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The 2007 Lake Tai pollution incidence between late May and early June was the result of pollution caused by cyanobacterium bloom in Lake Tai. Lake Tai belongs to Jiangsu province and is China’s third largest freshwater lake that historically, has been beset by recurring episodes of cyanobacteria blooms, with increasing coverage areas and frequency in recent years. The subsequent cyanobacterial decomposition that released septic odorous compounds created great societal concerns regarding water supplies and treatment, especially for the population that rely on Lake Tai for their source of drinking water. The 2007 Lake Tai algae bloom in particular led to a water crisis that affected two million residents in the city of Wuxi.

Water Crisis Profile

The water crisis was brought to the public attention of Wuxi residents beginning the evening of 28 May 2007. Residents noticed that their tap water appear swampy, discolored in light yellow, and emitting a foul odor. Earlier that day, the Wuxi Water Company discovered that the Nan-quan water intake at the northeast part of Lake Tai, which goes to the source water plant, turned a brown color and smelled an offensive odor. This is indication that the odorous tap water experienced by Wuxi residents is sourced from Lake Tai. In the next few days, water quality deteriorated to the point that exceeded the water purification abilities of treatment plants. Various methods were attempted to alleviate the problem but the threshold odor number (TON) value still remained significantly above normal. The unusable tap water led to a shortage of bottled water and other bottled drinks in Wuxi, and a water crisis and local panic ensued.

Cause

Microcystis aeruginosa under the microscope

The 2007 crisis, as with the bloom incidents in previous years in Lake Tai, is attributed to the over-abundance of a species of cyanobacterium known as Microcystis aeruginosa forming discolored mats floating on the water surface. However, Yang et al. also cited the intrusion of black water agglomerate unknown in origin as the culprit of the 2007 crisis.

The odorous chemicals responsible for the water crisis is identified to be high concentrations of volatile organic sulfur compounds (VOSCs), which includes methyl thiols, dimethyl sulfide (DMS), dimethyl disulfide (DMDS), and dimethyl trisulfide (DMTS). The release of VOSCs from the decomposition of cyanobacterial bloom is not uncommon. Each year from 1987 to 2007, Lake Tai has suffered from cyanobacterial blooms with increasing duration and frequency due to eutrophication from agricultural run-off and from illegal discharges of industrial and domestic waste. The pollutants from industrial and domestic waste likely poisoned the cyanobacteria, whose decomposition was aided by the dense resident population of heterotrophic microorganisms that were nourished by high levels of organic matter, nitrogen, and phosphorus runoff. Organic sulfur reserves in the cyanobacteria then converted into VOSCs during its decomposition, and this large amounts of VOSCs produced the black water agglomerate previously mentioned. This combination of stresses tipped the ecosystem equilibrium of Lake Tai past a critical threshold, ultimately triggering the 2007 algae blooms and water crisis.

Adverse Effects

Effects on the Lake Water

Together with releasing large amounts of VOSCs into Lake Tai, the anaerobic decomposition of the cyanobacterial blooms produced other related repercussions including anoxic water, increased nutrient loading, and decreased pH. Studies showed that Tai Lake contained low dissolved oxygen (DO) levels because the decomposition process depleted the available oxygen. Decay of the cyanobacteria further increased nitrogen and phosphorus levels multiple fold, exacerbating nutrient loading into the lake. Microorganisms fed off of the nutrients released from cell lysis and produced large amounts of carbon dioxide during their respiration, causing the decrease in water pH.

Effects on Human Health

A number of health risks are associated with Microcystis exposure in Lake Tai. The organism is able to incorporate into the food chain, causing poisoning of fish, birds, invertebrates, plants, mammals, and humans.

Acute and chronic exposure to Microcystins through consumption of food and water contaminated by Microcystins pose serious risks to human health. The organism produces a common and potent hepatotoxic cyanotoxin that is widely present in eutrophic freshwaters and are capable of causing intoxications and death in exposed human populations. The reason they are hepatotoxic is because their main target is the liver, causing hepatocellular damage. Their proposed mechanism of action is through alterations of hepatic protein expression and inducing damage in lipid metabolites of both the liver and serum. Analysis of cytokine profiles indicate that Microcystin significantly disrupts fatty acid beta-oxidation and secretion of hepatic lipoproteins, which induces immune mediated inflammation of the liver and can lead to nonalcoholic steatohepatitis disease (NASH). Studies also suggests that prolonged exposure to Microcystins is carcinogenic due to their roles in inducing formations of neoplastic nodules, which are precursors for liver tumorigenesis. The pathway likely involves the inhibition of protein phosphatase (PP1 and PP2A) involved in catalyzing the hydrolysis of phosphate groups. This likely results in a disturbance in the chemical equilibrium between phosphorylated and dephosphorylated states, which in a series of chain reactions, upregulates the activity of transcriptional factors and protein kinases that ultimately led to uncontrolled cellular differentiation.

Emergency Water Treatment Process

Zhang et al. developed an emergency water treatment process that was implemented by the Wuxi Water Company on June 1, 2017. The plan involved adding potassium permanganate (KMnO4) to the intake of the pipeline so that the VOSCs could be oxidized during the source water’s six hour delivery to the treatment plant. After reaching the treatment plant, powered activated carbon (PAC) was added to break down KMnO4 and neutralize the contaminants. The dose of PAC and KMnO4 were closely monitored by an oxidation reduction potential (ORP) meter and adjusted accordingly. Within a day, the quality of water was returned to meeting all 106 requirements of the Chinese Standard for Drinking Water Quality, which is also comparable to that of the World Health Organization.

Aftermath and Government Response

Following the Wuxi water crisis, the local government enacted a series of measures to protect Lake Tai from future recurrences of pollutions. These include strict standards for the direction of industrial waste discharges into the city’s sewage collection system for processing in the wastewater treatment plant rather than into the lake, as well as measures to restore the wetlands along the lake shore. China’s central government has also become more vocal in environmental protection, resource saving, and environmentally conscious modes of production. On March 2008, the People’s Congress promoted the State Environmental Protection Agency to Ministry of Environmental Protection so they have more power to implement policies. In the 2008 revision, punishment for violating the Water Pollution Control Law also became more severe.

See Also

• Algae bloom
• Microcystis aeruginosa
• Eutrophication
• Wuxi
• Lake Tai