Evaluation of sludge handling using acidification and sequential aluminum coagulant recovery: case study of El-Sheikh Zayed WTP

Coagulation is one of many steps used in surface water treatment to remove suspended particles, colloids, organics, color, and microorganisms from water before drinking (Keeley et al. 2014a; Fouad et al. 2017a, 2017b). Coagulation is carried out by mixing a coagulant with raw water. Common coagulants used worldwide include aluminum sulfate (alum), ferric chloride, and poly-aluminum chloride. Huge quantities of sludge from water treatment plants are produced when coagulants are added to raw water. In Egypt, alum is the common coagulant used in water clarification. Egyptian treatment plants annually consume about 365,000 tons of aluminum sulfate in the water clarification process. More than a million tons of wet alum sludge is produced daily by water treatment in Egypt.

In many developing countries such as Egypt, a common practice for the disposal of water treatment sludge is by dumping it into water bodies or sanitary sewers (Nair & Ahammed 2015). Disposal of water treatment sludge into waterways is reported to be toxic to aquatic life (Xu et al. 2017). In Egypt, due to regulatory changes, water treatment sludge has to be disposed into landfills. Since the levels of pollutants in the water treatment sludge are relatively low, as the best quality raw water sources are generally selected for drinking water production, recycling of alum sludge may be a feasible option instead of disposal. A number of research efforts has been made, particularly in recent years, to use the water treatment sludge in many beneficial ways. These include its use in building and construction materials (Fouad et al. 2017a, 2017b), in wastewater treatment (Nair & Ahammed 2015), and for soil conditioning (Keeley et al. 2014a). Two different approaches have been tested for the use of water treatment sludge in water and wastewater treatment. In the first approach, a coagulant is recovered from the sludge and is used for water and wastewater treatment. In the second approach, wet or dry sludge itself is used as a coagulant or adsorbent for removal of different contaminants. Recovery of coagulant metal from the sludge is an attractive option and has been reported in studies (Keeley et al. 2014a, 2016). Generally, four coagulant recovery methods can be employed on the chemical sludge generated from a water treatment plant. These methods include acidification, basification, ion exchange, and membrane processes (Keeley et al. 2014b). Most of the studies reported recently used acidification for coagulant recovery due to its high efficiency and low cost compared to other methods (Chen et al. 2012; Fouad et al. 2017a). Several factors are known to affect coagulant metal recovery using acidification (Keeley et al. 2014b; Fouad et al. 2017a, 2017b). Coagulant recovery from water treatment sludge was found to minimize the operational cost of water purification, preserve natural materials, and follow the new laws and regulations regarding disposal of sludge by reusing it after recovery instead of disposal in waterways. In addition, this will also improve the environmental quality from bioaccumulation of aluminum in the environment if released without treatment, which will protect aquatic life and, hence, human health and life.

In this study, the authors have tried to explore the feasibility of aluminum recovery from sludge using acidification to produce efficient coagulant and determine the limitations of the sequential recovery process to produce water with acceptable quality as drinking water. Especially trihalomethane (THM) concentration as the organic matter accumulation through the recovery process is the major concern for water production using recovered coagulant solutions.

This research work tested different parameters that affect aluminum leaching from El-Sheikh Zayed water treatment plant (ESZ-WTP) sludge using hydrochloric acid. Moreover, optimum conditions detected were used to prepare an aluminum coagulant solution. The efficiency of the coagulant was compared to the commercial one and the effect of the two coagulants on the produced water quality was tested. Three sequential aluminum recoveries were studied for the recovered coagulant and the quality of the produced water was monitored.

ESZ-WTP consists of four main treatment phases: coagulation/flocculation; clarification using tube settler design; filtration using rapid sand filters; and pre- and post-chlorination for water disinfection. Aluminum sulfate (alum) was used in the coagulation of water at ESZ-WTP. Chlorine and coagulant doses were 5.5 and 30 mg/L, respectively, at the time of sludge sampling. Drying lagoons were used for drying the alum sludge produced and collected from the tube settler clarifiers. Three drying lagoons were operated in the following modes: one lagoon in the filling mode, one lagoon in the drying mode, and the last lagoon in the standby mode.

The current study was carried out through three main phases. In Phase I, sampling, preparation, and characterization of sludge were done. In Phase II, the recovery of aluminum from sludge was applied using hydrochloric acid. Six different parameters that affect the recovery process were studied. The optimum values of the recovery parameters were applied to obtain a solution that had the best recovery of aluminum, then characterized and tested in water clarification and compared with the virgin coagulant (aluminum sulfate). In Phase III, sequential coagulant recovery from sludge using hydrochloric acid was evaluated. To sum up the experimental work phases and experiments, Table 1 was designed to provide a summary of the experimental steps carried out throughout the research.

