Patent GB-1420421-A (GB)
Title: Nitrification of bod-containing water
IPC: C02F1/52
Filed: 1973-04-30
Abstract: 1420421 Activated sludge process UNION CARBIDE CORP 30 April 1973 [1 May 1972] 20355/73 Heading ClC An activated sludge process comprising two aeration zones in each of which zones sludge is recycled from the settled effluent to the aeration zone and in which a feed stream consisting at least in part of the solids-depleted effluent from the first aeration zone is passed to the second aeration zone is characterized in that: (a) there is passed to the second aeration zone a feed stream containing carbon food (biodegradable carbonaceous material), non-viable material and carbon food - consuming microorganisms amounting to a total BOD 5  content of 20-100 p.p.m. in addition to nitrogen food (biodegradable nitrogenous material); (b) both aerations are carried out in confined zones with a feed gas containing at least 50% (by volume) oxygen; (c) the oxygenating gas and the liquor in the second aeration zone are mixed and recirculated against each other during a liquor residence time in said zone of 30 to 240 minutes such that the oxygenated liquor has: (1) a volatile suspended solids (MLVSS) population comprising both 2-40% nitrogen food-consuming microorganisms and 98-60% carbon food-consuming microorganisms plus non-viable material; (2) a suspended solids content (MLSS) of 2000- 10,000 p.p.m.; and (3) an MLVSS/MLSS ratio of at least 0À4 : 1; (d) the dissolved oxygen content (DO) of the liquor in second aeration zone is maintained at least at 2 p.p.m.; (e) the carbon food, non-viable material and carbon food-consuming microorganisms/biomass ratio in the second aeration zone is maintained at 0À03-0À60 kg. BOD 5 /day Î [kg. volatile suspended solids; and (f) oxygen-depleted gas is discharged from the second aeration zone with an oxygen content of at least 20% (by volume). Referring to Fig. 2, in which the feed stream to the second zone consists entirely of the solidsdepleted effluent from the first zone; waste water, for example municipal sewage or waste from petrochemical or paper plants, enters chamber 10 through conduit 11 where it is agitated together with activated sludge recirculated through conduit 15 by agitation means 16. Oxygenating gas enters through conduit 12 and some of the oxygenating gas disengaged from the liquor body is withdrawn through conduit 19, compressed and returned to submerged sparger or diffuser 22 beneath agitator 16, thus being recirculated in contact with the liquor body. Oxygen-depleted gas is withdrawn through conduit 23. Effluent liquor leaves chamber 10 via overflow weir 25 and conduit 27 and enters clarifier 29. After settling, sludge is withdrawn and recycled to chamber 10 and solids-depleted effluent leaves via overflow weir 32 and conduit 34 and enters aeration chamber 110 and subsequently clarifier 129, the operation of which is similar to that of zone 10 and clarifier 29. Purified effluent leaves clarifier 129 via conduit 140. In other embodiments (not shown): (a) diffuser 22 is replaced by surface-type impellers which throw sheets of liquid into the gas space for recirculation against the gas; (b) the aeration zones are divided into a number of sub-zones having interconnecting passages for both gas and liquor; and (c) the feed stream to the second aeration zone is composed only partly of the solids-depleted effluent from the first zone, the remainder of the stream being a secondary BOD 5  source which may be (i) waste water by-passed around the first zone, (ii) sludge settled from the first zone effluent or (iii) liquor from the first zone.