What Is Eutrophication? A Professional Wastewater Term Explained
Eutrophication refers to the phenomenon, under the influence of human activities, in which nutrients required by organisms—such as nitrogen and phosphorus—enter slowly flowing water bodies such as lakes, rivers, and bays in large quantities, causing algae and other plankton to multiply rapidly, dissolved oxygen in the water to decline, water quality to deteriorate, and fish and other organisms to die in large numbers. Under natural conditions, lakes also transition from an oligotrophic to a eutrophic state, but this natural process is very slow. Eutrophication caused by human discharge of nutrient-containing industrial and domestic wastewater, however, can appear in a short time.
When eutrophication occurs, planktonic algae multiply in large numbers, forming algal blooms (a natural ecological phenomenon of massive algae proliferation in freshwater bodies). Because the color of the dominant planktonic algae differs, the water surface often appears blue, red, brown, milky white, etc. This phenomenon is called red tide or red bloom in the ocean.
In surface freshwater systems, phosphate is usually the limiting factor for plant growth, whereas in seawater systems it is often ammonia nitrogen and nitrate that limit plant growth and total productivity. The substances that cause eutrophication are often the limited nutrients in these water systems. For example, in normal freshwater systems phosphorus content is usually limited, so adding phosphate leads to excessive plant growth; whereas in seawater systems phosphorus is not deficient but nitrogen is limited, so adding nitrogen pollutants eliminates this limiting factor, leading to excessive plant growth. Domestic sewage, as well as wastewater from industries such as fertilizers, food, and farmland drainage, all contain large amounts of nitrogen, phosphorus, and other inorganic salts. After natural water bodies receive this wastewater, nutrients in the water increase, promoting the vigorous growth of autotrophic organisms—especially blue-green algae and red algae, whose individual numbers increase rapidly while other algal species gradually decrease. Algae in water bodies were originally dominated by diatoms and green algae; the massive appearance of blue-green algae is a sign of eutrophication. As eutrophication develops, the water eventually becomes dominated by blue-green algae. Algae reproduce quickly and have short growth cycles. After algae and other plankton die, they are decomposed by aerobic microorganisms, continuously consuming dissolved oxygen in the water, or decomposed by anaerobic microorganisms, continuously producing gases such as hydrogen sulfide, deteriorating water quality in two ways and causing large numbers of fish and other aquatic organisms to die. The residues of algae and other plankton, during decay, release large amounts of nitrogen, phosphorus, and other nutrients back into the water for a new generation of algae and other organisms to use. Therefore, even if the external nutrient source is cut off, a eutrophic water body can hardly self-purify or recover to normal.
Eutrophication affects water-body quality, reduces water transparency, makes it difficult for sunlight to penetrate the water layer, and thus affects photosynthesis of aquatic plants, possibly causing a supersaturated state of dissolved oxygen. Both supersaturation of dissolved oxygen and low dissolved oxygen in water are harmful to aquatic animals and cause large fish kills. At the same time, because of eutrophication, a large amount of algae with blue-green algae and green algae as dominant species grows on the water surface, forming a layer of 'green scum,' causing harmful gases produced by the anaerobic decomposition of organic matter accumulated at the bottom and biotoxins produced by some plankton to also harm fish. Because eutrophic water contains nitrate and nitrite, long-term drinking of water in which these substances exceed certain standards also causes poisoning and disease in humans and livestock.
After the formation of 'green scum,' submerged algae cannot receive sunlight and will respire the oxygen in the water, unable to photosynthesize. The oxygen in the water gradually decreases, and aquatic organisms also die due to insufficient oxygen. Dead algae and organisms then undergo oxidation in the water, at which point the water body also becomes very foul-smelling, and the water resource is polluted and can no longer be used.
Excessive nitrogen, phosphorus, and other nutrients in water bodies mainly come from untreated or incompletely treated industrial and domestic wastewater, organic garbage, and livestock and poultry manure, as well as agricultural fertilizers, of which the largest source is the large amount of fertilizer applied to farmland.
Farmland runoff carrying large amounts of ammonia nitrogen and nitrate nitrogen into water bodies changes the original nitrogen balance, promoting the rapid proliferation of certain algal species adapted to the new conditions, covering large water areas. For example, in some lake-branch waterways of southern China's water-network regions, large amounts of nitrogen flowing from farmland promote the massive proliferation of floating plants such as water peanut, water hyacinth, water lettuce, and duckweed, causing some river sections to affect navigation. After these aquatic plants die, bacteria decompose them, increasing organic matter in their water body, leading to further oxygen consumption and large fish kills. Recently, relevant U.S. research departments have found that domestic sewage and human and livestock manure with urea and ammonia nitrogen as the main nitrogen forms, after being discharged into water bodies, will turn the normal nitrogen cycle into a 'short-circuit cycle'—that is, the massive discharge of urea and ammonia nitrogen destroys the normal nitrogen–phosphorus ratio and causes the planktonic plant community in this water area to change completely. The original normal planktonic plant community was composed of diatoms, flagellates, and dinoflagellates, but these populations are almost completely replaced by blue-green algae, red algae, and small flagellates (genus Nannochloris, genus Stichococcus).
