Stream Pollution

Question 1

Waste 2 groups

Answer 1

Biodegradable wastes: wastes (mainly organic) readily broken down or
decomposed into simpler substances by biological action
of microbes. They are used by microbes as food
i.e. proteins.

Non-biodegradable wastes: wastes that cannot be decomposed by biological
action of microbes i.e. PCBs.

Question 2

Self-purification

Answer 2

Self-purification: is the ability of rivers to recover from the effects of pollution
naturally.

Self-purification depends on:
● The strength and volume of pollutants
● The stream discharge or flow rate
● The rearation

Non-biodegradable pollutants cannot be assimilated by natural streams
i.e. PCB, pesticides
They accumulate in the sediments.

Question 3

Reaeration

Answer 3

Reaeration: oxygen transfer between the air and the water.
Fast-flowing, shallow, turbulent streams are rearated more
effectively than slow, deep, meandering streams.

Question 4

Waste assimilation

Answer 4

The waste assimilation in a stream take place in two steps:

                    1.  The physical processes of mixing, dilution, and reaeration 
                     occurs 

                    2.  The biological processes occur (the microorganisms in the 
                    water metabolize the organic pollutants)   

When a point source of pollutant is discharged into the river, the two basic
physical
processes take place:
● Mixing
● Dilution
Mixing is not complete near the point of discharge.

Question 5

Waste plume

Answer 5

Waste plume: is the mixing zone near the point of discharge where mixing is
not complete.
The length of mixing zone depends on:
● Channel geometry
● Flow velocity
● Design of the discharge pipe

Question 6

Point-source, plug flow model

Answer 6

As the water and wastes flow downriver, it is assumed that they are uniformly mixed at any given cross section of the river, and it is assumed that there is no dispersion of wastes in the direction of flow. This referred as point-source, plug flow model.

Question 7

Oxygen transfer

Answer 7

The rate of oxygen transfer from air to water during the reaeration process
depends on:
● temperature
● oxygen deficit

Question 8

Oxygen deficit

Answer 8

Oxygen Deficit: is the difference between the actual DO concentration and the
saturation DO value.

                      Large deficit means faster the rate of oxygen transfer  
                          (reaeration curve) 
                        
                      The slope of the reaeration curve increases as the 
                      deoxygenation curve falls

The deoxygenation caused by microbial decomposition of wastes and oxygenation by reaeration are competing processes that are simultaneously removing and adding oxygen to a stream.

Question 9

Dissolved oxygen SAG curve

Answer 9

Dissolved Oxygen Sag Curve = Dissolved Oxygen Profile: is the graph of the
actual DO in the stream versus time.
It gives a profile view of the DO concentrations
along the length of the stream.

Initially, the rate of Deoxygenation exceeds the Rate of Rearation.
So that the oxygen profile begins to sag.
When most of the organics are decomposed, the rate of reaeration dominates
and the oxygen profile begins to rise towards its original level.

Question 10

Minimum DO occurs at

Answer 10

The minimum DO in a stream occurs when:

                                              Rate of Rearation = Rate of Deoxygenation

Question 11

Critical Point

Answer 11

At the critical point downstream, dissolved oxygen reaches its minimum value and river conditions ate at their worst.

Beyond the critical point, the remaining organic matter in the river has diminished to the point where oxygen is being added to the river by reaeration faster than it is being withdrawn by decomposition, and the river begins to recover.

Question 12

In very polluted water

Answer 12

When there is very heavy organic pollution in the water or very low streamflow,
the oxygen in the water may be completed depleted (DO = 0).

The sag curve intersects the horizontal axis at DO = 0, resulting in anaerobic or
septic conditions.

As the river gets more polluted, the oxygen sag curve drops below an
acceptable level.

In the extreme case, anaerobic conditions can occur.

Question 13

Zones of pollution

Answer 13

1. Zone of degradation: it forms below the point of waste discharge. This zone is characterized by floating solids, turbidity, the DO drops rapidly.
2. Zone of active decomposition: The DO level drops to about 40% of its saturation value. The water is heavily polluted. Higher forms of aquatic life die. Sludge deposits. Anaerobic conditions maybe occur and odors may be noticeable.
3. Zone of recovery: The rate of reaeration will exceed the rate of deoxygenation. The DO increases. The water is gradually clearing. No odors. Aquatic species. reappear.
4. Zone of clean water: this zone is characterized by clear water, high DO.
   The stream has recovered its original quality through a process of natural
   self-purification.