Eutrophication Treatment Challenges
By Professor Yin Daqiang 8 June, 2011
How will China go about treating eutrophication in their lakes? Professor Yin explains.
In China, Chemical Oxygen Demand (COD) is used as a key indicator of water quality and in the 11th Five year Plan a COD reduction target of 10% was established. Yet, whilst COD is a useful indicator of overall water quality and thus pollution, establishing targets for this parameter alone is unlikely to be sufficient to address the extent of pollution in many of China’s lakes and rivers, given the extent of nitrogen effluents being discharged from agriculture, industry and municipal sources.
In recent years eutrophication has worsened causing major crises such as the lake Taihu algae bloom, which disrupted the municipal water services for more than a month in Wuxi , a city with a population of five million. Recognising the problem and in an attempt to further address eutrophication, the 12th Five Year Plan (FYP) (2011-2015) introduced a new pollution target in the form of Ammonia Nitrogen (NH3-N). Unfortunately meeting pollution targets is by no means a simple matter , and reducing ammonia nitrogen alone, in unlikely to solve the eutrophication problem.
Professor Yin Daqiang from the College of Environmental Science and Engineering at Tongji University, has spent many years researching eutrophication in relation to the Lake Taihu basin and explains to Asia Water Project some of the key challenges in fighting the problem
1st Challenge: Non-point source pollution from rural communities and agriculture
Nitrogen and phosphorous pollution from a variety of sources continue to cause eutrophication in many of China’s lakes and rivers. The majority of both Total Phosphorous (TP) and Total Nitrogen (TN) discharges are thought to be from non-point sources (about 40-50%), making pollution control particularly challenging. To make matters worse, such pollution is on the rise.
Figure 1: The different sources of TN and TP pollutions in China.
Of the non-point source TP and TN emissions, untreated sewage from rural populations is the main contributor, followed by aquaculture and then agricultural run-off. The nature of such pollution raises key questions and challenges facing treatment options:
1. Improving municipal wastewater treatment for rural areas
|Pollutant||‘I-A’ Standard (mg/L)|
In sensitive areas such as the lake Taihu basin, the sewage treatment plant standard in force is currently at the most stringent ‘I-A’ level. However water quality has continued to deteriorate, largely as a result of the non-point source pollution from rural communities.
To treat sewage from small towns and rural communities, sewage needs first to be collected and then treatment requires low cost, easy to handle technology and process management.
A key question for decision makers focused on waste management policy is, given a significant rural population, whether to employ centralised or local treatment units. In China, due to the sheer scale of rural communities, as well as the extent of agriculture and poverty, this question is not easily answered. Indeed, coming up with a cost effective solution that can be easily operated, represents a significant challenge. It is understood that further studies are needed to determine the feasibility of a centralized processing model under such circumstances, given the challenges of sewage collection in rural areas.
2. Controlling and reducing agricultural non-point source pollution
In addressing agricultural pollution a number of measures which are challenging to implement and enforce, need to be considered:
- Promoting the planting of drought resistant crops
- Reducing pesticide use and fertilizer1
- Improving control of aquaculture
3. Improving the water quality of rivers discharging into the lake and strengthen ecological restoration
As an example, strengthening ecosystem measures, such as ecological restoration and biological control to improve the proportion of fresh fish, algae, etc.
4. Enforcement of environmental regulations
To improve eutrophication control across the Lake Taihu basin, it is important to ensure enforcement of regulations that strongly control nitrogen and phosphorous fertilization intensity , lower emissions and reduce agricultural nitrogen and phosphorous loss into the lake.
2nd Challenge: Design and operation of the biological nitrogen removal process
The aim of biological waste water treatment is the removal of biodegradeable organic matter. The biological process involves introducing microorganisms such as bacteria into waste water under carefully controlled conditions. The microorganisms consume the organic matter converting it to carbon dioxide, water and importantly energy, for growth and reproduction. The success of the process is dependent upon establishing a mixed community of microorganisms that will consume organic waste material, and that will aggregate (bioflocculation) and settle to produce a concentrated sludge (return activated sludge, or RAS) for recycling.
Conventional Activated Sludge 2 is a biological system, widely used around the world as a basis for waste water treatment. More sophisticated and notably more expensive methods use membrane technologies such as Membrane Bioreactor (MBR).
