Heavy Metals Removal From Anaerobically Digested Sludge

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ISBN 13 :
Total Pages : 122 pages
Book Rating : 4.:/5 (655 download)

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Download or read book Effect of Acid and Base Pretreatment on the Anaerobic Digestion of Excess Municipal Sludge written by and published by . This book was released on 2005 with total page 122 pages. Available in PDF, EPUB and Kindle. Book excerpt: The treatment and disposal of sewage sludge is a growing problem worldwide due to the continuing increase in sludge production from biological wastewater treatment plants and recent stringent regulations regarding sludge treatment and disposal. To date, the primary disposal methods for the excess sludge produced are landfilling, incineration and application in agricultural or forestland [1]. The high water content of the sludge makes incineration expensive and faces restrictions on disposal in modern landfills due to the tendency to reduce the amount of organic matter applied. Agricultural use seems to be the best options but several constraints exist due to health related problems such as odors, heavy metals content or pathogens and hazardous compounds that may be present in the sludge preventing their use in farming [2]. In this context, anaerobic sludge digestion represents a good sludge treatment alternative, decreasing the amount of solids present in the sludge with consequent reductions in disposal cost. Furthermore, we have to consider the advantage represented by energy recovery in the form of methane production that can be utilized as an energy source at the facility [3]. The present research project evaluates the combined use of hydrochloric acid (HCl) and sodium hydroxide (NaOH) for sludge chemical pretreatment to improve the performance of anaerobic digestion by extending the effectiveness of solids hydrolysis which is considered the rate-limiting step in the process [4]. The effect of temperature and the length of pre-treatment were initially evaluated to optimize the pretreatment process prior to use of any anaerobic treatment. Once the results of this experiment are analyzed, the performance of anaerobic digestion with chemical pretreatment using different combinations of primary and waste activated sludge pretreatment with either acid or base was studied. Two different SRTs for anaerobic digestion, 10 and 20 days, were investigated using duplicate reactors for each condition. Two reactors (number 1 and 6), used as control, were fed with a mixture of untreated primary and waste activated sludge in a mass ratio 1:1 with a combined VSS concentration around 30 g/l. Two reactors (number 2 and 4) were fed with a mixture of primary sludge pretreated with HCl and activated sludge pretreated with NaOH. The last two reactors (number 3 and 5) were fed with a mixture of primary sludge treated with NaOH and activated sludge treated with HCl. The same primary and waste activated sludge mass ratio as the one used to feed the control reactors were maintained in the four reactors fed with the chemically pretreated sludge. Monitored parameters were: TSS, VSS, total and soluble COD, Total Kjheldahl Nitrogen (TKN), Ammonia, Volatile Fatty Acids (VFA), pH, Phosphates, metals concentration, fecal coliforms concentration and gas production and composition. As expected, the results showed that 20 days SRT provided higher VSS and COD removal than 10 days SRT for all the three configurations evaluated. For both SRTs, the mixture of primary sludge treated with NaOH and activated sludge treated with HCl resulted in higher VSS and COD removal. VSS removal averaged from 37.2% in reactor 1 and 38.6% in reactor 6 for the 10 days SRT. Increasing the SRT to 20 days led to 46.6% and 44% VSS removal for reactor 1 and 6, respectively. Reactors 2 and 4 led to an overall average VSS removal of 44% for the 10 days SRT experiment which increased to around 50% in both reactors during the 20 days SRT experiment. For both SRTs, reactors 3 and 5 provided the highest average solids reduction with values around 46% for 10 days treatment and close to 53% for 20 days SRT (Chapter 7.2.1). It has to be considered that, in terms of maximum solids destruction, for the 10 days SRT experiment, a highest solids removal value of 47.8% was achieved in reactor 3 while 56.7% removal was obtained in reactor 5 when the SRT increased to 20 days. A similar trend was obtained when the COD removal has been considered. The two control reactors, 1 and 6, averaged from the 37.2% and 38.1% removal for the 10 days SRT experiment to values around 45% when the SRT was increased to 20 days. An overall average COD removal of 44.5 and 43.2% was achieved in reactors 2 and 4, respectively, for the 10 days SRT experiment and increased to around 47% in both reactors when the SRT was increased to 20 days. For both SRTs, reactors 3 and 5 provided the highest average COD removal with values around 45% for 10 days treatment experiment and close to 51% for 20 days SRT. For both retention times, all reactors exhibited good reproducibility in the replicates. Volatile fatty acids (VFAs) concentration was very low for both 10 and 20 days SRT in all reactors and weekly gas analysis exhibited a methane content of approximately 65%, meaning good overall performance of anaerobic digestion. The research proposed showed that anaerobic sludge digestion can be successfully used as sludge treatment alternative to reduce the amount of solids present in the sludge, with consequent reductions in disposal costs. Furthermore, compared with other sludge pretreatment options, the combined use of strong acid and strong base as chemical sludge pretreatment resulted in a good balance between solids reduction, process stability and cost reduction.