pH

The activity of photosynthesis requires the availability of a large amount of CO. En las lagunas facultativas o aeróbicas, el carbono disponible a partir del ion bicarbonato y la producción del ion OH hace que se produzcan periodos en los cuales el pH es relativamente alto. Así, en las últimas horas diurnas, se pueden alcanzar valores de pH superiores a 9. El desarrollo de un pH demasiado alto hace que la actividad bacteriana disminuya, se reduce la producción de CO y el proceso simbiótico se ve disminuido.

La utilización casi completa del CO disponible, por los procesos de fotosíntesis, puede incrementar el pH hasta llegar a 11.[6]​ La precipitación del calcio como carbonato, en aguas con alta dureza, ayuda a contener el incremento del pH, ya que la precipitación del carbonato de calcio a partir del bicarbonato presente en el agua se hace con la liberación de H:.

pH also affects the mortality of bacteria in stabilization ponds. Studies carried out in Portugal show that the mortality of bacteria is greater if the pH is greater than 8.5.[8] The pH value at which coliforms die varies from one author to another, but it is generally accepted that this limit is greater than 9.

lagoon depth

Optimal pond depth is influenced, to some extent, by the surface of the pond. It must be taken into account that aeration and photosynthesis preferably occur near the interface between air and liquid, which is where the greatest illumination necessary for the photosynthesis processes of algae is received. On the other hand, shallow ponds encourage the growth of emergent vegetation. As depth increases, irradiation and contact with atmospheric oxygen becomes less and anaerobic degradation processes of organic matter begin to occur, generating methane, which could be described by the reaction:

This reaction, in which {CHO} denotes general non-nitrogenous organic matter, is actually a series of biodegradation processes facilitated by certain anaerobic bacteria and which could be defined as a redox process in which both the oxidizing and reducing agents are organic substances [9]​ and which occurs mainly in the deepest part of the pond and in the sludge settled at the bottom. The subsequent biochemical oxidation of methane requires the contribution of more oxygen, which reduces the effectiveness of those lagoons based on aerobic processes:

In shallow lagoons methane (CH) escapes into the atmosphere, in deep lagoons it can demand a lot of oxygen from the upper layers, although the carbon dioxide (CO) formed can participate in the photosynthesis of algae. Therefore, it is important to seek a compromise in the design of ponds that also prevents the growth of undesirable vegetation. In most cases the lagoons have depths greater than 1 m, which is sufficient to prevent such growth. The intensity of the mixture is also a function of depth; At lower depths, the wind provides greater mixing and, in addition, there will be a greater surface area, which favors surface aeration. Depth also affects the temperature of the lagoon; the greater the depth, the less heat is lost, which allows for more intense decomposition. In surface load designs, the depth increases with retention time and constitutes a determining factor of the physical dimensions of the lagoon to provide specific fecal coliform removal efficiencies. Based on recent research, the tendency is to consider depths greater than 1.5 m with the objective of providing additional volume to store the sludge and because in some cases apparently the removal of BOD is more a function of the volumetric organic load than of the surface organic load.

Nutrients

Among the essential nutrients for algae growth are obviously carbon, nitrogen, phosphorus, calcium and magnesium.

Sludge sedimentation

The amount of sludge accumulated by sedimentation is extremely variable, depending on the concentration of settleable material contained in the wastewater and the rate of decomposition of the settled waste.

Some authors explain the influence of the sludge layer in this way:[10]​ Primary facultative lagoons: In these lagoons, part of the BOD reduction is due to the sedimentation of organic material. This sludge decomposes anaerobically, reducing its organic concentration and releasing the products of its anaerobic fermentation. Initially the volume of sludge increases but, in the long run, the BOD contributed by sedimentation is compensated by the BOD reduced by the anaerobic process, leaving the volume unchanged.

The bottom of a facultative lagoon behaves as a storage area for sludge in anaerobic decomposition. During the cold season, the sludge volume generally increases due to the decreased rate of anaerobic stabilization. On the contrary, in hot periods, such as summer, a reduction in volume will occur, as the decomposition rate increases.

