'Sinkair' entrainment aerator entraining headspace air in septage.
Since a balancing facilit y and rising main followed the prearation, it was decided to all ow the third stage of oxidation to occur in the balance facility with dissolved oxyge n carried over from the aeration of the second stage. Aeration of the first and second stages was achieved by contacting the septage with the air in the head space of the covered ta nks using 'Sinkair' entrainment aerators (transfer capacity at standard conditions, 7. 5 kg/ hr) . A controlled concurrent air flow of 120 m 3 / hr was drawn through the head space of first and second stages by a centrifugal fan extracting the foul air and discharging to a caustic scrubber for deodorising. The benefit of staged oxidation can be seen more clea rl y by comparing the upper and lower diagrams of figure 3. H ydraulic detention time is reduced from 10 hours for the single stage to two hours for the three stage and the oxygen transfer required is approximately halved.
H 2 S CONTROL When septage is aerated, a small proportion of the sulphide is stripped from solution as H ,S gas. KLA for H ,S transfer is 72% of
Caustic scrubber and vent stack for foul air disposal. KLA for oxygen transfer(!). On this basis less than IO OJo of H 2 S in
the septage is stripped in th e first stage of ~era ti on at Toukley. By arranging air flow concurrently as shown in Figure 3 most of the stripped H ,S redissolves in the second stage and after two stages only 3.3% of th e sulphide is left in the spent aeration air. A caustic scrubber designed al~ng conventional lines was in-
3 STAGES AS INSTALLED AT TOUKLEY 120M3/hr AIR IN""L-;E""T,---133M3/hr ------i911 5' =20mg/L
APPROX 5OOVPM H2S (0•09 kg/hr) 3RD STAGE
5 ' =omg/1S' =Omg/L H.R.T.= 40min. TO BALANCE - - - - - - - T A N K S & S.T.P.
SULPHIDE LOAD= 2·7 kg/hr S' OXIDATION RATE= 21 mg/L/hr 0 2 REQUIRED FOR S = 2 ·7 kg/hr FOR RESPN.= 0•5 kg/hr TOTAL 3·2 kg/hr VOLUME 90M 3
5'= 1•5mg/1.
S' =6m /L
S' LOAD= 0•8 kg/hr S' O.R.= 7mg/L/hr 02REQD FOR 5 =0•8kg/hr FOR RESPN.= O·S kg/hr TOTAL 1 •3 kg/hr VOLUME 90M 3
S' LOAD= S' O.R;:: 0 2REQD FORS = FOR RESPN.= TOTAL
0 ·2 kg/hr 2mg/L/hr 0 ·2 kg/hr 0·5kg/hr 0·7 kg/hr
VOLUME 90M 3
SINGLE PHASE FOUL AIR
133M 3/hr S'
= 2Omg/L 5' =Omg/L H.R.T.= 1Ohrs VOLUME 1350M 3
S' =Omg/L OXYGEN REQUIRED FOR SULPHIDE 2·7kg/hr FOR RESPIRATION 6•8 kg/hr TOTAL 9·5 kg/hr
Figure 3. Comparison of 3 stage a nd single sta ge sulphide oxidation Sulphide in - 20 mg/ L. Sulphide out - 0 mg/ L. (Worst possible instance - Toukley) 28
WAT E R June, 1986