The Study Of Aerobic Biological Treatment Of Low Concentration CMC Wastewater

CMC, the initials of "Carboxymethyl Cellulose (Sodium)", is a cellulose derivative with carboxymethyl groups (-CH2-COOH) bound to some of the hydroxyl groups. CMC, which is applied as thickening agent, emulsifying agent and so forth in various civil and industrial products, has been the most abundantly produced and most widely applied cellulose products currently and globally.The main chemical constituents of CMC wastewater are:water, sodium glycolate and sodium chloride, and there is a little raw cellulose (and some impurities), sodium hydroxide, ethanol and CMC in it. CMC wastewater is typical high concentration and high salinity organic wastewater, whose COD and salinity can at highest be above80000mg/L and150000mg/L respectively, and at lowest be around20000mg/L and40000mg/L respectively, so it must be effectively treated before being discharged into sewerage.The study of the dissertation originates from a project of aerobic biological treatment of low concentration CMC wastewater, and fouceses on the three serious problems found in the project by analysing on the causes and experimenting on the solutions.Firstly, the dissertation discusses a full scale project:in a specialty chemical factory located in Taixing City, Jiangsu Province, low concentration CMC wastewater, whose COD and salinity was20000mg/L and40000mg/L respectively, was treated with aerobic membrane biological reactors (MBR), and the target COD of the treated effluent was below500mg/L. After three-month acclimatization period, the biological treatment system showed a relatively ideal performance in the subsequent four-month operation period. Namely, the treated effluent met the discharge standard most of the time. However, three great problems were found during the two periods. The first problem was that the permeate flux of the ultrafiltration system descended too fast, and even faster as the salinity ascended. During operation, the regular physical wash (air-water backwash) was unable to effectively restore the permeate flux, so the intervals between chemical washes had to be shortened. Heightening the frequency and intensity of physical washes had some mitigation effect, but the effect was quite limited. The second problem was that the biological treatment system showed obvious untoward reaction to salinity shocks. Especially during the acclimatization period, the yet unstable biological system was very sensitive to the ambient salinity change, so the principle of increasing influent salinity gradually must be strictly followed. During operation period, the biological system showed better tolerance to salinity shock. However, the CMC wastewater fluctuated in water quality fairly widely, and a wave of salinity shock over35000mg/L still caused considerable damage to the biological system. The third problem was that huge amounts of big tawny bubbles piled up layer upon layer, overflowed out of the aerobic reactor and drifted everywhere blown by the wind. Although the foaming problem had less negative effect on the treatment efficiency of the biological system, it was destructive to the factory environment.Secondly, the causes of these problems are analysed from the viewpoint of the wastewater constituents. Upon review of research literature, sodium glycolate, the main organic constituent in CMC wasterwater, is inferred to be one probable cause of these problems, and the abscence of micronutrients in CMC wasterwater is probably the principal cause of the problems. The improvements to these problems are discussed from the viewpoint of the wastewater treatment process. With better shock tolerance and less suspended sludge, Moving Bed Biofilm Reactor (MBBR) is supposed to make improvements.Thirdly, a comparison upon improvement effects on filterability, settleability and salinity shock tolerance is made between the AS reactors with micronutrients added and the MBBR without micronutrients added. The addition of micronutrients is realized through two ways:adding natural water into the wastewater and adding chemical agents into the wastewater. The experiment results show:adding natural water into the wastewater notably improves the filterability and salinity shock tolerance, but fails to make improvement in settleability; adding chemical agents into the wastewater makes no improvement, but negative effects; the MBBR makes the worst filterability, even far worse than any of the AS reactors; all the three trial solutions make no improvement in treatment efficiency; the minimum Food to Microorganism (F/M) that can maintain the current activated sludge concentration is0.3-0.5kg COD/kg MLSS. The following conclusions are made on the basis of the experiment results:the micronutrients in natural water can effectively improve the filterability and salinity shock tolerance of the activated sludge for treating CMC wastewater, and then for full scale projects, mixing CMC wastewater with municipal sewage is expected to be a more practicable option of acquiring micronutrients and improving the performance of the activated sludge.Fourthly, whether mixing CMC wastewater with municipal sewage can improve the filterability is studied through bench scale experiment and intermediate scale experiment. The bench scale experiment results show:mixing municipal sewage can effectively improve the filterability of the activated sludge, but slightly less effective than adding natural water. In the intermediate scale experiment held in a CMC factory, high concentration CMC wastewater and municipal sewage was mixed by ratio1:3, and treated with a plug flow aerobic MBR. The SCOD and salinity of the wastewater was18000mg/L and32000mg/L respectively; the design organic loading was0.4kgSCOD/kgMLSS. The intermediate scale experiment results show:the COD of the treated effluent was constantly below1000mg/L; the ultrafiltration flux descended slowly, and the interval between chemical washes was more than two months.Fifthly, the trace elements in the natural water utilised in the experiments are qualitatively analysed and quantitatively determined by a Perkin Elmer Optima8300Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES), and the results could be the reference for natural water synthesis.Sixthly, the formation mechanism of stable foam is defined upon review of research literature: stable foam is formed under the synergy of foaming agent (surfactant) and foaming stabiliser. The causes of foaming problem in the aerobic treatment of CMC wastewater are analysed by aerating unfiltered, filtered CMC wastewater, the mixed liquid of unfiltered, filtered CMC wastewater and activated sludge. The following conclusions are made on the basis of the experiment results:as a surfactant, CMC is the foaming agent; as the suspended solid in CMC wastewater, natural cellulose and its impurities is the foaming stabiliser, which has very excellent foam stabilising effect. For full scale projects, there can be two solutions to the foaming problem:the first is to pre-aerate the CMC wastewater for a period of time and then skim the scum before aerobic biological treatment; the second is to filter the CMC wastewater before aerobic biological treatment.Seventhly, the dissertation discusses the building of a mathematical model which can model the treatment of CMC wastewater with activated sludge process within the framework of the Activated Sludge Model No.1(ASM1) developed by the International Water Association (IWA). ASM1is on the theoretical basis of the death-regeneration hypothesis, modelling8dynamic processes, involving13state variables, utilising5stoichiometric parameters and14kinetic parameters. After studying ASM1detailedly and analysing the water quality of CMC wastewater deliberately, the "CMC wastewater-ASM1", which simplifies the original ASM1from8dynamic processes to3, from13state variables to8, from5stoichiometric parameters to4, from14kinetic parameters to5, has been built. Among the model parameters,1toichiometric parameter and2kinetic parameters need measuring and calculating. After measurement and calculation, the heterotrophic yields YH, the heterotrophic maximum specific growth rates μM and the heterotrophic decay rates6H for CMC wastewater treatment at different ambient salinity within range from10000mg/L to80000mg/L have been acquired.