What occurs when a water oxygen level has been depleted?

10.2.3.1 Dissolved oxygen (DO)

DO is the amount of oxygen in aquatic environments that is accessible to fish, invertebrates, and all organism in the water. Most aquatic plants and animals require oxygen to survive; fish, for instance, cannot survive for long in water with dissolved oxygen less than 5 mg/L. The low level of dissolved oxygen in water is a sign of contamination and is an important factor in determining water quality, pollution control and treatment process. The DO in a saturated solution varies with the water temperature and elevation. For example, cold water has a higher DO than warm water. At sea level and at 20° C the DO value is 9.1 mg/L in freshwater. At a constant temperature, the higher the elevation, the lower the DO. The introduction of organic waste, especially domestic and animal sewage, industrial waste from the activities of paper mills, leather manufacturing, slaughterhouse sewage and crop wastewater, dramatically reduce the DO in water. The wastes in these industries cause oxygen demand, and they are broken down and decomposed by bacteria into oxygen. Most oxygen-demanding waste is organic waste. The oxidation of 3 of mg/L of carbon requires 9 ppm of soluble Oxygen. Dissolved oxygen is measured with an oxygen measuring device (DO-meter).

DO measurement

DO is measured by the azide modification of the Winkler method. The DO level in natural and wastewater depends on the physical, chemical, and biochemical activities in the water bodies. Oxygen is considered as poorly soluble in water. Its solubility is related to pressure and temperature. In freshwater, DO reaches 14.6 mg/L at 0 °C and approximately 9.1, 8.3, and 7.0 mg/L at 20, 25, and 35 °C, respectively, and 1 atm pressure. At temperatures of 20 and 30 °C, the level of saturated DO is 9.0-7.0 mg/L. Low oxygen in water can kill fish and other organisms present in water. For living organism, about 4 mg/L of minimum DO should be in water. The oxygen-depleting substances reduce the available DO. During summer months, the rate of biological oxidation is highly increased. Unfortunately, the DO concentration is at its minimum due to higher temperature.

The azide modification of the Winkler method.

10.3.3 Dissolved Oxygen

Adequate dissolved oxygen concentrations are critical during all phases of striped bass and hybrid culture. Low dissolved oxygen concentrations can result in slower growth and induce the stress response predisposing the animals to infectious disease. Monitoring of dissolved oxygen concentrations is complicated by the rate at which they can change. In heavily stocked raceways, tanks, or flow-through systems, for example, an interruption of oxygenation may result in critically low dissolved oxygen concentrations within minutes due to consumption by the culture animals. Management of dissolved oxygen concentrations in ponds must also consider the daily rhythms of concentrations characteristic of ponds (Boyd, 1979).

Striped bass and its hybrids have different dissolved oxygen requirements at different stages in their lives. Table 10.2 summarizes much of the available information on dissolved oxygen requirements. Striped bass also appear to require higher concentrations of dissolved oxygen relative to other temperate species. Generally, dissolved oxygen concentrations should be maintained as close to saturation as possible for best survival and growth.

Table 10.2. Dissolved oxygen (DO) requirements of striped bass.

23.3.4 Dissolved oxygen

DO determines the suitability of the stream for the survival of the fishes (Chang, 2005). The DO content <3 mg/L is fatal for fishes (Novonty, 2002). In cold water, DO level 4 mg/L is the limit to support the less tolerant species common to salmonid habitats and support the varying fish population (USEPA, 1986). In Bagmati River, DO ranged from 2.1 mg/L to 8.4 mg/L and such wide variation can be attributed to the higher organic load from densely populated urban area and direct sewage/industrial discharge without prior treatment (Bhatt et al., 2013). River water having DO <2 mg/L and BOD >15 mg/L are considered severely polluted making survival of aquatic organism difficult (Lai et al., 2013). In Bagmati River, the DO was found to be dropped sharply as the river moves toward urban area (Jorpati) (Table 23.2). This decrease in DO occurs in response to the increased organic load in the urban section of the river. The DO was found to increase where water turbulence was high, even though those increments in the DO level were not the sufficient support in most of the aquatic life (Khadka et al., 2015). The heavy input of wastewater and organic matter resulted into anoxic environment due to consumption of DO along the lower stretch of the river (Paudyal et al., 2016). As compared to 2007, the DO in 2018 was reported to be substantially decreased (Table 23.2). This decrease in DO could be the reason for the disappearance of fish species in the river. Similarly, Bhatt and McDowell (2007) reported the absence of benthic organisms in the lower urban section, though varieties of algae, fishes, and insects are reported in the upper section of the river. In the river, DO concentrations <4 mg/L were observed for 64.7% of the time (Kannel et al., 2007b).