On the other hand, in very warm climates, having a high rate of anaerobic decomposition, the oxygenation capacity of the aerobic layer may be exceeded and consequently the lagoon becomes a totally anaerobic lagoon. If you want to avoid this, you can think of some means to increase oxygenation. This, in already constructed lagoons, can be achieved through forced aeration.

The periodic extraction of sludge is one of the tasks included in the operation of the lagoons. The presence of floating "scum" in the lagoon, causing a bad odor, may be an indication that the sludge has accumulated excessively and must be removed.

In facultative lagoons, the sludge accumulation rate is estimated at 0.03 m³/inhabitant/year.[11]​.

Sludge is a byproduct of the wastewater treatment process. It occurs in both primary and secondary treatment processes. Biological sludge comes from the secondary treatment of domestic wastewater and is mainly excess biomass produced in biological processes and suspended mineral material. The treatment of sludge, due to its characteristics, presents some problems and difficulties; for its characteristics:

The increase in wastewater treatment has begun to generate as a consequence an increase in the quantities of sludge produced, creating the need to properly manage this byproduct to alter the environment as little as possible.[12]​.

In general, sludge is made up mainly of the elements that make up the effluent, the chemical additives used in the process and the bacterial mass that participates in the treatment. They have a humidity close to 80%, high contents of organic matter, reaching 65%, and important levels of macro and micronutrients; While organic matter serves as a soil conditioner, macro- and micronutrients serve as a source of nutrients for plants.[13]​ These positive aspects of sludge are offset by some negative factors, such as the presence of pathogenic microorganisms, undesirable concentrations of metallic elements and the possible presence of potentially toxic persistent organic compounds that have not been sufficiently studied in all climatic and social environments.[14]​[15]​.

Whatever the final destination of the sludge, it must undergo some stabilization process to minimize health risks. This is related to reducing humidity, reducing the potential for attracting vectors and reducing or eliminating the content of pathogenic microorganisms. The amount of sludge generated must be managed responsibly in the different treatment plants, for this it is vital to know its chemical, physical and bacteriological composition, in order to carry out adequate management.[16]​.

Winds

The mixing of waters in a stabilization lagoon depends on several factors, including the wind. The wind energy dissipated in mixing is a function of the extent of the lagoon; Therefore, large lagoons tend to have better mixing than small lagoons.

Mixing is important because it provides a more uniform distribution of temperature, oxygen, and algae throughout the pond.

The wind can thus help establish aerobic conditions down to the bottom of the lagoon. The maximum effect of the wind is obtained when a space free of obstacles for its action ("fetch") of 100 to 200 m is allowed. To favor the action of the wind, lagoons are built with their greatest length parallel to the predominant direction of the wind. To minimize short circuits caused by the action of the wind, the inlet-outlet axis of the lagoon must be aligned perpendicular to the predominant direction of the wind, screens must be placed to provide water flow conduction guides and improve the efficiency of the treatment.

In a slow-flowing, stratified lagoon, algae settle on the mud and photosynthetic oxygen production in the aerobic zone is reduced.

Sulfides

The existence of residual influent sulfur compounds affects the biota of the stabilization lagoons, like any biological process, by promoting a change from green algae to blue-green algae.

The reducing bacteria Desulfovibrio uses sulfate as an inorganic hydrogen acceptor and produces a large amount of sulfides in an anaerobic environment.

If the pH remains high, the reduced sulfur remains in solution as a disulfide ion. Some studies[17]​ indicate that sulfides are toxic to algae at concentrations of 6 to 7 mg/L; although concentrations of up to 8 mg/L, for a few days, do not appreciably affect BOD removal.

There are two types of bacteria that oxidize reduced sulfur compounds. The first group is colorless, strictly anaerobic bacteria, which use molecular oxygen as an electron acceptor. These bacteria are rare in stabilization lagoons and, if found, their optimal environment is the surface layer of the lagoon. The second group is the photosynthetic sulfur bacteria, strictly anaerobic, which use sunlight and sulfides as well as CO as a hydrogen acceptor. These bacteria impart a red or brown color to the water and appear when there are sulfides and anaerobiosis. Sulfide oxidation occurs in two stages:[17]​.