Excess phosphorus in water bodies mainly comes from fertilizers, agricultural waste, and municipal sewage. According to relevant data, in the past 15 years the phosphate content of surface water has increased 25-fold, and in the United States 60% of the phosphate entering water bodies comes from municipal sewage. In municipal sewage the main source of phosphate is detergents, which, besides causing eutrophication, also produce large amounts of foam in many water bodies. Excess phosphorus in water bodies comes on the one hand from external industrial and domestic wastewater, and on the other hand from its internal sources—that is, the bottom sediment of water bodies releases phosphate under reducing conditions, thereby increasing phosphorus content. Especially in some eutrophic lakes caused by nitrate, the discharge of municipal sewage makes it more complex, and the system will deteriorate rapidly; even if the addition of phosphate is stopped, the problem will not be solved. This is because over the years a large amount of phosphate-rich sediment has accumulated at the bottom, which, due to the protective layer of insoluble iron salts, normally does not participate in mixing. However, when the bottom water has low oxygen content and is in a reducing state (usually occurring during summer stratification), the protective layer disappears, causing phosphate to be released into the water.
Most scholars believe that the increase in concentrations of nutrients such as nitrogen and phosphorus is the cause of massive algae proliferation, with phosphorus being the key factor. The physical, chemical, and biological factors (such as sunlight, nutrient salts, seasonal changes, water temperature, pH, and the mutual relationships of organisms themselves) affecting algae growth are extremely complex. Therefore, it is difficult to predict the trend of algae growth and to define indicators of eutrophication. Generally adopted indicators are: nitrogen content in water exceeding 0.2–0.3 ppm, biochemical oxygen demand greater than 10 ppm, phosphorus content greater than 0.01–0.02 ppm, pH 7–9 in freshwater with bacterial count exceeding 100,000 per mL, and chlorophyll-a content—an indicator of algae quantity—greater than 10 ug/L.
The prevention and control of eutrophication is the most complex and difficult problem in water-pollution treatment. This is because: ① the complexity of pollution sources—the nitrogen and phosphorus nutrients causing water eutrophication come from both natural and anthropogenic sources, both exogenous and endogenous, which makes it difficult to control pollution sources; ② the high difficulty of nutrient removal—no single biological, chemical, or physical measure can completely remove nitrogen and phosphorus nutrients from wastewater. Conventional secondary biological treatment can only remove 30–50% of nitrogen and phosphorus.
Control exogenous nutrient input
The eutrophication of the vast majority of water bodies is mainly caused by the enrichment of externally input nutrients in the water body. If the external input of nutrients is reduced or cut off, the water body loses the possibility of nutrient enrichment. Therefore, the first priority should be to reduce or cut off external nutrient input and control exogenous nutrients, starting from controlling anthropogenic pollution sources. One should accurately investigate and clarify the main discharge sources of nutrients entering the water body, monitor the nitrogen and phosphorus concentrations in the wastewater and sewage discharged into the water body, calculate the annual total nitrogen and phosphorus discharge, and provide a reliable scientific basis for implementing measures to control exogenous nutrients.
For the treatment and remediation of eutrophic river and lake water bodies, various countries and regions adopt different physical, chemical, and biological methods for prevention, control, and restoration, and have achieved certain results. The main physical treatment methods include sediment dredging, water flushing, and mechanical aeration, etc. On the one hand, the engineering volume is huge and the operating cost is high; on the other hand, dredging sediments of heavily polluted rivers and lakes easily causes the suspended and diffuse release of bottom sediments, promoting the release of nitrogen and phosphorus nutrients and the metal ions they adsorb, thus exposing the water environment to the risk of secondary pollution from heavy-metal ions and nitrogen–phosphorus nutrients released from sediments. Chemical methods include adding coagulants and algicides; although they achieve certain effects in the short term, they also have problems of incomplete treatment and high cost, and especially cause secondary pollution and new ecological problems. The currently popular biological and ecological restoration uses measures such as microbial degradation, absorption and transfer by aquatic plants, or filtration and adsorption by ecological floating beds and filter beds to reduce ammonia nitrogen in water bodies. Although such methods avoid secondary pollution, they are greatly affected by the natural environment, have demanding conditions, and compared with other treatment technologies have the disadvantages of long cycles and slow results.
