Wastewater Glossary: Black and Odorous Water Bodies
What Are Black and Odorous Water Bodies?
Black and odorous water bodies are a severe form of water pollution. In practice the term describes water whose colour has become conspicuously abnormal and/or which gives off a strong, offensive odour, as a result of organic pollution that has exceeded the water body's self-purification capacity.
Rural Black and Odorous Water Bodies
Rural black and odorous water bodies are water bodies with conspicuously abnormal colour or a strong, offensive odour located within administrative villages (communities) outside the urban built-up area, that is, outside the urban development boundary.
Identification focuses on the ponds, ditches and canals in front of and behind villagers' houses and on water bodies that attract strong public complaints. The spatial scope is the natural village or villager group, extended outwards from the residential cluster by about 500 m in southern China and about 1,000 m in northern China. Water body types include rivers, lakes, reservoirs, ponds and ditches. In principle, water bodies with a surface area of 200 m2 or more are included in the survey, although water bodies that attract strong public complaints are not limited by this threshold.
Determination follows a three-step method: sensory judgement, public appraisal and water quality monitoring. The process stops as soon as a step returns a positive result. Sensory judgement relies on abnormal colour (blackening, whitening and so on) or abnormal odour (strong, offensive); the presence of either characteristic is sufficient. Where sensory judgement is disputed, the village committee organises consultation and a questionnaire survey with at least 20 valid responses; if more than 60% of respondents consider the water body "black" or "odorous", it is determined to be black and odorous. Where public appraisal still cannot settle the case, water quality is monitored for three indicators: transparency, dissolved oxygen and ammonia nitrogen. A water body is classified as black and odorous if any single indicator reaches its threshold, namely transparency below 25 cm, dissolved oxygen below 2 mg/L or ammonia nitrogen above 15 mg/L. For water bodies with a high sediment load, an exceedance of transparency alone does not lead to a black and odorous classification. Water bodies of 2,000 m2 or more enter the national supervision list; those below 2,000 m2 enter the provincial supervision lists.
Urban Black and Odorous Water Bodies
Urban black and odorous water bodies are water bodies inside the urban built-up area that present an unpleasant colour and/or emit an unpleasant odour. They are graded as lightly black and odorous or severely black and odorous according to the degree of pollution, using transparency, dissolved oxygen (DO), oxidation-reduction potential (ORP) and ammonia nitrogen (NH3-N) as indicators.
| Indicator | Lightly black and odorous | Severely black and odorous |
|---|---|---|
| Transparency | 25 to 10 cm | Below 10 cm (where water depth is under 25 cm, 40% of the actual depth) |
| Dissolved oxygen (DO) | 0.2 to 2.0 mg/L | Below 0.2 mg/L |
| Oxidation-reduction potential (ORP) | -200 to 50 mV | Below -200 mV |
| Ammonia nitrogen (NH3-N) | 8.0 to 15 mg/L | Above 15 mg/L |
Root Causes
The fundamental cause is organic pollution that exceeds the self-purification capacity of the water environment. Aerobic decomposition of organic matter pushes the oxygen consumption rate above the reaeration rate, so the water body becomes oxygen deficient. Organic degradation then becomes incomplete and slows down, while anaerobic biodegradation generates hydrogen sulphide, ammonia, mercaptans and other odour-causing substances and forms black-coloured matter at the same time. The result is the black and odorous condition.
Pollution Sources
Urban black and odorous water bodies are fed by point sources, non-point sources, internal sources and other sources.
Point sources: direct discharge from outfalls, combined sewer overflow during the rainy season, first-flush or dry-weather flow in separate stormwater pipes, and make-up water from unconventional water sources.
Non-point sources: the pollution load carried by rainfall and dispersed livestock and poultry breeding wastewater in peri-urban areas.
Internal sources: contaminated sediment, organisms, floating matter, suspended solids, bank-side litter, uncleared aquatic plants and algal blooms.
Other sources: non-compliant effluent from municipal wastewater treatment plants, accidental discharges from industrial enterprises and autumn leaf fall.
For rural water bodies the main sources are domestic sewage, domestic waste, livestock and poultry manure and agricultural residues entering the water, with a small number of cases caused by industrial waste and illegal wastewater discharge by enterprises. Non-point pollution from livestock and aquaculture, crop production and accumulated riverside domestic waste is the single largest contributor.
Hydraulic Conditions and Sediment Release
Insufficient hydrodynamic conditions, in other words poor flow, directly degrade water quality and aggravate the black and odorous condition. Large pollutant loads entering the water settle into river and lake sediment, so water quality keeps deteriorating, the aquatic ecosystem is damaged and the water body loses its self-purification capacity. Through a series of physical, chemical and biological processes the pollutants in the sediment are re-suspended and released continuously back into the water column, which is why deterioration recurs.
