The 2007 Taihu Lake incident raised awareness of the hazards of cyanobacterial blooms in China. The Ministry of Ecology and Environment has since made efforts to improve the country's ability to detect blooms and respond to them.

The discharge of nutrients such as nitrogen and phosphorus from urban sewage treatment plants still accounts for a large proportion of the total discharge of water pollutants in China’s water bodies and poses a great risk of water eutrophication, one of the leading causes of the algal blooms.

In April 2018, the Ministry of Ecology and Environment of China released the "Notice on Strengthening the Prevention and Control of Nitrogen and Phosphorus Pollution from Stationary Pollution Sources", pointing out the bottlenecks in the improvement of water quality, and various challenges in the national water pollution prevention and control. The discharge of nutrients such as nitrogen and phosphorus from urban sewage treatment plants currently accounts for a large proportion of the total discharge of water pollutants in China’s water bodies and poses a great risk of water eutrophication.

Many people still remember the 2007 cyanobacteria incident in Taihu Lake, China's third-largest freshwater lake, that caused the drinking water crisis in the city of Wuxi, Jiangsu province. The Taihu Lake eutrophication caused a massive bloom of the toxin-producing cyanobacteria Microcystis. Taihu was the city's sole water provider, leaving approximately two million people without drinking water for at least a week. This cyanobacterial bloom event began two months earlier than previously documented for Microcystis blooms in Taihu. This was attributed to an unusually warm spring. The prevailing wind direction during this period caused the bloom to accumulate at the shoreline near the intake of the water plant. Water was diverted from the nearby Yangtze River to flush the lake of the bloom. However, this management action was counterproductive, because it produced a current which transported the bloom into the intake, exacerbating the drinking water contamination problem. The severity of this microcystin toxin-containing bloom and the ensuing drinking water crisis were attributable to excessive nutrient enrichment; however, a multi-annual warming trend extended the bloom period and amplified its severity, and this was made worse by unanticipated negative impacts of water management. Long-term management must therefore consider both the human and climatic factors controlling these blooms and their impacts on water supply in this and other large lakes threatened by accelerating eutrophication.

Cyanobacteria often referred to as blue-green algae, are microscopic organisms ubiquitous in all types of water around the globe. They use sunlight to make their food in warm, nutrient-rich environments and can quickly multiply, resulting in blooms that spread across the water's surface.

These harmful algal blooms, often abbreviated to HABs and which are of concern when visible in lake water, can form at any time of the year but most often between spring and fall.

In 2007 a national survey by the Environmental Protection Agency found microcystin, a recognized liver toxin, and potential liver carcinogen, in one out of every three lakes that were sampled. Some strains of cyanobacteria can also produce neurotoxins, while most of the toxin-producing algae can cause gastrointestinal illness and acute skin rashes.

The 2007 Taihu Lake incident raised awareness of the hazards of cyanobacterial blooms in China. The Ministry of Ecology and Environment has since made efforts to improve the country's ability to detect blooms and respond to them.

Over the years, cyanobacteria in Taihu Lake have still occurred locally. The central and local governments have taken a large number of engineering and technical measures from pollution sources to reduce emissions, intercept nitrogen and phosphorus in rivers entering the lake, restore water ecology, and introduce clean water sources to control the cyanobacteria in the lake. In May 2021, the Jiangsu Provincial Department of Ecology and Environment, together with the Provincial Department of Finance, adjusted and implemented the regional plan for water pollution. The new plan requires that the cross-section of rivers in the land basin be implemented according to the Class III target. The total phosphorus index of the rivers entering the lake is further limited; the total nitrogen is included in the assessment factor for regional compensation of the water pollution in the Taihu Lake Basin to curb the eutrophication of the water body.

The new scheme puts forward higher requirements for the total nitrogen (TN) index in sewage treatment. At present, the index is mainly limited to the removal of organic matter and ammonia nitrogen. Therefore, it is necessary to implement deep denitrification treatment of the tailwater of sewage treatment plants.

The tailwater of sewage treatment plants generally has a low C/N ratio, low organic matter concentration, and high TN content. The main form of nitrogen in TN is nitrate nitrogen, so nitrate-nitrogen becomes the focus of advanced treatment. Most of the COD in the tailwater of the sewage treatment plant is non-biodegradable organic matter, so the BOD content is extremely low. According to the traditional biological denitrification theory, the BODs/TN ratio for the complete denitrification process is 2.68. The BODs/TN ratio required by the actual denitrification process is above 4, so the traditional biological denitrification process is difficult to apply to the deep denitrification of the tailwater of the sewage treatment plant, while the sulfur autotrophic denitrification process has the advantage of no external carbon source, less mud, and no secondary pollution, therefore it has become an important direction for denitrification of wastewater treatment plant tailwater.