11.3.2.1 Dissolved oxygen

DO varied throughout the year with mean daily DO generally much lower in the summer months (December to February) compared with the winter months, across all sites (Fig. 11.6a). Interestingly, the URBAN site had much higher mean daily DO levels across most of the year compared with either the WSUD or FOREST sites, possibly reflecting the higher baseflow at this site. The daily range in DO is often used as an indicator of stream health (Bunn et al., 2010), with lower daily ranges seen to be more typical of healthy streams. Across our sites, both the WSUD and URBAN sites had larger daily ranges across all seasons compared with the FOREST site (Fig. 11.6b). These DO results are consistent with how we understand water quality in urban creeks. Generally DO is much lower across the seasons because of increased respiration rates associated with higher organic loads from impervious surfaces in urban catchments (Blunder Creek) unless water flow is maintained (Stable Swamp Creek). Intuitively, upstream WSUD should reduce nutrient and organic inputs to urban streams and therefore reduce the likelihood of large DO fluctuations. However, our WSUD site is downstream of Forest Lake, a large stormwater retention basin, and while this system appears to positively influence some aspects of hydrology (as outlined previously), it is eutrophic and likely increases nutrient levels in the downstream tributaries. Interestingly, daily mean DO levels in the FOREST site were often extremely low, particularly in the summer (Fig. 11.6a) possibly reflecting the position of the sonde in a pool within the creek, the high organic load from riparian leaf litter, and the often low-flow conditions at the FOREST site between storm events. This suggests that at times during the year, conditions in the forested catchment will be just as severe as in the urbanized catchments. However, in the urbanized catchments, there are likely to be other drivers of poor health, including increased loads of labile organic carbon from road runoff, high heavy metals, and other pollutants that will exacerbate the ecosystem health impacts of low levels of DO.

C. Oxygen

Dissolved oxygen is generally measured using either of two methods; the Winkler Method or the Membrane-Electrode Method (APHA et al. 1998). Each method has specific advantages and disadvantages. The advantages of the Winkler Method are (1) when performed by experienced persons it can very accurately measure DO with great precision, and (2) it is relatively inexpensive to acquire the necessary titration burettes, sample bottles, and chemicals. The primary disadvantages of the Winker Method are (1) one cannot continuously monitor change in DO, but rather must rely on discrete measures, and (2) reducing or oxidizing materials dissolved in the water can interfere with accurate measurement of DO concentration. The advantages of the Membrane-Electrode Method are (1) ease of use, and (2) one can continuously monitor change in DO, especially in running waters that move water across the probe membrane. The primary disadvantage of the Membrane-Electrode Method relates to difficulties associated with instrument maintenance and calibration. The cost of oxygen measuring instruments has gone down significantly in the past decade. Where once a recording device and probe cost over $1500, a YSI Model 550A Dissolved Oxygen Meter (Range: 0 to 20 mg/L (0–200%); (±2% air sat.)or ±0.3 mg/L(±2% air sat.), is now available for under $500.

7.5 Dissolved Oxygen

Dissolved oxygen levels in water depend, in part, on the chemical, physical, and biochemical activities occurring in the water. Oxygen has a limited solubility in water directly related to atmospheric pressure and inversely related to water temperature and salinity. Low-dissolved oxygen levels can limit the bacterial metabolism of certain organic compounds.

On-site, dissolved oxygen is commonly measured using a membrane electrode of the polarographic type in a flow-through cell. The zero is commonly set using a saturated solution of sodium sulfite and the 100% saturated environment by holding the probe close to the surface of clean water. Below 1 ppm, electrodes provide only a qualitative measure of DO2 due to slow electrode response (Wilkin, McNeil, Adair, & Wilson, 2001).