The sludge was characterized by measuring: (1) total volatile solids by weight loss on ignition at 550 °C (Santisteban et al. 2004); (2) water content through drying 10 g of wet sludge at 105 °C for 2 hours and reweighing it (USEPA 2001); (3) total metals' content was determined through acid digestion according to the US-EPA Method 3050 B (USEPA 1996). It was applied through mixing 1 g of sludge with 10 mL of 1:1 HNO₃ and covering with a watch glass. The sample was heated to 95 °C ± 5 °C and refluxed for 10–15 min without boiling, then it was allowed to cool for 5 min. Afterwards, 5 mL of concentrated HNO₃ was added and the sample heated up to 95 °C ± 5 °C and refluxed. This process was carried out repeatedly until the entire sample was digested.

After digestion, aluminum, iron, magnesium, cobalt, manganese, vanadium, strontium, chromium, nickel, lead, sodium, potassium, calcium, and zinc were analyzed using inductively coupled plasma optical emission spectroscopy (Optima 8300 dv ICP-OES, manufactured by PerkinElmer, USA) according to the US-EPA Method 200.7 (USEPA 1994).

Five experiments were carried out to get the optimum conditions for coagulant preparation from sludge using hydrochloric acid, and were as follows.

In this experiment, the effect of hydrochloric acid concentration expressed as acid normality on aluminum recovery was studied. In this test, 500 mL of different hydrochloric acid solutions with different normality values was added to 5 g of alum sludge. The hydrochloric acid solutions were prepared at different normality values of 0.1, 0.5, 1.0, 1.5, and 2.0 N. Leaching with the different acid normality values was performed at a constant temperature of 25 °C. The acid solution and sludge were mixed at a constant rotational speed of 80 rpm for 20 min. After mixing, solutions were allowed to settle, then filtered. The filtrates were sampled to determine the concentration of recovered aluminum, hence, leaching efficiency.

Here, aluminum recovery was investigated using 1 N hydrochloric acid solution and 5, 10, 15, 20, and 25 g of alum sludge prepared as discussed before. The leaching of aluminum was performed at 25 °C. The acid solutions were mixed with the different masses of sludge at a constant paddle speed of 80 rpm for 20 min. The produced solutions were allowed to settle, then filtered, and aluminum was analyzed in filtrates and recovery efficiency was calculated.

In this trial, five mixing times were examined in five different jars. In each jar, 500 mL of 1.0 N hydrochloric acid solution was added to 5 g of sludge. Then, the solution was mixed at a constant speed of 80 rpm. The mixing was performed at a fixed temperature of 25 °C for 10, 20, 30, 40, and 50 min. At the end of the mixing, solutions were allowed to settle then filtered. The filtrates were sampled to determine the amount of recovered aluminum and the leaching efficiency.

In this experiment, the effect of solution temperature on aluminum leaching from alum sludge was investigated. Three different temperatures of 25, 40, and 60 °C were tested. The temperature was adjusted using a hot plate and a mercury thermometer. The temperature was kept constant during the mixing time. At each temperature, 500 mL of 1.0 N hydrochloric acid solution was added to 5 g of sludge mass. Then, the solution was mixed at a constant speed of 80 rpm for 20 min. At the end of mixing, solutions were allowed to settle then filtered. The filtrates were sampled to determine the recovered aluminum.

In this experiment, five mixing speeds were tested in five different jars. In each jar, 500 mL of 1.0 N hydrochloric acid solution was added to 5 g of sludge mass. Then, the solution was mixed at stirring speeds of 20, 40, 60, 80, and 100 rpm for 20 min. The effect of mixing speeds was performed at a constant temperature of 25 °C. After mixing the sludge with the acid solution, the suspension was allowed to settle then filtered as indicated before. The filtrates were sampled to determine the amount of dissolved aluminum.

This part of Phase II, consists of two experiments. In the first one, the optimum conditions obtained from the five experiments above were used to recover a coagulant in hydrochloric acid solution. In the second experiment, the recovered coagulant was tested against the commercial aluminum sulfate (alum) in the clarification of raw water sampled from ESZ-WTP using jar test.

