Black and Odorous Water Bodies: How They Are Defined, Graded and Fixed

Colour and smell are only the visible end of the problem. A black and odorous water body is one whose organic load has exceeded what the water can digest, so oxygen disappears, degradation stalls halfway, and the end products of anaerobic decay do the rest. The definition matters because it is also the entry ticket to a funded treatment programme.

Rural and urban cases are counted differently

In rural areas the term covers water bodies with conspicuously abnormal colour or a strong offensive odour inside administrative villages or communities, outside the urban built-up area and the urban development boundary. Surveys focus on the ponds, ditches and canals in front of and behind villagers' houses, plus any water body that attracts strong public complaints. The spatial scope is the natural village or villager group, extended about 500 m outward from the residential cluster in southern China and about 1,000 m in northern China. Rivers, lakes, reservoirs, ponds and ditches all count. In principle, water bodies with a surface area of 200 m2 or more enter the survey, though complaint-driven cases are not limited by that threshold.

Urban cases are water bodies inside the built-up area that show an unpleasant colour, emit an unpleasant odour, or both. They are graded as lightly or severely black and odorous rather than simply flagged, and the grading uses four indicators: transparency, dissolved oxygen, oxidation-reduction potential (ORP) and ammonia nitrogen.

Three steps decide whether a water body qualifies

Determination runs through sensory judgement, public appraisal and water quality monitoring, and stops as soon as one step returns a positive result. Sensory judgement relies on abnormal colour such as blackening or whitening, or abnormal odour; either one is enough. Where that judgement is disputed, the village committee organises consultation and a questionnaire with at least 20 valid responses, and if more than 60% of respondents call the water body black or odorous, the case is settled.

If public appraisal still cannot settle it, three indicators are monitored. A water body qualifies if any single indicator reaches its threshold: transparency below 25 cm, dissolved oxygen below 2 mg/L, or ammonia nitrogen above 15 mg/L. For water bodies carrying a high sediment load, a transparency exceedance alone does not trigger the classification, because suspended silt can block light without any organic failure. Water bodies of 2,000 m2 or more enter the national supervision list; smaller ones go on provincial lists.

Light and severe, on the same four axes

• Lightly black and odorous: transparency 25 to 10 cm, dissolved oxygen 0.2 to 2.0 mg/L, ORP between -200 and 50 mV, ammonia nitrogen 8.0 to 15 mg/L.
• Severely black and odorous: transparency below 10 cm, dissolved oxygen below 0.2 mg/L, ORP below -200 mV, ammonia nitrogen above 15 mg/L. Where water depth is under 25 cm, the transparency criterion becomes 40% of actual depth.

The grading is not academic. Severe cases usually need dredging and reaeration, while light cases can often be recovered by interception, flow restoration and planting alone.

Why oxygen runs out in the first place

Aerobic decomposition of organic matter pushes the oxygen consumption rate above the reaeration rate, so the water body turns oxygen deficient. Organic degradation becomes incomplete and slows down, and anaerobic biodegradation takes over, generating hydrogen sulphide, ammonia, mercaptans and other odour-causing substances while forming black-coloured matter at the same time. That combination is the condition itself, not a symptom of something else.

Load arrives from four directions. Point sources include direct outfall discharge, combined sewer overflow in the rainy season, first-flush or dry-weather flow in separate stormwater pipes, and make-up water from unconventional sources. Non-point sources carry pollution in rainfall runoff and in dispersed livestock and poultry wastewater around cities. Internal sources are the ones operators forget: contaminated sediment, organisms, floating matter, bank-side litter, uncleared aquatic plants and algal blooms. The fourth group covers non-compliant effluent from municipal plants, accidental industrial discharges and autumn leaf fall. In rural settings the dominant contributors are domestic sewage, domestic waste, livestock and poultry manure and crop residues, with non-point pollution from livestock, aquaculture and riverside waste the single largest share.

Poor flow turns a bad year into a permanent condition

Insufficient hydrodynamic conditions degrade water quality directly. A large pollutant load settles into sediment, water quality keeps sliding, the aquatic ecosystem is damaged and the water body loses self-purification capacity. Physical, chemical and biological processes then re-suspend and release those pollutants back into the column, which is why deterioration recurs after every clean-up that stops at the water surface.

The treatment route that is actually applied

Four principles govern the work: adapt measures to local conditions, favour affordable and economically applicable options with priority on resource reuse, lead with demonstration cases, and focus on practical results. County-wide systematic treatment and source control form the foundation, and the technical route runs through sewage interception, internal source treatment, water system connectivity and ecological restoration. Backfilling a water body to make it disappear is explicitly prohibited.

In practice that means rural domestic sewage treated with resource recovery and ecological methods, removal of domestic waste, control of livestock, aquaculture and crop-production pollution, and proper handling of industrial and municipal effluent. Contaminated sediment is dredged and remediated. Ecological revetments, constructed wetlands, floating islands, reeds and calamus, plus aeration equipment, restore self-purification capacity. Connectivity projects bring back ecological flow, with reclaimed water and rainwater diverted as ecological make-up. Smart governance layers IoT sensing and AI forecasting over plant, network and river so that drainage problems can be traced and pumping scheduled automatically.

Long-term upkeep decides whether the money was wasted. Routine patrols and cleaning run through the river and lake chief system, monitoring is reinforced with more than 200 additional national points on poorer-quality tributaries and small water bodies, and enforcement keeps industrial sources in compliance. Satellite remote sensing and big-data screening feed a closed loop of investigation, referral, verification, rectification and case closure.

The numbers so far

By the end of 2020, 98.2% of black and odorous water bodies in prefecture-level and above cities had been eliminated. During the 14th Five-Year Plan the scope widened from those built-up areas to county-level cities and rural areas. By the end of 2025, such water bodies in prefecture-level and above cities had been essentially eliminated, more than 90% of those in 397 county-level cities were gone, and more than 98% of the 5,042 larger rural cases nationwide had been treated. National targets aim at essentially eliminating the problem in county-level city built-up areas by 2025 and in rural areas by 2027, with lasting cleanliness in prefecture-level and above cities.