Tannic Acid-based Conditioning Technology For Improving Sludge Dewaterability And Mechanism Insight

Dewatering treatment is an essential section in the process of sludge treatment and disposal.However,due to the characteristics of high organic matter content,fine particles,and strong hydrophilicity,the dewaterability of sludge produced by municipal sewage treatment plants is very poor.After traditional conditioning and mechanical dewatering,the moisture of sludge cake is still as high as 80%.Therefore,how to enhance sludge dewaterability has become one of the research issues in the sludge treatment and disposal field in recent years.In order to enhance sludge dewatering performance,a new sludge conditioner and its mechanism were studied in this paper.Tannic acid(TA),a secondary metabolite of plants,was used as a basic conditioner here.Based on its characteristics of complexing and precipitating protein and reducing metal ions,different conditioning systems were constructed.The effects of various factors on sludge dewatering performance and biochemical properties were explored.The appropriate process parameters of the conditioning systems were discussed,and the mechanism of improving sludge dewatering performance by different conditioning systems was deeply investigated.The main results are as follows:(1)The effect of TA conditioning on sludge dewatering was studied.The results showed that TA was an effective sludge conditioner.The optimum dosage of TA was 0.15 mmol/g TS(total solid).Under the conditions of stirring speed of 200 r/min for 1 min and settling time of 15min.Capillary suction time(CST),specific resistance of filtration(SRF),and water content(Wc)of dewatered sludge cake were reduced by 81.3%,87.5%,and 13.4%,respectively.The analysis results showed that TA combined with extracellular protein and protein-like substances of sludge and settled,thus reducing the supernatant viscosity.In addition,TA also had bridging and netting effects,which could make the sludge particle size larger.These changes of sludge properties could enhance sludge dewaterability.(2)The effect of temperature on TA conditioning was studied.The results indicated that in the range of 25-55°C,the effect of TA on sludge conditioning was obviously enhanced.The optimal conditioning temperature was 55°C,and CST,SRF,and Wc were reduced by 84.5%,96.5%,and 19.9%,respectively.From the perspective of mechanism,the regulation of temperature can improve the efficiency of TA conditioning to remove extracellular protein,especially in slime and loosely bound layers,reducing the supernatant viscosity;the main interaction between TA and extracellular polymeric substances(EPS)was hydrophobic binding,which was enhanced with the increase of temperature.When the temperature exceeds 55°C,the thermal effect will increase the release of the biopolymers to the extracellular space,which weakens the effect of TA conditioning.(3)The relationship between p H and conditioned sludge by TA was studied.The results presented that at the optimal p H value of 4.0,CST,SRF,and Wc of TA treated sludge were reduced by 86.8%,96.3%,and23.6%,respectively.From the perspective of mechanism,the decrease of p H can promote TA conditioning to remove extracellular protein,decrease the supernatant viscosity,and increase of sludge particle size;in the acidic range,the increase of p H increases the binding sites between EPS and TA,and the removal of extracellular protein depended on the synergistic effect of TA and p H.Compared with TA conditioning alone and TA conditioning under temperature regulation,the Wc of sludge cake obtained by TA conditioning combined with p H regulation is lower.(4)The effect of TA conditioning enhanced by Fe³⁺on sludge was studied.The results showed that the optimum dosage of Fe³⁺was 0.9mmol/g TS.Under this condition,TA conditioning could further reduce22.2%CST,5.45%SRF,and 5.27%Wc in comparison with TA conditioning alone.Fe³⁺could strengthen TA conditioning to remove extracellular protein and reduce supernatant viscosity;iron ion can promote TA conditioning to increase sludge Zeta potential,enhance sludge flocculation effect,and enlarge sludge particle size;Fe³⁺reduces the binding sites between EPS and TA,while TA-Fe³⁺complex may promote the settlement and compression of TA aggregates,promote the release of water in the crosslinked biopolymer networks of sludge,make the surface morphology of sludge flocs became smoother and denser.It is a feasible method to enhance the effect of TA conditioning by using complementary cations and TA.(5)The effect of the coupled process of TA and Fe²⁺/persulfate on sludge dewatering was studied.The results showed that the addition of TA with molar ratio of TA/Fe²⁺of 0.25 into Fe²⁺(0.3 mmol/g TS)/persulfate(0.6 mmol/g TS)system can further reduce 61.5%CST,35.3%SRF and 6.4%Wc than Fe²⁺/persulfate conditioning.The increase of temperature will promote EPS solubilization and weaken the effect of the coupled process on sludge conditioning;acid p H can enhance the effect of the coupled process of TA and Fe²⁺/persulfate on sludge conditioning,but the final sludge p H will be too low,which is not conducive to subsequent disposal.Through Box-Behnken design method,the functional relationship model between conditioning parameters of the coupled process of TA and Fe²⁺/persulfate and sludge dewaterability was established.The optimized dosages were 0.43 mmol/g TS SPS,0.58 mmol/g TS Fe²⁺,and 0.14mmol/g TS TA.Under the above dosages,the Wc of sludge cake obtained by small-scale filter press is 62.1%,which is close to the effect of sludge deep-dewatering.(6)The mechanism of the coupled process of TA and Fe²⁺/persulfate on sludge conditioning for improving sludge dewatering was studied.The results showed that the coupled process of TA and Fe²⁺/persulfate enhanced the removal of EPS and protein-like substances,and further reduce the viscosity of sludge supernatant.The coupled process effectively deconstructed the main chain of EPS protein-peptide chains,cracked the secondary structure of EPS protein,changed the spatial conformation of the protein,and reduced the hydrophilic functional groups on the surface of sludge flocs.On the one hand,TA promoted the regeneration of Fe²⁺,enhanced the activation effect of persulfate,strengthened the cracking of sludge flocs and cell lysis,and facilitated the release of bound water in sludge gel-like structure.On the other hand,TA also enhanced the flocculation performance of the coupled process,increased the sludge particle size,and made the microstructure of sludge flocs denser;oxidation and re-flocculation were the main reaction mechanisms of the coupled process of TA and Fe²⁺/persulfate on sludge conditioning for the improvement of sludge dewaterability.Compared with direct flocculation,oxidation decomposition of sludge and re-flocculation conditioning had significant advantages,and the Wc of the sludge cake obtained after coupled conditioning and pressure filtration was lower.In order to satisfy the high standard of the sludge dewatering effect,the conditioning strategy of“pre-oxidation and re-flocculation”can be formulated in the future sludge dewatering treatment.