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Place: Tal Shachar reservoir, Israel
Date: 18 July 2021
Abstract
BlueGreen Water Technologies Ltd. performed a trial treatment of a cyanobacterial (blue-green algae) bloom in the Tal Shachar reservoir using a new flocculant-based product - Lake Guard Dew®. The agricultural reservoir, which went unused for the past several months, suffered from an outbreak of cyanobacteria (mainly Microcystis sp.) visible as green scum accumulated along the reservoir banks. The BlueGreen team assessed the situation of the reservoir and treated it on July 18th, 2021. Monitoring of water quality parameters; including dissolved oxygen (DO%), pH, chlorophyll-b and phycocyanin; was performed using a YSI Sonde over a period of six days (July 18th-23rd). Directly after the application of the treatment, aggregates (i.e., flocs) could be observed forming in the water. Furthermore, brown clumps, assumed to be dead algae, appeared on the water surface. The treatment significantly increased the amount of dissolved oxygen (DO%) but did not affect the pH. Chlorophyll-b and phycocyanin levels exhibited an increase and then a drop after the treatment. Overall, the transparency of the water seemed to improve in the days after treatment. This product may be effective for the treatment of water bodies that are not used for recreational activities, such as agricultural reservoirs. Further work is required to assess the duration of effect and environmental impact of this new product.
Background
Cyanobacteria blooms occur frequently around the globe and are considered a serious threat to animal and human health (Huisman et al. 2018). The occurrence of these blooms depends on certain environmental conditions, mainly high nutrient availability; but also, temperature, pH and light (Paerl and Otten 2013, Descy et al. 2016). Hypoxic conditions, induced by the rapid growth and decay of cyanobacteria cells, resulting in the death of aquatic life in large numbers. Moreover, cyanobacteria blooms produce biological toxins which affect the liver, digestive and nervous system of both humans and animals (Huisman et al. 2018). Thus, these blooms negatively influence many aspects of human life, including food security, tourism, health and the economy.
Lake Guard Dew® is a new product of BlueGreen Water Technologies Ltd. aimed at treating algal blooms in a variety of water bodies. This product is based on Aluminum sulfate (ALUM), a flocculant widely used for the treatment of wastewater in Israel and around the world. ALUM, like other
flocculants, helps separate suspended solids from the water through coagulation of fine particles. Soluble ALUM is a positively charged molecule that neutralized the negatively charged solid particles, causing them to come together in aggregates called “flocs” which usually settle to the bottom of the water body. These flocs can later be removed from the water through different filtering processes.
Flocculants have previously been considered for harvesting microalga and controlling algal blooms due to their high efficiency and applicability (Matter et al. 2019). The negatively charged surface of algal cells allows successful aggregation without causing lysis, resulting in the removal of cell mass from the water to the sediment while preventing the discharge of toxic metabolites. Removing algal bloom using flocculants can accelerate the ecological restoration of natural water bodies and improve the general state of the water (Pan et al. 2019, Song et al. 2021).
Treatment setup
Tal Shachar reservoir is an agricultural water reservoir of 12 acres located in Mateh Yehuda Regional Council in the central part of Israel (Fig. 1). The reservoir, which went unused for the past several months, suffered from an outbreak of cyanobacteria, mainly of the genus Microcystis sp. The cyanobacteria bloom was visible as green scum which accumulated along the reservoir banks (Fig. 2).
Fig. 1. A drone image of the Tal Shachar Reservoir (a) and its relative location on the map of Israel (b).
Fig. 2. A drone image of the Microcystis cyanobacteria bloom, visible as green scum accumulated along the reservoir banks.
Fig. 3. Application of Lake Guard Dew® in the Tal Shachar Reservoir using a motorboat. Lake Guard Dew® is visible on the surface of the water (as white stripes, an example is marked by a red arrow) right after application.
Methodology
The BlueGreen team arrived at the Tal Shachar Reservoir on the morning of Sunday, July 18th, 2021. Using a motorboat (Fig. 3a,b), 500 kg of Lake Guard Dew® was applied to the water between 09:30- 12:00. This product comes in granules that float on the surface of the water while slowly releasing the active ingredient (ALUM).
