| The amount of municipal sewage sludge produced increased annually,and high water content of sludge seriously restricted the subsequent disposal of sludge.Deeply dewatering technologies with the purpose of decreasing the water content of sludge have been focused on by many researchers around the world.Bioleaching technology deriving from microbial hydrometallurgy is a microbial method for treating sewage sludge with the aims of both removing sludge-borne heavy metals and enhancing the sludge dewaterability.Without the addition of any conditioner,the water content of sludge could be decreased to 60%by using bioleaching conditioning and chamber filter press.However,the presence of dissolved organic matters(DOM)in sludge seriously restricts the activities ofAcidithiobacillus species,thus handicapping the bioleaching of sludge.Although the addition of acid-tolerant yeast strains into sludge bioleaching could alleviat the toxicity of sludge DOM to Acidithiobacillus species through degrading sludge DOM,thus enhancing the bioleaching performance,the extremely acid circumstance in bioleaching system was not suitable for the growth of yeast strains.The amount of yeast strains was dramatically decreased after sludge bioleaching,which cannot effectively support the long-term operation of sludge bioleaching.On the contrary,an increase in total fungi in sludge was coincided with the degradation of organic acids during sludge bioleaching,indicating that fungi are able to grow well in bioleaching circumstance.Filamentous fungi in sludge can increase their biomasses by assimilating soluble and insoluble substances in sludge.Meanwhile,some filamentous fungi in sludge could entrap the solids particles from sludge and compress the sludge with their filamentous(hyphae)mycelia.But the solid content of sludge that can be treated by isolated filamentous fungi was as low as 1%,resulting in low efficiency of sludge treatment.Generally,the solid content of thicken sludge produced in wastewater treatment plants in our country was 2%-4%.To treat the sludge by using filamentous fungi,it is necessary to dilute the primary sludge with water to a desired solid content,about 1%.However,such dilution process will not only burden the entire project,but also restrain long-term operation.Therefor,in the present study,filamentous fungi which can improve the dewaterability of sludge with high solid content(3%-4%)was isolated,and the mechcisms responsible for enhancement of dewaterability of sludge treated by filamentous fungi were investigated,and the effect of combined bioleaching system driven by both filamentous fungi and Acidithiobacillus species on sludge dewaterability was explored,and then theoretical and technical support were proposed for efficient and low cost treatment.The main conclusions are as follows:(1)27 fungal strains were isolated from sludge,and seven fungal strains capable of promoting the dewaterability of sludge with high solid content were screened out according to their ability in enhancing sludge dewaterability.However,their optimum growth conditions were totally different.It was found that that all these seven fungal strains can grow well in weakly acid circumstance,but not survive in extremely acid circumstance(pH below 2).Only the filamentous fungus strain 1 can grow in the pH range of aerobic digestion of sludge.Consequentially,filamentous fungus strain 1 can grow well in sludge during fungal treatment.Importantly,most isolated filamentous fungi formed fungal pellets during their cultivation in pure culture,but the mycelia of filamentous fungus strain 1 dispersed evenly to entrap sludge particles.Therefore,filamentous fungus strain 1 was selected out as the most suitbale filamentous fungus for enhancing sludge dewaterability.This filamentous fungus was identified as Mucor through morphological identification and analysis of ITS rRNA sequences and named as Mucor sp.ZG-3.(2)During fungal treatment of municipal sewage sludge using Mucor sp.ZG-3,the dewaterability enhancement of treated sludge was influenced by the inoculation method and inoculum size of Mucor sp.ZG-3,and the solid content of sludge.It was found that the optimum inoculation method was mycelia inoculation and the optimum inoculum size was 10%.However,compared with the control treatment,the dewaterability enhancement of fungal treatment was not discrepant when the inoculum size was less than 10%.When the inoculum size was higher than 10%,the dewaterability of treated sludge even deteriorated,comparing to the original sludge.The performance of fungal treatment on enhancing sludge dewaterability was much more effective in treating sludge with high solid content than sludge with low solid content.The optimum solid content in sludge for fungal treatment was about 4%.Under the optimized conditions,the specific resistance to filtration(SRF)of sludge can be decreased by 75.1%after being treated using Mucor sp.ZG-3 and the dewaterability of treated sludge was obviously improved.Meanwhile,the COD value of sludge supernatant was decreased,and the treated sludge still exhibited good settleability.