Figure 2 : Simple Conventional Activated Sludge Wastewater Treatment System, 2009
Source: Conventional Activated Sludge Systems, Information Leaflet, Kruger, Veolia Water
Under aerobic conditions, the Conventional Activated Sludge process removes first organic matter and suspended solids followed by some phosphorus. For wastes with high nitrogen and phosphorus additional processes should be employed.
In China, the Conventional Activated sludge system is thus generally followed by an additional anaerobic phase which facilitates nitrogen removal. Although relatively cheap, the process requires careful design and management, relying on balancing retention time of the waste water and microorganisms in each phase (anaerobic, anoxic, aerobic) as well as ensuring appropriate volumes of waste water to ensure effective treatment.
The optimum combination of the phases, residence time and adapted recirculation to manage the effectiveness and efficiency of the microorganisms, are key factors in achieving nitrogen and phosphorus removal.
Based on this system, china’s sewage treatment plants designed for TN and TP removal can be divided into five main categories, one of the most popular of which is A2/O:
- Activated Sludge improved technology
- A/O ( Anaerobic and Oxidation) process,
- A2/O ( Anaerobic Anoxic and Oxidation ) process,
- Oxidation ditch water treatment unit ; and
- Sequenced Batch Reactors–(SBR) and its improved process.
The A2/O system employs additional treatment phases based on reducing the microorganism’s exposure to oxygen in order to maximize nitrogen removal3.
Figure 3: A2/O process flow diagram
Source: Dictionary of Water and Waste Management, Second Edition
There are however challenges in ensuring effective nitrogen removal through the above process, potentially creating difficulties in meeting pollution removal targets.
To remove nitrogen, the wastewater must go through two stages. Firstly the microorganisms oxidise the ammonia to nitrite and nitrate – this is an aerobic process known as nitrification. Then the waste water undergoes denitrification whereby anaerobic microorganisms derive their oxygen by reducing the nitrite and nitrates in the wastewater to nitrogen gas. In this phase, there may be situations4where there are insufficient organic compounds in the waste water to facilitate this process. As a result, the denitrification process itself can become a barrier to nitrogen removal.
3rd Challenge: 12th FYP only targets ammonia nitrogen, omitting other nitrogen compounds
Although the 12th FYP has established emission reduction targets for ammonia nitrogen, other polluting nitrogen compounds such as nitrates and nitrites as well as Total Kejeldahl Nitrogen (TKN) (which collectively comprise TN) are not addressed. Ideally it is hoped that China will progress beyond just a specific ammonia nitrogen target and will also establish targets for TN in the future.
However TN in itself presents a challenge. In order to remove TN, the treatment process needs to include denitrification. As noted above, this process itself can be a challenge, in addition to which , the operational and technological requirements demand more expertise than just nitrification. This is problematic for small water treatments plants which will more typically be built in rural areas. It is thus relatively easy to meet the standard for ammonium nitrogen but is more difficult to increase the denitrification efficiency and thus to treat all the nitrogen compounds present in the waste water.
1 In its August 2010 Report: The Real Cost of Nitrogen Fertilizer, Greenpeace noted that 67% of the nitrogen fertilizer applied in agriculture is lost. Moreover agriculture is also responsible for 57% nitrogen water pollution.
2The essentials of the activated sludge process are: the aeration of wastewater in the presence of aerobic microorganisms, the removal of biological solids from the wastewater by sedimentation and the recycling of the settled biological solids back into the aerated wastewater (source: Civil Engineering Group, The History of Activated Sludge, August 2010. Dr Harlan Bengston).
3 The A2/O method is the simplest synchronized nutrient removal process with an optimal retention time. The alternation of anaerobic, aerobic, anoxic operating conditions, inhibit the propagation of filamentous bacteria to overcome the risk of sludge bulking. Sludge bulking occurs when bacteria die and aggregate potentially interrupting the system. This aggregation is monitored using a Sludge Volume Indicator (SVI). The anaerobic, anoxic and aerobic phases are strictly separated in which is conducive to growth and reproduction of different microbial strains and eventually to obtain nitrogen and phosphorus removal with good results.
4 Nitrate removal by simultaneous sulfur utilizing autotrophic and heterotrophic denitrification under different organics and alkalinity conditions: batch experiments. SE Oh, MS Bum, YB Boo, A Zubair and IS Kim, Department of Environmental Science and Engineering, Kwangju Institute of Science and Technology, Kwangju 500-712, Korea.
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