Effects of agricultural waste-based conditioner on ultrasonic-aided activated sludge dewatering

Abstract

The effects of agricultural wastes on ultrasonic-aided activated sludge dewaterability were investigated in this study. Wheat straw powder (WSP), corn stalk powder (CSP) and rice hull powder (RHP) were used as physical conditioners. Several indicators, including capillary suction time (CST), specific resistance to filtration (SRF) and the dewatering process were adopted to characterize the sludge dewaterability. Soluble protein and filtrate polysaccharide were also characterized to estimate their function on sludge dewatering. The results showed that sludge dewaterability was greatly improved by adding WSP or CSP under an ultrasonic intensity of 28 kHz. The SRF was reduced from 1.0 × 10⁹ S² g⁻¹ to 0.4 × 10⁹ S² g⁻¹ (or less) with a dosage of more than 0.75 g/g dry solid (DS). The moisture content of the sludge cake decreased from 93% to 80% and from 94% to 79% by adding WSP and CSP with ultrasonication. However, no visible enhancements were observed in sludge dewaterability by adding RHP. Moreover, the addition of these agricultural wastes contributed to an increase in the high heating value of dewatered sludge, and ultrasonication further improved the sludge low heating value by reducing the moisture content. The synergistic mechanism of sludge conditioned by agricultural wastes and ultrasonication was attributed to agricultural wastes forming a permeable and rigid lattice structure and ultrasonication cracking the sludge structure.

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1. Introduction

Sludge production is on the rise in China due to the enforcement of the Chinese legislation in small town sewage treatment. The treatment and disposal of surplus sludge produced in waste water treatment plants (WWTPs) is very challenging. One of the key problems related to the activated sludge is the high moisture content, which is as high as 95%. The expense of transporting and handling the raw activated sludge may account for 60% of the total running cost of a whole WWTP.¹ Therefore, before the final disposal of sludge by incineration or landfilling, it is necessary to make the sludge moisture content as low as possible, in an economical way. Hence, the energy consumed during incineration, the cost of transportation, and the amount of land used in landfilling can be reduced significantly. However, owing to the rigid biological gel structure and strong water–sludge interaction in the sludge, breaking down the sludge and releasing the water is particularly difficult. Over the last decade, many technologies have been developed to enhance the sludge dewaterability, including chemical agent conditioning, physical conditioning, and physico-chemical conditioning. In particular, solar photocatalytic treatment,² biophysical drying and thermo-degradation,³ electrolysis⁴ and co-conditioning by heat and CaCl₂ (ref. 5) are all on the list. Most of these processes proved to enhance the sludge dewaterability by disrupting the extracellular polymeric substances (EPS), which are the major components of activated sludge flocs and have a strong affinity for water molecules.^6,7 The water entrapped in the EPS-structure was bound mainly by polysaccharides and proteins, which were the main components of the EPS.⁸ Considering the highly hydrated nature of EPS, it is believed that the sludge dewatering efficiency can be increased by degrading the EPS. Although they exhibit some enhancement effects, these technologies are still far from being industrially implemented due to problems such as high capital cost and operational expense.

In recent years, ultrasonication has been proposed to improve the sludge dewaterability and settleability because it can disrupt the sludge floc structure and release bound water.^7,9 Moreover, it is eco-friendly in comparison to acid or alkaline treatment, metal ion addition and electro-chemical treatment. Moreover, ultrasound equipment can be easily integrated into and operated in wastewater treatment plants.¹⁰ Ultrasonication exerts most of its physical and chemical effects through the phenomenon of cavitation.¹¹ When ultrasonic pressure waves propagate through water, gas and vapor bubbles will be generated and collapsed violently and rapidly, which leads to high shear forces. It was reported that ultrasonic-assisted Fenton treatment presented obvious advantages over the Fenton-treated sludge in disrupting the floc structure of textile-dyeing sludge.¹² Huan et al. found that sludge dewaterability was improved by ultrasonication and FeCl₃ only when sludge disintegration was 2–5%.¹¹ However, ultrasonication significantly decreased sludge dewaterability under some conditions such as during treatment with large specific energy dosages.^13,14

