Formation And Control Of Disinfection By-products During Municipal Wastewater Treatment And Resue By Membrane Bioreactors

Water scarity is one of the most important factors that influence the global economical development. It has been the top three problems that human society have to overcome in the future 50 years. Municipal wastewater reclamation and resue is an efficient way to improve the water resource usage, which can not only reduce the water pollution problem but also relieve the water scarity issue. However, monitor and control of healthy and ecological risks during reclaimed water reuse is important. In this study, membrane bioreactor (MBR) was used for municipal wastewater treatment and reuse. The MBR performance in municipal wasterwater treatment, formation characterisitics of disinfection by-products (DBPs) during chlorine disinfection of MBR effluents were studied. The effects of disinfection conditions and MBR effluent dissolved organic matter (DOM) characteristics on DBP formation and speciation were also investigated. To investigate the influences of microbial metabolic characterisics in MBR system on effluent DBP formation, two different soluble microbial products (SMP) released at different microorganism growth phases: biomass associated products (BAP) and utilization associated products (UAP) were separately produced to investigate their DBP formation during chlorination. Formation of various carbonaceous and nitrogenous DBPs during chlorine and monochloramine disinfection of recycled municipal wastewater produced by MBR was studied and quantified using differential ultraviolet (UV) absorbance and fluorescence spectral parameters. Formation mechanisms of halogenated DBPs were proposed. Powdered activated carbon (PAC) was added to the MBR to reduce the DBPs formation. Effects of PAC addition on MBR performance, MBR effluent DOM properties and DBP formation charaterisics were investigated. The main results were summarized as follows:1. MBR was efficient in removal of suspended solids and biodegradable organic matters. High quality reclaimed water that meet with various reuse standsrds can be produced though the MBR system. Compared with the raw municipal wastewater, MBR effluent DOM had more high molecular weight (MW) and hydrophobic acid components. The percentage of DOM with MW>100 kDa was increased from 20% to over 50, and the proportion of hydrophobic acid (HoA) was increased from 25% to over 40%. In addition, the MBR effluent had high trihalomethane formation potential (THMFP). Particularly, MBR effluent had high formation potential of brominated THMs (Br-THMs), which might be due to the high level of Br in MBR effluents. Increasing chlorine dose and reaction time resulted in higher THM yields, which increased the formation of chlorinated THMs (Cl-THMs), but decreased the yield of Br-THMs.2. MBR effluent DOM were mostly composed of high MW fractions, more than 50% of the DOM exhibited MW higher than 30 kDa. The high MW DOM exhibited higher SUVA than low MW components. About 52% of the DOM in MBR effluents were hydrophilic substances (HiS). HoA were the primary hydrophobic components in MBR effluent DOM, accoutting for 38% of the total MBR effluent DOM. Hydrophobic components exhibited higher SUVA than HiS. Particularly, HoA had the highest SUVA value, which was about 2.75 L/mg·m. Chlorine reactivity of DOM decreased with the decrease of MW, and MW> 30 kDa fractions produced over 55% of total THMs in chlorinated MBR effluent. Hydrophobic organics had much higher THMs formation reactivity than HiS. Particularly, HoA exhibited the highest chlorine reactivity and contribu ted up to 71% of total THMs formation. Meanwhile, low-MW and hydrophilic DOM were susceptible to produc e Br-THMs. Of the fluorescent DOM in MBR effluent, aromatic moieties and humic acid-like had higher THM formation reactivity; while soluble microbial product-like materials and fulvic acid-like materials had high bromine incorporation ability. The fluorescence intensity of humic acid-like materials had a positively linear correlation with THM formation reactivity; while that of soluble microbial product-like had a negative linear correlation with THM formation reactivity.3. The microorganisms at endogenous phase produced more SMP. Compared with utilization associated products (UAP) that were derived from the biodegradation of original substrate in microbial growth