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Anaerobic Digestion Safety

Safety programs set forth by the City in accordance with all Federal, State, and Local regulations will be followed by employees. Check with your immediate supervisor if you are unsure of the safety requirements necessary to perform duties in this process area. Wastewater personnel are obligated to understand and follow all confined space requirements set forth by the City.

General Safety Precautions
  • Use lockout procedures when performing maintenance on any electrical or mechanical equipment prior to servicing.
  • Use extreme caution to avoid falls during entry and exit from buildings and basins.
  • Stairs and hand rails may become slippery when wet due to condensation.
  • Practice meticulous personal hygiene when work requires contact with chemicals, raw sewage and sludge.
  • Keep access hatches closed when not in use.
Process Specific Safety Precautions
  • The pressure relief and vacuum relief valves should be checked at least every 6 months to ensure proper operation. If either of these valves is defective, an explosive condition could exist.
  • Smoking and open flames are not permitted around the digester at any time.
  • All digester gas handling equipment must be maintained as recommended by the manufacturer or a dangerous condition could result.
  • If a digester must be shut down, the potential for explosion of digester gas increases. Gas production will continue at a slower rate even if the digester contents have been allowed to cool and feeding has stopped. Because of this situation, it is important that no source of ignition or open flame be allowed in the area.
  • When taking a digester out of service, the feed to the digester should be stopped. The digester contents should continue to be mixed through the mixing pumps. Solids should be digested for up to 1 to 2 weeks and gas production should be monitored to determine the stage of digestion. When gas production reaches zero, the digester gas valve should be closed to isolate the unit from the gas system.
  • After an explosion meter is used to test the atmosphere for explosive gas, the digester is ready to be opened. A gas ratio of 5.7 to 13.5 digester gas to air is explosive (lower and upper explosive limits). In order to open the digester to the atmosphere, the pressure and vacuum relief set points should be adjusted. Drain all solids to the drying beds for additional treatment.
  • When the sludge level is below the access hatch level, the digester hatches should be opened.
  • Explosion-proof ventilation equipment should be continuously in operation when work is being done in or around the digester. Forced ventilation should be used in the digester. After the forced ventilation is operating, the digester atmosphere should be tested for explosive gases and oxygen deficiency before anyone enters it.
  • A combustible gas indicator should be used to test for explosive gases. A reading can be taken by inserting a probe through a ventilation hole. An oxygen deficiency indicator should be used to test for oxygen deficiency.
  • Any work in the digester area will inevitably involve direct contact with sludge, which may contain pathogenic bacteria and viruses. If any personnel have an open wound, take precaution to avoid contact and possible infection.
  • When entering a digester, all personnel should wear safety hats and outer garments (such as coveralls) to prevent contact with sludge. Boots or non-sparking safety shoes should be worn to protect one's feet and to avoid sparks. Rubberized or waterproof gloves should be worn to prevent scratches or abrasions that could become infected.  
  • When working in the tank, personnel should be equipped with a safety harness.  
  • All pumps and piping directly connected to the digester should be flushed and drained.
  • While repairing all valves and internal digester components, the digester should be ventilated by the forced air fans as long as the unit is open and work is being performed.
  • A safety harness with a lifeline and an approved air supply or self-contained breathing apparatus should be used by any personnel entering a digester where gas is present or where oxygen is deficient. Two people must observe the operation.
  • Any tools or equipment should be lowered into the digester. No one should attempt to carry tools down a ladder into a digester. Non-sparking tools should be used when working in or around the digester.
  • After working in or around the digester, all protective equipment should be rinsed off or otherwise cleaned. All personnel should wash thoroughly with disinfectant soap and hot water.

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Last Updated: 9/16/2013 11:26:12 AM
Version 4.0.1