Ultra-clean denitrification technology for biochemical tailwater

Biochemical tailwater ultra-clean denitrification technology is an autotrophic denitrification technology. Autotrophic denitrification technology refers to the use of CO2, HCO3-, CO32-, etc. as inorganic carbon sources under the action of autotrophic denitrifying microorganisms to reduce Inorganic substances (H2, S, S2-, S2O32-, Fe, Fe2+, etc.) as electron donors, the process of reducing nitrate-nitrogen in water lacking organic carbon sources to N2. Compared with heterotrophic denitrification technology, this technology has the advantages of low treatment cost, low sludge yield, stable N2, and no secondary pollution.

The device used in the ultra-clean denitrification technology of biochemical tailwater can be composed of multi-stage reaction towers. The biochemical tailwater enters the reaction tower and passes through the packing, and a large amount of nitrogen is discharged from the gas outlet. During the operation, the water quality is stable, the effluent SS has no obvious change, and there is no secondary pollution.

This technology is suitable for the denitrification of the biochemical tailwater. It has strong removal capacity and strong load resistance during operation. It can reduce total nitrogen to below 4mg/L, has a strong impact capacity, and has better disposal measures for sludge bulking and poor sedimentation.

Technological Innovation and Highlights

No additional carbon source is required to avoid the risk of effluent COD exceeding the standard, stable effluent water quality;

The total nitrogen range of the influent water is 20-500mg/L, with a wide scope of application;

The multi-purpose single tank with concentrated functions, which can reduce total phosphorus and suspended solids while removing total nitrogen;

The composite filter material promotes the formation of a multi-phase denitrification and phosphorus removal reaction;

The total nitrogen removal rate is high, up to more than 95% of the total nitrogen removal rate, which can be used for extreme denitrification treatment;

Simple operation and maintenance, low backwash frequency;

Applicable to various reactors, the modification is simple, and only the filter material needs to be replaced.

A case study

This technology is applied to a company's wastewater upgrading project. The wastewater treatment plant in the chemical concentration area where the company is located implements the new standard DB32/939-2020. Although the existing high-efficiency ADFB anaerobic denitrification fluidized bed can effectively reduce the nitrate-nitrogen in wastewater from 1000-2000mg/L to below 200mg/L, it is difficult to reduce the discharge to the standard at low load, and due to the high concentration of COD in the effluent caused by the excessive carbon source added during the denitrification process, the company's outlet will no longer receive catalyst production wastewater. Therefore, the catalyst wastewater needs to be upgraded and transformed, and after the treatment reaches the standard, it will be sent to the external outlet of the power department designated by the company.

In this project, the anaerobic denitrification ADFB tower is used as the front-end treatment process of the system, and a two-stage A/O+ secondary sedimentation tank is built to reduce the TN to 15mg/L. The secondary sedimentation tank adopts a radial-flow sedimentation tank, and two new aeration biological filter tanks, one is used for filtration, and the other is filled with autotrophic denitrification filter material, which is used as a guaranteed unit for the total nitrogen in tailwater to meet the standard.

The project has achieved initial results within 24 hours after the transformation, and the design index can be reached within 48 hours; the completion time for the domestication of autotrophic denitrifying microorganisms is short, and the fluctuation of the incoming water has little impact on the system, the operation is stable, the quality of the effluent is stable, and no secondary pollution occurs.

The water quality of drinking water sources guarantees the normal life needs of the masses, but with the development of the economy, the nutrient enrichment of nitrogen and phosphorus in lakes and reservoirs is becoming more and more serious. For the pollution problem of excessive nitrogen in the water body, the ultra-clean denitrification technology of biochemical tail water has very important practical significance in the application of actual water body treatment due to its ultra-high and stable treatment effect and low treatment cost.

The full information on the risks posed by the eutrophication, and consequently, algal blooms, is important for protecting public health, both about the consumption of drinking water and exposure to toxins through recreation on lakes. Potential exposure to cyanotoxins is of public health concern and blooms particularly pose a threat to dogs entering lakes. If a person or a pet comes in contact with water that may contain harmful bacteria, the Centers for Disease Control and Prevention advises immediate rinsing with fresh water. Dogs should not be allowed to lick the contaminated water off their fur, the CDC adds, and a veterinarian should be called right away. Anyone swallowing water near a harmful algal bloom should immediately call a doctor or poison control center.