Basic Wastewater Treatment Terminology: What Is a Stabilization Pond (Oxidation Pond)?
A stabilization pond uses solar energy as its primary energy source. By planting aquatic plants in the pond and carrying out aquaculture and waterfowl farming, an artificial ecosystem is formed. Driven by solar energy (provided by solar radiation), through the material migration, transformation, and stepwise energy transfer and conversion along multiple food chains in the pond, the organic pollutants entering the pond are degraded and transformed. Finally, not only are pollutants removed, but they are also recovered as resources in the form of aquatic plants, aquaculture, and waterfowl; the purified wastewater can also be recovered and reused as a renewable resource, combining wastewater treatment with utilization and realizing the reclamation of wastewater.
The artificial ecosystem forms multiple food chains by planting aquatic plants and raising fish, ducks, geese, etc. It includes not only decomposer organisms (bacteria and fungi) and producer organisms (algae and other aquatic plants), but also consumer organisms such as fish, shrimp, shellfish, snails, ducks, geese, and wild waterfowl. These three groups cooperate to treat and utilize the pollutants in the wastewater more effectively. If appropriate numerical and energy ratios are maintained among the trophic levels, a sound multi-level ecological balance system can be established. When wastewater enters such a stabilization pond, its organic pollutants are not only degraded and purified by bacteria and fungi; their final degradation products-some inorganic compounds serving as carbon, nitrogen, and phosphorus sources-with solar energy as the primary energy input, participate in the metabolic processes of the food web and migrate and transform step by step from lower to higher trophic levels, finally turning into products such as aquatic crops, fish, shrimp, mussels, geese, and ducks, thereby yielding considerable economic benefits.
In China, especially in water-scarce and arid regions, biological oxidation ponds are an effective method for the resource utilization of wastewater, so stabilization pond treatment has become a new technology vigorously promoted in China.
At the same time, the associated supporting processes and equipment are also being continuously optimized and upgraded.
The wastewater treated by stabilization ponds can be used for agricultural irrigation, and aquatic plants and aquaculture can also be cultivated in the treated water. The organic matter in the wastewater is converted into aquatic crops, fish, waterfowl, and other products for people's use or other purposes. Taking into account the revenue from comprehensive utilization, it may even reach a balance of payments or a surplus.
Wind energy is one of the important auxiliary energy sources for stabilization ponds. With appropriate design, natural aeration and oxygenation by wind energy can be achieved in the pond, thereby saving electricity and reducing treatment energy consumption. In addition, no complex mechanical equipment or devices are needed in stabilization ponds, which makes their operation more stable and maintains good treatment performance, and their operating cost is only 1/5-1/3 of that of conventional wastewater treatment plants.
In addition, this technology also has wide application and practice in related fields.
Another advantage of stabilization pond wastewater treatment technology is that it produces a small amount of sludge, only 1/10 of that produced by the activated sludge process. The sludge generated in the front-end treatment system can be sent to the lotus pond or reed pond in the ecosystem, or to nearby farmland, where it is used and consumed as organic fertilizer. For a pond system with an anaerobic pond or alkaline pond at the front end, the sludge fermentation pit at the bottom of the anaerobic or alkaline pond causes the sludge to undergo acidification, hydrolysis, and methane fermentation, thereby converting organic solid particles into liquid or gas, enabling near-zero sludge discharge.
In many Chinese cities the BOD concentration of wastewater is very low, below 100 mg/L, making it impossible for the activated sludge process, especially the biological oxidation ditch, to operate normally, whereas stabilization ponds can not only effectively treat high-concentration organic water but also treat low-concentration wastewater.
At the same time, the associated supporting processes and equipment are also being continuously optimized and upgraded.
Aquatic plant pond-planting some fibrous vascular aquatic plants in the pond, such as reeds, Alternanthera philoxeroides, water lettuce, and water hyacinth, can effectively remove pollutants from the water, especially with good removal of nitrogen and phosphorus.
In addition, many factors need to be considered in practical engineering applications.
The principle of the anaerobic pond is the same as other anaerobic biological treatment processes, relying on the metabolic function of anaerobic bacteria to degrade organic substrates. The reaction is divided into two stages: first, acid-producing bacteria hydrolyze complex macromolecular organic matter into simple organic matter (organic acids, alcohols, aldehydes, etc.); then methanogenic bacteria use these organic substances as nutrients in an anaerobic fermentation reaction, producing methane and carbon dioxide, etc.