After completing the five experiments in the first part of Phase II, the optimum values of the recovery parameters obtained from Part I of Phase II were applied to prepare a solution that had the optimum recovery of aluminum, hence used as a coagulant in the treatment process. The percent of aluminum recovered in the coagulant solution was measured. In addition, the recovered solution was evaluated by comparing its composition with the commercial alum that was used in ESZ-WTP at the time of the study.

In this experiment, raw water samples were collected from the intake of ESZ-WTP to perform the testing. Jar tester was used to evaluate the performance of recovered coagulant solution versus commercial alum in clarification of raw water. The commercial alum and recovered coagulant were added to 1 L of raw water at the same doses, which were 20, 25, 30, 35, 40, and 45 mg/L. Jar test was performed by filling six rounded jars of 1 L capacity for each by raw water, then coagulant was added and mixed with raw water at a mixing speed of 120 rpm for 2 min. Then, the water was mixed slowly at a mixing speed of 40 rpm for 18 min, which simulates the operating conditions in ESZ-WTP. The water was allowed to settle for 10 min before samples were collected from the supernatant. The collected samples of raw and clarified water were analyzed for pH, turbidity, temperature, conductivity, total dissolved solids, total alkalinity, chlorides, aluminum, iron, manganese, total THMs, algal count, total plate count, and total coliform bacterial count. All analyses of samples were performed according to Standard Methods for the Examination of Water and Wastewater (APHA, AWWA, WEF 2012).

In this phase, sequential coagulant recovery was studied in order to evaluate the accumulation of different contaminants in the recovered coagulants and to determine how long the sludge would be recyclable without affecting the quality of the treated water. The experiments in this phase were carried out using jar test (Reid & Edison 2004). Water samples were collected and analyzed according to Standard Methods for the Examination of Water and Wastewater (APHA, AWWA, WEF 2012). Four different experiments were the building blocks for this phase and they were as follows.

Jar test was carried out by applying optimum doses of virgin aluminum sulfate and chlorine on raw water and the produced sludge was subjected to recovery using hydrochloric acid as explained before to produce the first aluminum coagulant. This process was sequentially repeated twice to produce and evaluate the second and third recovered coagulants. The treated water each time was collected and evaluated through analyzing pH, turbidity, temperature, conductivity, total dissolved solids, total alkalinity, chlorides, aluminum, iron, manganese, total THMs, algae count, total plate count at 35 °C, and total coliform bacteria.

Alum sludge was characterized to explore the feasibility of coagulant recovery using hydrochloric acid and its application as an alternative for the commercial coagulant. The composition of alum sludge from ESZ-WTP is a reflection of the structure of the virgin coagulant used in the treatment process and the contaminants in raw water. The analysis of sludge collected from ESZ-WTP showed that the average values of the water and organic contents were 86% and 25%, respectively. In addition, Table 2 shows the concentrations of metals in the sludge collected from ESZ-WTP. As shown, the major metal constituents of the sludge were aluminum 13.3%, iron 2.2%, and calcium 1.2%. These are nearly the same constituents of the commercial alum used in the treatment process. This indicates that the main source of metals in the sludge is the coagulant used in the treatment and not from raw water.

The following part shows the results obtained from the different experiments (Experiments 1–5) carried out in Phase II, Part I to determine the optimum conditions for aluminum leaching from sludge.

It was also proved by Fan et al. (2013) that leaching rate increases greatly initially from waste super-alloys with increasing the stirring speed to 250 rpm. Nevertheless, when mixing speed is elevated more than 250 rpm, the leaching rate of Ni and Co decreases greatly because materials will be rotating with the solution and the effect of stirring decreased.

From the experiments of Phase II, Part I the optimum parameters of aluminum recovery from sludge were obtained. These parameters include a hydrochloric acid concentration of 2.0 N, an acid to sludge ratio of 1.83 on a mass basis, a mixing speed of 80 rpm, at a temperature of 40 °C and a mixing time of 40 minutes. The use of this combination of optimum values resulted in a higher recovery of aluminum from the sludge, which reached 82.4%. Table 3 shows the metals' content in the recovered coagulant solution compared with the commercial alum. It is obvious from Table 3 that the metals' concentrations in the commercial alum are higher than that in the recovered coagulant. This assures that the main source of metals in the recovered coagulant is the commercial one. In addition, the reduction in metals' concentrations of the recovered coagulant against the virgin solution is explained through two points: first, not all the metals are precipitated and removed in the sludge; and second, the leaching process did not perform 100% recovery for all the metals in the sludge.