(3)The sludge dewaterability was improved during fungal treatment through degrading sludge EPS and capturing sludge particles by Mucor sp.ZG-3.It was found that the content of slime EPS or polysaccharide content in slime EPS significantly influenced the dewaterability of sludge.The degradation of slime EPS and polysaccharide in slime EPS by Mucor sp.ZG-3 contributed on the improvement of sludge dewaterability during the fungal treatment of sludge using Mucor sp.ZG-3.Moreover,Mucor sp.ZG-3 was useful in entrapping small sludge particles by its mycelia to form much bigger sludge flocs.Sludge SRF was negatively correlated with the percentage content of sludge flocs with the size of larger than 100 ?m.The destruction degree of sludge flocs larger than 100 ?m was decreased from 50%to 20%after the addition of Mucor sp.ZG-3,and the inhibition of the destruction of sludge flocs larger than 100 ?m is helpful for the sludge dewaterability enhancement.(4)Sludge contains many organic matters,such as DOM.Mucor sp.ZG-3 can assimilate sludge DOM to promote its growth.More than 65.2%of sludge DOM was degraded after 3 days of fungal treatment.Among them,L-DOM,which is extremely toxic to A.ferrooxidans LX5,has a 76.2%of degradation,and H-DOM,which is not toxic to A.ferrooxidans LX5,also had some reduction.Meanwhile,low molecular weight organic acids in sludge DOM were also degraded.Formic acid and acetic acid,which have toxicity to Acidithiobacillus species,were completely assimilated by Mucor sp.ZG-3,and the concentration of propionic acid that is weakly toxic to Acidithiobacillus species was dramatically decreased by up to 52.5%.As a result,the toxicity of treated sludge DOM to A.ferrooxidans LX5 has been obviously allerviated.The oxidization efficiency of Fe2+by A.ferrooxidans LX5 reached 90%in only 4 days in the medium with treated sludge DOM.Therefore,filamentous fungus Mucor sp.ZG-3 can be used as a DOM degrader to alleviate the toxicity of DOM to Acidithiobacillus species,thus improving the activities of Acidithiobacillus species.(5)The sludge dewaterability was improved by the individual inoculation of either A.ferrooxidans LX5 or Mucor sp.ZG-3.However,the simultaneous inoculation of both A.ferrooxidans LX5 and Mucor sp.ZG-3 did not further improve the enhancement extent of sludge dewaterability,compared to their individual inoculation,because of the restricted activities of these two microorganisms.When filamentous fungus Mucor sp.ZG-3 and A.ferrooxidans LX5 were sequentially inoculated into sludge at day 0 and the end of day 1 of treatment,respectively,the sludge specific resistance(SRF)achieved was decreased to 1.00×1012 m/kg,which was 25.9%lower than that achieved by conventional bioleaching driven by A.ferrooxidans LX5 alone.Mucor sp.ZG-3 firstly improved sludge dewaterability and alleviated the toxicity of sludge DOM through degrading sludge DOM,which would create a preferential circumstance for the growth of LX5,and then A.ferrooxidans LX5 could achieve the highest activity to further improve the sludge dewaterability.Filamentous fungus Mucor sp.ZG-3 is advantageous over Rhodotorula mucilaginosa R30 in enhancing sludge dewaterability during sludge bioleaching with the sequential inoculation.(6)During the sludge bioleaching with the sequential inoculation of Mucor sp.ZG-3 and A.ferrooxidans LX5,the amount of energy source was a rate limiting step in the overall bioleaching process.With the increase of energy source addition,the sludge dewaterability was gradually improved.The addition of 4 g/L energy source was found to be optimum for enhancing sludge dewaterability.Under the optimum concentration,sludge SRF was decreased by 96.9%to 2.600×1011 m/kg,which was still 40.0%lower than that in conventional bioleaching process with the inoculation of A.ferrooxidans LX5 alone and the addition of 10 g/L energy source.However,the sludge dewaterability was not further improved but deteriorated,with a further increase in energy source addition beyond 4 g/L.Therefore,the inoculation of filamentous fungus Mucor sp.ZG-3 into sludge bioleaching will not only improve sludge dewaterability,faciliting the subsequent disposal of dewatered sludge cake,but also lower the expense of sludge treatment through reducing the addition of energy source,thus providing a novel technology to deeply dewater sewage sludge. |
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