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chains or colonies of bacteria that don't settle or sink to the bottom as they should). Bulking is caused by changes in <br />wastewater strength or quantity. When too much water/wastewater is added to the system, the bacteria can run out of food or <br />become overloaded. Bulked bacteria remain suspended in the liquid and can clog the outflow. <br />A fixed -film reactor has bacteria growing on a specific surface medium and air is provided to that part of the tank. The <br />bacteria can grow on any surface including fabric, plastic, Styrofoam, and gravel. Decomposition is limited to this area, and <br />settling occurs in a second chamber. This design is expensive, but the effluent is of consistently high quality, and bulking is <br />uncommon. There is no need for a return mechanism because the bacteria stay on the film (Figure 3). <br />In a sequencing batch reactor, aerobic decomposition, settling, and return occur in the same chamber. Air is bubbled <br />through the liquid during the decomposition cycle. The bubbler shuts off, and the wastewater goes through a settling cycle <br />(Figure 4). Once the bubbler turns back on, the tank reenters the decomposition cycle, and settled bacteria mixes back into the <br />aerobic environment. After settling of bacteria and solids, the treated effluent is discharged to the soil treatment system. <br />Bacteria settle out more consistently in this kind of tank, but since it has more moving parts and requires a controller, it has <br />more potential for mechanical and electrical failure. <br />Placement <br />ATUs require very little installation space, which allows placement flexibility. A typical ATU space requirement is 25 square <br />feet for a 3 -bedroom home. Most ATUs are located after a septic tank. However, because of strict permitting requirements <br />this is not practical for onsite systems. <br />Designing Aerobic Treatment Units <br />When sizing an ATU, consider the amount of water generated by the home, the addition of oxygen, the concentration of <br />organic matter in the wastewater, and the settling characteristics of the chosen system. Proper design of these systems depends <br />on the waste strength. Siting and construction considerations are the same as those for typical onsite treatment systems. <br />Final Disposal of Wastewater <br />Some ATUs produce the same high-quality effluent as large wastewater treatment plants that discharge to surface water. <br />Therefore, in an onsite system, effluent is sent to a soil treatment system for final treatment. Options for a soil treatment <br />system include trenches, mounds, and drip distribution. <br />Pressure distribution (rather than gravity) to the soil system is necessary, since the effluent from ATUs contains very little <br />organic matter. The effluent is so "clean," a biomat layer does not form the way it does in a soil treatment system receiving <br />effluent from septic tanks. A pressure distribution network is needed to apply effluent evenly throughout the system. <br />System Classification <br />The soil treatment system may last longer when receiving effluent from an aerobic tank than it would receiving effluent from a <br />septic tank, since ATUs remove more solid matter from the wastewater. It may be possible to make the soil treatment systems <br />somewhat smaller than those used for conventionally pretreated effluent. It also may be possible to reduce the vertical <br />separation to the seasonally high water table or bedrock. Changes to size or vertical separation would put these systems into <br />the "performance" classification, requiring local approval and an operating permit. An operating permit requires a monitoring <br />and mitigation plan and the installation of a flow meter. <br />Operation and Maintenance <br />All routine operation and maintenance practices suggested for any onsite treatment system apply to ATUs. ATUs are more <br />maintenance intensive than septic tanks. A maintenance contract is strongly recommended and required for standard use <br />according to Minnesota Rules Chapter 7080. Depending on the local governmental unit requirements and the <br />recommendations of the manufacturer, the system may require quarterly to yearly maintenance. Maintenance includes <br />