Investigation on the effective disposal of sludge from a water treatment plant

Large quantities of sludge are generated during the drinking water treatment process, which employs coagulation, sedimentation, and filtration processes for water purification (Chen et al. 2010; El-Didamony et al. 2014). With the rapid growth of population, the demand for potable water is escalating (Ahmad et al. 2016; Lim et al. 2016). The major sources of waste are sedimentation basins and filter backwashes. Alum coagulation sludge, which is high in viscous metal hydroxides, comprises large quantities of small particles. These are among the most difficult forms of sludge to handle because of their low settling rate, low permeability to water, and thixotropic characteristics. Contamination of surface water is caused primarily by the discharge of effluents by industries. Industries discharge different pollutants in various concentrations causing a temporal variation in raw water quality. Due to this variation, it is necessary to change the dosing of chemicals, hence creating qualitative and quantitative changes in the composition of water treatment sludge (WTS; Ahmad et al. 2017).

Disposal of sludge from water treatment plants has become an important issue of global environmental concern due to problems relating to land and toxicity of such waste components (Nguyen et al. 2022). Investigations were conducted to convert sludge into a reusable resource such as fertilizer in agriculture and in brick manufacturing.

Dumping dewatered sludge into landfills is a common practice in many countries and leachate from such dumping can contaminate groundwater (Babatunde & Zhao 2007; Sales et al. 2011; Ahmad et al. 2017). Acid leaching, base leaching, ion exchange, and membranes are four of the available methods to recover aluminum from sludge (Ooi et al. 2018; Hassan Basri et al. 2019). Acid leaching is the most preferable method due to the high efficiency and low cost involved in the process. According to previous investigations, the recovery rate of aluminum from sludge was reported between 40 and 100% (Ramadan et al. 2008; Rodríguez et al. 2010; Ooi et al. 2018).

A number of investigations have been conducted in manufacturing bricks incorporating WTS during the last two decades (Babatunde & Zhao 2007; Chiang et al. 2009; Cherifi et al. 2011; Huang & Wang 2013; Kizinievič et al. 2013). However, a negative public opinion has arisen on the occupational health of brick manufacturers and end users, prompting the necessity of investigating the mobility of hazardous chemicals in bricks from sludge. It has been found that either the removal or the fixation of hazardous chemicals into the metrics is the best option to avoid potential health hazards (Huang et al. 2005; Chiang et al. 2009; Boltakova et al. 2017). The removal of hazardous chemicals is not practical and is costly due to the complexity and inhomogeneity between different sludge samples insinuating to rely on suitable fixation strategies to prevent the exposure.

Thirty samples of WTS were collected from the BDWTP during different water turbidity periods and mixed together to obtain a homogeneous sample. Three clay samples were collected randomly from a brick manufacturing site at Kelaniya, Sri Lanka, and used for manufacturing bricks. Fifty bricks were manufactured by mixing collected clay and homogeneous WTS at different ratios. A composite sample was made from a number of discrete samples that were collected from a body of brick material and combined into a single sample to consider this single sample to be a representative of the components of that body material.

Thirty sludge samples were subjected to the determination of metal ions through a dry ashing method. In this method, about 5.0 g of WTS sample was air-dried, then grounded and passed through a 1 mm stainless-steel sieve, and then stored in a plastic bottle. Before analysis, the samples were dried at 65 °C for 48 h. About 1 g (accuracy 0.01 g) from each sample was weighed and dry-ashed. The samples were digested in an aqua-regia solution for 1 h and filtered through a 0.45 μm filter paper. The filtrate was quantitatively transferred to a 100 mL volumetric flask and topped up by adding distilled water. These solutions were stored in plastic bottles at a temperature of 4 °C till the analysis was completed. The thawed and vortexed samples were tested for As, Al, Fe, Pb, Ni, Cr, and Mn by using atomic absorption spectroscopy (AAS).

Sludge samples were collected and subjected to the alum recovery process, which consists of two main processes, alkaline extraction and acid extraction (Ooi et al. 2018). Five sludge samples representing various turbidity conditions were placed on a watch glass and kept for 3 h at 220 °C in an oven. Then, sludge was finely ground.

Approximately 1.0 g of a dry sludge sample was added into each 50 mL of 0.05 M sulfuric acid and the pH was monitored. The suspension was stirred using a magnetic stirring plate for 45 min. The prepared sample was kept overnight and filtered quantitatively using a 55 μm filter paper. The same procedure was followed with a 1.0 M NaOH basic solution.

The filtrate was transferred into a 500 mL volumetric flask and brought to the final volume by using distilled water. Then, the pH of filtrate was measured by using a pH meter and stored in a clean plastic bottle at 4 °C till the completion of the analysis.

Approximately 5 g of dry sludge was added into 500 mL of distilled water and the above procedure was followed for the suspensions.

Approximately 16 mL of aqua-regia solution was added into 2.5 g of a dry sludge sample and the suspension was stirred for 2 h while heating at 300 °C. The prepared sample was kept overnight and filtered using a 55 μm filter paper, quantitatively transferred into a 500 mL volumetric flask, brought to the final volume by using distilled water, and stored in a clean plastic bottle at 4 °C till the analysis. The Al content was determined according to the HACH DR 2800 Spectrophotometer procedure manual, Method 8012.

