Vocabulary Pond Treatment Aerobic Bacteria Bacteria which will live and reproduce only in an environment containing oxygen which is available for their respiration (breathing), namely atmospheric oxygen or oxygen dissolved in water. Oxygen combined chemically such as in water molecules (H 2 O), cannot be used for respiration by aerobic bacteria. Anaerobic Bacteria Bacteria that live and reproduce in an environment containing no free or dissolved oxygen. Anaerobic bacteria obtain their oxygen supply by breaking down chemical compounds which contain oxygen, such as sulfate (SO 2-4 ). Facultative Pond The most common type of pond in current use. The upper portion (supernatant) is aerobic, while the bottom layer is anaerobic. Algae supply most of the oxygen to the supernatant. Overturn The almost spontaneous mixing of all layers of water in a reservoir or lake when the water temperature becomes similar from top to bottom. This may occur in the fall/winter when the surface waters cool to the same temperature as the bottom waters and also in the spring when the surface waters warm after the ice melts. Percolation The movement or flow of water through soil or rocks. Photosynthesis The process in which organisms, with the aid of chlorophyll (green plant enzyme), convert carbon dioxide and inorganic substances into oxygen and additional plant material, using sunlight for energy. All green plants grow by this process. Riprap Broken stones, boulders, or other materials placed compactly or irregularly on levees or dikes for the protection of earth surfaces against the erosive action of waves. Parallel Operation Wastewater being treated is split and a portion flows to one treatment unit while the remainder flows to another similar treatment unit. - 18 -
Series Operation Wastewater being treated flows through one treatment unit and then flows through another similar treatment unit. Purpose of Ponds Ponds are used to stabilize wastewater through natural biological processes. They may be used as the only method of treatment or act in a way to Polish water treated by another treatment process. Process Description Wastewater flows into the pond full of organic and inorganic wastes. Organic material is broken down by naturally present bacteria. Heavier organic matter will sink and undergo decomposition at the bottom of the pond in an anaerobic environment. Lighter organic material will be broken down while in suspension. Inorganic material will settle and remain on the bottom of the pond until it is removed by scheduled cleaning. Nutrients, such as phosphorous and nitrogen, are used by green algae during their metabolic processes. Typical Removal Efficiencies for Facultative Ponds Water Quality Indicator Typical Removal Efficiency Dissolved Organic Solids 80% Suspended Solids 90% Biochemical Oxygen Demand (BOD) 50-90% Bacteria 90% - 95% - 19 -
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Pond Classification Aerobic Pond Dissolved oxygen is present throughout the depth of the pond Anaerobic Pond Dissolved oxygen IS NOT present in the pond Facultative Pond Upper layer of water is aerobic because it contains dissolved oxygen while the bottom has no dissolved oxygen and is therefore anaerobic (Most Common Type) Advantages of Pond Treatment 1. Expensive equipment not required 2. Highly trained operators not required 3. Construction costs are low 4. Treatment is equal to some conventional treatment processes 5. Adaptable to changing loads 6. Energy cost is low 7. Has few sludge handling problems 8. Most trouble free of all treatment processes Disadvantages of Pond Treatment 1. May produce foul odors 2. Require large area of land 3. Weather affects treatment process 4. May contaminate ground water 5. May have high Suspended Solids in effluent Aerobic Process in Upper Layer of Facultative Ponds 1. Organic material converted to cell mass and CO 2 by aerobic bacteria 2. Algae take up CO 2 and nutrients during photosynthesis and release O 2 3. DO sustains aerobic bacteria population and the process continues Anaerobic Process in Lower Layer of Facultative Ponds 1. Organic material is converted to nitrogen, organic acids and CO 2 by bacteria called acid producers 2. Bacteria group called methane fermenters breakdown the acids and other products to form methane gas and alkalinity 3. Most of the organic load is removed in this lower layer - 21 -
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ph fluctuations Due to Photosynthesis/Respiration During daylight Algae takes up CO 2 which reduces ph During night Algae respirates and gives off CO 2 which reduces ph Operation Guidelines 1. ph should be maintained above 7.5 if possible (soda ash can be added if necessary to raise ph) 2. minimize scum build-up and accumulation 3. minimize odors a. recirculation from aerobic units b. surface aerators c. addition of sodium nitrate as oxygen source d. chlorination in recirculation line (may negatively affect biological process) 4. Do not allow weeds to grow on pond edges a. Reduces mosquito population b. Helps reduce scum accumulation points 5. Operate ponds in parallel if organic loading is heavy Practice Questions True/False 1. If detention time is long enough, most ponds can meet fecal coliform standards 2. Algae produces oxygen during the day and uses oxygen during the night 3. The growth of desirable algae can be inhibited if sufficient nutrients are not in the wastewater 4. The lower layer of a facultative pond usually aerobic while the upper layer is mostly anaerobic 5. For best results, grab samples should be analyzed after a stabilization period of several hours has passed - 23 -