Introduction

The discharges of industrial and domestic wastewater had increased significantly nowadays due to the fast industrialization and over growth of human population. Due to this reason, huge quantities of sludge may be produced from wastewater treatment plants in many countries. For example, the total amount of sludge is expected to reach 13 million tons in European countries by 2020 (Samolada and Zabniotou 2014).

The water content of the activated sludge was reported more than 95% (Thomas and Rolf 1997). The pollutants in wastewater pose dangerous threat to human and aquatic environment (Devi and Saroha 2016). Thus, the purification of wastewater is very important to avoid critical global scarcity and shortages of water. The biological wastewater treatment is one of the most commonly any economically used methods for the degradation of organic contaminants by utilization of bacterial metabolism (Al-Dawery 20162017; Nielsen et al. 2010). As a result of the microorganism growth, formation of the activated sludge takes place (Ahmed and Wahid 2014). The amount and bioactivity of the activated sludge are the core factors for a successful operation of the wastewater treatment plant (Han et al. 2012; Ahmed 2018). However, excessive amount of solid sludge would have serious problems in sludge disposing management and a negative impact on the environment and economic concerns (Appels et al. 2008; Abe, et al. 2011; Hait and Tare 2011).

Sludge dewatering is an important and at the same time a very difficult task; thus, the optimization of sludge treatment is a great challenge. In addition, the presence of the bacterial extracellular polymeric substance in the activated sludge causing water-biomass bonding and leading to a negative influence on the water-solid separation (Poxon and Darby 1997; Mikkelson and Kieding 2002; Liu and Fang 2003). Related to this study, large efforts have been done by many researchers to develop and improve the purification of wastewater especially that which contains organic pollutants and application of biological treatment such as inorganic ions and organic polyelectrolyte (Al-Dawery 2015); Li, et al. 2016; Kozminykh et al. 2016), membrane bioreactor (Lin et al. 2012; Skouteris et al. 2012; Smith, et al. 2012), collagen dispersions (Davis and Maffia 1995), enzymatic treatment (Nelson et al. 1988), ultrasonic pretreatment (Yin, et al. 2004), ozonation pretreatment (Ahmed 2019), activated sludge treatment (Ahmed and Wahid 2014), freezing/thaw treatment (Wu, et al. 2001; Zheng, et al. 1998), etc. Most of the sludge treatment methods are used for sludge dewatering and the widely used mechanisms that are proposed for the sludge particles flocculation. For example, there are three major theories for flocculation and sludge dewatering based on using cationic ions; these are alginatation sludge; double layer, and cation bridging (Sobeck and Higgins 2002).

In addition to the importance of sludge dewatering for many countries that are suffering from water scarcity, the wastewater treatment plants produce biosolid. Application of biosolids is useful if treated sufficiently, can be applied for soil improvement, and supplies plant with the necessary nutrients. The biosolids were in rich of organic materials and fertilizers. Also, the handling and management of sludge cost can reach up to 50% of the total operating cost of wastewater treatment plant.

Organic polyelectrolytes are the most effective sludge conditioning and dewatering that form a long-chain network and adsorb sludge particles leading to flocculation and rapid settling (Al-Dawery 2017). The huge amount of produced activated sludge requires large quantities and high concentrated polyelectrolytes. Reduction of application of chemicals and sustainable cost-effective method is a challenge for the operation of sewage treatment. Manufacturing cost of polyelectrolyte is approximately 3000–5000 USD per ton. The effective dosages of polyelectrolyte concentration for sufficient sludge conditioning and dewatering are 50–100 ml depending on solid content of the sludge. The generation per capita per day of wastewater characteristics would be estimated from measured collected wastewater flow, the population, and concentration data. The results indicate that there is a huge amount of wastewater produced daily.

Pollution from wastewater in terms of capita values, the conception of pollution would be understandable for many citizens. However, the changes in living standards and technologies application for wastewater treatment would suggest re-examination of the pollution per capita loadings. On the other hand, considerable problems in environment are caused by discharge and sludge produced from wastewater. The increase in population would increase pollution of water, and imposing of restrictive limits by local authorities would lead to the need for new treatment technologies. Thus, it is very important that the discharged wastewater be assessed and current treatment technologies be improved. Accordingly, additional cost would be needed for covering new technologies, handling and sludge management, transportation, plant operation, etc. Worldwide, the capacity of wastewater treatment plants is varying from country to country as shown in Table 1.

Table 1 Wastewater volumetric flow rate treated in some countries
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The aim of this research was to evaluate the effectiveness of using supernatant fluid that produced from previously conditioned sludge for treatment of a fresh municipal wastewater and also to study the impact of this method on reducing dosage of polyelectrolyte and decreasing cost of conditioning sludge and its operation.

