Organic Pollution of Streams

The first successes of modern environmental engineers and scientists were in protecting human and environmental health from the effects of discharging human wastes (sewage) to streams. The concerns were transmission of waterborne disease and gross degradation of the receiving water body due to excessive organic loading. Sewage discharge affects streams by adding toxins, pathogens, suspended solids, nutrients, and readily biodegradable organic carbon. These produce a cascade of effects, mostly related to reduced oxygen concentration in the water column. The oxygen depletion, in turn, is due to augmented heterotrophic oxygen consumption in the water column and in the sediments and to chemoautotrophic oxidation of ammonia. Recall that the nitrogenous oxygen demand of typical domestic wastewaters can be very high, since nitrification of each milligram of NH3-N to NO—-N consumes 4.57 mg of oxygen.

DO Sag Equation If the organic pollution comes from only a single point source, the stream gradually recovers with distance downstream in a fairly predictable way. The master variable in this situation is oxygen. The changes in oxygen can be described by a model involving the following variables with their typical units (note that g/m3 is equivalent to mg/L):

C dissolved oxygen concentration (also called DO; g/m3).

Cs saturated DO, or oxygen concentration the water would have if it were in equilibrium with the atmosphere. Values decrease with temperature, salinity, and altitude. Example values in pure water at sea level range from 14.6 g/m3 at 0oC to 9.2 g/m3 at 20oC. L biochemical oxygen demand (BOD; g/m3).

D DO deficit, the amount the DO is below saturation; equal to Cs — C.

The major processes affecting oxygen levels in streams are:

U oxygen uptake rate (g/m3 per day); a combination of heterotrophic

(organic carbon) and autotrophic (ammonia) biochemical oxidation. B benthic oxygen demand (g/m2 ■ day), due to heterotrophic and auto-

trophic oxidation in the sediment. Also called sediment oxygen demand (SOD).

R reaeration rate (g/m2 pre day); the rate at which oxygen enters the water from the atmosphere by physicochemical mass transfer. P specific photosynthetic oxygen production rate (g/m3 ■ day), by green plants and algae.

At the point that a parcel of water mixes with the discharge, the mixture has a DO of C0 and a BOD of L0. The DO at this point corresponds to an initial deficit D0 = Cs — C0. As the parcel continues downstream, its DO changes in response to a balance between the sources of oxygen, R and P, and the depletion by U and B:

dt h h where h = V/A is the average depth of the water column, or the volume of a section of a stream, V, divided by its plan area, A. Note from the units that reaeration and benthic consumption are normalized to the area, since they represent transport of oxygen across the top and bottom surfaces of the water column, respectively.

At the same time that oxygen is disappearing according to equation (15.2), the BOD in the water column is being depleted as oxygen is consumed by the uptake process:

Much can be learned from a simplified model that neglects photosynthesis and benthic oxygen demand. Recall that rivers tend to be allochthonous to begin with, and this will be more so for polluted streams. Benthic demand may be quantitatively important, but it behaves similarly to the water column BOD and does not greatly change the qualitative behavior of the model. Thus, only U and R remain in the model. Oxygen uptake can be described as a first-order decay:

The parameter kj is the rate coefficient for decay of BOD. This is sensitive to the temperature and biodegradability of the BOD. The k1 values must be determined experimentally either in the laboratory or, preferably, in field experiments. To measure kj in the field, BOD bottles containing samples of river water are submerged directly in the stream being modeled for 2 to 6 hours. Initial and final DO measurements yield a rate of oxygen consumption, U. Dividing by the BOD yields a value for k1. Biodegradation rate coefficients have been measured for numerous industrial chemicals in laboratory experiments (see Appendix B).

Usually, parallel experiments will be done with clear glass bottles and with black-painted ones. The difference between the light and dark bottles gives an estimate of phy-toplankton photosynthesis, and the dark bottle measures respiration. This is done either to validate the assumption of neglecting photosynthesis or to quantify it for including in the model.

Combining equations (15.3) and (15.4) and solving the resulting differential equation produces the typical exponential or first-order decay of BOD in the stream (Figure 15.21 a):

Figure 15.21 Changes downstream from a point of organic loading such as a sewage discharge. (Based on Connell and Miller, 1984.)

Note that the distance downstream, x, in Figure 15.21 corresponds to the distance traveled in time, t, at a stream velocity, v, by the transformation x = vt.

Reaeration is also first order, with respect to the DO deficit. That is, the rate at which oxygen enters the water is assumed to be proportional to how far the water is below saturation:

The parameter k2 is the coefficient of reaeration. It is controlled by turbulent mixing in the water. Correlations are available to estimate the value of k2 from hydraulic parameters of the stream such as its depth, velocity, and hydraulic slope.

