EDS Water Management The training equipment The EDS Water Management is a modular training equipment which simulates the core processes of water and wastewater treatment plants. The EDS Wastewater Treatment System Festo Didactic 11/2013
Wastewater treatment Introduction to the learning module Wastewater treatment In nature, water bodies normally have the power to purify themselves due to the presence of certain microorganisms such as bacteria and algae, which decompose the organic compounds in the wastewater, breaking and transforming them into simple substances such as carbon dioxide or nitrogen. If the microorganisms have enough time to decompose organic compounds and consequently keep their concentrations within certain limits, then the self-purification process does the job. Treatment becomes necessary as soon as discharge volume and consequently, concentrations rise. In this case, bacteria growth and the oxygen demand in the water will also increase. Wastewater treatment accelerates the natural decomposition by optimising the conditions for decomposing bacteria. The efficient and economic layout of a wastewater treatment plant requires a careful design based on aspects such as flow rates, components of the raw wastewater and organic load, end use of the treated water, economic viability, site area available for installation and climatic patterns as temperature and rainfall. This is why, there is no standard solution, but the selection of processes and the type of wastewater treatment plant will vary depending on each specific case. However, the usual process of treating domestic wastewater can be divided into the following standard stages: Pre-treatment Primary or physical treatment Secondary or biological treatment Tertiary treatment Generic wastewater treatment process (basis source:university of Pretoria, Prof. EMN Chirva) 2 Festo Didactic GmbH & Co. KG
Secondary or biological treatment Once passed the pre-treatment and primary treatment stages, the aim of secondary or biological treatment is to reduce the organic material in the wastewater. Secondary treatment has been designed based on the biological process of self-purification - mentioned above - which occurs naturally. It helps to prevent water bodies from being polluted by natural born wastewater. In the corresponding processes, the biodegradable organic matter from domestic wastewater acts as nutrient for a bacterial population which is provided with oxygen in controlled conditions. In summary, the biological treatment, is so to say, an accelerated natural oxidation process were biodegradable organic matter is decomposed by bacteria. Biological wastewater treatment subsequently avoids the entry of contaminants which could induce a lack of oxygen in water bodies. The most common aerobic processes with suspended biomass, are the aerated lagoon and the activated sludge or aeration tank which is treated in detail in this workbook. EDS Aeration Tank (B401) from the top Festo Didactic GmbH & Co. KG 3
Exercise: Aeration Problem description Bacteria are used to clean wastewater. While doing this, the microorganisms use dissolved oxygen (O 2 ) to oxidize dissolved organic substances to CO 2. Oxygen naturally dissolves in water keeping the balance between the oxygen content in water and oxygen concentration in the surrounding air. The equilibrium between the oxygen in air and water remains constant, if there is no oxygen consumption. Oxygen concentration in water then depends on the O 2 concentration in air, temperature and air pressure. Training notes Equilibrium of oxygen in water is about 7 to 9 mg/l near sea level with 21% O 2 in air and a temperature of about 20 C. In an activated sludge process, one strives to achieve an optimal oxygen content of 1.5 to 2.0 mg/l in the aeration tank. This concentration is sufficient for the bacteria and at the same time saves energy, which is crucial as aeration consumes 50 80% of electrical energy in a wastewater treatment plant. Note For more information about the energy demand of aeration, see workbook: Energy optimisation in water and wastewater treatment plants Layout The air blower at the bottom of the aeration tank (Tank B401) injects air bubbles via Motor M 402. As air bubbles from the aerator rise through the aeration tank, oxygen dissolves into the water. The quantity of air injected can be controlled. An oxygen sensor is installed in the tank to measure the concentration of oxygen dissolved in water. The FluidLab Water Management software displays the values. The oxygen consumption by microorganisms in a real activated tank can be simulated by adding sodium sulphide (Na 2 SO 3 ) in various concentrations. Learning outcomes When you have successfully completed this exercise, you can: Explain the basic function of aerobic wastewater treatment using the activated sludge processes. Measure and name the amount of oxygen dissolved in water without bubble aeration. Describe the significance of oxygen injection into the aeration tank. Explain the benefits of constantly measuring dissolved oxygen in an aeration tank. 4 Festo Didactic GmbH & Co. KG
Tasks 1. Oxygen concentration without aeration Motor M402 for aeration in Tank B401 is switched off for this task. Use the oxygen sensor to measure the concentration of dissolved oxygen in Tank B401 a) How much oxygen is dissolved in the water without aeration? Without aeration the oxygen content is between 6 up to 9 mg/l. It is not zero! 2. Aerate and measure O 2 concentration Switch on Motor M402 for aeration and set the delivery rate to 100%. Measure the oxygen concentration with the aid of the oxygen sensor again. a) What is the level of dissolved oxygen in Tank 401? Without consumption a balance between oxygen concentration in air and in water takes place, only depending on temperature, air pressure and concentration of oxygen in air. As a result, at equilibrium, further aeration does not lead to higher oxygen concentration in water. The following graph shows that even with excessive aeration (100%, green line), the oxygen concentration (red line) doesn t reach the setpoint of 9 mg/l (black line) but levels at about 8.5 mg/l O 2. Aeration without oxygen consumption (Pop-up menu oxygen-sensor control) Festo Didactic GmbH & Co. KG 5
3. Simulate oxygen consumption Switch off the aerator motor M402. Add 50 ml sodium sulphite solution 10% (Na 2 SO 3 ) to Tank B401. Start Pump 401 with 1.5 l/min Training notes For calculations: 1 kg Na2SO3 consumes 125 g O2. a) Observe the oxygen concentration for approximately 10 minutes. The added sodium sulphite consumes dissolved oxygen and the oxygen content in Tank 401 drops. Progress of oxygen concentration after once adding Na 2 SO 3 without aeration using Fluid Lab - PA closed loop b) What would this mean in the real world? This corresponds to oxygen being consumed by microorganisms in the aeration tank during the decomposition of biodegradable contaminants in the wastewater. Without aeration the oxygen level drops dramatically. 6 Festo Didactic GmbH & Co. KG
4. Aerate with oxygen consumption a) Switch the aerator pump back on. b) Periodic addition of sodium sulphite (Na 2 SO 3 ) causes continued oxygen consumption, thus simulating the oxygen demand of bacteria eating up organic substances in a biological treatment of wastewater. c) What course does the oxygen concentration take? d) Try to keep a constant oxygen level by controlling the aerator manually ( hand ). Why is it difficult to control the system? Due to the slow chemical reaction there always is a delay between action and reaction, which means to increase or decrease aeration has no enduring influence on the oxygen concentration, as the oxygen consumption overlays the oxygen solution. Note The EDS Water Management allows the aeration to be software-controlled. You find specific exercises for closed-loop control of aeration on in workbook of Monitoring, controlling and optimising operations and similar exercises with a focus on aerations energy demand in workbook Energy optimisation in water and wastewater treatment plants. Sample solution: Controlling of aeration using a two-step controller (red: dissolved oxygen concentration green: aeration) using Fluid Lab - PA closed loop (source: workbook Monitoring, controlling and optimising operations ) Festo Didactic GmbH & Co. KG 7
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