Materials and methods

Materials

The SNF slightly high cationic polyelectrolyte used for sludge conditioning was provided by manufacturer SNF FLOERGER, France. SNF is organic coagulants and flocculants for water treatment. SNF molecular structure is shown in Fig. 1. The conditioning solution was prepared by mixing 1 g of polyelectrolyte with 1 liter of deionized water and then stirred using the orbital shaker for at least 24 h. Different concentrations of polymer solution were used during operating the jar test.

Fig. 1
figure 1

Chemical structure of SNF polyelectrolyte

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Sludge sample

Sludge samples were collected from the aeration tank of Nizwa wastewater treatment plant in Nizwa, Sultanate of Oman. Samples were conditioned with SNF polyelectrolytes and supernatant of previously conditioned sludge. The average values of the total suspended solids (TSS), volatile suspended solid (VSS), and fixed suspended solid (FSS) of the collected samples, conductivity, and pH are presented in Table 2 and were analyzed according to APHA 1998. The calculation of TSS in the wastewaters was as follows: 20 ml of sludge sample was firstly filtered using a pre-weighed filter paper on a porcelain crucible and then placed in drying oven for 1 h at 105 °C. After drying, the sample was weighed for TSS. The sample was then dried in a muffle furnace for 1 h at 550 °C. The dried sample was weighed for FSS.

Table 2 The characteristics of the collected samples sludge
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Flocculation test

The coagulation and flocculation of the activated sludge were evaluated using jar test to determine the optimum dosage of conditioning chemicals. Six cylindrical glass jars were used, and each contained 1 liter of sludge suspension. Different dosages of polyelectrolyte were added to each jar simultaneously. The solutions in the six jars were stirred using Armfield (USA) variable speed mixer. Two mixing rates were performed, rapid mixing at rate 200 rpm for 2 min to simulate proper and fast mixing followed by slow rate mixing at 90 rpm for 30 min to promote flocculation process. Floc formation was observed during the slow mixing.

Sludge volume index

Settling properties of the collected sludge samples were characterized using the total suspended solid (TSS) and the sludge volume index (SVI) as described by 2540D and 2710D methods according to APHA standard methods (APHA 1998). The SVI test requires pouring a 1000 mL of sludge fluid into graduated cylinder. Collect a gallon of sludge, mix gently, and after 30 min, record the volume to which the sludge is settled. The SVI formula is given in Eq. 1.

$$ SVI\ \left(\frac{mL}{gTSS}\right)=\frac{Se\mathrm{t} tled\ sludge\ volume\ \left(\frac{mL}{L}\right)}{Suspended\ solids\ concentration\ \left(\frac{mg}{L}\right)}x1000\ \left( mg/g\right) $$
(1)

The treated sludge samples using jar test were poured into a 1 liter graduated cylindrical glass vessel and allowed to settle; the interface position between the sludge and the supernatant was measured during settling.

Sludge volume index is the measured volume that is occupied by 1 g of sludge after 30 min of settling. The data of SVI were used to calculate the improvement of settling of the conditioned sludge samples with pure polyelectrolyte and supernatant separately using the following Eq. 2.

$$ \% of\ improved\ SVI=\frac{SVI\ of\ fresh\ sample- SVI\ of\ conditioned\ sample}{SVI\ of\ \mathrm{f} resh\ sample}x100 $$
(2)

Zeta potential

Zeta potential is the measure of the electrostatic magnitude or charge attraction/repulsion between sludge particles and is considered as fundamental parameters measuring the effect of stability (Shaikh et al. 2017). A high value of zeta potential indicates highly stable system either with a positive or negative surface charge, for which strong electrostatic between the particles is incorporated.

Electrical charges of the supernatant of the conditioned activated sludge samples were measured using Malvern Instruments Zetasizer (UK). This device performs three times zeta analysis for each sample as loaded into its capillary cell; the average values were used.

Results and discussion

The samples collected from wastewater treatment plant at the city of Nizwa, Oman, showed high SVI as shown in Table 1. Due to the bulking sludge phenomenon as a result of high organic content and presence of the extracellular polymer increase SVI and lead to low settling rate (Pere, et al. 1993; Sanin and Vesilind 1996). Cationic polyelectrolyte showed a considerable impact on dispersed negatively charge sludge particles, hence on settling and dewatering. Due to this reason, SNF polyelectrolyte with slightly positive charge density was used for conditioning for the selected sludge with different total suspended solids.

Fresh SNF polyelectrolyte for sludge conditioning

Fresh SNF polyelectrolyte for sludge conditioning was selected, and several experiments were carried out in order to examine the effects of addition of the pure SNF polyelectrolyte on the sludge settling and characteristics of sludge volumes.