The water quality of the reservoir was measured daily using a YSI Sonde, once just before the application of Lake Guard Dew® (July 18th) and then over the course of five days (July 19th-23rd).
Sampling took place every morning at 09:15 in the area adjacent to the reservoir pumping station, which was reached using a canoe (Fig. 4). Due to a malfunction in the motorboat, which required the acquisition of an alternative vessel, sampling on the second day (July 19th) took place using a bucket on the reservoir bank (sampling here on occurred using a canoe). The YSI Sonde registered 10-20 readings at each sampling point around the pumping station (nine sampling points on average), of the following parameters: temperature, dissolved oxygen (DO), pH, conductivity, chlorophyll-b and phycocyanin (representative of green algae and cyanobacterial biomass, respectively). The ratio of chlorophyll-b to phycocyanin from each sampling day was later used to evaluate the relative change in cyanobacteria biomass before and after the Lake Guard Dew® treatment.
Fig. 4. The main sampling area adjacent to the reservoir pumping station. This area was reached using a canoe.
Since there were uneven samples sizes between the measuring days and since the data did not exhibit a normal distribution, the YSI Sonde readings were averaged for each day. Additionally, water samples were collected daily from July 19th – 23rdand sent to AMINOLAB facility located in Nes Tziona, Israel, for examination. The lab tested for concentrations of Nitrogen ammonia, Phosphate PO4-3, Kjeldahl nitrogen (TKN) , Nitrate nitrogen N-NO3-, Nitrite nitrogen N-NO2-, and general nitrogen. Due to the high cost of lab testing, samples were limited and insufficient for proper statistical analyses. All graphs are presented using the tidyverse package (Wickham 2016) in R version 4.1.1 (2021-08-10).
Results and discussion
Within a relatively short period of time (two days) from the application of Lake Guard Dew®, flocs could be observed forming in the water (Fig 5a,b). Furthermore, brown clumps of 10-50 cm, assumed to be dead algae (Fig 5b), appeared on the water surface.
Fig. 5. Tal Shachar Reservoir two days after the application of Lake Guard Dew® (a). Flocs could be observed in the water (b) as well as brown clumps of 10-50 cm, assumed to be dead algae (d,c).
The treatment had an apparent large impact on the amount of dissolved oxygen (DO%), which increased drastically after the first day of application and then decreased subsequently (Fig.6a). pH values seemed to remain relatively stable (Fig. 6b) while chlorophyll-b and phycocyanin concentrations seemed to increase slightly the day after application and then fall back down again (Fig. 6c,d).
Fig. 6. The effect of the Lake Guard Dew® treatment on dissolved oxygen DO% (a), pH (b), chlorophyll-b (ug/L) (c) and phycocyanin (ug/L) (d) over the course of three days. Bars indicate means ± SE.
Results of water samples tested by the AMINOLAB facility revealed that Nitrogen ammonia decreased 29.27%, Phosphate PO4-3 decreased 14.29% and Nitrite nitrogen N-NO2- decreased 48.57%. (Fig. 7a,b,e). Further sampling is planned to provide a sufficient sampling size for statistical analyses.
Fig. 7. The effect of the Lake Guard Dew® treatment on Nitrogen ammonia (a), Phosphate PO4-3 (b), Kjeldahl nitrogen (TKN) (c), Nitrate nitrogen N-NO3- (d), Nitrite nitrogen N-NO2- (e), and general nitrogen (f). Nitrogen ammonia decreased 29.27%, Phosphate PO4-3 decreased 14.29% and Nitrite nitrogen N-NO2- decreased 48.57%. A larger sample size is required to provide statistical proof of these observations.
Overall, the transparency of the water seemed to improve with the use of flocculant in the days after treatment. This makes sense given that flocculants separate suspended solids from the water, including algae. The water indeed became clearer, yet brown clumps (assumed to be dead algae) appeared on the water surface. Thus, this method may be relatively effective for treating water bodies that are not used for recreational activities, such as agricultural reservoirs. Although this was not evident in the timescale of the current trial, DO% might drop at some point since the flocs may be a source of oxygen demand as they decay. Further work is required to assess the duration of effect and environmental impact of this product before use in practical application.
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