Enhanced Degradation Of PAM In The BES And System Manipulation

The wide application of water-soluble polyacrylamides (PAM) causes serious environmental pollution. Biological treatment of PAM is a kind of economic and environmental method. However, the common aerobic or anaerobic treatments receive very limited efficiency due to the recalcitrance to the microbial degradation. Bioelectrochemical system (BES) is a novel wastewater processing technology which achieves effective wastes removal through the generation of bio-electricity. Based on above, this thesis aims at exploring the feasibility of BES in the treatment of PAM for enhanced degradation efficiency.A linear partially hydrolyzed PAM (HPAM) with viscosity-average molecular weight of 5×106 and hydrolysis degree of 25% was used as the substrate to cultivate electro-active microbial communities for the HPAM degradation. The results showed that the bacteria in the BES could utilize HPAM as the sole carbon and nitrogen source and simultaneously generate electricity. Both the anode-attached and planktonic bacteria contributed to the electricity generation, while the anode-attached community exhibited stronger electron transfer ability than the planktonic one. Electricity generation was gradually enhanced when the concentration of HPAM increased from 250 mg L-1 to 500 mg L-1, with the maximum current density of 110 mA m-2 obtained at 500 mg L-1 of HPAM. However, significantly decreased power outputs were observed when the concentration of HPAM was further increased to 750 mg L-1, implying an inhibiting effect of excessive HPAM on the bacteria which might be attributed to the hindrance of high solution viscosity on the mass transfer between bacterial cells and medium.The closed-circuit operation gradually decreased the concentration of NH4+-N to 0.1 ±0.08 mg L-1. Meanwhile, the COD was removed by 32.5% from 933±12 mg L-1 to 630±4 mg L-1. The viscosity of solution was decreased from 23.01±0.15 mPa s to 21.00±0.10 mPa s. The concentration of NH4+-N in the reactor operated with open circuit was stabilized at ca.1.2 mg L-1 through the whole batch. The viscosity of solution was also decreased to a very slight extent and only 7.4% of COD removal efficiency was achieved with the open-circuit mode. It is obvious the closed-circuit operated BES performed better than its open-circuit counterpart, suggesting the generation of bioelectricity was able to enhance the biodegradation of HPAM in the BES.The molecular structures of HPAM and its degradation products were identified by gel permeation chromatography, nuclear magnetic resonance spectroscopy, X-ray photoelectron spectroscopy and infrared spectroscopy. Structure analysis suggested the bacteria could hydrolyze the -CONH2 side chains to produce NH4+-N which was then utilized as a nitrogen source. The chain of HPAM was partially degraded in the BES, producing polymeric products with lower molecular weight. The degradation of HPAM is not conducted in the way of "chain cleavage", but was a "chain shortening" process accompanied by the formation of ether bond. The two α-[-CH-] in the "head to head" structure were firstly oxidized to [-C-OH], and then the [CH2-C-OH] bonds in the carbon chain were cleaved. Small organic acids were produced which could pass through the bacterial membranes and used as carbon source by the bacteria. Finally, dehydration reaction occurred between the -OH groups on the two [-CH2OH] to form an ether bone in the chain.Polyacrylic acid (PAA) and PAM with different molecular weights were respectively treated in the BES reactor to investigate the influences of side chain group and molecular weight on the degradation of PAM. Very similar degradation efficiencies were observed for the PAA and PAM with molecular weight of 5×106, suggesting the side chain groups is of negligible influence on the degradation abilities of bacteria in the BES. The PAM with molecular weights of 1.0 x 105,1.5 x 106 was also partly degraded in the BES, which follows the same degradation pathway as the PAM with molecular weight of 5×106, in which the the carbon chain was attacked on the "head-to-head" structure.