Improving Fermented Supernatant Quality And Sludge Dewaterability Of Waste Activated Sludge Alkaline Fermentation

The production of volatile fatty acids(VFAs)from waste activated sludge(WAS)fermentation to supply the biological nutrient removal(BNR)system has shown a promising alternative process for WAS management.For that purpose,fermented supernatant from WAS alkaline fermentation(AF)was a proven method.However,besides the advantage,it has shown some drawbacks:high impurities(carbon and nutrients),degrading sludge dewaterability,and a considerable amount of NaOH required.The thesis's main goal was to investigate the prospect of acidifications and magnesium chloride addition during WAS fermentation on those of AF shortcoming.The first experiment steps investigating the uses of MgCl₂ during WAS AF.The results revealed that a higher MgCl₂ concentration inhibited acidogenesis,while the purity of VFAs improved and the maximum percentage of VFAs accounted for 52.92%(w/w)of soluble chemical oxygen demand(SCOD)at 120 mmol/L Mg²⁺.A phosphate removal efficiency of 81.22%was obtained at 15 mmol/L MgCl₂ concentration,while only a 14.77%increase was observed when further MgCl₂ was added(120 mmol/L MgCl₂).Capillary suction time(CST)values decreased sharply from 4410.20 s to207.30 s with the increase of MgCl₂.A similar trend was observed in bound-water,and the minimum value was 85.56±0.06%.The distribution of extracellular polymeric substances(EPS)and rheological profiles of the fermented sludge were also analyzed,and the results confirmed that a high MgCl₂ concentration had a positive effect on sludge dewatering.The subsequent experiment set was dosed 120 mmol/L MgCl₂ to investigate the effects of different methods and times on the performance of WAS AF.The investigation revealed that maximum SCOD was achieved without magnesium addition,indicating magnesium addition,regardless of the dosing method or time,could constrain the sludge hydrolysis process.Fermented sludge dewaterability was improved with magnesium addition,as based on results of CST and bound water.The optimal magnesium dosage was reached at R2,following WAS AF,in which little VFA reduction(67.62 mg COD/L),higher P removal efficiency(95.41%),and lower alkali consumption were achieved.Further,682.19 mg COD/L of the released PN was precipitated,suggesting that soluble PN was effectively recovered from sludge with magnesium addition after fermentation.The follow-up investigation conducted a comprehensive comparison of WAS anaerobic acidic at p H 5.00(R5.0),and alkaline fermentation at pH 10.00(R10.0)for VFAs,biopolymer accumulation,fermented sludge dewaterability,nutrients,and metals release has been evaluated.The results presented that AF was able to accelerate the biopolymer release and was further beneficial to increase VFAs production(2901.33mg COD/L).However,these excessive releases of biopolymers(such as 1708.81 and398.90 mg COD/L of protein/PN and polysaccharide/PS,respectively)increased bio-refractory organic C,nitrogen(N),and phosphorus(P)loading.Total phosphorus(TP)concentration was 117.84±15.07 mg/L in R_10.0,including 89.34±1.49 mg/L of PO₄^3--P and 28.50±6.79 mg/L of organic P(OP).Nevertheless,the TP concentration release was only 1.65±0.03 mg/L at R_5.0,and OP was dominant(1.16±0.02 mg/L;70.33%).Most of the total nitrogen(TN)at R_5.0 was ammonium(98.45%),while 85.32%of TN was ammonium at R_10.0,referring to 58.22 mg/L of N left,which was recognized as dissolved organic nitrogen(DON)matter.Further,Al concentration was released more than other metals at all conditions,and the maximum concentration was reached to134.52 mg/L at R_10.0,2.99 times higher than at R_5.0.The concentration of other metals,such as Ca,Fe,and Mg,were below 50.00 mg/L at all conditions and were released more at R_5.0.The fermented sludge filterability severely deteriorated at R_10.0,which was supported by the results of normalized capillary suction time(n CST)and specific resistance to filtration(SRF).Based on these aspects,VFAs from WAS via acidic fermentation may be an appropriate way to apply as a carbon source for the BNR process directly.A novel strategy of initiating fermentation at a pH of 10.00 was developed to improve VFAs purity,P removal efficiency,and fermented sludge dewaterability during WAS fermentation.Although VFAs concentration was lower(1.69±0.09 g COD/L)when the p H was only initially adjusted to pH 10.00(R_IA)relative to when the pH was maintained at 10.00 on a daily basis(R_DC),the purity of VFAs in the fermented liquid was improved(58.48%).Furthermore,the release of TP in R_IA was 5.90 times lower than that in R_DC(139.37 mg/L).The n CST and SRF in R_IA decreased to 42.23%and40.70%,respectively,suggesting that the dewaterability of fermented sludge also improved.The amount of alkali needed was 17.44 kg for each ton of TS in R_IA,which was 5.49 times lower than in R_DC.Thus,approximately 45.44 USD was saved in operational costs for each ton of TS processed in R_IA.These results indicated that VFAs production via initial p H 10.00 fermentation was a robust and cost-efficient way to apply as carbon resource into WWTP.To sum up,the findings have revealed that both treatments during WAS fermentation have multiple merits and proven to adjust the VFAs purity,nutrient(P)reduction,and enhance the sludge dewaterability.However,during fermentation,the p H treatment is recommended for the lower chemicals(Na OH,MgCl₂)amount needed.