Research Area

The study site was located in Petch Siam village, Bangkok, Thailand (13°53´00´´N-100°38´02´´E). The shallow pool was small, and highly eutrophic with dense phytoplankton blooms. It had lost its recreational value and function. The shallow pool had a water surface area of approximately 472 m² and an average depth of 0.7 m. The influx of nutrients was primarily from the drainage system of a village and from duck feeding.

Application of Polyaluminum Chloride

Commercial-grade polyaluminum chloride (PAC) (Al₂O₃>30%) was purchased from Thai PAC Industry Company Limited, Bangkok. The experiment was conducted in situ during July-August 2021. We used a dosage of PAC at 30 g/m² and therefore, approximately 15 kg of PAC was applied to this water body in total. PAC powder was added to water at a ratio of 1:5 in a 200 l plastic container and mixed well to dissolve. Then the PAC in solution was sprayed on the shallow pool surface using a spray unit. Water samples were taken in four consecutive weeks: week one (before PAC application), week two (during PAC application), week three (one week after PAC application) and week four (two weeks after PAC application).

Water Quality Analysis

Water samples were collected from five sampling points at two depths (surface water and near-bottom water). We monitored the water depth (m) and environmental variables, consisting of temperature (ºC), transparency (cm), pH, dissolved oxygen (DO, mg/l), conductivity (µs/cm), and total dissolved solids (TDS, mg/l) using a multiparameter analyzer (Consort 633) and an EXO4-Port water quality sonde. In addition, water samples were collected in plastic bottles, kept in plastic containers at 4ºC, and then analyzed for chemical variables in the laboratory. Total nitrogen (mg/l), total phosphorus (mg/l), alkalinity (mg/l), biological oxygen demand (BOD₅, mg/l), and chlorophyll a (µg/l) were analyzed in triplicate (Table 1). The microcystin content (µg/l) in the water was determined using a Microcystins-Adda ELISA kit (Abraxis, USA) and the absorbance was read at a wavelength of 450 nm using a microplate spectrophotometer with two replicates. Before analysis, all water samples were filtered through glass microfiber filter grade GF/C diameter 47 mm. The TN-to-TP ratio was calculated to identify the nutrient limiting factor(s), with a ratio lower than 10 indicating that phosphorus is a limiting factor, and a ratio higher than 17 indicating that nitrogen is a limiting factor. A ratio between 10 and 17 indicates that either nitrogen or phosphorus could be a limiting factor [18].

Phytoplankton community and structure were determined using a 20 µm plankton net. A sample (1 L) of water was collected from the center of the shallow pool and then poured through a phytoplankton net. Samples were preserved using four percent formaldehyde and identified to the species level. Plankton was counted and calculated for density and composition in the laboratory using a Sedgwick-Rafter counting chamber under a compound light microscope.

Sediment Quality Analysis

In this study, sediment samples were collected from five sampling points and were analyzed for various properties: soil texture using a hydrometer, total nitrogen and total phosphorus contents (g/kg) using digestion, organic matter content (%) using the Walkley and Black titration method, and total aluminum content (g/kg) using atomic absorption spectrophotometry (Table 1). To study floc formation and accumulation on the bottom sediment, a gravity corer was used to take surface sediment samples from five sampling points before, during, and after PAC application. The surface sediment layer was studied by direct observation and photographs.

Statistical Analysis

Differences in water quality and environmental factors during the experiment were analyzed based on one-way ANOVA using the homogeneity of variance test. If the value was greater than 0.05, then the data were equally distributed, whereas if the value was 0.05 or less, the distribution was unequal. Differences were determined through the robust test of the equality of means (Welch). For the ANOVA analysis and the Welch test, a p value less than 0.05 determined a significant difference between variables. Then, further analysis of the different trial sets was performed with post hoc multiple comparisons. For case of equal distribution (equal variances assumed), Tukey’s test was applied. If the distribution of the data was not equal (equal variances not assumed), Dunnett’s T3 test was applied. Statistical analysis was performed using the IBM SPSS Statistics software, authorized user version 22 (USA). Results were presented as mean±standard deviation.

Water Quality

The current study highlighted the use of PAC and compared the water quality before, during, and after its application to restore a hypereutrophic urban shallow pool in Thailand. Before PAC application, this shallow pool was hypereutrophic with exceptionally high concentrations of nitrogen and phosphorus (Table 2) and this resulted in a high concentration of chlorophyll a, indicated by severe phytoplankton blooms. Transparency depth was low due to high turbidity and chlorophyll a values. The water quality had also deteriorated due to high BOD₅ values. The TN:TP value showed that the pool was nitrogen limited. In this shallow pool, phosphorus was more abundant than nitrogen and this may result from urban runoff and discharges from surrounding areas. During and after PAC application, the overall water quality improved markedly (Fig. 1). The water pH in this study decreased from 8.9±0.1 to 7.3±0.1 similar to [19] showing that the PAC themselves reduced the water’s pH from approximately 8.15 to 7.6-7.8 due to the coagulant addition; however, it was still in the neutral and natural range. The phosphorus concentrations were markedly reduced and significantly different from those before the application of PAC and this could be explained by chemical precipitation between Al⁺³ and PO₄^-3 which can remove phosphorus from the water column [10, 15]. A reduction of BOD₅ was observed since PAC hydrolyzes into positively charged aluminum which neutralizes the negative charges of colloids/organic contaminants/algal cells causing particles to form larger flocculate particles that sink to the pool bottom [11,12]. The total aluminum concentrations were not significantly different before and after PAC application. This study also detected microcystin (average concentration 0.19±0.07 µg/l). During application of PAC, the microcystin content tended to increase. However, two weeks after application, the microcystin content had decreased, possibly because of the reduction in the levels of cyanobacteria. Microcystin content in the current study was comparable to other studies both in Thailand and other countries. For example, in Lake Oubeira, Algeria, where a predominant bloom of cyanobacteria occurred, the amount of microcystin toxin was in the range 0.028-13.4 µg/l, with the highest values in September [20].

