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■Cleanouts shall be sealed with a screwed cap or plug secured under a tamperproof(bolt-dawn)cover.Cleanout <br /> spacing shall be approximately 300'.Conventional open-channel manholes will not be allowed except where desired <br /> to site a flume for flow measurement. <br /> SULFIDE CONTROL <br /> The normal hydrogen sulfide content of gravity sewers in the Pacific Northwest is approximately zero.The hydrogen sulfide content <br /> of septic tank effluent from the specified withdrawal zone is also generally negligible. <br /> However,effluent sulfate readily reduces to sulfide during periods of detention in the STEP service piping,pressure sewers,and <br /> inverted siphons.These systems reduce all available sulfate over time,resulting in hydrogen sulfide concentrations well exceeding 10 <br /> mg/1. <br /> Because of corrosion,odor,and safety concerns,STEP discharges into unarmored gravity sewers shall not exceed 0.1 mg/l hydrogen <br /> sulfide content. <br /> L Common Pressure Sewers <br /> ■STEP system designs shall include effective controls to prevent the development of hydrogen sulfide in flooded <br /> service lines,pressure sewers,and flooded sections of small-diameter gravity sewers. <br /> ■Pressure sewers shall be oxygenated by means of air injection into the head(low point)of each common sewer <br /> collector line.End-of-pipe chemical oxidation systems are relatively expensive and will not usually be approved. <br /> ■Air injection rate shall be 2 sef n per inch diameter.Air supply shall be a receiver-mounted compressor rated at <br /> the static head on the system at the point of injection.Static head shall be computed as the sum of all ascending <br /> segments in the line being aerated. <br /> IN No automatic air release valves shall be installed.Manual air release valves and automatic vacuum release valves <br /> may be installed where warranted in thejudgerrent of the engineer. <br /> ■Air injectors shall be V copper tubing and saddle-mounted corporation stop.Adjacent to the corporation stop, <br /> injector piping shall contain a suitable check valve,needle valve,airflow meter with pressure gauge,and an isolation <br /> valve and pressure reducer at the receiver,along with necessary unions and drip legs for condensate.All fittings <br /> shall be suitable for air service at the rated pressure of the compressor. <br /> ■Airflow meters shall be calibrated in efrn,with range suitable for the computed acf n(actual cu.ft.per min). <br /> Flowmeters sized for scfm are usually too large and will not be approved.Pressure-adjusted meters should not be <br /> used as the installed working pressure cannot be predicted with sufficient accuracy. <br /> ■Compressor and injector assembly shall be secured in a locked vault.Compressors and vaults should be muffled, <br /> silenced,and soundproofed.Compressors may be installed below grade in noise-sensitive areas.Receivers shall be <br /> fitted with automatic drain valves for condensate purge. <br /> ■Spare or standby compressors will not be,required for STEP systems. <br /> ■End-of-pipe aeration alone,or air-stripping alone,is generally unable to reduce the sulfide content of STEP <br /> sewers to 0.1 mg/l,and shall not be relied on for sulfide control.However,an approved commercial air-stripper <br /> vented through an.activated carbon filter may be installed as a polishing process.This process may be installed prior <br /> to discharge into any gravity sewer where odor and safety may be a concern,at the engineer's or OWNER's <br /> discretion. <br /> ■New sulfide control methods will be considered.and evaluated on their merits. <br /> 2.Unflooded Sections of Common Small-Diameter Gravity Sewers <br />