Waste management plants play a significant role in civil works and ensuring good public health. Waste management processes include landfills, aerobic treatment systems and anaerobic treatment systems. While these systems operate differently, the presence or absence of oxygen is critical in determining their effectiveness. Aerobic waste treatment systems utilize oxygen which is then taken by microorganisms in filters to break down undesired chemical compounds. I.e., oxidation ponds entail a combination of processes by algae and bacteria whereby algae produces oxygen which is then used by the bacteria to break down compounds and giving up carbon dioxide which is used up by the algae. This is cycle of processes which helps to dismantle undesired compounds. In this regard, it is important to determine the appropriate rate of aeration and the specific points that yield maximum results. Therefore this study involved oxygen uptake analyses in aeration trains of a Springfield regional wastewater treatment facility.
Background
Biological waste treatment in activated systems relies on microorganisms which break down the dissolved organic compounds in the wastewater using dissolved oxygen. The oxygen uptake rate is essential in wastewater treatment which determines the rate of oxygen utilization by the microorganisms. The amount of oxygen used is expressed in mg/L (ppm) per hour. The determination of the oxygen uptake rate helps to assess how active the microorganisms are and whether they are utilizing the oxygen provided to breakdown the dissolved organic substances in the effluent. Since the solids are not controlled in an aerated stabilization basin, and the suspended solids is a function of biological activity, biochemical oxygen demand (BOD) and loading, only the uptake rate of dissolved oxygen is determined for aerated stabilization basins. However, for activated sludge systems, the dissolved oxygen uptake rate is used to measure specific oxygen uptake rate. SOUR refers to the amount of oxygen consumed by 1g of the suspended solids in one hour in the activated sludge (Sikdar and Irvine p 495-499). DOUR is a significant parameter in measuring slug loading because increase in loading (BOD) increases the oxygen uptake rate. OUR is also used to detect the level of toxicity. The presence of toxins reduces the uptake rate of oxygen. If the oxygen uptake rates are already known, it is possible to detect the presence of toxins by checking for decrease in uptake rates. The test results are important in screening waste water for toxicity and improving the management of spills. During interpretation of results, the trends over a given time are considered important than individual numbers. Thus, the tests must be conducted regularly to enhance the relevance of the results (Dukta and Liu pp 150-152). Typically, it is advisable that samples are obtained from 3 points from the system during the retention time. Mostly, samples taken between 48 and 96 hours after effluent has entered the system are useful in predicting and addressing problems before they exacerbate. DOUR is calculated by drawing a graph of dissolved oxygen of the sample in every ten minutes using a dissolved oxygen probe. However, time intervals might differ but there should be a 1mg/L difference between the start and the end. The sample to be measured should be in an air tight bottle so that no oxygen from the outside gets in.
Procedure.Wear appropriate clothing, footwear, helmet, safety glasses, and gloves. The first step involved filling 2 liter container 75% full at the first Sampling Station. The container was closed and shaken vigorously for 30 seconds. Then, DO probe was inserted into the container halfway deep in the liquid and used to stir. When the DO level reached 8.0 mg/L, DO readings were taken every thirty seconds for 8 minutes. The readings were recorded on a worksheet. This procedure was repeated for all the six sampling stations. After recording all the data, calculations were done in an excel worksheet, the DO probe cleaned and returned to the laboratory.
The results obtained from the tests are shown below.
Time in Minutes Dissolved Oxygen (Mg/L)
0.00 8.25
0.5 7.88
1.0 7.53
1.5 7.21
2.0 6.92
2.5 6.66
3.0 6.38
The figures above show the test results from the six stations. For every station, the oxygen uptake rate was calculated by determining the slope of the graph. For instance, for station one, the OUR was
(8.25 - 3.91) / (0.0 8.0) = -0.53
Therefore, the oxygen uptake rate for station 1 was 0.53mg/L/minute. Similarly, the specific oxygen uptake rate was determined and found to be 11.63 (mg/g/)hr. OUR is converted to SOUR by dividing it with MLVSS. This is to help obtain a clearer representation based on the magnitude of microbes in the sludge. This process was repeated for all the six stations and the data recorded.
Summary and Recommendations
The values obtained for SOUR and OUR for each station provide significant information needed for the analysis and better management of the waste water treatment. If the value of SOUR obtained is more than the recommended optimal range, that would be interpreted to mean that the aeration system F/M ratio is on the rise thus resulting to a lot of food (BOD load) and insufficient microbes (MLVSS). Also, this could be interpreted to mean that the sludge is too young and if the situation is not corrected quickly, suspended particles that take long to settle will be carried over with the discharge effluent. Similarly, the opposite happens when the specific oxygen uptake rate value drops below the optimal value. In such cases, it means that there is inadequate food to sustain microbes growth which could lead to pinpoint floc effect after the sludge settling too quickly (Dukta and Liu p 150).
From the results of the tests, the SOUR values for stations 1,2, and 3 were above 8 while station 4 ,5, and 6 had SOUR values less than 8. However, only station 3 had an approximate value of 8 which is the optimal level. Stations 1 and 2 had SOUR values of 1 and 6 respectively. This means that the sludge had a lot of food but insufficient microbes. As a result, there is the likelihood that the suspended solids that take long to settle will be carried over by the discharge effluent. On the other hand, stations 4,5, and 6 had SOUR values of 4.7, 4.3, and 3.9 respectively. These values are below the recommended 8 meaning that the sludge has inadequate food to sustain the growth of the microbes.
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