Inorganic and organic physical conditioners were frequently applied to condition sludge for dewatering because physical conditioners possess strong advantages in reducing chemical conditioning requirements, which increases the solid content of cake and enhances the deposition on filter media.^15,16 In many places, these substances are used in large amounts, and are usually discarded as agriculture and industry affiliate products. Some researchers tried to recycle the substances used to condition the sludge. Chen conditioned sludge using sulfuric acid-modified coal fly ash.¹⁷ The mineral waste increased the ash amount and lowered the calorific value of dewatered sludge. With regard to organic conditioners, mainly fibrous raw materials, such as wood or agricultural residues, which consist of three dominating polymers, namely, cellulose, hemicelluloses and lignin, could supply enough heating value. Lin et al.¹⁸ and Ding et al.¹⁹ recycled wood chips or wood chips and wheat dregs for sludge processing and found that these conditioners enhanced the sludge filtration performance and increased the energy content of the filter cake. It has also been found that the application of physical conditioners could reduce the use of chemical conditioners and the cost of the process while still achieving the same level of dewatering.^20,21 However, when physical conditioners are used as the sole conditioner (without any chemical conditioner), a much higher dose of conditioners is required in order to make a significant improvement in sludge dewaterability.^16,22,23

In this study, the combined effects of ultrasonication and agricultural wastes on the SRF and CST of sludge were investigated. The heating value of dry sludge was determined. A modified filter unit was used to measure the sludge dewatering performance. Finally, the contribution of agricultural wastes in improving the ultrasonic-aided sludge dewatering performance was discussed.

2. Materials and methods

2.1 Experimental materials

The sludge was taken from the secondary sedimentation tank of the Wenchang waste water treatment plant in Harbin, China. The collected sludge was pretreated by rinsing three times with tap water and sieving through a 30 mesh sifter to get rid of large particles. Then, the sludge sample was immediately transferred to the lab and stored in a plastic container at 4 °C prior to use. Sludge characteristics are shown in Table 1.

Table 1 Raw sludge characteristics

Parameter	Unit	Value
pH	—	7.2–7.6
Moisture content	%	96.7–97.8
Total suspended solids	mg L^−1	25–29
Volatile suspended solids	Mg L^−1	14–16
Viscosity	Pa s	3.34–3.83
SRF	S^2 g^−1	0.94 × 10^9 to 1.03 × 10^9

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Three types of agricultural wastes, including rice hull powder (RHP), wheat straw powder (WSP) and corn stalk powder (CSP), were selected to condition activated sludge. The agricultural wastes were dried at 60 °C for 24 h in an electric thermostatic drying oven, and then sieved through a 30 mesh sifter to remove the bulk particles.

2.2 Experimental procedures

At the beginning, 800 g activated sludge (about 97% water) was put in each of six 1 L beakers. WSP, CSP and RHP were expressed as a weight percentage of the additive based on the original dry solid (DS) content of the sludge and were dosed in the levels of 0, 0.50, 0.75, 1.00, 1.25 and 1.50 g/g DS. The mixture was homogenized by stirring at 260 rpm for 1 min and then at 160 rpm for another 1 min. Then, half the mixture in each beaker was dumped in a 500 mL conical flask. The remaining sludge was agitated at 160 rpm for 15 min and was set as the control. Sludge samples in conical flasks were placed in an ultrasonic cleaner (KQ2200DA, China), which emitted ultrasonic energy at 28 kHz through the bottom of the reactor. The sludge samples were irradiated in an ultrasonic field for 15 min, which was reported to be the most cost-effective method for disrupting sludge.²⁴

2.3 Analytical methods

The CST was monitored by a CST instrument (Triton, Model 304M, Essex, UK). Mixed liquor suspended solids (MLSS), mixed liquor volatile suspended solids (MLVSS), and moisture content were measured by the gravimetric method.²⁵ The soluble fraction referred to the filtrate passing through a 0.45 μm filter membrane. Soluble protein was determined using the rapid Lowry method (Shanghai, China) with bovine serum albumin as the standard. The anthrone–sulfonic acid method²⁶ was employed to assess the content of polysaccharide in the filtrate with glucose as the standard reference.

