The performance of sludge blanket clarifier against conventional settler under high water turbidity conditions

Usually, preliminary disinfection by ozone, chlorine dioxide, or chlorine improves the removal of small concentrations of algae in conventional sedimentation tanks (Plummer & Edzwald 2002). However, high concentrations of algae causes many problems and increases the consumption of disinfectants in treatment plants (Gerde et al. 2014). Further, a high concentration of algae cause many other problems such as color, odor, taste and toxic compounds, as well as reducing the efficiency of water treatment and increasing the clogging of filters (Chen et al. 1998; Teixeira & Rosa 2006; Henderson et al. 2008).

In regions with a hot climate, algae removal by sedimentation is not easy (Tumsri & Chavalparit 2011). Although high temperatures increases the efficiency of sedimentation for most particles due to the decrease in fluid viscosity, it has a negative effect on the removal efficiency of algal cells (Ma & Liu 2002). At high temperatures, most algae cells swell and release large quantities of gases that reduce the coagulation of these cells. Conversely, at low temperatures, better algal removal is achieved as production of gases due to algae photosynthesis is decreased, but low temperatures decrease the efficiency of the sediment particles due to a decrease in fluid viscosity (Wu et al. 2007). Therefore, algae removal is improved at temperatures of 10°C and below, but removal of solids is improved only at temperatures of 20°C and above (Al-Layla et al. 1974; Wells & LaLiberte 1998; Mkpenie et al. 2007; Goula et al. 2008).

Although algae and clay colloids are the same size, ranging from 0.001 to 1.0 microns, the density of algae (1.2 g/cm³) is generally lower than that of clay (2.6 g/cm³) (Han & Kim 2001). Compared to clay colloids, algae consumes a large amount of alum (Al-Layla et al. 1974). Further, the presence of algae in sediments leads to changes in sludge characteristics and difficulty in its removal (Thomsen & McGlathery 2006).

Recently, numerous studies were carried out on the removal and control of algae in water treatment plants (WTPs). These studies include: air floatation (Dahlquist & Kulesza 2001), electrocoagulation, or electrocoagulation combined with electro-flotation (Gao et al. 2010). However, the application of these studies is very limited in the field (Kawakami et al. 2016). Compared to conventional sedimentation, flotation is considered to be a more efficient process for algae removal (Chen et al. 1998; Teixeira & Rosa 2006; Henderson et al. 2008).

Sludge blanket clarifier technology is very popular all over the world and is a good solution for algae removal. The main advantages of sludge blanket clarifiers are that they can produce a high quality effluent at minimum cost, as flocculation and clarification can be accomplished within a single reactor (Lin et al. 2004; Head et al. 1997). The use of sludge blanket clarifiers for clean water production was developed in the early nineties (Bare et al. 1975; Gates & McDermott 1968). Recently, this technique has been enhanced with surface sludge cones for the removal of excess sludge, called a flatbed clarifier. Flatbed clarifiers are an updated version of sludge blanket clarifiers that have good potential for upgrading sedimentation tanks, especially for algae removal.

Ibshan is a large WTP located in north Egypt, which receives high turbidity water (turbidity = 30–40 NTU and algae concentration up to a billion cells per litre). Ibshan WTP contains three different rectangular clarifiers, consisting of a flatbed clarifier, gravity sludge hoppers, and a conventional sludge scraper.

The main objective of this study was to determine the sludge accumulation as well as the removal efficiency for turbidity and algae for the aforementioned settlers based on real observations in the field. So, this study aims to compare the performance of a sludge blanket clarifier, scraper settler, and hopper zone settler with respect to their removal efficiency, solids accumulation patterns and alum dose under high turbidity water conditions with a high algae concentration. The observation of these clarifiers was undertaken in two phases. The first phase was carried out during maintenance, while all clarifiers were empty for cleaning. After cleaning, the second phase was carried out over 12 months of operation.

Data in this study was collected from the field and confirmed using a laboratory-scale set up, as explained below.