2. Dissolved Oxygen

Dissolved oxygen in the aquatic habitat is an important requirement for survival. The mean requisite for dissolved oxygen by ostracodes falls within a very narrow margin of 7.3–9.5 mg/L[70]. In general, the concentration of dissolved oxygen in the water is broad. Candona subtriangulata has the highest minimum oxygen requirement of 5.6 mg/L. A high number of species (34 out of 43, in Canadian habitats; L. D. Delorme, unpublished data) can tolerate low dissolved oxygen concentrations (Fig. 19.10). Those species which require a minimum 3 mg/L dissolved oxygen in the water are Cytherissa lacustris, Cytheromorpha fuscata, Ilyocypris gibba, Limnocythere ceriotuberosa, L. herricki, L. itasca, L. verrucosa, and Potamocypris variegata. Delorme[66] found that Fabaeformiscandona caudata could survive in Lake Erie even though the hypolimnion has summer dissolved oxygen content below its lower tolerance limit of 2.8 mg/L. Its survival was accomplished by having a short life cycle allowing it to produce eggs before the onset of anoxia. This survival mechanism has been in place for some time in Lake Erie as indicated by the presence of fossil shells in the lake sediments[67]. Contrary to F. caudata, C. subtriangulata and Cytherissa lacustris have become locally extinct in the central basin of Lake Erie because they have a 1-year life cycle and cannot reach sexual maturity as they require a minimum dissolved oxygen content of 5.6 and 3.0 mg/L respectively. Cytherissa lacustris is making a comeback with a reduction in anoxia. Newrkla[195] found that C. lacustris could survive at 1 mg O2/L in the laboratory on a glass substratum and minimal food (20°C and 20 hr exposure). Nine of the 43 species examined in a previous study[70] can tolerate near-zero dissolved oxygen (below the detection limit of the Winkler Azide Method, APHA 1965) for short periods. Candona decora, Cyclocypris ampla, C. sharpei, and Cypria ophtalmica have been recovered from water of zero dissolved oxygen from a small river in early December (L. D. Delorme, unpublished data). The river had been covered by about m of ice, suggesting that these waters had been anoxic for some time. Fox and Taylor[94,95] have found by experimentation that some ostracodes survive longer at oxygen levels below air saturation of 21%.

Dissolved Oxygen

Dissolved oxygen is one of the most critical parameters of concern with any kind of aquaculture operation, so the aquaculturist should be familiar with the dynamics of dissolved oxygen concentrations. Measuring dissolved oxygen is fairly straightforward in that there are commercially available oxygen meters or chemical test kits that provide the materials for performing a Winkler titration in the field. The ability to measure dissolved oxygen is essential in deep semi-intensive ponds or in situations of netpen or fishcage aquaculture with fish held at high stocking densities. In these relatively intensive systems, the high stocking densities require the ability to monitor dissolved oxygen and to have methods to aerate the water available. However, with proper care, artisanal methods of aquaculture can be carried out without the need of directly measuring oxygen concentrations in the water. There are a number of simple secondary criteria that can be used to assess the health of the pond and potential for oxygen depletion problems (hypoxia).

Although the atmosphere is 20 percent oxygen, it has a very low solubility in water, and its solubility decreases with increasing temperature and salinity (Table 12). For example, a tropical freshwater fishpond at sea level with a temperature of 35°C will be at 100 percent oxygen saturation, with a dissolved oxygen concentration of 6.949 mg/L. Altitude also affects oxygen solubility. The maximum saturation concentration of dissolved oxygen must be reduced by about 12 percent for each 1,000 m of altitude (Creswell 1993). Based on solubility characteristics alone, there is a greater probability of hypoxia at higher temperatures and salinities. This is compounded by the Q10 effect of temperature on organisms in the pond driving up oxygen demand.

TABLE 12. Oxygen solubility (mg/L) in waters of various temperatures and salinities at sea level

Although oxygen will diffuse into surface waters from the atmosphere at rates of 1 to 5 mg/L daily, the primary source of oxygen in most natural water bodies is photosynthesis from phytoplankton and aquatic plants, which ranges from 5 to 20 mg/L daily. Respiratory losses of oxygen include respiration by plankton (5 to 15 mg/L daily), respiration by fish (2 to 6 mg/L daily), respiration by benthic organisms (1 to 3 mg/L daily), and diffusion of oxygen into the air (1 to 5 mg/L daily). Since oxygen is being produced only during the daylight hours, and respiration is occurring on a 24-hour basis, there is a diurnal (day-night) fluctuation in dissolved oxygen concentration in ponds, with minima at sunrise (Figure 22). In a well-managed artisanal pond, most of the respiration is due to the phytoplankton and plants that represent most of the biomass, not the fish. During the daytime the plants are respiring, but there is generally a net production of oxygen in the pond during the day.

Fertilizing a pond generally increases the primary productivity of a pond by increasing the density of phytoplankton in the water. This practice increases the amount of food available for zooplankton and grazing fish, but it also increases the probability of hypoxia during the nighttime hours by increasing the respiring biomass. During the daytime the net production of dissolved oxygen proceeds at an accelerated rate, but the final concentration of dissolved oxygen attained is the maximum saturation level based on temperature and salinity (Figure 22). Further details about pond fertilization will be covered in subsequent sections.