Based on this, industry experts have also carried out extensive research and improvement.
(5) It is generally placed at the head of the pond system as a pretreatment facility, followed by facultative ponds, aerobic ponds, or even advanced treatment ponds for further treatment, which can greatly reduce the volume of the subsequent facultative and aerobic ponds.
In addition, many factors need to be considered in practical engineering applications.
It has a good removal effect on high-temperature, high-concentration organic wastewater, such as from the food, biopharmaceutical, petrochemical, slaughterhouse, livestock, aquaculture, pulp and paper, brewing, and pesticide industries. It also has a certain removal effect on chemical substances such as alcohols, aldehydes, phenols, and ketones, as well as on heavy metals.
In addition, many factors need to be considered in practical engineering applications.
(3) Influent quality: the organic loading in the influent must not be too high. The concentration of organic acids in the system should be less than 3000 mg/L; the influent sulfate concentration should not exceed 500 mg/L; influent BOD:N:P = 100:2.5:1; C:N is generally about 20:1; the pH should be between 6.5 and 7.5; the influent must not contain toxic substances, and the concentration of heavy metals and harmful substances must not be too high, in accordance with the 'Code for Outdoor Drainage Design.'
In addition, many factors need to be considered in practical engineering applications.
The minimum allowable BOD surface loading of the pond must be specified. According to actual conditions, the minimum allowable loading of anaerobic ponds in China is: North-300 kg BOD5/(10^4 m2-d); South-800 kg BOD5/(10^4 m2-d).
The design of anaerobic ponds for municipal sewage abroad generally adopts this method, and many industrial wastewater anaerobic ponds in China also use this method. According to the design parameters for municipal sewage anaerobic ponds in seven U.S. states, the BOD volumetric loading is generally 0.2-0.4 kg BOD5/(m3-d), with some individual ranges being larger, for example, Montana uses a design parameter of 0.032-1.6 kg BOD5/(m3-d). The design loading for industrial wastewater should be determined through experiments; the pilot results of anaerobic pond treatment of meat-processing wastewater in China are shown in the table below:
In addition, many factors need to be considered in practical engineering applications.
When the anaerobic pond treats wastewater with high VSS, it is advisable to design using the VSS volumetric loading. According to foreign data, the design parameters of several anaerobic ponds treating industrial wastewater are as follows:
In addition, this technology also has wide application and practice in related fields.
Effective water depth h1: 3.0-5.0 m. If the depth is too great, although it favors the formation of anaerobic conditions, it will make the water temperature at the bottom of the pond too low, which is also unfavorable to the reaction.
Sludge storage thickness h2: >=0.5 m. The sludge volume of municipal sewage anaerobic ponds is calculated at 50 L per person per year, and the sludge removal cycle is generally 5-10 years.
In addition, this technology also has wide application and practice in related fields.
4) Inlet and outlet: the anaerobic pond inlet is set at the bottom, 0.6-1.0 m above the pond bottom, so that the influent mixes with the bottom sludge. The inlet pipe diameter is generally 200-300 mm; for oily wastewater, the inlet pipe diameter should be not less than 300 mm. The outlet pipe should be below the water surface, with a submergence depth of not less than 0.6 m, and should be below the scum layer or freezing layer. Generally both the inlet and outlet should be no fewer than two; when the pond bottom width is less than 9 m, a single inlet may also be used.
Because the anaerobic pond is usually at the head of the stabilization pond system and intercepts a large amount of sludge, there should be at least two ponds in parallel for alternate sludge removal; the area of a single pond should not exceed (0.8-4)x10^4 m2.
The effective depth is between 1.0-2.0 m. The upper layer is the aerobic zone; the middle layer is the facultative zone; the pond bottom is the anaerobic zone, where the settled sludge undergoes anaerobic fermentation. The facultative pond is the most commonly used treatment system among various types of treatment ponds.
The facultative pond is the most common type of stabilization pond. Its effective depth is generally 1.0-2.0 m, divided into three layers from top to bottom: the upper aerobic zone, the middle facultative zone (also called the transition zone), and the anaerobic zone at the pond bottom (see Figure 3). The purification principle of the aerobic zone is basically the same as that of the aerobic pond. Algae perform photosynthesis, producing oxygen and ample dissolved oxygen. Organic matter is oxidized and decomposed by aerobic heterotrophic bacteria; in the facultative zone the dissolved oxygen supply is rather tight, with low and intermittent content. There exist heterotrophic facultative bacteria that can use the small amount of dissolved oxygen in the water to oxidize and decompose organic matter, and at the same time, under anaerobic conditions, can use NO3- and CO32- as electron acceptors for anaerobic metabolism.