As can be seen from the equations above, this type of photosynthesis does not produce oxygen or BOD reduction.

dissolved oxygen

Depending on each lagoon, the surface oxygenated layer presents a variation in the amount of dissolved oxygen (DO), presenting its maximum in the daytime hours, due to the action of photosynthesis, and may even reach supersaturation, and presenting a minimum in the nighttime hours.

During the day, values ​​of 36 mg/L have been measured. To account for DO variations, sampling of a stabilization pond should cover 24-hour periods.

The surface aerobic layer of a facultative lagoon acts as a barrier, retaining the anaerobic bottom water at greater depths, containing HS and CH. The gaseous products of the decomposition of the sludge deposited at the bottom are oxidized as they pass through the upper layer of the lagoon. If the availability of oxygen is reduced in this stratum, bad odors may occur. The depth of the layer with DO presence, or the depth of the oxypause, depends, among others, on the surface charge of the lagoon, temperature, solar radiation, photosynthesis and the degree of surface re-aeration.

solar radiation

Solar radiation comprises all the direct, diffuse or dispersed light radiation received on a horizontal surface, per day. It can be expressed in Langleyds/d, calories per cm² per day (cal/cm².d). The complete meteorological stations have instruments for the continuous recording of solar radiation.

The solar radiation received depends on the season of the year, latitude, cloud cover, and atmospheric pollution.

The absorption of solar energy in the stabilization and maturation lagoons is very important, since through the photosynthesis of algae it favors the destruction of pathogenic organisms, reduces the color of the lagoon and favorably influences the water temperature. The absorption of solar radiation is selective, in natural waters the absorption of the disinfectant ultraviolet band (253.7 nm) is appreciable, and produces a decrease in the natural color of approximately 20% per month.[18]​.

The heat capacity of water or specific heat, or in other words, the amount of heat required to raise 1 gram of water one degree Celsius in temperature, at atmospheric pressure is 1 cal/gC or 4.186 J/gC.

The lagoon acts as a natural means of disinfection thanks, mainly, to exposure to sunlight for a relatively long period and the possible production of antibiotic substances by algae.[18]​.

Visible radiation with wavelengths between 400 and 700 nm is considered the energy available for photosynthesis.

Temperature

The activity of bacteria has an optimal temperature; if the water is at lower or higher temperatures, the activity will be reduced. In general, it is considered that with an increase of 10 °C, the activity of bacteria doubles, reaching a maximum of 35 to 38 °C.

Cold water organisms, psychrophilous organisms, live at temperatures of 0 to 10 °C and have, in general, a low growth rate. Both aerobic and anaerobic decomposition proceed slowly at low temperatures. Mesophilic organisms may also be present in the water, which develop at temperatures between 10 and 40 °C, and thermophiles at temperatures between 40 and 70 °C, which show different purifying activity, depending on the temperature. This, along with sunlight, are the most influential physical factors on photosynthesis. Diatomaceous algae have a lower critical temperature to carry out the photosynthesis process than blue-green algae, which are more tolerant of warm waters. In general, green algae are tolerant to a wide range of temperatures. In stabilization lagoons, the maximum growth rate of mesophilic algae occurs between 30 and 35.C.[19]​.

Temperature has an important effect on the mixing of water in the lagoon, especially if we observe the formation of a thermocline or layer of water that separates a hot upper zone from a colder lower zone. The main source of heat in a lagoon is solar radiation; This heat is exchanged through evaporation and with influent and effluent flows, as well as with precipitation. On sunny days the amount of heat stored in the lagoon increases during daylight hours, during days without sun and at night the lagoon loses heat to its surroundings. The upper two meters of the lagoon absorb approximately 50% of the solar radiation, heating it up. When the influent water is cold, having a greater density, it stays at the bottom and the hot water stays on the surface.