Combining (15.4) and (15.6) with (15.2) and (15.4) yields a differential equation which can be solved for initial conditions L0 and D0, resulting in the Streeter-Phelps or DO sag equation (D = Cs — C is the DO deficit):

Figure 15.21a shows the shape of this curve. Qualitatively, the DO initially decreases rapidly when U > R/h. However, U is decreasing as L decreases (less food available for respiration), and R increases as C decreases farther below saturation. Ultimately, if the initial pollution is not too great, these two rates will approach each other, and the curve bottoms out. The DO deficit at this point is called the critical deficit, Dc, and occurs at a distance downstream called the critical time of travel, tc:

The critical deficit can then be found by substituting the result of equation (15.8) into

As the BOD continues to decrease farther downstream, the stream starts to recover. If the critical DO concentration, Cc = Cs — Dc is low enough, it can form a barrier to the movement of fish or other oxygen-sensitive organisms. The critical concentration is often used for regulatory purposes to indicate the seriousness of the pollution. In the regulatory process of waste load allocation, models of the river are manipulated to determine how much pollution must be reduced at the source to meet some minimum DO standard at the critical point.

Other Chemical Changes and Effects Other changes occur as one travels downstream of the discharge. Biological effects are described below. Chemical changes include increases in nutrient concentrations. As shown in Figure 15.21b, ammonia increases due to mineralization of organic nitrogen and raw ammonia in the wastewater. The ammonia is gradually depleted as nitrification converts it ultimately into nitrate. The nitrate may be removed by phytoplankton uptake, and then either stays in the water column as organic nitrogen or is settled to the benthic zone. If the benthic zone is anoxic, nitrate may be removed from the water column to use to oxidize buried organics by denitrification.

Sedimentation of suspended solids near a discharge can smother benthic organisms. Sedimentation of suspended solids from the wastewater, plus biomass produced by growth on dissolved and colloidal material, can biodegrade in the sediment. The oxygen consumed in this way contributes to the benthic oxygen demand. Besides removing oxygen from the overlying water column, benthic oxygen demand reduces oxygen availability to the benthos. Benthic invertebrates are particularly sensitive.

Oxygen enters the sediment in a mass transport process similar to oxygen transport from the atmosphere to the water in reaeration. In this case the rate of transport is controlled by diffusion through the pores of the sediment and by the concentration in the water. If the oxygen demand is high enough or the rate of transport sufficiently low, the oxygen will only penetrate to a certain depth before it is consumed completely. Thus, there will be an oxic layer covering an anoxic layer, where denitrification replaces oxygen respiration. The depth of transition between the oxic and anoxic layers is controlled by a balance between the oxygen concentration in the water and the benthic oxygen demand.

Figure 15.21a shows the shape of this curve. Qualitatively, the DO initially decreases rapidly when U > R/h. However, U is decreasing as L decreases (less food available for respiration), and R increases as C decreases farther below saturation. Ultimately, if the initial pollution is not too great, these two rates will approach each other, and the curve bottoms out. The DO deficit at this point is called the critical deficit, Dc, and occurs at a distance downstream called the critical time of travel, tc:

At greater depths in the sediment, nitrate may be depleted and anaerobic conditions would prevail. Under these conditions respiration uses sulfate and carbonate as electron acceptors, producing H2S and methane. The H2S can diffuse back up through the sediment and enter the water column, where it can exert a toxic effect. The LC50 of H2S (the concentration that kills half the organisms in a laboratory test) to several freshwater invertebrates has been measured to range from 11 to 1070 mg/L. Some zoobenthos can live in anoxic sediments because they circulate water containing oxygen mechanically from the surface of the sediment.

Ultimately, the organic matter becomes oxidized and no longer affects the river. However, many of the nutrients remain. In fact, degradation of organic-rich sediments may release previously stored nutrients. Thus, the high organic-loading condition of the stream can give way to a eutrophic situation, with photosynthesis exceeding the baseline level upstream of the discharge.

The Streeter-Phelps equation is a steady-state solution. If it is necessary to take photosynthesis into account, P should be modeled with a diurnal function. This would result in DO concentrations that oscillate, reaching a minimum in the morning. It is even possible to achieve supersaturation (C > Cs) during sunny days if the amount of pollution is not great. The addition of organic pollution increases the ratio of heterotrophic to photosyn-thetic activity simply by providing food for the heterotrophs. However, photosynthesis may also be reduced directly by the effect of increased turbidity in the water, which can limit light availability.

Individual and Ecological Effects Fish and larger invertebrates breathe through gills and have a circulatory system to bring oxygen-enriched blood to the tissues. When oxygen becomes depleted, these organisms can increase the pumping of water over the gills in order to extract more. Organisms such as fish that can readily move in response to oxygen have been found to avoid areas where concentrations have dropped below 3 to 5 mg/L.

Small invertebrates such as zooplankton obtain their oxygen by diffusion through the skin, without specialized organs; some do not even have circulatory systems. Some organisms are adapted to low-oxygen conditions. Annelid worms have special high-capacity hemoglobin. Chironomid larvae have a high concentration of hemoglobin, giving them a red color. Table 15.12 shows some of the oxygen limitations observed experimentally with aquatic animals.

Systems with large organic carbon input are called saprobic, as opposed to eutrophic, which refers primarily to excess nitrogen and phosphorus. In eutrophic waters primary production exceeds respiration. In saprobic waters the reverse is true. As organic pollution

TABLE 15.12 Dissolved Oxygen Limits for Several Organisms


Temperature (0C)

Type of Test

DO (mg/L)

Brown trout (Salmo trutta)

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