Water Journal : Water Journal March 2011
refereed paper sewer processes water MARCH 2011 85 (pink). Here, similar days refer to days that had less than 3mm rain and an average temperature within 1.5°C of the post-wash samples. Note that this combined data is expressed as the mean values on a 24- hour time axis to allow for appreciation of diurnal patterns. Optimised Caustic Wash: pH profile The dosing was successfully increased to pH 11.5, while still maintaining the Malabar WWTP outfall pH below the licence limit of 8.5 (Figure 6). While the initial pH in the gravity main was higher for this stronger wash, it is evident that the dilutions and reactions returned the pH to acceptable levels by the time the flow reached Malabar WWTP. The Caustic Wash was repeated twice over summer, using this increased dose pH to maximise biofilm removal, while still complying with WWTP licensing conditions and minimising the risk of ammonia release in the networks. Due to the stringent control plan and strategic timing, there were no detrimental effects to any assets or stakeholders or the environment, either during or after any of the Caustic Washes. At the receiving Malabar WWTP the alkaline flow did not cause any processing problems. Optimised Caustic Wash: H2S profile The greater caustic dose and resulting elevated pH removed a significant portion of biofilm, reducing gas levels far more effectively. The wash yielded an initial reduction of up to 85% of pre-wash H2S levels (Figure 7), which then hovered around a 70% reduction for a further 20 days, until the LAP had to be shut down for other operational reasons. Note again that this data is expressed as the mean values on a 24-hour time axis. Both the magnitude and duration of the effect was improved significantly with this higher pH dose, as was suggested by the initial literature review. For this system, the ideal dose limit was found to attain pH 11.5, maximising the result while complying with the effluent licence regulations and not releasing ammonia in the network. At this optimal target pH, the calculated dose of caustic required was approximately 5,500L at 50% w/w concentration; however, the actual required quantity was found to vary significantly depending on the quality of the effluent on the day. Figure 5: Hydrogen sulfide gas after the initial Caustic Wash. Figure 6: pH in the network for optimised Caustic Wash. Figure 7: Hydrogen sulfide gas after the pH 11.5 Caustic Wash. A comparative trend is shown in Figure 8, illustrating Washes 1, 2 and 3, with dosing pHs of 10.7, 11.25 and 11.51 respectively.
Water Journal April 2011