In shallow lagoons, the continuous wind induces a circulation producing mixing of the waters which in turn produces an approximately uniform temperature profile. On the other hand, in the absence of winds and with a warm climate, transient thermal stratifications can develop and a thermocline or layer can form with a sudden change in temperature. This may disappear as the temperature drops during cold, windy weather.

The relationship between ambient temperature and water temperature in a lagoon varies depending on the general environmental conditions, as can be seen in the following empirical formulas.

Where:.

The water temperature in the lagoons is 2 to 3.C higher than the ambient temperature in winter and 2 to 3.C lower than the ambient temperature in summer.[22]​.

Temperature also affects the mortality rate of coliforms. In general, the mortality rate of fecal coliforms increases when temperature increases.[23]​[24]​.

Infiltration

Initial infiltration, in non-waterproofed lagoons, can be important in arid areas. However, infiltration is reduced over time thanks to the sedimentation of sludge, algae and bacteria. In porous soils, gaps can seal in about 3 months.[20]​

Percolation in a lagoon is a function of many variables and it is very difficult to predict its value, even with deep soil studies. The natural sealing or waterproofing of lagoons occurs based on three mechanisms:

Evaporation

Evaporation can be important in dry, hot and windy areas, but generally it does not exceed the contribution of rainwater too much, except in areas where there is a deficit in the water balance, such as in desert areas of the Peruvian coast, in the Bolivian highlands, among others.

Lagoon geometry

The shape of the stabilization pond basically depends on the topography. They can have any geometric shape, but ponds with uniform, continuous edges that prevent water stagnation and the formation of dead zones or the incidence of short circuits are preferred.

The lagoons can be square, (length/width ratio of 1/1), but rectangular with length/width ratio of 2/1 to 4/1 are preferred, to encourage plug flow and ensure better performance.

Some authors recommend parallel units to facilitate maintenance, allow modular expansion when flow increases, and the possibility of interconnection or effluent recirculation. They also recommend providing flow trains for flows less than 5000 m³/d, a minimum of 3 cells in series or maximum sizes of 16 ha.[25]​.

BOD and Dissolved Solids

The production of effluents, with high BOD, in many stabilization lagoons, is the result of growth of suspended biomass in the lagoon and not of the escape of BOD from the tributary through it; as confirmed by DVO tests on filtered and unfiltered effluents.

The above enhances the capacity of lagoons to treat wastewater, but also emphasizes the need to properly stop the algal and bacterial biomass of the effluent, if high quality effluents in terms of BOD and total suspended solids are desired.

Studies carried out in the lagoons of San Juan[26][27]​ indicate that for loads of 450 to 1100 kg BOD/inhabitant, in primary lagoons, the suspended BOD is around 70% of the soluble BOD and that in secondary lagoons, with loads of 50-200 kg BOD/inhabitant, the suspended BOD is greater than the soluble BOD. The ratio of total BOD to soluble BOD is, however, very variable and there are studies where this ratio ranges between 1.2 and 8.3.

If a stabilization lagoon manages to produce settleable biomass that is easy to retain, the effluent will be of very good quality. In general, it can be assumed that a single small lagoon releases large amounts of biomass and that several lagoons in series allow greater sedimentation of biomass and, therefore, an effluent with greater removal of suspended solids. To determine the optimal conditions to obtain this purpose, even more research and study of the processes is required.

In the European Union the requirements for effluents from stabilization lagoon systems are:[28]​.

Additionally, for effluents that discharge into waters sensitive to eutrophication, the following is required:

Retention time

Retention time is the most important parameter in determining the removal of fecal coliforms in a lagoon and therefore in the elimination of pathogenic organisms from wastewater. For example, in the studies carried out in the San Juan lagoons, it has been determined that for a complete removal of protozoa, two stabilization lagoons in series are required with a total retention period of 36.7 days. Helminths were eliminated in two lagoons in series with a retention time of 5.5 days, and Salmonella could not be completely eliminated in two lagoons in series with a total retention time of 36.7 days. The dependence of bacterial mortality on retention time has been demonstrated in numerous studies.[8][27]​.

pH

The activity of photosynthesis requires the availability of a large amount of CO. En las lagunas facultativas o aeróbicas, el carbono disponible a partir del ion bicarbonato y la producción del ion OH hace que se produzcan periodos en los cuales el pH es relativamente alto. Así, en las últimas horas diurnas, se pueden alcanzar valores de pH superiores a 9. El desarrollo de un pH demasiado alto hace que la actividad bacteriana disminuya, se reduce la producción de CO y el proceso simbiótico se ve disminuido.