There is no dissolved oxygen in the anaerobic zone. Suspended solids in the influent, as well as organic solids produced by the death of algae, bacteria, plants, etc., sink to the pond bottom, forming a 10-15 cm thick sludge layer, where anaerobic microorganisms carry out anaerobic fermentation and methanogenic fermentation to decompose the organic matter. Generally, 30% of the BOD can be removed in the anaerobic zone.
In addition, from the perspective of industry development, market demand is also driving technological progress.
(2) Pretreatment and influent quality requirements: if the facultative pond is used as the first stage, certain pretreatment measures are required. The specific provisions are the same as for the anaerobic pond, the only difference being that the facultative pond requires influent BOD:N:P = 100:5:1.
It is generally calculated by an empirical method, namely the BOD surface loading method. The BOD surface loading is closely related to the average winter temperature. The table below shows the main design parameters recommended for municipal wastewater facultative ponds based on the achievements of China's Seventh Five-Year Plan scientific and technological research project:
In addition, this technology also has wide application and practice in related fields.
The aerobic pond is a sewage aerobic biological treatment pond where bacteria and algae coexist. It is shallow, generally 0.3-0.5 m. Sunlight can directly penetrate to the pond bottom; bacteria, protozoa, and algae exist in the pond, and dissolved oxygen is provided by algal photosynthesis and wind-driven mixing, while aerobic microorganisms degrade the organic matter.
The degradation process of organic matter in the aerobic pond is essentially the conversion of dissolved organic pollutants into inorganic matter and solid organic matter-bacterial and algal cells. Aerobic bacteria use the oxygen in the water to oxidize and decompose organic pollutants through aerobic metabolism into inorganic substances CO2, NH4+, and PO43-, and synthesize new bacterial cells. Algae, on the other hand, use the carbon dioxide, inorganic nutrients, and water provided by aerobic bacteria, together with light energy, to synthesize organic matter and form new algal cells, releasing oxygen that in turn supplies the oxygen needed by aerobic bacteria in their metabolism. In the aerobic pond, algae are the producers and aerobic bacteria are the decomposers. In addition, the zooplankton present in the aerobic pond feed on bacteria, algae, and organic debris and are the primary consumers. Producers, decomposers, and consumers, together with the pond water, form an aquatic ecosystem that completes the cycling and transfer of matter and energy in the system, thereby purifying the incoming sewage.
The algae in the pond, besides providing dissolved oxygen for the aerobic degradation of sewage through photosynthesis, can also remove nitrogen and phosphorus nutrients from the sewage and adsorb some organic matter.
Algal photosynthesis causes the dissolved oxygen and pH of the pond water to vary diurnally. During the day, the oxygen released by algal photosynthesis exceeds the oxygen demand of bacteria degrading organic matter, so the dissolved oxygen concentration in the pond water is very high and can reach saturation. At night, algae stop photosynthesis, and due to biological respiration consuming oxygen, the dissolved oxygen concentration in the water drops, reaching its lowest at dawn. After sunlight returns, dissolved oxygen gradually rises again. The pH of the aerobic pond is related to the CO2 concentration in the water and is affected by the CO2 equilibrium of the carbonate system in the pond water.
It is worth noting that the technologies and standards in this field are also constantly evolving and being refined.
It is suitable for removing nutrients and treating dissolved organic matter; because of its good treatment effect, it is mostly used in series after other stabilization ponds for further treatment, treating the effluent from secondary treatment.
In addition, from the perspective of industry development, market demand is also driving technological progress.
With a pond depth greater than 2 m, artificial aeration is used to supply oxygen, and the entire pond is in an aerobic state. Aerated ponds are generally divided into two types: aerobic aerated ponds and facultative aerated ponds.
It does not rely mainly on natural purification processes but uses artificial supply of oxygen, usually by installing aerators on the pond surface. In essence, it is a process between the extended aeration method of the activated sludge process and the stabilization pond.
Based on this, industry experts have also carried out extensive research and improvement.
2) The artificial oxygenation equipment of aerated ponds is the same as for other aerobic processes. For example, the blower aerators, surface aerators, and horizontal-shaft brush aerators widely used in the activated sludge process and oxidation ditch process can all be used for oxygenation in aerated ponds.