From Fig. 1, the chlorophyll a concentration before PAC application was 887±48.33 µg/l. During the application of PAC, the chlorophyll a concentration was reduced by more than one-half (377.52±15.64 µg/l). After one and two weeks of PAC application, the chlorophyll a concentration remained lower than the initial concentration (327.80±29.32 and 392.92±45.39 µg/l, respectively). The trend for turbidity was similar to the chlorophyll a concentration, with a reduction after application of PAC. Reductions in the levels of chlorophyll a and turbidity after application of PAC, are possible through the mechanism of flocculation [15, 18] (Fig. 2). The reductions in chlorophyll a and turbidity resulted in increased transparency depth [21].

The water quality results for the near-bottom samples are presented in Table 3. Overall, the water quality changes were similar to those for the surface water. Before PAC application, the BOD₅ value was higher than after PAC application. DO concentrations increased after PAC application. The total nitrogen, total phosphorus, and total aluminum contents tended to increase during PAC application, but the differences due to the PAC application were not significant. In this study, the reduction of phosphorus was around 32% after one week of treatment, which was similar to a study in Poland that observed a reduction of TP content (28%) at the bottom of Lake Klasztorne Małe [22].

During and after PAC application, we observed no dead fish or dead organisms. Therefore, it can be stated that PAC application at the level used had no or low toxicity effects on the freshwater fauna. Acute toxicity of PAC was studied with juvenile zebrafish (Danio rerio) at approximately 2–3 months of age; the results showed that the mean±standard deviation of the LC50 at 96 hours was 749.7±30.6 mg/l [23]. The actual content of PAC used in the current study was much lower than that of LC50. However, polyaluminium chloride should be used with caution since some studies showed that PAC has a negative impact on charophytes, invertebrate and microbial community [24-26]. Aluminium in water of pH lower than six and higher than eight can occur in the soluble form (e.g., Al³⁺ ions) and can be harmful to freshwater fauna [24, 27].

Phytoplankton Assemblages

The number species of phytoplankton varied between 11 and 19 in this shallow pool. Before PAC application, the total density of phytoplankton was 843,400 units/l (Table 4), and the predominant species were Spirulina sp. and Microcystis sp. During PAC application, the total density of phytoplankton was reduced to 217,825 units/l and remained low toward the end of experiment. This could be explained that intact phytoplankton cells were removed by the formation of flocculation and sedimentation of algal cells [28, 29]. The dominant phytoplankton species shifted to Spirulina sp., Oscillatoria sp., Navicula sp., and Anabaena sp. PAC may have a particularly adverse effect on cyanobacterial cells, and were able to remove them from the water column [30]. Overall, the phytoplankton reduction was more than 70% after application of PAC. A study in Feldberger Haussee, Germany also revealed a similar result in that following the treatment of PAC, the cyanobacteria disappeared and resulted in alteration of the phytoplankton community structure [31]. We also observed that flocs developed and were suspended in the surface water for a few hours during application of PAC. This is consistent with a laboratory experiment showing that high doses (≥16 mg Al/l) of PAC resulted in large flocks of cyanobacteria aggregating in the top of test tubes [32].

Sediment Properties

The sediment characteristics are presented in Table 5. The texture of the sediment was clay. The sediment particles were fine and loosely amorphous. Most sediment parameters (organic matter and total nitrogen) remained constant throughout the experiment. It was observed that during the application of PAC, flocs formed and sank to the bottom of the shallow pool. The approximate height of the flocs was 10-15 cm (Fig. 3). One week after PAC application, the formed flocs had disappeared from the sediment surface. The organic matter content was in the range 7.86-11.06%, which was quite high and this may have been due to the high accumulation of organic materials such as phytoplankton cells in the water body, which can become a crucial internal nutrient source. Therefore, it is suggested that the increase in the total organic carbon of the sediments can be minimized by planting submerged macrophytes [33]. The total phosphorus content tended to decrease after application of PAC. In contrast, the total aluminum content significantly increased after the addition of the PAC and this may help reduce the PO₄^3--P release by P-binding [34] with aluminium from internal sediment under anoxic conditions [35-37]. The increased aluminum content was directly caused by application of PAC in the shallow pool. This was consistent with a previous study showing the presence of flocs in the surface sediment at a depth of 0-10 cm during application of PAC [38, 39]. Binding efficiencies of aluminum can be enhanced when aluminum was applied directly into the sediment [40]. Lastly, it is suggested that long term monitoring of water quality should be conducted after the end of restoration to investigate the duration of PAC’s effects since the longevity of the clear water state after chemical treatment varies from one year to several decades [41] depending on the input of exogenous nutrient loading.

Fig. 2.

Flocculation of phytoplankton and suspended particles in the surface water in the pool during application of PAC.

Fig. 1.

Marked improvement in water quality parameters (chlorophyll a, turbidity, and transparency) before and after application of PAC in a hypereutrophic pool. Bar charts present mean values with standard error bars; different letters (a,b,c) above bars indicate means that are significantly different at p<0.05.

Fig. 3.

Water-sediment interface in corer: a) one week before application of PAC, b) during application of PAC with presence of flocs, c) one week after application of PAC and d) two weeks after application of PAC.