odors will develop. If this occurs, you should check for flow problems within the sanitary system upstream of the treatment plant.

HYDROGEN ION CONCENTRATION (pH VALUE) TEST

The measure of acidity or basicity (alkalinity) of something is called the pH. The effect of pH on some parts of wastewater treatment makes it an important test. A low pH of domestic wastewater may mean that the wastewater is septic, or it can mean that industrial or commercial acid wastes are entering the system. A pH of 6.5 to 8 is about right for treatment plant influent. Test results showing a very high or low pH may mean someone is breaking sewer use regulations. Sudden changes of 0.5 or more on the pH scale may mean that operating problems are starting. Grab samples should be taken for pH tests.

SETTLEABLE SOLIDS TEST

The settleable solids test on wastewater can tell the operator a lot about what kind of wastewater is coming into the plant and how the solids are settling. Also, the settleable solids test can help the operator estimate the volume of sludge to be expected in the clarifier.

Either grab or composite samples will work for this test. The test is done using an Imhoff cone. The Imhoff cone (fig. 10-1) can be either glass or plastic. It can hold 1 liter and is marked off in milliliters (ml).

Before running the test, you should allow the sample to settle for 45 minutes. After 45 minutes, you should run a glass or plastic rod gently down the inside of the cone and turn it to loosen solids clinging to the sides. Settling should then continue for another 15 minutes. The depth of the solids in the bottom is then read from the scale and recorded as milliliters of settleable solids per liter of wastewater.

ACTIVATED SLUDGE SETTLEABILITY TEST

The settleability test is often used with all kinds of activated sludge plants to find the amount of solids in aeration units. The results help the operator to decide when to waste sludge and to find the rate of sludge return. The activated sludge settleability test can be run in a 1,000 ml graduated cylinder or in any clear, widemouthed container. The container should be ruled off into 10 units such as centimeters, milliliters, or inches. The sample is poured into the cylinder or jar up to the top mark and allowed to settle. Readings are taken from time to time to find settling rates. The sample should be allowed to settle for about 30 minutes.

FIVE-DAY BIOCHEMICAL OXYGEN DEMAND (BOD5 ) TEST

The BOD5 test is the most important test for finding the polluting strength of a wastewater. It is the most widely used way to check how the treatment plant is running. The BOD5 test indirectly measures the amount of organic material in the sample. Either grab or composite samples may be used for this test.

NPDES permits often state that influent and effluent flow-proportional composite samples be taken for the BOD5 test. Normal domestic wastewater coming into the plant should be in a 200 to 300 mg/l BOD5 range. The effluent must comply with the plant's

NPDES permit. To run the test, the amount of oxygen is measured in a portion of diluted wastewater, and another portion like the first one is stored at 20C for 5 days. The glass bottles shown in figure 10-1 are used for this test. During the 5 days, the microorganisms eat the organic matter in the wastewater and use oxygen at the same time. At the end of 5 days, the amount of oxygen consumed by the microorganisms times the dilution factor of the sample gives the sample's 5-day BOD. The dilution factor is the number of milliliters of dilution water added to a given number of milliliters of sample.

CHEMICAL OXYGEN DEMAND (COD) TEST

Like the BOD5 test, the chemical oxygen demand (COD) test finds the amount of oxygen required to consume the organic matter in a wastewater sample. The COD test does not measure the amount of oxygen used by the microorganisms. It uses a strong, chemical concentrated sulfuric acid silver sulfate solution. It is a good operating control test because the results can be obtained in as little as 1 hour. COD test results are equal to or greater than BOD5 test results. The chemical used in the COD test attacks more organisms in the wastewater than the slower BOD5 organisms. BOD5 data can often be related to COD data by a multiplying factor. For instance, the 200 to 300 mg/l BOD5 of normal