Stabilization Ponds: Sizing Anaerobic, Facultative and Aerobic Stages as Resource Units

A treatment plant that behaves like a farm

A stabilization pond uses solar energy as its primary energy source. Plant aquatic vegetation in the pond, run aquaculture and waterfowl alongside it, and an artificial ecosystem forms. Driven by solar radiation, organic pollutants entering the pond are degraded and transformed through material migration, transformation and stepwise energy transfer along multiple food chains. The end products are not waste but harvestable resources: aquatic plants, fish, waterfowl, and water clean enough to reuse. Treatment and utilisation are the same operation, which is the whole appeal of the technology.

That framing matters most where water is scarce. In China, particularly in water-scarce and arid regions, biological oxidation ponds are treated as a route to wastewater resource utilisation rather than as cheap disposal. Wind is the second energy input, providing natural aeration with no mechanical equipment in the pond, and running cost lands at only 1/5-1/3 of a conventional wastewater treatment plant.

Sludge is the other quiet advantage. Production is small, roughly 1/10 of the activated sludge process. What is generated can go to the lotus pond or reed pond in the ecosystem, or to nearby farmland, where it is consumed as organic fertiliser. For a pond system with an anaerobic or alkaline pond at the front end, the sludge fermentation pit at the bottom lets sludge undergo acidification, hydrolysis and methane fermentation, converting organic solid particles into liquid or gas and approaching zero sludge discharge.

Four pond types, four jobs

Anaerobic pond

The principle is the same as any other anaerobic biological treatment: rely on the metabolic function of anaerobic bacteria to degrade organic substrates, in two stages. Acid-producing bacteria hydrolyse complex macromolecular organic matter into simple organic matter, organic acids, alcohols, aldehydes; then methanogenic bacteria use those products as nutrients in anaerobic fermentation, producing methane and carbon dioxide. It suits high-temperature, high-concentration organic wastewater from the food, biopharmaceutical, petrochemical, slaughterhouse, livestock, aquaculture, pulp and paper, brewing and pesticide industries, and it also removes some alcohols, aldehydes, phenols, ketones and heavy metals. It is generally placed at the head of the pond system as pretreatment, which greatly reduces the volume needed for the facultative, aerobic and advanced ponds that follow.

Influent discipline is strict, and it is where most failures start. Organic loading must not be too high: organic acids in the system should stay below 3000 mg/L, influent sulfate must not exceed 500 mg/L, influent BOD:N:P should be 100:2.5:1 with C:N generally around 20:1, pH should sit between 6.5 and 7.5, and the influent must contain no toxic substances. Heavy metals and harmful substances must comply with the “Code for Outdoor Drainage Design”.

Design is by BOD surface loading, which must be specified as a minimum allowable figure. In China the minimum allowable loading for anaerobic ponds is 300 kg BOD5/(10^4 m2·d) in the North and 800 kg BOD5/(10^4 m2·d) in the South. Where volumetric loading is used instead, municipal sewage anaerobic ponds in seven U.S. states generally run 0.2-0.4 kg BOD5/(m3·d), with Montana designing to 0.032-1.6 kg BOD5/(m3·d); industrial wastewater loading should be determined by experiment, and when the wastewater carries high VSS the VSS volumetric loading is the better basis.

Geometry follows function. Effective water depth h1 is 3.0-5.0 m; deeper favours anaerobic conditions but makes bottom water too cold, which also hurts. Sludge storage thickness h2 is 0.5 m or more; municipal sewage sludge accumulates at about 50 L per person per year and is removed on a 5-10 year cycle. The inlet sits 0.6-1.0 m above the pond floor so incoming water mixes with bottom sludge, with inlet pipe diameter generally 200-300 mm and not less than 300 mm for oily wastewater. The outlet must stay below the water surface with submergence of not less than 0.6 m and beneath the scum or freezing layer. Generally both inlet and outlet should number at least two, though a single inlet is acceptable when the pond bottom width is under 9 m. Because the anaerobic pond intercepts most of the sludge, run at least two ponds in parallel for alternate cleaning, and keep a single pond under (0.8-4)x10^4 m2.

Facultative pond

The workhorse of the family, and the most commonly used treatment pond. Effective depth is generally 1.0-2.0 m, in three layers. The upper aerobic zone behaves much like an aerobic pond: algae photosynthesise, producing oxygen and ample dissolved oxygen, and aerobic heterotrophic bacteria oxidise and decompose organic matter. The middle facultative zone, also called the transition zone, runs short of dissolved oxygen, low and intermittent; heterotrophic facultative bacteria there use what little dissolved oxygen exists to oxidise organic matter and, under anaerobic conditions, also respire using NO3- and CO32- as electron acceptors.