WTS was mixed with clay. After thoroughly mixing sludge, clay, and water, the bricks were molded using a steel mold and assembled bricks were stacked in the field for air drying. In dry seasons, 3 or 4 days of air drying is sufficient before burning. Then, these bricks were loaded into the kiln and burnt with wood fire for 12–14 h. Burnt brick was allowed to cool.

WTS was mixed with clay in proportions of 25%:75% and 50%:50% by volume. As controls, clay bricks were prepared without using WTS. For each test, 50 bricks were prepared using a mixture of sludge and clay, kept for air drying for 4 days together with normal bricks, and then loaded to the kiln for burning. After cooling and visible inspection for cracking, the bricks were subjected to the testing of dimensions, compressive strength, water absorption, and efflorescence.

Five dry WTS samples (each weighing 5.0 g) were placed in plastic containers and 250 mL of distilled water was added to each container and placed in a jar tester for 1 and 18 h, respectively, for settling. Then, 100 mL of the supernatant in each sample was withdrawn and a nitric acid digestion method was used. The solutions were stored in plastic bottles at 4 °C till testing. The thawed samples were vortexed and tested for metal ions by using AAS.

Five dry sludge samples of weight 5.0 g were placed in a glass beaker and 250 mL of distilled water was added to each container and placed on a jar tester for 1 and 18 h, respectively, to settle the particles. Then, the supernatant of each sample was pipetted out and stored in clean plastic bottles at 4 °C for the determination of inorganic parameters and COD.

Six different samples were subjected to an analysis of inorganic and COD. Each anionic parameter was analyzed with respect to program numbers at selected wavelengths with different spectrophotometric methods listed in Table 1, and a standard solution series was prepared for each parameter for quantification. The COD value was determined by the open reflux method.

About 20.0 g of WTS and clay samples that were used for manufacturing bricks and manufactured brick samples were air-dried, then grounded and passed through a 1 mm stainless-steel sieve, and then stored in plastic bottles. Each sample was dried at 65 °C for 48 h prior to the analysis. About 1 g (accuracy 0.01 g) from each sample was weighed in a crucible. Then, the dry ashing method was used (Hseu 2004) and digested in an aqua-regia solution. Then, the solution was filtered through a 0.45 μm filter and quantitatively transferred to a 100 mL volumetric flask. The solution was then stored in plastic bottles at 4 °C. The thawed samples were vortexed and tested for metal ions by using AAS.

Three manufactured brick samples were placed in plastic containers and 3 L of distilled water was added to each container and kept 18 h, respectively, for settling. Then, 100 mL of the supernatant in each sample was pipetted out followed by the nitric acid digestion method using analytical grade nitric. During acid digestion, each supernatant was added with 2 mL of concentrated nitric acid and heated in a hot plate at 90–95 °C until the volume reduced to 15–20 mL. Then, the solution was filtered using a 0.45 μm filter and volume up to 100 mL. The solution was stored in plastic bottles at temperatures less than 4 °C till the analysis was completed. The thawed samples were vortexed and tested for metals ions by using AAS.

Thirty water treatment sludge samples were collected during different turbidity seasons in Kelani River. The statistic tool R i386 3.2.1 box-plot analysis, Shapiro-Wilk normality test and non-parametric and parametric test was performed for the data set to determine our data was match with standard values.

The clay samples collected were mainly composed of laterite-based soil and characterized by water-insoluble oxides such as Fe₂O₃, Al₂O₃, SiO₂, and TiO₂. Among them, SiO₂ was the most dominant oxide followed by Fe₂O₃ and Al₂O₃ (Nayanthika et al. 2018). The selected analyzed total metal content in the clay samples is presented in Table 2.

WTS had Mn, Al, and As compared with clay samples, clay was rich in K, Cd, Pb, and Cr compared with WTS, and the bricks formed intermediate levels depending on the mixing composition. Based on the data, it was clear that the recovery of Al prior to the formation of the bricks and the fixing of hazardous metals into the bricks were the best options for the effective use of sludge with minimum effect on the environment.

After a visible inspection for cracking, no cracks were observed in WTS after it was mixed with clay in proportions of 25%:75% samples. Dimensions, compressive strength, water absorption, and efflorescence were tested in this sample, and the results are given in Table 3. The results of manufactured bricks were compared under the handmade category with the requirement of SLS 39:1979. Except for the parameter of dimensions, all other required parameters complied with all types of constructions under the Type II category. The bricks that were manufactured by using 100% clay were measured under laboratory conditions, and the results of the dimensions were found to be the same as those of WTS that was mixed with clay in proportions of 25%:75% samples. For brick manufacturing, the manufacturer had used a traditional molder and dimension errors existed in the molder.

