Bioacidification Treatment For Woolscouring Effluent Through Acidithiobacillus Thiooxidans

Woolscouring effluent （WSE） is a refractory wastewater due to its poorbiodegradability. Although solid-liquid separation is recognized as an efficient approach,unusual stabilization caused by strong electrostatic repulsion between the micelles makessoluble and insoluble substances to form a stable emulsion, resulting in WSE difficult to berealized phase separation. It is found that sulfuric acid can demulsify woolscouring effluentto make suspended solid flocculate and then causes the solid-liquid separation. In this study,bioacidification through oxiding sulfur into sulfuric acid by Acidithiobacillus thiooxidanswas used to substitute chemical acid to treat WSE in light of its friendly environmentaleffect.In a preliminary study, it was found that the COD removal efficiency of WSE derivedfrom the sludge flocculation by ferric sulfate was higher than that of aluminum sulfate andferrous sulfate. The addition of 5g/L of ferric sulfate in WSE could remove 59.4% of totalCOD in WSE. The reaction time affected obviously the COD removal efficiency, and about15% of COD was further removed after shaken for 12h at 30℃in a horizontal shaker.Theoptimal pH of WSE for flocculation ranged from 5 to 6. Among chemical agents fortreating WSE, sulfuric acid could destroy readily the scatter state of organic soild in WSE.When the addition of sulfuric acid was more than 0.2%（v/v）, Zeta potential of WSE tendedtowards 0mv, while that of raw WSE was as low as -47.3mv. Besides, suspended solid inWSE exhibited a good dewaterability, as indicated by lower specific resistance rate ofsuspended solid （SS）. The maximal COD removal of 75% was achieved when pH wasreduced to about 3.00.To investigate bioacidification effect on destabilizing the emulsion, batch experimentsin shake flasks were performed over 12 days at 30℃in a horizontal shaker. The resultsshowed that suspended solids in WSE would flocculate and precipitate rapidly with theincrease of incubation time. Especially, when pH of WSE was declined to 3.00, 80% ofwater bound in the emulsion turned into free water. Moreover, solid dispered in wastewater was easily dewatered, as indicated by the decline of its specific resistance from more than9.81×10¹³m/kg to 5.6×10¹¹ m/kg. As a result, organic pollutant was removed with thesettlement of sludge. The maximal COD removal and wool wax removal reached 91.4％and 92.9％, respectively. 25％of COD was biodegraded and residual 66％of COD wasremoved by demulsification and precipitation arisen by bioacidification. Therefore, thedemulsification of bioacidification played an important role in treating the scour effluentemulsion, rather than aerobic biological degradation.The optimum conditions of bioacidification treatment were studied in a series ofcontrolled batch experiments. The results indicated that the optimum growth ofAcidithiobacillus thiooxidans appeared at 30℃and with 0.4-0.8％of sulfur powder asenergy substance. It was unnecessary for exogenous addition of N and P in bioaeidificationsystem because N and P hardly affected the process of bioacidification treatment. Inaddition, the detergent in WSE didn’t harm to the activity of Acidithiobacillus thiooxidans.Pre-acidification had been proved to be helpful in accelerating the process ofbioacidification and reducing HRT. In addition, the HRT of bioacidification treatment withpre-acidification through recycling bioacidified wastewater was shorter than that withpre-acidification through mineral acid. A great deal of Acidithiobacillus thiooxidans andother aerobic microbial contained in the recycled wastewater was responsible for thisphenomenon. It only took 32h to complete bioacidification reaction procedure （pH3.00）under the recycle ratio （bioacidified wastewater/fresh wastewater） of 4, while HRT of thetreatment without pre-acidification was as long as 8 to 9d. The advantage of bioacidifiedwastewater recycle was attributed to create a more satisfactory circumstance forAcidithiobacillus thiooxidans through decline of initial pH, which also shortened the courseof bioacidification treatment. When the initial pH of mixed wastewater was pre-acidified to5.40, the maximal treatment capacity was obtained. Successive 10 batch bioacidificationtreatments through bioacidified wastewater recycle exhibited a higher COD removal ofWSE.In light of successful shaking flask trials, bioacidification treatment of WSE in a 10 Lcontineous stirred tank reactor （CSTR） was also conducted with an aim to validate thefeasibility of bioacidification treatment of WSE in a big reactor instead of flask. It wasfound that when the initial pH of WSE was declined to 5.40 by adjusting with bioacidifiedwastewater, HRT of 33h was gained. 90％of COD was removed after gentle centrifuging （200×g） of treated liquor after treatment. At the same time, the solid in treatedwastewater was easily dewatered by centrifuge. It was found that the increase of solidcontent in WSE was not disrupt bioacidification treatment if the increase of HRT. The betterlinear relationship between solid content and HRT was described as follow: y=6.9×x+15,which was useful to predict the treatment time under a given solid content.The sludge generated in bioacidification treatment of WSE could be reused as organicmanure resource because it contained 46.3％of organic matter, 24.5％of wool, and18.6g/kg of total potassium. While the residue COD in supernate from bioacidificationtreatment was as high as 4,000mg/L, the further treatment was required. The WSEtreatment methods used in the solid-liquid separation of WSE led to different properties ofthe supemate, which was exhibited by the COD removal efficiency through absorption ofactivated carbon. The activated carbon adsorption removal efficiency of COD in thesupernate produced by bioacidification treatment was much higher than that producedthrough chemical acidification or PAM flocculation method. The different content of smallmolecular weight and hydrophobic organic fraction was responsible for the abovephenomoneon. The absorption capacities of activated carbon for COD in differentsupemates derived from bioacidification treatment, chemical acidification and PAMflocculation were 100mg/g, 70mg/g, and 43mg/g, respectively. Furthermore, it was found ina column experiment that the increase of the flow rate of the supemate to be treated wouldresult in a lower COD absorption efficiency by activated carbon column because, at thehigher flow rate, the contact time between activated carbon and wastewater was shortened.At the flow rate of 5mL/min, 15mL/min and 25mL/min, the absorption efficiency ofactivated carbon column was 86％, 73％and 48％, respectively. In order to regenerateactivated carbon column, 1mol/L NaOH was used to elute the COD-saturated the column.The number of regeneration was at most 4 times for activated carbon column in this pattern.It is concluded that the treatment for concentrated WSE through demulsification bybioacidification exhibits the feasibility of engineering application due to its high pollutantremoval and operation stability.