SRF was measured by a modified filter set-up (as Fig. 1 shows) using eqn (1):

α = 2bpA²/(μC) (1)

where α is the specific resistance of the sludge, S² g⁻¹; p is the filtration pressure, Pa; A is the effective filtration area, cm²; b is the slope of the T/V–V line equation, where T is the filtrate time (s) and V is the volume of filtrate (mL); μ is the viscosity of the filtrate, g cm⁻¹ s⁻¹ and C is the filter cake quality per milliliter of filtrate, g DS per mL filtrate, which can be calculated using eqn (2):

C = 1/[C_i/(100C_i) − C_f/(100 − C_f)] (2)

where C_i is the initial moisture content of the sludge suspension (%) and C_f is the final moisture content of the filtered sludge cake (%).

Fig. 1 Modified compression filter device for SRF and dewatering.

The experimental set-up for filtration tests mainly comprised a nitrogen cylinder, a closed cell with filter paper (filtering threshold 16 mm) on the bottom, an electronic analytical balance with a graduated cylinder on it, and a computer to record data. Initially, 100 mL conditioned sludge was poured into the closed cell and the pressure was then adjusted to 100 kPa. The computer recorded the weight of the filtrate every 5 seconds during the filtration test. The filtration lasted for 15 min or stopped when the sludge cake fractured. The moisture content values of the sludge before and after filtering were measured. Then, the SRF could be calculated. This set-up could depict the relationship between filtration time and filtrate volume. It was more accurate in pressure adjusting in comparison with a vacuum filtration installation. The measurement of sludge dewaterability was undertaken using the same filtration set-up. Sludge samples were filtered for 30 min under a pressure of 100 kPa.

The total solids and the moisture content of sludge samples were determined after evaporation at 105 °C for 24 h. The volatile combustible solids and ash in the sludge samples were determined in a covered crucible after combustion at 650 °C for 5 h. Samples, after drying at 105 °C, were analyzed for heating value using elemental analysis and were expressed as kJ kg⁻¹ of the dry total solids on a dry basis.

Elemental analysis was performed in an elemental analyzer (multi EA® 5000, Jena, Germany). The relationship between the observed high heating value (kJ kg⁻¹) and element (C, H, O, S and N) contents of the sludge can be estimated using eqn (3),²⁷ and the low heating value can be derived from the high heating value on a dry basis and the cake moisture content, as shown in eqn (4):^18,19

High heating value = 430.2[C] − 186.7[H] − 127.4[N] + 178.6[S] + 184.2[O] − 2379.9 (3)

Low heating value = high heating value × (1 − moisture content) (4)

where [C], [H], [N], [S], and [O] are the weight percentages of the corresponding elements.

The surface areas of agricultural wastes were measured by Brunauer–Emmett–Teller surface area measurement (Micromeritics ASAP2020, USA).