La utilización casi completa del CO disponible, por los procesos de fotosíntesis, puede incrementar el pH hasta llegar a 11.[6]​ La precipitación del calcio como carbonato, en aguas con alta dureza, ayuda a contener el incremento del pH, ya que la precipitación del carbonato de calcio a partir del bicarbonato presente en el agua se hace con la liberación de H:.

pH also affects the mortality of bacteria in stabilization ponds. Studies carried out in Portugal show that the mortality of bacteria is greater if the pH is greater than 8.5.[8] The pH value at which coliforms die varies from one author to another, but it is generally accepted that this limit is greater than 9.

lagoon depth

Optimal pond depth is influenced, to some extent, by the surface of the pond. It must be taken into account that aeration and photosynthesis preferably occur near the interface between air and liquid, which is where the greatest illumination necessary for the photosynthesis processes of algae is received. On the other hand, shallow ponds encourage the growth of emergent vegetation. As depth increases, irradiation and contact with atmospheric oxygen becomes less and anaerobic degradation processes of organic matter begin to occur, generating methane, which could be described by the reaction:

This reaction, in which {CHO} denotes general non-nitrogenous organic matter, is actually a series of biodegradation processes facilitated by certain anaerobic bacteria and which could be defined as a redox process in which both the oxidizing and reducing agents are organic substances [9]​ and which occurs mainly in the deepest part of the pond and in the sludge settled at the bottom. The subsequent biochemical oxidation of methane requires the contribution of more oxygen, which reduces the effectiveness of those lagoons based on aerobic processes:

In shallow lagoons methane (CH) escapes into the atmosphere, in deep lagoons it can demand a lot of oxygen from the upper layers, although the carbon dioxide (CO) formed can participate in the photosynthesis of algae. Therefore, it is important to seek a compromise in the design of ponds that also prevents the growth of undesirable vegetation. In most cases the lagoons have depths greater than 1 m, which is sufficient to prevent such growth. The intensity of the mixture is also a function of depth; At lower depths, the wind provides greater mixing and, in addition, there will be a greater surface area, which favors surface aeration. Depth also affects the temperature of the lagoon; the greater the depth, the less heat is lost, which allows for more intense decomposition. In surface load designs, the depth increases with retention time and constitutes a determining factor of the physical dimensions of the lagoon to provide specific fecal coliform removal efficiencies. Based on recent research, the tendency is to consider depths greater than 1.5 m with the objective of providing additional volume to store the sludge and because in some cases apparently the removal of BOD is more a function of the volumetric organic load than of the surface organic load.

Nutrients

Among the essential nutrients for algae growth are obviously carbon, nitrogen, phosphorus, calcium and magnesium.

Sludge sedimentation

The amount of sludge accumulated by sedimentation is extremely variable, depending on the concentration of settleable material contained in the wastewater and the rate of decomposition of the settled waste.

Some authors explain the influence of the sludge layer in this way:[10]​ Primary facultative lagoons: In these lagoons, part of the BOD reduction is due to the sedimentation of organic material. This sludge decomposes anaerobically, reducing its organic concentration and releasing the products of its anaerobic fermentation. Initially the volume of sludge increases but, in the long run, the BOD contributed by sedimentation is compensated by the BOD reduced by the anaerobic process, leaving the volume unchanged.

The bottom of a facultative lagoon behaves as a storage area for sludge in anaerobic decomposition. During the cold season, the sludge volume generally increases due to the decreased rate of anaerobic stabilization. On the contrary, in hot periods, such as summer, a reduction in volume will occur, as the decomposition rate increases.