Treatment Principles and Technical Route
Treatment follows four principles: adapt measures to local conditions and apply differentiated strategies; favour affordable, economically applicable options and prioritise resource reuse; lead with typical demonstration cases and focus on practical results. County-wide systematic treatment and control of pollution at source are the foundation. The basic technical route is built around source control and sewage interception, internal source treatment, water system connectivity and ecological restoration.
Source Control and Sewage Interception
Measures cover rural domestic sewage (with resource recovery and ecological treatment preferred), removal of domestic waste, prevention and control of livestock and poultry breeding pollution, control of aquaculture pollution, control of crop-production pollution, and treatment of industrial and municipal wastewater. Backfilling a water body to make it disappear is strictly prohibited.
Internal Source Treatment
Contaminated sediment is dredged, cleaned up and remediated.
Ecological Restoration
Ecological revetments, ecological purification such as constructed wetlands and floating islands, planting of reeds and calamus, and aeration equipment are used to restore the self-purification capacity of the water body.
Water System Connectivity and Flow Maintenance
Water system connectivity projects restore ecological flow, and reclaimed water and rainwater are diverted for ecological make-up water to improve mobility.
Smart Governance
IoT sensing and AI algorithms support a collaborative plant-network-river management and smart operation platform. Connected sensing devices monitor water quality, flow and other parameters in real time, and model-based forecasting is used to schedule treatment plant loading and pumping station operation intelligently. The result is automatic tracing of drainage problems, rapid response to risks and precise control of ecological make-up water.
Long-Term Management and Supervision
Once treatment is complete, a long-term mechanism is needed to prevent relapse. The river and lake chief system is used as the platform for routine patrols and cleaning, for operating and maintaining pollution control facilities, and for dedicated action on plastic waste. Monitoring and supervision continue to be strengthened, with more than 200 additional national monitoring points in relatively poor-quality tributaries and small water bodies, supported by provincial points. Enforcement is tightened so that industrial sources discharge in compliance. Public concerns are handled through the national ecological and environmental complaints platform, while satellite remote sensing and big-data screening are used to collect leads, with a closed-loop workflow of investigation, referral, verification, rectification and case closure. Treated water bodies are monitored periodically for transparency, dissolved oxygen and ammonia nitrogen, the results are published, and public reporting and oversight channels remain open.
Progress
By the end of 2020, 98.2% of the black and odorous water bodies in prefecture-level and above cities had been eliminated. During the 14th Five-Year Plan period the scope of supervision was extended from the built-up areas of prefecture-level and above cities to those of county-level cities and to rural areas. By the end of 2025, black and odorous water bodies in prefecture-level and above cities had been essentially eliminated and more than 90% of those in 397 county-level cities had been eliminated; more than 98% of the 5,042 larger rural black and odorous water bodies nationwide had been treated. By early 2026 the elimination rate in county-level city built-up areas exceeded 90%, and completion rates in prefecture-level and above cities and county-level cities in the Pearl River basin and elsewhere were above 90%.
National planning targets essentially eliminate black and odorous water bodies in county-level city built-up areas by 2025 and in rural areas by 2027, and aim for lasting cleanliness in the built-up areas of prefecture-level and above cities. County-level and rural black and odorous water body treatment continues to be promoted in subsequent policy documents, and provinces such as Anhui and Shandong have issued their own action plans with deadlines, investigation lists and full-area surveys combining field inspection, remote sensing, drones and AI recognition.
Technology and Standards
Treatment of black and odorous water bodies is a major part of the water pollution prevention and control campaign, and a large body of research and engineering practice has produced systematic solutions covering treatment technology, smart applications, treatment models and long-term mechanisms.
One example is a technology developed by the School of Environment at South China Normal University that calcines municipal sewage sludge at high temperature to produce water-purifying ceramsite, which is then modified by loading nano zero-valent iron so that it adsorbs pollutants and degrades harmful substances at the same time. The approach recovers resources from sludge and has been applied to black and odorous water treatment in Gaotian Village, Sihe Town, Xinyi, Maoming, Guangdong, where pond water quality gradually improved to Class IV of the surface water standard by 2025.
A second example comes from Ye County in Henan Province, where a four-colour (red, yellow, blue, green) graded management method gives every small water body in the county its own register entry with a unified number and mapped location. Colour labels reflect treatment status and correspond to different grid management levels, which allows measures to be tailored and updated dynamically. Officials took the lead and villagers contributed funds and labour, creating a multi-stakeholder mechanism that has improved long-term upkeep.