3. Results and discussion

3.1 Effects of ultrasonication and agricultural wastes on sludge dewaterability

As shown in Fig. 2, the CST of conditioned sludge increased with the increasing dosing amounts of WSP, CSP and RHP in the control trials. The CST of activated sludge ranged from 77.4 s to 103.3 s and from 75 s to 104.5 s in the presence of WSP and CSP in the ultrasonication trials, respectively (as seen in Fig. 2(a) and (b) and Table 3). In particular, the CST grew slowly with WSP and CSP doses of less than 1 g/g DS. When WSP and CSP were dosed at more than 1.0 g/g DS, the CST of the control sludge was raised significantly from about 107 s to 198 s and 238 s, respectively. However, the CST increased slowly in the ultrasonication trials (as shown in Fig. 2(c)), and the values for ultrasonic treated sludge were lower than those of the control trials, especially for low doses (<1 g/g DS). The sludge CST values for the control and ultrasonic sets were similar to those dosed with RHP. The CST values ranged from 107.7 s to 161.1 s and from 79.3 s to 165.1 s in the control and ultrasonic sets, respectively (as shown in Table 3). Moreover, from the perspective of the CST, the sludge dewaterability was apparently not enhanced by adding these agricultural wastes. This was ascribed to the agricultural waste addition, which significantly increased the sludge concentration and prolonged the time required for sludge sedimentation.²⁸ From Fig. 2, it can be observed that the moisture content of dewatered sludge cake declined with increasing doses of all three agricultural wastes. Moreover, the moisture content of sludge dewatered with the aid of ultrasonication was lower than that treated by stirring only at the same dosages of WSP, CSP and RHP. Nevertheless, the CST was not in concordance with the moisture content of dewatered sludge in the experiment. In other words, a higher CST value did not always correspond to a larger moisture content value for dewatered sludge. Therefore, the CST could not be used to evaluate the dewatering performance in this study. Other researchers also found that the CST test was not able to fully characterize sludge dewatering.^29,30 This method can be regarded as an empirical index for flocculation optimization in engineering practices rather than a quantitative index of dewatering devices.³¹

Fig. 2 Effect of agricultural waste dosage on the CST and the moisture content of activated sludge: (a) WSP, (b) CSP and (c) RHP.

Table 2 The physical properties of the agricultural waste

	Bulk density/kg m^−3	Surface area/m^2 g^−1	Heating value/kJ kg^−1
WSP	158.3	0.73	15928.94
CSP	149.4	0.76	19857.90
RHP	318.2	0.23	19274.60

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Table 3 The minimum and maximum values of each experimental variation

Index		WSP		CSP		RHP
		Control	Ultrasonic	Control	Ultrasonic	Control	Ultrasonic
a HV means heating value.
SRF/×10^9 S^2 g^−1	min	0.63	0.17	0.65	0.22	0.64	0.58
	max	1.66	1.02	1.63	1.01	1.61	1.10
CST/s	min	102.3	77.4	100.9	75.0	107.7	79.3
	max	196.5	103.3	206.6	104.5	171.1	165.1
Moisture content/%	min	81.8	79.1	85.0	80.2	84.8	82.5
	max	94.3	94.1	94.3	93.2	93.6	93.01
Filtrate volume/mL	min	50.1	52.2	48.7	52.2	50.1	52.0
	max	57.3	69.4	53.6	65.6	53.0	56.9
High HV^a/×10^3 kJ kg^−1	min	12.5	12.5	12.5	12.5	12.5	12.5
	max	14.4	14.7	16.9	16.9	16.6	16.4
Low HV/×10^3 kJ kg^−1	min	0.8	0.8	0.8	0.9	0.8	0.9
	max	2.5	3.0	2.5	3.2	2.4	2.7
Protein/mg g^−1 VSS	min	3.62	5.07	3.48	4.98	3.81	4.77
	max	10.54	12.66	10.58	12.07	7.59	9.93
Polysaccharide/mg g^−1 VSS	min	0.96	1.29	0.88	1.36	0.82	0.96
	max	3.18	3.47	4.58	5.61	1.45	1.97