On the other hand, in very warm climates, having a high rate of anaerobic decomposition, the oxygenation capacity of the aerobic layer may be exceeded and consequently the lagoon becomes a totally anaerobic lagoon. If you want to avoid this, you can think of some means to increase oxygenation. This, in already constructed lagoons, can be achieved through forced aeration.

The periodic extraction of sludge is one of the tasks included in the operation of the lagoons. The presence of floating "scum" in the lagoon, causing a bad odor, may be an indication that the sludge has accumulated excessively and must be removed.

In facultative lagoons, the sludge accumulation rate is estimated at 0.03 m³/inhabitant/year.[11]​.

Sludge is a byproduct of the wastewater treatment process. It occurs in both primary and secondary treatment processes. Biological sludge comes from the secondary treatment of domestic wastewater and is mainly excess biomass produced in biological processes and suspended mineral material. The treatment of sludge, due to its characteristics, presents some problems and difficulties; for its characteristics:

The increase in wastewater treatment has begun to generate as a consequence an increase in the quantities of sludge produced, creating the need to properly manage this byproduct to alter the environment as little as possible.[12]​.

In general, sludge is made up mainly of the elements that make up the effluent, the chemical additives used in the process and the bacterial mass that participates in the treatment. They have a humidity close to 80%, high contents of organic matter, reaching 65%, and important levels of macro and micronutrients; While organic matter serves as a soil conditioner, macro- and micronutrients serve as a source of nutrients for plants.[13]​ These positive aspects of sludge are offset by some negative factors, such as the presence of pathogenic microorganisms, undesirable concentrations of metallic elements and the possible presence of potentially toxic persistent organic compounds that have not been sufficiently studied in all climatic and social environments.[14]​[15]​.

Whatever the final destination of the sludge, it must undergo some stabilization process to minimize health risks. This is related to reducing humidity, reducing the potential for attracting vectors and reducing or eliminating the content of pathogenic microorganisms. The amount of sludge generated must be managed responsibly in the different treatment plants, for this it is vital to know its chemical, physical and bacteriological composition, in order to carry out adequate management.[16]​.

Winds

The mixing of waters in a stabilization lagoon depends on several factors, including the wind. The wind energy dissipated in mixing is a function of the extent of the lagoon; Therefore, large lagoons tend to have better mixing than small lagoons.

Mixing is important because it provides a more uniform distribution of temperature, oxygen, and algae throughout the pond.

The wind can thus help establish aerobic conditions down to the bottom of the lagoon. The maximum effect of the wind is obtained when a space free of obstacles for its action ("fetch") of 100 to 200 m is allowed. To favor the action of the wind, lagoons are built with their greatest length parallel to the predominant direction of the wind. To minimize short circuits caused by the action of the wind, the inlet-outlet axis of the lagoon must be aligned perpendicular to the predominant direction of the wind, screens must be placed to provide water flow conduction guides and improve the efficiency of the treatment.

In a slow-flowing, stratified lagoon, algae settle on the mud and photosynthetic oxygen production in the aerobic zone is reduced.

Sulfides

The existence of residual influent sulfur compounds affects the biota of the stabilization lagoons, like any biological process, by promoting a change from green algae to blue-green algae.

The reducing bacteria Desulfovibrio uses sulfate as an inorganic hydrogen acceptor and produces a large amount of sulfides in an anaerobic environment.

If the pH remains high, the reduced sulfur remains in solution as a disulfide ion. Some studies[17]​ indicate that sulfides are toxic to algae at concentrations of 6 to 7 mg/L; although concentrations of up to 8 mg/L, for a few days, do not appreciably affect BOD removal.

There are two types of bacteria that oxidize reduced sulfur compounds. The first group is colorless, strictly anaerobic bacteria, which use molecular oxygen as an electron acceptor. These bacteria are rare in stabilization lagoons and, if found, their optimal environment is the surface layer of the lagoon. The second group is the photosynthetic sulfur bacteria, strictly anaerobic, which use sunlight and sulfides as well as CO as a hydrogen acceptor. These bacteria impart a red or brown color to the water and appear when there are sulfides and anaerobiosis. Sulfide oxidation occurs in two stages:[17]​.