From the field, real data over 12 months were taken from Ibshan WTP, located in north Egypt. Ibshan WTP is large compound plant that has three different settlers to treat a discharge of 800 l/s. Figures 1 and 2 show aerial photos and a schematic diagram of Ibshan WTP and the parameters of each settler. The raw water entering this plant has high levels of both turbidity and algae, up to 40 NTU and 10⁹cells/liter, respectively. Ibshan WTP includes three different settlers run in parallel: a sludge blanket clarifier, conventional settler with scraper, and sludge hopper settler with discharges of 400 l/s, 200 l/s, and 200 l/s respectively. The sludge blanket clarifier, conventional settler with scraper, and sludge hopper settler were designed for retention times of 85, 120, 170 min respectively and a surface loading of 67.5, 48, 34 m³/m²/day respectively. Every year, during maintenance, water is discharged from the above aforementioned tanks and the sludge accumulation within each tank is observed and removed. So, the field data have been included in the analysis of the removal efficiency of turbidity and algae as well as the sludge accumulation patterns in each tank after one year's operation.

To supplement the field findings, laboratory-scale experiments were carried out using a small-scale setup under continuous conditions. Figures 3 and 4 illustrate the flow diagram and schematic of the bench-scale setup that was used in the present study. A laboratory-scale settler with a volume of 120 L was operated as a conventional settler and sludge blanket under separate conditions (Figures 3 and 4). The settler consisted of a glass basin 60 cm long, 50 cm wide and 40 cm high to handle a volume of 120 liters of water. A digital monitor was connected to measure total dissolved solids (TDS), conductivity, temperature, and pH. The laboratory experiments were conducted in this settler using conventional techniques, and then the same settler was converted to sludge blanket mode. Powdered clay and algae were used as two sources of turbidity that reached 80 NTU separately. The pH and oxygenation levels were controlled at 7.0 and 7.0 mg/L, respectively. In each case, the source of turbidity was initially clay only, then the clay content was gradually reduced and the algal content was gradually increased until the source of the turbidity was converted to algae only. So, the experiment was conducted with differing ratios of clay to algae under the same overall turbidity value in the conventional settler and sludge blanket phases. The primary characteristics of the initial synthetic raw water using powdered clay was as follows: NTU = 80, pH ≈ 7.0, dissolved oxygen (DO) ≈ 7.0, temperature ≈ 25–30°C, without any algae. However, the primary characteristics of the synthetic raw water containing algae were as follows: NTU = 80, algae concentration ≈109 cells/l temperature ≈ 25–30°C, DO ≈ 7.0 without any clay. The settler was run under different ratios of clay to algae solutions from 0.00 to 100% under a fixed turbidity of 40 NTU.

The results of this study were divided into two parts: field observation and laboratory experiments. From the field, sludge accumulation, turbidity removal, and algae removal were observed. From the laboratory, removal of both turbidity and algae were observed.

Figures 5–8 show the sludge accumulation pattern of each tank after one year of continuous operation. It is clear that highest accumulation of sludge occurred inside that settler with the sludge scraper, whilst the sludge blanket system achieved the lowest accumulation of sludge. Field data confirm that the sludge blanket tanks did not contain sludge accumulated at the bottom, except for some insignificant groups that do not reduce the efficiency of tank removal.

Sludge accumulated within the tank containing a sludge scraper from an initial point of 0 cm to 80 cm, gradually moving from the tank inlet to the tank outlet in the form of a wedge prism. The main reasons for sludge accumulation in this tank are stopping the scraper even only occasionally, or slowing down its movement and passing the scraper in the reverse direction over that settled sludge (Figures 5 and 6). Frequent accumulation of sludge layers at the tank bottom produce a wedge sludge prism growing at the end of the tank.

However, in the tank of longitudinal sludge hopper, the sludge accumulated along the tank bottom uniformly in the form of the longitudinal hopper shape as shown in Figure 5. In this tank, annual accumulation occurred along the bottom, reaching a depth of 1 meter at the tank walls by the end of the year. The main reasons for sludge accumulation in this tank are that the slope angle is insufficient for collection of all sludge or the increasing friction of the tank hopper surface increases accumulation (Figure 7).

Finally, in the sludge blanket tank, little accumulation of sludge was observed along the tank bottom, especially under the sludge cones used for sludge collection as shown in Figure 6. So, the sludge blanket tank did not contain much sludge accumulated at its bottom, except insignificant amounts that did not reduce the efficiency of turbidity removal (Figure 8).