The position of the interface between supernatant liquid and sediment after jar test was measured during settling as a function of time after being poured into the cylindrical flasks with various dosages of the polymer. The results of settling and SVI of the conditioned sludge samples with different TSS were shown in Figs. 2 and 3, respectively. It can be seen from Fig. 2 that the SNF polymer had a larger effect on the reduction of the sediment thickness, showing 50% reduction in the sediment thickness. Thus, it can be indicated that a significant settling with the greatest effect being achieved for the least amount of TSS, settling was improved by 50% for sample with TSS 1.265 g L−1. Similarly, the results showed improvement by almost 45–65% in SVI for the conditioned sludge sample as shown in Fig. 3 and Fig. 4. The highest improvement in SVI was for lower TSS. However, TSS content was largely affecting SVI; the results indicated a higher SVI 550,000 for lower TSS 1.265 g L−1 and a lower SVI 150,000 for higher TSS 6.7 g L−1. The high suspended solid is considered electrically stable according to Greenwood and Kendall 1999, leading to aggregation of sludge particles. Other researcher Al-Dawery 2015, found sludge SVI improvement due to effects of suspended solid and polyelectrolyte on settling and rheological properties of municipal activated sludge.

Fig. 2
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Sludge settling using fresh polyelectrolyte with different TSS

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Fig. 3
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SVI using fresh polyelectrolyte with different TSS

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Fig. 4
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SVI improvement of conditioned sludge samples using fresh polyelectrolyte and supernatant with different TSS

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The mechanism of bonding particles may be referred to Van der Waals force attraction (Al-Dawery 2017). The use of low concentration of polyelectrolyte leads to better settling especially in samples with low TSS 1.265 g L−1, compared to samples with higher TSS 6.7 g L−1; bridging was governed by the mechanism of attraction of particles. The successive addition of polyelectrolyte caused small repulsion between particles resulting higher level of settling, and thus the optimum conditioning necessarily happened with high concentration of polyelectrolyte.

Sludge conditioning with supernatant from first conditioning

For economical and cost-effective treatment operations and for reducing amount of used polyelectrolyte, supernatant from the previous conditioning process was used for the sludge conditioning instead of using pure polyelectrolyte. The results were presented in Figs. 5 and 6 for different TSS. It can be observed that the use of supernatant showed a significant impact on sludge settling as good as fresh polyelectrolyte, especially for treatment of sample with low solid content. The obtained settling referred to the partial availability of non-consumed polyelectrolyte in the supernatant that extracted from treated sludge sample with low TSS 1.265 g L−1. Regarding the results of settling zone and final sediment thickness value, as can be seen from Fig. 5, the supernatant has a larger effect on the reduction of the sediment thickness, which resulted in almost a 65% reduction in the sediment thickness for sample with low TSS 1.265 g L−1. The results showed a reduction by almost 60% in SVI for sample with low TSS as shown in Figs. 4 and 6a, while improvements of 30% were obtained for samples with higher TSS, from Fig. 4 and 6b,c. The improved settling using supernatant over that of pure polymer may be due to less repulsion between sludge particles caused by excessive addition of pure polymer.

Fig. 5
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Settling using supernatant from first conditioned sludge with different TSS

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Fig. 6
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SVI using supernatant from first conditioned sludge with different TSS

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Sludge conditioning with supernatant from second conditioning

In a similar procedure, supernatant from the second conditioning sludge was used for treatment of fresh sludge samples. The results showed no improvement on sludge settling and SVI at all cases as shown in Figs. 7 and 8. These results indicated that the whole polyelectrolyte had been consumed during first and second sludge treatment. The negative effect would be explained due to the repulsion between non-treated fine particles in the supernatant and those in the fresh sludge samples which had same negative particle charges.

Fig. 7
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Settling using supernatant from second conditioned sludge with different TSS

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Fig. 8
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SVI using supernatant from second conditioned sludge with different TSS

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Sludge conditioning using mixed pure polyelectrolyte with exhausted supernatant

Sludge conditioning using mixed fresh polyelectrolyte with exhausted supernatant from second treatment was examined for sludge sample with different TSS. Results of using 25% fresh polyelectrolyte and 75% of supernatant for conditioning sludge sample with TSS 1.265 g L−1 were plotted in Fig. 9. Better settling and lower SVI can be observed compared to results of using only exhausted supernatant shown in Figs. 6-a and 7-a. Results showed almost a 47% reduction in the sediment thickness and improvement by almost 16% in SVI. Sludge conditioning with cationic polyelectrolytes improves sludge settling, and SVI was achieved but slightly less than the findings of Nguyen (Nguyen et al. 2008). The results of similar test using 50% pure polyelectrolyte and 50% of supernatant for conditioning of two separate sludge samples with 1.265 g L−1 and TSS 6.7 g L−1 were plotted in Figs. 10 and 11, respectively. Both settling and SVI were improved in both cases compared to results presented in Figs. 7 and 8. For sample with TSS = 1.265 g L−1, the results showed improvement of 57% in sediment settling and 30% in SVI, while for sample with TSS = 6.7 g L−1, the results showed improvement of 49% in sediment settling and 46% in SVI.