The leachate capability of 1 kg of WTS was estimated and it contained nitrate (2,150 mg/kg), nitrite (50 mg/kg), sulfate (16,000 mg/kg), phosphate (2,200 mg/kg), ammonia (370 mg/kg), and COD (2,800 mg/kg) (Table 4). Also, a relatively high concentration of heavy metals Pb (0.2 mg/kg), Cd (0.1 mg/kg), Cr (0.9 mg/kg), and Ni (1 mg/kg) were leached into the environment. Micronutrients like Fe (4 mg/kg), Zn (0.5 mg/kg), and Cu (0.1 mg/kg) (Table 4) were also present in the leachate.

Table 4

Measured inorganic, organic chemical, and heavy metal parameters of sludge leachate

Sample number .	-N .	-N .	.	NH3-N .	.	COD .	Pb .	Cd .	Cr .	Fe .	Zn .	Cu .	Ni .
	Mean (mg L−1) .
4	50	0.7	38	7	300	40	0.005	0.003	0.013	0.029	0.006	ND	0.020
15	50	0.1	47	5	100	56	0.001	0.001	0.017	0.038	0.007	0.001	0.017
18	60	0.4	43	9	200	72	0.002	0.002	0.016	0.015	0.012	0.002	0.025
21	30	2.5	49	11	600	48	0.001	0.001	0.019	0.030	0.010	0.004	0.019
26	70	1.2	42	5	400	64	0.013	0.001	0.022	0.285	0.013	0.004	0.020
Average	43	0.98	43.8	7.4	320	56	0.004	0.002	0.017	0.080	0.010	0.003	0.020
Amount leached (mg/kg)	2,150	50	2,200	370	16,000	2,800	0.2	0.1	0.9	4.0	0.5	0.1	1.0

Sample number .	-N .	-N .	.	NH3-N .	.	COD .	Pb .	Cd .	Cr .	Fe .	Zn .	Cu .	Ni .
	Mean (mg L−1) .
4	50	0.7	38	7	300	40	0.005	0.003	0.013	0.029	0.006	ND	0.020
15	50	0.1	47	5	100	56	0.001	0.001	0.017	0.038	0.007	0.001	0.017
18	60	0.4	43	9	200	72	0.002	0.002	0.016	0.015	0.012	0.002	0.025
21	30	2.5	49	11	600	48	0.001	0.001	0.019	0.030	0.010	0.004	0.019
26	70	1.2	42	5	400	64	0.013	0.001	0.022	0.285	0.013	0.004	0.020
Average	43	0.98	43.8	7.4	320	56	0.004	0.002	0.017	0.080	0.010	0.003	0.020
Amount leached (mg/kg)	2,150	50	2,200	370	16,000	2,800	0.2	0.1	0.9	4.0	0.5	0.1	1.0

View Large

The chemical parameters varied with the raw water quality, amount of chemical dosing, and time of retained sludge in the sedimentation tanks such as the percentage of the sedimentation values opening, desludging time, and frequency (Husillos Rodríguez et al. 2011). A significant variation in sulfate concentration was observed in different samples, which varied between 100 (mg L⁻¹) and 600 (mg L⁻¹) due to different levels of alum dosing in various turbidity conditions.

The metal content of the leachate samples was determined, and only quantifiable amounts of Pb were found in Sample 1 (0.19 μg/kg) and in Sample 2 (0.32 μg/kg). No leachate was found for Al, Cd, Na, K, Zn, Cu, and Mn.

The mean value of the cation exchange capacity (36.7) represents clay loam grade in soil texture category and can be further categorized into elite type by considering the clay category. The results obtained indicated the presence of various types of metals in varying concentrations and also emphasized that heavy meals were present in the brick samples (Table 2). Pb and As metals were in relatively low percentage when compared with raw materials due to evaporation at high temperatures in the kiln. The metal K was high in the brick samples due to contact with dry ash during the manufacturing process.

The result of manufactured bricks demonstrated that sludge can be used to produce good-quality bricks for various engineering applications in construction and building by fulfilling the SLS 79:2003 standard. Sludge clay burnt bricks can be successfully produced using water treatment plant sludge as a supplement for clay. The proportion of sludge in the mixture was affecting the quality of brick. The heavy metal leachability of manufactured bricks was also analyzed and it was found that more than 99% of heavy metals were fixed in these bricks; therefore, the manufacture of bricks is an environmentally benign process.

This investigation mainly focused on the recovery of alum and manufacturing bricks. The recovery of the aluminum percentage in WTS was comparatively low due to the effect of polymer. Therefore, the recovery of aluminum must be done prior to the sludge treatment process. Leachate analysis confirmed that 99% of the heavy metals present in the sludge firmly bound to the manufactured bricks. The metal content of the leachate samples was determined, and only quantifiable amounts of Pb were found in Sample 1 (0.19 μg/kg) and in Sample 2 (0.32 μg/kg). No leachate was found for Al, Cd, Na, K, Zn, Cu, and Mn. Therefore, the manufacture of bricks using sludge is an environmentally friendly process.

The authors of this manuscript would like to thank the staff of the Biyagama Water Treatment Plant – National Water Supply and Drainage Board – for helping them in the process of data collection.

Data cannot be made publicly available; readers should contact the corresponding author for details.

The authors declare that there is no conflict.