The accumulated sludge in each tank has reduced the sedimentation zone, thus decreasing the removal efficiency. Figure 9 shows the turbidity removal efficiency of three settlers over 12 months of operation after sludge discharge from these tanks. It is clear that, for that tank with a sludge scraper, the removal efficiency reduced from 86.1 to 62.1% after one year of continuous operation, as shown in Figure 9. However, in the tank containing sludge hoppers a reduction in sedimentation efficiency was observed from 93 to 79%. Finally, in the tank containing a sludge blanket, the removal efficiency was only reduced from 94 to 88% after one year of continuous operation.

Algae removal was not recorded over the whole year, just turbidity, in Ibshan WTP. However, the removal efficiency of algae and turbidity were recorded in the first three months after the cleanup of these settlers. Figures 10 and 11 show the effluent turbidity and algae content respectively in the first three months of operation after cleaning took place in the settlers. Figures 10 and 11 show that, even in clean tanks, the sludge scraper and longitudinal sludge hopper tanks showed a low removal rate of both turbidity and algae compared to the sludge blanket. Although these tanks were run with a long retention time (170 minutes) and low surface loading (34 m³/m²/d) their removal efficiency was low, compared to the sludge blanket tank, which was operated with a shorter retention time and higher surface loading of 85 minutes and 67.5 m³/m²/d, respectively (Figures 10 and 11). In other words, the sludge blanket clarifier not only treated double the volume of water compared to the tank with a longitudinal sludge hopper, but also had a higher removal efficiency for both turbidity and algae.

The above results led the authors to re-measure the DO in each tank under a high algae concentration and review again the density of algae cells. It was found that the majority of submerged algae had a density of less than 1.2 g/cm³, which fits with results previously published by several other researchers such as Oliver et al. (1981), while Hu (2014) confirmed that most floating algae have a density of less than 1.0 g/cm³. Field measurements confirmed that, under a high algae concentration (10⁹ cell/l), solar radiation can easily penetrate the upper surface layers of the conventional tank, so a high algae concentration increases the level of DO inside this tank. DO values changed inside the same tank from 6.75 mg/l during the day to 8.55 mg/l at night. Therefore, the optimum alum dose was also re-estimated under different DO values (6.75 to 8.55 mg/l). It was also found that the optimum alum doses were not the same under different DO values. Therefore, penetration of light into the conventional settler excites the algae to produce high DO values, which allows the algae to overcome the optimal alum dose and leads to algae floating near the tank surface, thus reducing the removal efficiency of such tanks. Conversely, in the sludge blanket clarifier, the sludge blanket prevents light from penetrating the clarifier, therefore limiting algae activity. Moreover, it is easy to remove most types of algae when moving the water stream towards the surface of the tank regardless of the type of algae (floatable or submersible).

To supplement the above findings, Figure 12 shows turbidity removal efficiency after mixing clay and algae experimentally. In fact, these experiments were carried out outdoors with the settler subjected to solar radiation. It is clear that increasing the algal content in the raw water decreases the removal efficiency in conventional settlers, but no effect was seen on the performance of the sludge blanket clarifier. Further, Figures 13 and 14 show the removal efficiency of conventional and sludge blanket settlers under high turbidity conditions using clay and algae. It is clear that the performance of sludge blanket settlers is better than that of conventional settlers, even when operating at a short retention time and with high surface loading.

In conventional sedimentation tanks that are equipped with scrapers, sludge accumulates and hardens as a sediment layer due to several reasons. These reasons include: stopping the scraper for long periods and passing the scraper in the reverse direction over the settled sludge. Frequent accumulation of sludge layers at the tank bottom produces a wedge sludge prism growing at the end of the tank. This accumulated sludge reduces the sedimentation zone within the tank, resulting in a decrease in removal efficiency from 86.1 to 62.1%.

In sedimentation tanks using sludge hoppers, annual accumulation occurs along the bottom and reaches a depth of 1 meter at the tank walls by the end of year, which reduces sedimentation tank efficiency from 93 to 79%.

Laboratory experiments and extensive field data confirm that sediment tanks using a sludge blanket do not contain sludge accumulated at the bottom, except for some insignificant settling that does not reduce the efficiency of turbidity removal.

With respect to total turbidity, the average removal efficiency of these settlers were 88, 79 and 62.1% for the sludge blanket, sludge hoppers, and sludge scraper, respectively.

The authors wish to acknowledge the assistance given by the lab assistant and all the plant staff at Ibshan WTP, as well as all members of the Kafr El-Sheikh Water Company.