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As shown in Table 3, the ranges of the SRF values were 0.63–1.66, 0.65–1.63 and 0.64–1.61 × 10⁹ S² g⁻¹ in control trials and 0.17–1.02, 0.22–1.01, 0.58–1.10 × 10⁹ S² g⁻¹ in ultrasonic-aided sets for WSP, CSP and RHP, respectively. As shown in Fig. 3, for the tested agricultural wastes, the SRF of all the sludge samples declined with increasing doses, and the sludge dewatering performance was visibly enhanced with the aid of ultrasonication. Some other organic additives, e.g. wood chips and wheat dregs, were also reported to reduce the SRF of sludge.¹⁶ Both WSP and CSP also showed good performances in enhancing the sludge dewaterability (as shown in Fig. 3(a) and (b)). It is known that sludge with an SRF of less than 0.4 × 10⁹ S² g⁻¹ is easy to dewater. Despite the values decreasing after adding WSP or CSP, the SRF values were still more than 0.58 × 10⁹ S² g⁻¹ in the control trials (as shown in Table 3), indicating that the conditioned sludge was still difficult to dewater without the aid of ultrasonication. When the WSP or CSP dose increased to 0.75 g/g, the SRF of the conditioned sludge was less than 0.4 × 10⁹ S² g⁻¹, which is nearly half the value in the controlled trial, implying a strong capacity to improve the sludge dewaterability. However, further increasing the conditioner dose just resulted in a very minor reduction in the SRF. The combination of ultrasonication and organic physical conditioner showed a synergistic effect on reducing the SRF. While RHP and ultrasonication did not make the SRF decline to the critical value (0.4 × 10⁹ S² g⁻¹), the SRF values were much higher than 0.4 × 10⁹ S² g⁻¹ in the presence of both RHP (0–1.5 g/g DS) and ultrasonic radiation. In general, the results demonstrated that adding physical conditioners during mechanical sludge dewatering could improve the sludge dewaterability by reducing the SRF.^16,32 It was reported that the optimum dose of external conditioners was the value beyond which the SRF decrease becomes insignificant.^23,33 Therefore, the optimum dosages of WSP and CSP were in the range of 0.75–1.0 g/g DS. In the dewatering process, there was a positive correlation between the moisture content of the dewatered sludge cake and the SRF, indicating that sludge could be dewatered easily with a small SRF.

Fig. 3 Effect of agricultural waste dosage on the SRF of activated sludge: (a) WSP, (b) CSP and (c) RHP.

3.2 Effects of ultrasonication and agricultural wastes on MLSS/MLVSS and filtrate volume

Physical conditioners could improve the sludge dewatering performance, but could also increase the solid component of sludge in the meantime. The MLSS and the MLVSS were measured in the ultrasonic sets and are shown in Fig. 4. Both the MLSS and the MLVSS exhibited proportional correlations with the dosages of WSP, CSP and RHP. The ratio of MLVSS/MLSS tended to a steady value as the dosage of conditioner increased beyond the optimum value (as shown in Fig. 4), which indicated that an excess of organic physical conditioners made limited gains in sludge dewaterability at the expense of huge final sludge amounts. As shown in Fig. 4(a), the inflection point of the ratio of MLVSS/MLSS curve appeared at the WPS dosage of 1.00 g/g DS. Analogously, the inflection points occurred at the dosages of 0.75 and 1.25 g/g DS for CPS and RHP, respectively. Considering the ratio of MLVSS/MLSS of conditioned sludge, the optimal physical conditioners' dosages were 1.00, 0.75 and 1.25 g/g DS for WSP, CSP and RHP, respectively.

Fig. 4 Effect of agricultural waste dosage on MLSS, MLVSS and MLVSS/MLSS ratio: (a) WSP, (b) CSP and (c) RHP.

The variations of filtrate volumes are shown in Fig. 5. Table 3 shows the minimum and maximum values. The filtrate volume initially increased with conditioner dosage for all the tested agricultural wastes, and then decreased as the dose surpassed the optimum value. The result was similar to that reported by Lin et al. who investigated the performance of rice shell or bagasse as sludge conditioners.¹⁸ Ultrasonication made the sludge release more water than the control trial under the same experimental conditions. When the dosage of WSP was around 0.75 g/g DS, the ultrasonication-aided group achieved the highest filtrate volume at 68.4 mL, which was 21.3% higher than that in the control. With regard to CSP or RHP, the filtrate volumes were about 65.6 and 56.9 mL, corresponding to 22.3% or 7.3% higher than those obtained without ultrasonic aid.

Fig. 5 Effect of agricultural waste dosage on FV: (a) WSP, (b) CSP and (c) RHP.

