Dissolved Oxygen
INTRODUCTION
Dissolved oxygen is a useful parameter used to determine the level of effluent in waste water treatment. Living organisms in water bodies require oxygen to break down organic matter. These organisms consume the dissolved oxygen in water and give out carbon dioxide. Oxygen does not readily dissolve in water and gets depleted quickly by increased micro-organism activity. To maintain optimal oxygen levels in a water body, the oxygen replenishing rate from the air should be equal to dissolved oxygen use by micro-organism in the water. A curve drawn to represent dissolved oxygen levels down a stream from a point of effluent discharge would be a sag curve. Dissolved oxygen levels would be lowest at point of effluent discharge and increase steadily downstream as micro-organism activity decreases and more oxygen dissolves from the air into the water body. A sag curve that drops sharply at a given point indicates higher oxygen demand and a point of effluent discharge into the water.
Biochemical oxygen demand (BOD) tests are carried out to determine the quantity and condition of effluent discharged to a water body. Waste water from a treatment plants is subjected to these tests to determine the level of wastes. BOD tests are based on the principle that oxygen demand for the biodegradation process will be directly equal to amount of effluents in the waste water. Oxygen levels are directly related to effluent levels, a low oxygen level mean high effluent levels and high oxygen level translates to low effluents in the water.
Dissolved oxygen (DO) is used to determine the level of waste water treatment that will be required. The initial DO readings of a waste water sample are taken together with all the accompanying parameters. The sample is then incubated and further readings are taken after a week. The decrease in the DO represents the amount of oxygen used up by microorganism to degrade the organic substrate in the sample.
DO levels in water bodies are determined by physical factors such as temperature, pressure, surface disturbance, turbulence and concentration of chemical ions in the water. DO level measurements are done to avoid drainage of all oxygen in water bodies. Minimum DO levels required are 5mg\L in water. Waste water treatment is done with the aim of maintaining the DO around this value and involves breakdown of organic matter so that it is no longer ingestible by microorganisms.
MEASUREMENT OF DISSOLVED OXYGEN LEVELS (WINLKER TEST)
Winlker Test is used to determine the DO levels in a water sample. During this test, other factors that accelerate or slow degradation process such as temperature are kept constant. It is important that estimated DO in the test sample must be a minimum of 1mg/L otherwise the test results would be invalid.
There are many variables which can influence the test results. To guard against this, there exists a set of test procedures outlined in the CE 321 Environmental Engineering and Design Laboratory Manual.
The BOD test was conducted using three samples namely the raw influent, lagoon effluent, and wetlands effluent. The dilution water used in the test was tested and treated to ensure optimum conditions for the microorganisms in terms of pH and nutrients.
The sample is divided into two and the readings in one half of sample are taken while the other is incubated together with a blank. A blank is a bottle containing the dilution solution and is used as a control measure. If there is DO change in the blank during incubation, this renders the result invalid. After incubation period of seven days, the sample and the blank were tested again for DO and readings taken again.
RESULTS AND DISCUSSION
The difference in initial and final DO readings represents how much oxygen will be used in breakdown of organic matter. The test results are considered reliable if there is a minimum DO decrease of 2mg/L. Table 1 below represents actual data calculated. Raw data is presented in the Appendix A.
In the seven day Biochemical Oxygen Demand (BOD7) test, the NPDES limits for the various effluents average at 30mg/L. The lagoon effluent NPDS range from 70 to 100 mg/l in the BOD7 test. Readings for the wetlands normally average at 30mg/L. All the negative test results from Table 1 above are disregarded as these represent calculation errors. Readings for the raw effluent average at 200mg/L which is outside the normal effluent range of 70 to 100 mg/L. The Lagoon effluent reads at 103mg/L, which also falls outside the NPDES range. Wetlands produced average readings of 20mg/L.
The lagoon readings are the closest to the expected ranges. Some factors mentioned earlier in this report could have contributed in the inconsistency of test results with the expected outcomes. These include temperature and pressure. Temperature affects BOD tests as it has a direct relationship with the BOD coefficient factor.
Solids
INTRODUCTION
Solids removal is an essential part of waste water treatment. Solids determine the type of treatment to be undertaken on waste before it is released to a water body. Waste water is categorized according to its constituent solids. Constituent solids also determine the condition of the treatment plants. Conditions of the treatment plants are adjusted such that their waste water solid removal capacity matches that of the feed. Waste water solids comprise of both organic and inorganic solids. Organics solids are more prevalent and are of much importance as they influence the BOD levels. Discharge of waste water with high organic solid content will raise the BOD levels in a water body which will negatively impact on the aquatic life present by lowering the DO.