Fig. 9
figure 9

Test using 25% fresh polyelectrolyte and 75% supernatant with TSS = 1.265 g L−1

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Fig. 10
figure 10

Test using 50% fresh polyelectrolyte and 50% supernatant with TSS = 1.265 gL−1

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Fig. 11
figure 11

Test using 50% fresh polyelectrolyte and 50% supernatant with TSS = 6.7 g L−1

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Results of using 25% fresh polyelectrolyte and 75% of supernatant were mentioned in Fig. 9. A better settling and improvement in SVI were obtained compared to results of using only exhausted supernatant. Increasing concentration of fresh polyelectrolyte improved the conditioning process as shown in Fig. 10 for samples with TSS 1.625 g L−1 compared with Fig. 11 for samples with TSS 6.7 g L−1.

Effect of zeta potential

The results of zeta potential of the supernatant of the settled sludge using various concentrations of the polyelectrolyte are presented in Fig. 12. In this regard, the results showed that the fresh sludge sample had slightly negative initial zeta values −3.6 mV for sample with low TSS 1.265 g L−1 but under conditioning process by both pure polyelectrolyte and supernatant. The negative zeta values of sample were largely reduced and reached to an average value of +4 mV using pure polymer and + 5.9 mV using supernatant from first conditioning process. These results indicated the significant effect of polymer and supernatant on the surface charges of sludge samples especially those with low TSS. As shown, the results revealed that setting properties were improved respecting the positive measurement of zeta. This behavior explained that the dominant mechanism of flocculation into larger flocs was governed by the interparticle bridging rather than by the charge neutralization.

Fig. 12
figure 12

Zeta potential using sludge sample with TSS = 1.265 g L−1

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Therefore, the addition of a pure cationic polyelectrolyte and first supernatant lowered the zeta potential as shown in Fig. 12-a and b, while the opposite, an increase in the zeta potential was seen with the use of second supernatant (Fig. 12-c). The mechanism of this behavior may be referred to van der Waals force attraction as explained in the above results.

Cost-effective utilization of supernatant for sludge treatment

The use of supernatant for sludge treatment could reduce the cost of using pure polyelectrolyte for such treatment. The cost of which ranges between 2000 and 3000 USD per ton.

As per the experimental study, using the maximum dosage 100 mg polyelectrolyte per one liter of wastewater would require 100 g polymer/m3 of wastewater. Accordingly, the total amount of polyelectrolyte for treatment of one million m3/day is 100 tons d−1 which cost around 300,000 USD d−1. However, considering the use of supernatant, the cost would be reduced by more than 50%. This can be considered as a sustainable economic method for pollution reduction especially in the developing countries.

It was found that the application of maximum dosage 100 mg polyelectrolyte per liter of wastewater for treatment, this would require 100 g polymer/m3 of wastewater. Accordingly, the total amount of polyelectrolyte required for the treatment of 100 liter per day needs only 30 USD. However, we considered that the use of supernatant would reduce the cost of treatment more than 65%. In terms of fresh polyelectrolyte, it requires huge money and management of sludge solids so that sustainable economic method for pollution reduction could be achieved especially in the developing countries.

Conclusions

The application of supernatant polyelectrolyte can be considered as sustainable and cost-effective techniques for sludge conditioning and solid reduction especially in the developing countries. A high positively charged SNF polyelectrolyte was used for sustainable treatment selected for wastewater treatment. The findings of this study compared to polymer and the supernatant indicated significant effluence on the sludge volume index (SVI) and sludge settling. Both settling and SVI were improved by 50% and 60%, respectively.

  • The utilization of supernatant for sludge treatment would reduce the cost of sludge conditioning by almost 50%.

  • The results of using supernatant improved sludge settling by 50%, and SVI improved by 60% especially for sludge samples with low TSS.

  • Using mixed pure polyelectrolyte and supernatant gave better settling results compared to that of using only supernatant.

  • Increases in zeta potentials were observed for the case of using pure polymer and supernatant.

  • The obtained results showed that using supernatant could be attributed to utilize the excess polyelectrolyte particles remaining in the supernatants or that unattached particles, hence, reducing cost of operation.