In Phase II, Part II of this study, the optimum doses of the recovered coagulant solution and commercial alum were applied to the raw water using jar test. The analyses of the treated water by the optimum doses of recovered coagulant solution and commercial alum are reported in Table 4. As shown, the quality of the treated water using recovered coagulant solution and commercial alum was almost similar. Turbidity was 1.4 and 1.9 NTU when using commercial alum and recovered aluminum solution, respectively. This reveals that the recovery process did not affect leached coagulant efficiency in turbidity removal. This agrees with the findings of Keeley et al. (2014a), who reported that the recovered coagulants were capable of treating water and removed 60–70% of raw water turbidity for river and upland reservoir waters.

One of the main issues regarding alum sludge recycling using acidification is the accumulation of disinfection by-products, especially THMs, in sludge and their reintroduction into water being treated (Keeley et al. 2014b). As shown in Table 4, the concentration of the THMs in the treated water using the recovered solution is 0.09 mg/L, which is slightly higher than that when commercial alum is used (0.07 mg/L). Chloride concentration is also greater in the water treated using the recovered coagulant, which is attributed to the chloride ions introduced by the hydrochloric acid used in the recovery process. Another observation on the treatment using the recovered coagulant is the biological aspects of water including algal count; total plate count at 37 °C and total coliform bacteria were found to show a little difference in treated water using recovered coagulant if compared to the water produced using the commercial coagulant. Taking into account a fixed disinfectant dose and biological properties of raw water, the increase in algae and bacteria may reflect a small reduction in the efficiency of recovered coagulant in removing these organisms physically by coagulation and sedimentation.

This part discusses the results obtained from Experiments 1–4 of Phase III to evaluate sequential coagulant recovery and its effect on the resulting water quality.

As mentioned before, the main risk of using recovered coagulants is the accumulation of contaminants in sludge that are recycled in the recovery process of aluminum coagulants. To complete the picture regarding using recovered coagulants in water treatment safely without affecting the produced water quality, sequential (repeated) aluminum recovery was carried out by applying the optimum conditions for recovery using hydrochloric acid and then the produced waters were analyzed and are expressed in Table 5.

As shown in Table 5, although repeated recoveries do not show a significant change in the overall quality of treated waters, they show a remarkable increase in the total THMs. It can be seen that the highest concentration of the THMs appears using the third recovered coagulant; however, the water is still within acceptable limits for THM value for drinking water which is 0.1 mg/L. These findings are compatible with Keeley et al. (2014a), who reported that organic matter removed from surface water by a coagulation process is concentrated in the sludge. Acidification of alum sludge solubilizes organic matter as well as aluminum from sludge (Keeley et al. 2014b). In addition, performing the coagulant recovery process sequentially allows organic matter to be solubilized and reintroduced to water, then precipitated during coagulation and precipitation into sludge, then transferred to the recovered solutions again. In other words, the organic matter accumulation is directly proportional to the number of coagulant recoveries. Therefore, the increase in THMs is not due to THMs in sludge because they were already decomposed during the drying process in the preparation step (WHO 2005). The main cause is due to the organic content in sludge, which leached to the recovered solutions by acidification and acts as a precursor for THM formation during the disinfection process. This indicates that the maximum possible sequential times for coagulant recovery from ESZ-WTP sludge is three times, and then a virgin coagulant should be used to avoid accumulation of organic pollutants in the treated water. Thus, the recovery process will be applied on the sludge for three cycles of operation. The repeated recovery of aluminum coagulant from the sludge will reduce the amount used of virgin coagulant by about 66%. In addition, it will reduce the area needed for drying lagoons, as less sludge will be dumped into these lagoons.

The recovery of aluminum from sludge generated in water treatment can be achieved using hydrochloric acid. The aluminum recovery using the optimum conditions obtained through the study by adding hydrochloric acid of 2.0 N to dry sludge at an acid to sludge mass ratio of 1.83 and mixing them at 80 rpm for 40 minutes at a temperature of 40 °C results in a recovery efficiency of 82.4%. The recovered coagulant solution was analyzed and found to be similar in composition to the commercial alum. According to our study, the maximum possible sequential recoveries for coagulant from ESZ-WTP sludge are three times, then a noticeable increase in THMs will appear in the resulting water, so a virgin coagulant should be used after three times' recovery to refresh the sludge before applying the recovery process again. Recovered coagulant performance shows no significant deviation from commercial coagulant with three times' sequential recoveries, which empowers its usage on a large scale in Egypt.

We acknowledge the holding company for water and wastewater in Cairo, Egypt; especially the quality and environmental affairs general department and the drinking water reference laboratory and the wastewater reference laboratory for the support they gave the research team to accomplish this article.