Measurement of Water Pollution

Read this article to learn about the measurement of water pollution in sewage waste water with special reference to organic matter and pathogenic organisms.

Measurement of Organic Matter of Sewage:

The organic matter present in the sewage is regarded as biologically active, if it can be oxidized by the bacteria.

On the other hand, the organic matter resistant to bacterial degradation is considered as biologically inactive. For the purpose of sewage treatment, the biologically active organic matter is important.

The commonly used laboratory methods for the measurement of organic matter in sewage are given below:

i. Biochemical oxygen demand (BOD)

ii. Chemical oxygen demand (COD)

iii. Total organic carbon (TOC)

iv. Theoretical oxygen demand (ThOD).

Biochemical oxygen demand (BOD):

Biochemical oxygen demand is the most widely used parameter to measure the organic pollution in sewage as well as surface water. BOD basically involves the measurement of dissolved oxygen (DO) utilized by the microorganisms for the biochemical oxidation of organic matter. The demand for oxygen and the process of oxidation depends on the type and quantity of organic matter, temperature and type of the organism used.

In general, biochemical oxygen demand is measured for an incubation period of five days (hence appropriately referred to as BOD₅) at a temperature of 20°C. If the organic content of the sewage is high, it needs to be diluted for the measurement of BOD. Further, for waste water with less population of microorganisms, seeding with bacterial culture is necessary.

BOD indicates the amount of organic matter present in the sewage. Thus, the more is organic content, the higher is the BOD (Refer Table 56.2 for BOD values of different types of sewages). If the available oxygen (dissolved O₂) is less than the BOD, the organic matter decomposes anaerobically, putrefies and produces foul smell. Thus, BOD is a measure of nuisance potential of sewage.

Limitations of BOD:

1. BOD measures only biodegradable organic matter.

2. A high concentration of bacterial load is required.

3. For measuring BOD of toxic waste water, pretreatment is necessary.

4. Requires long period of incubation i.e. 5 days.

Despite these drawbacks, BOD is very widely used world over for practical and economic reasons.

Chemical oxygen demand (COD):

Chemical oxygen demand refers to the oxygen equivalents of organic matter that can be oxidized by using strong chemical oxidizing agents. Usually, potassium dichromate in the presence of a catalyst, in acidic medium is employed for this purpose. The overall reaction is given below.

Organic matter + Cr₂O₇^2- + H⁺ → Cr³⁺ + CO₂ + H₂O

When compared with BOD, COD oxidizes more organic compounds; hence COD values are higher than BOD values. Chemical oxygen demand can be determined in just three hours, in contrast to BOD requiring five days. Some workers determine COD and calculate BOD. This is possible since there exists a reasonably good correlation between COD and BOD.

Total organic carbon (TOC):

Measurement of total organic carbon is required when the concentration of organic matter is very low. TOC can be determined by oxidizing organic carbon, in the presence of a catalyst to CO₂, which can be measured.

Theoretical oxygen demand (ThOD):

The organic matter of sewage is mainly composed of carbohydrates, proteins, fats and products of their decomposition. If the chemical formulae of the organic matter (i.e. individual compounds) are known, the theoretical oxygen demand can be calculated.

Detection of Pathogenic Organisms of Sewage:

Human beings infected with disease-causing microorganisms (i.e. pathogens) are the carriers and they can discharge pathogenic organisms in waste water. Several species of bacteria, viruses, protozoa and helminths are responsible for diseases in humans. A selected list of the pathogenic organisms and the major diseases is given in (Table 56.1).

Among the diseases (water-borne diseases) originating from sewage, typhoid, paratyphoid, cholera, gastroenteritis, diarrhea, and dysentery are important. All these diseases are highly infectious and are responsible for the death of thousands of people in countries (particularly developing ones) with poor sanitation.

Indicator or index organisms of sewage:

It is tedious and time consuming to isolate and identify pathogenic organisms in water waste. Some other non-pathogenic bacteria of sewage polluted water are employed for this purpose e.g. E. coli, Streptococcus faecalis, Clostridium perfingens, Klebisella sp. These organisms are collectively referred to as indicator or index organisms.