Although the moisture content of the sludge cake after dewatering was lower with more physical conditioner in this study, the workload for the sludge disposal would double as the doses of WSP, CSP and RHP exceeded 1.00 g/g DS. The dry agricultural waste absorbed part of the moisture, which could not be squeezed out under a relatively low pressure. The results of MLSS/MLVSS and filtrate volume indicated that agricultural waste should be added in an appropriate strategy. Otherwise, the effect of physical conditioners will be counterproductive in reducing the disposal amount of sludge.

3.3 Effects of ultrasonication and agricultural wastes on the amounts of protein and polysaccharide in filtrate

EPS, presumed to be predominant in protein and polysaccharide, are regarded as one of the most important factors that influence the dewatering characteristics of wastewater sludge.^34,35 As shown in Fig. 6, the concentrations of soluble protein and polysaccharide in the filtrate increased after dosing agricultural waste in this study, indicating that the agricultural wastes released some proteins and polysaccharide into the aqueous phase. Moreover, ultrasonication could disintegrate the EPS and release more protein and polysaccharide into the water (as shown in Fig. 6 and Table 3). Sludge flocs were disrupted in a relatively short time and then the interstitial water in the sludge was released into the aqueous phase, which resulted in a lower moisture content in the ultrasonic-aided dewatered sludge. The amounts of soluble protein and polysaccharide in the presence of WSP and CSP were both slightly higher than in samples containing RHP. This was ascribed to more adventitious organic matters being present in WSP and CSP. Moreover, these matters did not influence the dewaterability, as illustrated by the SRF (Fig. 3), because they were different from the proteins and polysaccharides present in the EPS and could dissolve in water and pass through the filtering medium easily. Protein and polysaccharide contents could not only increase the sludge viscosity but could also decrease its dewaterability, as illustrated by the CST.^36,37 In this study, the soluble protein/polysaccharide concentration was correlated to the CST, but did not show any negative impact on reducing the SRF of the conditioned sludge.

Fig. 6 The soluble protein and polysaccharide contents of filtrates with different conditioning methods and additives.

3.4 Effects of agricultural wastes on heating values

As the agricultural waste dosage increased, the heating value of the dry sludge also increased (as shown in Fig. 7). In the controlled trials (Table 3), the high heating values of the dewatered sludge varied from 12.5 × 10³ kJ kg⁻¹ to 14.4 × 10³, 16.9 × 10³ and 16.6 × 10³ kJ kg⁻¹ when WSP, CSP and RHP, respectively, were added as the conditioners with a dose of 1.5 g/g DS. When ultrasonication was applied in the dewatering process, the high heating value increased from 12.5 × 10³ kJ kg⁻¹ to 14.7 × 10³, 16.9 × 10³ and 16.4 × 10³ kJ kg⁻¹ in the presence of WSP, CSP and RHP, respectively, with a dose of 1.5 g/g DS (as shown in Table 3). The results implied that the added conditioners would endow the sludge with higher heating values, which were not associated with ultrasonic aided conditioning.

Fig. 7 Effect of agricultural waste dosage on heating value: (a) WSP, (b) CSP and (c) RHP.

As far as the capacity to amplify the high heating values of the sludge was concerned, the priority of the tested agricultural wastes was in the order CSP > RHP > WSP. This resulted from their own inherent calorific values. Some other researchers reported that the organic physical conditioners increased the heating value of the sludge by 28.4% with wood chips and wheat dregs (300%)¹⁸ and 21.6% with individual wood chips (100%).¹⁹ These organic physical conditioners exhibited a similar effect in amplifying the high heating value of dry sludge. Therefore, this could reduce the fuel cost and treatment fees when combustion is used as the final disposal method of dewatered sludge. For inorganic physical conditioners, such as fly ash and ordinary Portland cement, however, they may result in an obvious increase in the net sludge yield and a sharp decrease in the calorific value due to large dosages.¹⁵ The high heating value was not influenced by ultrasonication with the same additive, as ultrasonication changed only the sludge structure not the composition.

