Water Journal : Water Journal March 2011
refereed paper sewer processes water MARCH 2011 89 at the WWTP inlet (Figure 1). The results show that the proposed dosing of oxygen, nitrate and ferric chloride was effective in controlling the dissolved sulfide levels to an average of 1mg S/L. With the dosing of magnesium hydroxide, the average hydrogen ion concentration at the WWTP inlet could be raised 3.3 x 10-9, which gives a pH of 8.5. As discussed earlier, some changes in the wastewater composition critical to N and P removal in WWTP were observed (Figure 2). Oxygen and nitrate both oxidise the hydrogen sulfide produced in sewer biofilm. Sulfide is oxidised both chemically and biologically with oxygen, while nitrate can only oxidise it biologically. The rate of sulfide oxidation with nitrate is thus lower than that with oxygen, especially in large pipes where chemical oxidation plays a major role. Consequently, nitrate requires a longer contact time for complete sulfide oxidation as compared to oxygen. In addition to the oxidation, both oxygen and nitrate promote heterotrophic activity in biofilm, thereby oxidising a significant amount of organic matter in sewage. The addition of both oxygen and nitrate thus resulted in reduced levels of volatile fatty acids (VFA) in the feed (Figure 2B). The impact was much more pronounced in the case of nitrate than in the case of oxygen. Simulation results revealed that, compared to oxygen, a much larger amount of nitrate (in terms of electron- accepting capacity) was required to achieve the same level of sulfide control, resulting in more consumption of organic carbon through denitrification. This was because of the longer contact time needed for sulfide oxidation as described above. It is worth mentioning here that the amount of VFA consumed would vary depending upon the amount of oxidant used and the location of its dosing. Dosing of ferric salts resulted in hydrogen sulfide precipitation in the form of FeS precipitates (Figure 2C), which would enter the WWTP. Laboratory studies reported in Gutierrez et al. (2010) have shown that a negligible fraction of FeS particles will be retained in the primary settling tank in this case, as the FeCl3 was dosed close to the WWTP inlet, resulting in a short contact time insufficient for FeS colloids to form large-sized particles. Once the FeS particles enter the WWTP, FeS precipitates get oxidised to Fe3+ and SO42- in the aeration tank and the Fe3+ thus formed results in the precipitation of PO43- (Gutierrez et al., 2010). The dosing of magnesium hydroxide caused elevated pH levels (Figure 2D). The IWA Activated Sludge Model No. 2d (ASM2d) (Henze et al. 2000) was Figure 1: Dissolved sulfide levels with the dosing of different chemicals. %#% %#( &#% &#( '#% 0694 !31B" % ( &% &( '% *6==;8@43 /?8C34 !95 /$," )1=486:4 .AB54: 6:742>6;: -6><1>4 3;=6:5 +4<<62 3;=6:5 Figure 2: Changes in wastewater composition due to chemical dosing in sewer (A) Comparison of dissolved H2S levels with various chemical dosing (B) Effects of oxygen/nitrate dosing on VFA (C) Ferric chloride dosing producing FeS (D) Mg(OH)2 dosing elevating pH Baseline data are those predicted by the SeweX model without any chemical dosage. %#% %#* &#% &#* '#% :BE? !><O" + , - . &% H4 0<K?DBF? 6<@F?KBME AO>JGNB>? >GKBF@ # %#% %#* &#% &#* '#% % &% '% (% 3?9 !E@$5" 3?JJB= >GKBF@ " %#% %#* &#% &#* '#% % '% )% +% -% &%% ;3/ !E@ 182$5" 0<K?DBF? 8NO@?F BFC?=LBGF 7BLJ<L? >GKBF@ ! %#% %#* &#% &#* '#% % ) - &' &+ 0<K?DBF? 8NO@?F BFC?=LBGF 7BLJ<L? >GKBF@ 3?JJB= >GKBF@ 6<@F?KBME AO>JGNB>? >GKBF@ 4'9<I !E@ 9$5"
Water Journal April 2011