| Membrane bioreactors (MBR), known as a combination of the conventionalactivated sludge process and the membrane filtration, have been increasingly used inmunicipal and industrial wastewater treatment because of its excellent solid-liquidseparation efficiency, high quality effluent, small footprint and reduced sludgeproduction. The wastewater generated from petroleum refinery industries contains alarge number of toxic contaminants from hydrocarbons, aromatic organic compounds toheavy metals, which can result in environmental and human health effects.Treatment of refinery wastewater using submerged MBR has been proved to befeasible. Therefore, this work was to evaluate the influence of different PAC dosageson the pollutants removal efficiencies of refinery wastewater and membrane foulingmitigation potential in membrane bioreactors. The effect of different powderedactivated carbon (PAC) dosages (0,1,2and5g/L) on the refinery wastewater treatmentin bench scale (1L) membrane bioreactors was examined. The reactors were run forapproximately130days at a sludge retention time (SRT) of25d and hydraulic retentiontime (HRT) of24h. The removal efficiency of COD increased with PAC dosageincreasing. Microtox tests showed that effluent quality with respect to toxicity wasbetter in all the MBRs with PAC addition than the MBR operated without PAC.The short term membrane filtration tests indicate that the addition of PACdecreased the membrane fouling resistance. Sludge volume index (SVI) decreasingwith the increasing PAC dosage might contribute to the improvement of foulingpropensity during MBR processes. Additionally, in order to understand the mechanismsof COD removal in the system, the fate of COD was determined using a mass balancemodel. The results indicate that biodegradation was main mechanism in reactorswithout or with low dosages of PAC (1and2g/L). In contrast, at a higher dosage ofPAC (5g/L), the influent COD was removed by the combined mechanisms ofdegradation by biomass and adsorption by PAC.In order to reduce external energy usage and even to become energy self-sufficient, organic carbons in wastewater become a source of energy to be extracted and convertedinto biomass. They can be digested to produce methane gas or incinerated to release theenergy originally present in wastewater, saving not only on energy and aeration costsbut providing ecological benefits. In this case, extremely short SRT and HRT areneeded. However, limited works were conducted to study the performance efficienciesof MBR at the conditions of short SRT and HRT. In the present work, thecharacteristics of activated sludge and membrane fouling potential were investigatedunder the short SRT of2days and HRT of1hour. Results show the MBR was capableof achieving excellent quality effluent at the extremely short SRT and HRT. All theCOD removal efficiencies were more than80%during the MBR operation. It wasfound that the reduction of HRT took more effects on sludge properties, includingmixed liquor suspended sludge concentrations, sludge settle ability, the fractions andconcentrations of soluble microbial products and membrane performance. Thetransmembrane pressure (TMP) increased significantly with HRT decreasing. Therefore,the feasibility on municipal wastewater treatment by MBR under the conditions ofextremely short HRT is still a problem, though shorter HRT may produce morebiomass.The standard curve method was performed alongside the H-point standard additionmethod of SMP analysis on many different samples including both artificial and realwastewater samples. Similar errors in the standard curve and H-point standard additionanalyses of artificial samples indicate that the matrix effects are lower (or virtuallyeliminated through the use of formula) in the artificial samples. The large difference inconcentration of each SMP in actual wastewater as determined by standard curvemethod compared to the H-point standard addition method indicates a significantamount of matrix interference. This matrix interference may be eliminated through theuse of H-point standard addition and formulae for carbohydrates. The lack of viableresults from the H-point standard addition of protein and humics in the wastewatersamples indicate strong matrix effects which could not be fully corrected by Lowrymethod.In water and wastewater treatment processes, membrane cleaning is of paramount economic and scientific importance and has significant impact on process operations.Chlorine solution is usually employed for the membrane cleaning in MBRs because itscheapness and efficiency. However, the use of chlorine can lead to the formation ofby-products such as THM in presence of organic carbon in solution, which has harmfuleffects on both environment and human health. Hence, hydrogen peroxide, biocatalystagent (BOCs) and chlorine were employed to remove long-term membrane fouling inorder to figure out an alternative cleaning agent in this work.The removal rate of membrane fouling resistance results showed that the cleaningefficiencies of sodium hypochlorite solution were still better than other agents. Thelong-term usage of Chlorine might damage the membrane surface because of the highconcentration, and will pollute the environment. With lower optimal concentration(50-100mg/L), the cleaning effect of hydrogen peroxide was slightly lower than thechlorine, whereas, the product is more friendly with the environment. Many conditionsmay influent the organic matters removal efficiency of hydrogen peroxide, includingagent concentration, pH, cleaning time. The results of this experiment illustrated thatthe best cleaning time was24h and the optimum pH was8.5. The results of infraredspectroscopic analysis (FTIR) indicated that both hydrogen peroxide and chlorineremoved polysaccharides efficiently, which is the main membrane foulant. Using BOCsas cleaning agent, however, seldom reduction of organic matters on membrane surfacewas observed. Therefore, under the optimum cleaning conditions, includingconcentrations, cleaning time and pH value, hydrogen peroxide can be used as aharmless cleaning agent instead of chlorine to remove long-term membrane fouling. |
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