As for the low heating value, this was dependent on the high heating value as well as the moisture content of the sludge. In particular, the low heating value increased with increasing dose of physical conditioners and decreasing moisture content value. As shown in Table 3, the low heating values in the control trials were magnified by 203%, 253% and 216% as a result of adding WSP, CSP and RHP, respectively, with a dose of 1.5 g/g. When ultrasonication was applied, the low heating values were amplified by 311%, 291% and 229% for WSP, CSP and RHP (1.5 g/g), respectively. This was ascribed to a higher dewatering performance in the presence of ultrasonication, which might reduce the sludge moisture content and thus augment the low heating values.

3.5 Mechanism of activated sludge co-conditioning by agricultural wastes and ultrasonication

The mechanism of activated sludge co-conditioning by agricultural waste and ultrasonication is shown in Fig. 8. Free water, interstitial water and capillary water adhered tightly to the sludge flocs (Fig. 8(a)). At the beginning of dewatering, the solids and particles gathered making the sludge larger and denser. In this phase, most free water and partial interstitial water were released from the sludge. Furthermore, the sludge became highly compressible and the EPS contracted tightly to package partial water and blocked the passageway for water leaking out. In the presence of ultrasonic energy, the sludge was destroyed to some extent, resulting in the release of EPS and moisture (Fig. 8(b)), because ultrasonication produced a type of sponge effect and facilitated the migration of moisture through natural channels created by wave propagation.²⁸ In addition, other ultrasonic effects, such as acoustic streaming, local heating, interface instabilities, agitation and cavitation, may also be beneficial for solid/liquid separation.³⁸ However, as a consequence of the strong compressibility of the sludge, individual ultrasound exhibited a minor effect on improving the sludge dewaterability. Because there were not enough permeable pores to transport water in the compressed sludge in the dewatering process, a lot of water remained trapped inside the sludge cake.

Fig. 8 The mechanism of activated sludge conditioning aided by: (a) none; (b) individual ultrasonication; (c) individual agricultural wastes; (d) ultrasonication together with agricultural wastes.

As shown in Fig. 8(c), the agricultural wastes acted as skeleton builders to improve the sludge structure. They formed a permeable and rigid lattice structure in the sludge cakes.¹⁵ In conclusion, the cooperative implementation of ultrasonication and agricultural wastes facilitated the sludge dewatering (Fig. 8(d)). The change in the conditioned sludge structure was correlative to the smaller bulk density and the larger surface area of agricultural wastes (Table 2). A lower bulk density contributes to a looser structure in the sludge, which may provide more channels for water to get through. A larger surface area may help the agricultural wastes to absorb and accommodate more water. The water in the agricultural wastes could be easily squeezed out by pressure. For example, with a lower bulk density and a higher surface area, WSP and CSP exhibited superior performances in enhancing sludge dewaterability compared to RHP.

4. Conclusions

As discussed above, the following conclusions could be obtained.

(1) Ultrasonication could greatly improve the dewaterability of agricultural waste-conditioned sludge. WSP and CSP were more effective in enhancing the effect than RHP due to a lower bulk density and a higher surface area.

(2) In the presence of WSP or CSP, the SRF of co-conditioned sludge was reduced to half the value for sludge conditioned by individual agricultural wastes. Moreover, ultrasonication could reduce the increase in CST caused by dosing these agricultural wastes. The optimum dosage range was 0.75–1.00 g/g DS.

(3) Ultrasonication did not change the high heating value, but increased the low heating value greatly by improving the dewaterability of sludge conditioned by agricultural wastes.

(4) The mechanism of the co-conditioning was that agricultural wastes played the role of skeleton builder, which supplied more permeable pores, and ultrasonication made the sludge crack and release more interstitial water and capillary water.

Acknowledgements

This study was jointly supported by the National Natural Science Foundation of China (51038003, 51278143), the Science and Technology Research Project of Department of Education of Heilongjiang Province (12531037), the Industry-University-Research Collaboration Project of Guangdong Province & Chinese Ministry of Education (2012B091000029), the Key Breakthrough Project of Guangdong & Hong Kong (2012BZ100021), and the Industry and Industry-University-Research Collaboration Project of Chancheng District (2012107101169).

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