Increasing the density of phytoplankton in the water also limits the depth of photosynthesis and thus oxygen production. In ponds with very dense phytoplankton blooms, photosynthesis may be occurring only in the top 10 cm of the epilimnion. In this situation, the fish are forced to the surface and there is risk of an algal die-off that would cause the pond to experience severe hypoxia or anoxia (no oxygen), resulting in a fish kill. Using a Secchi disk one could simply determine relative phytoplankton densities. A limnological Secchi disk is a simple flat, round device that is 25 cm in diameter and painted white and black on opposing quadrants. The Secchi disk is lowered into the pond and the depth at which the disk disappears from sight is noted. Clearer waters result in larger Secchi depths than do turbid waters. It is recommended that for shallow artisanal ponds, the optimum Secchi depths range from 40 to 50 cm. As a general rule, most ponds should contain enough dissolved oxygen to support hardy species of fish at a depth two to three times the Secchi depth. So an artisanal pond 1 meter in depth should have sufficient oxygen levels to support fish all the way to the bottom. If the Secchi depths are less than 40 cm, cease fertilization and partially drain the pond and replace with fresh water.

Fish selected for culture in artisanal ponds should be tolerant to low dissolved oxygen. The most frequently cultured fish in artisanal ponds, carps and tilapias, prefer dissolved oxygen levels to be in excess of 4 mg/L. These fish will tolerate transient overnight hypoxia to 1 mg/L, but continuous exposures to dissolved oxygen concentations less than 4 mg/L is stressful and can increase the likelihood of disease.

25.1 The Nature of Wastewater (Sewage)

The cloaca maxima, the “biggest sewer” in Rome, had at one time enough capacity to serve a city of one million people. This sewer, and others like it, simply collected wastes and discharged them into the nearest lake, river or ocean. This expedient made cities more habitable, but its success depended on transferring the pollution problem from one place to another. Although this worked reasonably well for the Romans, it does not work well today. Current population densities are too high to permit a simple dependence on transference. Thus, modern-day sewage is treated before it is discharged into the environment. In the latter part of the nineteenth century, the design of sewage systems allowed collection with treatment to lessen the impact on natural waters. Today, more than 15,000 wastewater treatment plants treat approximately 150 billion liters of wastewater per day in the United States alone. In addition, septic tanks, which were also introduced at the end of the nineteenth century, serve approximately 25% of the U.S. population, largely in rural areas.

Domestic wastewater is primarily a combination of human feces, urine and “graywater.” Graywater results from washing, bathing and meal preparation. Water from various industries and businesses may also enter the system. People excrete 100–500 grams wet weight of feces and 1–1.3 liters of urine per person per day (Bitton, 2011). Major organic and inorganic constituents of untreated domestic sewage are shown in Table 25.1.

Table 25.1. Typical Composition of Untreated Domestic Wastewater

From Pepper et al. (2006b).

The amount of organic matter in domestic wastes determines the degree of biological treatment required. Three tests are used to assess the amount of organic matter: biochemical oxygen demand (BOD); chemical oxygen demand (COD); and total organic carbon (TOC).

The major objective of domestic waste treatment is the reduction of BOD, which may be either in the form of solids (suspended matter) or soluble. BOD is the amount of dissolved oxygen consumed by microorganisms during the biochemical oxidation of organic (carbonaceous BOD) and inorganic (ammonia) matter. The methodology for measuring BOD has changed little since it was developed in the 1930s.

The 5-day BOD test (written BOD5) is a measure of the amount of oxygen consumed by a mixed population of heterotrophic bacteria in the dark at 20°C over a period of 5 days. In this test, aliquots of wastewater are placed in a 300-ml BOD bottle (Figure 25.1) and diluted in phosphate buffer (pH 7.2) containing other inorganic elements (N, Ca, Mg, Fe) and saturated with oxygen. Sometimes acclimated microorganisms or dehydrated cultures of microorganisms, sold in capsule form, are added to municipal and industrial wastewaters, which may not have a sufficient microflora to enable the BOD test to be carried out. In some cases, a nitrification inhibitor is added to the sample to determine only the carbonaceous BOD.

Dissolved oxygen concentration is determined at time 0, and, after a 5-day incubation, by means of an oxygen electrode, chemical procedures (e.g., Winkler test) or a manometric BOD apparatus. The BOD test is carried out on a series of dilutions of the sample, the dilution depending on the source of the sample. When dilution water is not seeded, the BOD value is expressed in milligrams per liter, according to the following equation (APHA, 1998).