The spatial and temporal distribution of nutrients input into water bodies such as lakes is very complex. Nitrogen and phosphorus elements in water bodies may be absorbed and utilized by aquatic organisms, or exist in dissolved salt form in water, or settle through complex physical, chemical, and biological reactions and continuously accumulate in bottom sediments, or be released from bottom sediments into the water. To reduce the endogenous nutrient load and effectively control internal phosphorus enrichment in lakes, different methods should be adopted according to different situations.
(1) The treatment and remediation of eutrophic river and lake water bodies is an urgent need for social and economic development, urban landscape, and ecological environment construction, with dual economic and environmental benefits.
(2) Significantly improve the treatment effect of eutrophic river and lake water bodies, greatly shorten the treatment cycle, and effectively reduce treatment costs.
(3) Restore the use functions of water bodies and effectively alleviate the serious shortage of water resources in China.
(4) Improve residents' living environment and raise the quality of people's lives.
Many foreign countries have recognized that the government's tertiary treatment of sewage—removing and recovering nitrogen and phosphorus from point-source sewage for reuse—is the most advanced, economical, and effective positive measure to prevent and control eutrophication.
This includes excavating bottom sediments, performing deep-water aeration of water bodies, diluting by water injection, and laying plastic on the sediment surface, etc. Excavating sediments can reduce or even eliminate potential internal pollution sources; deep-water aeration can periodically or irregularly supplement oxygen through artificial deep-lake-bottom aeration, so that no anaerobic layer appears at the water–sediment interface, keeping it aerobic and helping to inhibit phosphorus release from sediments. In addition, where conditions permit, injecting water with low phosphorus and nitrogen concentrations into the lake can dilute nutrient concentrations.
This is a method including coagulation–sedimentation and the use of chemical agents to kill algae—for example, many kinds of cations can effectively precipitate phosphorus from aqueous solution, of which the most valuable are the relatively cheap iron, aluminum, and calcium, all of which can form insoluble precipitates with phosphate and settle. For example, Long Lake in western Washington State, USA, is a eutrophic water body; in October 1980, aluminum salts were added to the lake to precipitate phosphate. In the fourth summer after adding aluminum salts, the phosphorus concentration in the lake water dropped from the original 65 ug/L to 30 ug/L, and the lake water quality improved markedly. In the chemical method, there is also a method of using algicides to kill algae. This method is suitable for water bodies with algal blooms covering the lake. After the algicide kills the algae, the algae decay and decompose and still release phosphorus, so the killed algae should be removed promptly, or appropriate chemicals should be added again to settle the phosphate released from algae decay.
A method that uses aquatic organisms to absorb and utilize nitrogen and phosphorus elements for metabolic activities to remove nitrogen and phosphorus nutrients from water bodies. Some countries have begun to test large-scale aquatic plant wastewater-treatment systems to purify eutrophic water bodies. Large aquatic plants include many species such as water hyacinth, reed, narrow-leaf cattail, Canadian pondweed, multi-spike watermilfoil, Nitella, and pennywort, which can be suitably selected according to different climatic conditions and pollutant properties. The characteristics of aquatic plants purifying water are that they take large aquatic plants as the main body, with plants and rhizosphere microorganisms in symbiosis, producing a synergistic effect to purify sewage. Through direct plant absorption, microbial transformation, physical adsorption, and sedimentation, nitrogen, phosphorus, and suspended particles are removed, and heavy-metal molecules are also degraded. Aquatic plants generally grow fast, and after harvesting can be processed as fuel, feed, or fermented to produce biogas. This is an important measure for treating lake eutrophication at home and abroad.
In recent years, some countries have adopted biological measures to control eutrophication and have achieved relatively obvious results, known as biomanipulation. For example, Germany has adopted biological control in recent years and successfully improved the water quality of an artificial lake (average depth 7 m). The method is to stock the lake annually with carnivorous fish species such as pike and bass to prey on the small fish that eat zooplankton; after a few years these small fish are significantly reduced, while zooplankton (such as Daphnia) increase, thereby reducing the amount of phytoplankton that is their food, and the transparency of the entire water body improves accordingly, with bacteria reduced and the depth distribution of the oxygen balance improved. It was also found that the phytoplankton population changed, with the proportion of blue-green algae growth increasing because they cannot be preyed upon by zooplankton; to control the growth of such algae, silver carp can be stocked.
Reduce endogenous nutrient load