As can be seen from the equations above, this type of photosynthesis does not produce oxygen or BOD reduction.

dissolved oxygen

Depending on each lagoon, the surface oxygenated layer presents a variation in the amount of dissolved oxygen (DO), presenting its maximum in the daytime hours, due to the action of photosynthesis, and may even reach supersaturation, and presenting a minimum in the nighttime hours.

During the day, values ​​of 36 mg/L have been measured. To account for DO variations, sampling of a stabilization pond should cover 24-hour periods.

The surface aerobic layer of a facultative lagoon acts as a barrier, retaining the anaerobic bottom water at greater depths, containing HS and CH. The gaseous products of the decomposition of the sludge deposited at the bottom are oxidized as they pass through the upper layer of the lagoon. If the availability of oxygen is reduced in this stratum, bad odors may occur. The depth of the layer with DO presence, or the depth of the oxypause, depends, among others, on the surface charge of the lagoon, temperature, solar radiation, photosynthesis and the degree of surface re-aeration.

solar radiation

Solar radiation comprises all the direct, diffuse or dispersed light radiation received on a horizontal surface, per day. It can be expressed in Langleyds/d, calories per cm² per day (cal/cm².d). The complete meteorological stations have instruments for the continuous recording of solar radiation.

The solar radiation received depends on the season of the year, latitude, cloud cover, and atmospheric pollution.

The absorption of solar energy in the stabilization and maturation lagoons is very important, since through the photosynthesis of algae it favors the destruction of pathogenic organisms, reduces the color of the lagoon and favorably influences the water temperature. The absorption of solar radiation is selective, in natural waters the absorption of the disinfectant ultraviolet band (253.7 nm) is appreciable, and produces a decrease in the natural color of approximately 20% per month.[18]​.

The heat capacity of water or specific heat, or in other words, the amount of heat required to raise 1 gram of water one degree Celsius in temperature, at atmospheric pressure is 1 cal/gC or 4.186 J/gC.

The lagoon acts as a natural means of disinfection thanks, mainly, to exposure to sunlight for a relatively long period and the possible production of antibiotic substances by algae.[18]​.

Visible radiation with wavelengths between 400 and 700 nm is considered the energy available for photosynthesis.

Temperature

The activity of bacteria has an optimal temperature; if the water is at lower or higher temperatures, the activity will be reduced. In general, it is considered that with an increase of 10 °C, the activity of bacteria doubles, reaching a maximum of 35 to 38 °C.

Cold water organisms, psychrophilous organisms, live at temperatures of 0 to 10 °C and have, in general, a low growth rate. Both aerobic and anaerobic decomposition proceed slowly at low temperatures. Mesophilic organisms may also be present in the water, which develop at temperatures between 10 and 40 °C, and thermophiles at temperatures between 40 and 70 °C, which show different purifying activity, depending on the temperature. This, along with sunlight, are the most influential physical factors on photosynthesis. Diatomaceous algae have a lower critical temperature to carry out the photosynthesis process than blue-green algae, which are more tolerant of warm waters. In general, green algae are tolerant to a wide range of temperatures. In stabilization lagoons, the maximum growth rate of mesophilic algae occurs between 30 and 35.C.[19]​.

Temperature has an important effect on the mixing of water in the lagoon, especially if we observe the formation of a thermocline or layer of water that separates a hot upper zone from a colder lower zone. The main source of heat in a lagoon is solar radiation; This heat is exchanged through evaporation and with influent and effluent flows, as well as with precipitation. On sunny days the amount of heat stored in the lagoon increases during daylight hours, during days without sun and at night the lagoon loses heat to its surroundings. The upper two meters of the lagoon absorb approximately 50% of the solar radiation, heating it up. When the influent water is cold, having a greater density, it stays at the bottom and the hot water stays on the surface.

