Water Pollution

STOP - You must read Chapter 4 before doing this lesson

Introduction

Water Pollution is any physical, biological, or chemical change in water quality that adversely affects living organism or makes water unsuitable for desired uses. Standard College Dictionary defines a pollutant as "that which pollutes". To pollute is "to make unclean or impure". How do such definitions apply to our waters? Scientists, representatives of industry, and government agencies have been struggling with this questions for the last 50 years.

""Of all our
natural resources
WATER
has become
the most precious"
Rachel Carson, Silent Spring

Whatever it is - used oil down a storm drain, excess fertilizers washed off a golf course, or goose poop washed into the river by rain - if it upsets the way that bacteria eat, insects reproduce, or fish swim upstream, then it is polluting the river or stream. So water pollution is anything that makes people, plants, or animals sick or affect their good health in some other way.

Most people think that "water pollution" is something that comes only from big factories along rivers. We have actually learned that water pollution comes from many places we do not even think of as a possible source. "Nonpoint source" pollution causes the majority of the problems we have in our waters today. Nonpoint source is define as a source of water pollution that it is not easy to be point out. The classic example is water run-off from farms, fields, streets, etc. On the other hand "point source" pollution comes from places that are easy to pinpoint such as a pipe from an industry discharging in a river, lake or stream.

Types of Water Pollution

Infectious Agents - this included pathogenic bacteria, viruses, parasites and other pathogens. The major source of these agents is animals or human feces. Some of the diseases can be thyphoid fever, cholera, dysentery, enteritis, hepatitis, schistosomiasis, etc.

Oxygen-Demanding Wastes - Biochemical Oxygen Demand (BOD), Chemical Oxygen Demand (COD), Dissolved Oxygen (DO), Oxygen sag.

DO- oxygen in water that is available to support aquatic life.
BOD- is an indication of how much putrescible organic material is present in the water. A low BOD indicates a good water quality and a high BOD reflects polluted conditions.
COD- a measure of the amount of oxygen in water chemically rather than biologically, consumed under specific conditions. It is usually higher than the BOD of water.
Oxygen sag- down stream from a point source, such as a municipal sewage plant discharge, a charasteristic decline and restoration of water quality can be detected by measuring dissolved oxygen content. Upstream from a pollution source, oxygen levels support normal populations of clean-water organisms. Futher downstream of the pollution source the water may become so oxygen-depleted that only the most resistant microorganisms and invertebrates can survive.

Plant Nutrients - Eutrophication, which is an increase in the nutrients levels in the water and an increase in biological productivity. Usually due to enrichment with phosphates and nitrates. It is more common on lakes and ponds.

Toxic Inorganic Materials - these include heavy metals such as mercury, cadmium, and lead.

Organic Materials - such as DDT, PCB (polychlorinate byphenils), dioxin, and other pesticides.

What influences the quality of surface waters?

It depends on land use, location and sources of natural and manmade pollution, and the natural process of purification, such as sedimentation, sunlight, aeration, nitrification, and dilution. Normal drinking water treatment has little effect on some chemicals contaminants. So, heavily polluted water sources should be avoided as drinking water supplies.

Drinking Water Treatment

Drinking water treatment consist of four major steps: sedimentation, coagulation-flocculation, filtration, and disinfection.

Sedimentation - incoming raw water is detained in a quite pond or tank for at least 24 hours to allow heavy suspended material to settle out by gravity. The water is then move to another tank where coagulation-floculation takes place.

Coagulation-flocculation - adding a coagulant such as alum (aluminum sulfate) to the water permits particles to come together and results in the formation of a flocculent mass (floc) or heavy precipitate, which agglomerates microorganism, suspended particles and colloidal matter. These floc are sttle out by gravity.
These first two steps remove from 90 to 98 percent of the bacteria, viruses and protozoa. Also, around 80 percent of the color and turbidity are removed.

Filtration - the water from the coagulation-flocculation tank is then pass through beds of sands or chrushed anthracite coal to reduce even more the concentration of remaining suspended solids, including many cells and protozoa.
A well-operated plant will remove 98 to 99.5 percent of the coliform bacteria, protozoa, and viruses, as well as some organic and inorganic chemicals in the raw water. Even though the water may look clean and without suspended particles some pathogenic bacteria or viruses may still present in the water. The next step is design to decrease the chance that we get sick with our drinking water supplies.

Disinfection - is the most important method utilized for killing pathogens in water. It is most commonly done with chlorine, but ozone and ultraviolet (UV) light can also be use for disinfection. Disinfection does not render the water sterile however and some pathogens are resistant to the normal levels of chlorine used.
Disinfection in the absence of the preceding steps is not highly effective because organic materials and suspended solids in the water interfere with the germicidal action of the chlorine.

Aeration - In many water treatment plants, preliminary treatment also includes aerating the water to remove iron and dissolved gases with the purpose to improve taste and odor of the water.

Fluorination - today many drinking water treatment plants add fluoride to the finished water to reduced the incidence of tooth decay.

This type of drinking water treatment does not remove such pollutants as radioactive elements, many organic chemicals and some heavy metals. A common problem around the US today is that the high pollution of many water supplies overpower the ability of the current purification plants to entirely remove pollutants.

Fecal coliform as indicators of water contamination

To ensure that the water treatment process is working efficiently laboratory tests of finished water samples are carried out on regular basis. Historically, the presence of appreciable number of coliform bacteria in a water sample has been used as indicator that the water is unsafe to drink. Fecal coliform serve as indicators that the water is contaminated with fecal material and hence potentially hazardous.

In other words, the presence of fecal coliform in he water samples indicate that less abundant but more harmful orgaism such as dysentery bacillus or hepatitis virus might also be present. Fecal coliforms are indicative of fecal contamination or sewage pollution.

Coliform bacteria are not normally considered pathogenic. But, some strains of Escherichia coli can cause "travelers diarrhea" and gatroenteritis.

Trihalomethanes (THM's)

These are organic dhemical that are form at the drinking water treatment plant itself when chlorine, added to the water for disinfection, reacts with natural organic compounds or matter in the water. Such substances (THM's) includes chloroform, bromoform, dichloro-bromomethane. Any drinking water supply which has been chlorinated is likely to have certain amount of THM's. The presence of chloroform has been detected in almost every water system tested for this chemical. Chloroform, at least in high doses, is known to cause liver, kidney disorders, central nevous system problems, birth defects, and cancer.

Chloramines, chlorine dioxide and ozone are all non-THM's-producing disinfectants which could be used in cities having problems with THM's, but they are more expensive and less efficient than chlorination.

EPA: Office of water web site.

EPA: Drinking Water web site.