The bottom anaerobic zone has no dissolved oxygen. Influent suspended solids and organic solids from dead algae, bacteria and plants sink and build a 10-15 cm sludge layer, where anaerobic microorganisms carry out anaerobic and methanogenic fermentation. Generally about 30% of the BOD is removed in this zone. When a facultative pond is the first stage it needs the same pretreatment as an anaerobic pond, the only difference being that influent BOD:N:P is set at 100:5:1. Design normally uses the empirical BOD surface loading method, which correlates closely with average winter temperature.

Aerobic pond

Shallow, generally 0.3-0.5 m, so sunlight penetrates straight to the bottom. Bacteria, protozoa and algae coexist; dissolved oxygen comes from algal photosynthesis and wind-driven mixing while aerobic microorganisms degrade the organic matter. The degradation is essentially conversion of dissolved organic pollutants into inorganic matter and solid organic matter, bacterial and algal cells. Aerobic bacteria oxidise pollutants through aerobic metabolism into CO2, NH4+ and PO43- and synthesise new cells; algae take the carbon dioxide, inorganic nutrients and water the bacteria supply, add light energy, synthesise organic matter and release the oxygen the bacteria need. Algae are the producers, aerobic bacteria the decomposers, zooplankton feeding on bacteria, algae and organic debris the primary consumers, and together with the pond water they close the cycle of matter and energy.

Algae also strip nitrogen and phosphorus nutrients and adsorb some organic matter, which is the second reason they are wanted. Note the diurnal swing, because it catches people out: daytime photosynthesis releases more oxygen than bacterial degradation consumes, pushing dissolved oxygen to saturation; at night photosynthesis stops and respiration pulls dissolved oxygen to its lowest point just before dawn. pH tracks the CO2 concentration through the carbonate system equilibrium.

Aerated pond

Where pond depth exceeds 2 m, artificial aeration supplies oxygen and the whole pond stays aerobic. Aerated ponds divide into aerobic aerated and facultative aerated types. They do not rely mainly on natural purification; aerators installed on the pond surface supply the oxygen, which puts the process somewhere between the extended aeration method of activated sludge and a stabilization pond. The oxygenation equipment is the same as elsewhere: blower aerators, surface aerators and horizontal-shaft brush aerators from activated sludge and oxidation ditch practice all work here.

Nutrient removal and the reuse case

Planting fibrous vascular aquatic plants in the pond, reeds, Alternanthera philoxeroides, water lettuce, water hyacinth, removes pollutants effectively, with particularly good nitrogen and phosphorus removal. Maturation and advanced treatment ponds suit nutrient removal and dissolved organic matter and are mostly used in series after other stabilization ponds, polishing the effluent from secondary treatment.

The record is long. The first recorded pond system was built in Texas in 1901, and more than 50 countries now use stabilization pond systems, among them France with over 1,500, West Germany with over 2,000 and the United States with over 20,000; 40% of Malaysia’s total industrial wastewater is treated this way. China began researching stabilization ponds for sewage in the 1950s, and policy has been supportive since: the National Environmental Protection Agency once allocated RMB 3 million to fund reconstruction and expansion of the Qiqihar ponds, and by 1990 China had built 118 stabilization ponds with a daily sewage treatment capacity of 1.9 million tons.

Three Chinese cases show the range. Suihua City: 130,000 m2, volume 380,000 m3, two pumping stations plus four simple ones, 19,500 m of brick anti-seepage channel and 6,000 mu of irrigation. Seventeen years of irrigating farmland with the purified sewage caused no pollution of soil, vegetables or groundwater; the water carries 22.5 mg/L nitrogen, 2.93 mg/L phosphorus and 6.0 mg/L potassium, saving 31,430 kg of chemical fertiliser and 1 million tons of groundwater annually, with vegetable yields up an average of 50,000-80,000 jin, maturing 7-10 days earlier and adding 250,000-300,000 yuan a year. Yima Coal Industry Group’s Changcun mine: over 400,000 yuan converted an abandoned pit into an oxidation pond of 150,000 m3 treating 82,000 m3 of mine water per month, easing the site’s water shortage and yielding over 20,000 kg of fresh fish a year. Gaotang Paper Group: 12 million yuan built an oxidation pond covering over 600 mu, running around the clock, letting treated papermaking wastewater serve aquaculture and irrigation and returning up to 80% of the volume to production.

Where ponds make sense

Low-BOD wastewater defeats the activated sludge process. In many Chinese cities BOD runs below 100 mg/L, too thin for the activated sludge process and especially the biological oxidation ditch to operate normally, whereas a stabilization pond handles both high-concentration organic water and low-concentration wastewater. Add the small sludge output, the fertiliser value of pond-bottom sludge, and the fit with agriculture, animal husbandry and aquaculture, and the case is strongest where land is plentiful and population sparse. Western China, with few people and large areas, is the obvious example, and the application prospects there remain broad.