(Eq. 25.1)BOD(mg/L)=D1−D5P

where:

D1=initial dissolved oxygen (DO), D5=DO at day 5, and

P=decimal volumetric fraction of wastewater utilized.

If the dilution water is seeded:

(Eq. 25.2)BOD(mg /L)=(D1−D5)−(B1−B5)fP

where:

D1=initial DO of the sample dilution (mg/L)

D5=final DO of the sample dilution (mg/L)

P=decimal volumetric fraction of sample used

B1=initial DO of seed control (mg/L)

B5=final DO of seed control (mg/L), and

f=ratio of seed in sample to seed in control= (% seed in D1)/(% seed in B1).

Because of depletion of the carbon source, the carbonaceous BOD reaches a plateau called the ultimate carbonaceous BOD (Figure 25.2). The BOD5 test is commonly used for several reasons:

To determine the amount of oxygen that will be required for biological treatment of the organic matter present in a wastewater

To determine the size of the waste treatment facility needed

To assess the efficiency of treatment processes

To determine compliance with wastewater discharge permits

The typical BOD5 of raw sewage ranges from 110 to 440 mg/L (see Example Calculation 25.1). Conventional sewage treatment will reduce this by 95%.

Example Calculation 25.1

Calculation of BOD

Determine the 5-day BOD (BOD5) for a wastewater sample when a 15-ml sample of the wastewater is added to a BOD bottle containing 300 ml of dilution water, and the dissolved oxygen is 8 mg/L. Five days later the dissolved oxygen concentration is 2 mg/L.

Using Eq. 25.1:

BOD(mg/ L)=D1−D5PD1=8mg/LD5=2mg/LP =15ml300ml=5%=0.05BOD5=8−20.05 =120mg/L

Chemical oxygen demand (COD) is the amount of oxygen necessary to oxidize all of the organic carbon completely to CO2 and H2O. COD is measured by oxidation with potassium dichromate (K2Cr2O7) in the presence of sulfuric acid and silver, and is expressed in milligrams per liter. In general, 1 g of carbohydrate or 1 g of protein is approximately equivalent to 1 g of COD. Normally, the ratio BOD/COD is approximately 0.5. When this ratio falls below 0.3, it means that the sample contains large amounts of organic compounds that are not easily biodegraded.

Another method of measuring organic matter in water is the TOC or total organic carbon test. TOC is determined by oxidation of the organic matter with heat and oxygen, followed by measurement of the CO2 liberated with an infrared analyzer. Both TOC and COD represent the concentration of both biodegradable and nonbiodegradable organics in water.

Pathogenic microorganisms are almost always present in domestic wastewater (Table 25.2). This is because large numbers of pathogenic microorganisms may be excreted by infected individuals. Both symptomatic and asymptomatic individuals may excrete pathogens. For example, the concentration of rotavirus may be as high as 1010 virions per gram of stool, or 1012 in 100 g of stool (Table 25.3). Infected individuals may excrete enteric pathogens for several days or as long as a few months. The concentration of enteric pathogens in raw wastewater varies depending on the following:

Table 25.2. Types and Numbers of Microorganisms Typically Found in Untreated Domestic Wastewater

From Pepper et al. (2006b).

Table 25.3. Incidence and Concentration of Enteric Viruses and Protozoa in Feces in the United States

The incidence of the infection in the community

The socioeconomic status of the population

The time of year

The per-capita water consumption

The peak incidence of many enteric infections is seasonal in temperate climates. Thus, the highest incidence of enterovirus infection is during the late summer and early fall. Rotavirus infections tend to peak in the early winter, and Cryptosporidium infections peak in the early spring and fall. The reason for the seasonality of enteric infections is not completely understood, but several factors may play a role. It may be associated with the survival of different agents in the environment during the different seasons. Giardia, for example, can survive winter temperatures very well. Alternatively, excretion differences among animal reservoirs may be involved, as is the case with Cryptosporidium. Finally, it may well be that greater exposure to contaminated water, as in swimming, is the explanation for increased incidence in the summer months.

Concentrations of enteric pathogens are much greater in sewage in the developing world than in the industrialized world. For example, the average concentration of enteric viruses in sewage in the United States has been estimated to be 103 per liter (Table 25.4), whereas concentrations as high as 105 per liter have been observed in Africa and Asia.

Table 25.4. Estimated Levels of Enteric Organisms in Sewage and Polluted Surface Water in the United States

From U.S. EPA (1998).