Among the indicator organisms, the rod-shaped bacteria, commonly known as coliform organisms (E. coli, Aerobacter sp) are the most commonly used. The intestinal tract of man is very rich in coliform bacteria. It is estimated that each person discharges about 100-400 billions of coliform organisms per day.

Therefore, the detection of coliform organisms is a clear indication of fecal contamination and thus the presence of pathogenic organisms. On the other hand, the absence of these organisms indicates that the water is free from disease-causing organisms. Detection of coliform organisms is based on the fact that a great majority of water-borne diseases have fecal origin.

Laboratory Methods for Detection of Coliform Organisms:

There are three methods commonly employed for the detection of coliform organisms in water samples-multiple tube-fermentation technique, membrane filter technique and colilert technique.

Multiple-tube fermentation (MTF) technique:

This is basically a test that involves acid fermentation. Lactose broth fermentation is tested at around 35°C for 1-2 days in a series of tubes, involving sequential tests-presumptive test, confirmed test and completed test.

Presumptive test:

The waste water sample is serially diluted. One ml of the sample of each dilution is then transferred to each of the five fermentation tubes. These tubes contain lactose culture medium and inverted gas collection tubes (Fig. 56.1). At the end of 24 hour incubation period, if there occurs gas collection in the inverted tube, the test is positive indicating the presence of coliform organisms.

Confirmed test:

If no gas formation occurs in the presumptive test, the incubation is continued for another 24 hours. If gas formation is detected now, the test confirms the presence of coliform bacteria. And if no gas is produced even in the confirmed test, it can be assumed that the samples are free from coliform.

Completed test:

In this, a sample from positive coliform test is grown on eosin methylene blue (EMD) agar. The incubation is carried out at 35°C for 24 hours. The presence of E. coli can be detected by the occurrence of greenish metallic sheen on the plates.

Membrane filtration technique (MFT):

The presence of coliform organisms in water can be detected by using cellulose acetate ester with a pore size of 0.3 to 0.5µm. For this purpose, the membrane is first sterilized for 20 minutes at 80°C. The filtration is carried out in aseptic conditions under vacuum. As the filtration occurs, the bacteria are held on the membrane surface (Fig. 56.2). The trapped bacteria are dried and stained with a dye such as erythrosine, and detected.

Advantages and disadvantages:

The membrane filtration technique is rapid and can be carried out outside the laboratory (fields) also. This is in contrast to multiple tube fermentation technique. The major limitation MFT is that it cannot be used for turbid and/or heavily polluted water. This is because the membrane pores get clogged.

Colilert technique:

Although commonly used, both the tests described above (MFT and MF) sometimes give false-positive and false-negative results. Colilert technique is a recent and novel technique designed to specifically detect E. coli and other coliform bacteria. This is based on the principle that when certain substrates are digested by microorganisms, chromogens (coloured substances) are produced. Thus, when O-nitrophenyl β-D-galactopyranoside (ONPG) is used, it is converted to a yellow compound by coliform bacteria. With the substrate 4-methylumbelliferyl β-D-glucuronide, E. coli produces a fluorescent compound.

Techniques to Distinguish Fecal From Non-Fecal Bacteria:

A positive laboratory test for coliform bacteria need not necessarily be due to pathogenic organisms, originating from intestines. For instance, Enterobacter (Aerobacter) aerogenes, found in decaying plant materials can ferment lactose and give a positive coliform test. Thus, the presence of E. aerogenes in water does not indicate contamination with coliform bacteria.

A series of laboratory tests are available to differentiate fecal from non-fecal organisms. These techniques are collectively referred to as IMViC test, with the following connotations.

I stands for indole test

M represents methy red test

V is for Voges-Proskauer test

C stands for citrate test

(Note: i is used only for phonetic purpose).

These four tests are used to distinguish E. coli from Enterobacter aero genes. E. coli gives positive indole and methyl red tests, and indicates that the source of pollution is of fecal origin. On the other hand, Enterobacter aero genes gives positive Voges-Proskauer and citrate tests which shows that the water pollution is due to non-fecal origin.