Waste water with high organic solids concentration is treated by means of microorganisms. Microorganisms are introduced into the waste water, and ample oxygen is availed. After a given duration, the solid organic matter is depleted but the microorganisms still remain. They are removed from the waste water through settling in a settling tank where they sink to the bottom and are sucked off to be recycled.
Organic solids in waste water can be categorized into three groups; total suspended solids, fixed suspended solids and volatile suspended solids. The amount of suspended solid is expressed in weight per weight per volume of solids in the sample. Total suspended solids cannot pass through the filter. Fixed suspended solids are the solids that cannot be filtered and cannot burn when ignited. Volatile solids are gotten by subtracting fixed suspended solids from total suspended solids.
SUSPENDED SOLID TEST
The total suspended solids (TSS), fixed suspended solids (FSS), and volatile suspended solids (VSS) contained in a sample are determined in the laboratory by the use of Suspended Solids Test Procedure. This procedure is explained in full details in CE 321 Environmental Engineering and Design Laboratory Manual. In this test, samples from three stages of the water treatment procedure were used. The three stages included the Raw Effluent, Lagoon Activated Sludge, Lagoon Effluent, and Final Effluent. The nature of these samples varied from solid to a clear liquid.
TEST PROCEDURE
An aluminum weighing dish and a glass fiber filter paper were washed thoroughly and dried to safeguard against error by removing all sticking matter. Then both the weighing dish plus filter were weighed, and value recorded as W1. The filter paper is was removed from the aluminum dish and placed in one of the containers in the vacuum Filter Apparatus as shown in figure 2. The waste water sample was then poured into the container on top of the filter paper and the vacuum pump was activated sucking away all the liquid in the sample and leaving behind solid constituents on the filter paper.
The filter paper was placed back on the weighing dish and heated to 103oC in an oven for one hour until its weight (W2), leveled off and became constant. The weighing dish was then placed in an oven at 550oC for twenty minutes until constituents ignited. The weight of the resulting sample, W3 was then recorded by weighing on a desiccator.
Figure 2: Vacuum Filter Apparatus
Results and Discussion
Results and Discussion
All sample weights recorded at various stages of the test are displayed at Appendix A.
The values that were determined using this procedure are Total Suspended Solids, Fixed Suspended Solids, and Volatile Suspended Solids. Equations 1, 2, and Equation3 respectively were used to obtain the values.
The last equation, equation 4, was used to determine the TSS removal efficiency by comparing values between initial TSS and final TSS.
Equation 1: Total Suspended Solids
TSS=W2-W1*1000 mLL*1000 mggmL of sample filtered
Where:
W1=mass of prepared filter
W2=mass of the filter plus dry sample
Equation 2: Volatile Residue
Volatile Residue=W2-W3*1000 mLL*1000 mggmL of sample filtered
Where:
W2=mass of the filter plus dry sample
W3=mass of the filter pad after ignition
Equation 3: Fixed Residue
Fixed Residue=W3-W1*1000 mLL*1000 mggmL of sample filtered
Where:
W1=mass of prepared filter
W3=mass of the filter pad after ignition
The values obtained using these equations are recorded in Appendix B. Negative results obtained indicate flaws in the experiment and calculations. Errors could have been caused by taking wrong readings during weighing or additional matter from the environment such as oil from people handling the sample.
Equation 4: Percentage Removal Efficiency
Percentage Removal Efficiency=TSSin-TSSoutTSSin*100
Some values obtained using this equation contains errors too. The values used in the calculation, TSS, FSS, and VSS were the source of the errors
Fixed Suspended Solids and Volatile Suspended Solids are shown in Table2.
According to Table 2 above, the values of TSS, FSS and FVS should decrease as the sample moves along the treatment plant with a slight deviation in the Lagoon Activated Sludge. Weight increases at the Lagoon Activated Sludge because of the addition of microorganisms. The values used to calculate the above averages are free from the negative errors found in appendix B.
According to NPDES standards, the final effluent should have TSS average values of 30 mg/L at winter condition. From Table 2, the TSS average is 35mg/L and so the effluent does not meet the NPDES standards.
Works Cited
Davis, M., &Masten, S. (2014). Principles of environmental engineering and science (3rd ed.). New York, NY: McGraw-Hill.
Murgel, George (2010). CE 321 Environmental Engineering and Design Laboratory Manual (9th ed.). Boise, ID: Prepared by George Murgel. Boise State University.
Appendix A
Appendix B