phase, the biomass associated products (BAP) that were released during biomass decay in the endogenous phase exhibited lower SUVA value. High MW biopolymers were the primary components of both BAP and UAP; however compared with BAP, the average MW of UAP was lower, which had components with MW less than 500 Da. UAP was considered to contain more proteins and aromatic polypeptides and amino acids; while BAP contained more polysaccharides and humic substances including fulvic acid-like and humic acid-like materials. Aliphatic primary amines were found in BAP, while aromatic secondary amines were found in UAP. BAP had higher formation reactivity of halogenated carbonaceous and nitrogen ous DBPs including THMs, haloketones (HKs), haloacetonitriles (HANs) and trichloronitromethane (TCNM). Particularly, the trichloroacetonitrile (TCAN) yield of UAP was about 276 times higher than BAP. However, the nonhalogenated species N-nitrosodimethylamine (NDMA) yield of UAP was twice higher than that of BAP.4. Chlorination resulted in extremely high yields of THMs and HANs. Monochloramine disinfection decreased total halogenated DBP yields by one order of magnitude, while doubled the NDMA formation. THMs were the prmairy DBP species after chlorination and chloramination. Extending reaction time increased DBP formation.The differential log-transformed absorbance theory could be applied to quantify the disinfection processes of biologically treated wastewater, which was available for both chlorination and chloramination. The differential log-transformed UV spectral parameters including DLnA350 (differential logarithm of absorbance at 350 nm) and DSloop300-340 (slope differential of the log-transformed spectra at wavelength range 300-340 nm) had strong exponential correlations to total THMs (THM4) formation and linearly correlated to total HANs (HAN4) yield during disinfection of MBR effluents. The two spectroscopic parameters(DLnA350 and DSloop300-340) that were related to halogenated DBP yields involved two independent wavelength ranges, meaning that DBPs were derived from two types of DOM in MBR treated municipal wastewater.The EEM fluorescence intensities decreased after chlorine and monochloramine disinfection. The differential Excitation/Emission Matrix (EEM) fluorescence spectral parameters including DI(λ280λ310) (differential fluorescence intensity for proteins) and DΦi,n (i=T,Ⅱ and Ⅲ; differential of integrated EEM intensities for gross DOM, humic acid-like and fulvic acid-like components) had strong exponential correlations to total THMs (THM4) formation and linearly correlated to total HANs (HAN4) yield during disinfection of MBR effluents.Based on the results, it can be proposed that aromatic proteins and humic substances were the primary DBP precursors in MBR effluents, which can produce DBPs through multiple-step complex reactions. At the initial period of halogenation reaction, no DBPs were formed. And HANs were generated before THMs.However no quantitative correlation was found between NDMA yields and the studied UV absorbance/EEM fluorescence spectroscopic parameters.5. PAC addition enhanced the MBR performance for municipal wastewater treatment, which improved the MBR effluent water quality and decreased the DBP precursors in MBR effluents. PAC addition improved the aggregation of sludge congeries and alleviated membrane fouling in MBR system. Hydrophobic DOM especially hydrophobic acids in PAC/MBR effluent (50%) were more than MBR effluent (42%). DOM with molecular weight＜1 kDa constituted 12% of PAC/MBR effluent DOM, which was less than that of MBR effluent (16%). Data obtained from excitation and emission matrix fluorescence spectroscopy revealed that PAC/MBR effluent DOM contained more simple aromatic protein, but had less fulvic acid-like and soluble microbial by-product-like.PAC addition had no significant influence on THMFP, but greatly reduced the formation of bromine-containing THMs during chlorination of effluents. PAC also increased THM formation reactivity of effluent DOM.52% of the THM precursors in the MBR effluent were rapid reaction reagent. PAC addition significantly reduced the rapid reaction reagent,82% of the THM precursors in PAC/MBR effluent were slow reacting substances, which resulted in a slow formation rate of THMs during chlorination of MBR effluents. Within 24 h chlorination, THM formation in PAC/MBR effluent was less than MBR effluent.