In shallow lagoons, the continuous wind induces a circulation producing mixing of the waters which in turn produces an approximately uniform temperature profile. On the other hand, in the absence of winds and with a warm climate, transient thermal stratifications can develop and a thermocline or layer can form with a sudden change in temperature. This may disappear as the temperature drops during cold, windy weather.

The relationship between ambient temperature and water temperature in a lagoon varies depending on the general environmental conditions, as can be seen in the following empirical formulas.

Where:.

The water temperature in the lagoons is 2 to 3.C higher than the ambient temperature in winter and 2 to 3.C lower than the ambient temperature in summer.[22]​.

Temperature also affects the mortality rate of coliforms. In general, the mortality rate of fecal coliforms increases when temperature increases.[23]​[24]​.

Infiltration

Initial infiltration, in non-waterproofed lagoons, can be important in arid areas. However, infiltration is reduced over time thanks to the sedimentation of sludge, algae and bacteria. In porous soils, gaps can seal in about 3 months.[20]​

Percolation in a lagoon is a function of many variables and it is very difficult to predict its value, even with deep soil studies. The natural sealing or waterproofing of lagoons occurs based on three mechanisms:

Evaporation

Evaporation can be important in dry, hot and windy areas, but generally it does not exceed the contribution of rainwater too much, except in areas where there is a deficit in the water balance, such as in desert areas of the Peruvian coast, in the Bolivian highlands, among others.

Lagoon geometry

The shape of the stabilization pond basically depends on the topography. They can have any geometric shape, but ponds with uniform, continuous edges that prevent water stagnation and the formation of dead zones or the incidence of short circuits are preferred.

The lagoons can be square, (length/width ratio of 1/1), but rectangular with length/width ratio of 2/1 to 4/1 are preferred, to encourage plug flow and ensure better performance.

Some authors recommend parallel units to facilitate maintenance, allow modular expansion when flow increases, and the possibility of interconnection or effluent recirculation. They also recommend providing flow trains for flows less than 5000 m³/d, a minimum of 3 cells in series or maximum sizes of 16 ha.[25]​.

BOD and Dissolved Solids

The production of effluents, with high BOD, in many stabilization lagoons, is the result of growth of suspended biomass in the lagoon and not of the escape of BOD from the tributary through it; as confirmed by DVO tests on filtered and unfiltered effluents.

The above enhances the capacity of lagoons to treat wastewater, but also emphasizes the need to properly stop the algal and bacterial biomass of the effluent, if high quality effluents in terms of BOD and total suspended solids are desired.

Studies carried out in the lagoons of San Juan[26][27]​ indicate that for loads of 450 to 1100 kg BOD/inhabitant, in primary lagoons, the suspended BOD is around 70% of the soluble BOD and that in secondary lagoons, with loads of 50-200 kg BOD/inhabitant, the suspended BOD is greater than the soluble BOD. The ratio of total BOD to soluble BOD is, however, very variable and there are studies where this ratio ranges between 1.2 and 8.3.

If a stabilization lagoon manages to produce settleable biomass that is easy to retain, the effluent will be of very good quality. In general, it can be assumed that a single small lagoon releases large amounts of biomass and that several lagoons in series allow greater sedimentation of biomass and, therefore, an effluent with greater removal of suspended solids. To determine the optimal conditions to obtain this purpose, even more research and study of the processes is required.

In the European Union the requirements for effluents from stabilization lagoon systems are:[28]​.

Additionally, for effluents that discharge into waters sensitive to eutrophication, the following is required:

Retention time

Retention time is the most important parameter in determining the removal of fecal coliforms in a lagoon and therefore in the elimination of pathogenic organisms from wastewater. For example, in the studies carried out in the San Juan lagoons, it has been determined that for a complete removal of protozoa, two stabilization lagoons in series are required with a total retention period of 36.7 days. Helminths were eliminated in two lagoons in series with a retention time of 5.5 days, and Salmonella could not be completely eliminated in two lagoons in series with a total retention time of 36.7 days. The dependence of bacterial mortality on retention time has been demonstrated in numerous studies.[8][27]​.