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Watertightness of the tanks is critical. For this reason, existing septic tanks are usually replaced with new tanks including a new building sewer. Projects in which existing tanks have been used have experienced excessive wet weather flows. Vacuum testing is often used to test watertightness. The typical acceptance criterion is less than 2.5 cm loss of mercury vacuum after 5 minutes with an initial vacuum of 10 cm. The test is run after connections are made and include the manholes. |
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In some SDGS systems, modifications have been made to the tank outlet to control maximum flow rates. This has been advocated to allow smaller diameter pipe to be used for the collection mains. Surge tanks installed after the interceptor tanks were used in the past, but they created excessive head loss in the system requiring unnecessary excavation and odour problems. Flow control devices are available which can be mounted inside the interceptor tank to maintain more uniform flow discharges. The freeboard available in the tank is used for the necessary storage. Most SDGS systems, however, have not used flow control devices. |
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9.3.5
Odour and Corrosion Problems |
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Odours and corrosion are the most commonly reported problems associated with SDGS. Odours typically occur at lift stations, air release valves and house plumbing stack vents. Odours are most pronounced where turbulence occurs to release the dissolved gases in the settled septic waste. Odours at lift stations are often successfully abated by installing drop inlets that extend below the pump off level. Drop inlets eliminate most of the turbulence. Other successful control measures include soil odour filters, and airtight wet well covers with vents that extend 35 m above grade. Soil odour filters are the usual method to eliminate odors from air release valves. |
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Odours at individual connections usually originate from turbulence in the collection main, most often on steep runs where hydraulic jumps occur at a change in slope. The odours which are released flow back up the sewer due to the 'chimney' effect and exit at the stack vents of the connections at the highest elevation. Successful corrective measures have included placing running traps in the service laterals or extending the main further uphill and terminating it with an air vent and soil odour filter. |
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