in sewage sludge treatment was indicated by, for example: rice biochar, rice powder, lignite, gypsum, wood chips and wheat bran [8,15,28,39,41,43]. The impact of energetic waste on the effectiveness of sludge dewatering is also under examination, but in some cases, physical conditioners do not improve sludge dewatering in a significant way. Physical conditioners are inert substances which only influence the change of mechanical strength and the permeability of sludge during filtration [38]. In order to improve the efficiency of sludge conditioning, skeleton builders are often modified with the use of chemical reagents. The dual conditioning methods by means of physical and chemical conditioners are also applied. Chen et al. [4] examined the effectiveness of sewage sludge dewatering after the application of coal fly ash modified with the sulphuric acid. Kuglarz et al. [14] also proved the positive impact of dual conditioning with the use of coal fly ash and Praestol 610 BC cationic polyelectrolyte on the sewage sludge dewaterability. The main mechanism of sewage sludge conditioning with the use of skeleton builders after chemical modification includes: charge neutralization, adsorption, bridging and agglomeration of flocs [16]. The literature review confirms that dual conditioning methods are superior in comparison to the single method. Chen et al. [4] proved that the coal fly ash after chemical modification can decrease the moisture content to a greater degree in comparison to raw ash. Wu et al. [38] also showed that the rice husk biochar after modification with the use of FeCl₃ can decrease the sludge moisture content and SRF, respectively, by approximately 25 and 91%. In comparison, raw rice husk biochar decreased the aforementioned parameters, accordingly by 4 and 60%.

In literature, there are only some papers concerning the influence of biomass ashes on the sewage sludge dewatering. In our previous studies, the sludge conditioning with wheat straw, beech wood and willow tree ashes were examined [35,36]. Results of Wójcik et al. [35] showed that the addition of biomass ash into wastewater sludge could decrease the moisture by approximately 10–25%, depending on the method of dewatering. Due to the increasing consumption of biomass in the energy sector, the ash is an inexpensive and easily-obtained material, which can be used as a physical conditioner. However, the dual sewage sludge conditioning with the application of biomass ash and cationic polyelectrolyte was not examined. Due to this fact, the effectiveness of the aforementioned method is not known.

In this paper, physical conditioning with the use of biomass ash was examined. The dual conditioning with the application of biomass ash as well as cationic polyacrylamide was also assessed. The influence of conditioning methods on selected parameters of sewage sludge, for example, the moisture content, pH and specific resistance to filtration, was determined. The effectiveness of the aforementioned methods was compared with the chemical conditioning by means of cationic polyacrylamide alone.