Testing Wastewater Treatment Efficiency

1/Biochemical Oxygen Demand (BOD)

The biochemical oxygen demand test has been used widely by regulatory agencies to gauge overall treatment plant efficiencies. The traditional BOD measurement of the plant influent, grit removal influent, and the final effluent gives the most common measure of treatment plant efficiency. The BOD of wastewater is a common indicator of the fraction of organic matter that may be degraded by microbial action at a given time period at a temperature of 20 degrees Centigrade. The test is related to the oxygen that would be required to stabilize the waste after discharging to a receiving body of water. The drop in BOD from grit removal effluent to final effluent is usually used in calculating the solids growth rate in the aeration tank. The BOD test is too slow to provide timely information to the operator for control purposes. It can, however, provide the operator with the historic results of previous operating conditions. Tests for BOD are to be made on composite samples daily. BOD tests run for at least 20 days are also to be made on the effluent periodically to determine the oxygen requirements of the nitrogen compounds present in the effluent.

2/Chemical Oxygen Demand (COD)

Chemical oxygen demand is another means of measuring the pollutional strength of wastewater. By using this method, most oxidizable organic compounds present in the wastewater sample may be measured. COD measurements are preferred when a mixed domestic-industrial waste is entering a plant or where a more rapid determination of the load is desired. The chemical oxygen demand test has a major advantage over the biochemical oxygen demand analysis because of the short time required for performance - a few hours as opposed to five days for the standard BOD test. Since this test can be run in several hours, it gives the operator a more timely idea of what is entering the plant and how the plant is performing. This permits closer operational control of the treatment process.Generally, COD values are higher than BOD values. The reason is that biochemical oxygen demand measures only the quantity of organic material capable of being oxidized, while the chemical oxygen demand represents a more complete oxidation. Typical COD values for domestic waste range from 200 - 500 mg/L.

3/Total Organic Carbon (TOC)

Total organic carbon measurements have been used as a method for determining pollutional levels of wastewater for many years. The organic carbon determination is free of many of the variables involved in the COD and BOD analyses, with somewhat more reliable and reproducible data being the result. The need for rapid determination of wastewater strength has led to the development of organic carbon analyzers and their introduction into some treatment plant laboratories. All of the available instruments measure the organic carbon content of aqueous samples, although there are several methods by which this is accomplished. The TOC values will generally be less than COD values, because a number of organic compounds may not be oxidized in the total organic carbon analysis. Typical values of TOC for domestic waste range from 100 - 300 mg/L.

4/Total Oxygen Demand (TOD)

Another method of measuring organic matter in wastewater involves the oxidation of the sample to stable end products in a platinum-catalyzed combustion chamber. Total oxygen demand is determined by measuring the oxygen content of the inert carrier gas, nitrogen. TOD measurements are becoming more popular because of their quickness in determining what is entering the plant and how the plant is responding. Analysis time is approximately 5 minutes. The results obtained generally will be equivalent to those obtained in the COD test.

5/Solids Determinations

Laboratory determinations of suspended solids (SS) in the influent, primary effluent, and final effluent are standard measurements used to indicate treatment plant efficiency. The SS measurements are used in calculating the sludge volume index (SVI) and sludge density index (SDI) - both important control tools. There is a distinction between total suspended solids (TSS) and total volatile suspended solids (TVSS). TSS measures both the active bacterial mass and the inert materials in the waste or mixed liquor. TVSS is a more accurate estimate of the mass of active microorganisms in the mixed liquor and is the parameter to be used in calculating the food-to-microorganism (F:M) ratio.

6/Sludge Density Index / Sludge Volume Index

To determine what the return sludge pumping rate should be and to get some idea of sludge settling characteristics, sludge indices have been proposed. One of the most common is the Donaldson Index, SDI:

SDI = (MLSS (%) x 100) / % volume MLSS after 30 min settling

The other common index is the Mohlman Index, SVI:

SVI = % MLSS volume after 30 min / % MLSS (mg/L MLSS) = ml settled sludge x 1000

These indices relate the weight of sludge to the volume the sludge occupies. They show how well the liquids-solids separation part of the activated sludge system is performing its function on the biological floc that has been produced and is to be settled out and returned to the aeration tanks or wasted. The better the liquid-solids separation is, the smaller will be the volume occupied by the settled sludge and the lower the pumping rate required to keep the solids in circulation.

7/Sixty-Minute Settling Test

The 60 minute settling test is a reasonable approximation of what is happening in the final settling tank. So that solids do not accumulate in the final settling tank, they must be removed at an average rate equal to that at which they are applied.

8/pH

pH is a method of expressing the acid condition of wastewater. The pH scale ranges from approximately 1 - 14, with a pH of 1 - 7 considered the acid range and 7 - 14 considered the base range. pH 7 is defined as neutral. pH is a vital tool of the wastewater treatment plant operator when determining unit operations.

9/Alkalinity

This is a measure of a wastewater's capacity to neutralize. The bicarbonate, carbonate, and hydroxide ions are the primary contributors to alkalinity. The determination of alkalinity levels at various points in a plant will be an aid to the proper understanding and interpretation of the treatment process. For example, if chemical addition is used to coagulate wastewater for solids removal, hydrogen ions may be released and cause the pH to decrease. Alkalinity will tend to neutralize the acids formed and permit coagulation to proceed in the proper pH range. Some other processes dependent on pH are disinfection, digestion, and sludge preparation and conditioning.