The first recorded pond system was built in the United States in 1901 in Texas. More than 50 countries around the world are now using stabilization pond systems; among them, France has over 1,500 stabilization ponds, West Germany over 2,000, and the United States over 20,000. Stabilization ponds are also widely used in developing countries. For example, 40% of Malaysia's total industrial wastewater is treated using stabilization ponds.
Because stabilization ponds are economical and energy-saving and can realize the reclamation of wastewater, they have received high attention from the Chinese government. Research on using stabilization ponds to treat sewage in China began in the 1950s. The Chinese government has always adopted encouraging and supportive measures for stabilization ponds. The National Environmental Protection Agency once allocated RMB 3 million to fund the reconstruction and expansion of stabilization ponds in Qiqihar. By 1990, China had built 118 stabilization ponds with a daily sewage treatment capacity of 1.9 million tons.
Besides treating domestic sewage from small and medium-sized towns, stabilization ponds are also widely used to treat various industrial wastewaters. In addition, because stabilization ponds can form composite ecosystems and the sludge at the pond bottom can be used as high-efficiency fertilizer, stabilization ponds have found increasing application in wastewater treatment in agriculture, animal husbandry, and aquaculture. Especially in the western regions of China, where there are few people and large areas of land, the application prospects of oxidation pond technology are very broad.
1. Suihua City oxidation pond. This pond covers an area of 130,000 m2 with a volume of 380,000 m3, two pumping stations, four simple pumping stations, 19,500 m of brick anti-seepage channels, and an irrigation area of 6,000 mu. According to water quality analysis of the sewage pumping station and the oxidation pond, the purification effect is obvious and can meet the farmland irrigation water quality standard. Using this purified sewage to irrigate farmland for 17 years has caused no pollution to soil, vegetables, or groundwater, and the purified sewage contains 22.5 mg/L nitrogen, 2.93 mg/L phosphorus, and 6.0 mg/L potassium, saving 31,430 kg of chemical fertilizer and 1 million tons of groundwater each year. From plot experiments and large-area surveys, all types of vegetables increased in yield, averaging an annual increase of 50,000-80,000 jin, maturing 7-10 days earlier, with an annual income increase of 250,000-300,000 yuan. Therefore, the use of oxidation ponds to treat sewage in small and medium-sized cities is an important water and fertilizer resource for agricultural production, and can also improve soil fertility and increase vegetable yields.
2. Changcun Coal Mine oxidation pond of Yima Coal Industry Group Co. To eliminate the pollution of the Jianhe River by mine water, over the past few years the mine has invested more than 400,000 yuan to convert an abandoned pit into an oxidation pond with a volume of 150,000 m3, treating 82,000 m3 of mine water per month. This new approach to controlling mine water pollution not only alleviated the severe water shortage in the mining area but also produced good economic and social benefits. The mine water treated by the oxidation pond can be used as industrial water above and below ground, and fish can also be farmed in the pond, with an annual fresh fish output of over 20,000 kg. Today, the area around the oxidation pond is shaded by green trees and has become a well-known scenic spot.
3. Gaotang Paper Group oxidation pond. To solve the sewage discharge problem, the group invested 12 million yuan to build a second-phase pollution-control project-the oxidation pond-covering over 600 mu, operating around the clock. This enables the company's treated papermaking wastewater to be used for aquaculture and agricultural irrigation, and up to 80% of the water volume to be recycled back into production, thereby realizing the harmless comprehensive utilization of papermaking wastewater and making the company a zero-pollution-discharge enterprise.
The Dongying City sewage treatment and utilization ecological project was completed in October 2000, with a designed treatment capacity of 100,000 t/d, covering about 110 hectares, with a total investment of 67 million yuan. It is a sewage treatment plant with a relatively complete stabilization pond treatment system designed and applied both domestically and abroad. The sewage quality is shown in the table below:
Adapting to local conditions, the project properly renovated and partitioned an existing reservoir into treatment units such as high-efficiency anaerobic ponds, aerated ponds, and aerated fish-culturing ponds; and built the nearby saline-alkali wasteland into ecological utilization units such as fish ponds, lotus ponds, and reed ponds.
Based on this, industry experts have also carried out extensive research and improvement.
(2) Grit chamber: each unit has a planar design size of 12.0 m x 3 m, an effective water depth of 1.2 m, a hopper inclination angle of 55 degrees, and a hopper height of 1.85 m.
